Integrated microlens coupler for photonic integrated circuits

Abstract

An example apparatus and method to form an integrated microlens coupler of the present disclosure are disclosed. The apparatus includes a substrate, a waveguide formed on the substrate to propagate a light beam in a horizontal direction, a self-aligned containment ring formed on the substrate, and an optical element formed on the substrate between the waveguide and free space, wherein the self-aligned containment ring defines a boundary and a location of the optical element on the substrate, wherein the optical element is to transform the light beam in the waveguide to a desired optical mode in the free space.

Description

INTEGRATED MICROLENS COUPLER FOR PHOTONIC INTEGRATED CIRCUITSCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to United States Provisional Patent Application Serial No. 63 / 484,378, filed February 10, 2023, which is herein incorporated by reference in its entirety.FIELD OF THE INVENTION

[0002] The present disclosure describes an optical interface between waveguides and free-space optical beams using batch fabricated microlenses that can be self-aligned to the optical waveguides.BACKGROUND

[0003] Photonic integrated circuits (PIC) are a microchip containing two or more photonic components that form a functioning circuit. PIC utilizes light to carry information signals on optical wavelengths, as opposed to electrons used by electronic integrated circuits. PICs and silicon photonics are widely used in data centers, telecommunication networks, quantum computing, neural and Al (artificial intelligence) computing, LiDARs (light detection and ranging) and other optical sensors.BRIEF DESCRIPTION OF THE DRAWINGS

[0004] The teaching of the present disclosure can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:

[0005] FIG. 1 depicts a side view of an integrated microlens coupler (IMC) of the present disclosure that is connected to an optical waveguide;

[0006] FIG. 2 depicts a top view of the example IMC of the present disclosure;

[0007] FIG. 3 depicts an example schematic of the optical beam profile (top) and the finite-difference time-domain (FDTD) simulation of the optical field (bottom) showing the optical beam from the input waveguide is collimated bythe IMC to a collimated beam in free space;

[0008] FIG. 4 depicts a three-dimensional drawing of the example IMC of the present disclosure that is used to couple light between a waveguide and an optical fiber;

[0009] FIG. 5 depicts the electric field profiles of output light from the example IMC and the fundamental mode of the optical fiber;

[0010] FIG. 6 depicts a side view of an example IMC with a self-aligned ring to confine the boundary of the microlens of the present disclosure;

[0011] FIG. 7 depicts a top view of the example IMC with a self-aligned ring of the present disclosure;

[0012] FIG. 8 depicts side views of the example IMC during an example fabrication process;

[0013] FIG. 9 depicts top views of the example IMC corresponding to the side views illustrated in FIG. 5;

[0014] FIG. 10 depicts the scanning electron micrograph (SEM) of a microlens with integrated waveguide and self-aligned ring and the measured and fitted profile of the microlens;

[0015] FIG. 11 depicts an example of two integrated microlens couplers connected by a loopback optical waveguide for characterizing the optical coupling with optical fibers;

[0016] FIG. 12 depicts the measured optical spectra of light coupled from optical fiber to waveguide and back to fiber through two example integrated microlens couplers;

[0017] FIG. 13 depicts an example of free-space coupling with an expanded optical beam to increase the misalignment tolerance;

[0018] FIG. 14 depicts a three-dimensional drawing of an example of free- space coupling with an expanded optical beam to increase the misalignment tolerance;

[0019] FIG. 15 depicts a side view of the example microlens coupler to couple light from a silicon waveguide to an optical fiber through an intermediate lower-index waveguide;

[0020] FIG. 16 depicts a top view of the example microlens coupler to couplelight from a silicon waveguide to an optical fiber through an intermediate lower- index waveguide;

[0021] FIG. 17 depicts the side view of the example microlens coupler with a footprint of a racetrack shape;

[0022] FIG. 18 depicts a top view of the example microlens coupler with the footprint of the racetrack shape;

[0023] FIG. 19 depicts a side view of the example microlens coupler with a footprint of a lollipop shape;

[0024] FIG. 20 depicts a top view of the example microlens coupler with the footprint of the lollipop shape;

[0025] FIG. 21 depicts a side view of the example IMC that directly couples light from a silicon waveguide to a fiber;

[0026] FIG. 22 depicts a top view of the example IMC that directly couples light from a silicon waveguide to a fiber;

[0027] FIG. 23 depicts simulation results of the example IMC with a silicon waveguide;

[0028] FIG. 24 depicts simulated transmission optical spectra of the example IMC with a single-mode AI2O3 waveguide to optical fibers in the visible regime;

[0029] FIG. 25 depicts examples of the insertion of free-space optical elements such as optical isolators between the microlens and the optical fiber of the present disclosure;

[0030] FIG. 26 depicts an example of hybrid free-space and guided waveguide photonic system with microlens couplers between waveguides and free space beams;

[0031] FIG. 27 depicts an example of hybrid free-space and guided-wave photonic integrated circuits of the present disclosure; and

[0032] FIG. 28 depicts another example of hybrid free-space and guided- wave photonic integrated circuits of the present disclosure.

[0033] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.DETAILED DESCRIPTION

[0034] The present disclosure provides examples of an integrated microlens coupler for photonic integrated circuits and methods for fabricating the same.

[0035] As noted above, PIC and silicon photonics are widely used in data centers, telecommunication networks, quantum computing, neural and Al (artificial intelligence) computing, LiDARs (light detection and ranging) and other optical sensors. One persistent challenge in mass production of PIC is the attachment of fibers in optical packaging due to the large mismatch in their mode sizes: the mode size of optical fiber is about 10 micrometers while that of silicon photonic waveguides is sub-micrometer.

[0036] Grating couplers and edge couplers with spot size converters were frequently used. However, they suffer from some drawbacks. The grating couplers have very narrow optical bandwidth, work with only one polarization, and are very sensitive to small variations in fabrication. Dual polarization grating couplers have been reported, however, at the expense of higher optical loss and more complicated design and fabrication. Edge couplers with mode size converters have been reported, sometimes in conjunction with V-grooves etched on silicon substrate for alignment of optical fibers. Fabrication of edge couplers requires either deposition of thick dielectric layers or patterning of fine structures (meta waveguides). Precise etching of waveguide facets and V- grooves further complicates the fabrication process.

[0037] The present disclosure describes the use of integrated microlens coupler (IMC) to couple light from waveguides to collimated beams in free space. The free-space beam can be tailored to match the mode size of single mode optical fibers, and therefore achieve low coupling loss without physical contacts.

[0038] In addition, the IMC can be batch fabricated on wafers and lithographically aligned to the waveguides. Previous microlens fabrication processes would require serial production near the edge of the wafer or substrate using a 3D printing process. The 3D printing process is a linear process where one microlens is printed at a time. This can be a timeconsuming and inefficient process compared to the batch fabrication process of the present disclosure.

[0039] Furthermore, the present disclosure allows the free-space beam diameter to be tailored to match the application requirements. For example, it can be expanded to couple to fiber collimators with larger misalignment tolerance. The IMC can also be used to construct integrated photonic circuits that combine both optical waveguides and free-space beams.

[0040] One application of the present disclosure is optical packaging of photonic integrated circuits with optical fibers. Another application is to couple photonic integrated circuits with free-space targets, such as trapped ions or atoms in quantum systems.

[0041] A schematic of the integrated microlens coupler (IMC) is illustrated in FIGs. 1 and 2. FIG. 1 illustrates a side view of an example IMC 100 of the present disclosure and FIG. 2 illustrates a top view of the example IMC 100 of the present disclosure. FIGs. 1 and 2 may be referred to simultaneously in the following description.

[0042] In an embodiment, the IMC 100 comprises an optical waveguide 104 (also referred to simply as a waveguide 104) and a microlens 106 (also referred to as an optical element 106) formed on a substrate 102. The microlens 106 may be formed on the substrate 102 between the optical waveguide 104 and free space 118. The free space 118 may be an air interface that is free of any optical elements or structures.

[0043] The optical waveguide 104 is directly attached to the microlens 106. Light 108 (also referred to as a beam of light 108, light rays 108, optical beam 108, light beam 108, and the like) propagates across the microlens 106 in the horizontal direction represented by an arrow 114 that is parallel to a top planar surface 116 of a substrate 102. This contrasts with the common use of microlenses with light propagating in the vertical direction (perpendicular to the top planar surface 116 of the substrate 102).

[0044] The light 108 exiting the microlens 106 can be collimated, focused, or engineered to match a desired optical profile by tailoring the shapes, dimensions, positions as well as the refractive index profiles of the opticalwaveguide 104 and the microlens 106. The example depicted in FIG. 1 shows the light 108 collimated by the microlens 106 and coupled to an optical fiber 110. The microlens 106 transforms the optical mode of the light 108. The light 108 exits the microlens 106, travels across the free space 118, and into the optical fiber 110.

[0045] The optical mode of the light 108 may be defined as the field pattern of the light 108. The microlens 106 of the present disclosure may transform the optical mode of the light 108 into a field pattern (e.g., size, diameter, location, collimation pattern, etc.) that matches the desired optical mode for the optical fiber 110 that is receiving the light 108 from the microlens 106.

[0046] Referring to FIG. 2, the optical waveguide 104 may have tapered section 112. For example, the optical waveguide 104 may have a first width 120, a tapered section 112, and then a second width 122 that is greater than the first width 120. The tapered section 112 is discussed in more details below.

[0047] FIG. 3 illustrates an example schematic of an optical beam profile of the light 108 and a finite-difference time-domain (FDTD) simulation 202 of the optical field that shows the optical beam of light 108 from optical waveguide 104 is collimated by the microlens 106 to a collimated beam in the free space 118. In the example illustrated in FIG. 3, the IMC 100 may comprise an Sll-8 waveguide 104 and a polymer microlens 106 with a refractive index of 1.59.

[0048] In an embodiment, the thickness of the waveguide 104 is approximately 2 micrometers, and the first width 120 is tapered from approximately 2 micrometers to a second width 122 of approximately 6 micrometers. The FDTD simulation 202 shows the light 108 from the waveguide 104 expanding inside the microlens 106 due to optical diffraction. The optical beam 108 is also tilted upward (e.g., away from the top planar surface 116 of the substrate 102) so the optical beam 108 is entirely above the substrate 102. This is achieved by tailoring the refractive index profiles of the optical waveguide 104 (n2), upper cladding layer or air interface (m), lower cladding layer or the substrate 102 (04), and the microlens 106 (ns).

[0049] In this design example, = 1 (air), ns = 1 .55 (SU-8), ns = 1 .59 (AZ 4620 photoresist polymer), and n4 = 1.46 (silicon dioxide (SiOs)). The indexprofile, ri2 ~ ns > ri4 > ni, results in tilting and expansion of the optical beam 108 in the microlens 106. The output beam of light 108 is collimated when the light 108 exits the microlens 106 with a radius of approximately 23 micrometers. A center 124 of the microlens 106 (illustrated in FIG. 2) is offset by approximately 2 micrometers below a center 126 of the waveguide 104 (illustrated in FIG. 1 ) to steer the output beam of light 108 to be parallel to the substrate 102. In other words, the center 124 of the microlens 106 is located approximately 2 micrometers below the center 126 of the waveguide 104. The design parameters here simply illustrate an example of a practical implementation. It is understood that other combinations of materials, dimensions, shapes, and refractive index profiles can be chosen to optimize the performance as well as increase the compatibility with the packaging process.

[0050] FIG. 4 shows a three-dimensional (3D) rendering of the IMG 100, including the light 108 coupled from the waveguide 104, through the microlens 106 and then into the optical fiber 110. The beam of light 108 is expanded and elevated inside the microlens 106 and collimated by the microlens-air interface.

[0051] FIG. 5 illustrates electric field profiles of light 108 that is output from another example of an IMG 500 and a fundamental mode of the optical fiber 110. FIG. 5 depicts another IMG 500 with an anti-reflective (AR) coating 512 and an optical fiber 510 with an AR coating 514. The IMC 500 may be similar to the IMC 100 in that includes a substrate 502, a waveguide 504, and a microlens 506.

[0052] The AR coatings 512 and 514 may serve two purposes; the AR coatings 512 and 514 reduce the back-reflection, as well as the IMC waveguide-to-fiber coupling loss. In an example, the AR-coating 512 may include a single magnesium fluoride (n=1 .37) layer having a thickness of approximately 239 nanometers. The design parameters of the AR coating 512 provided here simply illustrate an example of a practical implementation and various materials having different thickness can be deployed.

[0053] FIG. 5 also shows optical field profiles 516 and 518 simulated in 3D FDTD software. The profile 516 shows the simulated field profile after light 108 exits the I MC 500. The profile 518 is the profile of the fundamental mode of thefiber 510. The two field profiles 516 and 518 are well matched, indicating that the coupling from the IMC 500 to the fundamental mode of the fiber 510 is efficient. A graphical representation 520 of the profiles 516 and 518 also indicate that the profiles 516 and 518 are well matched.

[0054] FIG. 6 and FIG. 7 illustrate one of the salient features of the IMC 100 in how the microlens 106 can be lithographically aligned to the optical waveguides 104 using wafer-scale processing. FIG. 6 illustrates a side view of the example IMC 100 with a self-aligned containment ring 602 and FIG. 7 illustrates a top view of the example IMC 100 with the self-aligned containment ring 602. FIGs. 6 and 7 may be referred to simultaneously.

[0055] In one embodiment, the IMC 100 may be formed using the selfaligned containment ring 602. The self-aligned containment ring 602 defines the boundaries of the microlens 106 with a “retention wall” that is self-aligned to the optical waveguide 104, as illustrated in FIGs. 6 and 7.

[0056] In this embodiment, the self-aligned containment ring 602 is made of the same material as the waveguide 104, and both are patterned by the same lithographic step. Alternatively, the self-aligned containment ring 602 and the waveguide 104 can be patterned separately and aligned to each other through alignment markers on the lithographic masks. The overlay misalignment of modern lithographic equipment is usually better than 50 nm.

[0057] In one example, the microlens 106 formed by reflow (see below in FIGs. 8 and 9) is confined by the outer edge of the self-aligned containment ring 602 due to surface tension. In this embodiment, the self-aligned containment ring 602 is a physical structure. Alternatively, the surface wettability of the photoresist or polymers of the microlens 106 can be patterned. For example, a hydrophilic area surrounded by a hydrophobic ring can also confine the microlens 106 during the reflow process.

[0058] FIG. 8 illustrates a side view of an example fabrication process 800 of the IMC 100 using the reflow technique. FIG. 9 illustrates a top view of the example fabrication process 800 of the IMC 100 using the reflow technique.FIGs. 8 and 9 may be referred to simultaneously as the process 800 is described below.

[0059] At block 802, the optical waveguide 104 and the retention wall or selfaligned containment ring 602 are patterned by photolithography on a substrate 102. FIG. 9 illustrates how the self-aligned containment ring 602 is formed to define the boundary of the microlens 106 that will eventually be formed.

[0060] At block 804, a thick polymer material 810 (e.g., AZ 4620 or AZ 40XT photoresist polymer) is spun-coated and patterned by photolithography. In one embodiment, the polymer material 810 is applied in a cylinder shape as seen by a combination of the side view in FIG. 8 and the top view of FIG. 9. The edges of the polymer cylinders 810 are positioned on top of the self-aligned containment ring 602 for better wetting.

[0061] Finally, at block 806 the microlenses 106 are formed by increasing the temperature above the reflow temperature, causing the polymer cylinder 810 to form the spherical shape by surface tension. Though photoresist is used in this embodiment, it is understood that other polymer materials such as polyimide can be used to form the microlens 106. It is also possible to transfer the lens shape to hard materials such as silicon nitride, oxynitride, or aluminum oxide by dry etching (e.g., reactive ion etching).

[0062] FIG. 10 shows a scanning electron micrograph (SEM) 1000 of the IMC 100. A measured lens profile 1002 and a fitted lens profile 1004 (height map) agrees well with the predicted shape based on surface tension calculation.

[0063] FIG. 11 illustrates an example of two integrated IMCs 100-1 and 100- 2. FIG. 11 shows an example setup for characterization of fiber-to-waveguide coupling. Two microlenses 100-1 and 100-2 are connected to each other via an Sil-8 waveguide 104. After aligning two optical fibers 110-1 and 110-2, which are part of the same fiber array unit, the IMC coupling loss can be estimated from the fiber-to-fiber loss measurement. Because the Sll-8 waveguide loss is small, most of the loss can be attributed to the fiber coupling.

[0064] FIG. 12 illustrates graphs 1202 and 1204 representing measured optical spectra of light coupled from setup illustrated in FIG. 11 . The graph 1202 shows the measured optical spectra for the Transverse Electric (TE) and Transverse Magnetic (TM) polarizations with wavelengths spanning from 1260to 1360 nm (O-band). The graph 1204 shows the measured optical spectra for the TE and TM polarizations with wavelengths spanning from 1525 to 1575 nm (C-band). The spectra were obtained with a tunable laser as the source of light. The spectra show that the IMC coupling is broadband, exhibits low polarization dependent loss and, for this particular device, the minimum loss was measured to be 0.9 dB.

[0065] FIG. 13 shows a schematic of an IMC 1300 that couples light 108 into a beam of larger mode field diameter (MFD). Increasing the MFD leads to larger alignment tolerance between the optical fiber 110 and the IMC 1300. Larger mode size is obtained by increasing a size of the microlens 106 to a larger microlens 106. With a microlens radius of approximately 48 micrometers and a microlens height of approximately 43.5 micrometers, the target MFD of approximately 20 micrometers is obtained. The materials, refractive indices and waveguide dimensions remain the same as those described above, in FIG. 1.

[0066] To efficiently couple the large MFD beam exiting the IMC 1300 into the optical fiber 110, the MFD of the optical fiber 110 either may be increased to match that of the beam of light 108, or a second optical element 1302 (e.g., a lens) can be added, as shown in FIG. 13. The second optical element 1302 reduces the MFD of the beam of light 108 so that it matches the smaller MFD of the optical fiber 110. The design parameters here simply illustrate an example of a practical implementation. It is understood that other combinations of materials, dimensions, shapes, and refractive index profiles can be chosen to optimize the performance as well as increase the compatibility with the packaging process.

[0067] FIG. 14 shows a 3D rendering of the example IMC 1300 designed for large MFD. Light 108 is coupled from the waveguide 104, expands and tilts upwards inside the microlens 106 and collimated into a horizontal beam with large MFD at the microlens-air interface. The second optical element 1302 (e.g., a plano-convex lens) at a fiber facet of the optical fiber 110 decreases the MFD of the beam such that it matches the MFD of the fundamental mode of the optical fiber 110.

[0068] In silicon photonics, the light 108 can be coupled to silicon waveguides, which has a substantially higher refractive index (3.45) than the polymer waveguides. FIGs. 15 and 16 illustrate a side view and a top view, respectively, of an example an IMG 1500 with a silicon waveguide 1502. FIGs. 15 and 16 can be referred to simultaneously in the following description.

[0069] In the embodiment shown in FIGs. 15 and 16, light (not shown) is first coupled from the silicon waveguide 1502 to the polymer (SU-8) waveguide 104 by double inverse tapers formed by tapered ends 1504 and 1506 (illustrated in the top view in FIG. 16), and then coupled to optical fiber 110 through the microlens 106, as described earlier. The double inverse tapers may include a tapered end 1504 of the silicon waveguide 1502 and a tapered end 1506 of the optical waveguide 104. The tapered end 1504 of the silicone waveguide 1502 and the tapered end 1506 of the optical waveguide 104 may overlap to form the double inverse tapers 1504 and 1506, as shown in the top view illustrated in FIG. 16.

[0070] In one embodiment, the silicon waveguide 1502 may be approximately 200 nm thick, and the width is tapered from approximately 400 nm to 75 nm. The SU-8 waveguide 104 is placed at a distance 1508 that is approximately 400 nm above the silicon waveguide 1502 and is tapered from approximately 1 micrometer to 2 micrometers. In an example embodiment, a length 1510 of the tapered end 1504 of the silicon waveguide 1502 and a length 1512 of the tapered end 1506 of the optical waveguide 104 may both be approximately 500 micrometers.

[0071] The dimensions and materials of the IMC 1500 may be the same as the IMC 100 described above. The design parameters here simply illustrate an example of a practical implementation. It is understood that other combinations of materials, dimensions, shapes, and refractive index profiles can be chosen to optimize the performance as well as increase the compatibility with the packaging process.

[0072] In some embodiments, longer free propagation length inside the microlens is needed. FIGs. 17 and 18 show a side view and a top view, respectively, of an IMC 1700 with a non-spherical shape. FIGs. 17 and 18 mayhave a substrate 102, an optical waveguide 104, and an optical fiber 110 similar to the IMC 100, described above. However, the IMC 1700 may have a microlens 1706 to provide a longer free propagation length for the light 108.

[0073] In an embodiment, the microlens 1706 may have a footprint that has the shape of a racetrack as can be seen in the top view of FIG. 18. In other words, the shape of the outer perimeter 1708 when viewed from above may have a racetrack shape. Said another way, when looking at the top view of FIG. 18, the microlens 1706 may have a length 1712 that is not equal to a width 1710. For example, the length 1712 may be greater than the width 1710 to form the racetrack shape.

[0074] The microlens 1706 can be fabricated using the same process as the spherical microlens 106 described in FIGs. 8 and 9 above. The free expansion propagation length and the microlens radius of curvature are design parameters that can be optimized individually for different output beam sizes. For example, a racetrack-shaped microlens with 310 micrometers total length, 129 micrometer radius of curvature and 126 micrometer maximum height (e.g., a dimension “h” represented by a line 1714 in FIG. 17) will produce an output beam size of 80 micrometers mode field diameter.

[0075] A radius of curvature may be defined as a radius of a circle used to form the circle that includes the curve of the ends of the racetrack-shaped microlens 1706. For example, if a radius of a circle that would perfectly match, or lie on top of, the curve of the ends of the racetrack-shaped microlens 1706 would be the radius of curvature of the racetrack-shaped microlens 1706.

[0076] The materials and corresponding indices of refraction in this example are assumed to be the same as those in FIG. 3. The design parameters here simply illustrate an example of a practical implementation. It is understood that other combinations of materials, dimensions, shapes, and refractive index profiles can be chosen to optimize the performance as well as increase the compatibility with the packaging process.

[0077] FIGs. 19 and 20 show another embodiment of an IMC 1900 with a non-spherical shape. FIGs. 19 and 20 show a side view and a top view, respectively, of an IMC 1900 with a non-spherical shape. FIGs. 19 and 20 mayhave a substrate 102, an optical waveguide 104, and an optical fiber 110 similar to the IMC 100, described above. However, the IMC 1900 may have a microlens 1906 to provide a longer free propagation length for the light 108.

[0078] In an embodiment, the microlens 1906 may have a footprint that has the shape of a lollipop as can be seen in the top view of FIG. 20. In other words, the shape of the outer perimeter 1908 when viewed from above may have a lollipop shape.

[0079] The microlens 1906 can be fabricated using the same process as the spherical microlens 106 described in FIGs. 8 and 9 above. The microlens total length and the microlens radius of curvature are design parameters that can be optimized individually for different output beam sizes. For example, using the definition of the length, radius of curvature, and height described above with respect to FIGs. 17 and 18, a lollipop-shaped microlens with approximately 310 micrometers total length, approximately 129 micrometer radius of curvature, and approximately 126 micrometer maximum height will produce an output beam size of approximately 80 micrometers mode field diameter. The materials and corresponding indices of refraction in this example are assumed to be the same as those in FIG. 3. The design parameters here simply illustrate an example of a practical implementation. It is understood that other combinations of materials, dimensions, shapes, and refractive index profiles can be chosen to optimize the performance as well as increase the compatibility with the packaging process.

[0080] FIGs. 21 and 22 illustrate a side view and a top view, respectively, of a schematic of an IMC 2100 that directly couples light from a silicon waveguide 2104 to an optical fiber 110. In an embodiment, the IMC 2100 comprises a single-mode silicon waveguide 2104 and a microlens 106. The silicon waveguide 2104 is directly attached to the microlens 106.

[0081] As shown in the top view in FIG. 22, from the point where the silicon waveguide and the microlens are connected, the width of the silicon waveguide 2104 is tapered by a predetermined length to form tapered sections 2110 and 2112. The beam of silicon waveguide 2104 is coupled to the microlens 106during the tapered sections 2110 and 2112, expanded through the microlens 106, and finally coupled to the optical fiber 110.

[0082] In one embodiment, the silicon waveguide 2104 may be a 1xN coupler, as shown in the top view of FIG. 22. The 1xN coupler of the silicon waveguide 2104 may allow placement of multiple silicon waveguides 2104 to adjust the divergence of the output beam in the horizontal direction.

[0083] FIG. 23 shows the simulation results of the IMC 2300 with a silicon waveguide 2104. The IMC 2300 here comprises a single-mode silicon waveguide 2104 and a polymer microlens 106 with a refractive index of 1.68. A thickness 2120 of the waveguide 2104 is approximately 200 nanometers, and a width of the tapered section 2122 is tapered from approximately 400 nanometers to 100 nanometers. The length of the tapered section 2122 is 20 micrometers. The FDTD simulation 2302 shows the light 108 from the waveguide 2104 expanding inside the microlens 106 is due to optical diffraction. The beam of light 108 is also tilted upward so the optical beam of light 108 is entirely above the substrate 102. In this design example, ni = 1 (air), ns = 1 .68 (AZ 4620 of the microlens 106), n4 = 1.46 (SiC>2 of the substrate 102), and ns = 3.45 (Si of the silicon waveguide 2104).

[0084] The output beam of light 108 is collimated when the light exits the microlens 106 with a radius of approximately 20 micrometers. The center of the microlens 106 is offset by approximately 4 micrometers below the center of the waveguide 2104 to steer the output beam of light 108 to be parallel to the substrate 102.

[0085] The design parameters here simply illustrate an example of a practical implementation. It is understood that other combinations of materials, dimensions, shapes, and refractive index profiles can be chosen to optimize the performance as well as increase the compatibility with the packaging process.

[0086] The IMC concept can be extended to visible light waveguide. FIG. 24 shows a graph 2400 that represents the simulated visible light transmission spectra of an IMC with a single-mode AI2O3 waveguide to an optical fiber. Although coupling to two specific fibers is simulated (3 pm core S405-XP and 3.5 pm core S630-HP), coupling between any mode size fiber in the visiblespectrum is possible through selection of the waveguide and microlens dimensions.

[0087] Broadband, low-loss coupling to a 3 pm core fiber from 400 nm - 680 nm is accomplished using an 8.1 pm radius polymer microlens. A 10.4 pm radius polymer microlens is used for broadband low-loss coupling into a 3.5 pm core fiber from 630 nm - 860 nm. Both designs use a 200 nm thick single mode AI2O3 waveguide clad symmetrically in SiC>2, but it is understood that the IMC in this example is compatible with other visible light waveguide materials. The simulated coupling loss (~ 2 dB) can be further reduced by optimizing the design.

[0088] The IMC embodiments described herein provide a low-loss interface between waveguides and free space to allow insertion of bulk optical element such as optical isolators. FIG. 25 shows the schematic of another example IMC 2500 with a silicon waveguide 2504 combined with a bulk optical element 2502, such as an optical isolator. The parameters and the material of the silicon waveguide 2504 and the IMC 2500 may be similar to or the same as the IMC 2100 illustrated in FIGs. 21 and 22 and described above.

[0089] After the beam is entirely above the substrate 102 and collimated when exiting the microlens 106, the bulk optical element 2502 (e.g., an optical isolator) prevents unwanted optical reflections, which is important when the waveguide 2504 is connected to lasers. This significantly benefits applications that require low optical noise and a stable optical frequency.

[0090] FIG. 26 shows another embodiment of an IMC 2600. In an embodiment, the IMC 2600 may include a hybrid free-space and guided waveguide photonic system with microlens couplers between waveguides and a free-space optical element 2502. For example, the IMC 2600 may include a first waveguide 2604-1 directly coupled to a first microlens 106-1 formed on a substrate 102 and a second waveguide 2604-2 directly coupled to a second microlens 106-2 formed on the substrate 102. The waveguides 2604-1 and 2604-2 and the microlenses 106-1 and 106-2 may be formed on the substrate 102 using the wafer-processing techniques described to form the IMC 100, as illustrated in FIGs. 8 and 9. In another embodiment, the waveguides 2604-1and 2604-2 may be silicon waveguides similar to the silicon waveguide 2104 illustrated in FIGs. 21 , 22, and 23.

[0091] In an embodiment, the bulk optical element 2502 may be located in the free space between the first microlens 106-1 and the second microlens 106- 2. The bulk optical element 2502 may be an optical isolator.

[0092] After the beam is entirely above the substrate 102 and collimated when exiting the microlens 106-1 , the bulk optical element 2502 (e.g., an optical isolator) can prevent unwanted optical reflections. Subsequently, the beam is focused by another microlens 106-2 and coupled into another waveguide 2604- 2. This system can decrease optical noise and maintain a stable optical frequency, and effectively combine with other photonics integrated circuits.

[0093] FIG. 27 illustrates another example of an IMC 2700. In an embodiment, the IMC 2700 may include a hybrid free-space and guided-wave photonic integrated circuits. In this embodiment, the light 108 from a waveguide 2704 is collimated by an aspherical optical element or lens 2706. The waveguide 2704 may be directly coupled to the aspherical optical element or lens 2706. The aspherical optical element or lens 2706 may be fabricated similarly to the way the microlens 106 is fabricated using wafer-processing methods, as described above in FIGs. 8 and 9.

[0094] Subsequently, a free space optical element 2708 will reflect the collimated light 108 and focus the light 108 above the substrate 102. Owing to the collimated light 108, the whole system has high tolerance when the light 108 propagates from the aspherical optical element 2706 to the free space optical element 2708.

[0095] FIG. 28 depicts another example of an IMC 2800. In an embodiment, the IMC 2800 may include a hybrid free-space and guided-wave photonic integrated circuits. In this embodiment, the IMC 2800 may include a waveguide 2804 directly coupled to the microlens 106.

[0096] First, the IMC 2800 is used to collimate light 108 from the waveguide 2804. Depending on the material and dimensions of the waveguide 2804 and IMC 2800, such a system can be designed for the visible and / or the IR spectrum.

[0097] Then, optical beams of light 108 are redirected by another example of a free space optical element 2802. The free space optical element 2802 may be a reflector or reflective element A surface of the reflective element may have a concave curve that can redirect the light 108 and / or focus the light 108.

[0098] While FIG. 28 shows a focusing reflector, the surface can be tailored to produce a collimated beam of light 108 as well. The non-spherical free space optical element 2802 can be fabricated by 3D printing. The use of an IMC 2800 with the on-spherical free space optical element 2802 (e.g., a focusing reflector) holds promise for qubit addressing in scalable trapped ion quantum computing systems, among many other applications.

[0099] Thus, the present disclosure provides various embodiments of an IMC for photonic integrated circuits. The IMCs of the present disclosure may be formed to tailor the optical mode of beams of light for the desired optical mode of an optical fiber. The IMCs of the present disclosure may transform the beams of light to travel in free space between the microlens of the IMC and the optical fiber with low coupling loss and without physical contacts.

[0100] In addition, the IMCs of the present disclosure can be batch fabricated on wafers using wafer-processing methods, such as photolithography. This allows many IMCs to be fabricated simultaneously, rather than in a serial fashion as disclosed by previous IMC fabrication techniques. This may lead to scalable production that is more efficient and cheaper than previous production methods.

[0101] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

IN THE CLAIMSWhat is claimed is:

1. An apparatus, comprising: a substrate; a waveguide formed on the substrate to propagate a light beam in a horizontal direction; a self-aligned containment ring formed on the substrate; and an optical element formed on the substrate between the waveguide and a free space, wherein the self-aligned containment ring defines a boundary and a location of the optical element on the substrate, wherein the optical element is to transform the light beam in the waveguide to a desired optical mode in the free space.

2. The apparatus of claim 1 , wherein a refractive index profile of the waveguide is approximately equal to a refractive index profile of the optical element, wherein the refractive index profile of the waveguide and the refractive index profile of the optical element are each greater than a refractive index profile of the substrate, and wherein the refractive index profile of the waveguide, the refractive index profile of the optical element, and the refractive index profile of the substrate are each greater than a refractive index profile of air.

3. The apparatus of claim 2, wherein the light beam is tilted upward by the optical element to be entirely above the substrate.

4. The apparatus of claim 1 , further comprising: an optical fiber coupled to the optical element to receive the light beam that exits the optical element.

5. The apparatus of claim 4, further comprising:a second optical element located between the optical element and the optical fiber.

6. The apparatus of claim 5, wherein the second optical element comprises at least one of: a lens, an optical isolator, or a reflective element.7 The apparatus of claim 1 , further comprising: an anti-reflective coating on the optical element.

8. The apparatus of claim 1 , wherein the waveguide and the self-aligned containment ring are formed via a photolithography process on the substrate.

9. The apparatus of claim 1 , wherein the substrate comprises silicon and the apparatus further comprises: a silicon waveguide that is tapered from a first width to a second width, wherein the second width is smaller than the first width and located in the substrate and below a tapered portion of the waveguide.

10. The apparatus of claim 1 , wherein the optical element comprises a spherical three-dimensional shape.11 . The apparatus of claim 1 , wherein the optical element comprises a non- spherical three-dimensional shape.

12. The apparatus of claim 11 , wherein the non-spherical three-dimensional shape comprises at least one of: a racetrack shape or a lollipop shape.

13. The apparatus of claim 1 , wherein the waveguide comprises a 1 x N silicon waveguide that is directly coupled to the optical element.

14. A method, comprising:patterning a substrate via photolithography with a waveguide and a selfaligned containment ring, wherein the self-aligned containment ring defines a boundary of an optical element onto a substrate via photolithography; applying a polymer onto the self-aligned containment ring; and heating the polymer to a temperature above a reflow temperature of the polymer to cause the polymer to form a spherical shape via surface tension.

15. The method of claim 14, wherein the patterning comprises: patterning on the substrate a plurality of waveguides and a plurality of self-aligned containment rings to form a plurality of integrated microlens couplers on the substrate.

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