Phase tuning in waveguide arrays
The wavelength response of an arrayed waveguide grating can be tuned, in accordance with various embodiments, using a beam sweeper including one or more heaters to shift a lateral position of light focused by the beam sweeper at an interface of the beam sweeper with an input free propagation region of the arrayed waveguide grating.
1. A system comprising:
an arrayed waveguide grating (AWG) comprising a plurality of waveguides connected between a first input free propagation region (FPR) and a first output FPR; and
a beam sweeper for tuning a wavelength response of the AWG, the beam sweeper comprising
a second input FPR,
a second output FPR adjoining and optically coupled to the first input FPR of the AWG,
at least three waveguides connected between the second input FPR and the second output FPR, the at least three waveguides and the second output FPR being configured to focus light propagating in the at least three waveguides from the second input FPR to the second output FPR at a focus at an interface between the second output FPR and the first input FPR, and
at least one pair of forward and backward heaters laterally overlapping with the at least three waveguides, each of the heaters configured to impart an incremental phase shift between the light propagating in the at least three waveguides to thereby shift a lateral position of the focus, the phase shift imparted by the forward heater being of an opposite sign than the phase shift imparted by the backward heater, whereby the forward heater and the backward heater shift the lateral position of the focus in mutually opposite directions.
2. The system of claim 1 , wherein the at least three waveguides of the beam sweeper are fewer in number than the waveguides of the AWG.
3. The system of claim 1 , wherein the at least three waveguides of the beam sweeper are arranged, in a region immediately preceding the second output FPR, along rays emanating from a common center at the interface of the second output FPR with the first input FPR.
4. The system of claim 3 , wherein the plurality of waveguides of the AWG are arranged, in a region immediately following the first input FPR, along rays emanating from the center.
5. The system of claim 1 , wherein the at least three waveguides of the beam sweeper are all equal in length.
6. The system of claim 1 , wherein each of the heaters is configured to impart a constant incremental phase shift between all pairs of adjacent waveguides of the at least three waveguides.
7. The system of claim 6 , wherein each of the heaters is configured to heat a heated region spatially overlapping the at least three waveguides to a substantially uniform temperature so as to impart a uniform phase shift per unit length of heated waveguide, the heated region being shaped and positioned such that heated waveguide portions increase, between all pairs of adjacent waveguides, by constant length increments.
8. The system of claim 7 , wherein the at least three waveguides are uniformly spaced and the heated regions are triangular in shape.
9. The system of claim 6 , wherein spacings between the at least three waveguides are selected based on a temperature distribution associated with the heaters to achieve the constant incremental phase shift.
10. The system of claim 1 , wherein each of the heaters comprises a heating filament winding across an area laterally overlapping the at least three waveguides in a region to be heated.
11. The system of claim 10 , wherein the heating filaments comprises segments of equal and constant width connected by metal connections.
12. The system of claim 1 , comprising a plurality of pairs of heaters, each pair comprising a forward heater and a backward heater, at least some of the forward heaters or at least some of the backward heaters being configured to operate in parallel.
13. The system of claim 1 , wherein the heaters laterally overlap with a section of the at least three waveguides in which the at least three waveguides all have a common curvature.
14. The system of claim 13 , wherein the heaters laterally overlap with straight sections of the at least three waveguides.
15. The system of claim 1 , wherein the AWG and the beam sweeper are implemented in a photonic integrated circuit comprising a silicon-on-insulator (SOI) substrate including a silicon handle, a buried oxide layer disposed on top of the silicon handle, a silicon device layer disposed on top of the buried oxide layer, and a cladding layer disposed on top of the silicon device layer.
16. The system of claim 15 , further comprising a back-etched region in the silicon handle underneath the pair of heaters.
17. The system of claim 16 , wherein the back-etched region has a constant width across the at least three waveguides.
18. The system of claim 15 , further comprising thermal-isolation channels formed in at least one of the silicon device layer or the cladding layer surrounding heated portions of the at least three waveguides.
19. The system of claim 15 , wherein the waveguides of the AWG and the sweeper are formed within the silicon device layer.
20. The system of claim 15 , wherein the heaters comprise heating segments embedded in the cladding layer.
21. The system of claim 20 , further comprising temperature-sensing elements embedded in the cladding layer in between the heating segments.
22. The system of claim 21 , further comprising thermal isolation trenches in the cladding layer in between adjacent heating segments and temperature-sensing elements.
23. The system of claim 15 , wherein the heaters comprise doped, resistive regions in the silicon device layer.
24. The system of claim 1 , wherein the AWG is bidirectional, the system further comprising a second sweeper comprising
a third input FPR,
a third output FPR adjoining and optically coupled to the first output FPR of the AWG,
at least three waveguides connected between the third input FPR and the third output FPR, the at least three waveguides and the third output FPR being configured to focus light propagating in the at least three waveguides from the third input FPR to the third output FPR at a focus at an interface between the third output FPR and the first output FPR, and
at least one pair of forward and backward heaters laterally overlapping with the at least three waveguides, each of the heaters configured to impart an incremental phase shift between the light propagating in the at least three waveguides to thereby shift a lateral position of the focus, the phase shift imparted by the forward heater being of an opposite sign than the phase shift imparted by the backward heater, whereby the forward and backward heaters shift the lateral position of the focus in mutually opposite directions.
25. A system comprising:
an arrayed waveguide grating (AWG) comprising a plurality of waveguides connected between a first input free propagation region (FPR) and a first output FPR; and
a beam sweeper for tuning a wavelength response of the AWG, the beam sweeper comprising
a second input FPR,
a second output FPR adjoining and optically coupled to the first input FPR of the AWG,
at least three waveguides connected between the second input FPR and the second output FPR, the at least three waveguides being arranged, in a region immediately preceding the second output FPR, along rays emanating from a common center point at the interface of the second output FPR with the first input FPR so as to focus light propagating in the at least three waveguides from the second input FPR to the second output FPR at a focus at the interface, and
at least one heater laterally overlapping with the at least three waveguides, each of the heaters configured to impart an incremental phase shift between the light propagating in the at least three waveguides to thereby shift a lateral position of the focus.
26. A method comprising:
tuning a wavelength response of an arrayed waveguide grating (AWG) with a beam sweeper comprising an input FPR, an output FPR adjoining and optically coupled to an input FPR of the AWG, at least three waveguides connected between the input FPR and the output FPR of the beam sweeper, and a pair of forward and backward heaters laterally overlapping with the at least three waveguides, the tuning comprising:
focusing light propagating in the at least three waveguides from the input FPR of the beam sweeper to the output FPR of the beam sweeper at a focus at an interface between the output FPR of the beam sweeper and the input FPR of the AWG, and
using one of the forward or backward heaters to impart an incremental phase shift between the light propagating in the at least three waveguides to thereby shift a lateral position of the focus,
wherein the phase shift imparted by the forward heater is of an opposite sign than the phase shift imparted by the backward heater, whereby the forward heater and the backward heater shift the lateral position of the focus in mutually opposite directions.