Photonics: Optimizing the design of a compact coupler

(Nanowerk News) Integrated circuits — the tiny silicon chips running inside computers, mobile phones and cameras — represent one of the most important innovations of the twentieth century. The development of integrated circuits has not only reduced the size and power consumption of various electronic devices, but also created a multi-billion-dollar industry. Scientists would like to replicate the success of integrated circuits in high-performance computing and telecommunications with photonic integrated circuits, but shrinking the size of various photonic components to fit on a tiny chip is not easy.
There are many basic components in a photonic integrated circuit. One is the multimode interference (MMI) coupler, a device that is responsible for splitting and coupling optical signals utilizing the interference of propagating light. It is possible to use, for example, a 2x2 MMI coupler — a coupler with two optical inputs and two optical outputs — for coupling waves in a laser. Most MMI couplers designed to date have a device length of over 100 micrometers, making them too large for use in high-density photonic integrated circuits.
Qian Wang at the A*STAR Data Storage Institute and co-workers have come up with a powerful and generic optimization program that they have applied to design an ultra-compact MMI coupler ("Ultracompact Multimode Interference Coupler Designed by Parallel Particle Swarm Optimization With Parallel Finite-Difference Time-Domain").
The optimization program combines the use of two algorithms: finite-difference time-domain (FDTD) calculation, a popular computation method for modeling the electrodynamics in photonic devices, and particle swarm optimization (PSO), an intelligent optimization scheme that searches iteratively for the best solution to a problem. Simply combining FDTD and PSO, however, is not efficient and results in impractical computation times. The optimization program parallelizes the FDTD and PSO algorithms to cut down the amount of time needed to find the best solution. Running the optimization program on 12 parallel processors, for example, is almost ten times faster than running the same program on a single processor.
The researchers tested the performance of their optimization program in finding the most compact design for 2×2 MMI couplers with coupling ratios of 50:50 and 90:10. The numerical results showed that in both cases, it is possible to shrink the MMI coupler down to about ten micrometers on an indium phosphide chip with low excess loss. They verified their results by comparison with those obtained from a full simulation using a parallel three-dimensional FDTD-only method. They found good agreement between both sets of results. The findings demonstrate the potential of the optimization program in designing compact components in photonics integrated circuits.
Source: A*STAR