Integrated polarization controllers

Polarization management is a key functionality in many photonic applications including telecommunications, polarization diversity circuits and sensing, to name a few. Developing integrated circuits capable of reliably controlling polarization state would result in compact and low cost circuits with improved stability, compared with fiber or bulk optics solutions. Polarization rotators are a key building block of these circuits. Unfortunately, stringent fabrication tolerances make the integration of polarization rotators highly challenging. The main limitation arises from the need to tightly control the profile of the hybrid modes in the rotator waveguide as well as their relative phase shift during propagation. Both values are very sensitive to waveguide geometry variations, seriously hindering their practical application. We have developed a technology independent scheme that enables fabrication error compensation, substantially relaxing device tolerances. In our scheme, three polarization rotation waveguides are interconnected with two tunable phase shifters to correct geometry deviations. Interestingly, these phase shifters also enable dynamic wavelength tuning and output polarization extinction ratio selection. We also showed that, by adding an output phase shifter, we can control the relative phase. Hence, we can yield any desired output state of polarization. We have implemented this scheme in the silicon-on-insulator platform using simple waveguide heaters as tunable phase shifters. We experimentally demonstrated an unprecedented polarization extinction range of 40 dB (±20 dB). Furthermore, the device showed a 98% coverage of the Poincaré sphere with a tunability range covering the complete C-band. These results prove the potential of our scheme to alleviate the, otherwise, very stringent fabrication tolerances, overcoming the major limitation of current integrated polarization managing devices.