Abstract:
An apparatus comprising a plurality of alignment couplers, wherein the alignment couplers are equally spaced a first length apart from each other along a first surface, a plurality of photodetectors optically coupled to the plurality of alignment couplers, a memory, and a processor coupled to the photodetectors and the memory, wherein the memory comprises computer executable instructions stored in a non-transitory computer readable medium that when executed by the processor cause the processor to receive an electrical signal in response to at least one of the photodetectors detecting a first light, and determine an edge coupling alignment based on the electrical signal, wherein the edge coupling alignment is aligned when the electrical signal indicates two photodetectors of the plurality of photodetectors detect the first light, and wherein the edge coupling alignment is misaligned when the electrical signal indicates only one photodetector of the plurality of photodetectors detects the first light.

Description:
BACKGROUND 
     Edge coupling with edge couplers is a standard technique for coupling between single-mode fibers and photonic integrated circuit (PIC) devices such as optical switches, modulators, high-speed detectors, and interposers. Edge coupling provides a broadband response, offers low insertion loss (IL), and couples both transverse electric (TE) modes and transverse magnetic (TM) modes. Edge couplers include nano-tapered edge couplers, or spot size converters, and evanescent edge couplers. Evanescent edge couplers couple to optical fibers using a nano-taper that is coupled to a second, larger waveguide. The waveguide is formed of a polymer or an inorganic material such as silicon oxynitride (SiON) or an oxide. 
     Coupling efficiency is high when the mode field diameter (MFD) of a fiber and a waveguide are matched and when incoming light and outgoing light are aligned. Coupling efficiency is sensitive to misalignment, for example, between a fiber and a waveguide. As an example, a 0.5 micrometer (μm) lateral offset of an inverted-taper spot size converter (SSC) reduces a coupled light power output by half. 
     Optical coupling of multiple channels in an active PIC is challenging and expensive. Optical coupling of multiple channels is prone to signal drifting during the lifetime of operation. It is desirable to monitor edge coupling alignment between single-mode fibers and PICs during the lifetime of operation. 
     SUMMARY 
     In one embodiment, the disclosure includes an apparatus comprising a first photonic device comprising a first surface, and a plurality of alignment waveguides equally spaced a first length apart from each other and configured to guide a first light to a plurality of locations on the first surface, and a second photonic device comprising a plurality of alignment couplers equally spaced a second length apart from each other, wherein the first length and the second length are different, a plurality of photodetectors coupled to the alignment couplers and configured to detect the first light from the alignment couplers, and a controller coupled to the photodetectors and configured to determine an optical alignment between the first photonic device and the second photonic device when two of the photodetectors detect the first light, and determine an optical misalignment between the first photonic device and the second photonic device when only one of the photodetectors detects the first light. 
     In another embodiment, the disclosure includes an optical edge coupling method comprising positioning a first photonic device so that a first alignment waveguide from a plurality of alignment waveguides of the first photonic device is at least partially aligned with a first alignment coupler from a plurality of alignment couplers of a second photonic device, wherein the alignment waveguides are equally spaced a first length apart from each other along a first surface, and wherein the alignment couplers are equally spaced a second length apart from each other, detecting a first light from the alignment waveguides using at least one photodetector from a plurality of photodetectors of the second photonic device, and determining an alignment of the first photonic device and the second photonic device based on the detecting. 
     In yet another embodiment, the disclosure includes an apparatus comprising a plurality of alignment couplers, wherein the alignment couplers are equally spaced a first length apart from each other along a first surface, a plurality of photodetectors optically coupled to the plurality of alignment couplers, a memory, and a processor coupled to the photodetectors and the memory, wherein the memory comprises computer executable instructions stored in a non-transitory computer readable medium that when executed by the processor cause the processor to receive an electrical signal in response to at least one of the photodetectors detecting a first light, and determine an edge coupling alignment based on the electrical signal, wherein the edge coupling alignment is aligned when the electrical signal indicates two photodetectors of the plurality of photodetectors detect the first light, and wherein the edge coupling alignment is misaligned when the electrical signal indicates only one photodetector of the plurality of photodetectors detects the first light. 
     These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG. 1  is a schematic diagram of an embodiment of an optical system. 
         FIG. 2  is a schematic diagram of an embodiment of an optical system using edge coupling with edge coupling monitoring. 
         FIG. 3  is a flowchart of an embodiment of an edge coupling alignment method. 
         FIG. 4  is a flowchart of an embodiment of an edge coupling alignment monitoring method. 
         FIG. 5  is a schematic diagram of an embodiment of an optical system using Vernier edge coupling with edge coupling monitoring. 
         FIG. 6  is a flowchart of an embodiment of an edge coupling alignment method using Vernier edge coupling. 
         FIG. 7  is a schematic diagram of another embodiment of an optical system using Vernier edge coupling with edge coupling monitoring. 
         FIG. 8  is a flowchart of another embodiment of an edge coupling alignment method using Vernier edge coupling. 
         FIG. 9  is an embodiment of a device for implementing edge coupling alignment. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or later developed. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     Disclosed herein are various embodiments for providing optical edge coupling alignment and edge coupling monitoring capabilities. The efficiency of the optical alignment is monitored to detect misalignments, for instance those caused by thermo-mechanical induced stresses. For example, the coupling efficiency between a fiber coupler and an active PIC can be monitored during normal operation. Edge coupling alignment and monitoring capabilities of an active PIC can be provided by the active PIC itself. Edge coupling alignment with active edge coupling monitoring evaluates and monitors optical interconnect efficiency of active PICs. In multi-chip packages, stacked dies, and system packages applications, edge coupling alignment with active edge coupling monitoring the active PIC can be seen as a packaging platform that serves as a high-density substrate with a redistribution layer. A packaging platform allows incompatible technologies to be mixed onto the same platform for heterogeneous integration. Edge coupling alignment and active edge coupling monitoring may be implemented in optical system applications where opto-electronic packaging is used, or in applications with optical switches that have a large number of fiber channels such as metro networks or data centers. 
       FIG. 1  is a schematic diagram of an embodiment of an optical system  100 . Optical system  100  includes a fiber array  102 , a fiber coupler  104 , and an active PIC  106 . Optical system  100  is configured to communicate light along a light path from fiber array  102  to fiber coupler  104  and from fiber coupler  104  to active PIC  106  using a plurality of waveguides (e.g., single-mode optical fibers and/or multi-mode waveguides). Optical system  100  may be configured as shown or in any other suitable manner. 
     Fiber array  102  is configured to receive light and to guide the light to fiber coupler  104 . Fiber array  102  has a plurality of optical fibers optically coupled to fiber coupler  104 . The optical fibers may include single-mode waveguides and/or multi-mode waveguides. For example, fiber array  102  may be a V-groove assembly that is configured to carry a single-mode optical fiber core within the V-grooves of the V-groove assembly. Examples of fiber array  102  include, but are not limited to, fiber ribbons and V-groove assemblies. 
     Fiber coupler  104  is optically coupled to fiber array  102  and active PIC  106 . Fiber coupler  104  is configured to receive light from fiber array  102 , to reduce the MFD of the received light, and to guide the light to active PIC  106 . Fiber coupler  104  may also be referred to as an MFD converter or MFD reducer. Fiber coupler  104  is also configured to assist with edge coupling alignment between fiber coupler  104  and active PIC  106 . Examples of fiber coupler  104  include, but are not limited to, a lithography defined polymer waveguide, a lithography defined planar lightwave circuit (PLC) fan-in field reducer, an MFD-reducing or pitch-reducing assembly, an integrated polymer waveguide evanescently coupled to an on-chip waveguide, and a lensed fiber. 
     Active PIC  106  is configured to receive light from fiber coupler  104  and to use the light with one or more photonic devices. Further, active PIC  106  is configured to assist with the optical edge coupling alignment between fiber coupler  104  and active PIC  106  and to monitor the optical coupling alignment between fiber coupler  104  and active PIC  106  during operation. Examples of active PIC  106  may include, but are not limited to, an optical interposer, a photonic switch, and an optical transceiver. Active PIC  106  may use a lithographically-defined layout of single-mode and/or multi-mode waveguide elements to form a photonic circuit. Materials for constructing active PIC  106  include, but are not limited to, gallium arsenide (GaAs), indium phosphide (InP), lithium niobate (LiNbO3), lead zirconate titanate (PLZT), silicon nitride (SiN), silicon oxynitride (SiON), and polymers. 
       FIG. 2  is a schematic diagram of an embodiment of an optical system  200  using edge coupling with edge coupling monitoring. Optical system  200  includes a fiber array  204 , a fiber coupler  208 , and an active PIC  214 . Fiber array  204 , fiber coupler  208 , and active PIC  214  are configured similarly to fiber array  102 , fiber coupler  104 , and active PIC  106  in  FIG. 1 , respectively. Optical system  200  is configured to send light from fiber array  204  to fiber coupler  208  and from fiber coupler  208  to active PIC  214 . Optical system  200  is configured to provide edge coupling alignment between fiber coupler  208  and active PIC  214  and to monitor the edge coupling alignment between fiber coupler  208  and active PIC  214  during operation. Monitoring the edge coupling alignment may include detecting drift, warping (e.g., lateral, vertical, or horizontal), roll, and positional misalignments. Optical system  200  may be configured as shown or in any other suitable manner. 
     Fiber array  204  has a plurality of waveguides  206  and is configured to receive light from optical fibers  202  and guide the light to fiber coupler  208  using the waveguides  206 . Waveguides  206  may include single-mode waveguides and/or multi-mode waveguides. 
     Fiber coupler  208  has a plurality of signal waveguides  210  and one or more waveguide loops  212 . Fiber coupler  208  is configured to guide a first light from fiber array  204  to active PIC  214  using the signal waveguides  210 . Signal waveguides  210  may include single-mode waveguides and/or multimode waveguides. Fiber coupler  208  is also configured to assist with edge coupling alignment between fiber coupler  208  and active PIC  214  using the waveguide loops  212 . Waveguide loops  212  are positioned along the surface  250  of fiber coupler  208  that interfaces with active PIC  214 . Waveguide loops  212  are configured to receive a second light from a first alignment edge coupler, for example, alignment edge coupler  220 A, one of a pair of alignment edge couplers on active PIC  214 , and to guide the second light to a second alignment edge coupler, for example, alignment edge coupler  220 B, of the pair of alignment edge couplers on active PIC  214 . Waveguide loops  212  may include single-mode waveguides or multimode waveguides that are positioned along an edge of fiber coupler  208  that interfaces with active PIC  214 . In an embodiment, fiber coupler  208  has three waveguide loops  212 , such that a first waveguide loop  212  is positioned at about the center of fiber coupler  208  between signal waveguides  210 , and a second waveguide loop  212  and a third waveguide loop  212  are positioned outside of the outermost signal waveguides  210 . In another embodiment, one or more signal waveguides  210  can be positioned laterally outside of the second waveguide loop  212  or the third waveguide loop  212 . Waveguide loops  212  may further include mirrors, reflectors, or any other suitable component for guiding and redirecting light as would be appreciated by one of ordinary skill in the art upon viewing this disclosure. 
     Active PIC  214  includes signal couplers  216 , alignment couplers  220 A and  220 B, one or more photodetectors (PDs)  222 , one or more light sources  224 , a controller  228 , one or more surface grating couplers  226 , and one or more photonic devices  218 . In an embodiment, active PIC  214  is configured to receive a first light from signal waveguides  210  on fiber coupler  208  and to process the first light using photonic devices  218 . Active PIC  214  is also configured to assist with edge coupling alignment between active PIC  214  and fiber coupler  208  by providing a second light to waveguide loop  212  on fiber coupler  208  and receiving the second light from the waveguide loop  212  on fiber coupler  208  when the active PIC  214  and fiber coupler  208  are aligned. In another embodiment, active PIC  214  may be configured to receive the first light directly from fiber array  204  or from any other optical component. The transparency of waveguide materials used in active PIC  214  determines the wavelength of the second light that is used for aligning active PIC  214  and fiber coupler  208 . The second light may or may not use the same wavelength as the first light. Further, the second light may be a single-mode light that has a fundamental light mode or a multi-mode light that has a fundamental light mode and one or more higher-order light modes. 
     Signal couplers  216  may be nano-tapered edge couplers, evanescent edge couplers, or granting couplers. Signal couplers  216  are positioned along a surface  252  of the active PIC  214  that interfaces with fiber coupler  208 . Signal couplers  216  are configured to be optically coupled to signal waveguides  210  in fiber coupler  208 , to receive light from fiber coupler  208 , and to guide the light to one or more photonic devices  218 . 
     Alignment couplers may be nano-tapered edge couplers, evanescent edge couplers, or grating couplers. Alignment couplers  220 A and  220 B are configured to be optically coupled to waveguide loops  212  in fiber coupler  208 . One or more pairs of alignment couplers  220 A and  220 B are positioned along the surface  252  of active PIC  214  that interfaces with fiber coupler  208 . Alignment couplers  220 A are configured to guide light from light source  224  or surface grating coupler  226  to the waveguide loop  212  on fiber coupler  208 . Alignment couplers  220 B are configured to receive light from the waveguide loop  212  on fiber coupler  208  and to guide the light to photodetector  222 . 
     Photodetectors  222  are configured to detect light from alignment couplers  220 B and to generate an electrical signal in response to detecting light. For example, photodetectors  222  may be configured to generate an electrical current between about 1 microamp (μA) and about 10 μA in response to the received light. Examples of photodetectors  222  may include, but are not limited to, photodiodes or any other suitable device for detecting light as would be appreciated by one of ordinary skill in the art upon viewing this disclosure. Photodetectors  222  may be constructed using any suitable group IV semiconductors, group III-V materials, or any other suitable materials. 
     Light sources  224  are configured to receive control signals from controller  228 , to generate a light, and to send the light to alignment couplers  220 A. Examples of light sources  224  may include, but are not limited to, lasers, embedded lasers, vertical-cavity surface-emitting lasers (VCSELs), semiconductor optical amplifiers (SOAs), distributed feedback (DFB) lasers, and light-emitting diodes (LEDs). One or more light sources  224  may be monolithically integrated into controller  228 . 
     Surface grating couplers  226  are configured to receive light from an external light source and to guide the light to alignment couplers  220 A. Surface grating couplers  226  are configured to optically couple external light sources to alignment couplers  220 A. Surface grating couplers  226  may be optically joined to a common waveguide with light sources  224  and alignment couplers  220 A using optical combiners (e.g., Y-junction or multi-mode interference (MMI) combiners). Surface grating couplers  226  may include single-mode waveguides or multimode waveguides. In an embodiment, surface grating couplers  226  may be omitted. 
     Controller  228  is configured to communicate control signals and electrical signals with photodetectors  222 , light sources  224 , and photonic devices  218 . For example, controller  228  is configured to activate light sources  224  to generate lights to send to waveguide loops  212 , and to receive electrical signals from photodetectors  222  in response to the photodetectors  222  receiving or detecting light from waveguide loops  212 . Controller  228  may also be configured to process electrical signals from photonic devices  218  and photodetectors  222 . For example, controller  228  may determine whether active PIC  214  and fiber coupler  208  are aligned. Alternatively, controller  228  may be configured to communicate electrical signals from photonic devices  218  and photodetectors  222  to other devices for processing. Examples of controller  228  may include, but are not limited to, a complementary metal-oxide semiconductor (CMOS) flip-chip and a processor. Photonic devices  218  are configured to perform one or more photonic functions on light or an optical signal. For example, photonic devices  218  may be semiconductor circuits or chips that integrate multiple optical or opto-electrical components. 
       FIG. 3  is a flowchart of an embodiment of an edge coupling alignment method  300  for an active PIC (e.g., active PIC  214  in  FIG. 2 ). Method  300  can be implemented by a controller (e.g., controller  228  in  FIG. 2 ) on an active PIC to align a fiber coupler (e.g., fiber coupler  208  in  FIG. 2 ) and the active PIC for use in an optical system (e.g., optical system  100  in  FIG. 1 ). Method  300  may be implemented for testing photonic devices, for assembling or packaging a product, and for monitoring the edge coupling alignment between the photonic devices. Method  300  is used to determine whether the fiber coupler and the active PIC are aligned using waveguide loops (e.g., waveguide loops  212  in  FIG. 2 ) on the fiber coupler and alignment couplers (e.g., alignment couplers  220 A and  220 B in  FIG. 2 ) on the active PIC. 
     At step  302 , the active PIC uses a light source and a first alignment coupler from a pair of alignment couplers to provide a light to a waveguide loop. A light source or a surface grating coupler may provide light to the first alignment coupler. At step  304 , the active PIC uses a photodetector and a second alignment coupler from the pair of alignment couplers to determine whether the light from the waveguide loop is detected. When at least a portion of the light from the waveguide loop is detected at the second alignment coupler, a photodetector generates an electrical signal (e.g., an electrical current), and outputs the electrical signal to a controller. The controller may process the electrical signal to determine that the light from the waveguide loop is detected at the second alignment coupler. 
     At step  306 , the active PIC proceeds to step  308  when light is detected at the second alignment coupler from the waveguide loop; otherwise, the active PIC proceeds to step  312  when light is not detected from the waveguide loop. At step  308 , the active PIC determines that the photonic devices are aligned. Optionally at step  310 , the active PIC finely controls the alignment process based on the intensity of the light from the waveguide loop, for example, to maximize the amount of light detected. 
     Returning to step  306 , the active PIC proceeds to step  312  when light is not detected from the waveguide loop. At step  312 , the active PIC determines that the photonic devices are misaligned and returns to step  302 . The active PIC may also generate a signal to indicate the misalignment. In another embodiment, the active PIC may not return to step  302  when the active PIC determines that the photonic devices are misaligned. For example, the active PIC may generate an error signal and may terminate. 
     In an embodiment, the fiber coupler and the active PIC may be physically coupled to each other when the fiber coupler and the active PIC are aligned. For example, the fiber coupler and the active PIC may be coupled together using an adhesive, an epoxy, a solder joint, or any other suitable bonding technique as would be appreciated by one of ordinary skill in the art upon viewing this disclosure. 
       FIG. 4  is a flowchart of an embodiment of an edge coupling alignment monitoring method  400  for an active PIC (e.g., active PIC  214  in  FIG. 2 ). Method  400  can be implemented by a controller (e.g., controller  228  in  FIG. 2 ) on an active PIC to monitor the edge coupling alignment between the fiber coupler and the active PIC during operation. Method  400  may be implemented for monitoring edge coupling alignment while assembling or packaging a product, testing photonic devices, and using photonic devices. Method  400  is used to determine whether the fiber coupler and the active PIC are aligned using waveguide loops (e.g., waveguide loops  212  in  FIG. 2 ) on the fiber coupler and alignment couplers (e.g., alignment couplers  220 A and  220 B in  FIG. 2 ) on the active PIC. 
     At step  402 , the active PIC uses a light source and a first alignment coupler from a pair of alignment couplers to provide a light to a waveguide loop. A light source or a surface grating coupler may provide light to the first alignment coupler. At step  404 , the active PIC uses a photodetector and a second alignment coupler from the pair of alignment couplers to determine whether the light from the waveguide loop is detected. When at least a portion of the light from the waveguide loop is detected at the second alignment coupler, a photodetector generates an electrical signal (e.g., an electrical current), and outputs the electrical signal to a controller. The controller may process the electrical signal to determine that the light from the waveguide loop is detected at the second alignment coupler. At step  406 , the active PIC proceeds to step  408  when light is detected at the second alignment coupler from the waveguide loop; otherwise, the active PIC proceeds to step  410  when light is not detected from the waveguide loop. At step  408 , the active PIC determines that the photonic devices are aligned and the active PIC returns to step  302  for further monitoring of the edge coupling alignment between the active PIC and the fiber coupler. Monitoring the edge coupling alignment between the active PIC and the fiber coupler may be performed continuously or at predetermined time intervals during operation. Alternatively, the active PIC may terminate when monitoring the edge coupling alignment between the active PIC and the fiber coupler is not required. Returning to step  406 , the active PIC proceeds to step  410  when light is not detected from the waveguide loop. At step  410 , the active PIC determines that the photonic devices are misaligned and terminates. The active PIC may also generate a signal to indicate the misalignment. For example, the active PIC may generate an error signal and may terminate. 
       FIG. 5  is a schematic diagram of an embodiment of an optical system  500  using Vernier edge coupling with edge coupling monitoring. Optical system  500  includes a fiber array  504 , a fiber coupler  508 , and an active PIC  514 . Fiber array  504 , fiber coupler  508 , and active PIC  514  are configured similarly to fiber array  102 , fiber coupler  104 , and active PIC  106  in  FIG. 1 . Optical system  500  is configured to send light from fiber array  504  to fiber coupler  508  and from fiber coupler  508  to active PIC  514 . Optical system  500  is configured to provide edge coupling alignment between fiber coupler  508  and active PIC  514  and to monitor the edge coupling alignment between fiber coupler  508  and active PIC  514  during operation. Monitoring the edge coupling alignment may include detecting drift, lateral warping, roll, and positional misalignments. Optical system  500  may be configured as shown or in any other suitable manner. 
     Fiber array  504  has a plurality of waveguides  506 A and  506 B. Waveguides  506 A and  506 B may include single-mode waveguides and/or multi-mode waveguides. Fiber array  504  is configured to receive light from optical fibers  502 A and  502 B and to guide the light to fiber coupler  508  using waveguides  506 A and  506 B. 
     Fiber coupler  508  includes waveguides  510  and  512 , an optical splitter  516 , and a plurality of alignment waveguides  524 A- 524 K. Fiber coupler  508  is configured to receive a first light from waveguide  506 A using waveguide  512 , to distribute the first light among the plurality of alignment waveguides  524 A- 524 K using optical splitter  516 , and to output the first light to active PIC  514  using the plurality of alignment waveguides  524 A- 524 K. Waveguide  512  is an alignment fiber channel. Waveguide  512  extends from a first surface  554  of fiber coupler  508  and may be configured as a single-mode waveguide or a multi-mode waveguide. For example, waveguide  512  extends from an interface between fiber array  504  and fiber coupler  508 . Optical splitter  516  is shown as a 1:10 optical splitter for illustrative purposes. Optical splitter  516  may include any other suitable number of output channels as would be appreciated by one of ordinary skill in the art upon viewing this disclosure. Similarly, the plurality of alignment waveguides  524 A- 524 K may include any suitable number of waveguides. Alignment waveguides  524 A- 524 K extend to a second surface  550  of fiber coupler  508  and are configured to be about equally spaced a length, x, from each other. For example, alignment waveguides  524 A- 524 K extend to the interface (e.g., surface  550 ) between fiber coupler  508  and active PIC  514 . Further, fiber coupler  508  is configured to receive a second light from waveguide  506 B using waveguide  510  and to output the second light to active PIC  514  using waveguide  510 . Waveguide  510  is a signal waveguide or a data fiber channel. Waveguide  510  is configured to extend from the interface (e.g., the first surface  554 ) between fiber array  504  and fiber coupler  508  to an interface (e.g., the second surface  550 ) between fiber coupler  508  and active PIC  514 . Waveguide  510  may be configured as a single-mode waveguide or a multi-mode waveguide. 
     Active PIC  514  includes a plurality of alignment couplers  526 A- 526 K, a plurality of photodetectors  522 A- 522 K, a controller  528 , a photonic device  518 , and a coupler  520 . Active PIC  514  is configured to receive a first light from fiber coupler  508  at one or more of the plurality of alignment couplers  526 A- 526 K to determine whether the fiber coupler  508  and the active PIC  514  are aligned, and to determine a misalignment offset between the fiber coupler  508  and the active PIC  514  in accordance with the alignment couplers  526 A- 526 K that received the light from fiber coupler  508 . Further, active PIC  514  is configured to receive a second light from waveguide  510  on fiber coupler  508  when the fiber coupler  508  and active PIC  514  are aligned. Active PIC  514  and fiber coupler  508  are aligned when two of the alignment couplers  526 A- 526 K receive the first light from alignment waveguides  524 A- 524 K on fiber coupler  508 . For example, active PIC  514  and fiber coupler  508  are aligned when alignment couplers  526 A and  526 J receive the first light from alignment waveguides  524 A and  524 K on fiber coupler  508 , respectively. Active PIC  514  and fiber coupler  508  are misaligned when less than two of the alignment couplers  526 A- 526 K receive the first light from fiber coupler  508 . Alignment couplers  526 A- 526 K may be nano-tapered edge couplers, evanescent edge couplers, or grating couplers. Alignment couplers  526 A- 526 K are positioned along an interface (e.g., surface  552 ) between active PIC  514  and fiber coupler  508  and are configured to be equally spaced from each other. Alignment couplers  526 A- 526 K are configured to be optically coupled to alignment waveguides  524 A- 524 K when one or more of the alignment waveguides  524 A- 524 K are at least partially aligned with one of the alignment couplers  526 A- 526 K. 
     Alignment waveguides  524 A- 524 K and alignment couplers  526 A- 526 K are configured to operate similarly to a Vernier scale, where N graduations of alignment waveguides  524 A- 524 K covers N−1 graduations of alignment couplers  526 A- 526 K. Accordingly, the ratio of the spacing lengths of the alignment waveguides  524 A- 524 K to the spacing lengths of the alignment couplers  526 A- 526 K may be N:N−1. For example, assuming a decimal spacing, alignment waveguides  524 A- 524 K are positioned to cover about nine tenths of the spacing used for alignment couplers  526 A- 526 K. When alignment waveguide  524 A and alignment coupler  526 A are aligned, alignment waveguide  524 B is offset from alignment coupler  526 B about one-tenth of distance x between two neighboring channels on  514 , alignment waveguide  524 C is offset from alignment coupler  526 C about two-tenths of x, alignment waveguide  524 D is offset from alignment coupler  526 D about three-tenths of x, and so on. When alignment waveguide  524 A and alignment coupler  526 A are aligned, alignment waveguide  524 K is also aligned with  526 J. As such, when fiber coupler  508  and active PIC  514  are aligned, two of the alignment waveguides  524 A- 524 K, for example, alignment waveguides  524 A and  524 K, are aligned with two of the alignment couplers  526 A- 526 K, for example, alignment couplers  526 A and  526 J. 
     When fiber coupler  508  and active PIC  514  are misaligned, less than two of the alignment waveguides  524 A- 524 K are aligned with alignment coupler  526 A- 526 K. A misalignment offset can be determined when active PIC  514  and fiber coupler  508  are misaligned by using the alignment couplers  526 A- 526 K. For example, when fiber coupler  508  and active PIC  514  are misaligned by one-tenth of x, alignment waveguide  524 B is aligned with alignment coupler  526 B, when fiber coupler  508  and active PIC  514  are misaligned by two-tenths of x, alignment waveguide  524 C is aligned with alignment coupler  526 C, and so on. When alignment waveguide  524 B is aligned with alignment coupler  526 B, coupler  526 B receives light from alignment waveguide  524 B and photodetector  522 B generates an electrical signal. Receiving an electrical signal from photodetector  522 B indicates that the active PIC  514  and the fiber coupler  508  have a misalignment offset of about one tenth. Alternatively, alignment waveguides  524 A- 524 K and alignment couplers  526 A- 526 K are configured such that N graduations of alignment couplers  526 A- 526 K covers N−1 graduations of alignment waveguides  524 A- 524 K. Accordingly, the ratio of the spacing lengths of the alignment waveguides  524 A- 524 K to the spacing lengths of the alignment couplers  526 A- 526 K may be N−1:N. 
     Photodetectors  522 A- 522 K are optically coupled to alignment couplers  526 A- 526 K, respectively. Photodetectors  522 A- 522 K are configured to detect light from alignment couplers  526 A- 526 K, respectively, and to generate an electrical signal in response to detecting light. Examples of photodetectors  522 A- 522 K may include, but are not limited to, photodiodes or any other suitable device for detecting light as would be appreciated by one of ordinary skill in the art upon viewing this disclosure. Photodetectors  522 A- 522 K may be constructed using any suitable group IV semiconductors, group III-V materials, or any other suitable materials. 
     Controller  528  is configured to communicate control signals and electrical signals with photodetectors  522 A- 522 K and photonic device  518 . For example, controller  528  is configured to receive electrical signals from photodetectors  522 A- 522 K in response to photodetectors  522 A- 522 K detecting light. Controller  528  is also configured to process electrical signals from photodetectors  522 A- 522 K and photonic device  518 . For example, controller  528  is configured to determine that fiber coupler  508  and active PIC  514  are aligned when controller  528  receives electrical signals from two of the photodetectors  522 A- 522 K, for example, photodetectors  522 A and  522 J, and to determine that fiber coupler  508  and active PIC  514  are misaligned when controller  528  receives electrical signals from less than two of the photodetectors  522 A- 522 K. 
     In another embodiment, fiber coupler  508  and active PIC  514  are aligned when light is detected at a first photodetector. For example, fiber coupler  508  and active PIC  514  are aligned when light is detected by photodetector  522 A. Further, fiber coupler  508  and active PIC  514  are misaligned when light is detected at a photodetector other than the first photodetector. The other photodetectors may each be associated with a misalignment offset. Controller  528  is configured to determine that fiber coupler  508  and active PIC  514  are aligned when controller  528  receives an electrical signal from the first photodetector, for example, photodetector  522 A, and to determine that fiber coupler  508  and active PIC  514  are misaligned when controller  528  receives an electrical signal from any other photodetector. 
     Coupler  520  may be a nano-tapered edge coupler, an evanescent edge coupler, or a grating coupler. Coupler  520  is configured to be optically coupled to waveguide  510  on fiber coupler  508  when the fiber coupler  508  and the active PIC  514  are aligned. Coupler  520  is configured to receive a second light from waveguide  510  on fiber coupler  508  and to guide the second light to photonic device  518 . Photonic device  518  is configured to perform one or more photonic functions on light or an optical signal. For example, photonic device  518  may be a semiconductor circuit or a chip that integrates multiple optical or opto-electrical components. 
       FIG. 6  is a flowchart of an embodiment of an edge coupling alignment method  600  using Vernier edge coupling for an active PIC (e.g., active PIC  514  in  FIG. 5 ). Method  600  is implemented by a controller (e.g., controller  528  in  FIG. 5 ) to determine whether a fiber coupler (e.g., fiber coupler  104  in  FIG. 1 ) and an active PIC (e.g., active PIC  106  in  FIG. 1 ) are aligned. For example, method  600  may be used for testing photonic devices, for assembling or packaging a product, and/or for monitoring the edge coupling alignment. Method  600  determines whether a fiber coupler and an active PIC are aligned using Vernier edge coupling between a plurality of alignment waveguides (e.g., alignment waveguides  524 A- 524 K in  FIG. 5 ) and a plurality of alignment couplers (e.g., alignment couplers  526 A- 526 K in  FIG. 5 ). Further, method  600  may be employed to monitor the edge coupling alignment between the fiber coupler and the active PIC. 
     At step  602 , the active PIC detects light from at least one alignment waveguide on the fiber coupler via at least one alignment coupler from a plurality of alignment couplers. The alignment couplers are optically coupled to the photodetectors. Light from the alignment waveguides is guided by one or more alignment couplers and is detected using the photodetectors. The photodetectors generate an electrical signal in response to detecting the light and sends the electrical signal to the controller. 
     At step  604 , the active PIC determines whether light is detected at two alignment couplers from the plurality of alignment couplers. Active PIC proceeds to step  606  when light is detected from two alignment couplers; otherwise, active PIC proceeds to step  610 . For example, the controller determines how many electrical signals were received in response to photodetectors detecting light. 
     At step  606 , the active PIC determines that the photonic devices are aligned. At step  608 , the active PIC determines whether monitoring edge coupling alignment between the active PIC and the fiber coupler is required. The active PIC returns to step  602  when monitoring edge coupling alignment between the active PIC and the fiber coupler is required; otherwise, method  600  terminates when monitoring edge coupling alignment between the active PIC and the fiber coupler is not required. The active PIC may also generate a signal to indicate the alignment. The controller may have instructions that indicate whether the active PIC is configured to monitor edge coupling alignment between the active PIC and the fiber coupler. Monitoring the edge coupling alignment between the active PIC and the fiber coupler may be performed continuously or at predetermined time intervals during operation. 
     Returning to step  604 , the active PIC proceeds to step  610  when light is detected at less than two alignment couplers. At step  610 , the active PIC determines that the photonic devices are misaligned and returns to step  602  or, optionally, proceeds to step  612  and then returns to step  602 . The active PIC may also generate a signal to indicate the misalignment. Optionally, at step  612 , active PIC determines a misalignment offset between the active PIC and the fiber coupler. For example, the controller determines which photodetector sends an electrical signal in response to an alignment coupler receiving light and calculates a misalignment offset in accordance with the alignment coupler and photodetector. 
     In an embodiment, the fiber coupler and the active PIC may be physically coupled to each other when the fiber coupler and the active PIC are aligned. For example, the fiber coupler and the active PIC may be coupled together using an adhesive, an epoxy, a solder joint, or any other suitable bonding technique as would be appreciated by one of ordinary skill in the art upon viewing this disclosure. 
       FIG. 7  is a schematic diagram of another embodiment of an optical system  700  using Vernier edge coupling with edge coupling monitoring. Optical system  700  includes a fiber coupler  708  and an active PIC  714 . Fiber coupler  708  and active PIC  714  are configured similarly to fiber coupler  104  and active PIC  106  in  FIG. 1 , respectively. Optical system  700  is configured to send light from fiber coupler  708  to active PIC  714 , provide edge coupling alignment between fiber coupler  708  and active PIC  714 , and monitor the edge coupling alignment between fiber coupler  708  and active PIC  714  during operation. Monitoring the edge coupling alignment may include detecting drift, lateral warping, roll, and positional misalignments. Optical system  700  may be configured as shown or in any other suitable manner. 
     Fiber coupler  708  includes waveguides  710  and  712 , tapered coupler  730 , optical splitter  716 , and a plurality of alignment waveguides  724 A- 724 K. Fiber coupler  708  is configured to receive a first light from waveguide  732  on active PIC  714  using tapered coupler  730  and waveguide  712  to distribute the first light among the plurality of alignment waveguides  724 A- 724 K using optical splitter  716 , and to output the first light to active PIC  714  using the plurality of alignment waveguides  724 A- 724 K. Tapered coupler  730  may be a nano-tapered edge coupler, an evanescent edge coupler, or a grating coupler. Tapered coupler  730  is positioned along a first surface  750  of fiber coupler  708  and is optically coupled to waveguide  712 . Tapered coupler  730  is configured to widen towards the first surface  750  and may be configured as a single-mode waveguide or a multi-mode waveguide. Waveguide  712  is optically coupled to tapered coupler  730  and optical splitter  716 . Waveguide  712  may be configured as a single-mode waveguide or a multimode waveguide. Optical splitter  716  is shown as a 1:10 optical splitter for illustrative purposes. Optical splitter  716  may include any other suitable number of output channels as would be appreciated by one of ordinary skill in the art upon viewing this disclosure. Similarly, the plurality of alignment waveguides  724 A- 724 K may have any suitable number of waveguides. Alignment waveguides  724 A- 724 K extend to the first surface  750  of fiber coupler  708  and are configured to be about equally spaced a length, x, from each other. For example, alignment waveguides  724 A- 724 K are positioned at an interface between fiber coupler  708  and active PIC  714 . Further, fiber coupler  708  is configured to receive a second light, for example, from a fiber array, using waveguide  710  and to output the second light to active PIC  714  using waveguide  710 . Waveguide  710  is a data fiber channel and may be a single-mode fiber or multi-mode fiber. Waveguide  710  is configured to extend from a second surface  754  to the first surface  750 , which is the interface between fiber coupler  708  and active PIC  714 . 
     Active PIC  714  includes a plurality of alignment couplers  726 A- 726 K, a plurality of photodetectors  722 A- 722 K, a controller  728 , a photonic device  718 , waveguide  732 , and light source  734 . Active PIC  714  is configured to output a first light to fiber coupler  708  from light source  734 , to receive the first light from fiber coupler  708  at one or more of the plurality of alignment couplers  726 A- 726 K, to determine whether the fiber coupler  708  and the active PIC  714  are aligned, and to determine a misalignment offset between the fiber coupler  708  and the active PIC  714  in accordance with the alignment coupler that received the light from fiber coupler  708 . Further, active PIC  714  is configured to receive a second light from waveguide  710  on fiber coupler  708  when the fiber coupler  708  and active PIC  714  are aligned. Active PIC  714  and fiber coupler  708  are aligned when two of the alignment couplers  726 A- 726 K receive the first light from alignment waveguides  724 A- 724 K on fiber coupler  708 . For example, active PIC  714  and fiber coupler  708  are aligned when alignment couplers  726 A and  726 J receive the first light from alignment waveguides  724 A and  724 K on fiber coupler  708 , respectively. Active PIC  714  and fiber coupler  708  are misaligned when less than two of the alignment couplers  726 A- 726 K receive the first light from fiber coupler  708 . Alignment couplers  726 A- 726 K may nano-tapered edge couplers, evanescent edge couplers, or grating couplers. Alignment couplers  726 A- 726 K are positioned along a surface  752  between active PIC  714  and fiber coupler  708  and are configured to be equally spaced from each other. Alignment waveguides  724 A- 724 K and alignment couplers  726 A- 726 K are configured to operate similarly to alignment waveguides  524 A- 524 K and alignment couplers  526 A- 526 K in  FIG. 5 , respectively. Alignment couplers  726 A- 726 K are configured to be optically coupled to alignment waveguides  724 A- 724 K when one or more of the alignment waveguides  724 A- 724 K are at least partially aligned with one of the alignment couplers  726 A- 726 K. When fiber coupler  708  and active PIC  714  are misaligned, less than two of the alignment waveguides  724 A- 724 K are aligned with alignment coupler  726 A- 726 K. 
     Photodetectors  722 A- 722 K are optically coupled to alignment couplers  726 A- 726 K, respectively. Photodetector  722 A- 722 K are configured to detect light from alignment couplers  726 A- 726 K, respectively, and to generate an electrical signal in response to detecting light. Photodetectors  722 A- 722 K are configured similarly to photodetectors  522 A- 522 K in  FIG. 5 . 
     Controller  728  is configured to communicate control signals and electrical signals with photodetectors  722 A- 722 K and photonic device  718 . For example, controller  728  is configured to receive electrical signals from photodetectors  722 A- 722 K in response to photodetectors  722 A- 722 K detecting light. Controller  728  is also configured to process electrical signals from photodetectors  722 A- 722 K and photonic device  718 . For example, controller  728  is configured to determine that fiber coupler  708  and active PIC  714  are aligned when controller  728  receives electrical signals from two of the photodetectors  722 A- 722 K, for example, photodetectors  722 A and  722 J, and to determine that fiber coupler  708  and active PIC  714  are misaligned when controller  728  receives electrical signals from less than two of photodetectors  722 A- 722 K. 
     Light source  734  is configured to receive control signals from controller  728 , to generate a light, and to output the light to waveguide  732 . Examples of light sources  734  may include, but are not limited to, lasers, embedded lasers, VCSELs, SOAs, DFB lasers, and LEDs. In an embodiment, light sources  734  may be monolithically integrated into controller  728 . 
     Coupler  720  may be a nano-tapered edge coupler, an evanescent edge coupler, or a grating coupler. Coupler  720  is configured to be optically coupled to waveguide  710  on fiber coupler  708  when fiber coupler  708  and active PIC  714  are aligned. Coupler  720  is configured to receive a second light from waveguide  710  on fiber coupler  708  and to guide the second light to photonic device  718 . Photonic device  718  is configured to perform one or more photonic functions on light or an optical signal. For example, photonic device  718  may be a semiconductor circuit or a chip that integrates multiple optical or opto-electrical components. 
       FIG. 8  is a flowchart of another embodiment of an edge coupling alignment method  800  using Vernier edge coupling for an active PIC (e.g., active PIC  714  in  FIG. 7 ). Method  800  is implemented by a controller (e.g., controller  728  in  FIG. 7 ) to determine whether a fiber coupler (e.g., fiber coupler  104  in  FIG. 1 ) and an active PIC (e.g., active PIC  106  in  FIG. 1 ) are aligned. For example, method  800  may be used for testing photonic devices, for assembling or packaging a product, and for monitoring the edge coupling alignment. Method  800  determines whether a fiber coupler and an active PIC are aligned using Vernier edge coupling between a plurality of alignment waveguides (e.g., alignment waveguides  724 A- 724 K in  FIG. 7 ) and a plurality of alignment couplers (e.g., alignment couplers  726 A- 726 K in  FIG. 7 ). Further, method  800  may be employed to monitor the edge coupling alignment between the fiber coupler and the active PIC. 
     At step  802 , the active PIC uses a light source to provide a light to a tapered coupler. A light source or a surface grating coupler may provide light to the tapered coupler. At step  804 , the active PIC detects light from at least one alignment waveguide on the fiber coupler via at least one alignment coupler from a plurality of alignment couplers. The alignment couplers are optically coupled to photodetectors. Light from the alignment waveguides is guided by one or more alignment couplers and is detected using the photodetectors. The photodetectors generate an electrical signal in response to detecting the light and send the electrical signal to the controller. 
     At step  806 , the active PIC determines whether light is detected at two alignment couplers from the plurality of alignment couplers. Active PIC proceeds to step  808  when light is detected from two alignment couplers; otherwise, active PIC proceeds to step  812 . For example, the controller determines how many electrical signals were received in response to photodetectors detecting light. In an embodiment, the controller coarsely controls the alignment process until light is detected and then finely controls the alignment process to maximize the amount of detected light. 
     At step  808 , the active PIC determines that the photonic devices are aligned. At step  810 , the active PIC determines whether monitoring edge coupling alignment between the active PIC and the fiber coupler is required. The active PIC returns to step  802  when monitoring edge coupling alignment between the active PIC and the fiber coupler is required; otherwise, method  800  terminates when monitoring edge coupling alignment between the active PIC and the fiber coupler is not required. The active PIC may also generate a signal to indicate the alignment. The controller may have instructions that indicate whether the active PIC is configured to monitor edge coupling alignment between the active PIC and the fiber coupler. Monitoring the edge coupling alignment between the active PIC and the fiber coupler may be performed continuously or at predetermined time intervals during operation. 
     Returning to step  806 , the active PIC proceeds to step  812  when light is detected at less than two alignment couplers. At step  812 , the active PIC determines that the photonic devices are misaligned and returns to step  802  or, optionally, proceeds to step  814  and then returns to step  802 . The active PIC may also generate a signal to indicate the misalignment. Optionally, at step  814 , active PIC determines a misalignment offset between the active PIC and the fiber coupler. For example, the controller determines which photodetector sends an electrical signal in response to an alignment coupler receiving light and calculates a misalignment offset in accordance with the alignment coupler and photodetector. 
     In an embodiment, the fiber coupler and the active PIC may be physically coupled to each other when the fiber coupler and the active PIC are aligned. For example, the fiber coupler and the active PIC may be coupled together using an adhesive, an epoxy, a solder joint, or any other suitable bonding technique as would be appreciated by one of ordinary skill in the art upon viewing this disclosure. 
       FIG. 9  is a schematic diagram of a device  900 . The device  900  may be suitable for implementing the disclosed embodiments. For instance, the device  900  may be employed in a controller. Device  900  includes ports  910 , transceiver units (Tx/Rx)  920 , a processor  930 , and a memory  940  with an edge coupling alignment module  950 . Ports  910  are coupled to Tx/Rx  920 , which may be transmitters, receivers, or combinations thereof. The Tx/Rx  920  may transmit and receive data via the ports  910 . Processor  930  is configured to process data. Memory  940  is configured to store data and instructions for implementing embodiments described herein. The device  900  may also include electrical-to-optical (EO) components and optical-to-electrical (OE) components coupled to the ports  910  and Tx/Rx  920  for receiving and transmitting electrical signals and optical signals. 
     The processor  930  may be implemented by hardware and software. The processor  930  may be implemented as one or more central processing unit (CPU) chips, logic units, cores (e.g., as a multi-core processor), field-programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), and digital signal processors (DSPs). The processor  930  is in communication with the ports  910 , Tx/Rx  920 , and memory  940 . 
     The memory  940  includes one or more of disks, tape drives, and solid-state drives and may be used as an over-flow data storage device, to store programs when such programs are selected for execution, and to store instructions and data that are read during program execution. The memory  940  may be volatile and non-volatile and may be read-only memory (ROM), random-access memory (RAM), ternary content-addressable memory (TCAM), and static random-access memory (SRAM). Edge coupling alignment module  950  is implemented by processor  930  to execute the instructions for implementing various embodiments for implementing edge coupling alignment. 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.