Patent
1988-05-20
1991-01-29
Arnold, Bruce Y.
350405, G02F 101
Patent
active
049881698
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to an optical signal control method and apparatus. In particular, it relates to automatic control of polarisation of optical signals.
In general, optical waveguides do not preserve the polarisation state of light emerging from a length of waveguide: the polarisation varies with time.
A single mode fibre having circular symmetry about the axis of the core allows propagation of two orthogonally polarised modes. The fibre behaves as a birefringent medium due to differences in propagation speeds of the two modes resulting from anisotropy of the refractive index of the fibre core. A number of birefringence-inducing mechanisms are known. For example, a non-circular core introduces linear birefringence: the smaller transverse dimension of the fibre core becomes the fast axis of birefringence. Changing the electro-optic index, for example by applying an asymmetrical transverse stress, also induces linear birefringence, as does the application of a transverse electric field Linear birefringence may also be controlled by applying a voltage to electro-optic material such as lithium niobate. Alternatively, circular birefringence may be induced by means of a magnetic field along the length of the fibre (Faraday effect).
In many circumstances, it is desirable to be able to control the state of polarisation (SOP) of the modes in a fibre. One example is in coherent detection systems, where the polarisation states of the incoming signal and the local oscillator must be the same. It is therefore important to be able to control the SOP of one of the signals. Ideally, it should be possible to match any polarisation state to any other state, where both the initial and final states may vary.
Polarisation states can be represented on the Poincare sphere. This representation is fully described in published literature, e.g. Rashleigh: "Origins and Control of Polarisation Effects in Single Mode Fibres", J Lightwave Technology Vol. LT 1 No 2 June 1983 p. 312-331. Any general elliptical polarisation state such as shown in FIG. 1, where .psi.=.+-.arc tan b/a, is represented on the sphere by a single point S as shown in FIG. 2.
Horizontal and vertical polarisation states are represented by H and V respectively, and all linear states lie on the great circle HPVQ, where the latitude is zero. P and Q represent polarisation at .+-..pi./4 to the L and V states. L represents left hand circular polarisation and R right hand circular polarisation. Any state of polarisation is represented by a unique point on the sphere, where .psi.i and .phi.i of a particular elliptical polarisation state are represented by co-ordinates 2 .psi.i and 2 .phi.i on the sphere.
Birefringence causes a change in polarisation state from S to S' and thus a rotation about an axis passing through the centre of the sphere, through an angle which depends on the magnitude of the birefringence. Linear birefringence causes rotation about an axis lying in the plane HPVQ of FIG. 2.
Various methods have been proposed for controlling the SOP of a waveform propagating in single mode fibre. In a coherent optical communications system, a slight polarisation mismatch between the incoming signal and local oscillator signal causes a significant fall in received signal. The polarisation states of both signals may vary with time so if only one polarisation state is controlled, the controller must be able to transform any polarisation state S into any derived state S', where S, S' can lie anywhere on the Poincare sphere. Two birefringent elements in series are inadequate to transform S to S' for every S, S', but three elements may be adequate under certain circumstances. In practice a stress inducing birefringent element, for example, cannot apply a greater and greater stress to the fibre, or the fibre will break. It is therefore necessary to be able to reset or adjust birefringent elements, meanwhile maintaining the transformation of the SOP of the signal from the initial to final states, however those states may vary
REFERENCES:
patent: 4729622 (1988-03-01), Pavlath
Electronics Letters, vol. 22, No. 2, Jan. 16, 1986 (New York, U.S.), L. J. Rysdale: "Method of Overcoming Finite-Range Limitation of Certain State of Polarisation Control Devices in Automatic Polarisation Control Schemes", pp. 100-102.
Electronics and Communications in Japan, vol. 68, No. 6, Part 2: Electronics, Nov./Dec. 1985, Scripta Technica, Inc. (Silver Spring, Md., U.S.), Y. Kido: "Light Polarization Control Due to Electrooptic Effect", pp. 38-47.
Applied Physics Letters, vol. 35, No. 11, Dec. 79, American Institute of Physics, New York, U.S.), R. Ulrich: "Polarization Stabilization on Single-Mode Fiber", pp. 840-842.
Arnold Bruce Y.
British Telecommunications public limited company
Parsons David R.
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