Polarization controlling device comprising a beam splitter for

Abstract

For first and second polarization controlling elements (181, 182) driven by first and second driving voltages in a polarization controlling device, a beam splitter (15) has a dividing ratio controllable between 1:0 and 0:1 and divides an input polarized beam having an input polarization state. An output polarized beam is derived from first and second polarization controlled beams to have an output polarization state. While the dividing ratio is kept at 1:0 so that the second polarization controlled beam is null, the first driving voltage may approach either of a positive and a negative limit. In this event, a control unit (25) changes the dividing ratio to 0:1. Even while the second polarization controlled beam is null, the control unit controls the second driving voltage so that the both driving voltages are congruent modulo a unit voltage difference which makes each element carry out equivalent polarization control. The first driving voltage is likewise controlled. When the input polarization state optionally varies, the output polarization state can be defined by a mode filter, a laser diode, or two parameters descriptive of a coherent polarized beam. Irrespective of the input polarization state, the output polarization state can be defined by a signal beam which reaches the device with an optionally varying polarization state.