Low loss tunable ring resonator allpass filter

1. An apparatus, comprising:

an optical ring resonator having a waveguide ring including substantially straight waveguide segments and bent waveguide segments, the bent waveguide segments optically coupled to the substantially straight waveguide segments; and

wherein the bent waveguide segments have optical cores with substantially smaller cross-sectional areas than the substantially straight waveguide segments; and

wherein the bent waveguide segments change a propagation direction of received light by more than ½ of a right angle.

2. The apparatus of claim 1 , further comprising:

a substrate with a top surface, the optical ring resonator located along the top surface; and

wherein a lateral width of each bent waveguide segment varies adiabatically between a minimum width and a lateral width of the substantially straight waveguide segments.

3. The apparatus of claim 1 , wherein the bent waveguide segments are able to operate as substantially single-mode waveguides at a wavelength that the substantially straight waveguide segments operate as multi-mode waveguides.

4. The apparatus of claim 1 , wherein the ring further comprises a Mach-Zehnder interferometer connected between two of the bent waveguide segments.

5. The apparatus of claim 1 , wherein the substantially straight waveguide segments form more than ½ the length of the waveguide ring.

6. The apparatus of claim 1 , wherein the straight waveguide segments are coupled to the bent waveguide segments by a tapered waveguide segment.

7. The apparatus of claim 1 , wherein the waveguides have a semiconducting core.

8. The apparatus of claim 1 , further comprising an optical signal source configured to provide an optical signal to the optical ring resonator.

9. The apparatus of claim 1 , wherein the optical ring resonator is included in an arm of a Mach-Zehnder interferometer.

10. The apparatus of claim 9 , wherein a plurality of optical ring resonators are cascaded in the arm of the Mach-Zehnder interferometer.

11. The apparatus of claim 1 , wherein the optical ring resonator includes a plurality of coupled rings.

12. The apparatus of claim 11 , further comprising a phase adjuster to vary a coupling between coupled rings.

13. A method, comprising:

transmitting the optical signal through a multi-mode waveguide segment in a substantially straight segment of the waveguide;

then, changing a direction of propagation of the optical signal by transmitting the optical signal through a curved segment of the waveguide having a lateral width substantially smaller than a lateral width of the multi-mode waveguide segment; and

wherein the changing includes converting a propagation mode of the optical signal from a fundamental mode in the multi-mode waveguide segment to a different fundamental mode in the curved segment.

14. The method of claim 13 , wherein the lateral width of the curved segment varies adiabatically between a minimum lateral width and the lateral width of the multi-mode waveguide segment.

15. The method of claim 14 , wherein the minimum lateral width is about 0.5 μm.

16. The method of claim 14 , wherein the lateral width of the multi-mode waveguide segment ranges from about 1.0 to about 3.0 μm.

17. The method of claim 13 , wherein the multimode waveguide segment forms at least a portion of an optical ring resonator.

18. The method of claim 17 , wherein the optical ring resonator is included in an arm of a Mach-Zehnder interferometer.

19. The method of claim 17 , wherein an optical path length of the optical ring resonator is configured to be changed by a phase adjuster.

20. The method of claim 17 , wherein the optical ring resonator is cascaded with a second different optical ring resonator.