Wide shoulder, high order mode filter for thick-silicon waveguides

1. An optical filter for attenuating higher-order modes in an optical waveguide, the filter comprising:

a shoulder slab, wherein:

the shoulder slab is formed of a first material having a first index of refraction and disposed on a second material having a second index of refraction, the first index of refraction being higher than the second index of refraction; and

the shoulder slab defines:

a first end section having a first shoulder width,

an intermediate section, adjacent to the first end section along a direction of beam propagation, having a second shoulder width that is at least twice the first shoulder width, and

a far end section, adjacent to the intermediate section and opposite the first end section, along the direction of beam propagation; and

a waveguide ridge, formed of the first material and disposed atop the shoulder slab, wherein:

the waveguide ridge traverses the shoulder slab from the first end section to the far end section; and

the waveguide ridge is configured to guide light of a fundamental mode along the direction of beam propagation from the near end section to the far end section.

2. The optical filter for attenuating higher-order modes in an optical waveguide of claim 1 , wherein:

the waveguide ridge forms an input width in the near end section;

the waveguide ridge tapers adiabatically inward along the direction of beam propagation to a central region;

the central region defines a constant width along the direction of beam propagation; and

the waveguide ridge tapers adiabatically outward from the central region along the direction of beam propagation to the far end section.

3. The optical filter for attenuating higher-order modes in an optical waveguide of claim 1 , wherein the direction of beam propagation defined by the waveguide ridge is a straight line from the near end section to the far end section.

4. The optical filter for attenuating higher-order modes in an optical waveguide of claim 1 , wherein the second shoulder width is at least five times the first shoulder width.

5. The optical filter for attenuating higher-order modes in an optical waveguide of claim 1 , wherein a combined height of the shoulder slab and the waveguide ridge is greater than 0.5 μm and less than 2.0 μm.

6. The optical filter for attenuating higher-order modes in an optical waveguide of claim 1 , wherein the first material is crystalline silicon.

7. The optical filter for attenuating higher-order modes in an optical waveguide of claim 1 , wherein the shoulder slab is less than or equal to 100 microns long from the first end section to the far end section.

8. The optical filter for attenuating higher-order modes in an optical waveguide of claim 1 , wherein the optical filter further comprises:

a first waveguide optically coupled with the waveguide ridge and extending beyond the first end section in the direction of beam propagation; and

a second waveguide optically coupled with the waveguide ridge and extending beyond the far end section in the direction of beam propagation.

9. The optical filter for attenuating higher-order modes in an optical waveguide of claim 1 , wherein the shoulder slab forms beveled corners.

10. The optical filter for attenuating higher-order modes in an optical waveguide of claim 1 , wherein the first filter ridge and the second filter ridge are made out of the first material and then doped with a lossy material.

11. The optical filter for attenuating higher-order modes in an optical waveguide of claim 10 , wherein the shoulder slab comprises at least one of metal and Germanium.

12. The optical filter for attenuating higher-order modes in an optical waveguide of claim 1 , wherein the optical filter is symmetric along an axis running along the direction of beam propagation.

13. A method for filtering higher-order modes in a semiconductor waveguide, the method comprising:

in a first semiconductor waveguide, transmitting a beam of light along a direction of beam propagation, the beam of light having a fundamental mode and one or more higher-order modes;

coupling the beam of light along the direction of beam propagation into an optical filter;

attenuating, in the optical filter, the one or more higher-order modes along a direction of beam propagation while transmitting light of the fundamental mode along the direction of beam propagation, wherein the optical filter comprises:

a shoulder slab formed of a first material disposed adjacent a substrate material, the first material having a higher index of refraction than the substrate material, the shoulder slab defining a first section having a first width in the direction of beam propagation, and a second section adjoining the first section, the second section having a second width in the direction of beam propagation, the second width being at least twice the first width; and

a waveguide ridge, disposed adjacent to the shoulder slab and formed of the first material, for guiding the fundamental mode, wherein the optical filter tapers adiabatically along the direction of beam propagation; and

coupling the light of the fundamental mode into a second semiconductor waveguide, after the light of the fundamental mode has passed through the optical filter and the one or more higher-order modes are attenuated along the direction of beam propagation.

14. The method for filtering higher-order modes in the semiconductor waveguide of claim 13 , wherein attenuating the one or more higher-order modes comprises the waveguide ridge tapering adiabatically along the direction of beam propagation.

15. The method for filtering higher-order modes in the semiconductor waveguide of claim 13 , wherein the direction of beam propagation remains a straight line from the first semiconductor waveguide, through the optical filter and into the second semiconductor waveguide.

16. The method for filtering higher-order modes in the semiconductor waveguide of claim 13 , wherein the optical filter comprises crystalline silicon.

17. The method for filtering higher-order modes in the semiconductor waveguide of claim 13 , wherein a height of the waveguide ridge is between 0.5 and 2.0 μm.