Optical routing switch using symmetric liquid crystal cells

Abstract

An optical routing switch uses two liquid crystal cells that can produce offsetting rotations of the polarization of the input beam to provide fast, symmetrical switching. The input beam is first polarized and then passes through both liquid crystal cells in series. Both liquid crystal cells have two states (e.g., voltage-off and voltage-on) in which the beam polarization is rotated by predetermined angles (e.g., 0° and 90°), but in opposing rotational directions. A controller selectively rotates the LC cells through a sequence of steps, beginning with a “through” state in which both LC cells are in the first state. The polarization rotations provided by both liquid crystal cells offset one another so the beam polarization remains essentially unchanged. The LC cells can be rapidly switched to a “cross” state in which only one of the LC cells is changed to the second state and the polarization of the beam is rotated by a predetermined degree. The LC cells can then be rapidly switched back to the through state by changing both LC cells to their second state. Once again, the LC cells offset one another so that the beam polarization remains essentially unchanged. While remaining in the through state, both LC cells are allowed to return to the first state to complete the cycle. Both LC cells relax at the same rate and their offsetting polarization rotations cause the beam polarization to remain unchanged throughout the entire relaxation process. A polarization-dependent routing element (e.g., a polarized beamsplitter or birefringent element) routes the beam exiting the LC cells along either of two alternative optical paths based on the beam's polarization.