Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Patent
1989-01-03
1990-09-25
Arnold, Bruce Y.
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
350387, 350389, 455616, G02F 107, G02F 109
Patent
active
049589180
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to optical signal control methods and apparatus. The invention relates in particular, but not exclusively, to apparatus for processing one or more optical signals to produce a desired polarisation transformation on a Poincare sphere; to apparatus for adjusting the state of polarisation of one or both of two optical signals to achieve a chosen relationship between the states of the signals; and to apparatus for varying the state of polarisation of an optical signal by producing a desired polarisation transformation on the Poincare sphere; and to equivalent methods.
BACKGROUND AND SUMMARY OF THE INVENTION
A coherent optical communication system transmits information by modulating the phase, frequency or amplitude of an optical signal traveling down an optical fibre. At the receiving end of the fibre the information is recovered by interfering the arriving optical signal with a reference light beam produced by a local oscillator laser. For the two beams to combine effectively their polarisation states must be matched. Unfortunately the polarisation state of the received light may vary unpredictably with time owing to small disturbances to the optical fibre through which the signal has travelled. To avoid loss of transmitted information it is therefore necessary to transform the polarisation state of either the signal or the local oscillator so that their polarisation states are matched, and also to vary this transformation as the polarisation states of the signal and local oscillator change. A complete polarisation control system therefore requires transducers capable of altering the polarisation state of a light beam, and an algorithm designed to control the transducers so that polarisation matching can always be achieved. Most effort before now have concentrated on the transducers alone. Previous schemes have been based on fibre squeezers, fibre cranks and loops, and Faraday rotation, but none of the schemes proposed satisified all the requirements of coherent transmission systems.
One specical problem that has to be addressed by polarisation control schemes for optical communications is that of tracking a randomly varying polarisation state without encountering a range limit of the transducers. Cranks and loops can be arranged to provide endless control, although they are mechanicallly cumbersome and hence rather slow. More recently it has been proposed that three linearly birefringent elements and one circularly birefringent element provide endless control when cascaded together in series. A system using five squeezers to provide endless control has been demonstrated, although only four of these were required if either the input state or the output state were fixed.
Before setting out the present invention, it is useful to introduce the Poincare sphere as a tool for visualising transformations of polarisation states caused by birefringent elements.
Circularly symmetric single mode optical fibre is not strictly single mode, but supports two degenerate modes. If propagation along the fibre is in the Z direction then it is convenient to chose these two modes as those with the electric fields polarised in the X (horizontal) and Y (vertical) directions. Any propagation mode supported by the fibre may be represented as a sum of these two principle modes. In loose terms the electric field can be split into a horizontal component, Ex, and a vertical component, Ey, as shown in FIG. 1. The complete field is specified by the amplitude and phase of both Ex and Ey. The polarisation of the light in the fibre is determined by the relative phase and amplitude of Ex and Ey; it does not depend on the absolute phase of the signal or the total power in the signal. The polarisation state can therefore be determined by a single complex number .sigma. defined as horizontal components. Two diagramatic representations of a polarisation state are useful, namely the polarisation ellipse and the Poincare sphere. The polarisation ellipse is the ellipse traced out by adding hori
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patent: 4752120 (1988-06-01), Shimizu
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Electronics Letters, vol. 21, No. 20, Sep. 26, 1985, T. Okoshi et al.: "New Polarization-State Control Device: Rotatable Fiber Cranks", pp. 895-896.
Electronics Letters, vol. 22, No. 2, Jan. 16, 1986, L. J. Rysdale: "Method of Overcoming Finite-Range limitation of Certain State of Polarization Control Devices in Automatic Polarization Control Schemes", pp. 100-102.
Applied Physics Letters, vol. 35, No. 11, Dec. 1, 1979, American Institute of Physics (New York, US), R. Ultrich: "Polarization Stabilization on Single-Mode Fiber", pp. 840-842.
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Arnold Bruce Y.
British Telecommunications public limited company
Lerner Martin
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