Patent Publication Number: US-11649935-B2

Title: Coupling light source to photonic integrated circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. non-provisional application Ser. No. 17/007,308 filed Aug. 31, 2020, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to optics, and in particular to optical coupling of light sources. 
     BACKGROUND INFORMATION 
     Photonic systems which are sometimes referred to as photonic integrated circuits (PICs) typically include routing light emitted from light sources. Various photonic systems may include optical modulators, photodetectors, waveguides, and one or more light sources, for example. Cost, size, and/or efficiency improvements to photonic systems are desirable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
         FIG.  1 A  illustrates a portion of a device that includes a light director shaping illumination light from a light source for increased optical coupling efficiency into a waveguide, in accordance with aspects of the disclosure. 
         FIG.  1 B  illustrates a tilt angle with respect to a vector orthogonal to a plane of a waveguide, in accordance with aspects of the disclosure. 
         FIG.  1 C  illustrates a metalens as a light director for shaping illumination light, in accordance with aspects of the disclosure. 
         FIG.  2    illustrates a device that includes a bottom-emitting light source, a refractive spacing layer, and a microlens for shaping illumination light into shaped light incident on a light input grating, in accordance with aspects of the disclosure. 
         FIG.  3    illustrates a device that includes a light source and a microlens coupled with an emission aperture of the light source, in accordance with aspects of the disclosure. 
         FIG.  4    illustrates a device that includes a refractive wedge optic configured to impart a tilt angle to incident light, in accordance with aspects of the disclosure. 
         FIG.  5    illustrates a device including a tilt spacer to provide a tilt angle to illumination light emitted by a light source, in accordance with aspects of the disclosure. 
         FIG.  6    illustrates a device including a tilt spacer to provide a tilt angle to illumination light and a microlens providing a reduced divergence angle, in accordance with aspects of the disclosure. 
         FIG.  7    illustrates a top-down view of a tapered waveguide having a grating coupler, in accordance with aspects of the disclosure. 
         FIGS.  8 A- 8 K  illustrate an example fabrication process of an optical structure that increases light incoupling efficiency, in accordance with aspects of the disclosure. 
         FIG.  9    illustrates a process of fabricating a photonic integrated circuit, in accordance with aspects of the disclosure. 
         FIG.  10    illustrates an example head mounted device that may include photonic integrated circuits, in accordance with aspects of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of photonic systems are described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     In some implementations of the disclosure, the term “near-eye” may be defined as including an element that is configured to be placed within 50 mm of an eye of a user while a near-eye device is being utilized. Therefore, a “near-eye optical element” or a “near-eye system” would include one or more elements configured to be placed within 50 mm of the eye of the user. 
     In aspects of this disclosure, visible light may be defined as having a wavelength range of approximately 380 nm-700 nm. Non-visible light may be defined as light having wavelengths that are outside the visible light range, such as ultraviolet light and infrared light. Infrared light having a wavelength range of approximately 700 nm-1 mm includes near-infrared light. In aspects of this disclosure, near-infrared light may be defined as having a wavelength range of approximately 700 nm-1.6 μm. 
     In aspects of this disclosure, the term “transparent” may be defined as having greater than 90% transmission of light. In some aspects, the term “transparent” may be defined as a material having greater than 90% transmission of visible light. In some aspects, the term “transparent” may be defined as a material having greater than 90% transmission of infrared light. 
     The devices, substrates, and systems that are described in this disclosure are directed to increasing the coupling efficiency of light sources to photonic systems such as photonic integrated circuits (PICs). The devices described in this disclosure may also reduce the cost, size, and/or fabrication complexities associated with PICs. 
     Prior techniques for coupling light sources to waveguides include illuminating an input coupler (e.g. a grating) of a waveguide with illumination light from the light source. Since it generally increases the efficiency of the input coupler to tilt the illumination light, light sources such as LEDs and vertical-cavity side-emitting lasers (VCSELs) are soldered to electrical traces at an angle. However, since the resulting angle of the light source relies on the size of the solder ball, the time it takes for the solder to stabilize, and the angle accuracy ability of pick-and-place machines, the existing techniques do not provide consistent tilt angles for light coupling efficiency. The prior techniques may also require an additional wire-bond technique to electrically couple a second electrical contact (e.g. anode or cathode) of the light source to power the light source since one contact will be on the bottom of the light source to induce the spacing from the solder (to achieve the tilt angle) and the other contact will likely be on the top of the light source that requires a wire-bond process. In some contexts, it would be preferable for both the electrical contacts of the light source to be electrically coupled to electrical traces in the same fabrication process step (e.g. solder reflow). 
     Another issue in some prior arrangements is non-optimal divergence angles of light sources. For example light sources may have a divergence angle of up to 180 degrees. Many light sources have a Lambertian or Lambertian-esque divergence angles, for example. However, for better light coupling efficiency to an input coupler of a waveguide, the divergence angle of the illumination light from the light source would ideally be reduced. 
     In example of this disclosure, a light director (e.g. microlens or metalens such as a diffractive optical element) receives illumination light from the light source and shapes the illumination light to increase the efficiency of light that is incoupled into a waveguide. For example, the tilt angle of the illumination light may be adjusted to increase the input coupling efficiency. The tilt angle may be adjusted to between three and ten degrees, for example. The divergence angle of the illumination light may also be reduced by the light director. The divergence angle is reduced to approximately five degrees, in an example embodiment. The reduced divergence angle of the illumination light from the light source may be considered collimated light or near-collimated light in some examples. The light director may allow the light source (e.g. LED or VCSEL) to be mounted flat and allow the light source to be coupled to the photonic systems using conventional fabrication processing techniques. These and other embodiments are described in more detail in connection with  FIGS.  1 A- 10   . 
       FIG.  1 A  illustrates a portion of a device  100  that includes a light director shaping illumination light from a light source  150  for increased optical coupling efficiency into waveguide  170 , in accordance with aspects of the disclosure. Light source  150  includes emission aperture  156  and electrical contacts  151  and  152 . Electrical contact  151  may be an anode of the light source  150  and electrical contact  152  may be a cathode of the light source  150 , for example. Light source  150  may be an LED or a VCSEL. Light source  150  may be configured to emit visible light or non-visible light (including infrared light). Light source  150  may be configured to emit near-infrared light, in some aspects. In  FIG.  1 A , light source  150  is a top-emitting light source with coplanar electrical contacts on the same side of the emissions aperture. 
     Light source  150  is electrically coupled to device  100  by electrical traces (or electrical pads)  141  and  142 . In the illustration of  FIG.  1 A , electrical contact  151  is electrically coupled to trace/pad  141  and electrical contact  152  is electrically coupled to trace/pad  142 . In other embodiments (not illustrated) traces  141  and  142  may be disposed on an additional spacing layer to space light source  150  from light director layer  130 . In the illustration of  FIG.  1 A , light director layer  130  includes a refractive microlens  133  as the light director. Refractive microlens  133  may have a width of approximately twenty microns and a height of one micron. Refractive microlens  133  may have a spherical, aspherical, or freeform lensing surface  139  that provides optical power. In an example, the curvature of a spherical lens  139  has a radius of 80 microns. The entire light director layer  130  may be made from a refractive material such as amorphous silicon (a-Si), for example. The amorphous silicon may have a refractive index of approximately 3.5 at near infrared wavelength range for example. Microlens  133  may be formed of SiN, in an embodiment. In other embodiments (described below), the light director may be a metalens rather than a refractive microlens. For example, the light director may be a diffractive optical element (DOE), holographic light director, or otherwise. A metalens has a planar metasurface. The metasurface may have nanostructures in a periodic or non-periodic form such as an array of nano-pillars. The nano-pillar may have different diameters and a same height. 
     The illustrated microlens  133  includes an input side  139  that receives illumination light  163  and an output side  131 . Input side  139  has a curvature with optical power while output side  131  is planar, in the illustration of  FIG.  1 A . Output side  131  is coupled with a planar side  129  of cladding layer  121  of waveguide layer  120 . An anti-reflection (AR) coating may be disposed on input side  139  and/or output side  131  to increase optical efficiency by decreasing undesirable reflections. The AR coating may be a multi-layer dielectric coating tuned to the specific wavelength of illumination light  163 , for example. Silicon-nitride (SiN) may be utilized as an AR dielectric coating, in an embodiment. 
     Microlens  133  is disposed between light source  150  and waveguide layer  120 . Waveguide layer  120  includes waveguide  170 . Waveguide  170  includes light input coupler  172 . Light input coupler  172  is included in waveguide  170 . Light input coupler  172  is a grating that is formed of the waveguide material  170 , in  FIG.  1 A . In an example, light input coupler  172  is a grating formed of nano-pillars of a refractive waveguide material  170 . The nano-pillars may be formed by CMOS processes such as etching, masking, polishing, lithography, and/or vapor deposition, for example. A grating  172  is configured to incouple a particular wavelength of light into waveguide  170 . Therefore, grating  172  may be configured to specifically incouple the wavelength of light emitted by light source  150 . In the illustration of  FIG.  1 A , waveguide  170  is immersed in a cladding layer  121  to confine waveguide light propagating in waveguide  170 . Cladding layer  121  may have a lower index of refraction than waveguide material  170 . Cladding layer  121  may have a refractive index of approximately 1.45 and waveguide material  170  may have a refractive index of approximately 2.5. Cladding layer  121  may include silicon-oxide (SiO 2 ) and waveguide material  170  may be formed of silicon rich silicon-nitride, for example. 
     Optionally, waveguide layer  120  may include a reflector  180 . Reflector  180  is configured to increase optical efficiency by reflecting light that passes through the light input coupler  172  back to light input coupler  172  for a second chance (or more) of incoupling the light into waveguide  170 . Light input coupler  172  is disposed between the reflector  180  and the light director (microlens  133  in  FIG.  1 A ). Reflector  180  may be a metal layer or a diffractive reflector such as a distributed bragg reflector (DBR), for example. In an embodiment where reflector  180  is a diffractive reflector layer, the diffractive reflective layer may be specifically configured to reflect a near-infrared wavelength band of the illumination light (emitted by light source  150 ) back to the light input coupler  172  and configured to transmit light outside of the near-infrared wavelength band. The near-infrared wavelength band may have a linewidth of five nanometers or less. In the illustration of  FIG.  1 A , waveguide layer  120  is disposed over a substrate layer  110 . Substrate layer  110  may be considered a wafer in some contexts. Void  199  disposed between aperture  156  and microlens  133  may be an air gap or filled with materials suitable for antireflection coating purposes both for the microlens  133  and the light source  150 . 
     In operation, light source  150  illuminates the light director (microlens  133  in  FIG.  1 A ) with illumination light  163 . In  FIG.  1 C , the light director is a metalens  193 . The light director is configured to receive illumination light  163  and configured to direct illumination light  163  to light input coupler  172  as shaped light  167 . The light director is configured to tilt illumination light  163  to give shaped light  167  a tilt angle with respect to the light input coupler  172 . A dimension  124  between waveguide  170  and planar side  129  may be dimensioned with a depth of cladding layer  121  to optimize the illumination of light input coupler  172  with shaped light  167 . 
       FIG.  1 B  illustrates tilt angle ϕ  168  can be measured with respect to a vector  166  orthogonal to a plane of the waveguide  170 . As a result, shaped light  167  is incident upon light input coupler  172  at a tilt angle ϕ  168  to increase the intensity of waveguide light  179  that is incoupled by light input coupler  172  to propagate along waveguide  170 . Tilt angle ϕ  168  may be between three degrees and fifteen degrees. Tilt angle ϕ  168  may be between three degrees and ten degrees. Tilt angle ϕ  168  may be between five degrees and seven degrees, in some embodiments. 
     A middle  154  of emission aperture  156  may be offset by dimension  159  from a central optical axis  164  of refractive microlens  133  to impart the tilt angle ϕ  168  of the shaped light  167 . In one embodiment, dimension  159  is between four microns and eight microns. 
     Referring back to  FIG.  1 A , a light director (e.g. microlens  133  or metalens  193 ) may also be configured to give shaped light  167  a smaller divergence angle β  169  than an illumination light divergence angle θ  165  of illumination light  163 . For example, illumination light divergence angle θ  165  of illumination light  163  may be 15 degrees or more. When microlens  133  is the light director as in  FIG.  1 A , microlens  133  may provide a divergence angle β  169  to shaped light  167  that is between four degrees and seven degrees. In an embodiment, microlens  133  may provide a divergence angle β  169  to shaped light  167  that is less than ten degrees. In one example, divergence angle β  169  is five degrees. Divergence angle β  169  may be less than four degrees. Divergence angle β  169  may be considered collimated or near-collimated light, in some embodiments. 
     Therefore, the light director may shape illumination light  163  into shaped light  167  by imparting a tilt angle ϕ  168  and/or reduce illumination light divergence angle θ  165 . In one example, tilt angle ϕ  168  is approximately (within 1 degree of) five degrees and illumination light divergence angle θ  165  is approximately five degrees, for example. Shaping the illumination light  163  with a light director in one or both ways increases the optical efficiency of light input coupler  172 . 
       FIG.  1 C  illustrates a device  101  where the light director is a metalens (such as a diffractive element) rather than a refractive microlens, in accordance with aspects of the disclosure. Metalens  193  is included in light director layer  191  and may accomplish the same beam-shaping characteristics as described with respect to  FIG.  1 A  and microlens  133 . Metalens  193  may be a diffractive optical element (DOE), holographic light director, or otherwise. Metalens  193  may be a planar surface made of silicon-nitride, amorphous-silicon, titanium-oxide (TiO 2 ), or other suitable materials. Metalens  193  may be formed of nano-pillars having a same height but different diameters to keep metalens  193  a planar layer. While additional embodiments of this disclosure are illustrated with a refractive microlens as the light director, it is understood by those skilled in the art that the microlenses illustrated in this disclosure may be replace by metalenses that achieve the same optical characteristics as the microlenses. Therefore, those skilled in the art appreciate that the embodiments illustrated in  FIGS.  2 - 8    may instead include metalenses achieving the same optical functionality as the illustrated microlenses. 
       FIG.  2    illustrates a device  200  that includes a bottom-emitting light source  250 , a refractive spacing layer  240 , and a microlens  133  for shaping illumination light  263  into shaped light  267  incident on light input grating  172 , in accordance with aspects of the disclosure. Bottom-emitting light source  250  has emission aperture  256  disposed on a bottom side or the wafer of light source  250 . Traces, pads, or wire-bonds (not illustrated) may be electrically coupled to electrical contacts  251  and  252  to provide electrical power to light source  250  to generate illumination light  263 . Electrical contacts  251  and  252  may be coupled to the cathode and anode of light source  250 , respectively. 
     In device  200 , refractive spacing layer  240  spaces emission aperture  256  from microlens  133 . Refractive spacing layer  240  may be made of an optical polymer or silicon-oxide, for example. Refractive spacing layer  240  is disposed between the light director (microlens  133  in  FIG.  2   ) and light source  250 . Additionally, refractive spacing layer  240  may provide a top plane  249  offering a planar surface of refractive spacing layer  240  to place light source  250 . This planar surface assists in increasing the consistency of the placement of light source  250  with respect to microlens  133  and light input coupler  172 , which in turn, increases the light coupling efficiency of the optical system. Top plane  249  of the refractive spacing layer  240  may be parallel to emission aperture  256  of light source  250 . 
     In operation, light source  250  illuminates the light director (microlens  133  in  FIG.  2   ) with illumination light  263 . Illumination light  263  propagates through the refractive spacing layer  240  to become incident on microlens  133 . Microlens  133  is configured to receive illumination light  263  and configured to direct illumination light  263  to light input coupler  172  as shaped light  267 . Microlens  133  is configured to tilt illumination light  263  to give shaped light  267  a tilt angle with respect to the light input coupler  172 . Shaped light  267  may include the optical characteristics of shaped light  167  described with respect to  FIGS.  1 A- 1 C . 
       FIG.  3    illustrates a device  300  that includes light source  150  and a microlens  333  coupled with an emission aperture  156  of light source  150 , in accordance with aspects of the disclosure. In  FIG.  3   , microlens  333  is configured to give shaped light  367  a smaller divergence angle than would be emitted in the illumination light from aperture  156  of light source  150  if microlens  333  was not coupled with (e.g. disposed over) emission aperture  156 . For example, the illumination light of light source  150  would include the characteristics of illumination light  163  if microlens  333  was not coupled with emission aperture  156 . The illumination light propagates through microlens  333  and exits microlens  333  as shaped light  367 . An AR coating may be disposed on microlens  333  and/or a planar side  129  of cladding layer  121 . In addition to reducing the divergence angle of the illumination light, microlens  133  may also provide a tilt angle to shaped light  367 . For example, if emission aperture  156  of the light source  150  is offset from a central optical axis of the refractive microlens  333 , microlens  333  may also impart a tilt angle to shaped light  367 . The tilt angle may have the characteristics described with respect to tilt angle ϕ  168  and the divergence angle of shaped light  367  may have the same characteristics described with respect to divergence angle β  169 . 
     In operation, light source  150  illuminates the light director (microlens  333  in  FIG.  3   ) with illumination light. Microlens  333  is configured to receive the illumination light and configured to direct the illumination light to light input coupler  172  as shaped light  367 . Shaped light  367  propagates through void  399  and through cladding layer  121  to become incident on light input coupler  172 . Void  399  may be an air gap or filled with an index-matching optical material. Index-matching materials may be matched to layer  121 . If microlens  333  has a similar refractive index to layer  121 , then void  399  may have a refractive index lower than microlens  333 . Shaped light  367  may include the optical characteristics of shaped light  167  described with respect to  FIGS.  1 A- 1 C . 
       FIG.  4    illustrates a device  400  that includes a refractive wedge optic  431  configured to impart a tilt angle to incident light, in accordance with aspects of the disclosure.  FIG.  4    includes light source  450  having an emission aperture  456  that emits illumination light that illuminates microlens  433 . Microlens  433  is coupled with emission aperture  456  of light source  450 . A central optical axis of microlens  433  may be aligned with a center or middle of emission aperture  456 . Light source  450  may include the features described with respect to light source  150 . In  FIG.  4   , microlens  433  is configured to give shaped light  467  a smaller divergence angle than would be emitted in the illumination light from aperture  456  of light source  150  if microlens  433  was not coupled with (e.g. disposed over) emission aperture  156 . The illumination light propagates through microlens  433  and exits microlens  433  as shaped light  467 . Shaped light  467  propagates through void  499  before becoming incident on refractive wedge optic  431 . Void  499  may be filled with an index-matching optical material or be an airgap. 
     After propagating through void  499 , shaped light  467  encounters refractive wedge optic  431  of refractive layer  430 . Wedge optic  431  is configured to impart a tilt angle to shaped light  467  to generated re-shaped light  469 . The tilt angle of re-shaped light  469  may have the characteristics described with respect to tilt angle ϕ  168  and the reduced divergence angle imparted to shaped light  467  may have the same characteristics described with respect to divergence angle β  169 . An AR coating may be disposed on refractive wedge optic  431 . Refractive layer  430  may include amorphous silicon or other refractive material. In some embodiments, the tilt angle and/or divergence angle of illumination light is adjusted by optical modifications of the light source in combination with a light director also adjusting the divergence angle and/or tilt angle of the illumination light so that the beam shaping is split between the light source and the light director. 
       FIG.  5    illustrates a device  500  including a tilt spacer  559  to provide a tilt angle to illumination light emitted by light source  550 , in accordance with aspects of the disclosure. Light source  550  may be an LED or a VCSEL. Light source  550  may be a non-visible light source or a visible light source. Light source  550  may be an infrared light source or a near-infrared light source. Light source  550  is not a flip-chip light source and has electrical contact  551  on an opposite side of light source  550  as electrical contact  552 . Tilt spacer  559  props up electrical contact  552  and tilts light source  550 . As a result, emission aperture  556  of light source  550  is also tilted with respect to a plane of optical waveguide  170  and light input coupler  172 . Tilt spacer  559  provides sufficient tilt to give illumination light  563  a tilt angle that will increase the optical coupling efficiency of light input coupler  172 . In some embodiments, tilt spacer  559  may be an electrical trace to provide power to light source  550  and include copper or other conductive metal. Electrical contact  552  may be electrically coupled to tilt spacer  559 . Tilt spacer  559  may have a height of approximately 10 to 20 microns, in some contexts, to achieve a tilt angle for light source  550  of five to eight degrees. Tilt spacer  559  may be formed of polymer or SiO 2 . 
     Microlens  133  (or a metalens  193 ) acts as a light director configured to receive the illumination light  563  (already having a tilt angle) and direct the illumination light  563  to light input coupler  172  as shaped light  567 . Microlens  133  is configured to give shaped light  567  a smaller divergence angle than an illumination light divergence angle of illumination light  563 . In  FIG.  5   , tilt spacer  559  provides the tilt angle and microlens  133  provides the reduction of the divergence angle so that shaped light  567  is more efficiently coupled into waveguide  170  by light input coupler  172 . In contrast, for  FIG.  4   , microlens  433  reduces the divergence angle and refractive wedge optic  431  provides the tilt angle so that re-shaped light  469  is more efficiently coupled into waveguide  170  by light input coupler  172 . 
       FIG.  6    illustrates a device  600  including a tilt spacer  659  to provide a tilt angle to illumination light and a microlens  633  providing a reduced divergence angle, in accordance with aspects of the disclosure. Light source  650  and may include the features described with respect to light source  150  or light source  550 . Electrical contacts  651  and  652  may have the features of electrical contacts  551  and  552 . In  FIG.  6   , tilt spacer  659  props up electrical contact  652  and tilts light source  650 . As a result, emission aperture  656  of light source  650  is also tilted with respect to a plane of optical waveguide  170  and light input coupler  172 . Tilt spacer  659  provides sufficient tilt to give the illumination light outputted by emission aperture  656  a tilt angle that will increase the optical coupling efficiency of light input coupler  172 . Microlens  633  coupled with the emission aperture  656  of light source  650  reduces the divergence angle of the illumination light from light source  650  so that shaped light  667  has a reduced divergence angle. Consequently, tilt spacer  659  provides the tilt angle and microlens  633  provides the reduced divergence angle so that shaped light  667  has the optical characteristics described above to increase optical coupling efficiency to incouple shaped light  667  into optical waveguide  170  via light input coupler  172 . 
     In operation, light source  650  illuminates the light director (microlens  633  in  FIG.  6   ) with illumination light. Microlens  633  is configured to receive the illumination light and configured to direct the illumination light to light input coupler  172  as shaped light  667 . Shaped light  667  propagates through cladding layer  121  to become incident on light input coupler  172 . Shaped light  667  may include the optical characteristics of shaped light  167  described with respect to  FIGS.  1 A- 1 C . 
       FIG.  7    illustrates a top-down view of a tapered waveguide  770  having a grating coupler  772 , in accordance with aspects of the disclosure. Tapered waveguide  170  may be used as waveguide  170 , for example. Tapered waveguide  770  includes a grating coupler  772  configured to incouple a particular wavelength of light that is matched to a light source such as light source  150 . For example, shaped light  167  may be incident on aperture  760  of grating coupler  772  and grating coupler  772  incouples the shaped light into waveguide  770  as waveguide light  779 . Dimension  724  in  FIG.  7    may be approximately 10 microns, in some contexts. Tapered waveguide  770  is one way of incoupling a beam into a waveguide and other techniques may be employed to incouple light beams into a waveguide. 
       FIGS.  8 A- 8 K  illustrate an example fabrication process of an optical structure that increases light incoupling efficiency, in accordance with aspects of the disclosure. Advantageously, the illustrated fabrication process may be compatible with complementary metal-oxide semiconductor (CMOS) fabrication techniques to manufacture a monolithic PIC. 
       FIG.  8 A  illustrates an optical structure  800  having a substrate layer  810 . Substrate layer  810  may be considered a wafer having a size (e.g. diameter) of four inches to twelve inches and a thickness of 100-1000 microns, for example. Multiple PICs may be fabricated on the same wafer, in some contexts. Substrate layer  810  may be made from silicon or glass, for example. 
     In the optical structure  800  of  FIG.  8 B , a portion of cladding layer  821  is disposed on substrate layer  810 . Cladding layer  821  may be grown or deposited onto substrate layer  810 , depending on the materials of cladding layer  821  and substrate layer  810 . Cladding layer  821  may include silicon-oxide (SiO 2 ). 
     The optical structure  800  of  FIG.  8 C  shows reflector layer  880  formed on cladding layer  821 . Reflector layer  880  may be formed of a metal reflector layer. If reflector layer  880  is diffractive, layer  880  may be disposed on cladding layer  821  and a subtractive process (e.g. diamond turning or etching) may be used to form the diffractive structures in layer  880 , for example. Alternatively, an additive CMOS process may be used to building a diffractive structure as reflective layer  880 . 
     The optical structure  800  of  FIG.  8 D  illustrates an additional portion of cladding layer  821  formed over reflector layer  880 . 
     In the optical structure  800  of  FIG.  8 E , waveguide layer  870  is formed over cladding layer  821 . Waveguide layer  870  may be formed of silicon-nitride, for example. 
     In  FIG.  8 F , light input coupler  872  is formed in waveguide layer  870 . Forming light input coupler  872  may include a subtractive process (e.g. diamond turning or etching) to form nano-pillars for an input grating configured specifically to incouple the wavelength of light emitted by a particular light source (e.g. a VCSEL or LED). In an embodiment, layer  870  is a resin material having a refractive index that is imprinted to form light input coupler  872 . 
     The optical structure  800  of  FIG.  8 G  illustrates a remaining portion of cladding layer  821  deposited over waveguide layer  870  and filling in between the nano-pillars of grating  872 . 
       FIG.  8 H  shows light director layer  830  formed on cladding layer  821  and formed over the waveguide layer  870 . Light director layer  830  may be SiN. In some embodiments (not illustrated), an AR layer may be formed between cladding layer  821  and light director layer  830  to increase light propagation efficiency through a refractive light director layer  830 . 
     In  FIG.  8 I , microlens  833  has been formed from the light director layer  830 . Microlens  833  may have the features of microlens  133 , for example. Forming microlens  833  may include etching different depths using greytone lithography techniques to achieve the designed lensing surface of microlens  833 . An AR layer may be formed over the lensing surface of microlens  833  to increase the transmission of incident light. When a metalens is used rather than refractive microlens  833 , the metalens may be formed or placed over cladding layer  821  or the metalens may be formed into light director layer  830  in a subtractive process using known CMOS fabrication techniques. A metasurface of the metalens may be made of SiN and/or amorphous silicon, for example. The metasurface may include subwavelength nanostructures. Nanostructures such as nano-pillars having a same height but different diameter may be formed to generate a planar lens. 
     In  FIG.  8 J , electrical traces  841  and  842  are formed on optical structure  800 . Traces  841  and  842  may be gold or copper and coupled to a transistor having a gate receiving an electrical signal that modulates the electrical current through a light source to be added to optical structure  800 . Traces  841  and  842  may be considered contact pads to connect electrodes of a light source and may include multilayer metal structures to form the contact pads. 
       FIG.  8 K  illustrates light source  850  being coupled to traces  841  and  842 . In particular, electrical contact  851  of light source  850  is coupled to trace  841  and electrical contact  852  of light source  850  is coupled to trace  842 . Coupling light source  850  to traces  841  and  842  may include a pick-and-place machine placing light source  850  on traces  841  and  842  and allowing solder (not illustrated) to reflow to electrically couple to electrical contacts to the traces. Hence, optical structure  800  in  FIG.  8 K  illustrates a device that may be fabricated using CMOS fabrication techniques that increases optical coupling efficiency for light sources into waveguides. 
       FIG.  9    illustrates a process  900  of fabricating a photonic integrated circuit, in accordance with aspects of the disclosure. The order in which some or all of the process blocks appear in process  900  should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated, or even in parallel. 
     In process block  905 , a waveguide layer is formed. The waveguide layer include a waveguide (e.g.  870 ) immersed in in a cladding layer (e.g.  821 ). The waveguide includes a light input coupler (e.g.  872 ). In the context of  FIG.  8 K , the combination of cladding layer  821  and waveguide  820  may be considered the “waveguide layer” and reflector  880  may optionally be included in the waveguide layer. Waveguide layer  120  in  FIG.  1 A  is also an example waveguide layer. 
     In process block  910 , a light director layer is formed over the waveguide layer. The light director layer (e.g.  830 ) includes a light director. The light director may be a microlens or a metalens. A top plane of the waveguide layer is coupled with a bottom plane of the light director layer. For example, in the context of  FIG.  8 K , planar side  829  is a top plane of the waveguide layer and plane  839  is the bottom plane of light director layer  830 . 
     In process block  915 , a light source is disposed over the light director layer. The light source is positioned to illuminate the light director of the light director layer. The light director is configured to receive illumination light from the light source and direct the illumination light to the light input coupler as shaped light. The light director is configured to tilt the illumination light to give the shaped light a tilt angle with respect to the light input coupler. 
       FIG.  10    illustrates an example head mounted device  1000 , in accordance with aspects of the present disclosure. The devices and fabrication techniques described in this disclosure may be used to provide certain components to head mounted device  1000 . A head mounted device, such as head mounted device  1000 , is one type of smart device. In some contexts, head mounted device  1000  is also a head mounted display (HMD). Artificial reality is a form of reality that has been adjusted in some manner before presentation to the user, which may include, e.g., virtual reality (VR), augmented reality (AR), mixed reality (MR), hybrid reality, or some combination and/or derivative thereof. 
     The illustrated example of head mounted device  1000  is shown as including a frame  1002 , temple arms  1004 A and  1004 B, and a near-eye optical element  1006 A and a near-eye optical element  1006 B.  FIG.  10    also illustrates an exploded view of an example of near-eye optical element  1006 A. Near-eye optical element  1006 A is shown as including an illumination layer  1010  and a display layer  1020 . 
     As shown in  FIG.  10   , frame  1002  is coupled to temple arms  1004 A and  1004 B for securing the head mounted device  1000  to the head of a user. Example head mounted device  1000  may also include supporting hardware incorporated into the frame  1002  and/or temple arms  1004 A and  1004 B. The hardware of head mounted device  1000  may include any of processing logic, wired and/or wireless data interfaces for sending and receiving data, graphic processors, and one or more memories for storing data and computer-executable instructions. In one example, head mounted device  1000  may be configured to receive wired power and/or may be configured to be powered by one or more batteries. In addition, head mounted device  1000  may be configured to receive wired and/or wireless data including video data. 
       FIG.  10    illustrates near-eye optical elements  1006 A and  1006 B that are configured to be mounted to the frame  1002 . The frame  1002  may house the near-eye optical elements  1006 A and  1006 B by surrounding at least a portion of a periphery of the near-eye optical elements  1006 A and  1006 B. The near-eye optical element  1006 A is configured to receive visible scene light  1022  at a world side  1012  of the near-eye optical element  1006 A. The visible scene light  1022  propagates through optical element  1006 A to an eye of a user of the head mounted device on an eyeward side  1009  of optical element  1006 A. In some examples, near-eye optical element  1006 A may be transparent or semi-transparent to the user to facilitate augmented reality or mixed reality such that the user can view visible scene light  1022  from the environment while also receiving display light  1023  directed to their eye(s) by way of display layer  1020 . A waveguide  1025  included in display layer  1020  may be utilized to direct the display light  1023  generated by an electronic display in an eyeward direction, although other display technologies may also be utilized in display layer  1020 . In some implementations, at least a portion of an electronic display is included in the frame  1002  of the head mounted device  1000 . The electronic display may include an LCD, an organic light emitting diode (OLED) display, micro-LED display, pico-projector, or liquid crystal on silicon (LCOS) display for generating the display light  1023 . 
     In further examples, some or all of the near-eye optical elements  1006 A and  1006 B may be incorporated into a virtual reality headset where the transparent nature of the near-eye optical elements  1006 A and  1006 B allows the user to view an electronic display (e.g., a liquid crystal display (LCD), an organic light emitting diode (OLED) display, a micro-LED display, etc.) incorporated in the virtual reality headset. In this context, display layer  1020  may be replaced by the electronic display. 
     Illumination layer  1010  includes a transparent layer that may be formed of optical polymers, plastic, glasses, transparent wafers (such as high-purity semi-insulating SiC wafers) or any other transparent materials used for this purpose. A waveguide structure  1008  is configured to receive non-visible light (e.g. near-infrared light) from a non-visible light source. Hence, the devices and fabrication techniques described with respect to  FIGS.  1 A- 9    may be utilized in illumination layer  1010  to efficiently incouple light from a light source to a waveguide structure  1008 . Waveguide structure  1008  is configured to deliver the non-visible light from the non-visible light source to outcoupling element  1011 , in  FIG.  10   . Only one waveguide structure  1008  and one outcoupling element  1011  are illustrated in  FIG.  10   , although there may be a plurality of outcoupling elements in some implementations. Furthermore, multiple light sources, and multiple waveguides may be included in a PIC that is include in illumination layer  1010 . The one or more outcoupling elements  1011  are configured to outcouple the non-visible light propagating in waveguide structure  1008  as non-visible illumination light  1013  to illuminate an eye region. 
     The non-visible illumination light  1013  may be near-infrared light, in some aspects. The non-visible light source that generates the non-visible light for waveguide structure  1008  may include one or more of light emitting diode (LED), a micro light emitting diode (micro-LED), an edge emitting LED, a vertical cavity surface emitting laser (VCSEL), on-chip integrated laser, hybrid integrated laser, or a Superluminescent diode (S-LED). Depending on the architecture, a single light source or a light source array can be used. When a single light source is used, waveguide splitters can be used to distribute the light into multiple outputs. The light source may be buried in the frame so that is out of a FOV (field of view) of a user. When an array of light sources is used, each light source can supply one output so that no waveguide splitter is needed. A waveguide splitter may be used to split the power in one waveguide into multiple waveguides. For example, a Y shaped splitter can divide a single waveguide into two channels with balanced power or designed unbalanced power. A 1×2 MMI (multimode interferometer) coupler can function similarly to a Y splitter, a 1×4 MMI splitter can divide a single waveguide into 4 channels, and so on. A Mach-Zehnder interferometer can also be used for splitting optical power of a waveguide. 
     In some implementations, a combiner layer (not illustrated) is optionally disposed between display layer  1020  and illumination layer  1010  to direct reflected non-visible illumination light that has reflected from an eye region to a camera (e.g. camera  1070 ) to capture eye-tracking images. In some implementations, camera  1070  is positioned to image the eye directly by imaging the reflected non-visible illumination light reflecting from the eye region. Camera  1070  may include a CMOS image sensor. When non-visible illumination light  1013  is infrared light, an infrared filter that receives a narrow-band infrared wavelength may be placed over the image sensor so it is sensitive to the narrow-band infrared wavelength while rejecting wavelengths outside the narrow-band, including visible light wavelengths. 
     As shown in  FIG.  10   , outcoupling element  1011  and waveguide structure  1008  are disposed within the field-of-view (FOV) of a user provided by the near-eye optical element  1006 A. While outcoupling element  1011  may introduce minor occlusions or non-uniformities into the near-eye optical element  1006 A, outcoupling element(s)  1011  and waveguide structure  1008  may be so small as to be unnoticeable or insignificant to a wearer of head mounted device  1000 . Additionally, any occlusion from outcoupling element  1011  and waveguide structure  1008  may be placed so close to the eye as to be unfocusable by the human eye and therefore outcoupling element  1011  and waveguide structure  1008  will not be noticeable to a user of device  1000 . Waveguide structure  1008  includes a transparent (to visible light) dielectric material, in some implementations. Furthermore, outcoupling element  1011  and waveguide structure  1008  may be so small that even an observer (a person not wearing device  1000  but viewing device  1000 ) may not notice outcoupling element  1011  and waveguide structure  1008 . Outcoupling element  1011  may be smaller than 75 microns at it widest/longest dimension. In an implementation, outcoupling element  1011  may be smaller than 20 microns at its widest/longest dimension and waveguide structure  1008  may be approximately 1-10 microns wide and formed with transparent materials. Waveguide structure  1008  may be approximately 100 nm to 1 micron, in some implementations. Outcoupling element  1011  may be approximately 10 microns at its widest/longest dimension, in some implementations. In contrast, actual light sources positioned in illumination layer  1010  would have a footprint of approximately 100×100 microns or larger. 
     In some implementations, optical element  1006 A may have a curvature for focusing light (e.g., display light  1023 ) to the eye of the user. The curvature may be included in the transparent layer of illumination layer  1010 . Thus, optical element  1006 A may be referred to as a lens. In some aspects, optical element  1006 A may have a thickness and/or curvature that corresponds to the specifications of a user. In other words, optical element  1006 A may be considered a prescription lens. 
     Embodiments of the invention may include or be implemented in conjunction with an artificial reality system. Artificial reality is a form of reality that has been adjusted in some manner before presentation to a user, which may include, e.g., a virtual reality (VR), an augmented reality (AR), a mixed reality (MR), a hybrid reality, or some combination and/or derivatives thereof. Artificial reality content may include completely generated content or generated content combined with captured (e.g., real-world) content. The artificial reality content may include video, audio, haptic feedback, or some combination thereof, and any of which may be presented in a single channel or in multiple channels (such as stereo video that produces a three-dimensional effect to the viewer). Additionally, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, e.g., create content in an artificial reality and/or are otherwise used in (e.g., perform activities in) an artificial reality. The artificial reality system that provides the artificial reality content may be implemented on various platforms, including a head-mounted display (HMD) connected to a host computer system, a standalone HMD, a mobile device or computing system, or any other hardware platform capable of providing artificial reality content to one or more viewers. 
     The term “processing logic” in this disclosure may include one or more processors, microprocessors, multi-core processors, Application-specific integrated circuits (ASIC), and/or Field Programmable Gate Arrays (FPGAs) to execute operations disclosed herein. In some embodiments, memories (not illustrated) are integrated into the processing logic to store instructions to execute operations and/or store data. Processing logic may also include analog or digital circuitry to perform the operations in accordance with embodiments of the disclosure. 
     A “memory” or “memories” described in this disclosure may include one or more volatile or non-volatile memory architectures. The “memory” or “memories” may be removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules, or other data. Example memory technologies may include RAM, ROM, EEPROM, flash memory, CD-ROM, digital versatile disks (DVD), high-definition multimedia/data storage disks, or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transmission medium that can be used to store information for access by a computing device. 
     A computing device may include a desktop computer, a laptop computer, a tablet, a phablet, a smartphone, a feature phone, a server computer, or otherwise. A server computer may be located remotely in a data center or be stored locally. 
     The processes explained above may be implemented with computer software and hardware. The techniques described may constitute machine-executable instructions embodied within a tangible or non-transitory machine (e.g., computer) readable storage medium, that when executed by a machine will cause the machine to perform the operations described. Additionally, the processes may be embodied within hardware, such as an application specific integrated circuit (“ASIC”) or otherwise. 
     A tangible non-transitory machine-readable storage medium includes any mechanism that provides (i.e., stores) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine-readable storage medium includes recordable/non-recordable media (e.g., read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc.). 
     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.