Northern Telecom Limited

Northern Telecom Europe Limited

Timothy A Large

A transmitted (TX) signal received from the remote location is mixed in a polarization resolving beam splitter (10) with a local oscillator (LO) signal from a laser (15). The LO and TX signal components emerging from the beam splitter (10) are orthogonally polarized, but their polarization states are brought into alignment by passage through two birefringent elements (19, 20) before falling on two photodetectors (17, 18). Adjustment of a polarization controller (16) ensures that approximately equal LO power falls on each photodetector.

Optical mixing for heterodyne detection

A planar coil of single mode fiber has opposed ends twisted into orthogonal planes to form a non-planar coil 20 exhibiting circular birefringence. The amount of birefringence exhibited is modulated by means of an electromechanical transducer 34 which controls the distance separating two diametrically opposed portions of the coil. Two or more such controllers may be formed in tandem with intervening additional coils 50 providing quarter-wave linear birefringence.

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Papworth Everard, United Kingdom

Timothy A Large