| Mode decomposition technology is an important means to realize fiber laser mode field analysis and plays an important role in high quality fiber laser and large capacity fiber communication.The current mode decomposition methods based on near-field optical intensity analysis have poor precision and slow speed in the decomposition of complex mode fields with more than six eigenmodes.In order to solve the above problems,a transient fiber mode decomposition technique based on polarization phase-shifting point diffraction interference is proposed in this paper.The theory and simulation model of laser mode decomposition based on polarization point diffraction phase shifting interference are established.The optical fiber polarization point diffraction phase-shifting interferometry is combined with angular spectrum transmission to obtain the complex amplitude distribution of laser near field,and then the mode decomposition is realized.The problem of wave aberration caused by constructing actual optical path is avoided.In the simulation,the feasibility of the virtual 4f system is verified,and the measured mode fields of modes 6 and 10 are decomposed.The results show that the correlation number of light intensity distribution before and after mode decomposition is above 0.99.An experimental device of laser mode decomposition based on polarization phase-shifting point diffraction interference was built,and the experiment of less mode laser mode decomposition was carried out.The experimental results of 30 groups of 10 modes of low-mode laser mode decomposition show that the correlation number of light intensity distribution before and after mode decomposition is above 0.97,and the average operation time is 0.0641 s at300×300 pixel resolution.By further reducing the pixel resolution,mode decomposition can be completed within 0.0168 s.The beam quality of fiber laser is evaluated by mode decomposition and compared with that of commercial beam quality instrument.The results show that the measurement error of equivalent M~2 factor is less than 3%.The error analysis of laser mode decomposition based on polarization phase-shifting point diffraction interference is studied.The results show that the mismatched intensity of the reference and testing light will aggravate the effect of the azimuth error and phase delay error of the quarter wave plate on the mode decomposition.The radial dislocations of the reference and testing light introduces tilt and small magnitude coma.The axial dislocations of the reference and testing light introduces defocus.The fiber mode decomposition method based on polarization phase-shift diffraction interference is insensitive to defocus error.The laser mode decomposition method based on polarization phase-shifted diffraction interferometry is insensitive to defocus error.In conclusion,the feasibility and advantages of the laser mode decomposition method based on polarization phase-shifting point diffraction interference are verified by simulation and experiment,which provides a fast and accurate mode decomposition method for complex mode fields with the number of eigen modes greater than or equal to 10. |
