Theory of pulsed laser operated nonlinear directional coupler.

摘要

In this dissertation, a coupled mode theory is developed to describe the propagation of a laser pulse in a nonlinear directional coupler constructed from a semiconductor material. This theoretical study is important to an understanding of the switching characteristics of nonlinear couplers which are needed in the design and fabrication of optical signal processing devices operating at high speeds.;The theory is developed in a sufficiently general manner so as to be applicable to a wide range of couplers constructed from different semiconductor materials. Moreover, the theory is extended in this dissertation to the case of a coupler consisting of three channels. This latter extension is made in an attempt to improve the switching characteristics of the coupler.;All numerical calculations were carried out on a VAX 785 computer in computation times which do not exceed ten minutes. The resulting ease of computation associated with the present theory shows the theory to be superior to analyses based on the "beam propagation method" which require computation times on a supercomputer on the order of one hour.;The theory developed improves the previously used coupled mode theory of the nonlinear coupler in a number of ways. First, it eliminates the "steady state" assumption used in previous analyses by taking account of time dependent effects arising from the pulsed character of the input radiation. Second, it expresses the total coupler field in terms of the supermode fields of the total coupler structure rather than in terms of the normal mode fields of the separate channels. This is expected to lead to more accurate results in the case of strong coupling between the coupler channels. Third, it accounts for variations in the field within each channel perpendicular to the direction of propagation and fourth, it improves the description of the nonlinearity in the material used in the coupler by making use of a plasma theory to determine the nonlinear dependence of the change in the refractive index on the light intensity. The results of the theory are shown to be in good agreement with available experimental results.