Endless polarization control systems for coherent optics

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Abstract
In coherent optical systems or sensors, polarization matching between the superposed beams must be assured. The tracking range of automatic polarization control systems should be endless, i.e. any resets of finite range retarders, which transform the polarization, should cause no significant intensity losses. A variety of experimental systems including a computer as feedback controller are described. They include the minimum configuration of three fixed eigenmode retarders, i.e. the orientation of birefringence cannot be changed. These retarders are realized by fiber squeezers. Error-tolerant systems which contain more than the minimum number of elements, however, are better suited to cope with time variant retarder transfer functions, etc. A fourth fiber squeezer allows the losses of a nonideal systems to be kept to only 0.07 dB. Finally, for the first time, a closed loop system with two integrated optical retarders is described. These retarders have variable eigenmodes, i.e. adjustable birefringence orientation. An optimization procedure helps to idealize the device behavior. The system has less than 0.15 dB intensity losses, coupling and attenuation not included.<>