MICROMECHANICALLY-TUNABLE POLARIZATION ROTATOR FOR PHOTONIC INTEGRATED CIRCUITS

Abstract

An apparatus includes a polarization rotator or a polarization splitter. The polarization rotator and the polarization splitter each includes a first optical waveguide. The polarization rotator further includes a movable symmetry-breaking micro-electro-mechanical systems (MEMS) dielectric perturber separated from the first optical waveguide by a gap. The first optical waveguide and the MEMS dielectric perturber define a gap therebetween. The polarization rotator also includes a MEMS actuator moving the MEMS dielectric perturber so as to control the gap, thereby controlling polarization rotation within the first optical waveguide. The polarization splitter includes a second optical waveguide separated from the first optical waveguide by a gap. The polarization splitter also includes a MEMS actuator moving the first optical waveguide and/or the second optical waveguide so as to control the gap, thereby controlling polarization splitting between the optical waveguides.

Claims

1. An apparatus comprising: an optical waveguide; a movable symmetry-breaking micro-electro-mechanical systems (MEMS) dielectric perturber separated from said optical waveguide by a gap, said optical waveguide and said MEMS dielectric perturber comprising a gap therebetween; and a MEMS actuator moving said MEMS dielectric perturber so as to control the gap, thereby controlling polarization rotation within said optical waveguide.

2. The apparatus according to claim 1, wherein said optical waveguide comprises a length-wise axis, wherein said MEMS dielectric perturber comprises a constant perturber width along the length-wise axis.

3. The apparatus according to claim 2, wherein said optical waveguide comprises a waveguide width, wherein said constant perturber width is less than the waveguide width.

4. The apparatus according to claim 1, wherein said optical waveguide comprises a length-wise axis, wherein said MEMS dielectric perturber comprises a slanted perturber width along the length-wise axis.

5. The apparatus according to claim 4, wherein said optical waveguide comprises a waveguide width, wherein said optical waveguide comprises an input end and an output end, said slanted perturber width being one of less than the waveguide width toward the input end and less than the waveguide width toward the output end.

6. The apparatus according to claim 1, wherein said optical waveguide comprises a waveguide width and an output end, wherein said apparatus further comprises: a MEMS taper suspended above said optical waveguide and connected to said MEMS dielectric perturber, said MEMS taper comprising a taper width symmetrical along the length-wise axis, the taper width being less than waveguide width toward the output end.

7. The apparatus according to claim 6, further comprising: a plurality of MEMS anchors connected to said MEMS actuator, and a plurality of MEMS tethers respectively connecting said plurality of MEMS anchors to at least one of said MEMS dielectric perturber and said MEMS taper.

8. The apparatus according to claim 7, wherein at least one MEMS tether of said plurality of MEMS tethers comprises at least one etch hole wherein said etch hole selectively underetches said at least one MEMS tether, thereby suspending said MEMS dielectric perturber above said optical waveguide.

9. The apparatus according to claim 1, wherein said optical waveguide comprises one of silicon, silicon nitride (SiN), a dielectric, and a compound semiconductor, and wherein said MEMS dielectric perturber comprises one of silicon, silicon nitride (SiN), dielectric, and the compound semiconductor.

10. The apparatus according to claim 1, wherein said MEMS actuator comprises one of an electrostatic actuator, a gradient electric force actuator, a gradient optical force actuator, a piezo-actuator, and an electro-thermal actuator.

11. The apparatus according to claim 1, wherein said optical waveguide and said MEMS dielectric perturber comprise at least one wavelength of operation between ultraviolet (UV) to visible to infrared (IR).

12. The apparatus according to claim 1, wherein said optical waveguide comprises a propagating mode, said propagating mode comprising a shape, wherein said MEMS actuator moving said MEMS dielectric perturber changes the shape of the propagating mode.

13. An apparatus comprising: a first optical waveguide; a second optical waveguide separated from said first optical waveguide by a gap, said first optical waveguide and said second optical waveguide comprising at least one coupling condition; and a MEMS actuator moving at least one of said first optical waveguide and said second optical waveguide so as to control the gap, thereby controlling the at least one coupling condition, whereby controlling the at least one coupling condition controls polarization splitting between said first optical waveguide and said second optical waveguide.

14. The apparatus according to claim 13, wherein said first optical waveguide comprises one of silicon, silicon nitride (SiN), a dielectric, and a compound semiconductor, and wherein said second optical waveguide comprises one of silicon, silicon nitride (SiN), a dielectric, and the compound semiconductor.

15. The apparatus according to clam 13, further comprising: a plurality of MEMS anchors connected to said MEMS actuator, and a plurality of MEMS tethers respectively connecting said plurality of MEMS anchors to at least one of said first optical waveguide and said second optical waveguide.

16. The apparatus according to claim 15, wherein at least one MEMS tether of said plurality of MEMS tethers comprises at least one etch hole, wherein said etch hole selectively underetches said at least one MEMS tether, thereby suspending said second optical waveguide above said first optical waveguide.

17. The apparatus according to claim 13, wherein said first optical waveguide comprises one of silicon, silicon nitride (SiN), a dielectric, and a compound semiconductor, and wherein said second optical waveguide comprises one of silicon, silicon nitride (SiN), the dielectric, and the compound semiconductor.

18. The apparatus according to claim 13, wherein said MEMS actuator comprises one of an electrostatic actuator, a gradient electric force actuator, a gradient optical force actuator, a piezo-actuator, and an electro-thermal actuator.

19. The apparatus according to claim 13, wherein said first optical waveguide and said second optical waveguide comprise at least one propagating mode, wherein said at least one propagating mode comprises: one of a transverse-electric polarization and a transverse-magnetic polarization, and a respective at least one mode order, wherein said at least one coupling condition corresponds to said respective at least one propagating mode.

20. The apparatus according to claim 13, wherein said first optical waveguide and said second optical waveguide comprise at least one wavelength of operation between ultraviolet (UV) to visible to infrared (IR).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1A is a top plan view of an embodiment of an invention including a polarization-rotator.

[0037] FIG. 1B is a cross-sectional side view of an embodiment of an invention including a polarization-rotator, the view showing a gap between a dielectric perturber and a waveguide.

[0038] FIG. 1C is a cross-sectional side view of an embodiment of an invention including a polarization rotator, the view showing no gap between a dielectric perturber and a waveguide, i.e., a gap that has been closed.

[0039] FIG. 2A is a block diagram of an embodiment of the invention including a dielectric perturber having constant width.

[0040] FIG. 2B is a block diagram of an embodiment of the invention including a dielectric perturber having slanted width.

[0041] FIG. 3A is a top plan view of an embodiment of the invention including a slanted dielectric perturber.

[0042] FIG. 3B is a cross-sectional view of the embodiment of the invention through section cut 3B-3B shown in FIG. 3A.

[0043] FIG. 3C is a top plan view of an embodiment of the invention through section cut 3C-3C shown in FIG. 3A.

[0044] FIG. 3D is a top plan view of an embodiment of the invention through section cut 3D-3D shown in FIG. 3A.

[0045] FIG. 4A is a top plan view of an embodiment of the invention including a waveguide width taper.

[0046] FIG. 4B is a cross-sectional view of the embodiment of the invention through section cut 4B-4B shown in FIG. 4A.

[0047] FIG. 4C is a top plan view of an embodiment of the invention through section cut 4C-4C shown in FIG. 4A.

[0048] FIG. 4D is a top plan view of an embodiment of the invention through section cut 4D-4D shown in FIG. 4A.

[0049] FIG. 5A is a top plan view of an embodiment of the invention including a dielectric perturber, tethers, and anchors.

[0050] FIG. 5B is a top plan view of an embodiment of the invention including slanted dielectric perturber, a MEMS taper, tethers, anchors, and etch holes.

[0051] FIG. 6A is a block diagram of an embodiment of the invention including an MEMS actuator, which in turn includes a standard electrostatic actuator.

[0052] FIG. 6B is a block diagram of an embodiment of the invention including an MEMS actuator, which in turn includes a standard gradient electric force actuator.

[0053] FIG. 6C is a block diagram of an embodiment of the invention including an MEMS actuator, which in turn includes a standard gradient optical force actuator.

[0054] FIG. 6D is a block diagram of an embodiment of the invention including an MEMS actuator, which in turn includes a standard piezo-actuator.

[0055] FIG. 6E is a block diagram of an embodiment of the invention including an MEMS actuator, which in turn includes a standard electro-thermal actuator.

[0056] FIG. 7 is a block diagram of an embodiment of the invention including a polarization splitter.

[0057] FIG. 8A is a top planar view of an embodiment of the invention including a polarization splitter.

[0058] FIG. 8B is a is a cross-sectional view of the embodiment of the invention through section cut 8A-8A shown in FIG. 8A.

[0059] FIG. 9 is a top planar view of an embodiment of the invention including a polarization splitter, which further includes MEMS anchors and MEMS tethers.

DETAILED DESCRIPTION OF THE INVENTION

[0060] An embodiment of the invention includes a polarization rotator 100 and is described as follows with reference, by way of illustration, to FIGS. 1A, 1B, and 1C. The polarization rotator 100 includes a standard first optical waveguide 110 on a standard substrate 120. For example, the optical waveguide has a width on the order of 450 nm and has a height on the order of 220 nm. For example, the substrate 120 includes a standard insulator material, such as SiO.sub.2. The polarization rotator 100 further includes a movable symmetry-breaking MEMS dielectric perturber separated from the first optical waveguide 110 by an air gap 105. That is, the first optical waveguide 110 and the MEMS dielectric perturber define an air gap therebetween. In an embodiment of the invention, the gap is vertically situated between the first optical waveguide and the MEMS dielectric perturber, provided that the MEMS dielectric perturber breaks the symmetry of the waveguide cross-section. Similarly, in another embodiment of the invention, the gap 105 is horizontally situated between the first optical waveguide and the MEMS dielectric perturber, provided that the MEMS dielectric perturber breaks the symmetry of the waveguide cross-section. One of ordinary skill in the art will readily appreciate that, when the distance between the first optical waveguide 110 and the movable MEMS dielectric perturber is non-zero, there is an air gap of non-zero width between the first optical waveguide and the MEMS dielectric perturber, as shown by way of illustration in FIG. 1B. One of ordinary skill in the art will also readily appreciate that when the first optical waveguide 110 abuts the movable MEMS dielectric perturber, there is an air gap having no width, between the first optical waveguide 110 and the movable MEMS dielectric perturber, as shown by way of illustration in FIG. 1C. For the purpose of this patent application, symmetry-breaking is a term of art and is defined as being asymmetric relative to a length-wise axis and waveguide width of the first optical waveguide 110. By extension, for the purpose of this patent application, a symmetry-breaking MEMS dielectric perturber is a term of art and is defined as a dielectric perturber having a MEMS structure that interacts with an optical propagating mode in the optical waveguide in an asymmetric way so as to change a characteristic (e.g. polarization) of the propagating mode. MEMS-induced symmetry-breaking occurs by having the MEMS structure interact with only part of the propagating mode. For example, the symmetry-breaking MEMS dielectric perturber has a width on the order of 225 nm and a height on the order 220 nm.

[0061] For example, a planar cross-section of the symmetry-breaking MEMS dielectric perturber 130 having a constant perturber width along its length-wise axis is shown by way of illustration in FIG. 1A. Considering the planar cross-section of a symmetry-breaking MEMS dielectric perturber 130 with a constant perturber width, such a MEMS dielectric perturber is located more on one side of the length-wise axis than the other side thereof. As another example, a planar cross-section of the symmetry-breaking MEMS dielectric perturber 135 having a slanted perturber width along its length-wise axis (that is, increasing in width along the length-wise axis) is shown by way of illustration in FIG. 2A. For example, in an embodiment of the invention, the slanted perturber width tapers from zero to the full waveguide width, and the perturber starts off-center from the waveguide center axis. Considering the planar cross-section of a symmetry-breaking MEMS dielectric perturber 135 with a tapered perturber width, such a symmetry-breaking MEMS dielectric perturber asymmetrically increases in perturber width along the length-wise axis. The polarization rotator 100 also includes a MEMS actuator 140, such as shown in FIGS. 2A and 2B, moving the MEMS dielectric perturber so as to control the air gap 105, thereby controlling polarization rotation of an optical signal transmitted through the first optical waveguide 110.

[0062] Optionally, the first optical waveguide 110 includes a length-wise axis. The MEMS dielectric perturber includes a constant perturber width along the length-wise axis, and the optical waveguide comprises a waveguide width. The constant perturber width is less than the waveguide width.

[0063] Optionally, the first optical waveguide 110 comprises a length-wise axis. The MEMS dielectric perturber 135 has a slanted perturber width along the length-wise axis. For example, a dielectric perturber having a slanted perturber width has a width that increases from a minimum to a maximum along the length-wise axis of the first optical waveguide 110. Optionally, the first optical waveguide 110 includes a waveguide width. The first optical waveguide 110 includes an input end and an output end. The slanted perturber width is less than the waveguide width toward the input end or less than the waveguide width toward the output end. That is, in an alternative embodiment of the invention, the symmetry-breaking MEMS dielectric perturber 135 includes a slanted perturber width equal, or approaching equal, to the optical waveguide width at the optical waveguide's input end, and the slanted perturber width is less than the waveguide width at the optical waveguide's output end. For example, the symmetry-breaking MEMS dielectric perturber has a length on the order of 10.3 m. The slanted MEMS structure enables an adiabatic transition to the polarization rotation segment, and the MEMS taper enables an adiabatic transition between the optical waveguide and MEMS dielectric perturber.

[0064] Optionally, the first optical waveguide 110 includes a waveguide width and an output end. The polarization rotator 100 further includes a MEMS taper 150, such as shown by way of illustration in FIGS. 4A-4D, suspended above the first optical waveguide 110 and connected to the MEMS dielectric perturber. One of ordinary skill in the art will readily appreciate that FIGS. 4B-4D show an air gap between the MEMS taper 150 and the first optical waveguide 110. Enabling a low-loss transition between the polarization rotation segment of the polarization rotator 100 and the output waveguide optionally includes selecting the gap distance between the MEMS taper 150 and the first optical waveguide 110 including a non-zero distance or no distance. The MEMS taper 150 includes a taper width symmetrical along the length-wise axis. The taper width is less than waveguide width toward the output end. For example, the MEMS taper 150 has a length on the order of 20 m. Optionally, as shown by way of illustration in FIGS. 5A and 5B, the polarization rotator 100 further includes a plurality of MEMS anchors 160, 162, 164, 166, for example, connected to the MEMS actuator 140, which in turn is connected to the MEMS dielectric perturber and/or the MEMS taper 150. The polarization rotator 100 also includes a plurality of MEMS tethers respectively connecting the plurality of MEMS anchors 160, 162, 164, 166 to the MEMS dielectric perturber and/or the MEMS taper 150. Optionally, at least one MEMS tether of the plurality of MEMS tethers 170, 172, 174, 176 includes at least one etch hole 180. For ease of viewing, only one etch hole is provided a reference numeral in FIG. 5B. For ease of viewing, four etch holes are shown in FIG. 5B; one of ordinary skill in the art will readily appreciate that more or fewer etch holes per tether are suitably employed depending on the application. The at least one etch hole 180 selectively under-etches the at least one MEMS tether, thereby suspending the MEMS dielectric perturber above the first optical waveguide 110.

[0065] Optionally, the first optical waveguide 110 includes silicon, silicon nitride (SiN), a dielectric, or a standard compound semiconductor, and the MEMS dielectric perturber includes silicon, SiN, dielectric, or the compound semiconductor. For example, in alternative embodiments of the invention, the compound semiconductor includes indium phosphide (InP) or gallium arsenide (GaAs).

[0066] Optionally, the MEMS actuator 140 includes a standard electrostatic actuator 142, as shown, a standard gradient electric force actuator 144, a standard gradient optical force actuator 146, a standard piezo-actuator 148, or a standard electro-thermal actuator 149, as shown by way of illustration in FIGS. 6A-6E, respectively.

[0067] Optionally, the first optical waveguide 110 and the MEMS dielectric perturber include at least one wavelength of operation between ultraviolet (UV) to visible to infrared (IR). One of ordinary skill in the art will readily appreciate that wavelength of operation of the first optical waveguide 110 and the MEMS dielectric perturber is determined by the wavelength transparency of the materials chosen for the first optical waveguide 110 and the MEMS dielectric perturber.

[0068] For example, in operation, the first optical waveguide 110 includes a propagating optical mode. The propagating mode includes a shape. The MEMS actuator 140 moving the MEMS dielectric perturber changes the shape of the propagating mode. In an embodiment of the invention, changing the shape of the propagating mode changes the rotation of the polarization of the propagating mode.

[0069] Another embodiment of the invention includes a polarization splitter 200, as shown by way of illustration in FIGS. 7-9. The polarization splitter 200 includes a first optical waveguide 110 on a substrate 120. The polarization splitter 200 further includes a second optical waveguide 115 on the substrate 120 separated by gap from the first optical waveguide 110. In an embodiment of the invention, the gap is vertically situated between the first optical waveguide and the second optical waveguide. Similarly, in another embodiment of the invention, the gap 105 is horizontally situated between the first optical waveguide and the second optical waveguide. One of ordinary skill in the art will readily appreciate that when the first optical waveguide is located at a distance from the second optical waveguide, there is an air gap 105 between the first optical waveguide 110 and the second optical waveguide 115, the air gap having a non-zero width. One of ordinary skill in the art will readily appreciate that when the first optical waveguide abuts the second optical waveguide, the air gap 105 between the first optical waveguide 110 and the second optical waveguide 115 has no width. In an embodiment of the invention, the first optical waveguide 110 and the second optical waveguide 115 are identical waveguides having identical effective refractive indices. For example, the first optical waveguide 110 and the second optical waveguide 115 being such identical waveguides enables stronger optical coupling than if, for example, they had different effective refractive indices. The first optical waveguide 110 and the second optical waveguide 115 includes at least one coupling condition. The polarization splitter 200 also includes a MEMS actuator 140 moving the first optical waveguide 110 and/or the second optical waveguide 115 so as to control the size of the gap, thereby controlling the at least one coupling condition. Controlling the at least one coupling condition controls polarization splitting of a propagating optical mode between the first optical waveguide 110 and the second optical waveguide 115. For example, an incoming light signal enters the first optical waveguide 110, such as shown in FIG. 8A. By controlling the size of the gap between the first optical waveguide 110 and the second optical waveguide 115, the incoming light signal is split into a TE-polarized light signal in the first optical waveguide and a TM-polarized light signal in the second optical waveguide. Alternatively, in another embodiment of the invention, by controlling the size of the gap between the first optical waveguide 110 and the second optical waveguide 115, the incoming light signal is split into a TE-polarized light signal in the second optical waveguide and a TM-polarized light signal in the first optical waveguide.

[0070] Optionally, the first optical waveguide 110 includes silicon, SiN, a dielectric, or a standard compound semiconductor, and the second optical waveguide 115 includes silicon, SiN, a dielectric, or the compound semiconductor.

[0071] Optionally, the polarization splitter 200 further includes a plurality of MEMS anchors connected to the MEMS actuator and a plurality of MEMS tethers respectively connecting the plurality of MEMS anchors to the first optical waveguide and/or the second optical waveguide. Optionally, at least one MEMS tether of the plurality of MEMS tethers comprises at least one etch hole. Optionally, the etch hole selectively underetches the at least one MEMS tether, thereby suspending the second optical waveguide above said first optical waveguide. The MEMS anchors and/or MEMS tethers for the polarization splitter 200 are shown by way of illustration in FIG. 9.

[0072] Optionally, the first optical waveguide 110 includes silicon, silicon nitride, a dielectric, or a compound semiconductor, and the second optical waveguide 115 includes one of silicon, silicon nitride, the dielectric, or the compound semiconductor.

[0073] Optionally, the MEMS actuator 140 includes a standard electrostatic actuator 142, a standard gradient electric force actuator 144, a standard gradient optical force actuator 146, a standard piezo-actuator 148, or a standard electro-thermal actuator 149.

[0074] Optionally, the first optical waveguide 110 and the second optical waveguide 115 include at least one propagating optical mode. The at least one propagating mode includes a transverse-electric polarization or a transverse-magnetic polarization and a respective at least one mode order. The at least one coupling condition corresponds to the respective at least one propagating mode.

[0075] Optionally, the first optical waveguide 110 and the second optical waveguide 115 include at least one wavelength of operation between ultraviolet (UV) to visible to infrared (IR). One of ordinary skill in the art will readily appreciate that wavelength of operation of the first optical waveguide 110 and the second optical waveguide 115 is determined by the wavelength transparency of the materials chosen for the first optical waveguide and the second optical waveguide.

[0076] Another embodiment of the invention includes a polarization rotator 100 and is described as follows with respect to FIGS. 1-6E. The polarization rotator 100 includes 1) an input optical mode, e.g. a TE-polarized fundamental mode (TE.sub.0); an optical waveguide 110 supporting only fundamental modes of both polarizations (TE.sub.0 and TM.sub.0); a MEMS dielectric perturber 130 suspended above the waveguide 110 via a gap 105 and situated so that the MEMS dielectric perturber 130 covers only part of the width of the optical waveguide 110; a MEMS-induced symmetry-breaking effective index tuning of the waveguide across its width that induces a polarization rotation of a propagating optical mode; and a MEMS actuator 140 that enables the MEMS-waveguide gap to be varied to tune the interaction and to tune the polarization rotation angle by tuning the n.sub.eff of the optical mode in the narrow waveguide.

[0077] The MEMS dielectric perturber 130 locally tunes an effective index of a propagating waveguide mode. In an embodiment of the invention, actuation of the MEMS dielectric perturber is accomplished using a standard low-power electrostatic actuator 142. In another embodiment of the invention, actuation of the MEMS dielectric perturber is accomplished using a standard low-power gradient electric force actuator 144, 146, such as described in Marcel W. Pruessner, Dmitry A. Kozak, Nathan F. Tyndall, William S. Rabinovich, Venkatesh Deenadayalan, Michael L. Fanto, Stefan F. Preble, and Todd H. Stievater, Foundry-Processed Optomechanical Photonic Integrated Circuits, OSA Continuum 4 (4) 1215-1222 (2021), which is incorporated herein by reference. The MEMS deflection tunes the interaction between the MEMS and waveguide mode to modify the n.sub.eff. The width of the MEMS dielectric perturber 130 covering less than the width of the waveguide 110, for example, only half of the width of the waveguide, enables symmetry-breaking MEMS perturbation, which in turn enables polarization rotation.

[0078] In operation according to an embodiment of the invention, a TE.sub.0-mode is launched into the combined MEMS-waveguide structure. In an embodiment of the invention, for the MEMS-waveguide gap=0 nm case (i.e. when the MEMS structure rests on the waveguide), there is complete polarization conversion from TE.sub.0 to TM.sub.0, i.e. the TM.sub.0-overlap is maximized, and the TE.sub.0-overlap is minimized. As the MEMS-waveguide gap is increased, the polarization rotation decreases, and the TE.sub.0-overlap is maximized while the TM.sub.0-overlap is minimized. As the MEMS-waveguide gap decreases to 0 nm, there is a greater than 20 dB polarization extinction for TE-to-TM rotation. Similarly, as the MEMS-waveguide gap increases, the output becomes increasingly TE with >20 dB polarization extinction.

[0079] A polarization rotator according to an embodiment of the invention is exceptionally compact, for example, requiring a MEMS polarization rotator length of only l.sub.MEMS=4.1 m. However, for complete 90 polarization rotation (i.e., TE.sub.0-to-TM.sub.0 conversion) at MEMS-waveguide gap=0 nm, the insertion loss is greater than 3 dB. As the MEMS moves further away from the waveguide, the insertion loss approaches zero; in other words, as the MEMS-waveguide gap increases to infinity, the combined MEMS-waveguide structure behaves more and more akin to a standalone waveguide that supports both TE.sub.0 and TM.sub.0 modes but with no change in the polarization between the input and the output of the waveguide.

[0080] Another embodiment of the invention include an apparatus and is described as follows with respect to FIGS. 3A-3D and 4A-4D. This alternative embodiment of the invention includes components of the apparatus described above and shown in FIG. 1, albeit with the following modifications. This alternative embodiment of the invention includes a slanted micro-electro-mechanical systems dielectric perturber, for example, suspended above the waveguide via a gap 105 and situated so that it covers between 0 and 100% of the width of the optical waveguide. This alternative embodiment of the invention further includes an output MEMS width taper that enables a low-loss transition between the MEMS-waveguide polarization rotation region and the output waveguide. In an embodiment of the invention, the output MEMS width taper is actuated, i.e., moved by a standard actuator, so as to vary a gap distance between the output MEMS width taper and the optical waveguide. In an embodiment of the invention, the output MEMS width taper is actuated by the same actuator that actuates the MEMS dielectric perturber. In another embodiment of the invention, the output MEMS width taper is actuated by its own actuator, i.e., one distinct from the actuator that actuates the MEMS dielectric perturber. In another embodiment of the invention, the output MEMS width taper is a fixed structure such that the gap distance between the output MEMS width taper and the optical waveguide is fixed.

[0081] The slanted MEMS structure enables an adiabatic transition to the polarization rotation segment compared to an abrupt transition, such as shown in the apparatus of FIGS. 1B and 3. Additionally, the MEMS width taper at the output enables a low-loss transition between the polarization rotation segment and the output waveguide. For example, in a silicon-based embodiment of the invention including a l.sub.MEMS=10.3 m slanted MEMS length and a l.sub.taper=20 m MEMS width taper length and at a MEMS-waveguide gap of 0 nm, there is complete polarization rotation with an insertion loss of only 0.25 dB and a polarization extinction of 15 dB. At a MEMS-waveguide gap of 18 nm, there is no polarization rotation, i.e., an input TE.sub.0 mode remain TE.sub.0. One of ordinary skill in the art will readily appreciate that embodiments of the invention including different materials, such as silicon nitride, have different device lengths and/or geometries. Although the insertion loss is 1.2 dB for the no polarization case (gap=18 nm), the insertion loss is, for example, essentially zero with further increase in the MEMS-waveguide gap because as the MEMS-waveguide gap increases to infinity, the combined MEMS-waveguide structure behaves more and more akin to a standalone waveguide (i.e., one with no associated MEMS dielectric perturber). This alternative embodiment of the invention enables continuous polarization rotation angle tuning over 0-90. This alternative embodiment of the invention therefore exhibits both complete tenability and low insertion loss.

[0082] In an another embodiment of the invention, such as shown by way of illustration in FIGS. 5A and 5B, the polarization rotator includes standard tethers and standard anchors for support of the MEMS dielectric perturber.

[0083] In another embodiment of the invention, the polarization rotator includes standard tethers and standard anchors for support of the MEMS dielectric perturber. In this embodiment of the invention, polarization rotator further includes etch holes, whereby the overlap of the optical mode with the tether is reduced, and the optical loss is minimized.

[0084] The MEMS structures are, for example, actuated using standard gradient electric forces. Briefly, the suspended MEMS structures are modified to include two metal electrodes: one on either side of the waveguide. If the metal electrodes are at the same level as the waveguide (and hence below the MEMS structure), then application of a voltage across the two electrodes generates an electric field that interacts with the MEMS structure to generate a gradient electric force. This gradient electric force has a primarily vertical component that actuates the MEMS structure down towards the waveguide and reduces the gap 105.

[0085] Applicants recognized that, beyond polarization rotation, MEMS-waveguide structures can also be used for tunable polarization splitting similar to the fixed polarization splitting of the polarization rotator 100. Instead of a slanted MEMS dielectric perturber, polarization splitter 200 includes a directional coupler, which in turn includes two identical and moveable silicon waveguides 110, 115, as shown by way of illustration in FIGS. 7-9. One of ordinary skill in the art will readily appreciate that standard materials other than silicon are used in alternative embodiments of the invention. The separation or gap 105 between the waveguides 110, 115 are, for example, varied using one or more MEMS actuators 140 to tune the coupling condition. However, because TE- and TM-polarized light have different mode effective indices and mode confinement, there is a polarization-dependence to the coupling condition in addition to a gap-dependence. For example, beam propagation method (3D-BPM) simulations show that actuating the MEMS from gap=65 nm to 100 nm results in a TE-polarized signal being switched from BAR to CROSS. Similarly, a TM-signal will be switched from CROSS to BAR when gap=65 nm.fwdarw.100 nm. This reconfigurability indicates that an embodiment of the invention including the polarization splitter 200 enables continuous tuning of the polarization splitting as the gap between the waveguides is varied.

[0086] One or more embodiments of the invention include silicon waveguides and silicon MEMS structures for symmetry-breaking and polarization rotation. Silicon has a large refractive index (n.sub.Si=3.45 at =1550 nm), which makes the optical mode tightly-confined for the waveguide geometry considered (t.sub.Si=220 nm and w.sub.Si=450 nm). The tight optical confinement requires that the silicon MEMS dielectric perturber be brought into close proximity to the optical waveguide mode in order to effect polarization rotation. Indeed, complete tuning of the polarization state from 0-90 occurs for MEMS actuation over gap=0-20 nm. By moving to materials with a lower refractive index such as silicon nitride (n.sub.Si3N42.0), the optical mode will be less confined for similar waveguide dimensions. The lower confinement in turn means that any MEMS structure will start interacting with the optical mode at larger MEMS-waveguide gap so that polarization rotation will occur at larger distances. In practice, this benefit may ease the operation requirements and allow for a greater tolerance when actuating the MEMS structure to achieve a given polarization rotation. In addition to materials, adjusting the waveguide geometry can also modify the actuation distance (gap) that is required to achieve a desired polarization rotation. By increasing the polarization rotation length a weaker symmetry-breaking MEMS perturbation (i.e. larger gap) can still enable large polarization rotation. Finally, both material and geometry can also be used to design polarization rotators for specific wavelengths of operation. In particular, materials such as InP, GaAs, AlN, LiNbO3 and others may enable the MEMS polarization rotators to be coupled to electro-optic or nonlinear optical devices on-chip. Other materials such as Ge may enable the MEMS polarization rotators to operate at mid- or longwave infrared (MWIR or LWIR) wavelengths.

[0087] One or more embodiments of the invention include a standard electrostatic actuator, a gradient electric force actuator, or other standard actuator. An example of another standard actuator is a standard piezo-actuator. For such a piezo-actuator, although the displacement is generally small, fairly large strains can be induced using low voltages. The fairly large strains can be harnessed to enable large actuation distances (100's nm), as required in one or more embodiments of the present invention. Therefore, in an embodiment of the invention, piezo-actuation enables even lower voltage operation than MEMS n.sub.eff tuning devices.

[0088] An additional example of a standard actuator is a standard optical force actuator. The optical force actuator, for example, employs one of three optical forces that can be applied to the above-mentioned MEMS structures: 1) photothermal forces, in which light-induced heating causes deformation and actuation, 2) radiation pressure forces, in which the momentum of light reflected off a surface, e.g. a mirror, transfers photon momentum and results in an actuation force, and 3) gradient optical forces, in which changing the direction of light near a materials interface (i.e. focusing using a lens) can lead to a momentum change and an induced force. These optical forces are generally small, although they can have substantial effects on the dynamics of MEMS oscillators. For some applications, optical forces are useful for all-optical PICs because they do not require an external (electrical) source and any required laser light for actuation can be supplied via optical fiber.

[0089] One or more embodiments of the invention focus primarily on polarization rotation to convert an input signal with arbitrary polarization state into an on-chip signal with known polarization (i.e. TE- or TM-polarization). That is, the rate of polarization rotation is slow assuming the polarization state of the input signal remains constant or is only slowly-varying. For some applications, e.g. quantum key distribution, however, it is desirable to modulate the polarization of an optical signal. MEMS actuation enables MHz-rate modulation rates (potentially 10's-100 MHz) of the polarization. Therefore, an embodiment of the present invention find applications in quantum key distribution for secure communication channels.

[0090] Although a particular feature of the disclosure may have been illustrated and/or described with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Also, to the extent that the terms including, includes, having, has, with, or variants thereof are used in the detailed description and/or in the claims, such terms are intended to be inclusive in a manner similar to the term comprising.

[0091] As used herein, the singular forms a, an, and the do not preclude plural referents, unless the content clearly dictates otherwise.

[0092] As used herein, the term and/or includes any and all combinations of one or more of the associated listed items.

[0093] As used herein, the term about when used in conjunction with a stated numerical value or range denotes somewhat more or somewhat less than the stated value or range, to within a range of 10% of that stated.

[0094] All documents mentioned herein are hereby incorporated by reference for the purpose of disclosing and describing the particular materials and methodologies for which the document was cited.

[0095] Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departing from the spirit and scope of the invention. Terminology used herein should not be construed as being means-plus-function language unless the term means is expressly used in association therewith.

[0096] This written description sets forth the best mode of the invention and provides examples to describe the invention and to enable a person of ordinary skill in the art to make and use the invention. This written description does not limit the invention to the precise terms set forth. Thus, while the invention has been described in detail with reference to the examples set forth above, those of ordinary skill in the art may effect alterations, modifications and variations to the examples without departing from the scope of the invention.

[0097] These and other implementations are within the scope of the following claims.