Method for modifying the combining or splitting ratio of a multimode interference coupler

1. A method for modifying a splitting or combining ratio of a first multimode interference (MMI) coupler, comprising:

arranging the first coupler is arranged to convey light from one or several input waveguides to one or several output waveguides; and

arranging a film of a material over the first coupler, the film being strained so that a permanent force is applied by said film to a surface of the first coupler resulting in a non-uniform mechanical stress field within the material of an MMI region of the coupler, so that a refractive index profile in a material of the first coupler changes as a consequence of said force,

wherein said splitting or combining ratio is modified as a consequence of said changed refractive index profile.

2. The method according to claim 1 , wherein the first coupler has two input waveguides and/or two output waveguides.

3. The method according to claim 1 , wherein the modification of the splitting or combining ratio changes said splitting or combining ratio towards a value of 1.

4. The method according to claim 1 , wherein the strained film extends laterally away from the first coupler.

5. The method according to claim 1 , wherein the film is partly etched away after being arranged over the first coupler, so that said force after straining of the film material is different after said etching as compared to the same situation without etching.

6. The method according to claim 5 , wherein said etching away comprises etching away the film completely over one or several certain areas of the first coupler.

7. The method according to claim 1 , wherein the film in a first method step is arranged on the first coupler at a first temperature, and that the film, in a second method step, is then allowed to assume a second temperature, which is different from the first temperature, whereby the film expands or shrinks as a consequence of the change in temperature.

8. The method according to claim 1 , wherein the film in a first method step, is arranged on the first coupler in liquid or solid form, and that, in a second method step, the film material is subjected to a processing step which permanently alters at least one material property whereby the film expands or shrinks as a consequence of the change in material property.

9. The method according to claim 1 , wherein the film is arranged over the entire length, in the direction of light propagation, of the MMI region of the first coupler.

10. The method according to claim 1 , wherein the first coupler is arranged as a part of an integrated optical circuit, which integrated optical circuit also comprises at least one second multimode interference coupler, arranged to convey light from one or several respective input waveguides to one or several respective output waveguides wherein a film of a material is arranged over all of said first and second couplers, after which the film is strained so that a force is applied by said film to the surface of each of the first and second couplers, and so that the refractive index profile in the respective material of the first and second couplers changes as a consequence of said force, and wherein said respective splitting or combining ratio is modified as a consequence of said respective changed refractive index profile.

11. The method according to claim 1 , wherein the first coupler is arranged on a semiconductor wafer, which semiconductor wafer also comprises at least one second multimode interference coupler, arranged to convey light from one or several respective input waveguides to one or several respective output waveguides, wherein a film of a material is arranged over all of said first and second couplers on the wafer, after which the film is strained so that a force is applied by said film to the surface of each of the first and second couplers, and so that the refractive index profile in the respective material of the first and second couplers changes as a consequence of said force, and wherein said respective splitting or combining ratio is modified as a consequence of said respective changed refractive index profile.

12. The method according to claim 10 , wherein the first coupler and the at least one second couplers are manufactured from the same material structure.

13. A multimode interference (MMI) coupler system, comprising:

the coupler arranged to convey light from one or several input waveguides to one or several output waveguides; and

a circuit which comprises a film of a strained material arranged over the coupler and arranged to permanently apply a force to a surface of the coupler, the force resulting in a non-uniform stress field with a material of an MMI region of the coupler, and so that a refractive index profile in a material of the coupler, and as a consequence also a splitting or combining ratio of the coupler, is different as a consequence of said force as compared to the case without said force.

14. The coupler system according to claim 13 , wherein the coupler has two input waveguides and/or two output waveguides.

15. The coupler system according to claim 13 , wherein the film is arranged to only partly cover the coupler.

16. The coupler system according to claim 13 , wherein the coupler is part of an integrated optical circuit.

17. The coupler system according to claim 14 , wherein the integrated optical circuit which, apart from said coupler, comprises at least one other multimode interference coupler, and wherein the film is arranged over both said coupler the at least one other coupler.

18. The coupler system according to claim 13 , wherein the film material is a polymer which shrinks or expands with changing temperature.

19. The coupler system according to claim 13 , wherein the film is arranged over the entire length, in the direction (D) of light propagation, of the MMI region of the coupler.

20. The method according to claim 11 , wherein the first coupler and the at least one second couplers are manufactured from the same material structure.