Laser Printed Fiber Microlens For Fiber-diode Coupling By Direct Laser Writing

Laser diode (LD) is widely used in optical fiber communication, biomedicine, laser pumping, material processing and other fields, because of its merits such as small size, light weight, cheap price and so on. In many applications, laser diode is required to be coupled with the optical fiber. However, with wide light-emitting area LD generates an elliptical mode field, which does not match the circular mode field of optical fiber. That provides a low direct coupling efficiency between laser diode and optical fiber.In order to improve the coupling efficiency of laser diode and fiber, a common approach is to fabricate a microlens on a fiber endface for the single-mode fiber and LD to couple directly. Although there are some methods of microlens fabrication, like heating and drawing fiber, chemical etching, grinding followed with polishing, these methods more or less have disadvantages of complex fabrication process, time-consuming and high processing cost. In this study, we demonstrate a novel fabrication process for fiber microlens by laser direct writing, which is able to print a microlens on the fiber endface and allows a batch print process at a low-cost, high consistency and high fabrication rate.Using laser direct writing, fiber microlenses with different shapes are fabricated and their coupling efficiency is measured. A 650nm laser diode with far-field divergence angles of 6.5°(horizontal) ×35°(vertical), and an aspect ratio of 5 is used. A maximum coupling efficiency of 53.5% is obtained in the experiment.The shape of microlens affects the coupling efficiency. The radius of curvature of lens tip is described by using an inscribed circle. It’s found experimentally that lens height for maximized coupling is inversely proportional to the inscribed circle radius. For a maximum coupling efficiency, an optimized inscribed radius is 8.1μm, the lens height is found to be 50μm with a tolerance of +5/-0μm.Defining the separation between the LD and the fiber microlens tip as working distance, the coupling efficiency is dependent on working distance. In the experiment, coupling efficiency peaks when the working distance is at 15.8μm.The lateral misalignment and angular deviations between the LD and the fiber result in a decrease of the coupling efficiency. In the experiment, it is measured that for a 1-dB coupling loss increment, the tolerances of X, Y translational displacements are 2.5,1.2μm respectively and the tolerances of θX,θY angular deviations are 2.0,5.0 degrees, respectively.In this paper, the ray tracing method in ZEMAX is applied to simulate the coupling efficiency, and the simulation results are consistent with the experiment results.