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1 August 2021 A displacement probe using backscattering for profiling the ground surface form
In-Ung Song, Ho-Soon Yang, Jun-Gyu Park, Goeun Kim, Kyuman Cho, Sug-Whan Kim
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Proceedings Volume 11817, Applied Optical Metrology IV; 1181709 (2021) https://doi.org/10.1117/12.2594367
Event: SPIE Optical Engineering + Applications, 2021, San Diego, California, United States
Abstract
We present a new displacement probe for a gantry-type profilometer to measure the ground surface form. The system is based on heterodyne interferometry with an acousto-optic modulator (AOM) and I/Q demodulation scheme. When the light from the single-frequency laser diode passes through the AOM following the Bragg condition angle, it is divided into the non-diffracted zero-order and a diffracted first-order beam with a frequency shift as the amount of the AOM driving frequency. One beam is used for the test and the other for the reference beam. Orthogonally reflected beams pass through the AOM one more time, and these beams make a heterodyne signal with a double modulation frequency. In this case, the only backscattered beam from the ground surface satisfies the Bragg condition, producing the modulated interference. Therefore, the AOM tends to reduce the stray light noises from the other incidence angles, acting as a transmission filter for backscattered light. The interference light arrives at the high-speed photodiode and it is then demodulated using the I/Q demodulation to extract the phase value. Although the backscattered ray from the rough glass surface has a very low intensity, the I/Q interferometer can detect the signal because the phase can be acquired regardless of the intensity change. Preliminary experiments confirmed that the system can measure from backscattered light of ground glass surface form.
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In-Ung Song, Ho-Soon Yang, Jun-Gyu Park, Goeun Kim, Kyuman Cho, and Sug-Whan Kim "A displacement probe using backscattering for profiling the ground surface form", Proc. SPIE 11817, Applied Optical Metrology IV, 1181709 (1 August 2021); https://doi.org/10.1117/12.2594367
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KEYWORDS
Backscatter
Interferometers
Demodulation
Modulation
Profiling
Profilometers
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