Research On 2D-precison Multi-mode Fiber Coupler With High Efficiency

LIDAR is a research hotspot all over the world now as it has many outstanding advantages.For example,it has extremely high resolution,strong anti-interference ability,lighter volume and quality than microwave radar,and ability of detecting hidden targets such as mines and submarines that ordinary radars cannot.At present,in the LIDAR imaging,the non-scanning imaging method of the array detector overcomes some shortcomings of the single detector.But there is a certain gap between the pixels of the array detector,in order to make better use of laser energy and reduce optical energy loss,a fiber array coupler is usually used at the transmitting end to split the laser beam.Because of the fiber's advantages of light weight,good flexibility,anti-interference etc.,the large optical fiber array connected to the photodetector can avoid the fabrication difficulties of the large photodetector array and the requirements for large number of detectors.Fiber arrays and lens arrays have important applications in LIDAR systems.Precision coupling technology of 2D fiber array is one of the key technologies in the application of LIDAR system.In the coupler formed by the fiber array and the lens array,the coupling efficiency and reliability are affected by many factors.There are still many problems and shortcomings of the current 2D fiber arrays and their couplers:(1)the existing 1D fiber array products are relatively mature,but the 2D fiber array positioning errors are relatively large,and the main technology is monopolized by some foreign companies;(2)A small number of fiber optic array products that have recently appeared in China are only with flat fiber end faces.The problem is of low coupling efficiency when coupling with laser beams,leading to require high power laser sources in use;(3)The researches on 2D fiber-optic array couplers that have been reported in China are mostly limited to the theoretical design,and the low coupling efficiency is still the problem.Based on the above,we propose to design and develop a 2D fiber coupler with high coupling efficiency and high precision positioning.The main contents of the thesis include the following:At first,two kinds of 2D hole array nickel stencils with high-precision position are designed and manufactured by UV-LIGA process and electroplating process.And consistently two kinds of 2D optical fibers arrays of 8×8 and 7×7 are fabricated by the improved defining-position method.The principle and process of fabricating optical fiber array and polishing the end-face of fibers are described in detail.By testing the positioning accuracy of the two fiber arrays,the average error of the fiber positioning in the 8×8 fiber array is 6.6 ?m.According to analysis,the error mainly comes from the tolerance of the fiber coating.After removing the coating of fiber,the average positioning error of the 7×7fiber array reaches 0.9 ?m.The AFM test results show that the roughness of polished optical fiber end-face reaches 6.7 nm,which is less than one percent of the working wavelength and fully meets the requirements for use.After testing,the average coupling efficiency of the 8×8 fiber array is 38.9%.Secondly,a micro-lens array is designed and fabricated according to 7×7 fiber array.The lens was calculated,simulated and verified by means of geometrical optics theory of refraction lens,Zemax simulation software and light field theory,respectively.The determined lens material is PMMA,the clear aperture of the micro-lens unit is 0.48 mm,and the curvature radius is 0.9mm.A micro-lens array is fabricated using precision single point diamond turning method.After testing,the consistency and uniformity of the processed lens basically meet the requirements;the roughness of the top area of the micro-lens is 17 nm,less than ?/30@632.8nm,but the roughness of the edge part is significantly reduced;the curvature radius is 0.92 mm,which is higher than the design value out of 0.02 mm.In addition,we introduce a method of making a micro-lens array by a pattern transfer process.A theoretical model of transferring micro-lens array by etching is established,and the influence factors of surface shape evolution are analyzed;photoresist micro-lens array is fabricated by thermal reflow method,and ion beam etching is used to transfer the pattern on silicon substrate.The test and characterization results show that when the ion beam incident angle is 40°,the micro-lenses produced by etching have high accuracy,smooth surfaces,and good uniformity.Finally,the 2D fiber coupler is constructed and the coupling efficiency is tested.The effects of mechanical alignment error,end-face reflection,input beam divergence angle and off-axis distance on coupling efficiency are theoretically analyzed.By testing and calculating the output power and coupling efficiency of 49 fibers in the 7×7 array,the average coupling efficiency of the remaining 48 fibers is 57.51%,except the broken one fiber.From the statistical results,the coupling efficiency of each coupling unit is approximately normal distribution,and the data is relatively concentrated.Supported by the micro-lens array,the coupling efficiency is nearly doubled compared with the 8×8 fiber array without micro-lens array,even if the fiber diameter is reduced by about 4 times.