MODELING OF ROUGH-SURFACE EFFECTS IN AN OPTICAL TURNING MIRROR USING THE FINITE-DIFFERENCE TIME-DOMAIN METHOD

A finite-difference time-domain (FDTD) method is applied to calculate the forward-reflected and back-reflected powers of a guided mode from a rough turning mirror in a bent waveguide of a high-power laser array. Numerical simulation of this large structure (about 100 x 100 wavelengths) requires large computer memory and intensive computation. However, by segmenting this large problem into a number of smaller problems, the simulation region can be shrunk to a small area containing only the details of the rough-surface mirror. Moreover, by launching the incident wave judiciously, the computation time grows linearly as the length of the mirror. A farfield transformation of the calculated time-domain scattered field yields forward-reflected and back-reflected powers. Hence, the computer time needed to analyze this large turning-mirror system is reduced to about 3 min of CRAY time, compared to several hours for a brute-force approach using a full mesh. Therefore, the FDTD method is a fast and effective way to obtain the forward-reflected and back-reflected powers from a realistic rough turning mirror. Such information is useful for improving the laser array performance through design and process optimization, and the FDTD method is suitable for the computer-aided design of laser array structures.