A polarization independent liquid crystal assisted vertical coupler switch

Vertical directional couplers made of channel waveguides are very attractive structures for the fabrication of optical switches satisfying some of the requirements needed in the next generation optical networks due to their very short interaction length and their good performances [1]. Liquid crystals can be used in conjunction with such structures to provide them the property of reconfigurability needed for the switching operation. For this reason we experimentally investigated, in a previous work, a vertical voltage-controlled directional coupler made of a SSFLC layer placed between two BK7 glass ion-exchanged slab waveguides. Measurements on this prototype resulted in short switching times, low driving voltages and bistability [2]. In this work we demonstrate the feasibility of a 2x2 polarization independent switch with a similar structure. The coupling of light takes place between two channel diffused glass waveguides by means of a voltage-controlled nematic liquid crystal (NLC) layer placed between them. Two planar metallic electrodes are coated on the internal surface of each substrate, beside the single-mode waveguide to form a four-electrode configuration. These electrodes provide an external voltage control of the orientation of the NLC molecules, placed between them, causing the coupling of a single polarized component (TE or TM) of the waveguided field. The polarization to be switched is decided by the signs of the voltages applied at these four electrodes. A polarization independent operation is obtained cascading the TE- and the TM-switching structures. This device is suitable for the realization of large NxN switching fabrics in which a high number of vertical couplers is interconnected by input and output bent waveguides diffused on each substrate. In order to calculate the performances of the 2x2 polarization insensitive optical switch a 3-D finite difference beam propagation method algorithm (BPM) was used. The two cross-state sections (TE and TM) was designed to operate at lambda=1550 nm by varying some parameters and performing BPM simulations to achieve the lowest losses and crosstalk. We considered two 4 um wide Ag+-K+ ion-exchanged buried single-mode channel waveguides, diffused on two BK7 substrates, with maximum n of 0.05 and with a diffusion length of 1.5 um in the two orthogonal transversal directions x and y. An arbitrary NLC mixture (ne=1.6, no=1.45) was also considered. We found optimum values for the NLC layer thickness (3.1 um) and for the angle teta between the NLC director and the propagation direction. teta lies in the yz plane (xz plane) for the TE (TM) switch. We noticed also the existence of an optimum value of 7.75 um for the separation width between the two couples of electrodes. Calculated crosstalk and losses of the TE switch with the quasi-TE fundamental mode propagating in the input waveguide were –45.6 dB and 0.45 dB respectively with a coupling length of 143 um. The TM switch, with the fundamental quasi-TM propagating mode, exhibits a crosstalk of –29.1 dB and losses of about 1.7 dB with a coupling length of 131 um. Estimated average crosstalk and losses for the polarization insensitive cascaded switch were of –32 dB and 1 dB respectively. [1] B. Liu, A. Shakouri, P. Abraham, B. G. Kim, A. W. Jackson, and J. E. Bowers, Appl. Phys. Lett., vol. 72, no. 21, pp. 2637-38, 1998. [2] R. Asquini, A d’Alessandro, IEEE LEOS 2000, 13th Annual Meeting, vol. 1, pp. 119-20, 2000.