| 摘要 |
When sensing the presence of ultrasonic energy in a workpiece portion by illuminating such portion with light from a single-mode frequency stabilized laser, the reflected light, frequency modulated by the Doppler effect due to the ultrasonic energy induced deflection of the workpiece portion, is passed through a gas filled light filter. The frequency modulated light is amplitude modulated by the filter by selecting the frequency of the laser to cause a point of operation which is located along a steep slope portion of the absorption curve characteristic of the filter. To increase the steepness of the slope portion of the filter, the present invention discloses the use of laser light to saturate the filter in a part region.
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