Adiabatic polarization rotator-splitter

1. A system that includes a polarization rotator splitter (PRS), the PRS comprising:

a silicon nitride (SiN) rib waveguide core that includes a rib and a ridge that extends vertically above the rib, the SiN rib waveguide core having a total height h SiN from a bottom of the rib to a top of the ridge, a rib height h rib from the bottom of the rib to a top of the rib, a rib width w rib , and a top width w SiN of the ridge;

wherein:

the rib laterally extends beyond a first side of the ridge and beyond a second side of the ridge between opposing first and second sides of the rib;

the rib width w rib is the width of the rib from the first side of the rib to the second side of the rib;

the rib width w rib varies along at least a portion of a length of the SiN rib waveguide core;

the ridge of the SiN rib waveguide core comprises a first ridge that together with the rib and surrounding cladding forms a first rib waveguide;

the SiN rib waveguide core further comprises a second ridge that together with the rib and surrounding cladding forms a second rib waveguide;

the second ridge is spaced apart from the first ridge by a gap width w in a portion of the PRS;

the top width w SiN of the first ridge is constant through the portion of the PRS; and

the second ridge has a top width w SiN2 that tapers outward through the portion of the PRS.

2. The system of claim 1 , wherein the first ridge and the second ridge are separated by a port gap distance w port gap at an output of the PRS, the port gap distance w port gap at the output of the PRS being greater than the gap width w gap between the first ridge and the second ridge in the portion of the PRS.

3. The system of claim 1 , wherein:

the SiN rib waveguide core has a first section of length L 1 , a second section of length L 2 , and a third section of length L 3 ;

the rib is linearly tapered in each of the first and third sections;

the first ridge is linearly tapered in each of the first and second sections; and

at the beginning of the first section, the widths w rib and w SiN are equal to each other.

4. The system of claim 3 , wherein:

the portion of the PRS comprises a fourth section of length L 4 ; and

along the length L 4 of the fourth section, the rib width w rib is constant.

5. The system of claim 1 , wherein the gap width w gap is constant through the portion of the PRS.

6. The system of claim 1 , the PRS further comprising:

a first stage that includes the first rib waveguide;

a second stage coupled end to end with the first stage, wherein the second stage comprises the portion of the PRS with the first and second ridges of the first and second rib waveguides spaced apart by the gap width w gap ; and

a third stage coupled end to end with the second stage, wherein:

the third stage comprises the first rib waveguide and the second rib waveguide;

the first ridge tapers inward through a first portion of the third stage and has a constant width through a remaining portion of the third stage;

the second ridge tapers inward through the first portion of the third stage and has the constant width through the remaining portion of the third stage;

the rib has a constant width through the first portion of the third stage;

in the remaining portion of the third stage, the rib divides into a first sub-rib associated with the first ridge and a second sub-rib associated with the second ridge;

each of the first and second sub-ribs tapers inward through the remaining portion of the third stage.

7. The system of claim 1 , further comprising:

a SiN strip waveguide core having a first end continuously coupled to an input of the SiN rib waveguide core and a second end opposite the first end, the second end including a tapered end; and

an interposer waveguide disposed above or below the tapered end of the SiN strip waveguide core and aligned laterally and longitudinally with the tapered end to form an adiabatic coupler with the SiN strip waveguide core.

8. The system of claim 7 , wherein the PRS and the SiN strip waveguide core are formed in a SiN layer of a silicon (Si) photonic integrated circuit (PIC) and wherein the PRS includes a TE port and a TM port, the Si PIC further comprising:

a first wavelength division demultiplexer (demux) formed at least partially in the SiN layer of the Si PIC, the first demux including an input coupled to the TE port of the PRS; and

a second demux formed at least partially in the SiN layer of the Si PIC, the second demux including an input coupled to the TM port of the PRS.

9. A system that includes a polarization rotator splitter (PRS), the PRS comprising:

a first stage that includes a silicon nitride (SiN) rib waveguide core that includes a rib and a ridge that extends vertically above the rib, the SiN rib waveguide core having a total height h SiN from a bottom of the rib to a top of the ridge, a rib height h rib from the bottom of the rib to a top of the rib, a rib width w rib , and a top width w SiN of the ridge, wherein the rib width w rib varies along at least a portion of a length of the SiN rib waveguide core;

a second stage that includes:

a first strip waveguide core continuous with the SiN rib waveguide core and that laterally tapers inward from a first end of the first strip waveguide core to a second end of the first strip waveguide core; and

a second strip waveguide core spaced apart from the first strip waveguide core by a gap width w gap , the second strip waveguide core tapering outward from a first end of the second strip waveguide core to a second end of the second strip waveguide core; and

a third stage that includes:

a first S-bend waveguide core continuous with the first strip waveguide core; and

a second S-bend waveguide core continuous with the second strip waveguide core,

wherein output ends of the first and second S-bend waveguide cores are laterally separated by a greater distance than input ends of the first and second S-bend waveguide cores.

10. The system of claim 9 , wherein:

the SiN rib waveguide core has a first section of length L 1 , a second section of length L 2 , and a third section of length L 3 ;

each of the rib and the ridge of the SiN rib waveguide core is linearly tapered in each of the first, second, and third sections; and

at the beginning of the first section, the widths w rib and w SiN are equal to each other.

11. The system of claim 10 , wherein:

the second stage has a length L 4 ;

each of the first and second strip waveguide cores of the second stage is linearly tapered in the second stage through the length L 4 ; and

along the length L 4 of the second stage, the gap width w gap is constant.

12. The system of claim 9 , further comprising:

a SiN strip waveguide core having a first end continuously coupled to the SiN rib waveguide core and a second end opposite the first end, the second end including a tapered end; and

an interposer waveguide disposed above or below the tapered end of the SiN strip waveguide core and aligned laterally and longitudinally with the tapered end to form an adiabatic coupler with the SiN strip waveguide core.

13. The system of claim 12 , wherein the PRS and the SiN strip waveguide core are formed in a SiN layer of a silicon (Si) photonic integrated circuit (PIC) and wherein the PRS includes a TE port and a TM port, the Si PIC further comprising:

a first wavelength division demultiplexer (demux) formed at least partially in the SiN layer of the Si PIC, the first demux including an input coupled to the TE port of the PRS; and

a second demux formed at least partially in the SiN layer of the Si PIC, the second demux including an input coupled to the TM port of the PRS.

14. A system that includes a polarization rotator splitter (PRS), the PRS comprising:

a silicon nitride (SiN) rib waveguide core that includes a rib and a ridge that extends vertically above the rib, the SiN rib waveguide core having a total height h SiN from a bottom of the rib to a top of the ridge, a rib height h rib from the bottom of the rib to a top of the rib, a rib width w rib , and a top width w SiN of the ridge;

wherein:

the rib width w rib varies along at least a portion of a length of the SiN rib waveguide core;

the ridge of the SiN rib waveguide core comprises a first ridge that together with the rib and surrounding cladding forms a first rib waveguide that is included in a first stage of the PRS;

the SiN rib waveguide core further comprises a second ridge that together with the rib and surrounding cladding forms a second rib waveguide;

the second ridge is spaced apart from the first ridge by a gap width w gap ; and

the PRS further comprises a second stage that includes:

a first portion of the first rib waveguide; and

a first portion of the second rib waveguide;

the top width w SiN comprises a first top width w SiN1 of the first ridge;

the first top width w SiN1 of the first ridge is constant through the second stage;

the second ridge has a top width w SiN2 that tapers outward through the second stage; and

the gap width w gap is constant through the second stage.

15. The system of claim 14 , the PRS further comprising a third stage that includes:

a second portion of the first rib waveguide; and

a second portion of the second rib waveguide;

wherein:

the first ridge tapers inward through a first portion of the third stage and has a constant width through a remaining portion of the third stage;

the second ridge tapers inward through the first portion of the third stage and has the constant width through the remaining portion of the third stage;

the rib has a constant width through the first portion of the third stage;

in the remaining portion of the third stage, the rib divides into a first sub-rib associated with the first ridge and a second sub-rib associated with the second ridge;

each of the first and second sub-ribs tapers inward through the remaining portion of the third stage.

16. The system of claim 14 , further comprising:

a SiN strip waveguide core having a first end continuously coupled to the SiN rib waveguide core and a second end opposite the first end, the second end including a tapered end; and

an interposer waveguide disposed above or below the tapered end of the SiN strip waveguide core and aligned laterally and longitudinally with the tapered end to form an adiabatic coupler with the SiN strip waveguide core.

17. The system of claim 16 , wherein the PRS and the SiN strip waveguide core are formed in a SiN layer of a silicon (Si) photonic integrated circuit (PIC) and wherein the PRS includes a TE port and a TM port, the Si PIC further comprising:

a first wavelength division demultiplexer (demux) formed at least partially in the SiN layer of the Si PIC, the first demux including an input coupled to the TE port of the PRS; and

a second demux formed at least partially in the SiN layer of the Si PIC, the second demux including an input coupled to the TM port of the PRS.