Abstract:
A light shield may be formed in photonic integrated circuit between integrated optical devices of the photonic integrated circuit. The light shield may be built by using materials already present in the photonic integrated circuit, for example the light shield may include metal walls and doped semiconductor regions. Light-emitting or light-sensitive integrated optical devices or modules of a photonic integrated circuit may be constructed with light shields integrally built in.

Description:
TECHNICAL FIELD 
     The present disclosure relates to optical devices and modules, and in particular to photonic integrated circuits. 
     BACKGROUND 
     Photonic integrated circuits include multiple optical components integrated on a common substrate, typically a semiconductor substrate. The optical components may include arrays of elements such as waveguides, splitters, couplers, interferometers, modulators, filters, etc., and may have similar or different optical processing functions. Photonic integrated circuits may be built by bonding together several optical, electro-optical, or optoelectronic chips. Electrical driver chips may also be attached to optoelectronic chips and electrically coupled by solder bumps or wirebonds. 
     Structurally, photonic integrated circuits resemble electronic integrated circuits, with optical waveguides for conducting optical signals between different optical components. Due to integrated character of optical components and connections, photonic integrated circuits may be suitable for mass production to a similar degree integrated electronic circuits are, potentially allowing significant economy of scale. Silicon-based photonic integrated circuits in particular may benefit from a well-developed material, technological, and knowledge base of silicon-based microelectronics industry. 
     It may be desirable to reduce size of photonic integrated circuits to fit more circuits on a same semiconductor wafer. To achieve size reduction, individual circuit components need to be more densely packed. There is, however, a limit on how densely the components may be packed. When distances between the components are too small, optical crosstalk may result. The optical crosstalk occurs because light scattered from one component may be coupled to a nearby component, impacting that component&#39;s optical performance. Amplifiers, lasers, and photodetectors may be particularly sensitive to optical crosstalk caused by stray light from neighboring components. 
     One typical example of a light-scattering component is a Mach-Zehnder interferometer of an optical modulator. When light modes in two arms of the Mach-Zehnder interferometer are in counter phase, a Y-junction combiner combining the two arms does not couple light into the output waveguide of the Y-junction combiner. Instead, the light is coupled into a radiative mode, causing the light to scatter throughout the photonic integrated circuit. Another typical example of a light-scattering component is an in-coupler of light. An in-coupler disposed near an edge of a photonic integrated circuit may scatter light escaped the core of an input waveguide due to an optical misalignment, imperfection of the input optical mode, etc. The scattered light may become guided by various layers of the photonic integrated circuit, causing extensive “ringing”, i.e. optical crosstalk. 
     Thus, not only is optical crosstalk a limiting factor of miniaturization of photonic integrated circuits, it may also be a performance-degrading factor, and a significant design constraint. In prior-art photonic integrated circuits, the optical components are spaced apart to reduce the effect of optical crosstalk. This increases the overall dimensions of photonic integrated circuits, raising manufacturing costs. 
     SUMMARY 
     In accordance with an aspect of the present disclosure, a light shield structure may be formed between integrated optical devices of a photonic integrated circuit. Preferably, a light shield structure is formed using the very materials used to build the photonic integrated circuit, i.e. the materials already present in the circuit and compatible with the material system of the circuit. Metal layers, metal vias, and doped semiconductor regions may be used to surround light-sensitive and/or light-emitting integrated optical components or modules. Thus, a light shield may be integrally built in. 
     In accordance with an aspect of the disclosure, there is provided a photonic integrated circuit comprising a substrate, first and second integrated optical devices over the substrate, and a light shield structure between the first and second integrated optical devices. The light shield structure is configured to suppress optical crosstalk between the first and second integrated optical devices. For example, the light shield structure may include an opaque structure for suppressing i.e. absorbing, reflecting, scattering light propagating between the first and second integrated optical devices, such as a light emitting device and a photodetector. In a preferred embodiment, the opaque structure has optical transmission of less than 10%. 
     In one exemplary embodiment, the opaque structure may include a first opaque wall fully or partially surrounding the first integrated optical device, e.g. on all four sides, or on three sides when the first integrated optical device is disposed near an edge of a photonic integrated circuit. Openings may be provided in the first opaque wall for optical waveguides to extend through the openings. For silicon-based systems, the first opaque wall may include heavily doped silicon, e.g. doped at a carrier concentration of at least 10 18  cm −3 . 
     In one embodiment, the opaque structure is not coplanar with the first or second integrated optical devices. The opaque structure may include a metal structure disposed farther away from the substrate than the first integrated optical device, or closer to the substrate. The light shield structure may include a second opaque wall extending from the first opaque wall and surrounding the first integrated optical device. The light shield structure may also include a photonic crystal, a plasmonic structure, a random or semi-random scatterer, etc. 
     In accordance with another aspect of the disclosure, the light shield structure may include a dielectric layer and a channel or trench extending through the dielectric layer from the first opaque wall and surrounding the first integrated optical device. The channel or trench may be filled e.g. with metal or semiconductor, forming a second opaque wall extending from the first opaque wall. Furthermore, a light-shielding metal or semiconductor layer may be disposed over the first integrated optical device. The light-shielding metal or semiconductor wall may extend to the metal or semiconductor layer, thus providing a nearly complete integrated enclosure for the first integrated optical device. Similar light shielding structures may be provided around the second integrated optical device as well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings, in which: 
         FIG. 1A  is a plan view of a photonic integrated circuit of the present disclosure; 
         FIG. 1B  is a side cross-sectional view of the photonic integrated circuit of  FIG. 1A , taken in a plane B-B shown in  FIG. 1A ; 
         FIG. 2  is a three-dimensional partial cut-out view of a photonic integrated circuit including a metal light shield; 
         FIG. 3  is a three-dimensional partial cut-out view of a photonic integrated circuit including a semiconductor light shield; 
         FIG. 4A  is a frontal cross-sectional view of a shielded waveguide-coupled photodetector according to the present disclosure, wherein electrodes of the photodetector perform the light shielding function; 
         FIG. 4B  is a plan view of the shielded waveguide-coupled photodetector of  FIG. 4A ; 
         FIG. 5  is a top view of a shielded waveguide Y-junction according to the present disclosure; 
         FIG. 6  is a top view of a shielded edge coupler according to the present disclosure; 
         FIG. 7  is a top view of a shielded grating coupler according to the present disclosure, featuring an optional shielded serpentine waveguide; 
         FIG. 8  is a top view of a shielded optical device, the light shielding structure including a Bragg grating structure; 
         FIG. 9  is a frontal cross-sectional view of a shielded integrated optical device according to another aspect of the present disclosure; and 
         FIG. 10  is a frontal cross-sectional view of a photonic integrated circuit of the disclosure including and an opaque wall extending between the two integrated optical devices for reducing optical crosstalk between them. 
     
    
    
     DETAILED DESCRIPTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. 
     Referring to  FIGS. 1A and 1B , a photonic integrated circuit  100  of the present disclosure includes a substrate  150 , first  101  and second  102  integrated optical devices over the substrate  150 , and a light shield structure  108  between the first  101  and second  102  integrated optical devices. By way of a non-limiting example, the first integrated optical device  101  may include a slab optical waveguide section  121  coupled to input  151  and output  152  waveguides. The light shield structure  108  may include any opaque structure, e.g. a metal structure, configured to suppress optical crosstalk between the first  101  and second  102  integrated optical devices. In the embodiment shown in  FIGS. 1A and 1B , the light shield structure  108  includes a first opaque wall  131  surrounding the first integrated optical device  101 . An optional second opaque wall  132  may extend from the first opaque wall  131 , surrounding the first integrated optical device  101  as shown in  FIG. 1B . In one embodiment, a metal or semiconductor shield layer (not shown for brevity) may extend over the first integrated optical device  101  such that the second opaque wall  132  extends to the metal or semiconductor shield layer. 
     The first opaque wall  131  and/or second opaque wall  132  may include an optically absorbing material. Furthermore, the first opaque wall  131  and/or second opaque wall  132  may be at least partially reflecting, and/or scattering, to ensure that the first opaque wall  131  effectively functions as a light shield. In one embodiment, the first opaque wall  131  and/or second opaque wall  132  has optical transmission of less than 10%, and more preferably less than 5%, of the incoming and/or outgoing stray light. 
     Referring specifically to  FIG. 1A , the first opaque wall  131  may surround the first integrated optical device  101 , while leaving an opening for at least one waveguide, e.g. openings  141 ,  142  for the input  151  and output  152  waveguides, respectively. The term “surrounds” is understood herein as allowing for openings in a surrounding structure if required, e.g. the openings  141 ,  142  are provided in the first opaque wall  131  for the input  151  and/or output  152  waveguides. 
     Referring specifically to  FIG. 1B , the light shield structure  108  may be not coplanar with the first integrated optical device. In the embodiment shown, the light shield structure  108  does not extend to the plane of the first integrated optical device, being farther away from the substrate  150  than the first integrated optical device. This may be advantageous in embodiments where the light shield structure  108  includes a metal structure, and the first integrated optical device  101  includes a semiconductor structure under the metal. The light shield structure  108  may also be closer to the substrate  150  than the first integrated optical device  101 . The light shield structure  108  may include not only an opaque absorptive structure but also nano- and microstructures such as a photonic crystal, a plasmonic structure, or a random or semi-random scatterer, for example. 
     In some embodiments of the present disclosure, at least one of the first  101  and/or the second  102  integrated optical device may be manufactured on additional substrates bonded to the substrate  150 . Alternatively, at least one of the first  101  and/or the second  102  integrated optical device may be monolithically fabricated on the substrate  150 . Furthermore, in some embodiments, the first integrated optical device  101  may include a light emitting device such as a laser or a semiconductor optical amplifier (SOA) e.g. a reflective SOA and/or traveling-wave SOA, while the second integrated optical device  102  may include a receiver, a photodetector, etc.; or the other way around. The first  101  and/or second  102  integrated optical devices may be comprised of Si, SiO 2 , doped glass, SiON, SiN, InP, AlGaAs, GaAs, InGaAsP, InGaP, InAlAs, and InGaAlAs. By way of a non-limiting example, the substrate may include Si, GaAs and InP. 
     Referring to  FIG. 2 , a photonic integrated circuit  200  is a variant of the photonic integrated circuit  100  of  FIGS. 1A and 1B , and includes similar elements. The photonic integrated circuit  200  of  FIG. 2  includes a metal wall  231 . The metal wall  231  (only one half is shown in  FIG. 2  for clarity) may be disposed on the same layer as the first integrated optical device  101  and may surround the first integrated optical device  101 . A metal layer  113  may be disposed on top of the metal wall  231  over the first integrated optical device  101 , for extra protection against stray light. 
     In accordance with one aspect of the present disclosure, an integrated photodetector of a photonic integrated circuit may be optically shielded using an opaque wall structure made of the very material a photosensitive layer of the integrated photodetector is made of, although a doping level may be adjusted for better absorption of light. Referring to  FIG. 3 , a photonic integrated circuit  300  is a variant of the photonic integrated circuit  100  of  FIGS. 1A and 1B , and includes similar elements. The photonic integrated circuit  300  of  FIG. 3  includes an optically absorbing wall, e.g. a semiconductor opaque wall  331  surrounding the first integrated optical device  101  and shielding the first integrated optical device  101  from exterior light  309 . In one embodiment, the semiconductor opaque wall  331  is made of germanium. In another embodiment, the semiconductor opaque wall  331  is made of silicon doped to a carrier concentration of at least 10 18  cm −3 . Preferably, the semiconductor opaque wall  331  should have optical transmission of less than 10%, and more preferably less than 5% of the incoming stray light  309 . 
     Turning now to  FIGS. 4A and 4B , an integrated photodetector  400  of the present disclosure includes an isolating silicon substrate  402  including a buried oxide layer  403  on a silicon underlayer  401 , a slab optical waveguide  421 , and a photosensitive slab  422  optically coupled to the slab optical waveguide  421 . A first electrode  431  may be electrically coupled to the photosensitive slab  422  for conducting a photoelectric signal provided by the photosensitive slab  422  upon illumination with light guided by the slab optical waveguide  421 . The first electrode  431  may encircle or surround the photosensitive slab  422  as shown in  FIG. 4B , thus functioning as a light shield for absorbing or reflecting stray light  409  propagating towards the photosensitive slab  422 . A second electrode  432  may be disposed on top of the photosensitive slab  422 , thus shielding the photosensitive slab  422  from ambient light  488 . 
       FIGS. 4A and 4B  illustrate but one example of an electrode structure having direct current (DC) or radio frequency (RF) electrodes configured for usage as light shields. More generally, an optical device may be shielded by surrounding light-emitting or light-sensitive portions of the optical device with an electrode structure of the optical device, e.g. photodetector electrodes, modulator electrodes, etc. 
     Referring to  FIG. 5 , a photonic integrated circuit  500  is an embodiment of the photonic integrated circuit  100  of  FIGS. 1A and 1B , and includes similar elements. The photonic integrated circuit  500  of  FIG. 5  includes a substrate  502  and a first opaque wall  531 . The photonic integrated circuit  500  further includes a waveguide Y-junction  521  ( FIG. 5 ) as an embodiment of the first integrated optical device  101  ( FIG. 1B ). The first opaque wall  531  ( FIG. 5 ) of the photonic integrated circuit  500  may surround the waveguide Y-junction  521 , e.g. by repeating the shape of the waveguide Y-junction  521  to capture any light coupled into radiative modes. 
     Turning to  FIG. 6 , a photonic integrated circuit  600  is another embodiment of the photonic integrated circuit  100  of  FIGS. 1A and 1B , and includes similar elements. The photonic integrated circuit  600  of  FIG. 6  includes a substrate  602  and a first opaque wall  631 . The photonic integrated circuit  600  further includes an edge coupler  621 . The edge coupler  621  ( FIG. 6 ) may be disposed proximate an edge  607  of the substrate  602 . The first opaque wall  631  partially surrounds the edge coupler  621 , leaving the edge  607  available for coupling an optical beam  680  to the edge coupler  621  via an optional external lens  682 . A waveguide  651  is coupled to the edge coupler  621 . The waveguide  651  extends through an opening  641  in the opaque wall  631  for outputting the coupled optical beam  680 . 
     Referring to  FIG. 7 , a photonic integrated circuit  700  is yet another embodiment of the photonic integrated circuit  100  of  FIGS. 1A and 1B , and includes similar elements. The photonic integrated circuit  700  of  FIG. 7  includes a substrate  702  and a first opaque wall  731 . The photonic integrated circuit  700  further includes a grating coupler  721  for optically coupling to an external optical fiber or waveguide, not shown. The grating coupler  721  ( FIG. 7 ) corresponds to the first integrated optical device  101  ( FIG. 1B ). The first opaque wall  731  surrounds the grating coupler  721 . The first opaque wall  731  has an opening  741  to pass through a waveguide  751  optically coupled to the grating coupler  721 . In the embodiment shown, the waveguide  751  includes serpentine structure including a plurality of alternating turns  781 . At least one turn  781  may be provided. 
     First  771  opaque side walls and second  772  opaque side walls may be provided, as a part of an optical shield structure. The first  771  opaque side walls and second  772  opaque side walls run on both sides of the serpentine structure, so that first  771  opaque side walls and second  772  opaque side walls may absorb or redirect scattered light emitted by the waveguide  751 . The first  771  opaque side walls and second  772  opaque side walls may provide better stray light capturing than straight walls. Furthermore, a second opaque wall, not shown, may be disposed on the first opaque wall  731 , and/or on the first  771  and second  772  opaque side walls. 
     Referring now to  FIG. 8 , a photonic integrated circuit  800  is yet another embodiment of the photonic integrated circuit  100  of  FIGS. 1A and 1B , and includes similar elements. The light shield structure of the photonic integrated circuit  800  of  FIG. 8  includes a Bragg structure  871  on a substrate  802 . The Bragg structure  871  is configured for out-coupling stray light. The Bragg structure  871  may include a plurality of concentric or parallel walls in the first layer surrounding an integrated optical device  820 , as shown. 
     Turning to  FIG. 9 , a photonic integrated circuit  900  is yet another embodiment of the photonic integrated circuit  100  of  FIGS. 1A and 1B , and includes similar elements. The photonic integrated circuit  900  includes a substrate  902 , which includes a first dielectric layer  911 , such as silicon oxide, for example, on the substrate  902 . The photonic integrated circuit  900  further includes of an integrated optical device  908 . The integrated optical device  908  is disposed between the first dielectric layer  911  and a second dielectric layer  912 . A channel  990  extends through the second dielectric layer  912 , surrounding the integrated optical device  908  for absorbing or redirecting stray light. To improve stray light rejection, a metal wall  991  may be formed in the channel  990 . The metal wall  991  may extend through the second dielectric layer  912  running around the integrated optical device  908 . To further suppress optical crosstalk and reject stray light, a metal overlayer  992  may be disposed over the integrated optical device  908 . For better stray light rejection, the metal wall  991  may extend upwards to the metal overlayer  992 . 
     Referring now to  FIG. 10 , a photonic integrated circuit  1000  is a variant of the photonic integrated circuit  100  of  FIGS. 1A and 1B , and includes similar elements. The photonic integrated circuit  1000  of  FIG. 10  may include a SOI substrate  1002  including a buried oxide layer  1003  on a silicon underlayer  1001 , and first  1021  and second  1022  integrated optical devices fabricated on the SOI substrate  1002 . An opaque wall  1031  extends between the first and  1021  second  1022  integrated optical devices for suppressing optical crosstalk between the first  1021  and second  1022  integrated optical devices. Similar to the photonic integrated circuit  900  of  FIG. 9 , the photonic integrated circuit  1000  of  FIG. 10 , may include a metal overlayer  1092  over the integrated optical device  1021  and  1022 . For better stray light rejection, the opaque wall  1031  may extend from the substrate  1002  to the metal overlayer  1092 . 
     The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments and modifications, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.