Compact and fast response ultrasonic detection device based on two-wave mixing in a gallium arsenide photorefractive crystal

An improved phase demodulator for the detection of ultrasound based on two-wave mixing (TWM) in the diffusion regime in a 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. Another new feature of the device is a balanced receiver based on large area InGaAs detectors. 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 sensitivity to high frequency laser phase noise. The performance of the device operated with a cw and a pulsed laser source is described. A comparison with the conventional confocal Fabry-Perot in the transmission mode is also presented. As expected, the GaAs-based TWM photorefractive system is less sensitive than the Fabry-Perot at high frequencies but becomes more sensitive below some frequency value. For the 30 W pumping level used into the crystal and a 1 m long Fabry-Perot with 85% reflectivity mirrors, this value was found to be about 1.5 MHz. This feature is known to be useful in the cases where low frequencies have to be detected, higher frequencies being attenuated by the material. This is the case of coarse microstructure materials or viscous or mushy materials. This photorefractive configuration is also attractive like all photorefractive systems for compactness and the absence of any need for active stabilization. To obtain a sensitivity equal or better than that of the confocal Fabry-Perot in the transmission mode, this new setup can be used with an InP:Fe or a CdTe:V photorefractive crystal under an applied field.