Abstract
Photosensors utilizing the photo-EMF effects in semiconductor materials and devices have recently been deployed to detect ultrasound propagation in test materials,1,2 monitor the velocity of remote targets,3 and gauge optical path lengths1,3 with, in some cases, sub-wavelength precision.6 While it is capable of high precision velocity discrimination (~ 10 μm/sec), the velocity detection dynamic range/bandwidth of photo-EMF sensors is determined by the response times of the sensor materials which in turn are often limited by their dielectric relaxation times. Under the illumination of modest-power (~100 mW) continuous-wave laser sources, the dielectric relaxation time is on the order of 1 μs, leading to the Doppler frequency detection bandwidth of approximately 100 kHz. In order to characterize high-speed objects like air jets, it is necessary to enhance the detection bandwidth of photo-EMF sensors significantly. One less-desirable approach is to increase the power of the continuous-wave laser which unfortunately increases the costs, complexity, and stability requirements of the laser source. One obvious alternative is to deploy pulsed laser sources, e.g., Q-switched and mode- locked lasers, which offer peak power levels readily in excess of 1 kW without boosting up the overall external power consumption. Pulsed laser sources also lead to improved signal-to-noise ratios for signal detection due to the significantly increased laser power levels scattered back from the target under surveillance.
© 2002 Optical Society of America
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