Waveguide Photodetectors For Coherent Optical Communications

Abstract

One of the major problems which has been encountered in the renewed field of coherent optical communications is one of polarization control in conventional low-birefringence optical fiber. In standard coherent communications systems it is always necessary that the received signal be linearly polarized and aligned with the polarization axis of the local oscillator (LO). If this condition is not maintained, the performance of the coherent receiver rapidly decreases. This situation can rarely be maintained because fiber deformations cause random fluctuations in the received signal polarization state [1]. Methods of solving the polarization state problem include polarization maintaining fiber, polarization compensation of the LO using a number of electro-optic, magneto-optic, and mechanical techniques, or polarization diversity techniques [1,2]. Polarization maintaining fiber is still in an early stage of development and is likely to be an expensive alternative since existing fiber facilities will need to be completely replaced if coherent systems are to be implemented. Polarization compensation relies on electromechanical or electrooptical feedback which may not be desirable in communication systems due to added hardware and cost [1].