Design of an Ultrashort Directional Coupler with an SSFLC Coupling Layer

A design of a vertical directional coupler, operating at the wavelength λ= 632.8 nm, is presented. The coupler consists of a layer of the Hoechst ferroelectric liquid crystal mixture FELIX-M4851-025, homogeneously aligned and sandwiched between two single mode ion-exchanged BK7 glass waveguides. The design of the directional coupler was carried out by using the beam propagation method, taking into account also teflon alignment layer and ITO electrode contributions. Given the FLC mixture refractive indices, ne- 1.611 and no= 1.466 at λ= 632.8 nm and T= 25 °C, calculations show that less than 60 μm coupling length, with an extinction ratio higher than 21 dB can be obtained by using single mode waveguides with a refractive index change at the surface Δn= 0.013, an FLC layer 2 μn thick, and cone axis tilted by about 19° from the propagation direction of the lightwave.