Apparatus for monitoring the state of a liquid-crystal polarization

An optical switch having a liquid crystal (LC) polarization modulator to direct optical signals, the improvement comprising:

• a polarized reference light source located on a first side of the LC polarization modulator, said polarized reference light source including a source of light in the same general spectral region as a light beam to be input to the LC polarization modulator and a first polarizer configured to polarize light to a predetermined polarization;
• a first polarized light detector located on a second side of the LC polarization modulator, opposite the first side, and configured to detect light having a desired polarization, said first polarized light detector comprising a second polarizer configured to receive the polarized reference light after passing through the LC polarization modulator and to pass only light having the desired polarization and a detector located to receive light passing through the second polarizer;
• and
• a second polarized light detector located on the second side of the LC polarization modulator and configured to detect light having a polarization opposite the desired polarization, wherein the second polarized light detector comprises a third polarizer configured to receive the polarized reference light after passing through the LC polarization modulator and to pass only light having the polarization opposite the desired polarization and a detector configured to receive light passing through the third polarizer.