Electro-optic waveguide polarisation modulator

1. An electro-optic waveguide polarisation modulator comprising:

a waveguide core having first and second faces defining a waveguide core plane,

a plurality of primary electrodes arranged at a first side of the waveguide core plane and out of said plane, and

at least one secondary electrode arranged at a second side of the waveguide core plane and out of said plane, wherein the electrodes are configured to provide an electric field having field components in two substantially perpendicular directions within the waveguide core so as to modulate the refractive index thereof such that electromagnetic radiation propagating through the core is convertible from any first polarisation state to any second polarisation state.

2. An electro-optic waveguide polarisation modulator according to claim 1 comprising a plurality of the secondary electrodes within the second electrode layer.

3. An electro-optic waveguide polarisation modulator according to claim 1 wherein the primary electrodes comprise coplanar stripline electrodes adapted in use to provide an electric field which propagates in a longitudinal direction within the waveguide core at substantially the same speed as that of optical electromagnetic radiation propagating therein and being matched thereto.

4. An integrated optical waveguide system comprising a polarisation modulator according to claim 1 .

5. An optical communication network comprising a polarisation modulator according to claim 1 , wherein the modulator is adapted in use to compensate for polarisation distortion arising within said network.

6. An optical communication network according to claim 5 wherein the polarisation modulator is adapted in use to apply a polarisation compensation at at least one of a transmitter in the network, a receiver in the network, and at any position along which an optical signal is transmitted through the network.

7. An optical communication network according to claim 6 wherein the polarisation modulator is adapted in use to apply a polarisation pre-compensation at a transmitter in the network so as to compensate for polarisation distortion arising within said network.

8. A quantum cryptography system comprising a polarisation modulator according to claim 1 .

9. A quantum cryptography system according to claim 8 wherein the polarisation modulator is adapted in use to modulate an optical signal transmitted there-from.

10. A quantum cryptography system according to claim 9 wherein the polarisation modulator is adapted in use to modulate the transmitted optical signal such that said signal comprises a desired quantum cryptographic polarisation state having a compensating pre-change applied thereto to compensate for polarisation distortion arising within said system.

11. A quantum cryptography system according to claim 8 wherein the polarisation modulator is adapted in use to modulate an optical signal received there-by.

12. A light detection and ranging system (LIDAR) comprising a polarisation modulator according to claim 1 .

13. A method of modulating the polarisation of electromagnetic radiation using a polarisation modulator according to claim 1 comprising the steps of:

(i) inputting electromagnetic radiation having a first polarisation state into the polarisation modulator,

(ii) applying a first voltage V 1 between a first primary electrode and at least one secondary electrode and applying a second voltage V 2 between a second primary electrode and at least one secondary electrode, so as to provide an electric field component in a first direction within the waveguide core substantially perpendicular to the waveguide core plane so as to modulate the refractive index of the waveguide core in a direction substantially perpendicular to the first direction, and an electric field component in a second direction within the waveguide core substantially parallel with the waveguide core plane so as to modulate the refractive index of the waveguide core in directions inclined at angles ±45° to the first direction,

such that electromagnetic radiation propagating through the core is convertible from any first polarisation state to any second polarisation state.

14. A method according to claim 13 comprising the further step of:

(iii) outputting electromagnetic radiation having the second polarisation state from the polarisation modulator.

15. A method according to claim 13 wherein the magnitude of the modulation of the refractive index in the direction substantially perpendicular to the first direction within the waveguide core is proportional to the mean value of the first voltage V 1 and the second voltage V 2 .

16. A method according to claim 13 wherein the magnitude of the modulation of the refractive index in directions inclined at angles ±45° to the first direction within the waveguide core is proportional to the modulus of the difference between the first voltage V 1 and the second voltage V 2 .

17. A method of modulating the polarisation state of electromagnetic radiation propagating in an electro-optic waveguide core having first and second faces defining a waveguide core plane, comprising the step of providing within the waveguide core from a first position outside the waveguide core plane adjacent the first face and a second position outside the waveguide core plane adjacent the second face an electric field having field components in a first direction and in a second direction substantially perpendicular to the first direction such that the electromagnetic radiation propagating through the core is convertible from any first polarisation state to any second polarisation state.

18. A method according to claim 17 wherein the first direction is substantially perpendicular to the waveguide core plane.

19. A method according to claim 17 wherein the second direction is substantially parallel with the waveguide core plane.