Optical multiplexer

1. An optical multiplexer, comprising:

a plurality of input waveguides, each comprising an input slab portion and an input rib portion;

an output waveguide, comprising an output slab portion and output rib portion; and

a wavelength multiplexer element, coupled to each input waveguide and the output waveguide, the wavelength multiplexer element comprising a slab waveguide which includes a grating configured to multiplex signals of differing wavelengths, received from the input waveguides, into a multiplexed signal, and provide the multiplexed signal to the output waveguide,

wherein the input rib portion(s) of one or more of the input waveguides are tapered so as to decrease in width in a direction towards the slab waveguide of the wavelength multiplexer element,

wherein the input slab portions of the input waveguides and/or the output slab portion of the output waveguide have a height which is substantially equal to a height of the slab waveguide in the wavelength multiplexer element, and

wherein the wavelength multiplexer element is an echelle grating multiplexer element.

2. The optical multiplexer of claim 1 , wherein all of the input waveguides are tapered so as to decrease in width in a direction towards the slab waveguide of the wavelength multiplexer element.

3. The optical multiplexer of claim 1 , wherein the output waveguide is tapered so as to decrease in width in a direction towards the slab waveguide of the wavelength multiplexer element.

4. The optical multiplexer of claim 1 , wherein the or each input waveguide is tapered so that an optical mode within the or each input waveguide is confined to the input slab portion of that input waveguide at a point at which that input waveguide is coupled to the slab waveguide of the wavelength multiplexer element.

5. The optical multiplexer of claim 1 , wherein an optical mode of each input waveguide at least partially overlaps the optical mode of the adjacent input waveguide(s).

6. The optical multiplexer of claim 1 , wherein the wavelength multiplexer element is configured to perform coarse wavelength division multiplexing.

7. The optical multiplexer of claim 1 , operable over a wavelength range of at least 1.2 μm.

8. An optical multiplexer, comprising:

a plurality of input waveguides, each comprising an input slab portion and an input rib portion;

an output waveguide, comprising an output slab portion and output rib portion; and

a wavelength multiplexer element, coupled to each input waveguide and the output waveguide, the wavelength multiplexer element comprising a slab waveguide which includes a grating configured to multiplex signals of differing wavelengths, received from the input waveguides, into a multiplexed signal, and provide the multiplexed signal to the output waveguide,

wherein the input rib portion(s) of one or more of the input waveguides are tapered so as to decrease in width in a direction towards the slab waveguide of the wavelength multiplexer element, and

wherein an optical mode of each input waveguide at least partially overlaps the optical mode of the adjacent input waveguide(s).

9. The optical multiplexer of claim 8 , wherein the wavelength multiplexer element is an arrayed-waveguide grating.

10. An optical multiplexer, comprising:

a plurality of input waveguides, each comprising an input slab portion and an input rib portion;

an output waveguide, comprising an output slab portion and output rib portion; and

a wavelength multiplexer element, coupled to each input waveguide and the output waveguide, the wavelength multiplexer element comprising a slab waveguide which includes a grating configured to multiplex signals of differing wavelengths, received from the input waveguides, into a multiplexed signal, and provide the multiplexed signal to the output waveguide,

wherein the input rib portion(s) of one or more of the input waveguides are tapered so as to decrease in width in a direction towards the slab waveguide of the wavelength multiplexer element, and

wherein, for a waveguide of the plurality of input waveguides, the material of the input rib portion is continuous with the material of the input slab portion.

11. The optical multiplexer of claim 10 , wherein the wavelength multiplexer element is configured to perform coarse wavelength division multiplexing.

12. The optical multiplexer of claim 10 , operable over a wavelength range of at least 1.2 μm.

13. The optical multiplexer of claim 10 , wherein all of the input waveguides are tapered so as to decrease in width in a direction towards the slab waveguide of the wavelength multiplexer element.

14. The optical multiplexer of claim 10 , wherein the output waveguide is tapered so as to decrease in width in a direction towards the slab waveguide of the wavelength multiplexer element.

15. The optical multiplexer of claim 10 , wherein the or each input waveguide is tapered so that an optical mode within the or each input waveguide is confined to the input slab portion of that input waveguide at a point at which that input waveguide is coupled to the slab waveguide of the wavelength multiplexer element.

16. The optical multiplexer of claim 10 , wherein the input slab portions of the input waveguides and/or the output slab portion of the output waveguide have a height which is substantially equal to a height of the slab waveguide in the wavelength multiplexer element.

17. The optical multiplexer of claim 10 , wherein the wavelength multiplexer element is an echelle grating multiplexer element.

18. The optical multiplexer of claim 10 , wherein the wavelength multiplexer element is an arrayed-waveguide grating.

19. The optical multiplexer of claim 10 , wherein an optical mode of each input waveguide at least partially overlaps the optical mode of the adjacent input waveguide(s).