Rapid quantitative measurement of internal ultrasonic field in acousto-optic modulator via off-axis digital holography

Abstract

The internal ultrasonic field of an acousto-optic crystal determines the effects of acousto-optic modulator (AOM) devices. Conventionally, schlieren method is used for the visualization of field in AOM, but it can only provide an intensity image for qualitative visualization. Here, we employ the digital holographic interferometry (DHI) to quantify the ultrasonic field. To the best of our knowledge, this is the first time applying DHI to the measurement of ultrasonic field in the solid crystal. We compare it with the schlieren method, and analyze the consistency and advantages of quantitative measurement in experiments. In actual applications, the phase of ultrasonic field is seriously affected by the incident angle of object beam and the excitation time of crystal. Especially, only when the incident angle is within the crystal Bragg angle range, the phase inside the device can be accurately reconstructed. Moreover, the reconstructed phase of ultrasonic field explains the thermal expansion effect of the crystal and estimated its working distance 3 mm, which is never mentioned before and hinders the actual performance of AOM. And we also explore the relationship between the field and radio frequency (RF) voltages. These evidences could be utilized to improve AOM devices design and guide its practical applications.