The finite-difference time-domain (FDTD) algorithm has been used in simulation-based designs of many optical devices, but it fails to reproduce high-Q whispering gallery modes (WGMs). On the other hand, the nonstandard (NS) FDTD algorithm can accurately compute WGMs and can be used to make simulation-based designs of WGM devices. Wavelength splitters using the coupled resonator optical waveguides (CROWs) based on WGM couplings have recently attracted attention because they are potentially ultracompact. In this paper, we design a CROW wavelength splitter using NS FDTD simulations and demonstrate high interchannel extinction ratios of over 20？dB. 1. Introduction The finite-difference time-domain (FDTD) algorithm [1] has been used in simulation-based designs of optical devices such as optical fibers. However, the FDTD algorithm has not been used to design optical disc and ring resonators based on whispering gallery modes (WGMs) in the past, although these are essential building blocks of integrated optical circuits. This is due to failure of the conventional FDTD algorithm to reproduce the high-Q WGM resonances [2–4]. Instead the discontinuous Galerkin time-domain (DGTD) method [5] which can accurately calculate these resonances with lower memory consumption has been used, but its computational overhead is significantly higher than FDTD [6]. Recently, the nonstandard (NS) FDTD algorithm [7, 8] has been successfully used for high-accuracy WGM simulations with lower memory consumption and computational overhead [9, 10]. This is because high-accuracy difference operators are derived by optimizing to monochromatic wave propagation in the NS-FDTD algorithm and its temporal-spatial difference errors are considerably reduced by comparison with the conventional FDTD. Details of the NS-FDTD derivation are given in [8, 9]. Thus, the NS-FDTD algorithm can be used to make simulation-based designs of WGM devices. On the other hand, the coupled resonator optical waveguide (CROW) [11, 12] has attracted much attention in recent years because it combines characteristics of both resonator and waveguide and is potentially ultracompact. In particular, the optical wavelength splitter using the CROW [13, 14] is very small in comparison with conventional splitters. For example, the microring resonator-based splitter [15] has high interchannel extinction ratios (IERs), but its size is much larger than a wavelength (a few dozen μm) because the resonator employs total internal reflection with designs based on geometric optics theory. Other splitters using an arrayed waveguide
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