Modal Decomposition Technology Based On Correlation Filter Of Eigenmode Superposition For Optical Fiber Laser Beam

With the development of optical fiber technology,the power transmitted in optical fiber is significantly improved,and the high precision and high power system is widely used in military,communication,manufacturing and other fields.However,excessively high transmission energy will cause the nonlinear effect in the fiber.It is usually chosen to increase the core diameter and reduce the energy density,but increase the number of guided wave modes.In addition,when the beam is transmitted in multi-mode fiber,there is mode competition and mode coupling among multiple guided wave modes.The pump source of the laser,the service life of the resonator,and the aging of the supporting circuit components will reduce the beam quality of the beam,which will affect the performance of the optical system.Beam quality factor M2 and other parameters for evaluating beam quality can only reflect the transmission or focusing degree of the laser beam selectively,but cannot analyze the mode characteristics of the beam.This paper first analyzes the intrinsic mode characteristics in the optical fiber and the basic principle of beam diffraction transmission,and then deduces the formula to verify the theoretical correctness of the mode decomposition technique based on the principle of correlation filtering.In the simulation process of mode decomposition,combined with the two-step ABCD algorithm,the sampling unit size of far-field spectrum surface can be adjusted to improve the spot resolution.A liquid crystal phase modulator is used as a correlation filter to modulate the optical fiber outgoing beam so that the intrinsic modes of optical fiber internal transmission are separated spatially.The far field light intensity data processing algorithm is proposed.The detection light intensity of CCD is introduced into the computer.The light intensity distribution is obtained by combining the algorithm operation.The influence of defocusing and shifting error factors on the result of mode decomposition is analyzed.The experimental platform was built to realize the mode decomposition,and the influence of space carrier frequency component and defocusing error on the mode decomposition was observed experimentally.The above work is carried out to solve the difficult problems in simulation analysis and practical engineering application of pattern decomposition technology,among which the independent innovation content is as follows:1.Two-step ABCD algorithm is applied to far-field spot analysis of pattern decomposition.Because the sampling rate of Fourier transform spectrum is fixed,the far-field diffraction spot is too small and concentrated to be analyzed in simulation.The two-step ABCD algorithm can be used to adjust the size of the sampling unit of the spectrum surface,improve the spot resolution,ensure the accuracy of the light intensity data selected from the spot,and then ensure the accuracy of the model decomposition results.2.Liquid crystal phase modulator is selected to complete phase modulation,and the far-field optical intensity data processing algorithm is proposed to jointly realize mode decomposition.Based on the principle of correlation filtering it can be seen that there are amplitude modulation and phase modulation for optical fiber outgoing beams.The liquid crystal phase modulator can adjust the phase with high precision and flexible programmable operation.An algorithm for far field light intensity data is proposed,which is equivalent to the amplitude modulation of the beam.Only phase modulation can separate the light spots in frequency domain,and only the light intensity data processed by the algorithm can be used to calculate the relevant parameters of the eigenmode.3.Analyze the influence of defocusing and shifting factors on the model decomposition results.High order Gaussian beam and high order Bessel beam have focal shift characteristics.The influence of focal shift error factors is analyzed by simulation.In practical engineering applications,defocusing error is unavoidable for the detector's position and geometric focus.Through simulation and experiment,it is found that the high-order mode LP02 is more affected by defocusing error,and the error rate of mode decomposition results is less than 10% in the range of-0.25% to 0.25% compared with its five low-order modes with lower order.Finally,this paper summarizes the whole paper,and prospects the laser mode content measurement and the mode decomposition technology based on the relevant filtering principle,and analyzes the future development direction and the problems to be solved.