An integrated 2x2 SSFLC optical switch with channel ion-exchanged glass waveguides

The next generation networks demand for all-optical switches with good performances (i.e. low losses and crosstalk) in order to exhibit fundamental capabilities like automatic protection switching, network monitoring and wavelength routing [1]. Vertical directional couplers are very attractive structures for the fabrication of optical switches satisfying some of these requirements due to their very short interaction length and their good performaces [2]. For these reasons 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 [3]. Now we report a theoretical investigation about the possibility of using such structure to realize a 2x2 optical switch in which the coupling of light takes place between two channel diffused glass waveguides. This device can be used in larger NxN cascaded switches. In such switching matrices a high number of vertical couplers is interconnected by input and output bent waveguides diffused on each of the two glass substrates that contains the SSFLC. A pair of metallic electrodes coated on the internal surface of each substrate, near the waveguide, provides an external voltage control of the FLC orientation. In order to calculate the performances of this 2x2 optical switch a 3-D finite difference beam propagation method algorithm (BPM) was used. The cross-state switch was designed to operate at =1550 nm by varying some parameters and performing BPM simulations in order to achieve the lowest losses and crosstalk. An arbitrary SSFLC layer (ne=1.6, no=1.45) exhibiting a smectic cone angle of 45° was considered. We found optimum values for the SSFLC layer thickness (3 m) and its angle of alignment  respect to propagation direction ( =51.68°, nFLC()=1.537 for the TE polarized light). We noticed also the existence of an optimum value of 7.75 m for the separation width between the two couples of electrodes. The ion-exchanged buried channel waveguides, diffused on the two BK7 substrates, were 4 m wide, with maximum n of 0.05 and with a diffusion length of 1.5 m in the two orthogonal transversal directions. Calculated crosstalk and losses of the switch for the quasi-TE fundamental mode propagating in the waveguide were –41 dB and 0.47 dB respectively with a coupling length of only 142 m. In the bar-state (nFLC=1.4518) this optimized device exhibits a loss of only 0.01 dB. Such structure, that is realized using the low-cost technology of SSFLC cells and ion-exchanged glass waveguides, could meet the requirements of fast operation, low cost, low power consumption, scalability and compactness [4], needed to make all-optical networks. [1] S. Bregni, G. Guerra, A. Pattavina, in Communications World. Editor: N. Mastorakis. WSES Press, 2001. [2] 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. [3] R. Asquini, A d’Alessandro, IEEE LEOS 2000, 13th Annual Meeting, vol. 1, pp. 119-20, 2000. [4] G. I. Papadimitriou, C. Papazoglou, A. S. Pomportsis, IEEE J. Lightwave Technol., vol. 21, no. 2, pp. 384-405, 2003.