Quartz-Enhanced Photoacoustic Spectroscopy: A Review
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paper_title: High-Performance InP-Based Mid-IR Quantum Cascade Lasers

paper_content:

Quantum cascade lasers (QCLs) were once considered as inefficient devices, as the wall-plug efficiency (WPE) was merely a few percent at room temperature. But this situation has changed in the past few years, as dramatic enhancements to the output power and WPE have been made for InP-based mid-IR QCLs. Room temperature continuous-wave (CW) output power as high as 2.8 W and WPE as high as 15% have now been demonstrated for individual devices. Along with the fundamental exploration of refining the design and improving the material quality, a consistent determination of important device performance parameters allows for strategically addressing each component that can be improved potentially. In this paper, we present quantitative experimental evidence backing up the strategies we have adopted to improve the WPE for QCLs with room temperature CW operation.
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paper_title: External cavity quantum cascade laser

paper_content:

In this paper we review the progress of the development of mid-infrared quantum cascade lasers (QCLs) operated in an external cavity configuration. We concentrate on QCLs based on the bound-to-continuum design, since this design is especially suitable for broadband applications. Since they were first demonstrated, these laser-based tunable sources have improved in performance in terms of output power, duty cycle, operation temperature and tuneability. Nowadays they are an interesting alternative to FTIRs for some applications. They operate at room temperature, feature a high spectral resolution while being small in size. They were successfully used in different absorption spectroscopy techniques. Due to their vast potential for applications in industry, medicine, security and research, these sources enjoy increasing interest within the research community as well as in industry.
---
paper_title: Cavity Ring-Down Spectroscopy: Techniques and Applications

paper_content:

Preface List of contributors Glossary Chapter 1 - An introduction to cavity ring-down spectroscopy 1.1 Introduction 1.2 Direct absorption spectroscopy 1.3 Basic cavity ring down spectroscopy setup 1.4 A more refined picture 1.5 Fitting of cavity ring down transients 1.6 A few examples 1.7 Going beyond the standard pulsed CRDS experiment 1.8 Summary 1.9 References Chapter 2 - Cavity enhanced techniques using continuous wave lasers 2.1 Introduction 2.1 Properties of optical cavities and cw lasers relevant to cavity enhanced spectroscopy 2.3 Experimental methods for cw laser cavity enhanced spectroscopy 2.4 Spectroscopy with resonant cavities 2.5 Summary Chapter 3 - Broadband cavity ring-down spectroscopy 3.1 Introduction. 3.2 The time and wavelength evolution of a single ringdown event. 3.3 Two dimensional techniques: resolving broadband cavity output in time and wavelength. 3.4 One dimensional techniques: time or wavelength. 3.5 How to extract quantitative information from broadband spectra. 3.6 Optimising the sensitivity of a broadband measurement. 3.7 Applications of broadband cavity methods. 3.8 References . Chapter 4 - Cavity ring-down spectroscopy in analytical chemistry 4.1 Introduction 4.2 Condensed media CRDS 4.3 Evanescent-wave CRDS 4.4 Future trends and perspectives Chapter 5 - Cavity ring-down spectroscopy using waveguides 5.1. Introduction 5.2. The basic experiments 5.3. Optics and Instrumentation 5.4. Review of waveguide CRD literature 5.5. Conclusion and outlook 5.6. Acknowledgements Chapter 6 - Cavity ring down spectroscopy of molecular transients of astrophysical interest 6.1. Introduction 6.2. Experimental 6.3. Astronomical considerations 6.4. Results 6.5. Outlook Acknowledgements References Chapter 7 - Applications of cavity ring-down spectroscopy in atmospheric chemistry 7.1. Brief overview 7.2. Measurement of trace atmospheric species by CRDS 7.3. Laboratory based studies of atmospheric interest 7.4. Optical properties of atmospheric aerosol particles 7.5. Future developments Chapter 8 - Cavity ring-down spectroscopy for medical applications 8.1. Introduction 8.2. Trace gases in medicine and biology 8.3. Instrumentation for laser analytics of breath and other biological gas samples 8.4. Applications to life sciences 8.5. Conclusion and Perspectives 8.6. References Chapter 9: Studies into the growth mechanism of a-Si:H using in situ cavity ring-down techniques 9.1. Introduction 9.2. Gas phase CRDS on SiH x radicals 9.3. Thin film CRDS on dangling bonds in a-Si:H films (ex situ) 9.4. Evanescent wave CRDS on dangling bonds during a-Si:H film growth Chapter 10 - Cavity ring down spectroscopy for combustion studies 10.1. Introduction 10.2. General description of cavity ring down spectroscopy in flames 10.3. Experimental set-up 10.4. Quantitative concentration measurements in flames 10.5. Concentration profile determination 10.6. Specific difficulties in combustion studies 10.7. Case of particles: soot volume fraction determination 10.8. Conclusion and prospective References Appendix A Literature
---
paper_title: Molecular gas sensing below parts per trillion: radiocarbon-dioxide optical detection.

paper_content:

Radiocarbon (C) concentrations at a 43 parts-per-quadrillion level are measured by using saturatedabsorption cavity ringdown spectroscopy by exciting radiocarbon-dioxide (CO2) molecules at the 4:5 m wavelength. The ultimate sensitivity limits of molecular trace gas sensing are pushed down to attobar pressures using a comb-assisted absorption spectroscopy setup. Such a result represents the lowest pressure ever detected for a gas of simple molecules. The unique sensitivity, the wide dynamic range, the compactness, and the relatively low cost of this table-top setup open new perspectives for C-tracing applications, such as radiocarbon dating, biomedicine, or environmental and earth sciences. The detection of other very rare molecules can be pursued as well thanks to the wide and continuous mid-IR spectral coverage of the described setup.
---
paper_title: Photoacoustic Techniques for Trace Gas Sensing Based on Semiconductor Laser Sources

paper_content:

The paper provides an overview on the use of photoacoustic sensors based on semiconductor laser sources for the detection of trace gases. We review the results obtained using standard, differential and quartz enhanced photoacoustic techniques.
---
paper_title: Terahertz quantum cascade lasers

paper_content:

Six years after their birth, terahertz quantum-cascade lasers can now deliver milliwatts or more of continuous-wave coherent radiation throughout the terahertz range — the spectral regime between millimetre and infrared wavelengths, which has long resisted development. This paper reviews the state-of-the-art and future prospects for these lasers, including efforts to increase their operating temperatures, deliver higher output powers and emit longer wavelengths.
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paper_title: Cavity-Enhanced Direct Frequency Comb Spectroscopy: Technology and Applications

paper_content:

Cavity-enhanced direct frequency comb spectroscopy combines broad bandwidth, high spectral resolution, and ultrahigh detection sensitivity in one experimental platform based on an optical frequency comb efficiently coupled to a high-finesse cavity. The effective interaction length between light and matter is increased by the cavity, massively enhancing the sensitivity for measurement of optical losses. Individual comb components act as independent detection channels across a broad spectral window, providing rapid parallel processing. In this review we discuss the principles, the technology, and the first applications that demonstrate the enormous potential of this spectroscopic method. In particular, we describe various frequency comb sources, techniques for efficient coupling between comb and cavity, and detection schemes that utilize the technique's high-resolution, wide-bandwidth, and fast data-acquisition capabilities. We discuss a range of applications, including breath analysis for medical diagnosis, trace-impurity detection in specialty gases, and characterization of a supersonic jet of cold molecules.
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paper_title: Quantum cascade laser-based integrated cavity output spectroscopy of exhaled nitric oxide

paper_content:

A nitric oxide (NO) sensor employing a ther- moelectrically cooled, continuous-wave, distributed feedback quantum cascade laser operating at 5.47 µm (1828 cm −1 )a nd off-axis integrated cavity output spectroscopy was used to meas- ure NO concentrations in exhaled breath. A minimum mea- surable concentration (3σ) of 3.6 parts-per-billion by volume (ppbv) of NO with a data-acquisition time of 4 s was demon- strated. Five prepared gas mixtures and 15 exhaled breath samples were measured with both the NO sensor and for in- tercomparison with a chemiluminescence-based NO analyzer and were found to be in agreement within 0.6 ppbv. Exhaled NO flow-independent parameters, which may provide diagnos- tic and therapeutic information in respiratory diseases where single-breath measurements are equivocal, were estimated from end-tidal NO concentration measurements collected at various flow rates. The results of this work indicate that a laser-based ex- haled NO sensor can be used to measure exhaled nitric oxide at a range of exhalation flow rates to determine flow-independent parameters in human clinical trials.
---
paper_title: High-Performance InP-Based Mid-IR Quantum Cascade Lasers

paper_content:

Quantum cascade lasers (QCLs) were once considered as inefficient devices, as the wall-plug efficiency (WPE) was merely a few percent at room temperature. But this situation has changed in the past few years, as dramatic enhancements to the output power and WPE have been made for InP-based mid-IR QCLs. Room temperature continuous-wave (CW) output power as high as 2.8 W and WPE as high as 15% have now been demonstrated for individual devices. Along with the fundamental exploration of refining the design and improving the material quality, a consistent determination of important device performance parameters allows for strategically addressing each component that can be improved potentially. In this paper, we present quantitative experimental evidence backing up the strategies we have adopted to improve the WPE for QCLs with room temperature CW operation.
---
paper_title: Geometrical optimization of a longitudinal resonant photoacoustic cell for sensitive and fast trace gas detection

paper_content:

We present a quantitative discussion of the acoustic transmission line theory pertaining to experimental results from a resonant photoacoustic cell excited in its first longitudinal mode. Window absorption is optimally suppressed by buffer volumes and tunable air columns. The acoustic behavior of an ultrasensitive one inch condenser microphone is quantitatively described. A small and sensitive photoacoustic cell has been developed for intracavity use in a CO2 waveguide laser permitting measurements of ethylene down to 6 pptv (long term stability 20 pptv) with a time response of 2 s at a trace gas flow of 6 1/h. To demonstrate the fast time response within a biological application the instant ethylene release of a single tomato is measured.
---
paper_title: On-line laser photoacoustic detection of ethene in exhaled air as biomarker of ultraviolet radiation damage of the human skin

paper_content:

The exhaled air and volatile emission by the skin of human subjects were analyzed for traces of ethene (C2H4) by means of CO2 laser photoacoustic trace gas detection. Due to the extreme sensitivity of the detection system (6 part per trillion volume, 6:1012), these measurements could be performed on-line and noninvasively. Exhaled ethene was used as a biomarker for lipid peroxidation in the skin of human subjects exposed to ultraviolet (UV) radiation from a solarium. A change in the ethene concentration was already observed in the exhaled air after 2 min. Adaptation of the skin to UV exposure and direct skin emission could also be observed.
---
paper_title: Dual cantilever enhanced photoacoustic detector with pulsed broadband IR-source

paper_content:

Abstract The cantilever enhanced photoacoustic (CEPA) trace gas detection was combined with an electrically modulated broadband infrared (EMBIR) source. The high sensitivity of the detection method was further improved by using two cantilever sensors on the opposite walls of the photoacoustic cell in order to suppress noise. Methane (CH 4 ) gas was used to demonstrate the sensitivity of the method yielding a detection limit of 0.5 ppm with 5 s sample integration time employing an optical filter with a center wavenumber 2950 cm −1 . The achieved result is very good with a low power infrared source and enables the utilization of the benefits of such a source: low price and power consumption, easily controlled and simple electrical modulation of the radiation intensity and absence of the mechanical chopper and its noise. The structure of the source enables a relatively high pulse rate to be used as the modulating frequency. Optical filters can be used to select the wavenumber region for the selective detection of different gases.
---
paper_title: High-resolution photoacoustic and direct absorption spectroscopy of main greenhouse gases by use of a pulsed entangled cavity doubly resonant OPO

paper_content:

An entangled cavity doubly resonant optical parametric oscillator (ECOPO) has been developed to provide tunable narrow line width (<100 MHz) pulsed (8 ns) radiation over the 3.8–4.3 μm spectral range at a multi-kilohertz repetition rate with up to 100-W peak power. We demonstrate that coarse single mode tuning is obtained over the full spectral range of oscillation (300 cm−1), while automated mode-hop-free fine tuning is carried out over more than 100 GHz. High-resolution spectra of main greenhouse gases (CO2, N2O, SO2 and CH4) have been obtained in good agreement with calculated spectra from the HITRAN database. These experiments outline the unique capabilities of the ECOPO for multi-gas sensing based on direct absorption as well as photoacoustic spectroscopy.
---
paper_title: Molecular gas sensing below parts per trillion: radiocarbon-dioxide optical detection.

paper_content:

Radiocarbon (C) concentrations at a 43 parts-per-quadrillion level are measured by using saturatedabsorption cavity ringdown spectroscopy by exciting radiocarbon-dioxide (CO2) molecules at the 4:5 m wavelength. The ultimate sensitivity limits of molecular trace gas sensing are pushed down to attobar pressures using a comb-assisted absorption spectroscopy setup. Such a result represents the lowest pressure ever detected for a gas of simple molecules. The unique sensitivity, the wide dynamic range, the compactness, and the relatively low cost of this table-top setup open new perspectives for C-tracing applications, such as radiocarbon dating, biomedicine, or environmental and earth sciences. The detection of other very rare molecules can be pursued as well thanks to the wide and continuous mid-IR spectral coverage of the described setup.
---
paper_title: The potential of mid-infrared photoacoustic spectroscopy for the detection of various doping agents used by athletes

paper_content:

The feasibility of laser-photoacoustic measurements for the detection and the analysis of different isolated doping agents in the vapour phase is discussed. To the best of our knowledge, this is the first time that photoacoustic vapour-phase measurements of doping substances have been presented. Spectra of different doping classes (stimulants, anabolica, diuretica, and beta blockers) are shown and discussed in terms of their detection sensitivity and selectivity. The potential of laser spectroscopy for detecting the intake of prohibited substances by athletes is explored.
---
paper_title: Improved photoacoustic detector for monitoring polar molecules such as ammonia with a 1.53 μm DFB diode laser

paper_content:

A new differential photoacoustic (PA) detector equipped with band-rejecting acoustic filters has been developed for diode laser photoacoustics. The differential design provides good flow and electric noise suppression. The detector was used for monitoring ammonia in continuous flow operation. The first test measurements of synthetic air-ammonia mixtures were carried out using a 5 mW distributed feedback (DFB) diode laser at 1.53 μm, providing a sensitivity limit of about 300 ppbV. Because of the adsorption of ammonia molecules on the walls of the detector and the gas tubing, high flow rates were needed for the measurements in the 1–100 ppmV concentration range. The smallest concentration detected was 1 ppmV.
---
paper_title: Fiber-amplifier-enhanced photoacoustic spectroscopy with near-infrared tunable diode lasers

paper_content:

A new approach to wavelength-modulation photoacoustic spectroscopy is reported, which incorporates diode lasers in the near infrared and optical fiber amplifiers to enhance sensitivity. We demonstrate the technique with ammonia detection, yielding a sensitivity limit less than 6 parts in 10(9), by interrogating a transition near 1532 nm with 500 mW of output power from the fiber amplifier, an optical pathlength of 18.4 cm, and an integration time constant of 10 s. This sensitivity is 15 times better than in prior published results for detecting ammonia with near-infrared diode lasers. The normalized minimum detectable fractional optical density, alphaminl, is 1.8 x 10(-8); the minimum detectable absorption coefficient, alphamin, is 9.5 x 10(-10) cm(-1); and the minimum detectable absorption coefficient normalized by power and bandwidth is 1.5 x 10(-9) W cm(-1)/square root Hz. These measurements represent what we believe to be the first use of fiber amplifiers to enhance photoacoustic spectroscopy, and this technique is applicable to all other species that fall within the gain curves of optical fiber amplifiers.
---
paper_title: Photoacoustic Techniques for Trace Gas Sensing Based on Semiconductor Laser Sources

paper_content:

The paper provides an overview on the use of photoacoustic sensors based on semiconductor laser sources for the detection of trace gases. We review the results obtained using standard, differential and quartz enhanced photoacoustic techniques.
---
paper_title: Applications of Photoacoustic Spectroscopy to Problems in Dermatology Research

paper_content:

The technique of photoacoustic spectroscopy (PAS) was applied in two areas of dermatology research: 1) drug detection and drug diffusion rates in skin, and 2) thermal properties and water content of skin. The drug studies involved detection of the drug tetracycline in the skin and determination of the diffusion rate of the drug through the skin. The water content studies involved determining the thermal properties of the epidermis as a function of water content and the effect of the water concentration gradient across the epidermis. A multilayer model for the photoacoustic effect was developed to account for the nonuniform thermal properties of the intact skin arising from the water concentration gradient. This model was used to determine the width of the region comprising the diffusional barrier in skin. The width of the barrier region was found to correspond to that of the outermost layer of the epidermis, the stratum corneum. This finding coincides with previous research indicating that the stratum corneum comprises the primary barrier to the diffusion of water through the epidermis.
---
paper_title: Applications of quartz tuning forks in spectroscopic gas sensing

paper_content:

A recently introduced approach to photoacoustic detection of trace gases utilizing a quartz tuning fork (TF) as a resonant acoustic transducer is described in detail. Advantages of the technique called quartz-enhanced photoacoustic spectroscopy (QEPAS) compared to conventional resonant photoacoustic spectroscopy include QEPAS sensor immunity to environmental acoustic noise, a simple absorption detection module design, and its capability to analyze gas samples ∼1mm3 in volume. Noise sources and the TF properties as a function of the sampled gas pressure, temperature and chemical composition are analyzed. Previously published results for QEPAS based chemical gas sensing are summarized. The achieved sensitivity of 5.4×10−9cm−1W∕√Hz is compared to recent published results of photoacoustic gas sensing by other research groups. An experimental study of the long-term stability of a QEPAS-based ammonia sensor is presented. The results of this study indicate that the sensor exhibits very low drift, which allows da...
---
paper_title: QEPAS based ppb-level detection of CO and N_2O using a high power CW DFB-QCL

paper_content:

An ultra-sensitive and selective quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor platform was demonstrated for detection of carbon monoxide (CO) and nitrous oxide (N2O). This sensor used a state-of-the art 4.61 μm high power, continuous wave (CW), distributed feedback quantum cascade laser (DFB-QCL) operating at 10°C as the excitation source. For the R(6) CO absorption line, located at 2169.2 cm−1, a minimum detection limit (MDL) of 1.5 parts per billion by volume (ppbv) at atmospheric pressure was achieved with a 1 sec acquisition time and the addition of 2.6% water vapor concentration in the analyzed gas mixture. For the N2O detection, a MDL of 23 ppbv was obtained at an optimum gas pressure of 100 Torr and with the same water vapor content of 2.6%. In both cases the presence of water vapor increases the detected CO and N2O QEPAS signal levels as a result of enhancing the vibrational-translational relaxation rate of both target gases. Allan deviation analyses were performed to investigate the long term performance of the CO and N2O QEPAS sensor systems. For the optimum data acquisition time of 500 sec a MDL of 340 pptv and 4 ppbv was obtained for CO and N2O detection, respectively. To demonstrate reliable and robust operation of the QEPAS sensor a continuous monitoring of atmospheric CO and N2O concentration levels for a period of 5 hours were performed.
---
paper_title: Applications of Photoacoustic Spectroscopy to Problems in Dermatology Research

paper_content:

The technique of photoacoustic spectroscopy (PAS) was applied in two areas of dermatology research: 1) drug detection and drug diffusion rates in skin, and 2) thermal properties and water content of skin. The drug studies involved detection of the drug tetracycline in the skin and determination of the diffusion rate of the drug through the skin. The water content studies involved determining the thermal properties of the epidermis as a function of water content and the effect of the water concentration gradient across the epidermis. A multilayer model for the photoacoustic effect was developed to account for the nonuniform thermal properties of the intact skin arising from the water concentration gradient. This model was used to determine the width of the region comprising the diffusional barrier in skin. The width of the barrier region was found to correspond to that of the outermost layer of the epidermis, the stratum corneum. This finding coincides with previous research indicating that the stratum corneum comprises the primary barrier to the diffusion of water through the epidermis.
---
paper_title: Quartz-enhanced photoacoustic spectroscopy

paper_content:

A new approach to detecting a weak photoacoustic signal in a gas medium is described. Instead of a gas-filled resonant acoustic cavity, the sound energy is accumulated in a high- Q crystal element. Feasibility experiments utilizing a quartz-watch tuning fork demonstrate a sensitivity of 1.2x10(-7) cm(-1) W/ radicalHz . Potential further developments and applications of this technique are discussed.
---
paper_title: Applications of quartz tuning forks in spectroscopic gas sensing

paper_content:

A recently introduced approach to photoacoustic detection of trace gases utilizing a quartz tuning fork (TF) as a resonant acoustic transducer is described in detail. Advantages of the technique called quartz-enhanced photoacoustic spectroscopy (QEPAS) compared to conventional resonant photoacoustic spectroscopy include QEPAS sensor immunity to environmental acoustic noise, a simple absorption detection module design, and its capability to analyze gas samples ∼1mm3 in volume. Noise sources and the TF properties as a function of the sampled gas pressure, temperature and chemical composition are analyzed. Previously published results for QEPAS based chemical gas sensing are summarized. The achieved sensitivity of 5.4×10−9cm−1W∕√Hz is compared to recent published results of photoacoustic gas sensing by other research groups. An experimental study of the long-term stability of a QEPAS-based ammonia sensor is presented. The results of this study indicate that the sensor exhibits very low drift, which allows da...
---
paper_title: QEPAS based ppb-level detection of CO and N_2O using a high power CW DFB-QCL

paper_content:

An ultra-sensitive and selective quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor platform was demonstrated for detection of carbon monoxide (CO) and nitrous oxide (N2O). This sensor used a state-of-the art 4.61 μm high power, continuous wave (CW), distributed feedback quantum cascade laser (DFB-QCL) operating at 10°C as the excitation source. For the R(6) CO absorption line, located at 2169.2 cm−1, a minimum detection limit (MDL) of 1.5 parts per billion by volume (ppbv) at atmospheric pressure was achieved with a 1 sec acquisition time and the addition of 2.6% water vapor concentration in the analyzed gas mixture. For the N2O detection, a MDL of 23 ppbv was obtained at an optimum gas pressure of 100 Torr and with the same water vapor content of 2.6%. In both cases the presence of water vapor increases the detected CO and N2O QEPAS signal levels as a result of enhancing the vibrational-translational relaxation rate of both target gases. Allan deviation analyses were performed to investigate the long term performance of the CO and N2O QEPAS sensor systems. For the optimum data acquisition time of 500 sec a MDL of 340 pptv and 4 ppbv was obtained for CO and N2O detection, respectively. To demonstrate reliable and robust operation of the QEPAS sensor a continuous monitoring of atmospheric CO and N2O concentration levels for a period of 5 hours were performed.
---
paper_title: Quartz-enhanced photoacoustic spectroscopy

paper_content:

A new approach to detecting a weak photoacoustic signal in a gas medium is described. Instead of a gas-filled resonant acoustic cavity, the sound energy is accumulated in a high- Q crystal element. Feasibility experiments utilizing a quartz-watch tuning fork demonstrate a sensitivity of 1.2x10(-7) cm(-1) W/ radicalHz . Potential further developments and applications of this technique are discussed.
---
paper_title: Geometrical optimization of a longitudinal resonant photoacoustic cell for sensitive and fast trace gas detection

paper_content:

We present a quantitative discussion of the acoustic transmission line theory pertaining to experimental results from a resonant photoacoustic cell excited in its first longitudinal mode. Window absorption is optimally suppressed by buffer volumes and tunable air columns. The acoustic behavior of an ultrasensitive one inch condenser microphone is quantitatively described. A small and sensitive photoacoustic cell has been developed for intracavity use in a CO2 waveguide laser permitting measurements of ethylene down to 6 pptv (long term stability 20 pptv) with a time response of 2 s at a trace gas flow of 6 1/h. To demonstrate the fast time response within a biological application the instant ethylene release of a single tomato is measured.
---
paper_title: Air and gas damping of quartz tuning forks

paper_content:

Abstract The effects of air and gas pressure on the vibration of miniature quartz tuning forks are investigated. Beside damping, which decrease the Q factor of the resonator, the resonant frequency appears to vary quite linearly with the gas pressure. This basic effect could lead to the realization of a simple pressure sensing element. Analysis of the experimental data for the damping, 1/ Q , and the relative frequency shift, Δ f / f 0 , of the resonant frequency is made in terms of acoustic loss and fluid loading of the tuning fork. Direct correlation between Δ f / f 0 and the gas mass carried along is clearly shown, so that the expected sensitivity of the quartz tuning fork as a pressure sensor and its useful range of operation can be well defined.
---
paper_title: On-line laser photoacoustic detection of ethene in exhaled air as biomarker of ultraviolet radiation damage of the human skin

paper_content:

The exhaled air and volatile emission by the skin of human subjects were analyzed for traces of ethene (C2H4) by means of CO2 laser photoacoustic trace gas detection. Due to the extreme sensitivity of the detection system (6 part per trillion volume, 6:1012), these measurements could be performed on-line and noninvasively. Exhaled ethene was used as a biomarker for lipid peroxidation in the skin of human subjects exposed to ultraviolet (UV) radiation from a solarium. A change in the ethene concentration was already observed in the exhaled air after 2 min. Adaptation of the skin to UV exposure and direct skin emission could also be observed.
---
paper_title: Dual cantilever enhanced photoacoustic detector with pulsed broadband IR-source

paper_content:

Abstract The cantilever enhanced photoacoustic (CEPA) trace gas detection was combined with an electrically modulated broadband infrared (EMBIR) source. The high sensitivity of the detection method was further improved by using two cantilever sensors on the opposite walls of the photoacoustic cell in order to suppress noise. Methane (CH 4 ) gas was used to demonstrate the sensitivity of the method yielding a detection limit of 0.5 ppm with 5 s sample integration time employing an optical filter with a center wavenumber 2950 cm −1 . The achieved result is very good with a low power infrared source and enables the utilization of the benefits of such a source: low price and power consumption, easily controlled and simple electrical modulation of the radiation intensity and absence of the mechanical chopper and its noise. The structure of the source enables a relatively high pulse rate to be used as the modulating frequency. Optical filters can be used to select the wavenumber region for the selective detection of different gases.
---
paper_title: Cavity Ring-Down Spectroscopy: Techniques and Applications

paper_content:

Preface List of contributors Glossary Chapter 1 - An introduction to cavity ring-down spectroscopy 1.1 Introduction 1.2 Direct absorption spectroscopy 1.3 Basic cavity ring down spectroscopy setup 1.4 A more refined picture 1.5 Fitting of cavity ring down transients 1.6 A few examples 1.7 Going beyond the standard pulsed CRDS experiment 1.8 Summary 1.9 References Chapter 2 - Cavity enhanced techniques using continuous wave lasers 2.1 Introduction 2.1 Properties of optical cavities and cw lasers relevant to cavity enhanced spectroscopy 2.3 Experimental methods for cw laser cavity enhanced spectroscopy 2.4 Spectroscopy with resonant cavities 2.5 Summary Chapter 3 - Broadband cavity ring-down spectroscopy 3.1 Introduction. 3.2 The time and wavelength evolution of a single ringdown event. 3.3 Two dimensional techniques: resolving broadband cavity output in time and wavelength. 3.4 One dimensional techniques: time or wavelength. 3.5 How to extract quantitative information from broadband spectra. 3.6 Optimising the sensitivity of a broadband measurement. 3.7 Applications of broadband cavity methods. 3.8 References . Chapter 4 - Cavity ring-down spectroscopy in analytical chemistry 4.1 Introduction 4.2 Condensed media CRDS 4.3 Evanescent-wave CRDS 4.4 Future trends and perspectives Chapter 5 - Cavity ring-down spectroscopy using waveguides 5.1. Introduction 5.2. The basic experiments 5.3. Optics and Instrumentation 5.4. Review of waveguide CRD literature 5.5. Conclusion and outlook 5.6. Acknowledgements Chapter 6 - Cavity ring down spectroscopy of molecular transients of astrophysical interest 6.1. Introduction 6.2. Experimental 6.3. Astronomical considerations 6.4. Results 6.5. Outlook Acknowledgements References Chapter 7 - Applications of cavity ring-down spectroscopy in atmospheric chemistry 7.1. Brief overview 7.2. Measurement of trace atmospheric species by CRDS 7.3. Laboratory based studies of atmospheric interest 7.4. Optical properties of atmospheric aerosol particles 7.5. Future developments Chapter 8 - Cavity ring-down spectroscopy for medical applications 8.1. Introduction 8.2. Trace gases in medicine and biology 8.3. Instrumentation for laser analytics of breath and other biological gas samples 8.4. Applications to life sciences 8.5. Conclusion and Perspectives 8.6. References Chapter 9: Studies into the growth mechanism of a-Si:H using in situ cavity ring-down techniques 9.1. Introduction 9.2. Gas phase CRDS on SiH x radicals 9.3. Thin film CRDS on dangling bonds in a-Si:H films (ex situ) 9.4. Evanescent wave CRDS on dangling bonds during a-Si:H film growth Chapter 10 - Cavity ring down spectroscopy for combustion studies 10.1. Introduction 10.2. General description of cavity ring down spectroscopy in flames 10.3. Experimental set-up 10.4. Quantitative concentration measurements in flames 10.5. Concentration profile determination 10.6. Specific difficulties in combustion studies 10.7. Case of particles: soot volume fraction determination 10.8. Conclusion and prospective References Appendix A Literature
---
paper_title: High-resolution photoacoustic and direct absorption spectroscopy of main greenhouse gases by use of a pulsed entangled cavity doubly resonant OPO

paper_content:

An entangled cavity doubly resonant optical parametric oscillator (ECOPO) has been developed to provide tunable narrow line width (<100 MHz) pulsed (8 ns) radiation over the 3.8–4.3 μm spectral range at a multi-kilohertz repetition rate with up to 100-W peak power. We demonstrate that coarse single mode tuning is obtained over the full spectral range of oscillation (300 cm−1), while automated mode-hop-free fine tuning is carried out over more than 100 GHz. High-resolution spectra of main greenhouse gases (CO2, N2O, SO2 and CH4) have been obtained in good agreement with calculated spectra from the HITRAN database. These experiments outline the unique capabilities of the ECOPO for multi-gas sensing based on direct absorption as well as photoacoustic spectroscopy.
---
paper_title: The potential of mid-infrared photoacoustic spectroscopy for the detection of various doping agents used by athletes

paper_content:

The feasibility of laser-photoacoustic measurements for the detection and the analysis of different isolated doping agents in the vapour phase is discussed. To the best of our knowledge, this is the first time that photoacoustic vapour-phase measurements of doping substances have been presented. Spectra of different doping classes (stimulants, anabolica, diuretica, and beta blockers) are shown and discussed in terms of their detection sensitivity and selectivity. The potential of laser spectroscopy for detecting the intake of prohibited substances by athletes is explored.
---
paper_title: Improved photoacoustic detector for monitoring polar molecules such as ammonia with a 1.53 μm DFB diode laser

paper_content:

A new differential photoacoustic (PA) detector equipped with band-rejecting acoustic filters has been developed for diode laser photoacoustics. The differential design provides good flow and electric noise suppression. The detector was used for monitoring ammonia in continuous flow operation. The first test measurements of synthetic air-ammonia mixtures were carried out using a 5 mW distributed feedback (DFB) diode laser at 1.53 μm, providing a sensitivity limit of about 300 ppbV. Because of the adsorption of ammonia molecules on the walls of the detector and the gas tubing, high flow rates were needed for the measurements in the 1–100 ppmV concentration range. The smallest concentration detected was 1 ppmV.
---
paper_title: Fiber-amplifier-enhanced photoacoustic spectroscopy with near-infrared tunable diode lasers

paper_content:

A new approach to wavelength-modulation photoacoustic spectroscopy is reported, which incorporates diode lasers in the near infrared and optical fiber amplifiers to enhance sensitivity. We demonstrate the technique with ammonia detection, yielding a sensitivity limit less than 6 parts in 10(9), by interrogating a transition near 1532 nm with 500 mW of output power from the fiber amplifier, an optical pathlength of 18.4 cm, and an integration time constant of 10 s. This sensitivity is 15 times better than in prior published results for detecting ammonia with near-infrared diode lasers. The normalized minimum detectable fractional optical density, alphaminl, is 1.8 x 10(-8); the minimum detectable absorption coefficient, alphamin, is 9.5 x 10(-10) cm(-1); and the minimum detectable absorption coefficient normalized by power and bandwidth is 1.5 x 10(-9) W cm(-1)/square root Hz. These measurements represent what we believe to be the first use of fiber amplifiers to enhance photoacoustic spectroscopy, and this technique is applicable to all other species that fall within the gain curves of optical fiber amplifiers.
---
paper_title: Frequency dependent fluid damping of micro/nano flexural resonators: Experiment, model and analysis

paper_content:

This research systematically investigates the fluid damping for micromachined beam-type resonators with high resonant frequencies. The work is aimed to find a general fluid damping law which can quantitatively be used as a design tool for the resonance-based micro sensors by accurately predicting the quality factor for resonators operated in air or liquid. Micro-cantilevers with different dimensions are fabricated and tested in order to extract the damping characteristics. Combined with dimensional analysis, a novel linear fluid damping model is proposed, considering the effects from both device's dimension and the resonant frequency. Further by numerical analysis, the model is generalized to resonators with differently shaped cross-sections. Also the proposed fluid damping model provides an attractive way to use simple beam-type resonators as fluid viscosity sensors and air pressure sensors.
---
paper_title: Applications of Photoacoustic Spectroscopy to Problems in Dermatology Research

paper_content:

The technique of photoacoustic spectroscopy (PAS) was applied in two areas of dermatology research: 1) drug detection and drug diffusion rates in skin, and 2) thermal properties and water content of skin. The drug studies involved detection of the drug tetracycline in the skin and determination of the diffusion rate of the drug through the skin. The water content studies involved determining the thermal properties of the epidermis as a function of water content and the effect of the water concentration gradient across the epidermis. A multilayer model for the photoacoustic effect was developed to account for the nonuniform thermal properties of the intact skin arising from the water concentration gradient. This model was used to determine the width of the region comprising the diffusional barrier in skin. The width of the barrier region was found to correspond to that of the outermost layer of the epidermis, the stratum corneum. This finding coincides with previous research indicating that the stratum corneum comprises the primary barrier to the diffusion of water through the epidermis.
---
paper_title: Quantum cascade laser-based integrated cavity output spectroscopy of exhaled nitric oxide

paper_content:

A nitric oxide (NO) sensor employing a ther- moelectrically cooled, continuous-wave, distributed feedback quantum cascade laser operating at 5.47 µm (1828 cm −1 )a nd off-axis integrated cavity output spectroscopy was used to meas- ure NO concentrations in exhaled breath. A minimum mea- surable concentration (3σ) of 3.6 parts-per-billion by volume (ppbv) of NO with a data-acquisition time of 4 s was demon- strated. Five prepared gas mixtures and 15 exhaled breath samples were measured with both the NO sensor and for in- tercomparison with a chemiluminescence-based NO analyzer and were found to be in agreement within 0.6 ppbv. Exhaled NO flow-independent parameters, which may provide diagnos- tic and therapeutic information in respiratory diseases where single-breath measurements are equivocal, were estimated from end-tidal NO concentration measurements collected at various flow rates. The results of this work indicate that a laser-based ex- haled NO sensor can be used to measure exhaled nitric oxide at a range of exhalation flow rates to determine flow-independent parameters in human clinical trials.
---
paper_title: Applications of quartz tuning forks in spectroscopic gas sensing

paper_content:

A recently introduced approach to photoacoustic detection of trace gases utilizing a quartz tuning fork (TF) as a resonant acoustic transducer is described in detail. Advantages of the technique called quartz-enhanced photoacoustic spectroscopy (QEPAS) compared to conventional resonant photoacoustic spectroscopy include QEPAS sensor immunity to environmental acoustic noise, a simple absorption detection module design, and its capability to analyze gas samples ∼1mm3 in volume. Noise sources and the TF properties as a function of the sampled gas pressure, temperature and chemical composition are analyzed. Previously published results for QEPAS based chemical gas sensing are summarized. The achieved sensitivity of 5.4×10−9cm−1W∕√Hz is compared to recent published results of photoacoustic gas sensing by other research groups. An experimental study of the long-term stability of a QEPAS-based ammonia sensor is presented. The results of this study indicate that the sensor exhibits very low drift, which allows da...
---
paper_title: QEPAS based detection of broadband absorbing molecules using a widely tunable, cw quantum cascade laser at 8.4 μm.

paper_content:

Detection of molecules with wide unresolved rotational-vibrational absorption bands is demonstrated by using Quartz Enhanced Photoacoustic Spectroscopy and an amplitude modulated, high power, thermoelectrically cooled quantum cascade laser operating at 8.4 μm in an external cavity configuration. The laser source exhibits single frequency tuning of 135 cm-1 with a maximum optical output power of 50 mW. For trace-gas detection of Freon 125 (pentafluoroethane) at 1208.62 cm-1 a normalized noise equivalent absorption coefficient of NNEA=2.64×10-9 cm-1∙W/Hz1/2 was obtained. Noise equivalent sensitivity at ppbv level as well as spectroscopic chemical analysis of a mixture of two broadband absorbers (Freon 125 and acetone) with overlapping absorption spectra were demonstrated.
---
paper_title: Air and gas damping of quartz tuning forks

paper_content:

Abstract The effects of air and gas pressure on the vibration of miniature quartz tuning forks are investigated. Beside damping, which decrease the Q factor of the resonator, the resonant frequency appears to vary quite linearly with the gas pressure. This basic effect could lead to the realization of a simple pressure sensing element. Analysis of the experimental data for the damping, 1/ Q , and the relative frequency shift, Δ f / f 0 , of the resonant frequency is made in terms of acoustic loss and fluid loading of the tuning fork. Direct correlation between Δ f / f 0 and the gas mass carried along is clearly shown, so that the expected sensitivity of the quartz tuning fork as a pressure sensor and its useful range of operation can be well defined.
---
paper_title: QEPAS based ppb-level detection of CO and N_2O using a high power CW DFB-QCL

paper_content:

An ultra-sensitive and selective quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor platform was demonstrated for detection of carbon monoxide (CO) and nitrous oxide (N2O). This sensor used a state-of-the art 4.61 μm high power, continuous wave (CW), distributed feedback quantum cascade laser (DFB-QCL) operating at 10°C as the excitation source. For the R(6) CO absorption line, located at 2169.2 cm−1, a minimum detection limit (MDL) of 1.5 parts per billion by volume (ppbv) at atmospheric pressure was achieved with a 1 sec acquisition time and the addition of 2.6% water vapor concentration in the analyzed gas mixture. For the N2O detection, a MDL of 23 ppbv was obtained at an optimum gas pressure of 100 Torr and with the same water vapor content of 2.6%. In both cases the presence of water vapor increases the detected CO and N2O QEPAS signal levels as a result of enhancing the vibrational-translational relaxation rate of both target gases. Allan deviation analyses were performed to investigate the long term performance of the CO and N2O QEPAS sensor systems. For the optimum data acquisition time of 500 sec a MDL of 340 pptv and 4 ppbv was obtained for CO and N2O detection, respectively. To demonstrate reliable and robust operation of the QEPAS sensor a continuous monitoring of atmospheric CO and N2O concentration levels for a period of 5 hours were performed.
---
paper_title: Frequency dependent fluid damping of micro/nano flexural resonators: Experiment, model and analysis

paper_content:

This research systematically investigates the fluid damping for micromachined beam-type resonators with high resonant frequencies. The work is aimed to find a general fluid damping law which can quantitatively be used as a design tool for the resonance-based micro sensors by accurately predicting the quality factor for resonators operated in air or liquid. Micro-cantilevers with different dimensions are fabricated and tested in order to extract the damping characteristics. Combined with dimensional analysis, a novel linear fluid damping model is proposed, considering the effects from both device's dimension and the resonant frequency. Further by numerical analysis, the model is generalized to resonators with differently shaped cross-sections. Also the proposed fluid damping model provides an attractive way to use simple beam-type resonators as fluid viscosity sensors and air pressure sensors.
---
paper_title: Quartz-enhanced photoacoustic spectroscopy

paper_content:

A new approach to detecting a weak photoacoustic signal in a gas medium is described. Instead of a gas-filled resonant acoustic cavity, the sound energy is accumulated in a high- Q crystal element. Feasibility experiments utilizing a quartz-watch tuning fork demonstrate a sensitivity of 1.2x10(-7) cm(-1) W/ radicalHz . Potential further developments and applications of this technique are discussed.
---
paper_title: QEPAS detector for rapid spectral measurements

paper_content:

A quartz enhanced photoacoustic spectroscopy sensor designed for fast response was used in combination with a pulsed external cavity quantum cascade laser to rapidly acquire gas absorption data over the 1196–1281 cm−1 spectral range. The system was used to measure concentrations of water vapor, pentafluoroethane (freon-125), acetone, and ethanol both individually and in combined mixtures. The precision achieved for freon-125 concentration in a single 1.1 s long spectral scan is 13 ppbv.
---
paper_title: QEPAS based detection of broadband absorbing molecules using a widely tunable, cw quantum cascade laser at 8.4 μm.

paper_content:

Detection of molecules with wide unresolved rotational-vibrational absorption bands is demonstrated by using Quartz Enhanced Photoacoustic Spectroscopy and an amplitude modulated, high power, thermoelectrically cooled quantum cascade laser operating at 8.4 μm in an external cavity configuration. The laser source exhibits single frequency tuning of 135 cm-1 with a maximum optical output power of 50 mW. For trace-gas detection of Freon 125 (pentafluoroethane) at 1208.62 cm-1 a normalized noise equivalent absorption coefficient of NNEA=2.64×10-9 cm-1∙W/Hz1/2 was obtained. Noise equivalent sensitivity at ppbv level as well as spectroscopic chemical analysis of a mixture of two broadband absorbers (Freon 125 and acetone) with overlapping absorption spectra were demonstrated.
---
paper_title: Frequency dependent fluid damping of micro/nano flexural resonators: Experiment, model and analysis

paper_content:

This research systematically investigates the fluid damping for micromachined beam-type resonators with high resonant frequencies. The work is aimed to find a general fluid damping law which can quantitatively be used as a design tool for the resonance-based micro sensors by accurately predicting the quality factor for resonators operated in air or liquid. Micro-cantilevers with different dimensions are fabricated and tested in order to extract the damping characteristics. Combined with dimensional analysis, a novel linear fluid damping model is proposed, considering the effects from both device's dimension and the resonant frequency. Further by numerical analysis, the model is generalized to resonators with differently shaped cross-sections. Also the proposed fluid damping model provides an attractive way to use simple beam-type resonators as fluid viscosity sensors and air pressure sensors.
---
paper_title: Gas-phase photoacoustic sensor at 8.41 μm using quartz tuning forks and amplitude-modulated quantum cascade lasers

paper_content:

We demonstrate the performance of a novel infrared photoacoustic laser absorbance sensor for gas-phase species using an amplitude-modulated quantum cascade (QC) laser and a quartz tuning fork microphone. The photoacoustic signal was generated by focusing 5.3 mW of a Fabry–Perot QC laser operating at 8.41 μm between the tines of a quartz tuning fork which served as a transducer for the transient acoustic pressure wave. The sensitivity of this sensor was calibrated using the infrared absorber Freon 134a by performing a simultaneous absorption measurement using a 31-cm absorption cell. The power and bandwidth normalized noise equivalent absorption sensitivity (NEAS) of this sensor was determined to be D=2.0×10-8 W cm-1/Hz1/2. A corresponding theoretical analysis of the instrument sensitivity is presented and is capable of quantitatively reproducing the experimental NEAS, indicating that the fundamental sensitivity of this technique is limited by the noise floor of the tuning fork itself.
---
paper_title: Laser microphotoacoustic sensor of ammonia traces in the atmosphere

paper_content:

A microphotoacoustic highly selective sensor of ammonia is built. Main attention is paid to the operation mechanism of the acoustic sensor based on a quartz tuning fork. The optimal dimensions and configuration of the acoustic resonator are determined, which made it possible to increase the sensor sensitivity by two—three times compared to the sensitivity of the existing devices. The detector sensitivity for ammonia was 60ppb (0.05 mg m-3) for the measurement time of 10s and a 25-mW, 1.53-μm laser beam in the acoustic resonator.
---
paper_title: QEPAS detector for rapid spectral measurements

paper_content:

A quartz enhanced photoacoustic spectroscopy sensor designed for fast response was used in combination with a pulsed external cavity quantum cascade laser to rapidly acquire gas absorption data over the 1196–1281 cm−1 spectral range. The system was used to measure concentrations of water vapor, pentafluoroethane (freon-125), acetone, and ethanol both individually and in combined mixtures. The precision achieved for freon-125 concentration in a single 1.1 s long spectral scan is 13 ppbv.
---
paper_title: Applications of quartz tuning forks in spectroscopic gas sensing

paper_content:

A recently introduced approach to photoacoustic detection of trace gases utilizing a quartz tuning fork (TF) as a resonant acoustic transducer is described in detail. Advantages of the technique called quartz-enhanced photoacoustic spectroscopy (QEPAS) compared to conventional resonant photoacoustic spectroscopy include QEPAS sensor immunity to environmental acoustic noise, a simple absorption detection module design, and its capability to analyze gas samples ∼1mm3 in volume. Noise sources and the TF properties as a function of the sampled gas pressure, temperature and chemical composition are analyzed. Previously published results for QEPAS based chemical gas sensing are summarized. The achieved sensitivity of 5.4×10−9cm−1W∕√Hz is compared to recent published results of photoacoustic gas sensing by other research groups. An experimental study of the long-term stability of a QEPAS-based ammonia sensor is presented. The results of this study indicate that the sensor exhibits very low drift, which allows da...
---
paper_title: Application of acoustic resonators in photoacoustic trace gas analysis and metrology

paper_content:

The application of different types of acoustic resonators such as pipes, cylinders, and spheres in photoacoustics is considered. This includes a discussion of the fundamental properties of these resonant cavities. Modulated and pulsed laser excitation of acoustic modes is discussed. The theoretical and practical aspects of high-Q and low-Q resonators and their integration into complete photoacoustic detection systems for trace gas monitoring and metrology are covered in detail. The characteristics of the available laser sources and the performance of the photoacoustic resonators, such as signal amplification, are discussed. Setup properties and noise features are considered in detail. This review is intended to give newcomers the information needed to design and construct state-of-the-art photoacoustic detectors for specific purposes such as trace gas analysis, spectroscopy, and metrology.
---
paper_title: Theoretical analysis of a quartz-enhanced photoacoustic spectroscopy sensor

paper_content:

Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors are based on a recent approach to photoacoustic detection which employs a quartz tuning fork as an acoustic transducer. These sensors enable detection of trace gases for air quality monitoring, industrial process control, and medical diagnostics. To detect a trace gas, modulated laser radiation is directed between the tines of a tuning fork. The optical energy absorbed by the gas results in a periodic thermal expansion which gives rise to a weak acoustic pressure wave. This pressure wave excites a resonant vibration of the tuning fork thereby generating an electrical signal via the piezoelectric effect. This paper describes a theoretical model of a QEPAS sensor. By deriving analytical solutions for the partial differential equations in the model, we obtain a formula for the piezoelectric current in terms of the optical, mechanical, and electrical parameters of the system. We use the model to calculate the optimal position of the laser beam with respect to the tuning fork and the phase of the piezoelectric current. We also show that a QEPAS transducer with a particular 32.8 kHz tuning fork is 2–3 times as sensitive as one with a 4.25 kHz tuning fork. These simulation results closely match experimental data.
---
paper_title: Optimization of resonator radial dimensions for quartz enhanced photoacoustic spectroscopy systems

paper_content:

A finite element model for QEPAS systems has been developed that can apply to both on-axis and off-axis systems. The model includes the viscous and thermal loss on the acoustic resonator sidewalls, and these factors are found to significantly affect the signal to noise ratio. The model results are compared to experimental data and it is found that the model correctly predicts the optimal radial dimensions for resonator tubes of a given length. The model is applied to examine the dependence of signal-to-noise ratio on resonator diameter and sidewall thickness. The model is also applied to off-axis systems.
---
paper_title: QEPAS based ppb-level detection of CO and N_2O using a high power CW DFB-QCL

paper_content:

An ultra-sensitive and selective quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor platform was demonstrated for detection of carbon monoxide (CO) and nitrous oxide (N2O). This sensor used a state-of-the art 4.61 μm high power, continuous wave (CW), distributed feedback quantum cascade laser (DFB-QCL) operating at 10°C as the excitation source. For the R(6) CO absorption line, located at 2169.2 cm−1, a minimum detection limit (MDL) of 1.5 parts per billion by volume (ppbv) at atmospheric pressure was achieved with a 1 sec acquisition time and the addition of 2.6% water vapor concentration in the analyzed gas mixture. For the N2O detection, a MDL of 23 ppbv was obtained at an optimum gas pressure of 100 Torr and with the same water vapor content of 2.6%. In both cases the presence of water vapor increases the detected CO and N2O QEPAS signal levels as a result of enhancing the vibrational-translational relaxation rate of both target gases. Allan deviation analyses were performed to investigate the long term performance of the CO and N2O QEPAS sensor systems. For the optimum data acquisition time of 500 sec a MDL of 340 pptv and 4 ppbv was obtained for CO and N2O detection, respectively. To demonstrate reliable and robust operation of the QEPAS sensor a continuous monitoring of atmospheric CO and N2O concentration levels for a period of 5 hours were performed.
---
paper_title: An acoustic model for microresonator in on-beam quartz-enhanced photoacoustic spectroscopy

paper_content:

Based on a new spectrophone configuration using a single microresonator (mR) in "on beam" quartz-enhanced photoacoustic spectroscopy (QEPAS), referred to "half on beam QEPAS", a classical acoustic model originated from "orifice ended tube" was introduced to model and optimize the mR geometrical parameters. The calculated optimum mR parameters were in good agreement with the experimental results obtained in "half on beam" as well as conventional "on beam" QEPAS approaches through monitoring of atmospheric H2O vapor absorption. In addition, spectrophone performances of different QEPAS configurations (off beam, on beam and half on beam) were compared in terms of signal-to-noise ratio (SNR) gain.
---
paper_title: Gas-phase photoacoustic sensor at 8.41 μm using quartz tuning forks and amplitude-modulated quantum cascade lasers

paper_content:

We demonstrate the performance of a novel infrared photoacoustic laser absorbance sensor for gas-phase species using an amplitude-modulated quantum cascade (QC) laser and a quartz tuning fork microphone. The photoacoustic signal was generated by focusing 5.3 mW of a Fabry–Perot QC laser operating at 8.41 μm between the tines of a quartz tuning fork which served as a transducer for the transient acoustic pressure wave. The sensitivity of this sensor was calibrated using the infrared absorber Freon 134a by performing a simultaneous absorption measurement using a 31-cm absorption cell. The power and bandwidth normalized noise equivalent absorption sensitivity (NEAS) of this sensor was determined to be D=2.0×10-8 W cm-1/Hz1/2. A corresponding theoretical analysis of the instrument sensitivity is presented and is capable of quantitatively reproducing the experimental NEAS, indicating that the fundamental sensitivity of this technique is limited by the noise floor of the tuning fork itself.
---
paper_title: QEPAS spectrophones: design, optimization, and performance

paper_content:

The impact of design parameters of a spectrophone for quartz-enhanced photoacoustic spectroscopy on its performance was investigated. The microresonator of spectrophone is optimized based on an experimental study. The results show that a 4.4 mm-long tube with 0.6 mm inner diameter yields the highest signal-to-noise ratio, which is ∼30 times higher than that of a bare QTF at gas pressures between 400 and 800 Torr. The optimized configuration demonstrates a normalized noise-equivalent absorption coefficient (1σ) of 3.3×10−9 cm−1W/Hz1/2 for C2H2 detection at atmospheric pressure. The effect of the changing carrier gas composition is studied. A side-by-side sensitivity comparison between QEPAS and conventional photoacoustic spectroscopy technique is reported.
---
paper_title: Quartz-enhanced photoacoustic spectroscopy

paper_content:

A new approach to detecting a weak photoacoustic signal in a gas medium is described. Instead of a gas-filled resonant acoustic cavity, the sound energy is accumulated in a high- Q crystal element. Feasibility experiments utilizing a quartz-watch tuning fork demonstrate a sensitivity of 1.2x10(-7) cm(-1) W/ radicalHz . Potential further developments and applications of this technique are discussed.
---
paper_title: Laser microphotoacoustic sensor of ammonia traces in the atmosphere

paper_content:

A microphotoacoustic highly selective sensor of ammonia is built. Main attention is paid to the operation mechanism of the acoustic sensor based on a quartz tuning fork. The optimal dimensions and configuration of the acoustic resonator are determined, which made it possible to increase the sensor sensitivity by two—three times compared to the sensitivity of the existing devices. The detector sensitivity for ammonia was 60ppb (0.05 mg m-3) for the measurement time of 10s and a 25-mW, 1.53-μm laser beam in the acoustic resonator.
---
paper_title: Trace gas detection based on off-beam quartz enhanced photoacoustic spectroscopy: Optimization and performance evaluation

paper_content:

A gas sensor based on off-beam quartz enhanced photoacoustic spectroscopy was developed and optimized. Specifically, the length and diameter of the microresonator tube were optimized, and the outer tube shape is modified for enhancing the trace gas detection sensitivity. The impact of the distance between the quartz tuning fork and an acoustic microresonator on the sensor performance was experimentally investigated. The sensor performance was evaluated by determining the detection sensitivity to H(2)O vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient (1σ) of 6.2×10(-9) cm(-1) W/Hz(1/2) was achieved.
---
paper_title: Application of a widely electrically tunable diode laser to chemical gas sensing with quartz-enhanced photoacoustic spectroscopy.

paper_content:

A near-infrared diode laser with sample-grating distributed Bragg reflectors was used as a widely tunable spectroscopic source for multispecies chemical sensing. Quartz-enhanced photoacoustic spectroscopy was utilized to obtain high absorption sensitivity in a compact gas cell. CO2, H2O C2H2, and NH3 were monitored. A noise equivalent sensitivity of 8 x 10(-9) cm(-1) W(-1) Hz(-1/2)for NH3 detection was achieved, which corresponds to a NH3 mixing ratio of 4.4 parts in 10(6) by volume (ppmv) with a 1-s time constant and available 5.2-mW optical power in the gas cell.
---
paper_title: Laser-based systems for trace gas detection in life sciences

paper_content:

Infrared gas phase spectroscopy is becoming very common in many life science applications. Here we present three types of trace gas detection systems based on CO2 laser and continuous wave (cw) optical parametric oscillator (OPO) in combination with photoacoustic spectroscopy and cw quantum cascade laser (QCL) in combination with wavelength modulation spectroscopy. Examples are included to illustrate the suitability of CO2 laser system to monitor in real time ethylene emission from various dynamic processes in plants and microorganisms as well as from car exhausts. The versatility of an OPO-based detector is demonstrated by simultaneous detection of 13C-methane and 12C-methane (at 3240 nm) at similar detection limits of 0.1 parts per billion by volume. Recent progress on a QCL-based spectrometer using a continuous wave QCL (output power 25 mW, tuning range of 1891–1908 cm-1) is presented and a comparison is made to a standard chemiluminescence instrument for analysis of NO in exhaled breath.
---
paper_title: Off-beam quartz-enhanced photoacoustic spectroscopy

paper_content:

An off-beam (OB) detection approach is suggested and experimentally investigated and optimized for quartz-enhanced photoacoustic spectroscopy (QEPAS). This OB-QEPAS configuration, very simple in assembly, not only allows for use of larger excitation optical beams and facilitating optical alignment but also provides higher enhancement of photoacoustic signals than previously published results based on the common on-beam QEPAS under the same experimental conditions. A normalized noise equivalent absorption coefficient (1σ) of 5.9×10−9 cm−1W/Hz1/2 was obtained for water vapor detection at normal atmospheric pressure.
---
paper_title: NO trace gas sensor based on quartz-enhanced photoacoustic spectroscopy and external cavity quantum cascade laser

paper_content:

A gas sensor based on quartz-enhanced photoa- coustic detection and an external cavity quantum cascade laser was realized and characterized for trace nitric ox- ide monitoring using the NO R(6.5) absorption doublet at 1900.075 cm −1 . Signal and noise dependence on gas pressure were studied to optimize sensor performance. The NO concentration resulting in a noise-equivalent signal was found to be 15 parts per billion by volume, with 100 mW optical excitation power and a data acquisition time of 5 s.
---
paper_title: Coupling external cavity mid-IR quantum cascade lasers with low loss hollow metallic/dielectric waveguides

paper_content:

We report on the optical coupling between hollow core waveguides and external cavity mid-IR quantum cascade lasers (QCLs). Waveguides with 1000 μm bore size and lengths ranging from 2 to 14 cm, with metallic (Ag)/dielectric (AgI or polystyrene) circular cross-section internal coatings, have been employed. Our results show that the QCL mode is perfectly matched to the hybrid HE11 waveguide mode, demonstrating that the internal dielectric coating thickness is effective to suppress the higher losses TE-like modes. Optical losses down to 0.44 dB/m at 5.27 μm were measured in Ag/polystyrene-coated waveguide with an almost unitary coupling efficiency.
---
paper_title: Low-Loss Hollow Waveguide Fibers for Mid-Infrared Quantum Cascade Laser Sensing Applications

paper_content:

We report on single mode optical transmission of hollow core glass waveguides (HWG) coupled with an external cavity mid-IR quantum cascade lasers (QCLs). The QCL mode results perfectly matched to the hybrid HE11 waveguide mode and the higher losses TE-like modes have efficiently suppressed by the deposited inner dielectric coating. Optical losses down to 0.44 dB/m and output beam divergence of ~5 mrad were measured. Using a HGW fiber with internal core size of 300 µm we obtained single mode laser transmission at 10.54 µm and successful employed it in a quartz enhanced photoacoustic gas sensor setup.
---
paper_title: Ppb-level detection of nitric oxide using an external cavity quantum cascade laser based QEPAS sensor

paper_content:

Geometrical parameters of micro-resonator for a quartz enhanced photoacoustic spectroscopy sensor are optimized to perform sensitive and background-free spectroscopic measurements using mid-IR quantum cascade laser (QCL) excitation sources. Such an optimized configuration is applied to nitric oxide (NO) detection at 1900.08 cm(-1) (5.26 µm) with a widely tunable, mode-hop-free external cavity QCL. For a selected NO absorption line that is free from H(2)O and CO(2) interference, a NO detection sensitivity of 4.9 parts per billion by volume is achieved with a 1-s averaging time and 66 mW optical excitation power. This NO detection limit is determined at an optimal gas pressure of 210 Torr and 2.5% of water vapor concentration. Water is added to the analyzed mixture in order to improve the NO vibrational-translational relaxation process.
---
paper_title: Mid-infrared fiber-coupled QCL-QEPAS sensor

paper_content:

An innovative spectroscopic system based on an external cavity quantum cascade laser (EC-QCL) coupled with a mid-infrared (mid-IR) fiber and quartz enhanced photoacoustic spectroscopy (QEPAS) is described. SF6 has been selected as a target gas in demonstration of the system for trace gas sensing. Single mode laser delivery through the prongs of the quartz tuning fork has been obtained employing a hollow waveguide fiber with inner silver–silver iodine (Ag–AgI) coatings and internal core diameter of 300 μm. A detailed design and realization of the QCL fiber coupling and output collimator system allowed almost practically all (99.4 %) of the laser beam to be transmitted through the spectrophone module. The achieved sensitivity of the system is 50 parts per trillion in 1 s, corresponding to a record for QEPAS normalized noise-equivalent absorption of 2.7 × 10−10 W cm−1 Hz−1/2.
---
paper_title: Part-per-trillion level detection of SF6 using a single-mode fiber-coupled quantum cascade laser and a quartz enhanced photoacoustic sensor

paper_content:

We will report here on the design and realization of optoacoustic sensors based on an external cavity QCL laser source ::: emitting at 10,54 μm, fiber-coupled with a QEPAS spectrophone module. SF 6 has been selected as the target gas. Single ::: mode laser delivery through the prongs of the quartz tuning fork has been realized using a hollow waveguide fiber with ::: internal core size of 300 μm. The achieved sensitivity of the system was 50 part per trillion in 1 s corresponding to a ::: record for QEPAS normalized noise-equivalent absorption of 2,7•10 -10 W•cm -1 •Hz- 1/2 .
---
paper_title: Part-per-trillion level SF6 detection using a quartz enhanced photoacoustic spectroscopy-based sensor with single-mode fiber-coupled quantum cascade laser excitation.

paper_content:

A sensitive spectroscopic sensor based on a hollow-core fiber-coupled quantum cascade laser (QCL) emitting at 10.54 μm and quartz enhanced photoacoustic spectroscopy (QEPAS) technique is reported. The design and realization of mid-IR fiber and coupler optics has ensured single-mode QCL beam delivery to the QEPAS sensor. The collimation optics was designed to produce a laser beam of significantly reduced beam size and waist so as to prevent illumination of the quartz tuning fork and microresonator tubes. SF6 was selected as the target gas. A minimum detection sensitivity of 50 parts per trillion in 1 s was achieved with a QCL power of 18 mW, corresponding to a normalized noise-equivalent absorption of 2.7×10−10 W·cm−1/Hz1/2.
---
paper_title: Modulation cancellation method for measurements of small temperature differences in a gas

paper_content:

An innovative spectroscopic technique based on balancing and cancellation of modulated signals induced by two excitation sources is reported. For its practical implementation, we used quartz-enhanced photoacoustic spectroscopy as an absorption-sensing technique and applied the new approach to measure small temperature differences between two gas samples. The achieved sensitivity was 30 mK in 17 s. A theoretical sensitivity analysis is presented, and the applicability of this method to isotopic measurements is discussed.
---
paper_title: Spectroscopic measurements of isotopic water composition using a new modulation cancellation method

paper_content:

We report on the application of an innovative spectroscopic balancing technique to measure isotopologue abundance ::: quantification. We employ quartz enhanced photoacoustic spectroscopy in a 2f wavelength modulation mode as an ::: absorption sensing technique and water vapor as a test analyte. Isotope absorption lines with very close lower energy ::: levels and with the same quantum numbers have been selected to limit the sensitivity to temperature variations and ::: guarantee identical broadening relaxation properties. A detection sensitivity in measuring the deviation from a standard ::: sample δ 18 O of 1.4% o , in 200 sec of integration time was achieved.
---
paper_title: Modulation cancellation method in laser spectroscopy

paper_content:

A novel spectroscopic technique based on modulation spectroscopy with two excitation sources and quartz enhanced photoacoustic spectroscopy is described. We demonstrated two potential applications of this detection technique. First, we investigated the measurement of small temperature differences in a gas mixture. In this case, a sensitivity of 30 mK in 17 sec was achieved for a C2H2/N2 gas mixture with a 0.5% C2H2 concentration. Second, we demonstrated the detection of broadband absorbing chemical species, for which we selected hydrazine as the target molecule and achieved a detection limit of ∼1 part per million in 1 sec. In both cases, the measurements were performed with near-IR laser diodes and overtone transitions.
---
paper_title: Modulation cancellation method (MOCAM) in modulation spectroscopy

paper_content:

An innovative spectroscopic technique based on balancing and cancellation of modulated signals induced by two ::: excitation sources. We used quartz enhanced photoacoustic spectroscopy (QEPAS) in a 2f wavelength modulation mode ::: as an absorption sensing technique and employed a modulation cancelation approach for spectroscopic measurements of ::: small temperature differences in a gas mixture and detection of broadband absorbers. We demonstrated measurement of ::: small temperature differences in a C2H2/N2gas mixture with a sensitivity of 30 mK in 17 sec and detection of hydrazine, ::: a broadband absorbing chemical species, down to concentration of 1 part per million in volume in 1 sec. In both cases we ::: used near-infrared laser diodes and selected overtone transitions.nuscrip
---
paper_title: Modulation cancellation method for laser spectroscopy

paper_content:

We report on novel methods employing a modulation cancellation technique which result in a significantly increase in ::: the sensitivity and accuracy of trace gas detectors. This method can be applied for isotopomer abundance quantification, ::: temperature measurements and the detection of large molecules.
---
paper_title: Modulation cancellation method for measurements of small temperature differences in a gas

paper_content:

An innovative spectroscopic technique based on balancing and cancellation of modulated signals induced by two excitation sources is reported. For its practical implementation, we used quartz-enhanced photoacoustic spectroscopy as an absorption-sensing technique and applied the new approach to measure small temperature differences between two gas samples. The achieved sensitivity was 30 mK in 17 s. A theoretical sensitivity analysis is presented, and the applicability of this method to isotopic measurements is discussed.
---
paper_title: Ammonia detection by use of quartz-enhanced photoacoustic spectroscopy with a near-IR telecommunication diode laser

paper_content:

A gas sensor based on quartz-enhanced photoacoustic detection and a fiber-coupled telecommunication distributed-feedback diode laser was designed and characterized for trace NH3 monitoring at a 1.53-microm wavelength (overtone absorption region). Signal and noise dependence on gas pressure were studied to optimize sensor performance. The ammonia concentration resulting in a noise-equivalent signal was found to be 0.65 parts per million by volume with 38-mW optical excitation power and a lock-in amplifier time constant of 1 s. This corresponds to a normalized absorption sensitivity of 7.2 x 10(-9) cm(-1) W/Hz1/2, comparable with detection sensitivity achieved in conventional photoacoustic spectroscopy. The sensor architecture can be the basis for a portable gas analyzer.
---
paper_title: Impact of humidity on quartz-enhanced photoacoustic spectroscopy based detection of HCN

paper_content:

The architecture and operation of a trace hydrogen cyanide (HCN) gas sensor based on quartz-enhanced photoacoustic spectroscopy and using a λ=1.53 μm telecommunication diode laser are described. The influence of humidity content in the analyzed gas on the sensor performance is investigated. A kinetic model describing the vibrational to translational (V–T) energy transfer following the laser excitation of a HCN molecule is developed. Based on this model and the experimental data, the V–T relaxation time of HCN was found to be (1.91±0.07)10-3 s Torr in collisions with N2 molecules and (2.1±0.2)10-6 s Torr in collisions with H2O molecules. The noise-equivalent concentration of HCN in air at normal indoor conditions was determined to be at the 155-ppbv level with a 1-s sensor time constant.
---
paper_title: Modulation cancellation method for measurements of small temperature differences in a gas

paper_content:

An innovative spectroscopic technique based on balancing and cancellation of modulated signals induced by two excitation sources is reported. For its practical implementation, we used quartz-enhanced photoacoustic spectroscopy as an absorption-sensing technique and applied the new approach to measure small temperature differences between two gas samples. The achieved sensitivity was 30 mK in 17 s. A theoretical sensitivity analysis is presented, and the applicability of this method to isotopic measurements is discussed.
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paper_title: On-line laser photoacoustic detection of ethene in exhaled air as biomarker of ultraviolet radiation damage of the human skin

paper_content:

The exhaled air and volatile emission by the skin of human subjects were analyzed for traces of ethene (C2H4) by means of CO2 laser photoacoustic trace gas detection. Due to the extreme sensitivity of the detection system (6 part per trillion volume, 6:1012), these measurements could be performed on-line and noninvasively. Exhaled ethene was used as a biomarker for lipid peroxidation in the skin of human subjects exposed to ultraviolet (UV) radiation from a solarium. A change in the ethene concentration was already observed in the exhaled air after 2 min. Adaptation of the skin to UV exposure and direct skin emission could also be observed.
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paper_title: Influence of molecular relaxation dynamics on quartz-enhanced photoacoustic detection of CO2 at λ=2 μm

paper_content:

Carbon dioxide (CO2) trace gas detection based on quartz enhanced photoacoustic spectroscopy (QEPAS) using a distributed feedback diode laser operating at λ=2 μm is performed, with a primary purpose of studying vibrational relaxation processes in the CO2-N2-H2O system. A simple model is developed and used to explain the experimentally observed dependence of amplitude and phase of the photoacoustic signal on pressure and gas humidity. A (1σ) sensitivity of 110 parts-per-million (with a 1 s lock-in time constant) was obtained for CO2 concentrations measured in humid gas samples.
---
paper_title: Ultrasensitive gas detection by quartz-enhanced photoacoustic spectroscopy in the fundamental molecular absorption bands region

paper_content:

A trace gas sensor based on quartz-enhanced photoacoustic spectroscopy with a quantum cascade laser operating at 4.55 μm as an excitation source was developed. The sensor performance was evaluated for the detection of N2O and CO. A noise-equivalent (1σ) sensitivity of 4 ppbv N2O with 3 s response time to (1-1/e) of the steady-state level was demonstrated. The influence of the relevant energy transfer processes on the detection limits was analyzed. Approaches to improve the current sensor performance are also discussed.
---
paper_title: Photoacoustic phase shift as a chemically selective spectroscopic parameter

paper_content:

The phase information obtained in photoacoustic experiments can be used to separate the signals originating from chemical species with overlapping absorption spectra. This approach was applied to quantify parts per million CO levels in propylene using quartz-enhanced photoacoustic spectroscopy and a quantum cascade laser as an excitation source. The experimental data were used to evaluate V–T relaxation rates of CO and N2O in propylene.
---
paper_title: Quartz-enhanced photoacoustic spectroscopy

paper_content:

A new approach to detecting a weak photoacoustic signal in a gas medium is described. Instead of a gas-filled resonant acoustic cavity, the sound energy is accumulated in a high- Q crystal element. Feasibility experiments utilizing a quartz-watch tuning fork demonstrate a sensitivity of 1.2x10(-7) cm(-1) W/ radicalHz . Potential further developments and applications of this technique are discussed.
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paper_title: Spectroscopic measurements of isotopic water composition using a new modulation cancellation method

paper_content:

We report on the application of an innovative spectroscopic balancing technique to measure isotopologue abundance ::: quantification. We employ quartz enhanced photoacoustic spectroscopy in a 2f wavelength modulation mode as an ::: absorption sensing technique and water vapor as a test analyte. Isotope absorption lines with very close lower energy ::: levels and with the same quantum numbers have been selected to limit the sensitivity to temperature variations and ::: guarantee identical broadening relaxation properties. A detection sensitivity in measuring the deviation from a standard ::: sample δ 18 O of 1.4% o , in 200 sec of integration time was achieved.
---
paper_title: Quartz-enhanced photoacoustic spectroscopy

paper_content:

A new approach to detecting a weak photoacoustic signal in a gas medium is described. Instead of a gas-filled resonant acoustic cavity, the sound energy is accumulated in a high- Q crystal element. Feasibility experiments utilizing a quartz-watch tuning fork demonstrate a sensitivity of 1.2x10(-7) cm(-1) W/ radicalHz . Potential further developments and applications of this technique are discussed.
---
paper_title: Modulation cancellation method in laser spectroscopy

paper_content:

A novel spectroscopic technique based on modulation spectroscopy with two excitation sources and quartz enhanced photoacoustic spectroscopy is described. We demonstrated two potential applications of this detection technique. First, we investigated the measurement of small temperature differences in a gas mixture. In this case, a sensitivity of 30 mK in 17 sec was achieved for a C2H2/N2 gas mixture with a 0.5% C2H2 concentration. Second, we demonstrated the detection of broadband absorbing chemical species, for which we selected hydrazine as the target molecule and achieved a detection limit of ∼1 part per million in 1 sec. In both cases, the measurements were performed with near-IR laser diodes and overtone transitions.
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paper_title: Modulation cancellation method (MOCAM) in modulation spectroscopy

paper_content:

An innovative spectroscopic technique based on balancing and cancellation of modulated signals induced by two ::: excitation sources. We used quartz enhanced photoacoustic spectroscopy (QEPAS) in a 2f wavelength modulation mode ::: as an absorption sensing technique and employed a modulation cancelation approach for spectroscopic measurements of ::: small temperature differences in a gas mixture and detection of broadband absorbers. We demonstrated measurement of ::: small temperature differences in a C2H2/N2gas mixture with a sensitivity of 30 mK in 17 sec and detection of hydrazine, ::: a broadband absorbing chemical species, down to concentration of 1 part per million in volume in 1 sec. In both cases we ::: used near-infrared laser diodes and selected overtone transitions.nuscrip
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paper_title: Modulation cancellation method for laser spectroscopy

paper_content:

We report on novel methods employing a modulation cancellation technique which result in a significantly increase in ::: the sensitivity and accuracy of trace gas detectors. This method can be applied for isotopomer abundance quantification, ::: temperature measurements and the detection of large molecules.
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paper_title: Gas detection with evanescent-wave quartz-enhanced photoacoustic spectroscopy

paper_content:

Evanescent-wave gas sensing with tapered optical fibers (TOFs) and quartz-enhanced photoacoustic spectroscopy ::: (QEPAS) is reported. The evanescent field of TOFs with diameter down to sub-wavelength is utilized for photoacoustic ::: excitation in photoacoustic spectroscopy. A quartz tuning fork (QTF) with resonant frequency about ~32.75 kHz is used ::: to detect the generated pressure wave. A normalized noise equivalent absorption coefficient of 1.5×10 -6 cm -1 W/√Hz is ::: achieved for acetylene detection with a fiber taper with a waist diameter of 1.1 μm. It is found that QEPAS with TOFs of ::: sub-wavelength diameters exhibit comparable sensitivities with open path QEPAS but with additional advantages of ::: lower insertion loss, easier alignment, and multiplexing capability.
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paper_title: Long-period gratings in wavelength-scale microfibers.

paper_content:

We report the fabrication of long-period gratings (LPGs) in wavelength-scale microfibers with diameters from 1.5 to 3 microm. The LPGs were fabricated by use of a femtosecond IR laser to periodically modify the surface of the fibers. These LPGs have grating periods of a few tens of micrometers, much smaller than those in conventional optical fibers. A compact 10-period LPG with a device length of only approximately 150 microm demonstrated a strong resonant dip of >20 dB around 1330 nm. These microfiber LPGs would be useful in-fiber components for microfiber-based devices, circuits, and sensors.
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paper_title: Evanescent-wave photoacoustic spectroscopy with optical micro/nano fibers

paper_content:

We demonstrate gas detection based on evanescent-wave photoacoustic (PA) spectroscopy with tapered optical fibers. Evanescent-field instead of open-path absorption is exploited for PA generation, and a quartz tuning fork is used for PA detection. A tapered optical fiber with a diameter down to the wavelength scale demonstrates detection sensitivity similar to an open-path system but with the advantages of easier optical alignment, smaller insertion loss, and multiplexing capability.
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paper_title: Cutting-edge terahertz technology

paper_content:

Research into terahertz technology is now receiving increasing attention around the world, and devices exploiting this waveband are set to become increasingly important in a very diverse range of applications. Here, an overview of the status of the technology, its uses and its future prospects are presented.
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paper_title: Rotational Spectroscopy of Diatomic Molecules

paper_content:

1. General introduction 2. The separation of nuclear and electronic motion 3. The electronic hamiltonian 4. Interactions arising from nuclear magnetic and electric moments 5. Angular momentum theory and spherical tensor algebra 6. Electronic and vibrational states 7. Derivation of the effective hamiltonian 8. Molecular beam magnetic and electric resonance 9. Microwave and far-infrared magnetic resonance 10. Pure rotational spectroscopy 11. Double resonance spectroscopy Appendices.
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paper_title: Frequency and amplitude stabilized terahertz quantum cascade laser as local oscillator

paper_content:

We demonstrate an experimental scheme to simultaneously stabilize the frequency and amplitude of a 3.5 THz third-order distributed feedback quantum cascade laser as a local oscillator. The frequency stabilization has been realized using a methanol absorption line, a power detector, and a proportional-integral-derivative (PID) loop. The amplitude stabilization of the incident power has been achieved using a swing-arm voice coil actuator as a fast optical attenuator, using the direct detection output of a superconducting mixer in combination with a 2nd PID loop. Improved Allan variance times of the entire receiver, as well as the heterodyne molecular spectra, are demonstrated.
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paper_title: Terahertz spectroscopy: a powerful new tool for the chemical sciences?

paper_content:

Terahertz spectroscopy is only now beginning to make its transition from initial development by physicists and engineers to broader use by chemists, materials scientists and biologists, thanks to the increasing availability of commercial terahertz spectrometers. With the unique insights that terahertz spectroscopy can provide into intermolecular bonding and crystalline matter, it could prove to be an invaluable addition to the chemist's analytical toolset. This tutorial review aims to give an introduction to terahertz spectroscopy, its techniques, equipment, current applications and potential for the chemical sciences to a broad readership.
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paper_title: Chemical recognition in terahertz time-domain spectroscopy and imaging

paper_content:

In this paper, we present an overview of chemical recognition with ultrashort THz pulses. We describe the experimental technique and demonstrate how signals for chemical recognition of substances in sealed containers can be obtained, based on the broadband absorption spectra of the substances. We then discuss chemical recognition in combination with THz imaging and show that certain groups of biological substances may give rise to characteristic recognition signals. Finally, we explore the power of numerical prediction of absorption spectra of molecular crystals and illuminate some of the challenges facing state-of-the-art computational chemistry software.
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paper_title: Molecular Spectroscopy with TeraHertz Quantum Cascade Lasers

paper_content:

We have implemented quantum cascade lasers (QCLs) operating at about 2.5 THz in a spectrometer for high resolution molecular spectroscopy. One QCL has a Fabry-Perot resonator while the other is a distributed feedback laser. Linewidth and frequency tunability of both QCLs were investigated by mixing the radiation from the QCL with that from a 2.5 THz gas laser. Both were found sufficient for Doppler limited spectroscopy. Rotational tranistions of methanol were detected in absorption using a QCL as radiation source. Amplitude as well as frequency modulation of the output power of the QCL were used. The absolute frequency was determined simultaneously with the absorption signal by mixing a small part of the radiation from the QCL with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519 THz were measured in order to demonstrate the performance of the spectrometer.
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paper_title: Terahertz Spectroscopy: System and Sensitivity Considerations

paper_content:

Terahertz spectroscopy is a backbone method in many areas of research. We have analyzed typically employed THz spectroscopy systems and their sensitivity in a general comparative approach. Recent progress to reduce the data acquisition time by frequency multiplexing using a spectrometer with a THz quantum cascade laser is described. The performance of a spectrometer using a pulsed Ge THz laser with a few μs long integration time and recent progress to modulate the laser current within such a short pulse are presented. We also investigate the origin of random errors in intensity spectra of a THz TDS with the goal to identify common error sources in TDS systems to allow reduction of the total measurement time.
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paper_title: THz QCL-Based Cryogen-Free Spectrometer for in Situ Trace Gas Sensing

paper_content:

We report on a set of high-sensitivity terahertz spectroscopy experiments making use of QCLs to detect rotational molecular transitions in the far-infrared. We demonstrate that using a compact and transportable cryogen-free setup, based on a quantum cascade laser in a closed-cycle Stirling cryostat, and pyroelectric detectors, a considerable improvement in sensitivity can be obtained by implementing a wavelength modulation spectroscopy technique. Indeed, we show that the sensitivity of methanol vapour detection can be improved by a factor ≈ 4 with respect to standard direct absorption approaches, offering perspectives for high sensitivity detection of a number of chemical compounds across the far-infrared spectral range.
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paper_title: Nanowire-based field effect transistors for terahertz detection and imaging systems.

paper_content:

The development of self-assembled nanostructure technologies has recently opened the way towards a wide class of semiconductor integrated devices, with progressively optimized performances and the potential for a widespread range of electronic and photonic applications. Here we report on the development of field effect transistors (FETs) based on semiconductor nanowires (NWs) as highly-sensitive room-temperature plasma-wave broadband terahertz (THz) detectors. The electromagnetic radiation at 0.3 THz is funneled onto a broadband bow-tie antenna, whose lobes are connected to the source and gate FET electrodes. The oscillating electric field experienced by the channel electrons, combined with the charge density modulation by the gate electrode, results in a source-drain signal rectification, which can be read as a DC signal output. We investigated the influence of Se-doping concentration of InAs NWs on the detection performances, reaching responsivity values higher than 100 V W-1, with noise-equivalent-power of similar to 10(-9) W Hz(-1/2). Transmission imaging experiments at 0.3 THz show the good reliability and sensitivity of the devices in a real practical application.
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paper_title: High-resolution gas phase spectroscopy with a distributed feedback terahertz quantum cascade laser

paper_content:

The quantum cascade laser is a powerful, narrow linewidth, and continuous wave source of terahertz radiation. The authors have implemented a distributed feedback device in a spectrometer for high-resolution gas phase spectroscopy. Amplitude as well as frequency modulation schemes have been realized. The absolute frequency was determined by mixing the radiation from the quantum cascade laser with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519THz were measured in order to demonstrate the performance of the spectrometer.The quantum cascade laser is a powerful, narrow linewidth, and continuous wave source of terahertz radiation. The authors have implemented a distributed feedback device in a spectrometer for high-resolution gas phase spectroscopy. Amplitude as well as frequency modulation schemes have been realized. The absolute frequency was determined by mixing the radiation from the quantum cascade laser with that from a gas laser. The pressure broadening and the pressure shift of a rotational transition of methanol at 2.519THz were measured in order to demonstrate the performance of the spectrometer.
---
paper_title: A quartz enhanced photo-acoustic gas sensor based on a custom tuning fork and a terahertz quantum cascade laser

paper_content:

An innovative quartz enhanced photoacoustic (QEPAS) gas sensing system operating in the THz spectral range and employing a custom quartz tuning fork (QTF) is described. The QTF dimensions are 3.3 cm × 0.4 cm × 0.8 cm, with the two prongs spaced by ∼800 μm. To test our sensor we used a quantum cascade laser as the light source and selected a methanol rotational absorption line at 131.054 cm−1 (∼3.93 THz), with line-strength S = 4.28 × 10−21 cm mol−1. The sensor was operated at 10 Torr pressure on the first flexion QTF resonance frequency of 4245 Hz. The corresponding Q-factor was 74 760. Stepwise concentration measurements were performed to verify the linearity of the QEPAS signal as a function of the methanol concentration. The achieved sensitivity of the system is 7 parts per million in 4 seconds, corresponding to a QEPAS normalized noise-equivalent absorption of 2 × 10−10 W cm−1 Hz−1/2, comparable with the best result of mid-IR QEPAS systems.
---
paper_title: QEPAS detector for rapid spectral measurements

paper_content:

A quartz enhanced photoacoustic spectroscopy sensor designed for fast response was used in combination with a pulsed external cavity quantum cascade laser to rapidly acquire gas absorption data over the 1196–1281 cm−1 spectral range. The system was used to measure concentrations of water vapor, pentafluoroethane (freon-125), acetone, and ethanol both individually and in combined mixtures. The precision achieved for freon-125 concentration in a single 1.1 s long spectral scan is 13 ppbv.
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paper_title: Terahertz quartz enhanced photo-acoustic sensor

paper_content:

A quartz enhanced photo-acoustic sensor employing a single-mode quantum cascade laser emitting at 3.93 Terahertz (THz) is reported. A custom tuning fork with a 1 mm spatial separation between the prongs allows the focusing of the THz laser beam between them, while preventing the prongs illumination. A methanol transition with line-strength of 4.28 × 10−21 cm has been selected as target spectroscopic line. At a laser optical power of ∼ 40 μW, we reach a sensitivity of 7 parts per million in 4s integration time, corresponding to a 1σ normalized noise-equivalent absorption of 2 × 10−10 cm−1W/Hz½.
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paper_title: High-resolution photoacoustic and direct absorption spectroscopy of main greenhouse gases by use of a pulsed entangled cavity doubly resonant OPO

paper_content:

An entangled cavity doubly resonant optical parametric oscillator (ECOPO) has been developed to provide tunable narrow line width (<100 MHz) pulsed (8 ns) radiation over the 3.8–4.3 μm spectral range at a multi-kilohertz repetition rate with up to 100-W peak power. We demonstrate that coarse single mode tuning is obtained over the full spectral range of oscillation (300 cm−1), while automated mode-hop-free fine tuning is carried out over more than 100 GHz. High-resolution spectra of main greenhouse gases (CO2, N2O, SO2 and CH4) have been obtained in good agreement with calculated spectra from the HITRAN database. These experiments outline the unique capabilities of the ECOPO for multi-gas sensing based on direct absorption as well as photoacoustic spectroscopy.
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paper_title: The potential of mid-infrared photoacoustic spectroscopy for the detection of various doping agents used by athletes

paper_content:

The feasibility of laser-photoacoustic measurements for the detection and the analysis of different isolated doping agents in the vapour phase is discussed. To the best of our knowledge, this is the first time that photoacoustic vapour-phase measurements of doping substances have been presented. Spectra of different doping classes (stimulants, anabolica, diuretica, and beta blockers) are shown and discussed in terms of their detection sensitivity and selectivity. The potential of laser spectroscopy for detecting the intake of prohibited substances by athletes is explored.
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paper_title: A quartz enhanced photo-acoustic gas sensor based on a custom tuning fork and a terahertz quantum cascade laser

paper_content:

An innovative quartz enhanced photoacoustic (QEPAS) gas sensing system operating in the THz spectral range and employing a custom quartz tuning fork (QTF) is described. The QTF dimensions are 3.3 cm × 0.4 cm × 0.8 cm, with the two prongs spaced by ∼800 μm. To test our sensor we used a quantum cascade laser as the light source and selected a methanol rotational absorption line at 131.054 cm−1 (∼3.93 THz), with line-strength S = 4.28 × 10−21 cm mol−1. The sensor was operated at 10 Torr pressure on the first flexion QTF resonance frequency of 4245 Hz. The corresponding Q-factor was 74 760. Stepwise concentration measurements were performed to verify the linearity of the QEPAS signal as a function of the methanol concentration. The achieved sensitivity of the system is 7 parts per million in 4 seconds, corresponding to a QEPAS normalized noise-equivalent absorption of 2 × 10−10 W cm−1 Hz−1/2, comparable with the best result of mid-IR QEPAS systems.
---
paper_title: Terahertz quartz enhanced photo-acoustic sensor

paper_content:

A quartz enhanced photo-acoustic sensor employing a single-mode quantum cascade laser emitting at 3.93 Terahertz (THz) is reported. A custom tuning fork with a 1 mm spatial separation between the prongs allows the focusing of the THz laser beam between them, while preventing the prongs illumination. A methanol transition with line-strength of 4.28 × 10−21 cm has been selected as target spectroscopic line. At a laser optical power of ∼ 40 μW, we reach a sensitivity of 7 parts per million in 4s integration time, corresponding to a 1σ normalized noise-equivalent absorption of 2 × 10−10 cm−1W/Hz½.
---
paper_title: External cavity quantum cascade laser

paper_content:

In this paper we review the progress of the development of mid-infrared quantum cascade lasers (QCLs) operated in an external cavity configuration. We concentrate on QCLs based on the bound-to-continuum design, since this design is especially suitable for broadband applications. Since they were first demonstrated, these laser-based tunable sources have improved in performance in terms of output power, duty cycle, operation temperature and tuneability. Nowadays they are an interesting alternative to FTIRs for some applications. They operate at room temperature, feature a high spectral resolution while being small in size. They were successfully used in different absorption spectroscopy techniques. Due to their vast potential for applications in industry, medicine, security and research, these sources enjoy increasing interest within the research community as well as in industry.
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paper_title: THz quantum cascade designs for optimized injection

paper_content:

We report two different terahertz (THz) quantum cascade lasers based on bound-to-continuum transitions emitting at 2.8 and 3.8 THz. Novel design approaches have been explored with the purpose of optimizing the electron injection process. Both devices show large current dynamic ranges with output powers of several tens of mW, offering ample scope for future improvements.
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paper_title: Part-per-trillion level detection of SF6 using a single-mode fiber-coupled quantum cascade laser and a quartz enhanced photoacoustic sensor

paper_content:

We will report here on the design and realization of optoacoustic sensors based on an external cavity QCL laser source ::: emitting at 10,54 μm, fiber-coupled with a QEPAS spectrophone module. SF 6 has been selected as the target gas. Single ::: mode laser delivery through the prongs of the quartz tuning fork has been realized using a hollow waveguide fiber with ::: internal core size of 300 μm. The achieved sensitivity of the system was 50 part per trillion in 1 s corresponding to a ::: record for QEPAS normalized noise-equivalent absorption of 2,7•10 -10 W•cm -1 •Hz- 1/2 .
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paper_title: Submillimeter, millimeter, and microwave spectral line catalog.

paper_content:

This paper describes a computer accessible catalog of submillimeter, millimeter, and microwave spectral lines in the frequency range between 0 and 10,000 GHz (i.e., wavelengths longer than 30 μm). The catalog can be used as a planning guide or as an aid in identification and analysis of observed spectral lines. The information listed for each spectral line includes the frequency and its estimated error, the intensity, lower state energy, and quantum number assignment. The catalog has been constructed by using theoretical least-squares fits of published spectra lines to accepted molecular models. The associated predictions and their estimated errors are based on the resultant fitted parameters and their covariances.
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paper_title: Part-per-trillion level SF6 detection using a quartz enhanced photoacoustic spectroscopy-based sensor with single-mode fiber-coupled quantum cascade laser excitation.

paper_content:

A sensitive spectroscopic sensor based on a hollow-core fiber-coupled quantum cascade laser (QCL) emitting at 10.54 μm and quartz enhanced photoacoustic spectroscopy (QEPAS) technique is reported. The design and realization of mid-IR fiber and coupler optics has ensured single-mode QCL beam delivery to the QEPAS sensor. The collimation optics was designed to produce a laser beam of significantly reduced beam size and waist so as to prevent illumination of the quartz tuning fork and microresonator tubes. SF6 was selected as the target gas. A minimum detection sensitivity of 50 parts per trillion in 1 s was achieved with a QCL power of 18 mW, corresponding to a normalized noise-equivalent absorption of 2.7×10−10 W·cm−1/Hz1/2.
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paper_title: High power terahertz quantum cascade laser

paper_content:

We present high power terahertz quantum laser at about 3 THz based on bound-to-continuum active region design. At 10 K, corrected by the collection efficiency, the maximum peak power of 137 mW is obtained in pulsed mode. What’s more, we firstly introduce monolithically integrated THz quantum cascade laser (QCL) array and the maximum peak power increased to 218 mW after correction. In total, the array shows better performance than single device, implying cheerful prospect.
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paper_title: Mid-infrared fiber-coupled QCL-QEPAS sensor

paper_content:

An innovative spectroscopic system based on an external cavity quantum cascade laser (EC-QCL) coupled with a mid-infrared (mid-IR) fiber and quartz enhanced photoacoustic spectroscopy (QEPAS) is described. SF6 has been selected as a target gas in demonstration of the system for trace gas sensing. Single mode laser delivery through the prongs of the quartz tuning fork has been obtained employing a hollow waveguide fiber with inner silver–silver iodine (Ag–AgI) coatings and internal core diameter of 300 μm. A detailed design and realization of the QCL fiber coupling and output collimator system allowed almost practically all (99.4 %) of the laser beam to be transmitted through the spectrophone module. The achieved sensitivity of the system is 50 parts per trillion in 1 s, corresponding to a record for QEPAS normalized noise-equivalent absorption of 2.7 × 10−10 W cm−1 Hz−1/2.
---
paper_title: Influence of molecular relaxation dynamics on quartz-enhanced photoacoustic detection of CO2 at λ=2 μm

paper_content:

Carbon dioxide (CO2) trace gas detection based on quartz enhanced photoacoustic spectroscopy (QEPAS) using a distributed feedback diode laser operating at λ=2 μm is performed, with a primary purpose of studying vibrational relaxation processes in the CO2-N2-H2O system. A simple model is developed and used to explain the experimentally observed dependence of amplitude and phase of the photoacoustic signal on pressure and gas humidity. A (1σ) sensitivity of 110 parts-per-million (with a 1 s lock-in time constant) was obtained for CO2 concentrations measured in humid gas samples.
---
paper_title: NO trace gas sensor based on quartz-enhanced photoacoustic spectroscopy and external cavity quantum cascade laser

paper_content:

A gas sensor based on quartz-enhanced photoa- coustic detection and an external cavity quantum cascade laser was realized and characterized for trace nitric ox- ide monitoring using the NO R(6.5) absorption doublet at 1900.075 cm −1 . Signal and noise dependence on gas pressure were studied to optimize sensor performance. The NO concentration resulting in a noise-equivalent signal was found to be 15 parts per billion by volume, with 100 mW optical excitation power and a data acquisition time of 5 s.
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paper_title: Ultrasensitive gas detection by quartz-enhanced photoacoustic spectroscopy in the fundamental molecular absorption bands region

paper_content:

A trace gas sensor based on quartz-enhanced photoacoustic spectroscopy with a quantum cascade laser operating at 4.55 μm as an excitation source was developed. The sensor performance was evaluated for the detection of N2O and CO. A noise-equivalent (1σ) sensitivity of 4 ppbv N2O with 3 s response time to (1-1/e) of the steady-state level was demonstrated. The influence of the relevant energy transfer processes on the detection limits was analyzed. Approaches to improve the current sensor performance are also discussed.
---
paper_title: Part-per-trillion level SF6 detection using a quartz enhanced photoacoustic spectroscopy-based sensor with single-mode fiber-coupled quantum cascade laser excitation.

paper_content:

A sensitive spectroscopic sensor based on a hollow-core fiber-coupled quantum cascade laser (QCL) emitting at 10.54 μm and quartz enhanced photoacoustic spectroscopy (QEPAS) technique is reported. The design and realization of mid-IR fiber and coupler optics has ensured single-mode QCL beam delivery to the QEPAS sensor. The collimation optics was designed to produce a laser beam of significantly reduced beam size and waist so as to prevent illumination of the quartz tuning fork and microresonator tubes. SF6 was selected as the target gas. A minimum detection sensitivity of 50 parts per trillion in 1 s was achieved with a QCL power of 18 mW, corresponding to a normalized noise-equivalent absorption of 2.7×10−10 W·cm−1/Hz1/2.
---
paper_title: Photoacoustic phase shift as a chemically selective spectroscopic parameter

paper_content:

The phase information obtained in photoacoustic experiments can be used to separate the signals originating from chemical species with overlapping absorption spectra. This approach was applied to quantify parts per million CO levels in propylene using quartz-enhanced photoacoustic spectroscopy and a quantum cascade laser as an excitation source. The experimental data were used to evaluate V–T relaxation rates of CO and N2O in propylene.
---
paper_title: SEMICONDUCTOR LASER BASED TRACE GAS SENSOR TECHNOLOGY: RECENT ADVANCES AND APPLICATIONS Laser Based Trace Gas Sensor Technology

paper_content:

Recent advances in the development of sensors based on infrared diode and quantum cascade lasers for the detection of trace gas species is reported. Several examples of applications in environmental and industrial process monitoring as well as in medical diagnostics using quartz enhanced photoacoustic spectroscopy and laser absorption spectroscopy will be described.
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paper_title: Applications of quartz tuning forks in spectroscopic gas sensing

paper_content:

A recently introduced approach to photoacoustic detection of trace gases utilizing a quartz tuning fork (TF) as a resonant acoustic transducer is described in detail. Advantages of the technique called quartz-enhanced photoacoustic spectroscopy (QEPAS) compared to conventional resonant photoacoustic spectroscopy include QEPAS sensor immunity to environmental acoustic noise, a simple absorption detection module design, and its capability to analyze gas samples ∼1mm3 in volume. Noise sources and the TF properties as a function of the sampled gas pressure, temperature and chemical composition are analyzed. Previously published results for QEPAS based chemical gas sensing are summarized. The achieved sensitivity of 5.4×10−9cm−1W∕√Hz is compared to recent published results of photoacoustic gas sensing by other research groups. An experimental study of the long-term stability of a QEPAS-based ammonia sensor is presented. The results of this study indicate that the sensor exhibits very low drift, which allows da...
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paper_title: Diode laser-based photoacoustic spectroscopy with interferometrically-enhanced cantilever detection.

paper_content:

A novel sensitive approach to detect weak pressure variations has been applied to tunable diode laser-based photoacoustic spectroscopy. The sensing device consists of a miniature silicon cantilever, the deflection of which is detected with a compact Michelson-type interferometer. The photoacoustic system has been applied to the detection of carbon dioxide (CO2) at 1572 nm with a distributed feedback diode laser. A noise equivalent sensitivity of 2.8 x 10-10 cm-1WHz-1/2 was demonstrated. Potential improvements of the technique are discussed.
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paper_title: Optical gas sensing: a review

paper_content:

The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.
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paper_title: Intracavity quartz-enhanced photoacoustic sensor

paper_content:

We report on a spectroscopic technique named intracavity quartz-enhanced photoacoustic spectroscopy (I-QEPAS) employed for sensitive trace-gas detection in the mid-infrared spectral region. It is based on a combination of QEPAS with a buildup optical cavity. The sensor includes a distributed feedback quantum cascade laser emitting at 4.33 μm. We achieved a laser optical power buildup factor of ∼500, which corresponds to an intracavity laser power of ∼0.75 W. CO2 has been selected as the target molecule for the I-QEPAS demonstration. We achieved a detection sensitivity of 300 parts per trillion for 4 s integration time, corresponding to a noise equivalent absorption coefficient of 1.4 × 10−8 cm−1 and a normalized noise-equivalent absorption of 3.2 × 10−10 W cm−1 Hz−1/2.
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paper_title: High-Performance InP-Based Mid-IR Quantum Cascade Lasers

paper_content:

Quantum cascade lasers (QCLs) were once considered as inefficient devices, as the wall-plug efficiency (WPE) was merely a few percent at room temperature. But this situation has changed in the past few years, as dramatic enhancements to the output power and WPE have been made for InP-based mid-IR QCLs. Room temperature continuous-wave (CW) output power as high as 2.8 W and WPE as high as 15% have now been demonstrated for individual devices. Along with the fundamental exploration of refining the design and improving the material quality, a consistent determination of important device performance parameters allows for strategically addressing each component that can be improved potentially. In this paper, we present quantitative experimental evidence backing up the strategies we have adopted to improve the WPE for QCLs with room temperature CW operation.
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paper_title: Atmospheric CH_4 and N_2O measurements near Greater Houston area landfills using a QCL-based QEPAS sensor system during DISCOVER-AQ 2013

paper_content:

A quartz-enhanced photoacoustic absorption spectroscopy (QEPAS)-based gas sensor was developed for methane (CH4) and nitrous-oxide (N2O) detection. The QEPAS-based sensor was installed in a mobile laboratory operated by Aerodyne Research, Inc. to perform atmospheric CH4 and N2O detection around two urban waste-disposal sites located in the northeastern part of the Greater Houston area, during DISCOVER-AQ, a NASA Earth Venture during September 2013. A continuous wave, thermoelectrically cooled, 158 mW distributed feedback quantum cascade laser emitting at 7.83 μm was used as the excitation source in the QEPAS gas sensor system. Compared to typical ambient atmospheric mixing ratios of CH4 and N2O of 1.8 ppmv and 323 ppbv, respectively, significant increases in mixing ratios were observed when the mobile laboratory was circling two waste-disposal sites in Harris County and when waste disposal trucks were encountered.
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paper_title: Aircraft Emissions of Methane and Nitrous Oxide during the Alternative Aviation Fuel Experiment

paper_content:

Given the predicted growth of aviation and the recent developments of alternative aviation fuels, quantifying methane (CH4) and nitrous oxide (N2O) emission ratios for various aircraft engines and fuels can help constrain projected impacts of aviation on the Earth’s radiative balance. Fuel-based emission indices for CH4 and N2O were quantified from CFM56–2C1 engines aboard the NASA DC-8 aircraft during the first Alternative Aviation Fuel Experiment (AAFEX-I) in 2009. The measurements of JP-8 fuel combustion products indicate that at low thrust engine states (idle and taxi, or 4% and 7% maximum rated thrusts, respectively) the engines emit both CH4 and N2O at a mean ±1σ rate of 170 ± 160 mg CH4 (kg Fuel)−1 and 110 ± 50 mg N2O (kg Fuel)−1, respectively. At higher thrust levels corresponding to greater fuel flow and higher engine temperatures, CH4 concentrations in engine exhaust were lower than ambient concentrations. Average emission indices for JP-8 fuel combusted at engine thrusts between 30% and 100% of...
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Title: Quartz-Enhanced Photoacoustic Spectroscopy: A Review

Section 1: Introduction
Description 1: This section outlines the context and importance of trace gas detection, introduces different gas sensor types, and discusses the advantages of laser absorption spectroscopy over other techniques.

Section 2: Photoacoustic Spectroscopy
Description 2: This section describes the fundamentals of photoacoustic spectroscopy, the underlying optical absorption process, various noise sources, and the application areas of PAS.

Section 3: Quartz-Enhanced Photoacoustic Spectroscopy
Description 3: This section introduces quartz-enhanced photoacoustic spectroscopy (QEPAS), its benefits over regular PAS, and the principles of operation using quartz tuning forks.

Section 4: QEPAS Sensor
Description 4: This section details the configuration and working of a typical QEPAS sensor, including the optical components and methods for laser wavelength modulation.

Section 5: Quartz Tuning Fork: Resonant Properties and Noise
Description 5: This section explains the resonant properties and noise characteristics of quartz tuning forks used in QEPAS, discussing their mechanical and electrical modeling.

Section 6: Wavelength Modulation Detection
Description 6: This section elucidates the wavelength modulation technique used to improve the QEPAS signal-to-noise ratio and minimize external acoustic noise.

Section 7: Amplitude Modulation Detection for Broadband Absorbers
Description 7: This section describes the amplitude modulation technique applicable for the detection of broadband absorbers and the associated challenges and optimizations.

Section 8: On-Beam QEPAS
Description 8: This section discusses the design and functioning of on-beam QEPAS configurations, including the use of micro-resonators to enhance the photoacoustic signal.

Section 9: Off-Beam QEPAS
Description 9: This section introduces the off-beam QEPAS configuration, its advantages over on-beam setups, and details the optimization for different applications.

Section 10: Fiber-coupled QCL-QEPAS
Description 10: This section covers the integration of fiber-coupled quantum cascade lasers with QEPAS for flexible and compact sensor designs, briefing on hollow core waveguides.

Section 11: MOCAM Technique Combined with QEPAS
Description 11: This section explains the Modulation Cancellation Method (MOCAM) combined with QEPAS for applications like temperature and isotopic composition measurements of gas mixtures.

Section 12: Quartz-Enhanced Evanescent-Wave PAS
Description 12: This section introduces quartz-enhanced evanescent-wave PAS using tapered optical fibers, its configuration, and potential applications.

Section 13: Terahertz Spectroscopy for Gas Sensing
Description 13: This section discusses the application of terahertz spectroscopy for gas sensing, highlighting recent advancements and potential for high-resolution molecular spectroscopy.

Section 14: Extension of QEPAS Technique in THz Range: Custom-Made Tuning Fork
Description 14: This section elaborates on the extension of QEPAS to the THz range using a custom-made tuning fork with larger prong spacing to accommodate the THz laser beam.

Section 15: THz QEPAS Sensor for Methanol Detection
Description 15: This section presents a case study of a THz QCL-based QEPAS sensor for detecting methanol, discussing experimental setup, optimization, and results.

Section 16: Review of QEPAS-Based Trace Gas Detection
Description 16: This section reviews the performance of QEPAS sensors for various gases, summarizing the minimum detection limits and comparing it with other techniques.

Section 17: Long Term Stability of a QEPAS Sensor
Description 17: This section assesses the long-term stability and performance of QEPAS sensors using Allan variance analysis to quantify sensitivity drift and signal averaging limits.

Section 18: Comparison with Existing Optical Techniques and Perspective
Description 18: This section compares QEPAS with other optical detection techniques, discusses future prospects, and identifies key factors for achieving high sensitivity in optical gas sensors.

Section 19: Conclusions
Description 19: This section summarizes the key advancements and benefits of quartz-enhanced photoacoustic spectroscopy and its applications across various fields.