Measurement And Controlling Of The Complicated Optical Field In High-power Fiber Laser Based On Modal Decomposition

The further improvement of the power of the near-diffraction-limited high-power fiber laser is restricted by factors such as nonlinear effects.In order to increase the nonlinear effect threshold,the use of large-mode-field fiber is the most direct and effective method.However,increasing the core diameter will increase the number of eigenmodes,which may lead to the mode competition,the mode coupling,and the transverse-mode instabil-ity(TMI),etc.,and finally resulting in the deterioration of beam quality.In this paper,the study is focused on the model decomposition and the adaptive phase control of high-power fiber laser,based on the optical correlation filter decomposition(CFM)method.Theoretical analysises,numerical simulations and experimental verifications are applied to carry out related research,including the following contents:1.The theoretical analysis and numerical simulation of the mode decomposition(MD)based on the CFM are carried out.The basic principle of CFM method,transmission function design and the encoding process of computer generated hologram(CGH)are introduced in detail.Based on this,the numerical model of the whole process of CFM method is established.Taking 15/130 fiber and 25/250 fiber as examples,the crucial parameter selection and error sensitivity of CFM method are studied.2.The CFM MD experiment system is constructed,and the real-time MD of fiber laser based on CFM method is realized.In the experiment,the MD accuracy of this sys-tem was verified with low-power level multimode beam.Then,a collimated CGH design method for direct MD of high power fiber laser is proposed.This method is used to achieve the MD of high-power fiber laser system with mode instability,and the variation of the average mode component with power is measured.3.The adaptive phase correction based on MD is studied.The method of improving the beam quality by phase correction is studied.An integrated adaptive phase correction method based on MD is proposed.The 0^thorder diffraction beam,which carrying most of the laser poser,is phase-corrected,and on the other hand,the MD is performed on the+1^storder carrying a small portion of the laser power.In the phase correction experiment for dynamic multimode beams,the average far-field PIB was increased from 38.5%before correction to 61.8%after correction.4.A vector beam MD and polarization measurement method based on CFM is pro-posed.The modal content of the two orthogonal polarization components and the inter-polarization phase between them are respectively and simultaneously measured by the modal decomposition and the phase-shift interferometry(PSI).By using the correlation filter method,the MD and the PSI are integrated together and realized in a single-shot process.The real-time ability and the validity of this method are verified by experiments.It is shown that with this method,the polarization distribution of an arbitrary dynamic vector beam can be given in the form of polarization ellipses and Stokes parameters in real-time,at a rate of 25 Hz.This method provides an efficient way to investigate the relationship between the mode property and polarization property of the vector beam.