Elsevier

Ultrasonics

Volume 37, Issue 3, March 1999, Pages 257-259
Ultrasonics

Ultrasound detection by using a confocal Fabry–Perot interferometer with phase-modulated light

https://doi.org/10.1016/S0041-624X(98)00066-3Get rights and content

Abstract

The optical detection of ultrasound with a radio-frequency (RF) sideband type of optical discriminator is described. The sidebands are produced on a laser beam by an optical phase modulator. A confocal Fabry–Perot interferometer with a lens at the mid-point in the cavity has been used to provide frequency discriminator functions. The cavity is tuned to the maximum transmission point, and then the RF beat signal in the back-reflection beam is detected. The ultrasound signals are obtained from the phase changes between the detected RF signal and the modulation source. Several experimental results are presented.

Introduction

The laser generation and detection of ultrasound is a subject of interest in material characterization. There has been steady effort to improve the sensitivity of optical detection with laser interferometers. The use of a high-power laser can improve the sensitivity because the ultimate signal-to-noise ratio is limited by shot noise. However, the high-intensity beam may saturate an optical detector in interferometers that are normally operated at the half-maximum intensity of the interference patterns, leading to a loss in sensitivity.

Optical discriminators that are operated at the minimum-intensity fringe – i.e., the dark fringe – have been proposed for laser frequency stabilization [1], [2], where the discriminator functions are provided by stable Fabry–Perot cavities. In terms of industrial applications, a confocal Fabry–Perot interferometer is more useful than the plane Fabry–Perot one because the confocal type can receive many speckles scattered over a wide angle [3]. In this contribution, ultrasound detection by an optical frequency discriminator with a confocal Fabry–Perot interferometer has been demonstrated. In a conventional confocal Fabry–Perot interferometer, the minimum fringe intensity of back-reflection beam remains 50% of the input beam. To make darker interference patterns, a lens has been installed in the cavity. Experimental results of ultrasonic waveforms in a steel plate are presented.

Section snippets

Experimental

The optical discriminator is shown in Fig. 1. Concentric concave mirrors with 500 mm radii of curvature were separated by a distance of 1000 mm. The reflection coefficients of the mirrors were approximately 0.9. A lens with a focal length of 250 mm was placed at the mid-point of the two mirrors. The lens was coated on both sides with a dielectric anti-reflection coating that reduced the reflectance per surface to less than 0.25%. In this confocal cavity, a ray from one point on the mirror surface

Results

The response curves of the discriminator are shown in Fig. 2. The upper curve shows the intensity of the collimated reflection beam when the cavity length was swept by the PZT pusher. When the incoming beam was distorted by a plastic film, the minimum/maximum intensity ratio of the reflection beam was changed to be about 20%. This deterioration was mainly due to imperfect matching of the beam patterns between the direct reflection beam at the mirror and the outcoming beam from the cavity. The

Conclusions

It has been shown that laser-generated ultrasound can be measured with an optical discriminator based on a phase modulation sideband scheme. The discriminator is operated at a dark fringe point, where saturation of a photodetector can be avoided. In addition, it offers suppression of common modes of noise, such as that arising from fluctuations in the intensity of the laser source. Longitudinal and shear waves were measured in a steel sample. Future experiments with a higher power laser will be

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