Abstract:
A method and apparatus is provided for attaching a external optical component processing an optical beam to a PLC and optically aligning the external optical component with an optical element formed on the PLC. The method begins by securing the external optical component to a first side of a submount. A first side of a flexure element is secured to the first side of the submount. A second side of the flexure element is secured to a first side of the PLC on which the optical element is formed such that the external optical component and the optical element are in optical alignment to within a first level of tolerance. Subsequent to the step of securing the second side of the flexure element, a force is exerted on at least a second side of the submount to thereby flex the flexure element. The force causes sufficient flexure of the flexure element to optically align the external optical component and optical element to within a second level of tolerance that is more stringent than the first level of tolerance.

Description:
STATEMENT OF RELATED APPLICATION  
       [0001]     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/826,145, filed Apr. 15, 2004, entitled Active Optical Alignment And Attachment Thereto Of A Semiconductor Optical Component With An Optical Element Formed On A Planar Lightwave Circuit, which is incorporated herein by reference in its entirety. 
     
    
     STATEMENT OF RELATED APPLICATION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates generally to fiber optics, and in particular to an arrangement for providing optical alignment between an external optical component, such as a laser or LED, and an optical element formed or secured on a planar substrate.  
         [0004]     2. Background of the Invention  
         [0005]     In fiber optic technology there are many instances where it is necessary to optically align and optically couple light from an external optical component such as a semiconductor device and/or a micro electromechanical system (MEMS) to an optical component located or formed on a planar substrate. In Planar Lightwave Circuits (PLC) technology optical waveguides and other optical elements, such as mirrors, gratings, beam-splitters, are formed monolithically on silicon or glass wafers, using processing techniques similar to those used in the silicon microelectronics industry. Doped-silica waveguides are usually preferred because they have a number of attractive properties including low cost, low loss, fabrication process maturity and stability. Changes in dopants (such as P or Ge) concentration change the refractive index of the waveguide core and therefore the numerical aperture(NA) of the waveguide. This flexibility allows for optimized coupling to devices of different NA, for example, such as laser diodes, detectors, high NA and standard fibers. To maximize the coupling efficiency the component has to be accurately aligned within a narrow range around the optimum position. The cost of achieving proper alignment between the external optical component and an element on PLC is often high because it involves the use of expensive lenses and high precision actuators to accomplish the alignment. To achieve high efficiency coupling can be particularly hard in case of the single mode optics because the tolerance to misalignment is so small.  
         [0006]     The external optical components discussed includes lasers, Light Emitting Diodes(LED), semiconductor optical amplifiers, detectors, MEMS, filters, isolators, fibers, lenses, etc. To integrate the above-mentioned external optical components on PLC, there must be an alignment mechanism to optically couple the external optical components with the optical devices located on the PLC. Such devices can be either monolithically integrated on the PLC or be hybrid attached to the PLC platform.  
         [0007]     Some of the more difficult systems to align involve semiconductor lasers, because they have a highly diverging beam. A semiconductor laser that is to be optically aligned to a single-mode waveguide, which is the type commonly used in optical telecommunication systems, has a typical positional misalignment tolerance on the order of tenths of a micron.  
         [0008]     In a conventional alignment process in which a semiconductor device, such as a laser, is to be attached and optically aligned with an optical component, such as an optical fiber, the semiconductor device is first bonded to a submount. A weldable fixture is also attached to a submount. The optical fiber is secured to the weldable fixture. After securing the fiber to the fixture, the fixture is physically manipulated to achieve the desired coupling efficiency. The semiconductor device is not moved during the alignment process because it is electrically connected and thermally contacted for heat sinking to maintain stability. As a result the laser cannot be moved on the bonded submount. This technique requires specialized fixtures and flexures to enable bending in order to move the optical fiber back into position. The flexures are often complex structures enabling bending in various directions, and can be relatively expensive. If an active alignment technique is used, an optical signal is transmitted through the components and detected. Manipulating the optical fiber so that the transmission is at the highest possible level for the system, which indicates that the coupling efficiency is maximized, performs the alignment.  
         [0009]     This conventional approach becomes difficult, when the laser or LED have to be aligned to an element such as a facet of a waveguide formed on a PLC or to an optical element that is already securely attached to some other planar substrate. The PLC chip can be large and heavy and its manipulation and support can be difficult.  
       SUMMARY OF THE INVENTION  
       [0010]     In accordance with the present invention, a method and apparatus is provided for attaching a external optical component processing an optical beam to a optical element located on the PLC chip and optically aligning the external optical component with the latter optical element. The process starts by securing the external optical component to a first side of a submount. A first side of a flexure element is secured to the first side of the submount. A second side of the flexure element is secured to a first side of the PLC on which the optical element is located such that the external optical component and the optical element are in optical alignment to within a first level of tolerance. Subsequent to the step of securing the second side of the flexure element, a force is exerted on at least a second side of the submount to thereby flex the flexure element. The force causes sufficient flexure of the flexure element to optically align the external optical component and optical element to within a second level of tolerance that is more stringent than the first level of tolerance.  
         [0011]     In accordance with one aspect of the invention, an optical coupling efficiency of an optical beam propagating between the external optical component and optical element is monitored.  
         [0012]     In accordance with another aspect of the invention, the step of exerting a force is performed such that the coupling efficiency is maximized.  
         [0013]     In accordance with another aspect of the invention, the optical element is a facet of a planar waveguide formed on the PLC.  
         [0014]     In accordance with another aspect of the invention, the external optical component is a semiconductor laser.  
         [0015]     In accordance with another aspect of the invention, the external optical component is selected from the group consisting of a semiconductor laser, an LED, a semiconductor optical amplifier, a beam splitter, a thin film, a filter, a mirror, a birefringent material, a polarizer, and a diffractive element.  
         [0016]     In accordance with another aspect of the invention, the flexure element is formed from gold or a gold alloy, or other metal with low mechanical yield.  
         [0017]     In accordance with another aspect of the invention, the flexure has a preferred shape, and can be constructed as a monolithic element or from multiplicity of individual elements.  
         [0018]     In accordance with another aspect of the invention, the flexure element and submount are one element.  
         [0019]     In accordance with another aspect of the invention, the flexure element is formed from a thermally conductive material sufficient to serve as a heat sink for the external optical component.  
         [0020]     In accordance with another aspect of the invention, the second side of the submount on which the force is exerted is a back surface of the submount opposing the first side of the submount.  
         [0021]     In accordance with another aspect of the invention, the second side of the submount on which the force is exerted is an edge of the submount.  
         [0022]     In accordance with another aspect of the invention, the submount is mounted on the edge of a pocket etched into the first side of the PLC, whereas such pocket allows clearance for the external optical component.  
         [0023]     In accordance with another aspect of the invention, the submount is enclosed with a cover that mates with the first side of the PLC. In accordance with another aspect of the invention, the cover forms a hermetic seal with the first side of the PLC.  
         [0024]     In accordance with another aspect of the invention, the cover is formed from Kovar™, silicon, or Pyrex.  
         [0025]     In accordance with another aspect of the invention, the hermetic seal is established by a solder seal ring.  
         [0026]     In accordance with another aspect of the invention, a retaining disk is secured between the first side of the flexure element and the first side of the submount.  
         [0027]     In accordance with another aspect of the invention, the retaining disk has a diameter greater than a diameter of the flexure element.  
         [0028]     In accordance with another aspect of the invention, the flexure element and the retaining disk are formed from a common material.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]      FIG. 1  shows a semiconductor laser mounted on a submount before attachment to PLC platform.  
         [0030]      FIG. 2  shows a first embodiment of the optical apparatus constructed in accordance with the present invention in which an external optical component is attached to and optically aligned with an optical element formed on a PLC. The external optical element is selected to be a laser.  
         [0031]      FIGS. 3A-3C  show an embodiment of the optical apparatus constructed in accordance with the present invention in which a hermetic seal is enclosing the external optical component.  
         [0032]      FIG. 4  shows a second embodiment of the optical apparatus constructed in accordance with the present invention  
         [0033]      FIGS. 5A and 5B  show a third embodiment of the optical apparatus constructed in accordance with the present invention in which an external optical component is attached to and optically aligned with an optical element formed on a PLC.  
         [0034]      FIGS. 6A and 6B  show a fourth embodiment of the optical apparatus constructed in accordance with the present invention in which an external optical component is attached to and optically aligned with an optical element formed on a PLC. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0035]     The present invention describes a method and an apparatus for alignment of an external optical component such as a semiconductor laser to an optical element formed on a PLC. While the external optical component will be described below for illustrative purposes only as a semiconductor laser, the external optical component alternatively may comprise a variety of different active and/or passive elements that process an optical beam. For example, active devices include semiconductor lasers and amplifiers, LEDs, as well as more complex devices offering higher levels of functionality. Passive devices include, for example, beam splitters, thin films, filters, mirrors, birefringent material, polarizers, and diffractive elements. In the same spirit, the optical element formed on PLC will be for illustrative purposes a facet of a waveguide, but it could alternatively be a grating, a mirror, or other passive or active component formed on a PLC, or secured on PLC or other planar substrate.  
         [0036]     As detailed below, the external optical component is first attached to its own submount in a conventional manner and the resulting subassembly is bonded to the PLC via a deformable flexure element of low yield strength that allows for active optical alignment. Active alignment is then achieved by moving the external optical component into its proper position. In the present invention the external optical component is directly attached to its own submount and not the PLC. For example, in some circumstances, the external optical component is preferably aligned in an orientation that is upside-down in comparison to the conventional process. Among its other advantages, the invention eliminates the need for laser soldering or laser welding equipment. It also eliminates the need for hardware or fixtures to hold the PLC chip in place, which is more difficult due to the usually larger size of the latter. The invention allows attachment of the external optical component using conventional die-bonding equipment and allows pre-assembly of the external optical component with its submount by conventional pre-qualified means. The flexure element can be flexed to provide for active alignment of the external optical component to the PLC optical component after the bonding processes are completed, eliminating alignment losses caused by bonding. The flexure element may be, for example, a small gold shim made by hole punching a metal sheet and thus does not require any expensive machining.  
         [0037]      FIG. 1  shows a semiconductor laser  110  that is first attached to its own submount  111 , which in the case of a semiconductor laser is often aluminum nitride (AIN). Aluminum nitride is commonly used as a submount for semiconductor lasers because AIN has excellent thermal conductivity and is expansion matched to the GaAs material from which such lasers are formed, reducing stress which could otherwise alter the lasing wavelength of the device. In  FIG. 1  the laser is bonded in a standard way, such that the laser active ridge  116  is on top. Metal pads  113  and a metallized through hole  117  are provided for biasing the laser. The bottom of the laser is contacting pad  113 , and the top is connected to pad  114  using two wire bonds  115 .  
         [0038]     A flexure element  112  is also attached to the submount  111 . As detailed below, the flexure element  112  enables the laser active area  116  to be optically aligned to the waveguide  210  on PLC  211  in  FIG. 2  after the laser  110  has been bonded to the submount  111 . This is an important feature of the invention because the bonding process would otherwise cause misalignment and because during active alignment the bonded laser can be operated at room temperature since the bonding process that normally uses heat is not needed during the alignment step. The laser  110  and waveguide  210  can be optically aligned in an active alignment process by applying force to the laser submount  111  to thereby bend the flexure element  112  by an appropriate amount.  
         [0039]     The flexure element  112  is preferably made of a material that has low yield strength, meaning that it will bend but not tend to spring back. The flexure element  112  should also be stable and remain in position as long as sufficient force is not exercised. A preferred material for the flexure element  112  is gold or a gold alloy, which have low yield strength characteristics. Other exemplary materials with a low yield strength that may be employed are lead, nickel, nickel alloys, copper, silver and Kovar™. One advantage of gold is that it is compatible with a gold tin eutectic solder and can be die-bonded to bond it in position using performs of AuSn (80/20) eutectic solder, a solder commonly used to bond laser chips to AIN submounts. The absence of spring action and stability could be also achieved by proper design of a more complex multi-element flexure, which is also covered by this disclosure.  
         [0040]     The inventive attachment and optical alignment process begins by die-bonding and wire-bonding the laser  110  to the AIN submount  111  in a conventional manner, after which the resulting laser subassembly is aged and tested, also in a conventional manner.  
         [0041]     After the laser  110  is bonded, burned-in and tested on the AIN submount  111  in the aforementioned manner, the flexure element  112  is bonded to the AIN submount  111  at a location in front of the laser facet  110   a . The flexure element  112  may be bonded by the same technique used to bond laser  110  to the AIN submount  111 . That is, the same equipment can be used to bond both the laser  110  and the flexure element  112 . For example, an AuSn (80/20) eutectic solder may be used to establish both bonds.  
         [0042]     In addition to its low yield strength, the flexure element  112  should have a sufficiently high thermal conductivity to serve as a heat sink for the heat generated by laser  110 . This alleviates the need to attach any additional heat sinks or cooling elements such as a thermo-electric cooler (TEC) to the AIN submount  111 , which could adversely impact optical alignment by flexing the submount  111 . Of course, the thermal conductivity of the flexure element  112  can be increased, as needed by increasing its size along the dimensions that contact the AIN submount  111  and the PLC  211 . For example, if the flexure element  112  is configured in the shape of a disk, its diameter can be increased to increase its thermal capacity. The thickness of the flexure element  112 , however, is preferably about the same as the thickness of the laser  110  to facilitate initial alignment. Of course, the present invention encompasses flexure elements  112  of any shape and size and is not intended as a limitation on the invention.  
         [0043]     The resulting laser, flexure element and submount subassembly is next die-bonded to the PLC  211  also using, for example, a AuSn (80/20) solder. This step may be conveniently performed by inverting the sub-assembly so that the top of the laser  110  (usually the p side) and the top of the flexure element  112  are facing the PLC  211 . The top of the flexure element  112  is bonded to the PLC  211 , so that the laser  110  extends in front of the facet of the waveguide  210  in rough optical alignment, as shown in  FIG. 2 .  
         [0044]     As shown in  FIG. 2 , a wire bond  214  extends from the back of the AIN submount  111  to a point on the PLC  211  to complete the electrical circuit. The wire bond  214  serves as one electrical connection for the laser  110 . The other electrical connection to the laser  110  is established through the flexure element  112  itself. The bottom contacts for the laser  110  and the flexure element  112  are located on the same metal pad. The two resulting connections allow the laser  110  to be powered.  
         [0045]     Once the bonding process is complete, optical alignment between the laser  110  and waveguide  210  may be performed in an active manner. That is, the laser  110  is powered and aligned to the waveguide  210  by exerting a downward force on the back of the AIN submount  111  until the optical signal coupled into the waveguide  210  is maximized. Since a conventional die-bonder is generally able to initially place the laser  110  to within about 5-10 microns of its target position, the flexure element  112  only needs to bend sufficiently so that the laser  110  can be adjusted over these remaining 10 microns. It should be noted that because the aperture of waveguide  210  is significantly larger than the output aperture of the laser  110 , the coupling efficiency is not very sensitive on the angular misalignment between the axis of the laser  110  and the axis of the waveguide  210 , at least up to an angular misalignment of about 3 degrees in a typical application. In such an application the angular misalignment provided by exerting a force on the flexure element  112  will typically be less than about 0.1 degrees to achieve maximum coupling efficiency.  
         [0046]     Among the directions along which alignment must be achieved, the most sensitive are vertical alignments, such as up and down (i.e., in a direction perpendicular to the axis of waveguide  210  that also traverses the PLC chip  211 ) and side to side (i.e., in a direction perpendicular to the axis of waveguide  210  that is also parallel to the planes encompassing the submount  111  and the PLC  211 ). Up and down alignment is achieved by exerting a downward force on the AIN submount  111 , either in front of or behind the flexure element  112 . Exerting a force in front of the flexure element  112  causes the laser  110  to move down while exerting the force behind the flexure element  112  causes the laser to move upward. Side to side movements is accomplished by exerting a force on a side or edge of the AIN submount  111 . Since the linear displacements should be very small, the angular misalignment will be negligible.  
         [0047]     The present invention achieves multiple advantages with respect to the conventional bonding and optical alignment techniques. First, the invention enables the use of commercially available semiconductor lasers or other external optical components that are already mounted, burned-in, and pre-tested without the need for modifications. The laser or other external optical component is simply provided as a chip located on its own submount. Thus, the present invention advantageously makes use of a low cost, pre-qualified components. Moreover, the critical manufacturing process of the laser or other external optical component is completely separated from the alignment process. This is a key advantage, because doing otherwise might require a custom semiconductor device that would be highly specialized and thus much more expensive. A second key advantage is that the invention allows alignment to take place after the bonding steps are performed. In this way the process yield arising from alignment can be dealt with separately from the process yield arising from attachment. Since the alignment process is independent of the attachment process it does not affect the optimization of the attachment process. A third key advantage is that the alignment can be reworked, meaning that if for some reason proper alignment is not achieved, the optical components can always be repositioned by bending the flexure element until the alignment is satisfactory. A fourth key advantage is that the overall size of the assembly is relatively small, and in some cases may not be much larger than the size of the semiconductor laser itself. A fifth key advantage is that because the semiconductor laser rather than the waveguide is moved to achieve alignment, the invention allows many additional devices to be mounted and independently aligned on a single PLC chip, enabling larger scale integration of optical components.  
         [0048]     The assembly shown in  FIG. 2  may be finalized in the manner shown in  FIGS. 3A-3C  to provide a very compact, hermetically sealed component. In  FIG. 2 a  pocket  212  is shown etched in the PLC chip, such as it has a vertical facet  213  formed in the waveguide, to which the laser is coupled. The pocket is big enough as to accommodate part of the laser and the wire-bonds. Around the hole a metal ring  301  ( FIG. 3B ) is deposited that is sufficiently wide to encompass the hole and the laser on the submount mounted on the side of the hole. The metal ring is electrically isolated from the leads connecting the laser with the wire-bonding pads. A cap  302  made of a material close in expansion to silicon is fabricated, and can be soldered to the ring, so the laser is now hermetically sealed in the space formed between the pocket and the cap. One advantage of this approach is that the entire package does not have to be hermetic, but only a small part of it. It is significant, because this method allows using non-hermetic components simultaneously with the laser inside, further lowering the overall cost of the package  
         [0049]     In an alternative embodiment, as shown in  FIG. 4 , a laser is mounted on a PLC an edge of a PLC chip, such that the facet of the waveguide is either etched or polished. Here element  303  is an end facet of the waveguide  210 , and the laser is aligned to it.  
         [0050]     In other embodiments of the invention the AIN submount  111  to which the laser  110  or other external optical component is attached may be eliminated by directly attaching the laser  110  or external optical component to the flexure element  112 . That is, the flexure element  112  can serve as both the support for the external optical component and the component that is flexed during the active alignment process.  
         [0051]     Various embodiments of the optical-alignment flexure assembly are possible without deviating from the spirit of the invention. For example, by changing the laser position on the AIN submount, the AIN submount can be arranged at a 90 degree angle to the buried optical waveguide, as shown in  FIGS. 5A and 5B . This variation now makes the x direction (along the optical axis) more adjustable by displacing it further away from the attachment points of the flexures. The x-direction adjustment would take place by twisting the attachment points on the flexures rather than trying to sheer them. Another variation of the basic idea is an attachment scheme in which the buried optical waveguide is located opposite or across from the attachment point of the submount on another edge of the PLC. This is shown in  FIGS. 6A and 6B . With this geometry there is a well in the silicon bench that can accommodate the laser. The laser can then be aligned to the facet of the optical waveguide in a similar way as done before, except that the laser now points away from the flexure attachment point.  
         [0052]     In some embodiments of the invention connections to the contacts may be established as follows. A wirebond connects the p-contact of the laser to a pad on the AIN submount. This pad is holed through  117  the AIN (see  FIG. 1 ) to the backside of the AIN, which is coated with conductor as well as coating within the hole, making electrical connection to the backside. After assembly, the backside is then wirebonded down to a pad on the Si substrate, enabling an electrical path to the p-contact of the laser. The n-contact of the laser is simply made by conduction through the flexure itself to a pad on the PLC, eliminating the need for wirebonds.