Birefringence compensation in Sagnac and its quantum communication applications
Jan Bogdanski, Johan Ahrens, Mohamed Bourennane · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 2009
We present a new concept for birefringence compensation in Sagnac single mode fiber (SMF) interferometer, which has opened the door to secure multiuser quantum communications. The concept has been tested in both quantum key distribution (QKD) and secret sharing applications at total fiber loop lengths up to 150 km. Now, we present a Jones matrix based model of the concept as well as the visibility measurements confirming its practical validity. Sagnac based circular SMF network architecture is an attractive solution for both commercial telecom Intranets and backbone networks as well as for secure quantum information channels exploring QKD and secret sharing applications. Due to the difference in birefringence between the clockwise and counterclockwise transmission directions, Sagnac has not been so much explored by the quantum information community as other network architectures, especially the "plug & play". Our SMF Sagnac birefringence compensation uses polarization maintaining, horizontally aligned components only, except for the SMF fiber link. It converts the arbitrary elliptically polarized clockwise and counterclockwise light signals, transmitted over the SMF fiber, into the horizontal polarization. Thus, both the sent and received light signals have the same horizontal polarization. In order to avoid unnecessary power losses in the birefringence compensator, we have included a simple proportional-integral-derivative controller into our setup. The compensator provides a perfect birefringence compensation for the destructive interference, which leads to a high system visibility. The presented SMF Sagnac birefringence compensation could also be use in Sagnac metrology, for instance in very low power level sensing, requiring high measurement accuracy.