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Polarization Controlling Coatings For Optical Communications Applications

In free space optical communication systems, precise control of optical polarization state is essential. This is especially true where diplexing of transmit and receive paths depends on separation of orthogonal polarization states. It is also true since changes in polarization state in transmitter and receiver optical paths can lead to link losses, therefore influencing link margin and link bit error probability. We discuss the design, implementation and application of very high reflectance controlled retardance coatings for use at oblique incidence angles. These coatings allow the maintenance of highly circular or linear polarization states in a link. Coating design trade-offs are reviewed. Spectral bandwidth, angular sensitivity and thermal behavior are also discussed. Coatings were designed and produced having better than 2 degrees residual p state - s state retardance for 45 degree incidence. Coated components exhibited less than 0.3 degree retardance variation over their surfaces. Contamination sensitivity of these coatings was evaluated using a design code based approach. Contamination that produced only small changes in reflectivity was found to produce relatively large changes in relative retardance. Link performance can be optimized through use of properly designed and carefully executed controlled retardance coatings.