Automatic polarization controller having broadband, reset-free operation

Abstract

Wide optical bandwidth and broad wavelength tuning range are achieved in a reset-free, optical, automatic polarization controller by combining three controllable fractional wave elements in cascade and further by controlling the orientations of both outermost fractional wave elements to differ by a prescribed angular amount which is maintained substantially constant. The prescribed angular amount is defined to be between 0 and 2.pi., inclusively. Synchronous control of both outermost fractional wave elements maintains the prescribed angular difference to be maintained constant during operation of the polarization controller. The three fractional wave elements are described as an endlessly rotatable half-wave element and two synchronously rotatable quarter-wave elements wherein the half-wave element is placed between the quarter-wave elements. Each fractional wave element varies the orientation of linear birefringence along its optical wavepath and introduces a specified phase retardation. Embodiments of the polarization controller are realized using either distributed bulk optic devices or integrated electrooptic waveguide devices. Rotation of the elements is afforded by a feedback control circuit which monitors the output optical polarization and derives appropriate electrical drive signals to achieve the proper rotation of the elements.