Simplified Theory of the Multimode Fiber Coupler

Simplified Theory of the Multimode Fiber Coupler By K. OGAWA (Manuscript received October 29, 1976) We simplify the coupling theory between two contiguous, parallel, multimode step-index fibers, describe the coupling concept, and derive an upper estimate for the overall coupling efficiency between the two fibers. The maximum coupling obtainable, according to this estimate, is less than 72 percent (--1.5 dB). The coupling efficiency derived for short coupling lengths shows good agreement with experimental results. I. INTRODUCTION A multimode fiber tap-coupler is a useful component for certain optical communication systems such as the optical data bus. Unfortunately, it is not an easy matter to evaluate the simultaneous coupling process between the hundreds or thousands of modes. An analysis of the problem has been given by Snyder and others. 1-3 These authors analyzed only the coupling between certain mode pairs. Snyder 4 recently reported the total power transition between multimode fibers. However, his conclusion is based on H E i m modes for analyzing the crosstalk. We have derived a similar simplified expression for the total coupling between identical, contiguous, parallel, step-index, multimode fibers, which can expand to all modes under the restriction that two fiber cores are touching each other. We predict that the maximum coupling efficiency is less than 1.5 dB when all modes carry an equal amount of power. T h e distance between two fibers affects the coupling efficiency very seriously when fibers have large numerical apertures.