LAB_13 - Optical Sources and Transducers

LAB_13 - Optical Sources and Transducers - Lab 13 Optical...


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Lab 13 143 Optical Sources and Transducers Summary Photonics is the field of electronics involving semiconductor devices that emit and detect light. Computers, communications, information storage and retrieval, materials creation and processing, manu- facturing, quality control, medicine, genetic engineering, instrumentation, indeed virtually every field of engineering endeavor is being transformed by advances in this emerging field of lightwave engineering. This experiment is intended to provide the student with a brief introduction to a few optical sources and transducers. The optical spectrum is usually defined as including infrared, visible and ultraviolet radia- tion. We will study the characteristics of various LED’s and photodetectors, which operate in the opti- cal spectrum, focusing on such things as their range, sensitivity, and dynamic response. Educational Objectives After completing this lab a student should be able to: 1 . Use photodetectors and emitters. 2 . Measure the range, sensitivity and dynamic response of different photodetectors. 3 . Measure the performance characteristics of photodevices. "Nature and Nature's laws lay hid in the night. ‘God said let Newton be!’ and all was light." - Alexander Pope
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Lab 13 144 Background Information Wave-Particle Duality: At times, light behaves as if it consists of a stream of particles, and at oth- ers it behaves like waves. In the wave theory of light, the wavelength λ and frequency f of light are related by the equation: where c is the velocity of light 3.0 x 10 8 m/sec. On the other hand, in the particle theory, light behaves as if it were composed of particles called photons having an energy E related to their frequency f by the equation: where h = 6.624 x 10 -27 erg-sec is a fundamental new constant called Planck’s constant. Light Emitting Diodes Light Emitting Diodes (LED’s) are semiconductor diodes (PN junctions) that emit light when an adequate voltage is applied to their terminals. The forward voltage across any diode must exceed a threshold level before current can cross the junction. For silicon the threshold voltage is 0.6 volts, for gal- lium arsenide it is 1.3 volts. If the applied voltage is larger than this threshold, electrons cross the junction and combine with holes, thus giving off energy in the form of photons. The amount of light emitted by an LED is directly proportional to the current flowing through the LED. Because the light emitting diode converts an electrical current directly into light, it is more efficient than many other light sources. Altering the material can drastically change the wavelength (color) and response of the LED. Typ- ical materials used belong to binary combinations of Group III and IV elements of the periodic table such as Gallium Arsenide (GaAs), Gallium Phosphide (GaP), etc. Mixed crystals can also be fabricated using modern electronic manufacturing techniques to create a variety of LEDs ranging in wavelength from vis- ible to infrared. For example using various combinations of gallium, aluminum and arsenic one can man- ufacture LED’s having a central wavelength emission anywhere between 828 and 940 nanometers. The
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