Research On Large Divergence Angle Light Source For Interferometer Based On Large Numerical Aperture And Few-mode Fiber

Using optical fiber light source in the interference system can effectively filter high-frequency noise in the wavefront and obtain high-quality spherical waves.The divergence angle of the conventional fiber light source is often small,which makes the interferometer based on the fiber light source bulky and the interferogram background uniformity.In order to improve the divergence angle of the output beam of the interferometer fiber light source,this paper proposes the large divergence light source technology based on few-mode fiber with large numerical aperture.The theoretical relationship between the divergence angle of the light source and the parameters of the optical fiber is studied.The output divergence angle of the light source and the background uniformity of the interferogram can be improved effectively by using the fiber with larger numerical aperture and smaller core diameter.Limited by the fiber manufacturing process,the fibers with large numerical aperture and small core diameter are mostly few-mode fibers,which cannot be coupled with the laser efficiently,and will excite higher-order fiber modes,reducing the beam quality of the fiber light source.Combining the low-loss tapering conditions of the fiber and the theory of optical field transmission in the tapered fiber,the simulation analysis and experimental verification of low-loss tapering at the output end of large-aperture low-mode fiber are carried out.The experimental results show that the taper reduces the core diameter of the output end face,increases the divergence angle,and can effectively filter out high-order modes in the fiber and optimize the beam quality.The theory of optical power improvement of optical fiber source is studied.A standard single-mode fiber is used as the medium to couple the free-space laser into the few-mode fiber with large numerical aperture.There is mode field mismatch between the single-mode fiber and the few-mode fiber.In convention,the laser transmission efficiency of the fusion is only 79%.Hydrogen and oxygen flame is used to heat and expand the core of optical fiber fusion contact,and the laser transmission efficiency is increased to 92.4%.The polarization state control theory of fiber light source is studied.Based on the principle of fiber stress birefringence,fiber bending can cause phase delay of transmitted laser light.Combining the calculation of the number of linear loops and the control theory of the equivalent wave plate polarization,a rotatable coil polarization controller is used to coil the single-mode fiber,and the polarization control of heterogeneous fused fiber is successfully realized.Finally,a linearly polarized light output with a degree of polarization of 0.992 is obtained.At last,a set of optical path system of optical fiber light source with large divergence angle was set up in the laboratory.The measured divergence Angle of light source was 12.6°,and the beam quality evaluation factor M~2=1.008.Using this light source in a Fizeau interferometer,the uniformity of the interferogram formed(U=0.063)is significantly better than that of the standard single-mode fiber source(U=0.428).The results show that the large divergence angle light source based on large numerical aperture and few-mode fibers can significantly improve the uniformity of the interferogram background.