The relatively high (typically several MΩ) resistance of conventional GaAs photo-EMF detectors is one of the most important disadvantages of these devices, especially for laser ultrasonic detection. Indeed, to ensure the 10-30 MHz frequency band necessary for this application, one has to use feedback resistors in the input transimpedance amplifier of values less than 10 kΩ. This is clearly very far from optimal matching conditions, and the input noise of the bipolar-input operational amplifier (≈ 1 pA/Hz) proves to be dominating. Improving the current responsivity of the photodetector is of a vital importance in this situation. This can be done, in particular, in longitudinal configuration of the photodetector, or by using planar asymmetric interdigitated system of electrodes.

© 2001 Optical Society of America

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