Short response time fiber-coupled photorefractive ultrasonic demodulator

摘要

An improved phase demodulator for the detection of ultrasound based on two wave mixing (TWM) in semi-insulating photorefractive GaAs crystal is presented. A new optical layout is proposed in which the total laser power is injected into the crystal to reduce the grating buildup time. The device is then less sensitive to ambient vibrations or motion of the inspected part. The reduce the sensitivity of the device to amplitude fluctuations, a balanced receiver with a large etendue was developed. The measured rejection ratio of this balanced receiver is 50 dB. However, in this new optical configuration the signal and pump beam paths cannot be made equal which results in some snesitivity to high frequency laser phase noise. In this paper the results and performances of this novel configuration operating with CW and pulsed lasers are discussed. A comparison with the conventional confocal Fabry-Perot demodulator is also presented.