Optical coherent receiver having an optical hybrid configuration with reduced phase error

1. An optical coherent receiver, comprising:

a 90-degree optical hybrid configured to receive an input signal and a reference signal, and mix the input signal with four quadrature states associated with the reference signal to generate four output signals, wherein the 90-degree optical hybrid comprises:

a plurality of 3-dB couplers; and

a plurality of optical waveguides, wherein each optical waveguide of the plurality of optical waveguides couples two respective 3-dB couplers of the plurality of 3-dB couplers, and wherein each optical waveguide of the plurality of optical waveguides has a same optical path length,

wherein each optical waveguide of the plurality of optical waveguides comprises one or more respective bend sections,

wherein each optical waveguide of the plurality of optical waveguides is dimensioned according to a figure of merit (FoM) to reduce a phase error, and

wherein the FoM is defined by:

FoM

=

dneff

dw

⁢

L

,

wherein dneff denotes a variation in an effective refractive index of a waveguide transverse mode of an optical waveguide, L denotes a total length of the optical waveguide, and dw denotes a width variation of a width of the optical waveguide.

2. The optical coherent receiver of claim 1 , wherein each optical waveguide accumulates a same phase.

3. The optical coherent receiver of claim 1 , wherein each respective bend section comprises a respective continuous curvature defined by a respective curvature function to reduce the phase error, and

wherein each respective curvature function is configured to suppress an excitation of light to a higher order transverse electric (TE) mode.

4. The optical coherent receiver of claim 3 , wherein each respective continuous curvature has an effective radius that is configured to provide at least 35 dB suppression of the higher order TE mode.

5. The optical coherent receiver of claim 3 , wherein each respective curvature function includes at least one of:

a sin 1 curvature function,

a sin 2 curvature function,

a sin 3 curvature function,

a sin 4 curvature function, or

a Dolph-Chebyshev curvature function.

6. The optical coherent receiver of claim 1 , wherein the plurality of 3-dB couplers include:

a first 3-dB coupler configured to receive the input signal and generate a first pair of output signals based on the input signal;

a second 3-dB coupler configured to receive the reference signal and generate a second pair of output signals based on the reference signal;

a third 3-dB coupler coupled to the first 3-dB coupler by a first optical waveguide of the plurality of optical waveguides for receiving a first output signal of the first pair of output signals, and coupled to the second 3-dB coupler by a second optical waveguide of the plurality of optical waveguides for receiving a first output signal of the second pair of output signals, wherein the third 3-dB coupler is configured to generate a third pair of output signals based on the first output signal of the first pair of output signals and the first output signal of the second pair of output signals; and

a fourth 3-dB coupler coupled to the first 3-dB coupler by a third optical waveguide of the plurality of optical waveguides for receiving a second output signal of the first pair of output signals, and coupled to the second 3-dB coupler by a fourth optical waveguide of the plurality of optical waveguides for receiving a second output signal of the second pair of output signals, wherein the fourth 3-dB coupler is configured to generate a fourth pair of output signals based on the second output signal of the first pair of output signals and the second output signal of the second pair of output signals.

7. The optical coherent receiver of claim 1 , wherein the FoM is equal to 0 (zero) or substantially zero.

8. The optical coherent receiver of claim 7 , wherein the FoM is directly correlated to the phase error.

9. The optical coherent receiver of claim 1 , wherein each optical waveguide of the plurality of optical waveguides is dimensioned to minimize the FoM.

10. The optical coherent receiver of claim 1 , wherein each optical waveguide of the plurality of optical waveguides has an average width that is dimensioned such that a waveguide sensitivity to the FoM is reduced.

11. The optical coherent receiver of claim 1 , wherein each optical waveguide of the plurality of optical waveguides has an average width that is dimensioned such that a phase error sensitivity is reduced.

12. The optical coherent receiver of claim 1 , wherein each optical waveguide of the plurality of optical waveguides is a multi-mode waveguide.

13. The optical coherent receiver of claim 1 , wherein each optical waveguide of the plurality of optical waveguides is a fully etched waveguide having an average width that is dimensioned such that a waveguide sensitivity to the FoM is reduced.

14. The optical coherent receiver of claim 1 , wherein each optical waveguide of the plurality of optical waveguides comprises a shoulder and a ridge, wherein the ridge has an average width that is dimensioned such that a waveguide sensitivity to the FoM is reduced, and wherein the shoulder extends laterally from the ridge.

15. The optical coherent receiver of claim 1 , wherein each optical waveguide of the plurality of optical waveguides comprises at least one tapered portion having a tapered length configured to minimize insertion loss.

16. The optical coherent receiver of claim 1 , wherein a weighted FoM of the plurality of optical waveguides is 0.4-0.6 μm −1/2 .

17. An optical hybrid, comprising:

a first input configured to receive an input signal;

a second input configured to receive a reference signal;

a plurality of optical components configured to mix the input signal with the reference signal to generate a plurality of output signals; and

a plurality of optical waveguides, wherein each optical waveguide of the plurality of optical waveguides couples two respective optical components of the plurality of optical components, and wherein each optical waveguide of the plurality of optical waveguides has a same optical path length,

wherein each optical waveguide of the plurality of optical waveguides is dimensioned according to a figure of merit (FoM) to reduce a phase error, and

wherein the FoM is defined by:

FoM

=

dneff

dw

⁢

L

,

wherein dneff denotes a variation in an effective refractive index of a waveguide transverse mode of an optical waveguide, L denotes a total length of the optical waveguide, and dw denotes a width variation of a width of the optical waveguide.

18. The optical hybrid of claim 17 , wherein the FoM is less than one.

19. The optical hybrid of claim 17 , wherein each optical waveguide of the plurality of optical waveguides has an average width that is dimensioned such the FoM is reduced to an operating zone corresponding to a minimized phase error.

20. An optical hybrid, comprising:

a first input configured to receive an input signal;

a second input configured to receive a reference signal;

a plurality of optical components configured to mix the input signal with the reference signal to generate a plurality of output signals; and

a connecting waveguide network comprising a plurality of optical waveguide paths, wherein each optical waveguide path of the plurality of optical waveguide paths couples two respective optical components of the plurality of optical components, and wherein each optical waveguide path of the plurality of optical waveguide paths has a same optical path length,

wherein each optical waveguide path of the plurality of optical waveguide paths is dimensioned according to a figure of merit (FoM) to reduce a phase error, and

wherein the FoM is defined by:

FoM

=

dneff

dw

⁢

L

,

wherein dneff denotes a variation in an effective refractive index of a waveguide transverse mode of an optical waveguide path, L denotes a total length of the optical waveguide path, and dw denotes a width variation of a width of the optical waveguide path.