BPM analysis of integrated optical switches using polymeric optical waveguides and SSFLC at 1.55 m

Optical switches using polymeric waveguides and ferroelectric liquid crystals (FLC) have been modelled by using a 3D vectorial beam propagation method (BPM) at the wavelength of 1.55 micron used in optical communications. The structure is a vertical directional coupler consisting of a surface stabilised FLC (SSFLC), by using FELIX-M4851-025 mixture from Clariant, embedded between two polymeric waveguides and ITO layers as electrodes. The working principle of the switch is identical to the device reported in [1]. A polymeric buffer layer has been introduced to reduce losses due to ITO absorption. Polymeric waveguides and buffers used in our analysis can be obtained by different compositions of the P(PFS-GMA) (poly(pentafluorostyrene-co-glycidyl methacrylate)). Polymer films with different refractive indexes by varying the PFS amount (copolymer composition) were reported in literature [2]. We figured out an optimised device with an extinction ratio of more than 50 dB and losses of less than 1 dB with a coupling length of only 174 micron by using 3 micron thick polymeric waveguides with a refractive index of 1.475 at the wavelength of 1.55 micron. Such waveguides can be made by using a PFS content of 60 mol %. A buffer layer with a thickness of 6 micron and a refractive index of 1.462, corresponding to an amount of 100 % of PFS, has been included. Furthermore the device consists of a 5 micron FLC layer aligned by teflon layers with a thickness of 40 nm and ITO layers of 20 nm to drive it. The FLC cone axis is tilted by 51° from light propagation. The same device without buffer gives an extinction ratio of 29.33 dB and higher losses of 1.36 dB. Simulations show the benefits in terms of loss reduction due to the presence of the buffer. In particular it is possible to increase ITO thickness to have a more homogeneous and conductive layer without increasing optical losses. In fact ITO thickness of 40 nm in the device without buffers leads to losses of 5.55 dB that can be decreased to 1.35 dB by using buffer layers whose thickness is 6 micron. References [1] R. Asquini, A. d’Alessandro, “A bistable optical waveguided switch using a ferroelectric liquid crystal layer”, paper MM4, LEOS 2000, 13th Annual Meeting, Puerto Rico, 13-16 November 2000. [2] C. Pitois, S. Vukmirovic, A. Hult, D. Wiesmann, M. Robertsson, “Low-Loss Passive Optical Waveguides Based on Photosensitive Poly(pentafluorostyrene-co-glycidyl methacrylate)”, Macromolecules, 32, pp. 2903-2909, 1999.