Polarization-insensitive technique for coherent optical...

Details Polarization-insensitive technique for coherent optical... Polarization-insensitive technique for coherent optical...

H - Electricity

04

B

325/1

H04B 10/00 (2006.01)

Patent

CA 1298876

- 35 - POLARIZATION-INSENSITIVE TECHNIQUE FOR COHERENT OPTIONAL COMMUNICATION Abstract To exploit the potential advantages of coherent optical communications systems, the polarization states of the received optical signal and the local oscillator waves must be matched. A mismatch may severely degrade detection performance. These mismatches occur because the received signal state of polarization changes with time and along the ber. In this memorandum, we first review several existing techniques for handling this problem, such as polarization-maintaining fibers, polarization-state controllers and polarization-diversity receivers. We then describe several alternative techniques which rely on forcing the polarization state of either the transmitted signal or local oscillator to vary with time in a non-adaptive manner so that polarization-insensitive performance is obtained. The proposed schemes adopt a completely new approach which uses high-birefringence single-mode fibers to implement polarization switching. These techniques require only a single photodetector and give a fixed level of detection performance, with a power penalty relative to ideal of 3 dB. An important result of this work is a demonstration that polarization orthogonality between two signals launched into a long single- mode fiber is virtually preserved at the output. This permits us to consider polarization switching and/or modulation at the transmitter. A specific technique is presented in which polarization- insensitive heterodyne detection is achieved through data-induced polarization switching. The polarization switching is brought about by inserting a passive, birefringent optical device in the path of the transmitted FSK signal. The experimental results show that this technique suffers a power penalty of 3 to 4 dB when compared with a conventional FSK system with perfectly matched polarizations. This compares well with the expected penalty of 3 dB. This approach uses a single optical receiver and no additional receiver electronics, which could be a significant advantage for local-area networks.

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