CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of United States Provisional patent Applications Serial Nos. 62/985,125, filed March 4, 2020, and 63/075,627, filed September 8, 2020. Both of these applications are herein incorporated by reference in their entireties.

REFERENCE TO GOVERNMENT FUNDING

[0002] This invention was made with Government support under Grant No. 1827633, awarded by the National Science Foundation (NSF), Grant No. HR0011-19-2-0015, awarded by the Department of Defense/Defense Advanced Research Projects Agency (DOD/DARPA), and Grant No. DE-AR0000849, awarded by the Department of Energy Advanced Research Projects Agency- Energy (ARPA-E). The Government has certain rights in this invention.

FIELD OF THE DISCLOSURE

[0003] The present invention generally relates to optics, and more specifically relates to optical mode converters for efficiently coupling the light from a single mode optical fiber to an integrated waveguide.

BACKGROUND

[0004] Single-mode optical fibers have an effective refractive index of approximately 1.45 and a mode field diameter of approximately 10pm. On the other hand, integrated waveguides (such as silicon nitride or silicon-on-insulator waveguides) have an effective refractive index of approximately 2 or higher and a mode field diameter of less than 1 pm. The mismatch between the effective refractive indexes and mode sizes of the single-mode optical fibers and the integrated waveguides induces significant coupling losses if the integrated waveguides are directly coupled to the single-mode optical fibers. Thus, optical mode converters (also referred to as optical spot size converters or optical mode couplers) are typically used to couple light efficiently between single-mode optical fibers and integrated waveguides. SUMMARY OF THE DISCLOSURE

[0005] In one example, an apparatus includes a waveguide taper formed on a first substrate. The waveguide taper includes a first end to couple to an integrated waveguide having a first effective refractive index and a second end that is tapered in thickness and in width, the second end having a second effective refractive index that is lower than the first effective refractive index. The apparatus further includes an interposer waveguide formed on a second substrate, wherein a first end of a waveguide core of the interposer waveguide is positioned closely enough to the waveguide taper to allow for evanescent coupling of light between the interposer waveguide and the waveguide taper, and a single-mode optical fiber coupled to a second end of the waveguide core of the interposer waveguide.

[0006] In another example, an apparatus includes a waveguide taper formed on a first substrate. The waveguide includes a first end to couple to an integrated waveguide having a first effective refractive index and a second end that is tapered in thickness and in width, the second end having a second effective refractive index that is lower than the first effective refractive index. The apparatus further includes a single-mode optical fiber having a first end that is coupled to the waveguide taper that allows for evanescent coupling between the waveguide taper and the single-mode optical fiber. The single mode optical fiber includes a core and a cladding surrounding the core. A first end of the single mode optical fiber is polished at an angle to gradually expose the core.

[0007] In another example, an apparatus includes a plurality of waveguide tapers positioned along an optical axis on a first substrate. Each waveguide taper of the plurality of waveguide tapers includes a first end to couple to a respective first waveguide having a first effective refractive index and a second end having a second effective refractive index that is lower than the first effective refractive index. The apparatus further includes an interposer waveguide on a second substrate, wherein a first end of the interposer waveguide overlaps with the plurality of waveguide tapers and wherein a core of the waveguide taper is within a proximity to the plurality of waveguide tapers that allows for evanescent coupling of light between the interposer waveguide and the plurality of waveguide tapers, and a single-mode optical fiber coupled to a second end of the interposer waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The teachings of the present disclosure can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:

[0009] FIG. 1 illustrates a top view of an example of a device using evanescent couplers of the present disclosure to couple light between optical fiber arrays and integrated waveguide arrays;

[0010] FIG. 2 illustrates a cross sectional view of the device illustrated in FIG.

1 ;

[0011] FIGs. 3A and 3B illustrate details of the device of FIGs. 1 and 2 in greater detail;

[0012] FIGs. 4A-4C illustrate an example device in which the optical interposer couples to a single-mode optical fiber and to an integrated waveguide built on silicon photonics technology;

[0013] FIGs. 5A and 5B illustrate simulations of the evolution of the mode shape from the optical interposer of FIGs. 4A-4C to the silicon waveguide taper; [0014] FIGs. 6A and 6B illustrate an example device in which the optical interposer couples to a single-mode optical fiber and to a silicon nitride integrated waveguide formed on a silicon substrate with silicon dioxide;

[0015] FIGs. 7A and 7B illustrate simulations of the evolution of the mode shape from the optical interposer of FIGs. 6A-6B to the silicon nitride waveguide through the silicon nitride taper;

[0016] FIG. 8 illustrates a side view of an alternative example of the device of FIG. 6, in which the silicon nitride waveguide core of the integrated waveguide comprises a two-step taper;

[0017] FIG. 9A illustrates a side view of an example device in which the top cladding of the optical interposer is etched back to expose an evanescent optical field;

[0018] FIG. 9B illustrates a side view of a further example of the device of FIG. 9A, in which the optical interposer additionally includes a routing layer; [0019] FIGs. 10A and 10B illustrate simulations of the evolution of the mode shape from the optical interposer of FIG. 9A to the silicon photonics waveguide core of the integrated waveguide;

[0020] FIG. 11 illustrates a top view of an example of a device using evanescent couplers with polarization diversity;

[0021] FIG. 12 illustrates a cross sectional view of the device of FIG. 11 ;

[0022] FIGs. 13A and 13B illustrate top views of an example of the waveguide tapers of the device illustrated in FIGs. 11 and 12;

[0023] FIG. 14 illustrates a simulation of an optical mode converter similar to the optical mode converter illustrated in FIG. 13B;

[0024] FIG. 15 illustrates a simulation of a waveguide taper performed at a wavelength of approximately 1310nm;

[0025] FIGs. 16A and 16B illustrate simulated field propagation of the TE mode and TM mode, respectively for the first waveguide taper and the second waveguide taper of FIGs. 11 and 12;

[0026] FIG. 17 illustrates a simulation of the tolerance of the device of FIGs. 11 and 12 to manufacturing variation in adhesive thickness;

[0027] FIG, 18 illustrates a simulation of the tolerance of the device of FIGs. 11 and 12 to manufacturing variation in taper thickness;

[0028] FIGs. 19A and 19B illustrate an example of an optical interposer waveguide that includes at least one two-dimensional waveguide taper formed by thermal oxidation of a silicon waveguide;

[0029] FIGs. 20A-20B illustrate an example fabrication process for a two- dimensional waveguide taper;

[0030] FIGs. 21A-21 B illustrate cross sectional views of a waveguide taper created according to the thermal oxidation process illustrated in FIGs. 20A-20D; [0031] FIG. 22 illustrates the measured optical insertion loss of an optical interposer waveguide with a thermally oxidized two-dimensional waveguide taper for transverse electric and transverse magnetic polarizations; and [0032] FIGs. 23A-23B illustrate an example device that employs evanescent coupling between a two-dimensional waveguide taper and an angle-polished single-mode optical fiber. DETAILED DESCRIPTION

[0033] In one example, the present disclosure provides evanescent coupler mode converters. As discussed above, the mismatch between the effective refractive indexes and mode sizes of single-mode optical fibers and integrated waveguides induces significant coupling losses if the integrated waveguides are directly coupled to the single-mode optical fibers. As such, optical mode converters are typically used to couple light efficiently between single-mode optical fibers and integrated waveguides.

[0034] An optical mode converter comprises a waveguide that gradually changes the effective refractive index and mode size from the single-mode optical fiber to the integrated waveguide. Since the optical mode converter starts with a larger mode size to match the fiber, the optical mode converter often requires a cladding thickness of at least 20pm. In some cases (for example when part of the cladding on the device is to be etched to fabricate micro electro-mechanical system (MEMS) structures), it may not be desirable to fabricate such a thick cladding directly on the integrated waveguide substrate. In these cases, the optical mode converter can be fabricated on a different substrate. The optical mode converter is typically coupled to the single-mode optical fiber using end- face coupling. The optical mode converter is typically coupled to the integrated waveguide using end-face coupling or evanescent coupling. An evanescent coupling scheme may be used when low optical loss and large tolerance to misalignment are desired. Herein, the portion of the system positioned between the waveguide tapers and the single-mode optical fiber is referred to as an “optical interposer.”

[0035] Examples of the present disclosure provide an evanescent coupler that includes a waveguide taper whose thickness and width vary progressively in order to enhance evanescent coupling between the waveguide taper and the optical interposer, or between the waveguide taper and an angle-polished fiber. Although existing waveguide tapers sometimes vary the width, they do not vary the thickness; as a result, very sharp taper tips are typically required to achieve high coupling efficiency. The required sharp taper tips require advanced lithography and have tight fabrication tolerances. The two-dimensional waveguide taper of the present disclosure enables high coupling efficiency even with relatively large taper tips (e.g., as wide as 250 nm in a silicon-on-insulator waveguide).

[0036] In one example, a first end of the optical interposer is coupled to the single-mode optical fiber using end-face coupling. A second end of the optical interposer is configured to couple to the two-dimensional waveguide taper using coupling from evanescent fields. In one example, the optical interposer is fabricated separately from the integrated waveguide, and then flip chip bonded to the integrated waveguide using an adhesive or molecular bonding.

[0037] Further examples of the optical mode converter disclosed herein include different optical polarizations coupled into separate waveguides. For instance, the optical interposer may include a plurality of waveguide tapers positioned along the same optical axis (e.g., approximately, but not necessarily perfectly, aligned with the optical axis), where each taper of the plurality of waveguide tapers is designed to evanescently couple to a different mode or polarization of light, and, as such, each mode or polarization of light will be coupled to a different waveguide taper. The optical interposer in this example is coupled with evanescent fields in close proximity to an interposer waveguide including either a single waveguide or an array of waveguides. In some examples, an array of optical interposers may couple multiple channels, each including a plurality of waveguide tapers, on the same device.

[0038] A first end of the interposer waveguide has a mode size optimized to couple to the single-mode optical fiber using end-face coupling. A second end of the interposer waveguide couples to the waveguide tapers using coupling from evanescent fields.

[0039]

[0040] In one example, the evanescent coupler mode converter design comprises a multi-stage silicon photonic adiabatic coupler featuring polarization diversity. The multi-stage silicon photonic adiabatic coupler can split the polarization at the output of the evanescent coupler. In one example, the evanescent couple couples the transverse electric (TE) and transverse magnetic (TM) polarizations to different waveguide thicknesses at separate locations, and subsequently separates the TE and TM polarizations into two separate waveguides. [0041] Further examples of the disclosure provide a fabrication process for silicon evanescent couplers that does not require advanced (e.g., sub-250 nm) lithography. The waveguide taper dimensions may be controlled using a thermal oxidation process, which is easy to fabricate and can be adapted to different silicon photonics processes (including, for instance, silicon photonics MEMS processes).

[0042] All examples of the disclosed evanescent coupler optical mode converter may be optimized for minimal optical loss over a broad wavelength range. Typical nominal wavelengths can be 1.31 pm or 1.55pm. The typical required bandwidth can be at least 100nm. The evanescent coupler design supports both the TE and TM polarizations, without inducing significant coupling between the polarizations or between the fundamental and higher order modes. [0043] FIG. 1 illustrates a top view of an example of a device 100 using evanescent couplers (optical interposers) of the present disclosure to couple light between optical fiber arrays and integrated waveguides. In the example illustrated in FIG. 1 , the device 100 comprises an integrated waveguide 102, an array 104 of single-mode optical fibers, and an array 106 of optical interposer waveguides. [0044] In one example, the integrated waveguide 102 may comprise a silicon photonics chip comprising silicon integrated waveguides with different functionalities. The array 104 of single-mode optical fibers may be attached to a plurality of single-mode waveguides fabricated on a PLC substrate 108. Each of the single-mode optical fibers may be attached to the PLC substrate 108 on a V- groove fiber array.

[0045] The array 106 of optical interposer waveguides may be coupled to the array 104 of single-mode optical fibers. In one example (e.g., where light is to be coupled in or out of different sides of the integrated waveguide 102), the array 106 of optical interposer waveguides may comprise a plurality of optical interposer waveguides on a single device. The optical interposers may also perform pitch reduction from 127pm fiber spacing to a smaller spacing on the integrated waveguide 102.

[0046] FIG. 2 illustrates a cross sectional view of the device 100 illustrated in FIG. 1. As shown in FIG. 2, in one example, each individual optical interposer waveguide 112 of the array 106 of optical interposer waveguides may comprise a waveguide, where the waveguide comprises a waveguide core 116 and a surrounding cladding 118. A portion of the cladding 118 is tapered and etched back to expose the evanescent fields of the optical interposer waveguides close to the waveguide core 116 of optical interposer waveguide 112. For instance, the cladding 118 may be tapered in a portion of the cladding 118 which is on the top of the optical interposer waveguide 112 when the optical interposer waveguide 112 is fabricated, but on the bottom of the optical interposer waveguide 112 when the optical interposer waveguide 112 is assembled as illustrated in FIG. 2.

[0047] In one example, the optical interposer waveguide 112 may be physically attached to a a waveguide taper 102 using adhesive 114 (e.g., epoxy). The adhesive 114 may serve as additional cladding for the optical interposer waveguide 112, and the refractive index of the adhesive 114 may be chosen to match the refractive index of the cladding 118.

[0048] The integrated waveguide 102 in this example may be fabricated using silicon photonics technology, where a silicon waveguide core 120 is etched on top of a buried oxide (BOX) 122. The silicon waveguide core 120 tapers in the coupler region to perform gradual mode conversion between the low effective index of the optical interposer waveguide 112 and the higher effective index inside the silicon waveguide core 120. At least one mechanical bump stop 124 may be fabricated on the silicon waveguide core 120 to generate a constant bond gap when the device 100 is assembled.

[0049] FIGs. 3A and 3B illustrate details of the device 100 of FIGs. 1 and 2 in greater detail. In particular, FIG. 3A illustrates a top view of the device 100, while FIG, 3B illustrates a side view of the device 100.

[0050] As discussed above, the array 106 of optical interposer waveguides may be fabricated on a PLC substrate 108. In one example, the PLC substrate 108 may have an index contrast of approximately 1 .5 percent, and the waveguide core 116 of the optical interposer waveguide 112 may have a thickness of approximately 3.4pm. The waveguide core 116 of the optical interposer waveguide 112 includes a first end to couple to the single-mode optical fiber 110 and a second end to couple to the silicon waveguide taper 120 of the integrated waveguide 102. The waveguide taper 120 illustrated in FIGs. 3A and 3B is tapered in both thickness and width; this tapering in two dimensions helps the device 100 achieve high coupling efficiency.

[0051] In one example, the waveguide core 116 of the optical interposer waveguide 112 may be tapered to increase in width and/or thickness in a direction towards the first end of the waveguide core/ edge of the single-mode optical fiber 110 (or, put another way, tapered to narrow in width and/or thickness in a direction away from the second end of the waveguide core 116/integrated waveguide 102) to better match the fiber mode and improve the coupling efficiency. The cladding 118 forms a taper where the cladding 118 is etched back to expose the waveguide core 116 of the optical interposer waveguide 112. The thickness of the remaining cladding 118 in this example may be approximately 2pm.

[0052] The taper of the cladding 118 is designed to prevent unwanted reflections from the abrupt change in cladding thickness. In one example, the cladding 118, like the waveguide core 116, may be tapered to increase in width and thickness in a direction towards the first end of the waveguide core/edge of the single-mode optical fiber 110 (or, put another way, tapered to narrow in width and thickness in a direction away from the second end of the waveguide core 116/integrated waveguide 102). In one example, the cladding 118 directly contacts the waveguide taper 120 of the integrated waveguide 102.

[0053] As discussed above, the optical interposer waveguide of the present disclosure can couple to different integrated waveguide technologies, including silicon and silicon nitride (and other semiconductor and dielectric material) photonics. FIGs. 4A-4C, for instance, illustrate an example device 400 in which the optical interposer waveguide 412 (including a waveguide core 416) couples to a single-mode optical fiber 410 and to an integrated waveguide taper 402 built on silicon photonics technology. Specifically, FIG. 4A illustrates a cross sectional view of the device 400, while FIG. 4B illustrates a top view of the device 400, and FIG. 4C illustrates a side view of the integrated waveguide taper 402.

[0054] In one example, the waveguide core 420 of the integrated waveguide taper 402 comprises a two-level tapered structure which is optimized to improve the coupling efficiency to both TE and TM polarizations. The mechanical bump stops 424 are etched in the full thickness of the integrated waveguide’s core, which may have a thickness of approximately 220 nm in one example. In one example, the total length, of the tapered waveguide core 420 of the integrated waveguide taper 402 is optimized to be less than 1 mm. The mechanical bump stops 424 may be spaced to leave room for adhesive to flow between the waveguides of the array of integrated waveguides during the attachment of the optical interposer waveguide 412 to the integrated waveguide taper 402.

[0055] FIGs. 5A and 5B illustrate simulations of the evolution of the mode shape from the optical interposer waveguide 412 to the silicon waveguide core 420 of the integrated waveguide taper 402. In particular, FIG. 5A illustrates the evolution of the mode shape for the TE polarization, while FIG. 5B illustrates the evolution of the mode shape for the TM polarization. In both FIG. 5A and FIG. 5B, the side view cross section of the light intensity propagates from the single-mode optical fiber on the left to the integrated waveguide taper on the right. As illustrated, the coupling efficiency is greater than ninety-four percent for both polarizations.

[0056] As discussed above, the optical interposer waveguide of the present disclosure can also couple light to a silicon nitride integrated waveguide formed on a silicon substrate with S1O2 cladding. FIGs. 6A and 6B, for instance, illustrate an example device 600 in which the optical interposer waveguide 612 (including a waveguide core 616) couples to a single-mode optical fiber 610 and to a silicon nitride integrated waveguide formed on a silicon substrate 622 with silicon dioxide. Specifically, FIG. 6A illustrates a cross sectional view of the device 600, while FIG. 6B illustrates a side view of the device 600. The bump stops 624 in this example may have a thickness of approximately 580nm, and the cladding 618 surrounding the waveguide core 616 of the optical interposer waveguide 612 may be etched to a thickness t of approximately 1 pm. The silicon nitride waveguide core 620 of the integrated waveguide taper may be tapered similarly to the silicon example of FIG. 4, and the integrated waveguide taper may have a length of approximately 1 mm.

[0057] The design of the taper length for the waveguide core 620 of the integrated waveguide taper and the thickness of the cladding 618 may be optimized using a propagation simulation technique such as eigenmode expansion. An eigenmode expansion simulation calculates the coupling efficiency from the input at the single-mode optical fiber to the integrated waveguide, and vice-versa.

[0058] FIGs. 7 A and 7B illustrate simulations of the evolution of the mode shape from the optical interposer waveguide 612 to the silicon nitride waveguide 620 through the silicon nitride taper. In particular, FIG. 7A illustrates the evolution of the mode shape for the TE polarization, while FIG. 7B illustrates the evolution of the mode shape for the TM polarization. In both FIG. 7A and FIG. 7B, the side view cross section of the light intensity propagates from the single-mode optical fiber on the left to the integrated waveguide on the right. As illustrated, the coupling efficiency is greater than ninety-nine percent for both polarizations. [0059] FIG. 8 illustrates a side view of an alternative example of the device 600 of FIG. 6, in which the silicon nitride waveguide core 620 of the integrated waveguide taper comprises a two-step taper (i.e., including a first step 800 and a second step 802 that is etched further back than the first step), e.g., as opposed to a continuous taper as illustrated in previous examples.

[0060] In another example, the low index contrast waveguide core of the optical interposer waveguide may be created using thin layers of higher refractive index materials (e.g., silicon nitride) to create an effective material which has a low index contrast. The thickness, width, and number of the thin layers may be optimized to match the mode of the single-mode optical fiber. For example, the low index contrast waveguide core of the optical interposer waveguide may include two or more layers having thicknesses between 15nm and 25nm and widths between 6pm and 8pm . This structure can be optimized to match both the TE and TM polarized modes of the single-mode optical fiber, with efficiency greater than ninety-five percent. This structure can also be easily fabricated using planar microfabrication, since the thin high refractive index materials do not create a large topography on the device surface.

[0061] The top and bottom cladding of the low index contrast waveguide core of the optical interposer waveguide may be formed from a material with a refractive index similar to the refractive index of the cladding of the single-mode optical fiber, such as S1O2. In one example, the top and bottom cladding may have thicknesses of at least 5pm. The cladding layers and the thin layers of high refractive index material may be deposited on a substrate compatible with microfabrication processes, such as a silicon wafer. The thickness of the bottom cladding may be chosen to minimize the leakage of light to the substrate. The refractive index profile of the bottom cladding may be varied to further reduce the leakage of light to the substrate. For example, a first layer of bottom cladding with a refractive index may be deposited, and, subsequently, a second layer of bottom cladding with a refractive index r\2 may be deposited (where ni<n2). [0062] FIG. 9A illustrates a side view of an example device 900 in which the top cladding 918 of the optical interposer waveguide is etched back to expose an evanescent optical field. In one example, the waveguide core of the integrated waveguide 902 comprises silicon or silicon nitride, similar to the examples discussed above.

[0063] FIG. 9B illustrates a side view of a further example of the device 900 of FIG. 9B, in which the optical interposer waveguide additionally includes a routing layer 930. In one example, the routing layer 930 may comprise another thin layer of silicon nitride (e.g., having a thickness between 100nm and 200nm) to bring the evanescent field close to the waveguide core of the integrated waveguide 902. FIG. 9B, similar to FIG. 9A, also illustrates an etched back top cladding 918. However, in other examples, the routing layer 930 may be implemented without etching back the cladding 918.

[0064] FIGs. 10A and 10B illustrate simulations of the evolution of the mode shape from the optical interposer waveguide of FIG. 9A to the silicon photonics (or silicon nitride) waveguide core of the integrated waveguide 902. In particular, FIG. 10A illustrates the evolution of the mode shape for the TE polarization, while FIG. 10B illustrates the evolution of the mode shape for the TM polarization. In both FIG. 10A and FIG. 10B, the side view cross section of the light intensity propagates from the single-mode optical fiber on the left to the integrated waveguide on the right. As illustrated, the coupling efficiency is greater than ninety-five percent for both polarizations.

[0065] As discussed above, further examples of the evanescent coupler include different optical polarizations coupled into separate waveguides. In this case, the evanescent coupler may include a plurality of waveguide tapers positioned along the same optical axis (e.g., approximately but not necessarily perfectly, aligned with the optical axis), where each taper of the plurality of waveguide tapers is designed to evanescently couple to a different mode or polarization of light, and, as such, each mode or polarization of light will be coupled to a different waveguide taper. The waveguide tapers in this example are coupled with evanescent fields in close proximity to the optical interposer waveguide comprising either a single waveguide core or an array of waveguide cores. In some examples, an array of optical interposer waveguides may couple multiple channels on the same device.

[0066] The optical interposer waveguide may be coupled to the single-mode optical fiber using end-face coupling, as discussed above. Either end-face coupling or evanescent coupling can be used to couple the interposer waveguide to the integrated waveguide. Polarization diverse devices may also require a polarization splitter-rotator (PS R), since many silicon photonic devices operate in only one polarization. In one example disclosed herein, a plurality of waveguide tapers are deployed on silicon and directly split and rotate the polarization, without the need for a PSR.

[0067] FIG. 11 illustrates a top view of an example of a device 1100 using an evanescent coupler 1102 with polarization diversity. FIG. 12 illustrates a cross sectional view of the device 1100 of FIG. 11. In the example illustrated in FIGs. 11 and 12, an evanescent coupler 1102 comprises a silicon photonics chip including a plurality of tapered silicon waveguides, or “waveguide tapers” 1104 (e.g., at least a first waveguide taper 1104i and a second waveguide taper 1104 ₂ ) with different thicknesses and functionalities and a cladding 1114 on at least one side of the waveguide tapers 1104. The evanescent coupler 1102 is coupled to an interposer waveguide 1106. An array 1108 of single-mode optical fibers is also coupled to the interposer waveguide 1106 and may comprise a plurality of single-mode optical fibers attached on a V-groove array. A plurality of interposer waveguide arrays can be used on a single device if light is to be coupled in or out of different sides of the silicon photonics chip 1102.

[0068] The interposer waveguide 1106 may be attached to the evanescent coupler 1102 using adhesive. In one example, the interposer waveguide 1106 comprises a waveguide core 1110 and a cladding 1112 surrounding the waveguide core 1110. The waveguide core 1110 of the interposer waveguide 1106 may be brought close to the surface to expose the evanescent fields of the interposer waveguide 1106. The portions of the waveguide tapers 1104 that are positioned in the region of the interposer waveguide 1106 (e.g., overlap with the interposer waveguide 1106) perform gradual mode conversion between the low effective index of the single-mode optical fibers in the array 1108 of single-mode optical fibers and the higher effective index inside the silicon waveguide tapers 1104.

[0069] In the example illustrated in FIGs. 11 and 12, the light is coupled from a single-mode fiber of the array 1108 of single-mode optical fibers using the glass interposer waveguide 1106 and evanescent coupling to the silicon evanescent coupler 1102. The silicon evanescent coupler 1102 in this example comprises a plurality of waveguide tapers 1104, where each waveguide taper 1104 gradually increases the effective index from the index of the glass interposer waveguide 1106 to the effective index of the silicon photonics waveguide taper 1104.

[0070] In one example, the silicon layer thickness is approximately 220nm, and the minimum width of the silicon due to lithography resolution is approximately 250nm, which can be readily achieved in most silicon photonic foundries. This results in the TE mode having a larger refractive index than the TM mode. In order to facilitate a mode crossing in the evanescent coupler 1102 for both the TE and the TM modes, the TE section of the evanescent coupler (i.e., including the first waveguide taper 1104i) is etched thinner than the TM section (i.e., including the second waveguide taper 1104 ₂ ). For example, the first waveguide taper 1104i may be etched to a thickness of approximately 50nm, while the second waveguide taper 1104 ₂ may be etched to a thickness of approximately 100nm. Moreover, by using separate waveguide tapers 1110 for the TE and the TM sections of the evanescent coupler, the need for a polarization splitter is removed.

[0071] In this example, the waveguide tapers 1104 are tapered in both thickness and width. This two-dimensional tapering of the waveguides enables high optical coupling efficiency without the need for sharp taper tips which typically must be manufactured in advanced fabrication facilities.

[0072] FIGs. 13A and 13B illustrate top views of an example of the waveguide tapers 1104 of the device 1100 illustrated in FIGs. 11 and 12. In the example shown in FIG. 13A, the thinner first waveguide taper 1104i is routed separately from the thicker second waveguide taper 1104 ₂ to separate the TE and TM polarizations to different waveguides. Since the first waveguide taper 1104i is thinner than the second waveguide taper 1104 ₂ , a step thickness transition is used at the output of the first waveguide taper 1104i . The step thickness transition comprises a first layer or step 1116i, and second layer or step 1116 ₂ formed on top of the first step 1116 ₁ . The first step 1116 ₁ is thinner than the second step 1116 ₂ .

[0073] Depending on the designed width of the second waveguide taper 1104 ₂ , and due to the wide aspect ratio of the second waveguide taper 1104 ₂ , the TM mode may be converted into a TE1 mode. This conversion performs polarization rotation. For instance, as illustrated in FIG. 13B, a TE1 to TE0 optical mode converter 1118 may be positioned at the output of the second waveguide taper 1104 ₂ to produce a TE0 mode at the output of the second waveguide taper 1104 ₂ . The TE1 to TE0 optical mode converter 1118 may comprise an adiabatic coupler. A TE0 mode is typically used in silicon photonics. Thus, the optical mode converter may function as a polarization rotator.

[0074] FIG. 14 illustrates a simulation of an optical mode converter similar to the optical mode converter 1118 illustrated in FIG. 13B. The TE1 mode in FIG. 14 is rotated from the TM mode.

[0075] In one example, the lengths of the of the waveguide tapers 1104 are determined using an eigenmode expansion simulation (e.g., Lumerical MODE) at approximately 1330 nm (O-band). It has been experimentally determined that the minimum length needed for the first waveguide taper 1104i to achieve greater than ninety-nine percent efficiency is approximately 300pm, and the minimum length needed for the second waveguide taper 1104 ₂ to achieve the same efficiency is approximately 1000pm.

[0076] FIG. 15 illustrates a simulation of a waveguide taper performed at a wavelength of approximately 1310nm. In particular, FIG. 15 plots taper length versus insertion loss for both the first waveguide taper 1104i and the second waveguide taper 1104 ₂ of FIGs. 11 and 12 at approximately 131 Onm.

[0077] FIGs. 16A and 16B illustrate simulated field propagation of the TE mode and TM mode, respectively for the first waveguide taper 1104i and the second waveguide taper 1104 ₂ of FIGs. 11 and 12. As shown in FIG. 16B, the field propagation of the TM mode exhibits polarization rotation from the TMO mode to the TE1 mode. As discussed above, the TE1 mode polarization can be converted at the output of the second waveguide taper 1104 ₂ to the TEO polarization using an adiabatic coupler (e.g., optical mode converter 1118 of FIG. 13).

[0078] FIG. 17 illustrates a simulation of the tolerance of the device 1100 of FIGs. 11 and 12 to manufacturing variation in epoxy thickness. In particular, FIG. 17 simulates the insertion loss (in dB) of the device 1100 as a function of the epoxy thickness between the waveguide core 1110 of the interposer waveguide 1106 and the waveguide tapers 1104. In one example, the epoxy thickness may be as great as approximately 2pm. In another example, the epoxy thickness is approximately 1 pm or less. In the example illustrated in FIG. 17, an epoxy thickness of between approximately 0pm and approximately 2pm appears to be optimal.

[0079] A +/-0.02 epoxy index variation and a 1550 nm (C-band) operation were also simulated with negligible impact on the performance of the device 1100. Assuming that the ion-diffused waveguide core 1110 of the interposer waveguide 1106 and the single-mode optical fiber interface have an insertion loss of 1 dB, this would bring the total insertion loss to -1.5 dB or better, which is similar to previously demonstrated evanescent coupler devices. An advantage of the disclosed device, however, is that a silicon nitride layer is not required, and the TE and TM polarizations are already split at the output of interposer waveguide 1106, as described above.

[0080] FIG. 18 illustrates a simulation of the tolerance of the device 1100 of FIGs. 11 and 12 to manufacturing variation in taper thickness. In particular, FIG. 8 shows the tolerance map for both the TE and TM waveguide taper thicknesses. The -0.5 dB tolerance is shown to be +/- 20 nm for both the TE and TM waveguide tapers.

[0081] Thus, some examples of the present disclosed optical interposer waveguide may be configured to split the TE and TM polarizations. Using different waveguide taper thicknesses, the TE and TM waveguide tapers may be optimized separately for optimal fabrication tolerances. The optical interposer waveguide may exhibit a better than -0.5 dB insertion loss with +/- 20 nm tolerance on the waveguide taper thickness, +/- 0.02 tolerance on the epoxy index, and up to 2pm epoxy thickness. Moreover, the TM waveguide taper also rotates the polarization from the TM mode to the TE1 mode, which can easily be converted to the TEO mode with an adiabatic coupler. The optical interposer waveguide is therefore promising for use in silicon photonics devices that require low insertion loss as well as polarization diversity.

[0082] Further examples of the disclosure provide a fabrication process for silicon evanescent couplers that does not require advanced (e.g., sub-250 nm) lithography. The waveguide taper dimensions may be controlled using a thermal oxidation process, which is easy to fabricate and can be adapted to different silicon photonics processes (including, for instance, silicon photonics MEMS processes).

[0083] Evanescent coupling works by adiabatically transferring a mode between two waveguides through their evanescent fields. In one example, to create access to the evanescent fields from the single-mode optical fiber, a commercially available glass interposer waveguide with ion-diffused waveguides (which can include both buried and surface waveguides) may be used in the optical interposer waveguide.

[0084] FIGs. 19A and 19B illustrate an example of an optical interposer waveguide 1900 that includes at least one two-dimensional waveguide taper formed by thermal oxidation of a silicon waveguide. In one example, a glass interposer waveguide 1904 is adhered directly on top of a silicon photonics waveguide taper 1902 (e.g., using a layer of epoxy 1908 or other adhesive). The waveguide taper 1902 may have a thickness of approximately 220. In a further example, for optimal efficiency, the minimum width of the silicon waveguide taper 1902 is less than 50 nm. As shown, the waveguide taper 1902 may taper or narrow in a direction toward an array 1910 of single-mode optical fibers. Cladding 1906 may surround at least a portion of the waveguide taper 1902.

[0085] Thermal oxidation has been used previously to fabricate low-loss silicon rib waveguides and strip-to-rib waveguide transitions, as well as to achieve vertical thinning of silicon waveguides (e.g., to fabricate vertical tapers).

[0086] FIGs. 20A-20B illustrate an example fabrication process for a two- dimensional waveguide taper. In one example, the fabrication process uses thermal oxidation to reduce the lateral dimensions of the waveguide taper. As illustrated in FIG. 20A, the fabrication process starts with a silicon-on-insulator (SOI) wafer 2000. The SOI wafer 2000 may comprise a buried oxide layer 2010 (e.g., formed from silicon dioxide) and a layer of silicon 2006 on top of the buried oxide 2010. In one example, the SOI wafer 2000 has 220 nm thick device layer. A partial etch step defines at least one rib waveguide 2002. In one example, the partial etch step is approximately 100 nm deep.

[0087] As illustrated in FIG. 20B, a silicon nitride mask 2004 may be deposited over the etched SOI wafer 2000 and patterned. In one example, the silicon nitride mask is approximately 150 nm thick. The silicon nitride mask 2004 serves two functions: (1) to define the oxidized waveguide tapers; and (2) to protect the device waveguides from oxidation.

[0088] As illustrated in FIG. 20C, a deep anisotropic etch step may transfer the silicon nitride mask 2004 to the silicon 2006 of the SOI wafer 2000. In one example, the anisotropic deep etch is a 220 nm deep etch.

[0089] Lastly, as illustrated in FIG. 20D, the SOI wafer 2000 may be oxidized, and the silicon nitride layer 2004 may be removed using a heated phosphoric etch. Oxidation of the SOI wafer 2000 may take place in a furnace at approximately 1050°C for approximately four hours. The heated phosphoric etch may be performed at approximately 160°C for approximately twenty minutes. [0090] In one example, the oxidation process may be simulated using a three- dimensional process simulator. FIGs. 21 A-21 B illustrate cross sectional views of a waveguide taper created according to the thermal oxidation process illustrated in FIGs. 20A-20D. In particular, FIG. 21A illustrates a simulation of the waveguide taper using the three-dimensional process simulator, while FIG. 21 B illustrates a scanning electron microscope (SEM) image of the waveguide taper. [0091] In one example, the minimum taper dimension on the silicon nitride mask is approximately 250 nm, and the oxidation is calibrated to remove approximately 100 nm of silicon on each side of the waveguide taper. As shown in FIG. 21 A, the remaining taper width is less than 50 nm. FIG. 21 B shows the cross section of a fabricated waveguide, where the silicon nitride mask dimension was 400 nm. In this case, the waveguide taper’s width is reduced to approximately 200 nm. The thermal oxidation also reduces the thickness of the waveguide taper, because the oxygen can diffuse through the buried oxide layer. However, because the silicon nitride mask is relatively thick, the corners of the waveguide taper remain relatively square.

[0092] FIG. 22 illustrates the measured optical insertion loss of an optical interposer waveguide with a thermally oxidized two-dimensional waveguide taper for transverse electric and transverse magnetic polarizations. The test structure that serves as the basis for the measurements of FIG. 22 comprises a sixty-eight- channel glass interposer aligned to an array of oxidized two-dimensional waveguide tapers with a pitch of approximately 63.5 pm and tapers that are approximately 2 mm long. The test structure includes polarization splitter-rotators to determine the polarization dependent loss. The wavelength dependent insertion loss in the O band, including the polarization splitter-rotator, is shown in FIG. 22. Over a bandwidth greater than 50nm, the TE mode insertion loss is approximately 2.4 dB, while the TM mode loss is approximately 1 .8 dB.

[0093] T able 1 , below, breaks down the loss of each component of the optical interposer waveguide of FIGs. 19A and 19B. The differences between the TE and TM mode insertion losses are mostly explained by the difference in the propagation loss, and the nonzero index matching liquid gap. The adiabatic coupling is typically more sensitive to large gaps for the TE mode than for the TM mode. With packaging of the chip in a cleaner environment, the adiabatic gap loss can be reduced to 0 dB for the TE mode as well, since the waveguide taper is long enough to be adiabatic at zero gap. The waveguide-taper-to-rib- waveguide transition, the polarization rotator, and the polarization splitter all have relatively low insertion loss. The glass interposer waveguide can be improved with design changes, and the propagation loss can be reduced with process improvements.

Table 1 : Insertion loss of each component

[0094] The insertion loss compares favorably to other optical interposer waveguide designs, particularly considering the inclusion of the polarization splitter-rotator component. However, the disclosed design has the added advantage of being compatible with silicon photonics MEMS processing (e.g., as illustrated in FIGs. 19B, there is no top cladding on the waveguide taper 1902). Since the process uses a silicon nitride mask and thermal oxidation, the process can be easily adapted to other silicon photonic processes.

[0095] FIGs. 23A-23B illustrate an example device that employs evanescent coupling between a two-dimensional waveguide taper and an angle-polished single-mode optical fiber. For instance, FIG. 23A illustrates a cross sectional view of an evanescent coupler 2300 comprising a silicon photonics chip 2302. A single-mode optical fiber 2304, having a first end 2306 and a second end 2308, may be one of a plurality of single-mode optical fibers attached to a V-groove chip 230 to form an array of single-mode optical fibers.

[0096] As illustrated in FIG. 23B, which illustrates a more detailed, close-up view of a portion of the device illustrated in FIG. 23A, the single-mode optical fiber 2304 may comprise a core 2312 and a cladding 2314 surround the core. The first end 2306 of the single-mode optical fiber 2304 may be polished at an angle, as illustrated, to gradually expose the core 2312. The polished angle of the first end 2306 of the single-mode optical fiber 2304 allows for a greater surface area of the core 2312 to be exposed to the two-dimensional waveguide taper 2316 (e.g., as compared to an arrangement in which the first end 2306 of the single mode optical fiber 2304 is not polished at an angle), improving the evanescent coupling between the waveguide taper 2316 and the single-mode optical fiber 2304.

[0097] Although various examples which incorporate the teachings of the present disclosure have been shown and described in detail herein, those skilled in the art can readily devise many other varied examples that still incorporate these teachings.