Study On The Key Techniques Of Fiber Active Devices Packaging With Laser Welding

Packaging process remains a bottleneck for the cost-effective manufacturing of fiber-optic devices. The most important issues are the procedures of aligning a single mode fiber to the laser diode and fixing it in place automatically. The hearts of the fiber-optic alignment automation are high accuracy positioning stages and speedy alignment algorithms. Pulsed laser welding has proven to be the preferred bonding method. However, the post-weld-shift (PWS) during laser welding process results in a reduction in light coupling efficiency. The purpose of this work is to develop a sub-micron precision alignment and bonding process for laser welding technology in fiber-optic active devices packaging. In detail, delicate positioning stages, automated alignment algorithms, and compensating the PWS, are the main research topics. The contents are presented as follows:A multi-DOF alignment movement system combines coarse and fine adjustment is proposed. The system combines flexible hinge micro-displacement worktable with high resolution and the lead screw guide rail stages with long travel range, so the contradiction between the range and the resolution is settled and the two steps alignment with coarse and fine positioning is achieved. The all stages are actuated by step-motor, thus the system is low cost and easy controlling. The flexible hinge micro-displacement worktable designed in this work is driven by step-motor rather than widely used conventional piezoelectric actuator. The rotation-translation and translation-translation movement transformations are achieved by motor-spring, flexible hinge and elastic plate mechanisms. The output displacement is obtained by reducing the input displacement through elastic strain. The structure of the micro-displacement worktable is designed as parallel linkage. Thus the inter-coupling shift is minimized and the linearity of the displacement output is improved. The travel of the movement system is 50mm and the minimal resolution is 0.03μm. The resolution of the angular stages is 0.0001 deg. The fiber alignment positioning system offers the distinctive advantage of combining the ability to scan a wide optical field for initial light search and perform nanometer positioning for the peak coupling power optimization.The misalignments such as lateral (xy-axes) offsets, longitude (z-axis) gap, and angular deviation will result in rapid decrease of the coupling efficiency between light source and receiving fiber. These effects will be analyzed experimentally with the fiber-optic alignment stages developed in this work. The alignment tolerance and locating area are presented combined experimental results and theoretical analysis. The searching strategy consisting of coarse and fine alignment is developed, and the key of the optimization process is lateral alignment. The hill-climbing algorithm is widely used in current opto-electronics packaging industry because of its easy implementation. But its disadvantages are time-consuming and having the potential of missing the real peak. Novel fiber active alignment algorithms based on parabola approximation technique and pattern search method are developed to solve currently existing problems. Both simulation and experimental results for alignment optimization between a laser diode and a single-mode fiber will be presented. Comparison among different algorithms will be studied. Then, the optimization algorithm for maximum coupling efficiency is given. The algorithm combined out-spiral method for coarse alignment and mainly pattern search algorithm for fine alignment is used for fiber-optic alignment automation. The application results demonstrate that the process for locating the optimal coupling position with the algorithm developed in this work is speeded up; and the global convergence is much better than that of the hill-climbing algorithm.After studying on alignment stages and locating algorithms, the laser welding workstation for fiber active devices packaging such as laser diode packaging is designed. A three-dimensional finite element model for predicting the PWS in the case of a cylindrical package with fiber pigtail to a laser diode.is presented. The characteristics of PWS are analyzed theoretically with thermo-mechanical coupling. The PWS are observed when the energies of the three weld spot are unbalanced and the weld spot offsets are exist. The regularity that the PWS is depended on the energy comparision results of the three weld spots is found. In experimental research, the loss of the coupling power is measured and the preferred parameters of the laser welding are given for minimizing the PWS. The method to judge the direction of the PWS is proposed and is validated by experiments. The experimental analysis in the cases of unbalanced energies and weld spot offsets are carried out. The theoretic predicting is well confirmed by the experimental results. The directional thermal compensation by supplemental single-beam laser spot welding is achieved according to the characteristics analysis of the PWS. It can consistently achieve assembly tolerance less than 0.5μm and the coupling optical power can be increased to better than 95% of the original maximum throughput.