Stress measurement based on 1556 nm fiber laser frequency splitting effect
Authors: Chen Kai , Zhu Lian-Qing , Niu Hai-Sha , Meng Kuo , Dong Ming-Li ,   School of Instrument Science and Opto-electronics Engineering, Hefei University of Technology, Hefei 230009, China,   Beijing Engineering Research Center of Optoelectronic Information and Instruments, Beijing Key Laboratory for Optoelectronics Measurement Technology, Beijing Information Science and Technology University, Beijing 100016, China
Published: December 2020

When the optical component works in a complicated working environment such as a large temperature difference or a multi-modal state, internal stress accumulation is likely to occur, which affects the overall performance of the optical system. The stress-optical constant of optical component in the infrared range is one of the concerns of many optical systems. In this paper, a method of measuring the internal stress-induced birefringence of optical glass based on the frequency splitting effect of 1556.16 nm erbium-doped fiber laser is proposed. The planar dielectric lenticular and fiber Bragg grating (FBG) are selected to form a linear semi-open resonator by using an erbium-doped fiber as a gain medium, and a 976 nm semiconductor laser (LD) with a single-mode pigtail output for pumping. At the pump power of 200 mW, a laser output with a spectral center wavelength of 1556.16 nm, a spectral 3-dB bandwidth of 0.018 nm, and a longitudinal mode spacing of 40.77 MHz is obtained. The birefringence and its fast axis introduced into the cavity by the fiber itself are analyzed. The force sensing structure of the optical glass to be tested is placed in the cavity, and the frequency splitting of the laser after the stress loading on the optical glass in the cavity is compared with the scenario in the empty cavity, and the bifurcation and cavity birefringence caused by the external load are obtained by combining the Jones matrix transfer equation. The load on the optical glass is gradually increased from 0 to 20 N. In the stress loading process, the direction of the stress birefringence in the optical glass is parallel to the direction of the applied force. The frequency splitting of the inner cavity increases from 35.59 MHz to 35.77 MHz, which corresponds to a 679.18 nm—682.62 nm optical path difference of the cavity. The correspondence between stress and frequency splitting is understood according to the birefringence superposition model and frequency splitting, and the result can be traced back to the basic physical quantity-wavelength. Continuous loading of the optical glass results in a system with measurement repeatability better than 0.0459 MHz. The experiment is designed to avoid the uncorrected systematic error of the system induced by the sub-cavity effect. The non-aligned error equation is obtained, and the error is calibrated. The experimental results show that the sensitivity of the instrument for K9 glass is 22060 Pa/nm and the linearity is 99.44%. The method has no damage to the surface of the optical structure, nor occlusion nor influence of its normal in-service work. It is of significance for optical lens, structural measurement and error correction, and can be widely used for accurately measuring the birefringence of optical components in the infrared band.