Theoretical and experimental research on influence of cavity frequency difference in birefringent laser feedback system
Authors: Niu Hai-Sha  , Zhu Lian-Qing  , Song Jian-Jun  , Dong Ming-Li  , Lou Xiao-Ping  ,   School of Instrument Science and Opto-electronics Engineering, Beijing Information Science and Technology University, Beijing 100192, China;,   State Key Laboratory of Precision Measurement Technology and Instruments, Tsinghua University, Beijing 100084, China
Published: December 2020

The internal stress of glass material directly affects the processing quality of glass components and the service life of optical components. It is an important factor that relates to the overall system performance, safety, and reliability. Aerospace, precision optical systems, precision machining and other areas generally highly value the stress measurements of glass components. For example, the internal stress in the medium-glass material of precision imaging system will lead to the degradation of optical performance and reduce the image quality; the stress in the glass material used as the gain medium of high-power solid-state lasers not only directly affects the polarization state of the output light, but also shortens the service life of the laser; the stress concentration in the load-bearing glass of aircraft windshields, building glass curtain walls, etc., will cause serious accidents such as popping due to the reduction of glass mechanical properties. Therefore, the high sensitivity and large measurement range of stress detection technology has become a current research hotspot. Stress measurement techniques based on the birefringent external cavity laser feedback effect has received widespread attention due to its advanced and novel measurement principle. It is generally accepted in the traditional theory that the output phase of the laser in a feedback system is only determined by the phase retardation of birefringent element in an external cavity, and the measurement error is induced by the non-linear movement of external mirror. In this paper, the orthogonally polarized laser principle and the three-cavity equivalent model are combined to explain the influence of cavity frequency difference on the output of laser in feedback system. The frequency difference caused by the birefringence of the laser cavity is measured by comparing the intervals between adjacent longitudinal modes, and the frequency tuning feedback experiment is carried out. Theoretical analysis and experimental results show that the output phase of the laser is determined by the phase retardation of the external cavity, the frequency difference of the internal cavity, and the length of the external cavity. This conclusion is also confirmed by the measurement of the standard quarter wave plate. For a feedback system with an internal cavity frequency difference of 5 MHz and external cavity length of 150 mm, the phase difference induced by internal cavity frequency difference is about 0.573. The laser can output a single longitudinal mode below 40 MHz of the internal cavity frequency difference, and the length of the external cavity is generally larger than 150 mm when the actual system is designed, so the phase difference introduced by these two parameters cannot be ignored and must be calibrated. This study summarizes the phase characteristics of the orthogonally polarized laser under the joint of anisotropy feedback cavity, supplements the physical content of the laser feedback, and has great significance for accurate laser measurement of stress-birefringence, displacement, and distance.