A technology of absolute distance measurement based on multi-wavelength self-mixing interferometry of a three-wavelength optical fiber laser

A technology of absolute distance measurement based on the multi-wavelength self-mixing interferometry of a threewavelength optical fiber laser is presented and experimented. The optical fiber laser which can emit three wavelengths simultaneously is composed of a single fiber ring and three fiber branches. Each fiber branch includes a length of erbium-doped fiber and a fiber Bragg grating (FBG). The erbium-doped fiber is used as the gain medium while the FBG is used as a laser cavity reflective mirror and wavelength selector. Three independent laser cavities have been constructed in the single laser. As there is no laser mode competition, three wavelengths with stable power can be emitted simultaneously and the frequency stability of each wavelength can reach 10^-6 . Absolute distance measurement can be performed using multiple self-mixing interferometry of the three wavelengths. Based on the idea that the calculated decimal phase of the self-mixing interferometric signal of each wavelength should equal the measured decimal phase of the self-mixing interferometric signal of the same wavelength, absolute distance measurement can be realized. The nominal length of a gauge block has been used to prove the correctness of the measurement results by experiments. The standard deviation of ten times repeated measurements for a distance of 7 mm is 4.4 nm.