Method for modulating a carrier light wave

1. A method for modulating a carrier light wave so that a modulated light wave is achieved, the modulated light wave carrying information by means of symbols selected from a set of at least four different symbols, comprising:

leading the carrier light wave through a modulating interferometer comprising respective first and second parallel-coupled waveguiding modulation paths;

splitting the carrier light wave, and subsequently leading the split carrier light wave through said two paths, and then recombining the split carrier wave; and

for modulating each respective symbol, between said splitting and said recombining, phase shifting the respective light led through each respective path by a respective total variable part phase shift, the total variable part phase shift for each path being the sum of at least three respective variable part phase shifts,

wherein each respective variable part phase shift for each modulation state assumes one of two respective predetermined values,

wherein each symbol in said set of symbols is modulated using a respective combination of two such total variable part phase shifts,

wherein the modulation performed by the two paths is a PSK (Phase Shift Keying) scheme,

wherein said set of symbols comprises 2 N unique symbols,

wherein the light led through each respective path is phase shifted using exactly 2 N −1 variable part phase shifts,

wherein the respective difference between the respective predetermined values is the same for all variable part phase shifts,

wherein the two paths are also associated with a static phase shift offsetting the phase of the two paths compared to each other when recombined, and

wherein the static phase shift offsets the phase of the two paths when recombined by ±π radians.

2. The method according to claim 1 , wherein, for each variable part phase shift, said predetermined values are equal but of opposite sign.

3. The method according to claim 1 ,

wherein each respective path comprises a first group of variable part phase shifts, comprising 2 N−1 of the variable part phase shifts along the path in question, and a second group of variable part phase shifts, comprising the remaining 2 N−1 −1 variable part phase shifts along the path in question,

wherein the respective individual variable part phase shifts of the respective first variable part phase shift group of the first path are pairwise mapped to corresponding individual variable part phase shifts of the respective first variable part phase shift group of the second path, and

wherein for each such pair, the two variable part phase shifts are equally large but of opposite sign for all symbols in said set of symbols.

4. The method according to claim 3 , wherein the total variable phase shift of each first group of variable part phase shifts is

±

π

2

radians for all symbols in said set of symbols.

5. The method according to claim 1 , wherein the light led through each respective path, when recombined, is in phase for each symbol in said set of symbols.

6. The method according to claim 1 , wherein N is 2 and a number of symbols is four, or N is 3 and a number of symbols is eight.

7. The method according to claim 6 , wherein, in a phasor diagram, the symbols are equidistantly positioned along a circle, the center of which is located at the origin of the phasor diagram.

8. The method according to claim 1 ,

wherein each of said variable part phase shifts is achieved by a respective electrode for modifying the refractive index of the material of the respective path, and

wherein a control device applies a respective electrical signal to each respective electrode along both paths in order to achieve the corresponding modulating phase shift.

9. The method according to claim 8 , wherein the waveguiding material along each path is made from another material system than the control device.

10. A light wave modulating device, comprising:

a carrier light wave input;

a modulated light wave output; and

a modulating interferometer with first and second parallel-coupled waveguiding modulation paths, on which modulated light output the device is arranged to achieve a modulated output wave that carries information by means of symbols selected from a set of at least four different symbols,

the device being arranged to first split the incident carrier light wave, subsequently convey the light wave through said two paths and then to recombine the light wave,

wherein for modulating each respective symbol the device comprises a control device, arranged to control a respective set of at least three variable part phase shifting means along each of the first and second paths, each of the respective sets of variable part phase shifting means being arranged to phase shift the respective light led through the respective path, between said split and said recombining, by a respective total variable part phase shift,

wherein each respective variable part phase shifting means for each modulation state is arranged to impart a variable part phase shift to the light led through the path in question, the part phase shift being capable of assuming one of two respective predetermined values,

wherein the device is arranged to modulate each symbol in said set of symbols using a respective combination of two such total variable part phase shifts,

wherein the modulation performed by the two paths is a PSK (Phase Shift Keying) modulation scheme,

wherein said set of symbols comprises 2 N unique symbols,

wherein the light led through each respective path is phase shifted using exactly 2 N −1 variable part phase shifting means,

wherein the respective difference between the respective predetermined values is the same for all variable part phase shifting means,

wherein the device further comprises a static phase shifting means, arranged to offset the phase of the two paths compared to each other when recombined, and

wherein the static phase shifting means is arranged to offset the phase of the two paths when recombined by ±π radians.

11. The device according to claim 10 , wherein, for each variable part phase shift, said predetermined values are equal but of opposite sign.

12. The device according to claim 10 ,

wherein each respective path comprises a first group of variable part phase shifting means, comprising 2 N−1 of the variable part phase shifting means along the path in question, and a second group of variable part phase shifting means, comprising the remaining 2 N−1 −1 variable part phase shifting means along the path in question,

wherein the respective individual variable part phase shifting means of the respective first variable part phase shifting means group of the first path are pairwise mapped to corresponding individual variable part phase shifting means of the respective first variable part phase shifting means group of the second path, and

wherein for each such pair, the corresponding two variable part phase shifts are equally large but of opposite sign for all symbols in said set of symbols.

13. The device according to claim 12 , wherein the total variable phase shift of each first group of variable part phase shifting means is

±

π

2

radians for all symbols in said set of symbols.

14. The device according to claim 10 , wherein the control device is arranged to control the variable part phase shifting means so that the light led through each respective path, when recombined, is in phase for each symbol in said set of symbols.

15. The device according to claim 10 , wherein N is 2 and a number of symbols is four, or N is 3 and a number of symbols is eight.

16. The device according to claim 15 , wherein, in a phasor diagram, the symbols are equidistantly positioned along a circle, the center of which is located at the origin of the phasor diagram.

17. The device according to claim 10 ,

wherein each of said variable part phase shifting means is in the form of a respective electrode for modifying the refractive index of the material of the respective path, and

wherein the control device is arranged to apply a respective electrical signal to each respective electrode along both paths in order to achieve the corresponding modulating phase shift.

18. The device according to claim 17 , wherein the waveguiding material along each path is made from another material system than the control device.