Displacement measurement based on Fourier transform method with external laser cavity modulation

Smart optical sensors for the displacement measurement in submicron, have been developed by using the self-mixing effect of a laser diode. In this article, we propose a new type of self-mixing interferometry (SMI) to measure microscopic displacement. The phase of the SMI signal is modulated with external cavity length and demodulated by the Fourier transform analysis technique. Some errors for this method are discussed. We have demonstrated this system to measure the microscopic displacement of a remote target. The experiment results reveal that phase extraction error for the method is ∼2π×10−3 rad, and the displacement measurement error is approximately 8 nm.