PATENT DOCUMENT

Publication Number: US-10634843-B2
Application Number: US-201916271833-A
Country: US
Kind Code: B2

Title: Photonic integrated circuit with laser and isolator

Abstract:
An optoelectronic device includes a substrate, having a recess formed therein. An optical isolator is mounted in the recess. A laser includes a stack of epitaxial layers on the substrate and emits a beam of radiation toward the recess along a direction parallel to a surface of the substrate. A waveguide directs the beam emitted by the laser into the optical isolator.

Claims:
The invention claimed is: 
     
       1. An optoelectronic device, comprising:
 a substrate, having a device layer thereon and having a recess formed therein and extending at least through the device layer; 
 an optical isolator mounted in the recess and comprising an input face and an output face; 
 a laser comprising a stack of epitaxial layers on the substrate and configured to emit a beam of radiation toward the recess along a direction parallel to a surface of the substrate; and 
 a waveguide formed in the device layer and configured to direct the beam emitted by the laser toward the recess and into the input face of the optical isolator. 
 
     
     
       2. The device according to  claim 1 , wherein the waveguide has an entrance in proximity to the laser and an exit in proximity to the recess and is tapered along a direction of the beam between the entrance and the exit. 
     
     
       3. The device according to  claim 2 , wherein the exit of the waveguide is smaller than the entrance and has a transverse dimension that is less than a wavelength of the radiation emitted by the laser. 
     
     
       4. The device according to  claim 2 , wherein a transverse dimension of the waveguide widens adiabatically from the entrance to the exit. 
     
     
       5. The device according to  claim 2 , wherein the waveguide is tapered in both a horizontal dimension, parallel to the surface of the substrate, and in a vertical dimension, perpendicular to the surface of the substrate. 
     
     
       6. The device according to  claim 1 , wherein there is no lens between the laser and the optical isolator. 
     
     
       7. The device according to  claim 1 , wherein the input face of the optical isolator is positioned to receive the beam from the waveguide, and the beam exits the optical isolator through the output face, and wherein the device comprises one or more further photonic components on the substrate and a further waveguide configured to guide the beam from the output face of the optical isolator toward the one or more further photonic components. 
     
     
       8. The device according to  claim 7 , wherein there is no lens between the optical isolator and the further photonic components. 
     
     
       9. The device according to  claim 1 , wherein the optical isolator comprises a bulk Faraday rotator. 
     
     
       10. The device according to  claim 9 , wherein the optical isolator comprises an input polarizer positioned to receive the beam from the waveguide, and an output polarizer, which has a direction of polarization that is rotated relative to the input polarizer and through which the beam exits the optical isolator. 
     
     
       11. The device according to  claim 1 , wherein the substrate comprises a semiconductor material. 
     
     
       12. The device according to  claim 11 , wherein the semiconductor material comprises silicon. 
     
     
       13. The device according to  claim 1 , wherein the substrate comprises a dielectric material. 
     
     
       14. The device according to  claim 1 , wherein the recess has a closed bottom, and the optical isolator is mounted on the bottom of the recess. 
     
     
       15. The device according to  claim 1 , and comprising an optical interposer, wherein the recess passes through the substrate, and wherein both the substrate and the optical isolator are mounted on the optical interposer. 
     
     
       16. The device according to  claim 15 , wherein the optical interposer has an indentation, which is aligned with the recess in the substrate and in which the optical isolator is mounted. 
     
     
       17. An optoelectronic device, comprising:
 a substrate, having a recess formed therein; 
 a bulk optical isolator, comprising a single crystal of a magneto-optic material, having input and output faces, mounted in the recess; 
 a first tapered waveguide formed on the substrate and configured to direct a beam of radiation into the optical isolator through the input face; and 
 a second tapered waveguide formed on the substrate and configured to receive and guide the beam of radiation from the output face of the optical isolator to a further photonic component. 
 
     
     
       18. The device according to  claim 17 , wherein the first and second tapered waveguides comprise silicon nitride and are surrounded by one or more layers of silicon dioxide. 
     
     
       19. The device according to  claim 18 , wherein a portion of the one or more layers of silicon dioxide that is in proximity to the recess is surrounded by air. 
     
     
       20. A method for manufacturing an optoelectronic device, the method comprising:
 forming a recess in a substrate having a device layer thereon such that the recess extends at least through the device layer; 
 mounting an optical isolator, having an input face and an output face, in the recess; 
 depositing a stack of epitaxial layers on the substrate so as to define a laser, which is configured to emit a beam of radiation toward the recess along a direction parallel to a surface of the substrate; and 
 forming a waveguide in the device layer on the substrate so as to direct the beam emitted by the laser toward the recess and into the input face of the optical isolator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application 62/661,659, filed Apr. 24, 2018, and of U.S. Provisional Patent Application 62/756,608, filed Nov. 7, 2018. Both of these related applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to optoelectronic devices, and particularly to integrated photonic devices and methods for their fabrication. 
     BACKGROUND 
     Photonic integrated circuits (PICs) integrate multiple optical and optoelectronic functions in a single device, fabricated on a semiconductor substrate. These functions include both passive routing elements, such as waveguides, splitters, couplers and resonators; and active elements, such as tuners, modulators and detectors, as well as lasers. Some PICs incorporating lasers that are currently known in the art are fully implemented on III-V substrate, resulting in low-yield and high cost. Other PICs are made on silicon substrates using CMOS-compatible processes and are then integrated with a separately-fabricated laser (III-V) and isolator components, usually on an optical interposer. This latter approach allows utilization of the large-scale production, low cost and high yield that Si-photonics inherit from CMOS processing, but suffers from difficult, costly assembly based on “pick and place” tools. Recently, however, a number of foundries have developed the capability of integrating III-V laser dies on SOI (silicon on insulator) wafers, together with silicon-based waveguides and other optical and electro-optical components. 
     Optical isolators are commonly used in laser-based optical systems to prevent unwanted feedback into the laser cavity caused by back reflections from other system components. Components providing optical isolation are necessarily non-reciprocal to ensure that the laser can be protected from backwards-propagating optical fields while allowing the passage of forward-direction fields. The most common type of optical isolator is based on a Faraday rotator, which uses the magneto-optic effect to rotate the polarization of incident light. Optical isolators of this sort are generally referred to as Faraday isolators. Typically, the operating parameters of the Faraday rotator (choice of magneto-optic material, length, and magnetic field strength) are chosen to rotate the polarization by ±45° depending upon the direction of travel. Light enters the Faraday rotator through an input polarizer and exits through an output polarizer, which is rotated by 45° relative to the input. Any backward-propagating light that passes back through the output polarizer (in the opposite direction) will have its polarization counter-rotated by 45° by the Faraday rotator and will thus be blocked by the input polarizer. Faraday rotation can also be applied to unpolarized light, typically utilizing birefringent crystals to spatially separate polarization components. 
     SUMMARY 
     Embodiments of the present invention that are described hereinbelow provide improved integrated photonic devices and methods for producing such devices. 
     There is therefore provided, in accordance with an embodiment of the invention, an optoelectronic device, including a substrate, having a recess formed therein. An optical isolator is mounted in the recess. A laser, including a stack of epitaxial layers on the substrate, is configured to emit a beam of radiation toward the recess along a direction parallel to a surface of the substrate. A waveguide is configured to direct the beam emitted by the laser into the optical isolator. 
     In some embodiments, the waveguide has an entrance in proximity to the laser and an exit in proximity to the recess and is tapered along a direction of the beam between the entrance and the exit. In one embodiment, the exit of the waveguide is smaller than the entrance and has a transverse dimension that is less than a wavelength of the radiation emitted by the laser. Additionally or alternatively, a transverse dimension of the waveguide widens adiabatically from the entrance to the exit. Further additionally or alternatively, the waveguide is tapered in both a horizontal dimension, parallel to the surface of the substrate, and in a vertical dimension, perpendicular to the surface of the substrate. 
     Typically, there is no lens between the laser and the optical isolator. 
     In a disclosed embodiment, the optical isolator includes an input face, which is positioned to receive the beam from the waveguide, and an output face, through which the beam exits the optical isolator, and the device includes one or more further photonic components on the substrate and a further waveguide configured to guide the beam from the output face of the optical isolator toward the one or more further photonic components. Additionally or alternatively, there is no lens between the optical isolator and the further photonic components. 
     In some embodiments, the optical isolator includes a bulk Faraday rotator. In a disclosed embodiment, the optical isolator includes an input polarizer positioned to receive the beam from the waveguide, and an output polarizer, which has a direction of polarization that is rotated relative to the input polarizer and through which the beam exits the optical isolator. 
     In some embodiments, the substrate includes a semiconductor material, such as silicon. 
     Additionally or alternatively, the substrate includes a dielectric material. 
     In one embodiment, the recess has a closed bottom, and the optical isolator is mounted on the bottom of the recess. 
     In another embodiment, the device includes an optical interposer, wherein the recess passes through the substrate, and wherein both the substrate and the optical isolator are mounted on the optical interposer. In a disclosed embodiment, the optical interposer has an indentation, which is aligned with the recess in the substrate and in which the optical isolator is mounted. 
     There is also provided, in accordance with an embodiment of the invention, an optoelectronic device, including a substrate, having a recess formed therein. A bulk optical isolator, having input and output faces, is mounted in the recess. A first tapered waveguide is formed on the substrate and configured to direct a beam of radiation into the optical isolator through the input face. A second tapered waveguide is formed on the substrate and configured to receive and guide the beam of radiation from the output face of the optical isolator to a further photonic component. 
     In some embodiments, the first and second tapered waveguides include silicon nitride and are surrounded by one or more layers of silicon dioxide. In one embodiment, a portion of the one or more layers of silicon dioxide that is in proximity to the recess is surrounded by air. 
     There is additionally provided, in accordance with an embodiment of the invention, a method for manufacturing an optoelectronic device. The method includes forming a recess in a substrate and mounting an optical isolator in the recess. A stack of epitaxial layers is deposited on the substrate so as to define a laser, which is configured to emit a beam of radiation toward the recess along a direction parallel to a surface of the substrate. A waveguide is formed on the substrate so as to direct the beam emitted by the laser into the optical isolator. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic top and sectional views, respectively, of an optoelectronic device, in accordance with an embodiment of the invention; 
         FIGS. 2A and 2B  are schematic top and sectional views, respectively, of an optoelectronic device, in accordance with another embodiment of the invention; 
         FIGS. 3A , B and C are schematic top, front, and rear views, respectively, of a tapered waveguide, in accordance with an embodiment of the invention; 
         FIGS. 4A , B and C are schematic top, front, and rear views, respectively, of a tapered waveguide, in accordance with another embodiment of the invention; 
         FIG. 5  is a schematic sectional view of a tapered multi-layer waveguide, in accordance with an alternative embodiment of the invention; 
         FIGS. 6A and 6B  are schematic top and front views, respectively, of a tapered waveguide, in accordance with yet another embodiment of the invention; 
         FIGS. 7A and 7B  are schematic sectional and top views, respectively, of tapered waveguides formed on a substrate, in accordance with an embodiment of the invention; 
         FIGS. 8-10  are schematic sectional views of the tapered waveguides of  FIG. 7A , showing successive steps in preparation for insertion of an optical isolator in a recess between the waveguides, in accordance with an embodiment of the invention; 
         FIGS. 11A and 11B  are schematic sectional views, in vertical and horizontal planes, respectively, of an optoelectronic device comprising an optical isolator in a recess between tapered waveguides, in accordance with an embodiment of the invention; and 
         FIGS. 12A and 12B  are schematic sectional views, in vertical and horizontal planes, respectively, of an optoelectronic device comprising an optical isolator in a recess between tapered waveguides, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Although laser integration has the potential to significantly simplify and reduce the cost of PIC products, to do so it cannot rely on interposer-based assembly, in which each one of the elements (PIC, laser, isolator) is pick-and-placed and manually aligned via series of lenses to decrease optical loss. This patent application describes an alternative architecture for optical isolator integration that can overcome these limitations. 
     The recent development of silicon PICs incorporating lasers has created new opportunities for development of a wide variety of miniature optoelectronic devices. Integration of optical isolators with the laser and other photonic components, however, remains a major challenge. The proposed solutions to this problem fall into two general categories:
     Mounting a bulk Faraday isolator on or alongside the silicon chip. Because of the limited accuracy of mounting the isolator, devices that are known in the art use lenses to couple the laser beam (from a laser that is typically produced separately and mounted in place) into the isolator and out of the isolator into the PIC). In many cases, active alignment of the lenses is needed in order to compensate for deviations in the position and orientation of the isolator relative to the laser and PIC. Furthermore, the isolator, PIC and laser are commonly mounted on an interposer with limited precision, which further complicates the processing and increases the need for active lens alignment to minimize optical losses.   Integrated, waveguide-based optical isolators are typically formed by bonding or deposition of magneto-optical films and external magnets in close proximity to the photonic waveguide. This approach alleviates the problem of alignment, but suffers from high insertion loss of the films as compared to a single-crystal bulk isolator, as well as from high complexity, since integration of the magneto-optical material is incompatible with CMOS foundry processing and does not readily support the transverse-electric (TE) polarization of the laser.
 
The term “bulk” is used herein to refer to components that are separately manufactured and then placed on or alongside the PIC, as opposed to components that are fabricated as integral parts of the PIC. As noted above, bulk isolators typically comprise a single crystal of magneto-optic material and have roughly ten times less optical insertion loss than magneto-optic thin films, which are a few hundred nanometers thick and are grown on a lattice-mismatched substrate.
   

     Bulk optical isolators, such as miniature Faraday isolators, thus remain the solution of choice because of their simplicity, low insertion loss (typically 0.5 dB or less), and strong isolation. Embodiments of the present invention provide device designs and methods for fabrication that alleviate the problems of integration and alignment of such bulk optical isolators with PICs. The present embodiments are capable of achieving the necessary accuracy of alignment without using any lens between the laser and the optical isolator, as well as between the optical isolator and the rest of the components of the PIC with which the laser is integrated, and without requiring any sort of active alignment notwithstanding the error inherent in mounting the isolator. In the disclosed embodiments, the only element that requires pick-and-place assembly (possibly with relaxed precision relative to devices that are known in the art) is the isolator, and possibly a PIC. No lens, laser placement, or active alignment is involved. The disclosed devices and methods of fabrication fully utilize the advantages of laser integration on a silicon-based PIC, thus enhancing yield and reducing product cost. Although the disclosed embodiments relate specifically to Faraday isolators, the principles of the present invention may similarly be applied using isolators of other types with suitable properties. 
     In some of the embodiments that are described herein, an optoelectronic device comprises a semiconductor substrate in which a recess is formed. In the disclosed embodiments, the substrate comprises silicon (which is the best-developed and lowest-cost PIC material currently available), but alternatively, other suitable semiconductor materials may be used. A PIC is fabricated on the substrate, including a laser, comprising a stack of epitaxial layers on the substrate, which emits a beam of radiation along a direction parallel to a surface of the substrate. An optical isolator, such as a bulk Faraday rotator, is mounted in the recess. A waveguide, which is fabricated as part of the PIC and has an entrance in proximity to the laser and an exit in proximity to the recess, directs the beam emitted by the laser into the optical isolator. 
     In other embodiments, the waveguides for directing the laser beam into and out of the optical isolator are formed on a dielectric substrate, such as fused silica. After deposition of the waveguides and overlying layers, a recess is etched through the dielectric substrate, and the isolator is mounted in this recess. 
     The embodiments described below use two types of waveguides: high-refractive-index waveguides, typically made of silicon, and low-refractive-index waveguides, made of silicon nitride or silicon oxynitride, for example. In these embodiments, the laser is coupled to a silicon waveguide, and many of the passive photonic components, such as other waveguides, splitters, couplers, filters, and interferometers, as well as active photonic components, such as thermal tuners, modulators, and detectors, can be implemented within or in conjunction with silicon waveguides. (Detectors commonly use a Si—Ge platform). For coupling to the isolator, however, low-refractive-index waveguides are desirable. Therefore, the propagation mode of the radiation emitted by the laser is transferred from the high-index (Si) waveguide (n=3.4) into the low-index (silicon nitride or silicon oxynitride) waveguide (n=2.0, 1.6) for input to the optical isolator, and vice versa following the optical isolator. 
     Typically, the low-index waveguides and high-index waveguides are fabricated in different layers within the stack of thin-film layers on the substrate, so that the low- and high-index waveguides are separated in the vertical direction. Waveguide couplers are used to carry out the transfer between the different waveguides, as shown in the figures that follow. 
     In some embodiments, the low-index waveguide is tapered along a direction of the beam between the entrance and the exit of the waveguide. By proper design of the taper, the width of the laser beam exiting the waveguide toward the input face of the optical isolator is expanded (relative to the width entering the waveguide), while the divergence of the laser beam is reduced. At the output from the optical isolator, a similar waveguide, typically with its taper direction reversed relative to the waveguide at the input side, can be used to receive and guide the beam toward further photonic components on the PIC. This arrangement of tapered waveguides, which can be aligned very precisely (to within less than 10 nm) using state-of-the-art lithographic techniques, substantially reduces the sensitivity of the device to misalignment of the optical isolator (as well as of the PIC when mounted on an interposer) and thus enables the beam to be coupled into and out of the optical isolator without the use of lenses or active alignment. 
     Typically, the recess is formed in the substrate using manufacturing techniques that are known in the art, such as etching or milling. In some embodiments, the recess penetrates only part way through the substrate and thus has a closed bottom, on which the optical isolator is mounted. In some cases, however, creating a recess of this sort of sufficient depth can be difficult. Therefore, in other embodiments, the recess passes through the semiconductor or dielectric substrate, and both the substrate (with the PIC formed on it) and the optical isolator are mounted on an optical interposer, which may have an indentation, aligned with the recess in the semiconductor substrate, in which the optical isolator is mounted. 
     Device Designs with Semiconductor Substrates 
       FIGS. 1A and 1B  are schematic top and sectional views, respectively, of an optoelectronic device  20 , in accordance with an embodiment of the invention. Coordinate axes are shown at the right side of the figures for ease of reference. The Z-axis is taken to be the longitudinal axis of device  20 , running in the direction of the laser beam (as described below), while the X-axis runs transversely across the device. The X-Z plane, parallel to the upper surface of the device, is referred to, for the sake of convenience, as the horizontal plane. The Y-axis, perpendicular to the surface, is referred to as the vertical axis. The terms “horizontal” and “vertical” are used in this context solely for the sake of convenience, and device  20  may be operated in substantially any desired orientation. 
     Device  20  comprises a PIC, which is formed on a silicon substrate  22  (typically in the form of an SOI wafer). Photonic components are formed in a device layer  24  on substrate  22 , including a laser  26  and further downstream components  28 , including active and/or passive integrated components. Laser  26 , which comprises a stack of epitaxial layers within device layer  24 , emits a beam of radiation along the Z-direction toward a recess  30  that is formed in the substrate. Recess  30  is etched at least through device layer  24 , typically to a depth of 0.4 mm to 1 mm. 
     An optical isolator  32  is mounted in recess  30 , resting on the closed bottom of the recess. In this embodiment, optical isolator  32  is a Faraday isolator, comprising a bulk Faraday rotator  34 , having an input polarizer  36  at its input face, positioned to receive the beam from laser  26 , and an output polarizer  38  at its output face, with a direction of polarization that is rotated relative to the input polarizer. Faraday isolators of this sort, with dimensions on the order of a few millimeters or less, are available, for example, from II-VI Photonics, Inc. (Saxonburg, Pa.), and Deltronic Crystal Industries, Inc. (Dover, N.J.). Typically, output polarizer  38  is rotated by 45° relative to the input polarizer, as explained above. Consequently, isolator  32  will prevent any back-reflections, for example from downstream components  28 , from returning to laser  26 . 
     A waveguide  39  formed in device layer  24  guides the beam from laser  26  toward recess  30 . The beam is directed into optical isolator  32  by a tapered waveguide  40 , which is tapered along the Z-direction between the entrance and the exit of the waveguide. Typically, waveguide  39  comprises silicon, while waveguide  40  comprises a material with a low refractive index, such as a silicon nitride (for example Si 3 N 4 ) or silicon oxynitride (Si 2 N 2 O). (The index is “low” by comparison with silicon, but higher than the surrounding SiO 2  in device layer  24 .) In the pictured embodiment, waveguide  40  is formed in a higher level of the layer stack on substrate  22  than waveguide  39 , and a coupler  41  transfers the beam from waveguide  39  to waveguide  40 . Coupler  41  may comprise, for example, a segment of waveguide  39  that tapers along the Z-direction, overlaid by a segment of waveguide  40  with the opposite taper, as is known in the art, so that the laser mode is transferred from waveguide  39  to waveguide  40 . 
     In the present example, the exit of waveguide  40 , at recess  30 , is smaller than the entrance and has a transverse dimension that is less than the wavelength of the radiation emitted by laser  26 . As shown in the figures, it is desirable that waveguide  40  be tapered in both the horizontal (X) dimension and in the vertical (Y) dimension. Details of this design are described further herein below with reference to  FIGS. 3A-C . 
     By proper design of waveguide  40 , with a minimal exit dimension, it is possible to expand the spot size of the laser beam from about one or a few microns at the exit from laser  26  to several tens of microns at the entrance to optical isolator  32 . In the pictured embodiment, a further tapered waveguide  42 , with its taper direction reversed relative to waveguide  40 , narrows and guides the beam from the output of optical isolator  32  toward downstream components  28 . (Alternatively, other means, as are known in the art, may be used in coupling the optical radiation out of optical isolator  32 , depending on the requirements of the optical circuit.) This degree of taper mode expansion is sufficient to compensate for the expansion of the beam propagating in recess  30  and optical isolator  32 . Note that small displacements and/or rotations of optical isolator  32  within recess  30  will not affect the isolation performance beyond a small loss of optical power in addition to the insertion loss of the optical isolator itself. 
     Typically (although not necessarily), laser  26  emits a beam with horizontal (X-direction) polarization, which is guided toward recess  30  through waveguide  39  in a transverse electric (TE) mode (aligned along the X-direction). Following rotation of the polarization by optical isolator  32 , however, the beam will be polarized at a 45° angle relative to substrate  22  and will include both TE and TM (transverse magnetic) components in the downstream waveguide. This rotation can result in a loss of 3 dB in optical power due to leakage of the TM components into the substrate. To reduce this power loss, an additional polarization rotator (not shown) can be introduced following output polarizer  38 , to restore the horizontal polarization of the beam. Alternatively, optical isolator  32  may comprise a two-stage isolator, which outputs a beam with the same (horizontal) polarization as the input beam. 
     As a further alternative (not shown in the figures), the recess in substrate may be formed as a trench, for example by anisotropic etching in silicon, with sloping sides tilted by approximately 45° relative to the horizontal plane. Optical isolator  32  is mounted in the trench with a corresponding 45° rotation relative to the horizontal plane. The angles of rotation of polarizers  36  and  38  are adjusted accordingly. 
       FIGS. 2A and 2B  are schematic top and sectional views, respectively, of an optoelectronic device  50 , in accordance with another embodiment of the invention. Device  50  operates on principles similar to those of device  20 , as described above, and components having similar functions in device  50  to those in device  20  are labeled with the same indicator numbers. 
     In contrast to device  20 , device  50  comprises an optical interposer  52 , i.e., an additional substrate of sufficient size and surface quality to allow the other elements of device  50  to be mounted on it. Interposer  52  may comprise any suitable material, such as silicon or glass, for example. All optoelectronic components of device  50 , with the exception of optical isolator  32 , are formed on a PIC substrate  54 , comprising a suitable semiconductor material. Substrate  54  may comprise, for example, the upper silicon device layer of an SOI wafer; in this case, after fabrication of the optoelectronic components on the SOI, the lower silicon handling wafer is typically removed before bonding substrate  54  to interposer  52 . 
     A recess  56  in substrate  54  passes through the entire vertical thickness of the substrate, and optical isolator is thus mounted within recess  56  on interposer  52 . Depending on the height of optical isolator  32  relative to the thickness of substrate  54 , an indentation  57  may be formed in interposer  52  and aligned with recess  56  to accommodate the interposer. Recess  56  may enclose isolator on all sides (as does recess  30  in  FIG. 1A ); but alternatively, for ease of fabrication, recess  56  may be open on one side, as shown in  FIG. 2A . 
     A tapered waveguide  58  directs the beam from laser  26  into optical isolator  32  while expanding the spot size of the laser beam. The direction of taper of waveguide  58 , however, is opposite to that of waveguide  40  ( FIGS. 1A /B): The transverse dimension of waveguide  58  widens adiabatically from the entrance of the waveguide, in proximity to laser  26 , to the exit from the waveguide at recess  56 . A further tapered waveguide  60 , with its taper direction reversed relative to waveguide  40 , narrows adiabatically from the output of optical isolator  32  toward downstream components  28 . Details of these waveguides are described further hereinbelow with reference to  FIGS. 4A-C . 
     In alternative embodiments, waveguides  58  and  60  can be used in a device without an interposer, such as device  20 , and waveguides  40  and  42  can be used in a device with an interposer, such as device  50 . Furthermore, although  FIGS. 1A /B and  2 A/B show certain specific devices and component layouts for the sake of concreteness and clarity, the principles of the present invention, in terms of the use of waveguides to direct optical radiation into and out of an optical isolator, may similarly be applied in other devices and layouts. For example, optical isolators can be introduced, using waveguides in the manner described above, not only at the output from the laser, but also between other elements in a PIC or between the output of a PIC and an external fiber coupler in order to prevent back-reflections within the PIC or from external devices. All such alternative implementations are considered to be within the scope of the present invention. 
     Waveguide Details 
       FIGS. 3A , B and C are schematic top, front, and rear views, respectively, of tapered waveguide  40 , in accordance with an embodiment of the invention.  FIG. 3B  shows an entrance  70  to the waveguide (at the left in  FIGS. 1A /B), while  FIG. 3C  shows an exit  72  from the waveguide (in proximity to the optical isolator). As noted earlier, waveguide  40  typically comprises a low-index material (relative to silicon), such as silicon nitride or silicon oxynitride. The refractive index may be reduced still further, if necessary, by patterning waveguide  40  with a sub-micron grating structure, as is known in the art. 
     Entrance  70  has a width W SMW  that is chosen to receive and guide single-mode TE radiation, as output by laser  26  (through waveguide  39 ). (As noted earlier, laser  26 , waveguide  39 , and other components on the PIC are formed in the silicon device layer on the SOI wafer. To couple the mode from waveguide  39  into waveguide  40  and out of waveguide  42 , couplers  41  can be used, as described above.) Waveguide  40  tapers down to a minimum width W MIN  at exit  72 , which is typically the smallest width that can be produced using the applicable lithographic technology, currently approximately 50 nm for silicon PICs. It is desirable, as illustrated in the figures, that waveguide  40  taper in both the vertical and horizontal dimensions, so that at the end of the waveguide, the laser beam reaches the maximal achievable mode size with minimal astigmatism between the X and Y axis. Waveguide  42  has similar dimensions, but reversed along the Z-axis relative to waveguide  40 . 
       FIGS. 4A , B and C are schematic top, front, and rear views, respectively, of tapered waveguide  58 , in accordance with another embodiment of the invention. As in the preceding embodiment, an entrance  74  of waveguide  58 , shown in  FIG. 4B , has the width W SMW  needed to receive single-mode TE radiation from laser  26 . Waveguide  58  widens adiabatically to an exit  76 , shown in  FIG. 4C , with an output width W OUT  that is typically several tens of microns, as described above. Because the taper is adiabatic, the radiation propagates through the entire waveguide in the same TE mode in which it entered (typically the lowest mode, TE 0 ), without substantial reflection or energy transfer into a higher order-modes, thus exhibiting low optical loss. Any linear change in the width of the waveguide over a sufficiently long taper length can achieve the desired adiabatic behavior. Thus, for example, at a wavelength of 1.31 μm in silicon nitride (n=2.0), the wavelength in the material is 0.65 μm, and a taper that is over a hundred microns long with a linear variation of width will satisfy the requirements for adiabaticity. As a result, the TE 0  mode will be preserved with very small propagation loss, typically governed by the scattering. In one embodiment, a sub-linear increase of the width is used to decrease the loss even further. 
     Waveguides  40 ,  42 ,  58  and  60  can be produced by successive deposition and etching of multiple layers of the waveguide material. Producing structures of this sort with smooth vertical gradations, however, can be difficult. Alternative schemes that may be easier to implement are shown in the figures that follow. 
       FIG. 5  is a schematic sectional view of a tapered waveguide  80 , in accordance with an alternative embodiment of the invention. Waveguide  80  comprises a central layer  82  and upper and lower layers  84  of a guiding material, such as silicon nitride or silicon oxynitride. Layers  82  and  84  are separated by intermediate layers  86  of a lower-index material, such as silicon dioxide. The horizontal widths of layers  82  and  84  are tapered in the longitudinal (Z) direction, increasing from left to right in the view shown in  FIG. 4A . In addition, upper and lower layers  84  may be present only near the wide (right) end of the waveguide and removed entirely from the narrower (left) end. 
     This multi-layer structure (which may alternatively comprise more than three guiding layers) is designed to provide spherical mode expansion in the X-Y plane, which facilitates mode coupling to the zero-order Gaussian beam for free-space propagation in the recess and isolator. As a result, no astigmatism correction is needed at the receiving taper end, i.e., waveguide  42  or  60 , thus simplifying taper design and manufacturability. 
       FIGS. 6A and 6B  are schematic top and front views, respectively, of a tapered waveguide  90 , in accordance with yet another embodiment of the invention. The “ridge” configuration of waveguide  90  is useful in permitting gradual variation of the layer thickness (in the Y-direction) and width (in the X-direction) independently, and may also be advantageous in terms of reducing leakage to the substrate and supporting TM polarization. 
     Waveguide  90  comprises a base layer  94 , with an overlying ridge layer  96 . As in the preceding embodiments, layers  94  and  96  comprise silicon nitride, silicon oxynitride, or another suitable material of similar refractive index. Layers  94  and  96  are supported and surrounded by a lower-index material  92 , such as silicon dioxide. To achieve the desired guiding performance, base layer  94  can be about 100 nm thick in the vertical direction, and tapers horizontally to a width of about 100 nm at the narrow end of waveguide  90 , at the right side of  FIG. 6A . Ridge layer  96  can be about 450 nm thick in the vertical direction and similarly tapers in width over the wider (left) part of the waveguide, to a point midway along the length of the waveguide. 
     Alternative types of tapered waveguides that can be used in this context will be apparent to those skilled in the art after reading the present description and are considered to be within the scope of the present invention. 
     Device Designs with Dielectric Substrate 
     In an alternative embodiment of the present invention, which is illustrated in the figures that follow, the tapered waveguides for coupling light into and out of the isolator are formed on a dielectric substrate, rather than on a silicon substrate as described above. This approach is advantageous in achieving good coupling into and out of the low-index waveguides and ease of manufacture. Using this approach, the tapered waveguides may comprise a thin nitride layer, such as silicon nitride or silicon oxynitride, for example, surrounded by thick oxide layers above and below, with a thin oxide membrane (about 15 μm thick, for instance) at the edges of the waveguides facing the isolator, or free-standing thin nitride couplers with supporting ribs. The other components of the PIC are omitted from the figures that follow for the sake of simplicity; but they may similarly be formed by depositing suitable semiconductor and metal layers on the dielectric substrate. 
       FIGS. 7A and 7B  are schematic sectional and top views, respectively, of tapered waveguides  104  and  106  formed on a dielectric substrate  100 , in accordance with an embodiment of the invention. Substrate  100  comprises fused silica, for example, with a thick layer  102  (for example, about 1 μm thick) of SiO 2  deposited on the substrate. A thin layer of SiN is deposited over SiO 2  layer  102 , and is then etched to form tapered waveguides  104 , for example by a suitable photolithographic process, as is known in the art. The outer portions of waveguides  104  and  106 , to the left and right of the part of the device that is shown in these figures, may be coupled in the manner described above to high-index waveguides, such as silicon waveguides, which are formed over substrate  100  in other layers, adjacent to the SiN layer. 
       FIGS. 8-10  are schematic sectional views, showing successive processing steps in preparation for insertion of an optical isolator in a recess between waveguides  104  and  106 , in accordance with an embodiment of the invention. As shown in  FIG. 8 , an additional thick layer  108  of SiO 2  (again, about 1 μm thick) is deposited over waveguides  104  and  106 . An additional dielectric wafer  110 , for example, a wafer of fused silica about 700 μm thick, is then bonded over layer  108 . For this purpose, the upper surface of layer  108  may be polished, using chemical-mechanical polishing (CMP), for instance, and then bonded to wafer  110  by an oxide bond  112 . 
     As illustrated in  FIG. 9 , the upper side of wafer  110  is thinned, typically to a thickness of 100 μm or less, again using CMP. Wafer  110  is then bonded to an interposer  114 , comprising silicon, for example, by a polymer bond  116 , such as a suitable epoxy. A recess  118  is pre-formed in interposer  114  to accommodate the isolator. 
     Next, as illustrated in  FIG. 10 , the lower side of substrate  100  is thinned, as well, similarly by CMP to 100 μm or less. A recess  120 , in alignment with recess  118 , is then etched all the way through the dielectric layers, including both substrate  100 , wafer  110 , and the intervening oxide layers  102  and  108 . Deep reactive-ion etching (DRIE) may be used for this purpose, leaving a thin section of SiO 2  from layer  108  between the ends of waveguides  104  and  106  and recess  120 . 
       FIGS. 11A and 11B  are schematic sectional views, in vertical and horizontal planes, respectively, showing the final steps in production of an optoelectronic device  122 , in accordance with an embodiment of the invention, following the process illustrated in the preceding figures. Inner sidewalls  124  of recess  120  may be polished before insertion of the isolator in order to reduce light scattering. Isolator  32  is then inserted within recesses  120  and  118  and mounted on interposer  114 . Waveguide  104  guides light into isolator  32 , and waveguide  106  then collects and transmits the light that has passed through the isolator, in the manner described above. 
       FIGS. 12A and 12B  are schematic sectional views, in vertical and horizontal planes, respectively, showing an optoelectronic device  130 , in accordance with another embodiment of the invention. Device  130  is similar to device  122 , as described above, and components of device  130  that with the same structure and functionality as the corresponding components of device  122  are labeled with the same indicator numbers. Device  130  differs from device  122 , however, in that portions of the layers of SiO 2  that are in proximity to a central recess  140 , which contains isolator  32 , are surrounded by air. 
     The reason for this difference is as follows: The thickness of wafer  110  in device  122  (roughly 100 μm) can create difficulties in coupling SiN waveguides  104  and  106  to silicon waveguides  39 , because of the large vertical separation between the silicon and SiN layers. Furthermore, it can be difficult to connect the thick SiO 2  die to a much thinner silicon photonics die, which is typically no more than 8-15 μm thick. 
     To alleviate these difficulties, thin cladding layers  132  and  134  of SiO 2  (for example, 14 μm thick) are deposited above and below oxide layers  102  and  108 . To support the large mode produced by SiN waveguide  104 , without significant mode decay and loss into the interposer  136 , the portions of layers  132  and  134  that are in proximity to recess  140  are surrounded symmetrically by air cladding above and below recess  140 . To provide the desired air cladding above layer  132 , an interposer  136  is used, with a recess  138  that is wider in the X-direction than recess  140 , and thus wider than the distance between the ends of SiN waveguides  104  and  106 . In effect, the large recess  138  creates an undercut of more than 200 μm beneath cladding layer  132 . 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Metadata:
Filing Date: 20190210
Publication Date: 20200428
Grant Date: 20200428
Priority Date: 20180424
Inventors: BAYN, IGAL I.
SUTTON, ANDREW J.
SHPUNT, ALEXANDER
PELC, JASON S.
ARBORE, MARK A.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B2006/12157", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B2006/12116", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/13", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2006/12157", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/13", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2006/12061", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/1228", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2006/12121", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/1228", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/12004", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/12004", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2006/12121", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B2006/12061", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/12004", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B6/13", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2006/12121", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B2006/12116", "inventive": false, "first": false, "tree": "[]"}, {"code": "G02B6/1228", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2006/12157", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 68237630