Optical Directional Coupler Switch Using Domain Inversion Technology Ridwan Rafi Hossain, Mohammad Kibria Chowdhury, Ehsanul Matin Shujon, and M.S. Islam + Department of Electrical and Electronic Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh + E-mail:islams@eee.buet.ac.bd Abstract-The switching voltage reduction in optical directional coupler switch utilizing alternating is fully analyzed. It is shown that increasing the number of sections in electrode can reduce the switching voltage significantly. But high-speed operation of the switch is limited as the number of electrode-section increases. In this paper, the improvements of above problems are discussed. By the introduction of domain inversion technology in lithium niobate (LiNbO 3 ), lower switching voltage and high-speed operation of the optical directional coupler switch can be achieved. I. Introduction For an optical switch, achieving of perfect cross state and bar state to reduce the crosstalk and reduction of the switching voltage are required. A typical switching voltage at present for a wavelength of 1.5μm is about 5V. As it is difficult to fabricate exactly identical waveguides in the conventional directional coupler optical switch, so achieving perfect cross state is hard. The most popular type optical switch, alternating with electrodes divided into two sections, can achieve perfect cross state and bar state even though the two waveguides are not exactly identical [1]. However the switching voltage is high and high speed operation cannot be achieved. In this paper, reduction of switching voltage by dividing the coupler into multiple sections is analyzed. The switching voltage and interaction length of optical switch with alternating of five and six sections are calculated. Advantages and disadvantages of the division are discussed. Furthermore we propose the introduction of domain inversion technology in LiNbO 3 (hereafter referred as LN) for alternating type optical switch. The new design has advantages in forming multiple sections, high-speed switching and minimizing switching voltage. II. The Principle of Directional Coupler Switch The schematic of a conventional directional coupler switch is shown in Fig. 1. Here two waveguides with two parallel strip electrodes are formed in LiNbO 3. , The principle of operation of this switch is given in Fig. 2. If the interaction length, L equals the minimum conversion length, L o , the propagation constants 1 and 2 are equal. With no voltage applied between two electrodes the light is guided into one waveguide and outputs from another. This state is defined as cross state and we associate with the symbol . But if appropriate voltage is applied between two electrodes to satisfy 3 2 2 1 , the light outputs from the same waveguide. This state is defined as bar state and we associate with symbol . V Waveguide Electrodes L=L 0 P 1 (0) P 1 (L) P 2 (L) LiNbO 3 Fig. 1. The schematic diagram of an optical directional coupler switch. Fig. 2. The principle of operation of an optical directional coupler switch (single section). V = 0 ß / =0 V applied ß / = 2 3 25 % L o / 2 Cross state Electrode Guided Wave L o P 1 (0) P 1 (0) P 1 (L) P 2 (L) Bar state Waveguide 5th International Conference on Electrical and Computer Engineering ICECE 2008, 20-22 December 2008, Dhaka, Bangladesh 978-1-4244-2015-5/08/$25.00 (c)2008 IEEE 113