Frame-Transfer Gating Raman Spectroscopy for Time-Resolved Multiscalar Combustion Diagnostics

This invention could potentially benefit the development of advanced combustion systems such as gas turbine engines and internal combustion engines.

Accurate experimental measurement of spatially and temporally resolved variations in chemical composition (species concentrations) and temperature in turbulent flames is vital for characterizing the complex phenomena occurring in most practical combustion systems. These diagnostic measurements are called multiscalar because they are capable of acquiring multiple scalar quantities simultaneously. Multiscalar diagnostics also play a critical role in the area of computational code validation. In order to improve the design of combustion devices, computational codes for modeling turbulent combustion are often used to speed up and optimize the development process. The experimental validation of these codes is a critical step in accepting their predictions for engine performance in the absence of costprohibitive testing.

One of the most critical aspects of setting up a time-resolved stimulated Raman scattering (SRS) diagnostic system is the temporal optical gating scheme. A short optical gate is necessary in order for weak SRS signals to be detected with a good signal-to-noise ratio (SNR) in the presence of strong background optical emissions. This time-synchronized optical gating is a classical problem even to other spectroscopic techniques such as laser-induced fluorescence (LIF) or laserinduced breakdown spectroscopy (LIBS). Traditionally, experimenters have had basically two options for gating: (1) an electronic means of gating using an image intensifier before the charge-coupled-device (CCD), or (2) a mechanical optical shutter (a rotary chopper/mechanical shutter combination).

A new diagnostic technology has been developed at the NASA Glenn Research Center that utilizes a frame-transfer CCD sensor, in conjunction with a pulsed laser and multiplex optical fiber collection, to realize time-resolved Raman spectroscopy of turbulent flames that is free from optical background noise (interference). The technology permits not only shorter temporal optical gating (down to <1 μs, in principle), but also higher optical throughput, thus resulting in a substantial increase in measurement SNR.

The new technology is an experimental method (or scheme) for isolating true Raman spectral signals from flames using a single CCD detector. It does not use an image intensifier or a mechanical shutter. Individual electrical or optical devices employed in this method are not new; however, the diagnostic methodology itself, which utilizes a combination of existing devices for a particular application, is a novel concept.

The present methodology employs two key optical devices: a pulsed laser (nanosecond pulses) and a frame-transfer CCD sensor. Frame-transfer CCD sensors have been historically used to capture fast (microsecond timescale) transient events, such as Bose-Einstein condensate phenomena, over a short period of time (milliseconds). By their operation, the sensor area is exposed for a certain time and the charge is then transferred to the frame transfer area (or masking area) row-by-row, and is read out via a gain register or serial register. This is called “frame-transfer” readout or “kinetics” readout. The use of frame-transfer readout provides a very effective way of isolating true Raman signals from lasergenerated optical interferences in any combustion environment, in principle, without having to employ multiple CCD detectors or polarizer on the detection side.

Since laser-induced background emissions are unpolarized, unlike Raman scattering, which is polarized, they can be selectively isolated (and subtracted). While the theory of this polarization technique has been proposed previously, the implementation of this technique for time-resolved Raman diagnostics has not been matured. A principal reason is that an enabling technology that can increase the SNR was needed. When a flame receives two orthogonally polarized, but otherwise identical, laser pulses, Raman scattering can be observable only for the vertically polarized excitation pulse. The (unpolarized) laser-generated background emissions are observed regardless of the polarization state of the excitation pulses. If the two orthogonally-polarized laser pulses are separated in time so that they just fall onto a pair of consecutive sub-frames on the CCD sensor, subtracting the one (laser-generated background emission only) from the other (Raman signal plus background emission) results in a true Raman spectrum.

This work was done by Quang-Viet Nguyen, David G. Fischer, and Jun Kojima of Glenn Research Center.

