Research on non-coincidence detection of fiber optic circulators based on Hertz-level frequency-shifting heterodyne interferometry

Abstract

The fiber optic circulators in a form of point diffraction is a key component for coupling fiber and spatial optical paths in laser Doppler vibration measurement systems. The coupling efficiency and other performance parameters of fiber optic circulators are great significant for improving measurement accuracy and working distance of vibration measurement system. The conventional circulator coincidence detection methods include energy monitoring method and far-field coincidence monitoring method, which cannot be used to quantitatively analyze the fiber mismatch factors. Therefore, the consistency of the circulator coupling efficiency cannot be guaranteed. To solve these problems, a phase detection technology based on Hertz-level frequency-shifting heterodyne interferometry is proposed. The interferometry phase information is used to calculate the relative spatial positions of optical fibers, and this technology performs quantitative detection during the fiber alignment process. The interference wavefront formed by relative spatial positions of optical fibers was simulated and validated by experiments. The curves of coupling efficiency and wavefront PV value versus different kinds of alignment errors were simulated and analyzed. By fitting the interference wavefront with the Zernike polynomials, the correspondence between different kinds of alignment errors and Zernike coefficients was obtained. The value of Z2 (Zernike coefficient) can be used as the basis for judging whether there is transverse displacement in the Y direction. Similarly, Z3 corresponds to the transverse displacement in the X direction, Z4 corresponds to the longitudinal displacement in the Z direction and Z5 corresponds to the optical axis angle. Through this correspondence relationship, the quantitative separation and analysis of fiber mismatch factors are realized. The experimental results show that the measurement accuracy of this method for transverse displacement achieves better than 1μm. According to the phase diagram obtained from the experiment, Zernike coefficient fitting was performed, as shown in Figure 13 (a) and Table 3. The transverse displacement deviation, longitudinal displacement deviation, and angle deviation were calculated by the coefficients of Z2 to Z5. The fiber adjustment mechanism corrected the transverse displacement deviation, and the experimental phase diagram was shown in Figure 14. It provides a new detection method for fiber alignment and mismatch correction. Compared with the existing detection methods, the phase detection method based on Hertz-level frequency-shifting heterodyne interferometry solves the quantification problem of fiber coincidence adjustment. This method has the advantages of high measurement accuracy, compact detection structure and composition, and low detection cost. This method has great potential for application in the fields of optical fiber and spatial optical device alignment, optical system aberration detection, and planar wavefront detection.