Analysis of Ti:LiNbO 3 zero-gap directional coupler for wavelength division multiplexer/demultiplexer Pranabendu Ganguly * , Juran Chandra Biswas, Samir Kumar Lahiri Integrated Optics Laboratory, Advanced Technology Development Centre, Indian Institute of Technology, Kharagpur 721 302, West Bengal, India Received 3 May 2007; received in revised form 12 February 2008; accepted 13 February 2008 Abstract For the purpose of multichannel integrated-optical wavelength division MUX/DEMUX design, the wavelength tuning method of Ti:LiNbO 3 zero-gap directional coupler is analysed by effective-index based matrix method (EIMM). In this method, first, the 2D refrac- tive index profile of the Ti:LiNbO 3 zero-gap directional coupler is transformed into lateral 1D effective-index profile by WKB method. Finally, matrix method is applied to this effective-index profile and the propagation constants are computed from the resonance peaks of the excitation efficiency versus propagation constant characteristics. It has been shown that the channel wavelength tuning can be achieved by varying the Y-branching angle at the input and output of the device or by a slight variation of two-mode-section length. The method of electro-optic fine tuning of the channel wavelength and channel separation of the device has also been investigated. A cascaded structure with proper parameters for four-channel application is also simulated and the results are presented. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Zero-gap directional coupler; Wavelength division multiplexer/demultiplexer; Effective-index based matrix method; Ti:LiNbO 3 waveguide 1. Introduction Wavelength division multiplexing (WDM) is a useful technology for increasing the information capacity of optical fiber systems. In order to exploit such techniques to the fullest, single-mode fibers are the obvious choice owing to their broad bandwidth, low propagation loss, and compatibility to a variety of system components [1]. The realization of single-mode MUX/DEMUX’s has been accomplished by using wavelength dispersive elements like optical interference filter, optical diffraction gratings, and wavelength selective coupling between two adjacent wave- guides. Interference filter type MUX/DEMUX’s have been experimentally demonstrated by using fiber-optics [2] and microoptics, where GRIN-lenses [3] or ball lenses [4] have been implemented as beam collimators. These devices are considered to be of practical importance, if the transmis- sion system does not require more than about six channels with a spacing in the order of 30 nm. A greater number of channels, e.g., N = 20 [5], with a much narrower channel spacing, e.g., 1.35 nm [6], can be multiplexed by grating components. Here, microoptic techniques using GRIN- lenses [5], and guided wave approaches with gratings, waveguides, and photodetectors [7], integrated on one substrate, have been applied. The wavelength selective cou- pling has been utilized in all-fiber [8] and integrated optic [9] directional couplers. Depending on the symmetry of the coupler, the wavelength transmission curve is periodic [10] or exhibits a band-pass behavior [9]. Based on the practical advantage of fabrication, integrated optics offers the greatest potential of building compact MUX/DEMUX in a stable rugged structure with simplified assembling. Besides the concept of the integrated optic MUX/DEMUX mentioned above, another structure has been proposed and experimentally demonstrated [11] based on the wavelength dependence of two-mode-interference (TMI) in a two- moded waveguide. Similar to the symmetrical fiber coupler, transmission curve of a TMI-multiplexer is periodic, thus 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.02.014 * Corresponding author. Tel.: +91 3222 281936; fax: +91 3222 255303. E-mail address: pran@ece.iitkgp.ernet.in (P. Ganguly). www.elsevier.com/locate/optcom Available online at www.sciencedirect.com Optics Communications 281 (2008) 3269–3274