A novel compact wavelength-division multiplexer using highly dispersive waveguide-to-waveguide coupling Tzong-Yow Tsai * , Zhi-Cheng Lee, Yen-Cheng Fang, Ming-Hong Cha Lasers and Photonics Laboratory, Institute of Microelectronics, Department of Electrical Engineering, Advanced Optoelectronic Technology Center, National Cheng Kung University, #1 University Road, 701 Tainan, Taiwan Received 31 January 2005; received in revised form 10 January 2006; accepted 14 January 2006 Abstract A novel compact wavelength-division multiplexer using highly dispersive waveguide-to-waveguide coupling is designed, simulated and analyzed. The device consists of two very close single-moded waveguides that are periodically connected to form a mode-dependent dis- persive grating. It is demonstrated that the wavelengths over the edges of the photonic band gap contributed by the grating can be sep- arated in a very short propagation distance. Using the finite-difference time-domain method, the result shows that the wavelengths of 1570 and 1530 nm are separated by the grating in a coupler length of 57 lm which is much shorter than the required length of about 340 lm without grating assistance. The channel contrast of 20 dB and the insertion loss about 2 dB are achieved. Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.15.E; 42.79; 42.82.E Keywords: Finite-difference time-domain; Optical grating; Photonic bandgap; Two-mode interference; Wavelength-division multiplexer 1. Introduction Wavelength-division multiplexing (WDM) is an essen- tial technology for bandwidth broadening in today’s opti- cal communication. It can be realized in photonic integrated circuits (PICs) by a variety of techniques, such as co-directional couplers [1,2], micro-ring resonators [3,4], arrayed waveguide gratings (AWG) [5–7] and two- mode-interference (TMI) couplers [8,9]. TMI multiplexing is particularly interesting because of its simple configura- tion and easy fabrication. The operation of TMI multiplex- ing is based on the difference of self-imaging periods of the wavelengths k 1 and k 2 propagating in a two-moded wave- guide. The coupler length L c of k 1  k 2 demultiplexing can be approximated by L c ¼ p=jDbj; where Db ¼ðb 0;1  b 1;1 Þðb 0;2  b 1;2 Þ; ð1Þ where b j,k is the z-component propagation constant of the jth-order guided mode of k. Because of inherent weak wave- length dispersion, jDbj is usually very small and a long L c is therefore required. A few approaches have been reported for reducing L c [10–13]. In [10], only the fundamental and the second-order modes were excited by a symmetric input in a three-moded waveguide, and a large Db was then ob- tained by Db =(b 0,1  b 2,1 )  (b 0,2  b 2,2 ). A coupler length L c can be further reduced using a highly dispersive structure, such as a bulb-shaped waveguide [11] and a bent waveguide [12]. Nevertheless, the devices using the later two approaches generally suffer high insertion losses up to 5 dB and difficulty of theoretical modeling. Here, we propose a novel grating-assisted TMI multiplexer. Unlike the physics of well-known grating-assisted co-directional and contra- directional couplers, the proposed design exploits the opposite dispersions on the edges of a photonic band gap (PBG), where the wavelengths could be easily separated in 0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.01.035 * Corresponding author. Tel.: +886 6 275757562420; fax: +886 6 2345482. E-mail address: tzongyow@mail.ncku.edu.tw (T.-Y. Tsai). www.elsevier.com/locate/optcom Optics Communications 263 (2006) 197–200