光学工程讲义16-PHOTODETECTORS.ppt

PHOTODETECTORS Prof John Watson EG40GC Optical Engineering Lecture Notes CLASSIFICATION OF PHOTODETECTORS Classify detectors by mechanism of response to incident light detectors of photons detectors of heat Semiconductor detectors e-h pairs are created by excitation with incident light two types of semiconductor diode ? bulk semiconductor - light dependent resistor ? junction diode - pn diode, pin diode, phototransistor Photoemissive detectors ? electrons ejected from a photosensitive material on irradiation by light ? photomultiplier tube Thermal detectors ? heating effect of light raises the temperature of the irradiated material ? with the subsequent change in its electric properties ? thermopile, pyroelectric detector PERFORMANCE PARAMETERS Spectral Response All parameters vary with wavelength Match peak emission wavelength of source with peak response of detector Quantum Efficiency (?, QE) A measure of how many photoelectrons are produced for every photon incident on the photosensitive surface ? = (ne/nph) x 100% ne is the rate of photoelectron generation np is the incident photon rate Values in the range 5 to 30% are typical PERFORMANCE PARAMETERS Responsivity (S) The magnitude of the electrical signal output from a photodetector in response to a particular light power S = id/? [A W-1] S = Vd/? [V W-1] id is the output current of the detector Vd is the output voltage ? is the incident radiant power Response Time (tr) A measure of how long it takes a detector to respond to a change in light power falling on it usually measured with reference to a square input pulse both rise and fall times are often quoted A good working rule is choose detector with rise time of ~1/10 of shortest pulse duration to be detected PERFORMANCE PARAMETERS Noise Equivalent Power (NEP) All detectors produce a small output signal in darkness Sets a lower limit to the intensity of detected light In photomultipliers and semiconductor devices, the background signal is thermally g