Abstract:
An optoelectronic integrated circuit for coupling light to or from an optical waveguide formed in an optical device layer in a near-normal angle to that layer. In an embodiment, the integrated circuit comprises a semiconductor body including a metal-dielectric stack, an optical device layer, a buried oxide layer and a semiconductor substrate arranged in series between first and second opposite sides of the semiconductor body. At least one optical waveguide is formed in the optical device layer for guiding light in a defined plane in that device layer. Diffractive coupling elements are disposed in the optical device layer to couple light from the waveguide toward the second surface of the semiconductor body at a near-normal angle to the defined plane in the optical device layer. In an embodiment, an optical fiber is positioned against the semiconductor body for receiving the light from the coupling elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of copending U.S. patent application Ser. No. 15/069,532, filed Mar. 14, 2016, which is a continuation of U.S. patent application Ser. No. 13/370,886, filed Feb. 10, 2012. The entire contents and disclosure of U.S. patent application Ser. Nos. 15/069,532 and 13/370,886 are hereby incorporated herein by reference in their entireties. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with government support under Contract No. H90011-08-C-0102 (NOBS) (Defense Advanced Research Projects Agency (DARPA)). The government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    This invention generally relates to optoelectronic integrated circuits, or photonics chips, and more specifically, to optical coupling between a waveguide and a photonics chip. 
         [0004]    Optoelectronic integrated circuits (ICs) include both electronic and optical elements within a single chip. Typical electronic elements include field effect transistors, capacitors, and resistors; and typical optical elements include waveguides, optical filters, modulators, and photodetectors. Within a given optoelectronic IC, some of the electronic elements may be dedicated to handling tasks such as data storage and signal processing. Other electronic elements may be dedicated to controlling or modulating the optical elements. Including both types of elements on a single chip provides several advantages, which include reduced layout area, cost, and operational overhead. In addition, such integration yields hybrid devices, such as an opto-isolator. 
         [0005]    The integration of optical and electronic elements has been greatly facilitated by the maturation of today&#39;s semiconductor processing techniques. For instance, conventional processing techniques may be adapted to create silicon-based prisms, waveguides, and other optical devices. 
         [0006]    One device, however, that has been difficult to integrate is a silicon based laser or light source. As a result, most optoelectronic ICs are adapted to receive an externally applied light beam from a laser or an optical fiber. Unfortunately, introducing a light beam to an IC can often be difficult. For example, in order for an optoelectronic IC to accommodate a light beam, the spot size and the numerical aperture (NA) of the beam may need to be appropriately matched to optical elements within an IC. 
         [0007]    Two primary methods for coupling standard single-mode optical fibers to/from high-index contrast photonic platforms are (1) edge coupling based on inverse tapered waveguides, and (2) near-normal coupling based on resonant waveguide gratings. With the former approach, the inverse tapered waveguides (e.g., Si) are evanescently coupled to lower index contrast waveguides (e.g., SiON) with mode-field diameters larger than the high-index-contrast waveguides but still much smaller than those of standard single-mode optical fiber. 
         [0008]    This edge coupling approach may suffer from limited real estate available at the chip edge for optical I/O, limited I/O density limited to 1-D fiber arrays, and additional complexity (such as fiber tapers, lenses, etc.) required for interfacing the unmatched mode field diameters at the chip edge and fiber facet. The second approach alleviates many of these problems. First, it allows coupling of 2-D fiber arrays to 2-D layouts of waveguide gratings on the chip surface. Second, it can provide fiber-matched mode-field diameters in well-designed gratings. However, the intrinsic trade-off in the grating couplers between coupling efficiency and bandwidth, as well as a fundamental coupling efficiency limit of less than unity, plagues systems seeking to implement this technology. Additionally, the standard approach of coupling from the top of the photonic device layer is not compatible with CMOS-processing, which includes several layers of metals and dielectrics above the photonic device layer. 
         [0009]    In one prior art system, a metal mirror may be introduced above a grating to retransmit the uncoupled light back into the grating a second time for increased efficiency. In this implementation, the optical beam passes through the chip substrate, but the optical properties of the substrate are not leveraged for increased coupling performance. Instead, a complex lensing system is used near a remote fiber facet. Another prior art system implements prisms within and around the substrate to couple light into and out of the photonic waveguides from the bottom side of the chip. However, evanescent coupling schemes, rather than grating-based coupling schemes, are used, which require a costly thinning of the buried oxide layer to less than 250 nm (typically 1 to 3 um). 
       BRIEF SUMMARY 
       [0010]    Embodiments of the invention provide an optoelectronic integrated circuit for coupling light to or from an optical waveguide formed in an optical device layer in a near-normal angle to that layer. In an embodiment, the integrated circuit comprises a semiconductor body including a metal-dielectric stack, an optical device layer, a buried oxide layer and a semiconductor substrate arranged in series between first and second opposite sides of the semiconductor body. At least one optical waveguide is formed in the optical device layer for guiding light in a defined plane in the optical device layer. A plurality of diffractive coupling elements are disposed in the optical device layer to couple light from the waveguide to the second surface of the semiconductor body at a near-normal angle to the defined plane in the optical device layer. 
         [0011]    In an embodiment, an array of lenses is formed in the second surface of the semiconductor body to receive light from or to transmit light to the plurality of diffractive coupling elements. 
         [0012]    In an embodiment, a metallic reflective layer is formed in the metal-dielectric stack adjacent the diffractive coupling elements to redirect light to the diffractive coupling elements and therein enhance optical coupling efficiency. 
         [0013]    In one embodiment, the plurality of diffractive coupling elements comprise a grating patterned into the at least one optical waveguide to allow near-normal light reflection to and from the optical waveguide. 
         [0014]    In an embodiment, one or more mechanical elements are fabricated on the second surface of the semiconductor body to facilitate positioning an external optical fiber or optical waveguide on the semiconductor body in a position aligned with the array of lenses. 
         [0015]    In one embodiment, one or more grooves are formed in the second surface of the semiconductor body to reflect light from the diffractive coupling elements into a direction substantially parallel with the first and second surfaces of the semiconductor body to an edge of the semiconductor body. 
         [0016]    In an embodiment, at least one of said one or more grooves is a V-shaped groove extending into the semiconductor substrate from the second surface of the semiconductor body and aligned with the diffractive coupling elements. 
         [0017]    In one embodiment, one or more mechanical elements are fabricated on the semiconductor body to facilitate positioning an external optical fiber or optical waveguide on the semiconductor body in a position aligned to receive the light reflected by said one or more grooves. 
         [0018]    Embodiments of the invention provide a method of fabricating an optoelectronic integrated circuit. In an embodiment, the method comprises fabricating a semiconductor body including a metal-dielectric stack, an optical device layer, a buried oxide layer and a semiconductor substrate arranged in series between first and second sides of the semiconductor body. Fabricating this semiconductor body includes forming at least one optical waveguide in the optical device layer for guiding light in a defined plane in the optical device layer, and forming a plurality of diffractive coupling elements in the optical device layer to couple light from the waveguide to the second surface of the semiconductor body at a near-normal angle to said defined plane. 
         [0019]    In one embodiment, an array of lenses is formed in the second surface of the semiconductor body to receive light from or to direct light to the plurality of diffractive coupling elements. 
         [0020]    In an embodiment, a metallic reflective layer is formed in said metal-dielectric stack adjacent the diffractive coupling elements to redirect light to the diffractive coupling elements and therein enhance optical coupling efficiency. 
         [0021]    In one embodiment, the plurality of diffractive coupling elements are formed by patterning a grating into the optical waveguide to allow near-normal light reflection to and from the optical waveguide. 
         [0022]    In an embodiment, one or more mechanical elements are fabricated on the second surface of the semiconductor body to facilitate positioning an external optical fiber or optical waveguide against said third side of the semiconductor body in a position aligned to receive the light reflected by said at least one groove. 
         [0023]    Embodiments of the invention provide an optoelectronic transceiver module. The module comprises a semiconductor body including a metal-dielectric stack, an optical device layer, a buried oxide layer and a semiconductor substrate arranged in series between first and second sides of the semiconductor body. At least one optical waveguide is formed in the optical device layer for guiding light in a defined plane in the optical device layer. A plurality of diffractive coupling elements are disposed in the optical device layer to direct light from the waveguide toward the second surface of the semiconductor body at a near-normal angle to said defined plane. A fiber array connector is assembled on the semiconductor body, and this connector comprises an array of optical fibers arranged in a plane approximately parallel to the defined plane in the optical device layer for receiving said light from the waveguide. 
         [0024]    In one embodiment, the fiber array connector is assembled on the second surface of the semiconductor body, and the fiber array connector includes an array of lenses for focusing the light from the waveguide onto the fiber array. 
         [0025]    In an embodiment, the fiber array connector includes mechanical alignment elements for positioning the fiber array connector in a specified position on the semiconductor body. 
         [0026]    In one embodiment, the semiconductor body includes an array of lenses formed on the second surface of the semiconductor body to receive light from the plurality of diffractive elements. The mechanical alignment elements position the fiber array connector on the semiconductor body with the array of lenses of the fiber array connector aligned with the array of lenses of the semiconductor body. 
         [0027]    In an embodiment, the semiconductor body includes at least one groove formed in the second surface of the semiconductor body to reflect light from the diffractive coupling elements into a direction substantially parallel with the first and second surfaces of the semiconductor body to an edge of the semiconductor body, said edge being on a third side of the semiconductor body, between the first and second sides thereof. The fiber array connector comprises one or more mechanical alignment elements to facilitate positioning the fiber array connector adjacent said edge of the semiconductor body in a position aligned to receive the light reflected by said at least one groove. 
         [0028]    Embodiments of the invention provide backside optical coupling from optical waveguides fabricated on the front surface of a photonic chip through the semiconductor substrate. Embodiment of the invention use grating-based near-normal optical coupling schemes disposed on the front waveguide surface in combination with lens elements and alignment features integrated on the back surface of the substrate. In embodiments of the invention, the addition of metal mirrors disposed over the grating structure is used to enhance efficiency. The grating coupler is also compatible with polarization diversity schemes, mode-matching to specific optical fiber mode dimensions, and 2-D fiber array coupling. 
         [0029]    Embodiments of the invention can apply to passive photonic circuits, photonic structures implemented with electronics in a monolithic process, and photonic structures implemented with electronics through hybrid integration such as flip-chip or wire bonding. Embodiments of the invention are impervious to these details since these various embodiments all share in common the buried oxide layer beneath the photonic device layer, the substrate beneath the buried oxide, and the availability of metal above the photonic device layer. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0030]      FIG. 1  is a cross-section of an optoelectronic integrated circuit according to an embodiment of the invention. 
           [0031]      FIG. 2  shows the optoelectronic integrated circuit of  FIG. 1  in a packaged photonic optical transceiver. 
           [0032]      FIG. 3  illustrates an alternate embodiment of the invention, in which a V-shaped groove is formed in a surface of the photonic chip. 
           [0033]      FIG. 4  shows an embodiment of the invention in which an external waveguide or fiber is aligned with the direction of the light reflected by the waveguide grating. 
           [0034]      FIG. 5  illustrates the use of a metallic mirror in the embodiment of  FIG. 4 . 
           [0035]      FIG. 6  illustrates that the optical coupling in embodiment of the invention is bidirectional. 
           [0036]      FIG. 7  shows an embodiment in which lenses are formed in the bottom surface of the substrate of the photonic chip. 
           [0037]      FIG. 8  illustrates that the lens on the bottom surface of the chip may be comprised of a series of concentric rings. 
           [0038]      FIG. 9  shows an embodiment in which mechanical alignment features are formed on the substrate of the photonic chip. 
           [0039]      FIG. 10  illustrates a photonic chip having a series of waveguide gratings, and shows a series of external waveguides or fibers adjacent a bottom surface of the chip. 
           [0040]      FIG. 11  shows a photonic chip having a series of waveguide gratings, and shows a series of external waveguides or fibers adjacent a lateral edge of the chip. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    Embodiments of the invention provide an optical coupling structure that receives a light beam and couples the beam into a waveguide. In a reverse manner, the optical coupling structure may also move light away from the waveguide and out of an optoelectronic IC. The methods of fabricating such an optical structure described below implement a variety of conventional semiconductor processes and combinations thereof, which include: lithography, etching, thin-film deposition, and anti-reflective coatings. Moreover, some of the embodiments also include methods that may employ conventional wafer to wafer attachment/bonding processes. 
         [0042]    For simplicity, the description below and related figures describe an optical coupling structure that includes a silicon-based waveguide that consists of single crystalline silicon layer. In alternative embodiments the waveguide may be polycrystalline silicon and it may also comprise multiple layers with specific characteristics for each individual layer (i.e., doping, thickness, resistivity, etc.). The thickness of the waveguide may be tailored to accommodate one or more modes of a propagating light beam. In addition, although the described embodiments below use silicon-based optical elements, other types of high index materials (i.e., gallium arsenide, lithium niobate, indium phosphide, etc.) may replace silicon-based elements. Further, the waveguide and coupling regions may take on a variety of shapes and sizes. 
         [0043]      FIG. 1  is a cross-section of an optical coupling structure in accordance with an embodiment of the invention. This embodiment employs a silicon-on-insulator (SOI) wafer  100  (although other high-index-contrast material such as InP, SiN, etc. may be used) in which optical waveguides, represented at  102 , are patterned into the silicon device layer  104 . Typical single-mode waveguides are 0.2 um to 0.3 um in height by 0.4 um to 0.6 um in width. A buried oxide layer  106 , typically 1 to 3 um thick, exists on the SOI wafer beneath the silicon device layer and above the silicon substrate layer  110 , and a metal layer  112  exists above the device layer  104 . 
         [0044]    Both passive (e.g., wavelength-division multiplexers and demultiplexers, waveguide interconnects, power splitters and combiners, etc.) and active (e.g. modulators, detectors, switches, and phase shifters) components may be fabricated in the silicon device layer  104  together with electronic components used to interface the photonics and for other processing functions.  FIG. 1  also shows an external fiber or waveguide  114  that includes a connector  116 , a turning mirror  120  and one or more passive mechanical alignment elements  122 . 
         [0045]    In an embodiment, the substrate material for the photonic chip is Si; photonic circuits have been demonstrated to offer extremely high-density integration (nm-scale), CMOS and SiGe bipolar process compatibility, and low fabrication cost. Optical devices (modulators and detectors) have been demonstrated to operate at speeds up to 40 Gb/s and beyond. In addition, wavelength division multiplexing (WDM) can be straightforwardly incorporated into the Si photonic circuits, offering off-chip optical coupling of 10× to 100× more channels per fiber compared to parallel implementations. This feature takes full advantage of the integration capability of the Si photonic structures; because of their sub-micron dimensions, tens to hundreds of thousands of devices can be incorporated in a square centimeter, driving the need for dense, highly efficient, and broadband optical coupling schemes. 
         [0046]    In the embodiment shown in  FIG. 1 , a grating  130  is patterned into the photonic waveguides  102  in the device layer  104  so as to allow near-normal light reflection up/down from the waveguide. The substrate  110  is used to enhance performance by focusing the light into/out of the grating couplers via refractive lenses  132  etched into or bonded onto the bottom  134  of the substrate layer (back side of the wafer). The addition of metal mirrors  136  disposed over the grating structure  130  is used to enhance efficiency by redirecting the upward traveling light back toward the grating and redirecting this light along the desired optical path. 
         [0047]    In an embodiment, lens elements  132  are provided on the back surface  134  of the substrate  110  aligned on the optical path. An array of lenses are provided where each lens within the array corresponds to individual waveguide channels. The lens elements are designed to either collimate the output light from the photonic chip or to refocus the light into an external fiber or waveguide array. In the reverse path, the lens elements accept the light from the external fiber or waveguide array and focus the light onto the grating coupler which redirects the light into the photonic waveguides  102 . An embodiment of the invention employs lens elements lithographically fabricated on the back side of the wafer. Furthermore, mechanical alignment features  140  can also be fabricated on the back surface  134  to facilitate passive alignment to the external fiber or waveguide array  114 . 
         [0048]    The external fiber or waveguide connector  116  contains an array of optical fibers or waveguides arranged approximately parallel to the photonic chip surface, and also includes turning mirrors  120  to redirect light normal to the surface and into the fiber array. The fiber connector may or may not include an array of lens elements, one for each fiber element. The connector housing  116  can be fabricated from low-cost injection-molded plastic or lithographically patterned using a semiconductor substrate, such as a Si optical bench. 
         [0049]    The grating structure  130  may be implemented by etching sub-wavelength facets into a fraction of the full silicon layer thickness. For example, the etched areas may then be filled with dielectric material, such that a region of alternating silicon/dielectric facets results. Without further enhancements, the grating diffracts light in both the upward and downward directions in substantially equal amounts. 
         [0050]    Several methods can be used to improve coupling efficiency in the backside (through substrate direction). For example, the mirror  136  may be disposed at a distance of a half-wavelength above the grating  130 , where the wavelength includes the effect of the index of refraction of the dielectric layer between the grating and the mirror. Alternatively, the mirror  136  may be composed of a dielectric stack of alternating low-index and high-index layers. 
         [0051]    Another method of enhancing the efficiency is to blaze the grating  130 , which is to pattern the facets of the silicon grating such that the orientation of these facets provides substantial reflection in the backside (through substrate) direction. 
         [0052]    An alternate method to reduce the unwanted upward diffracted light is to dispose optically absorbing layers above the grating feature. This method does not improve efficiency, but will reduce unwanted scattered light which may result in optical crosstalk. 
         [0053]    The above discussion describes coupling from the waveguide and output at the back (substrate) side of the chip. The discussed methods also provide similar efficiency enhancements in the reverse direction—that is, input from the back of the chip and output into the silicon waveguide. 
         [0054]    An example of a fully packaged optical transceiver module, in accordance with an embodiment of this invention, is depicted in  FIG. 2 . The photonic chip  100  is flip-chip attached to an organic or ceramic package  140 . The fiber (or waveguide) connector  116  is assembled onto the back surface  134  using passive alignment features, for example as discussed above. A heat spreader  142  is also attached to the back surface of the photonic chip. The heat spreader also includes mechanical features to accommodate the fiber array connector. 
         [0055]      FIG. 3  illustrates an alternate embodiment. In this embodiment, one or more V-grooves  160  are fabricated into the rear or second surface  134  of the photonic chip and used to reflect light from the diffractive coupling elements into the substrate in a direction nearly parallel with the first and second surfaces  162 ,  134  of the chip to an edge  164  of the chip. This edge of the chip may be made optically smooth by a cleaving, polishing or etching step. 
         [0056]    An external fiber or waveguide  170  may be secured to the photonic chip at or adjacent to edge  164  to receive the light reflected by groove  160 . This external fiber or waveguide  170  may also be used to direct light into the chip  100 , toward groove  160 , which reflects this light toward grating  130 . As represented at  172 , one or more lenses may be disposed at or adjacent the end of fiber or waveguide  170  to focus light into or transmitted from the fiber or waveguide. 
         [0057]      FIGS. 4-11  show additional alternate embodiments. With the embodiment shown in  FIG. 4 , the external fiber or waveguide  202  is aligned with the direction of the light reflected by the waveguide grating  130 . In this embodiment, optical coupling is performed via the waveguide grating coupler using a refractive substrate with lensed  204  bottom face  134 . 
         [0058]    With the embodiment illustrated in  FIG. 5 , optical coupling is enhanced through top mirror  136  in the metal stack  112 . Optical coupling, as in the embodiment of  FIG. 4 , is performed via the waveguide grating coupler  130  using a refractive substrate with lensed  204  bottom face. This embodiment may also include a polarization diversity scheme via polarization-splitting waveguide grating and mode-matching to various fiber dimensions. 
         [0059]      FIG. 6  illustrates that the optical coupling is bidirectional—light may be transmitted from, as well as directed to, the external optical waveguide  202 . It may be noted, here too, that the embodiment may include a polarization diversity scheme via polarization splitting waveguide grating and mode-matching to various fiber dimensions. 
         [0060]      FIG. 7  shows an embodiment in which the lens  220  on the substrate  110  is formed in the substrate so that the lens does not project beyond the surface  134  of the substrate. 
         [0061]    As illustrated in  FIG. 8 , the lens  230  on the bottom surface  134  of the substrate  110  may be comprised of a series of concentric rings. 
         [0062]      FIG. 9  shows an embodiment in which mechanical alignment features  240  are formed on substrate  110 , around or adjacent lens  220 . 
         [0063]      FIG. 10  illustrates a photonic chip having a series of waveguide gratings  250 , and a series of external waveguides or fibers  252 . Each of the waveguides or fibers  252  is associated with one of the waveguide gratings  250  and may be used to direct light to or receive light from that one grating. 
         [0064]      FIG. 11  shows a photonic chip having a V-shaped groove  260 , similar to the groove  160  of  FIG. 3 , formed in substrate  110  of the chip. As shown in  FIG. 11 , this photonic chip also includes a series of waveguide gratings  262 , and a series of external waveguides or fibers  264  are associated with these gratings. These external waveguides or fibers are located adjacent a lateral edge  164  of the chip, between surfaces  162 ,  134 . Each of these external waveguides or fibers  264  are associated with one of the waveguide gratings and may be used to direct light to or receive light from that grating. 
         [0065]    While it is apparent that the invention herein disclosed is well calculated to achieve the features discussed above, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.