Best Paper Award

A Study on Topology Optimization of Nonlinear Optical Device Using Beam Propagation Method and Sensitivity Analysis Based on Adjoint Variable Method

Koyo MORI, Yasuhide TSUJI

[IEICE TRANS. ELECTRON., Vol. J101-C No. 5 MAY 2018]

  In recent years, information communication services have spread rapidly, and a further increase in speed and capacity of optical communication systems is required as a base network. In order to make best use of the high-speed nature of optical communication, it is important to construct an all-optical network, in which electrical processing is eliminated and optical signals are processed as they are. For this purpose, signal processing devices utilizing the Kerr effect, such as optical switches and an optical logic gates, have been actively investigated because the Kerr effect, which is one of the nonlinear optical effects, is expected to have a high-speed response.

  The development of optical devices has been done by the improvement of existing device structures and heuristic approaches so far, however in recent years, with the development of simulation techniques and the improvement of computer processing speeds and computer resources, research on the optimum design method utilizing computer simulations has been actively conducted and is utilized to design high-performance photonic devices. The optimal design methods are roughly classified into size optimization, shape optimization, and topology optimization. Among them, topology optimization has the highest design freedom and allows us to design device structures with arbitrary topology. In general, the design of nonlinear devices is more difficult than the design of linear devices, and if such a topology optimal design method can be applied to the design of nonlinear devices, it will be possible to develop completely novel optical devices that could not have been considered before.

  In this paper, a novel topology optimal design method for optical devices using the optical Kerr effect is proposed and developed. As a numerical analysis method, a beam propagation method that can efficiently analyze light propagating behavior in optical waveguide devices even if their device length is longer than several thousand times of the wavelength is employed. And as a structural representation in the design region, a density method that can represent a structure with an arbitrary topology is employed and their density parameters (design variables) are optimized by the gradient method. While topology optimal design methods can automatically generate novel and innovative device structures, the design variables to be optimized usually exceed hundreds of thousands or more and the efficient evaluation method of all the sensitivities which are the characteristic variations to the design variables is required. In this paper, the adjoint variable method, which can evaluate all the sensitivity at the cost of only two times of beam propagation analysis regardless of the number of design variables, is reformulated so that it can be applied to the sensitivity analysis of nonlinear devices. The effectiveness of the proposed approach is verified through the design of optical switches and optical logic gates using the optical Kerr effect. The design method proposed in this paper can be applied to the development of various next-generation devices in the field of photonics, so this paper can be highly evaluated as a paper suitable for the best paper award of the Institute of Electronics, Information and Communication Engineers.