Micro-Displacement Measurement Based on Dual Optical Feedback Self-Mixing Interference

Laser self-mixing interference (SMI) with dual external-cavity optical feedback exhibits a distinct characteristic: a change in the length of one external cavity periodically modulates the amplitude of the SMI signal produced by the other external cavity. Based on this characteristic, a micro-displacement measurement method is proposed, in which one feedback channel serves as the measurement channel and the external cavity length change is deduced from the amplitude of its SMI signal modulated by the other channel. Because the displacement information is encoded in the signal amplitude rather than only in the fringe phase, the method does not require phase unwrapping and therefore avoids the fringe-order ambiguity inherent in phase-based SMI displacement retrieval, while the amplitude readout is also less sensitive to intensity noise. A dual optical feedback SMI micro-displacement measurement system is constructed with a He-Ne laser (working wavelength 632.8 nm), a piezoelectric ceramic transducer (PZT), and a rotatable optical flat crystal. The displacement curve of the PZT moving platform is reconstructed from the measured SMI signals, and the change in the external cavity length induced by the rotation of the flat crystal is measured as well. The experimental results show that the measurement errors are 13.4–17.8 nm for the PZT feedback channel and 19.2–24.1 nm for the flat crystal feedback channel, both below 25 nm, demonstrating that the proposed method is a promising candidate for high-precision micro-displacement measurement.