Optical waveguide arrangements comprising an auxiliary waveguide-like structure
An optical waveguide arrangement is provided which comprises an active ridge waveguide structure 12 formed by etching of a semiconductor substrate 1, 2, 3 . There is also provided an auxiliary waveguide-like structure 8 formed on the substrate adjacent the active ridge waveguide structure 12 to control the etched profile of the active waveguide structure. The arrangement of the auxiliary structure 8 on the substrate controls the etched profile over the cross-section of the active waveguide structure 12 and along the length of the active waveguide structure 12 . Advantageously, this arrangement reduces or eliminates the disadvantages associated with etch-process induced asymmetries in the shape of closely spaced waveguides.
1. A multimode interference (MMI) coupler comprising an optical waveguide arrangement comprising:
an active ridge waveguide structure formed by etching of a substrate;
an auxiliary waveguide-like structure formed on the substrate adjacent to the active waveguide structure to control the etched profile over the cross-section of the active waveguide structure; and
an MMI region coupled with the active ridge waveguide structure,
wherein the active waveguide structure comprises an input active waveguide and/or an output active waveguide and the MMI region is coupled with the input active waveguide and/or the output active waveguide, the auxiliary structure being arranged adjacent at least one of the input and output active waveguides, and
wherein the active waveguide structure comprises two active waveguides, and the auxiliary structure is arranged such that a gap between the active waveguides is about the same as a gap between each active waveguide and a corresponding auxiliary waveguide in the auxiliary structure, and the gaps are defined such that an etched trench extends through at least a waveguide core layer of the substrate so as to provide a strong optical confinement for light within the active waveguide.
2. The MMI coupler of claim 1 , wherein the auxiliary structure is arranged on the substrate to impart a symmetric active waveguide profile.
3. The MMI coupler of claim 1 , wherein the active waveguide structure comprises one or more ridges and the auxiliary structure is arranged on the substrate so that each ridge in the active waveguide structure is symmetrical.
4. The MMI coupler of claim 1 , wherein the active waveguide structure comprises two or more ridges and the auxiliary structure is arranged on the substrate so that each ridge in the active waveguide structure is substantially the same shape.
5. The MMI coupler of claim 1 , wherein the maximum gap between the active waveguide and the corresponding auxiliary waveguide is about 10 μm.
6. The MMI coupler of claim 1 , wherein the auxiliary structure is arranged on the substrate to control the etched profile along the length of the active waveguide structure.
7. The MMI coupler of claim 6 , wherein the auxiliary structure is arranged to control the etched profile by variation of a gap between the active waveguide structure and the auxiliary structure.
8. The MMI coupler of claim 1 , wherein the substrate is a semiconductor substrate.
9. An optical waveguide arrangement comprising:
an active ridge waveguide structure formed by etching of a substrate; and
an auxiliary waveguide-like structure formed on the substrate adjacent to the active waveguide structure to control the etched profile over the cross-section of the active waveguide structure,
wherein the auxiliary structure is arranged on the substrate to control the etched profile along the length of the active waveguide structure,
wherein the auxiliary structure is arranged to vary the etched profile to produce a transition between a strongly guided active waveguide and a weakly guided active waveguide, and
wherein the etched profile of the weakly guided active waveguide comprises a shallowly etched trench extending through an upper confinement layer of the substrate, and the etched profile of the strongly guided active waveguide comprises a deeply etched trench extending through an upper confinement layer, a waveguide core layer and partially through a lower confinement layer of the substrate.
10. A method of manufacturing a multimode interference (MMI) coupler, the method comprising:
forming an active ridge waveguide structure by etching of a semiconductor substrate;
forming an MMI region coupled with the active ridge waveguide structure; and
controlling the etched profile along the cross-section of the active waveguide structure by forming an auxiliary waveguide-like structure on the substrate adjacent the active waveguide structure,
wherein the active waveguide structure comprises an input active waveguide and/or an output active waveguide and the MMI region is coupled with the input active waveguide and/or the output active waveguide, the auxiliary structure being arranged adjacent at least one of the input and output active waveguides, and
wherein the active waveguide structure comprises two active waveguides, and the auxiliary structure is arranged such that a gap between the active waveguides is about the same as a gap between each active waveguide and a corresponding auxiliary waveguide in the auxiliary structure, and the gaps are defined such that an etched trench extends through at least a waveguide core layer of the substrate so as to provide a strong optical confinement for light within the active waveguide.
11. The method of claim 10 , further comprising:
controlling the etched profile by providing symmetric ridges in the active waveguide structure.
12. The method of claim 10 , further comprising:
controlling the etched profile along the length of the active waveguide structure by varying the gap between the active waveguide structure and the auxiliary structure.
13. The method according to claim 10 , wherein the etched profile is produced by a dry etching technique.