Quasi-common-path laser feedback interferometry based on frequency shifting and multiplexing

Abstract

We present a quasi-common-path laser feedback interferometer based on frequency shifting and multiplexing. The interferometer uses two acousto-optic modulators to shift the frequency of the target-generated feedback light by 2 Omega. A properly aligned mirror is inserted into the feedback path to generate a feedback light frequency shifted by Omega. Phase variations of the two quasi-common-path feedback light beams are simultaneously measured through heterodyne demodulation with two different reference signals. Their subtraction accurately reflects the target displacement. Under typical room conditions, the system's short-period resolution is better than 2 nm, and its 3 min displacement accuracy is 8 nm.

Response of microchip solid-state laser to external frequency-shifted feedback and its applications

“…Owing to the limited bandwidth of the relaxation oscillation15, the microchip laser cannot respond to feedback light whose frequency shift is much larger than the relaxation oscillation frequency f R = 290 kHz at the pump level of P/P th = 2. Consequently, the two beams of frequency shifts Ω 1 , and Ω 2 can be omitted.…”

“…For micro-chip solid-state lasers, this value12 of the ratio reaches as high as ~10 6 . Consequently, these lasers can sense the extremely weak feedback power as low as 10 −12 , which permits broad application in fields such as tomography1213, microscopy14, interferometry15, vibrometry16, and velocimetry17. However, it is not yet clear to what amount of feedback power, the laser can be used for heterodyne phase measurement.…”

New method for lens thickness measurement by the frequency-shifted confocal feedback

“…The distance between the output end of the laser and the sample surface is called the external cavity length. Using the rate equation model, the optical power modulation of the frequency shift feedback is as followed [8]:…”

“…In particular, the frequency shift feedback effect of microchip laser was widely researched. Compared with the laser diode, it has advantages as small volume, simple pumping scheme and high sensitivity, and it has been utilized in the fields of velocity [7] and displacement measurement [8].…”

Refractive Index Measurement of Liquids by Double-Beam Laser Frequency-Shift Feedback

“…Laser frequency-shift feedback can be used to measure the displacement of non-cooperative target with nanometer resolution [9] due to its high sensitivity and heterodyne interference. In the refractive index measurement, the height variation of the liquid surface and the depth variation of the mirror under liquid are measured simultaneously, that is, the measurements of L and nL are both realized by laser feedback interference.…”