In this example, we will model a directional coupler based optical switch using liquid crystals (LCs).
A directional coupler based optical switch is considered in this example. Two identical single-mode slab polymeric waveguides sandwich an LC layer which controls the level of coupling between the waveguides. Fundamental TE mode (electric field is primarily y-polarized) is injected into waveguide 1 as shown below. The level of coupling is adjusted by the orientation distribution of the LCs. Ferroelectric liquid crystals with ordinary index no =1.462 and extraordinary index ne =1.546 is assumed in this example.
When the LC orientations are distributed as "state1" shown in the image below, the electric field Ey feels a smaller refractive index on average and the light guidance in the waveguide is reinforced, which means weak coupling between two waveguides. Due to the weak coupling, the light injected into the waveguide 1 can propagate in the waveguide 1 for a long distance, which can be used as a bar state. On the other hand, when the the LC direction is distributed as show in the "state2" below, the electric field Ey feels a larger refractive index on average and the light leakage into LC region is promoted, which means strong coupling between two waveguide. Due to the strong coupling, the light can travel between the waveguide back and forth in a shot distance, which can be used as a cross state.
The script file LC_optical_switch.lsf can be used to set the LC orientation for the cross or bar state for the LC_optical_switch.fsp simulation file. The variable "state" in the script file controls the LC state. If you set "state=1" or "state=-1", then you can set up "cross state" or "bar state", respectively.
When we choose "state=1", i.e. the cross state, the angle θ linearly increases toward the center of the LC region, and then linearly decreases toward the end of the other side.On the other hand, when we choose "state=-1", i.e. the bar state, the angle θ linearly decreases toward the center of the LC region, and then linearly increases toward the end of the other side as shown in the figure below
The figures below show the E field amplitude distribution in the waveguide for "cross"(top) and "bar" (bottom) states.
The movies below show the pulse propagation for both the cross and bar states. Note that the pulse length of the source was increased before running the simulation to generate the movie in order to more clearly see pulse.
- G. Ntogari, D. Tsipouridou, and E. E. Kriezis, "A numerical study of optical switches and modulators based on ferroelectric liquid crystal" J. Opt. A: Pure Opt. Vol. 7, pp. 82-87 (2005)