Abstract: A receiver for an optical transmission system, has a polarization diverse and phase diverse coherent optical receiver, and a digital adaptive equalizer for compensating for distortions in the optical signal introduced by the optical path. The entire field of the optical signal is mapped including phase and polarization information, to enable more complete compensation for impairments such as chromatic dispersion and PMD. Furthermore, it can also reduce the problems which have so far held back coherent optical detection from widespread implementation, such as polarization alignment and phase tracking. This can be applied to upgrade existing installed transmission routes to increase capacity without the expense of replacing the old fiber.

Abstract: A coherent optical intradyne receiver is described which comprises an optical image reject mixer capable of dividing, mixing and recombining signals from three inputs so as to produce an inphase and a quadrature channel. On each of the channels a same set of image products is cancelled and from which channels, phase modulated signals can be recovered.

Abstract: An optical coupler 10 comprising at least three fibres 12, 14 and 16, portions of the fibres being fused to each other to form a coupling region 30 operable to couple light therebetween. At least two of the fibres have inputs 18 and/or 20 and/or 22 and at least two of the fibres have outputs 24 and/or 26 and/or 28. In use, on illumination of at least two of the inputs light intensity is emitted from a plurality of the outputs having a substantially asymmetric phase relationship, such as, for example, a quadrature phase relationship between two of the outputs. The invention also provides for methods of making such a device.

Abstract: A PMD compensator has three compensation elements in series, each for providing variable differential group delay. Each compensation element provides compensation along a different orthogonal polarization axis. This enables PMD to be cancelled more completely.

Abstract: An optical coupler 10 comprising at least three fibres 12, 14 and 16, portions of the fibres being fused to each other to form a coupling region 30 operable to couple light therebetween. At least two of the fibres have inputs 18 and/or 20 and/or 22 and at least two of the fibres have outputs 24 and/or 26 and/or 28. In use, on illumination of at least two of the inputs light intensity is emitted from a plurality of the outputs having a substantially asymmetric phase relationship, such as, for example, a quadrature phase relationship between two of the outputs, The invention also provides for methods of making such a device.

Abstract: Generation of variable differential group delay An apparatus for generating variable DGD is particularly for use in a PMD compensator. The apparatus has first, second and third birefringent elements arranged in order between the input and output of the compensator and having first, second and third differential group delays (DGDs) in the ratio 1:2:1. The orientation of the PSPs of the signal in each element relatively to the principal axes of the element is controlled, such that a change in orientation between the first and second elements is equal and opposite to a change in orientation between the second and third elements. This arrangement provides symmetrical relative rotations of the signal PSPs and principal axes about the central birefringent element. In combination with the 1:2:1 ratio, it can be shown that compensation of any first order PMD can be achieved without the compensator introducing additional second order PMD.