Birefringence Measurement Based On Frequency Splitting Of Fiber Laser

With the development of aerospace technology and continuous breakthroughs in the performance of modern optical systems,the real-time online measurement of birefringence of optical materials has become a new research hotspot.There are abundant birefringence changes in optical fiber,and its birefringence measurement is very important to improve the overall performance of optical fiber system.The birefringence measurement technology based on laser frequency splitting effect can measure many birefringence and other external parameters.The traditional birefringence measurement technology based on laser frequency splitting effect is mostly based on He-Ne laser,which has a high requirement for the transmittance of the object,which limits the range of the sample;It is difficult to coupling with optical fiber and other waveguide systems.This paper is aimed at the birefringence measurement based on the frequency splitting of fiber laser.The mechanism of frequency splitting of multi longitudinal mode fiber is analyzed theoretically and simulated;The different birefringence types in the laser are measured;Different systems are designed and built,and their application is further expanded.The main research contents are as follows:1.The frequency splitting mechanism of the multi longitudinal mode fiber laser is studied.Through the study of the mode characteristics of the longitudinal mode orthogonal splitting of the multi longitudinal mode laser,the relationship between the frequency difference and the phase difference of the multi longitudinal mode frequency splitting is obtained.Combining birefringence Jones matrix and laser self consistent equation,the model of birefringence superposition in fiber cavity is deduced,which eliminates the nonlinear error which cannot be caused by the axis.The mechanism of beat frequency broadening of multi longitudinal mode laser is studied.The influence of gain curve on the longitudinal mode frequency deviation is analyzed by the mode traction theory.The broadening of beat frequency generated by frequency splitting in the uniform broadening fiber laser cavity is obtained.The system error caused by frequency splitting in multi longitudinal mode laser is estimated.The ion pair quenching phenomenon in the active fiber with high erbium-doped concentration is studied,and the gain instability caused by the ion quenching is analyzed.The effects of different quenching concentration and different quenching positions on beat frequency are simulated respectively,and then the rich frequency-domain evolution of beat frequency signal is explained.2.The measurement of birefringence of insertion device by fiber laser frequency splitting method is studied experimentally.An insertion birefringence and stress measurement system based on the frequency splitting effect of 1556 nm fiber laser is designed and built.The output characteristics of the half cavity frequency splitting fiber laser are studied.The polarization mode beat signal is measured in the cavity and cavity respectively under the stress loading.The experimental results show that there is inherent stress birefringence in the ordinary single-mode linear resonator.The sensitivity of the system is 22060 Pa/nm and the linearity is 99.44%.The thickness and refractive index of the wave plate are measured by using the same cavity structure system.The thickness and refractive index of the sample to be measured are obtained by fitting,and the error of the intrinsic refractive index measurement is 10^-5.3.The equivalent birefringence measurement of the external modulation of the polishing fiber is studied.The mechanism of intrinsic birefringence of asymmetric fiber is analyzed by the stress balance model.The nonlinear modulation law of the birefringence in the cavity by the external refractive index is theoretically analyzed,and the simulation of the equivalent birefringence is carried out by combining the finite element analysis.The equivalent birefringence and external refractive index measurement system of the polishing fiber cavity are designed and built.The equivalent birefringence is measured by using the superposition model of the external modulation birefringence of the polishing fiber.Through experimental research,the relationship between birefringence and external refractive index of the system is calibrated,and the measurement accuracy of external refractive index reaches 8.43×10^-5.The refractive index curve of glucose mass fraction and the change trend of thermo optical coefficient were measured.4.The circular birefringence measurement of all optical fiber torsional cavity is studied.The mechanism of circular birefringence in fiber laser is analyzed.The coupling coefficient of the weak guide fiber in torsion is derived by combining the theory of fiber perturbation and the coupling mode theory of fiber guide.Based on the direct-view solution method and the principle of laser self consistency,Jones matrix in the torsional state of laser is derived,and the separation model of intrinsic linear birefringence and circular birefringence in the cavity is obtained.A set of all optical fiber torsion cavity circular birefringence measurement system with 1551 nm band is designed and built.The output characteristics of laser are studied and the curve of the circular birefringence in the cavity is obtained with the change of the torsional angle.A method of simultaneous measurement of torsion tension double parameters is proposed.The results show that the torsion and stretching can be decoupled by the beat frequency and spectral measurement of the system.