Proceedings of the
9th International Conference of Asian Society for Precision Engineering and Nanotechnology (ASPEN2022)
15 – 18 November 2022, Singapore

Femtosecond Laser based Phase Locked Synthetic Wavelength Interferometer for Absolute Distance Measurement

Hyeokin Kang1, Woojeong Lee1 and Joohyung Lee1,#

1Department of Mechanical System Design Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu Seoul, Korea


A synthetic wavelength interferometer (SWI) with a long period wave generated through interference of waves which have similar frequencies has an advantage of avoiding 2π phase ambiguity in long distance measurement. However, during measuring the distance, a cyclic error occurs that deteriorate the resolution and accuracy. In this study, the synthetic waves having frequencies from tens of MHz to a GHz are generated by employing an Acousto-Optic Modulator (AOM) and a femtosecond laser. In addition, a phase locked method is proposed to remove cyclic error. The phase locked method is locking a difference of phase between measurement arm and reference arm of the SWI to a constant value. This can be implemented by utilizing a P-I controller to maintain voltage resulting from the phase difference between arms at a certain value. An output voltage from the P-I controller was converted to oscillation frequency using Voltage-Controlled Oscillator (VCO) principle. This oscillation frequency was input in the AOM as a modulation frequency. As a result, it is possible to lock the phase to a constant value by modulating the frequency of synthetic wave. It can also be used to measure the absolute distance by selecting a specific frequency of the synthetic wave. The advantage of this method is that the SWI can have high resolution over a wide range of distance measurement depending on the frequency, as well as eliminate cyclic error.

Keywords: Acousto-Optic Modulator, Cyclic error, Phase locked, Synthetic wavelength interferometer, Femtosecond laser

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