Digital Compensation for Optical Transmission System

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.

Description

FIELD OF THE INVENTION [0001] This invention relates to receivers for optical transmission systems, to digital adaptive equalizers for such receivers, to software for such equalizers, and to methods of offering a transmission service over such apparatus. BACKGROUND TO THE INVENTION [0002] Known optical transmission systems can be broadly categorized as direct detection, or coherent detection systems. In direct detection systems, at the receiver, the signal power is measured and therefore any phase and polarization information in the optical signal is ignored and lost. In coherent detection systems, the phase and / or polarization information is detected which enables the use of polarization and / or phase modulation as well as amplitude modulation, and so much greater information carrying capacity is possible, than in direct detection systems for a given optical signal to noise ratio. Direct detection systems have nevertheless dominated the market for long haul transmission systems due ...

AI Technical Summary

Benefits of technology

[0014] An additional feature of some embodiments is the transversal filter being adapted by iteration of a limited set of independent variables including three defining the orientation and magnitude of PMD, and one each for the chromatic dispersion, the orientation of the polarizing element at the receiver, and the phase of a local oscillator used for phase diverse detection in the optical receiver. Minimising the number of independent variables, to be fewer than the number of taps in the transversal filters, can increase the ch...

Problems solved by technology

In contrast coherent receivers require careful polarization alignment and phase tracking, which is difficult and can limit the cost/performance trade off.

Both coherent and direct detection systems are also limited in high capacity systems by distortions introduced by the optical path, mostly optical fiber.

Many complex solutions have been tried to ...

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