Abstract:
A method and structure for the mechanical and optical coupling of a plurality of individual planar optical devices ( 10, 20 ) to one another includes aligning the devices, bonding ( 35, 37, 38 ) the substrates ( 12, 22 ) of the planar devices ( 10, 20 ) to one another and subsequently fusing ( 40, 42, 44, 46 ) aligned optical waveguides ( 11, 29 ) utilizing a heat fusion technique. The substrates ( 12, 22 ) are bonded utilizing either heat fusion for a silica substrate or by a bonding adhesive for a silicon substrate. Once the aligned substrates ( 12, 22 ) have been mechanically bonded to one another, the optical pathways ( 11, 29 ) are coupled in one embodiment by heat fusion, preferably a focused CO 2  laser ( 30 ) which completes the mechanical and optical interconnection of multiple devices. It is possible with such alignment, bonding, and fusion processes to couple numerous planar optical devices ( 10, 20, 50 ) in an array which can subsequently be packaged for use in relatively complex optical networks. The resultant devices display a high level of reliability, are relatively inexpensive to manufacture, and display excellent operating characteristics.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon European Application Ser. No. 00401937.8, filed on Jul. 6, 2001, from which the benefit of priority is hereby claimed, and the full content which is incorporated herein by reference as though fully set forth.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method of manufacturing multiple complex planar optical devices and the resultant devices.  
           [0004]    2. Technical Background  
           [0005]    Optical networks used in the communications industry require many complex optical components, examples of which include N X M switches, gain-flattening filters, variable attenuators, add-drop multiplexers, splitters, and other components. An efficient manner in which to design and manufacture such components is to use planar optical devices, or chips, inasmuch as many optical functions can be implemented on the same substrate. Presently, integrating a large number of optical functions into a planar optical device is limited by the relatively small size of a silica substrate due to fabrication techniques such as chemical vapor deposition (CVD).  
           [0006]    In order to provide more complex optical devices, individual chips containing a number of optical functions have been mechanically joined to one another. One approach for connecting chips utiles a suitable mounting platform and optical fibers for coupling waveguides of individual chips to corresponding waveguides of an adjacent chip. The optical path adhesive employed to couple the optical fibers to the waveguides must exhibit a high degree of light transmittance to reduce propagation losses at the bond joints and must have a refractive index which closely matches that of the optical components being joined. In the past, optical adhesives, such as NOA61, available from Norland Products, Inc, New Brunswick, N.J. have been employed for coupling multiple optical function chips using optical fibers. There exists a need for joining different optical functioning devices to form complex networks in which the optical fiber interconnection between the chips and the mounting of chips to substrates, involving multiple complex steps, both of which result in relatively costly optical systems, are eliminated.  
         SUMMARY OF THE INVENTION  
         [0007]    The method and resultant structure of the present invention provides for the coupling of a plurality of individual planar optical components to one another by aligning and mechanically coupling the substrates of planar chips carrying optical waveguides thereon to one another and subsequently coupling the aligned optical waveguides, such as by heat fusion. Depending upon the substrate material, the substrates can, once precisely aligned to one another to align mating waveguides utilizing an active alignment process, be coupled by bonding utilizing either heat fusion in the case of a silica substrate; or by a bonding adhesive in the case of a silicon substrate. Once the aligned substrates have been mechanically coupled to one another, the optical waveguides are coupled utilizing, for example, heat fusion and preferably a focused CO 2  laser which completes the mechanical and optical interconnection of multiple devices. It is possible with such alignment and coupling by bonding and/or fusion to couple numerous chips in an array which can subsequently be packaged for use in relatively complex optical networks. The resultant devices display a high level of reliability, are relatively inexpensive to manufacture, and display excellent operating characteristics.  
           [0008]    Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.  
           [0009]    It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a perspective schematic view showing the alignment process for a pair of planar optical devices;  
         [0011]    [0011]FIG. 2 is a perspective schematic view of a first step in the process of coupling the planar optical devices shown in FIG. 1; and  
         [0012]    [0012]FIG. 3 is a perspective schematic view of a second step in the coupling of the planar optical devices shown in FIGS. 1 and 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]    Referring initially to FIG. 1, there is shown a first planar optical device or component  10 , such as a splitter, which is to be coupled to a second planar optical device  20 , such as a switch. Each of the planar optical devices  10  and  20  can be manufactured as chips utilizing conventional manufacturing techniques, such as CVD or the like. The devices  10  and  20  are typically manufactured on a silica (SiO 2 ) substrate ( 12  and  22  respectively), although as described below, they can also be manufactured on a silicon (Si) substrate, which alters the manufacturing process as described in detail below.  
         [0014]    The devices  10  and  20  illustrated in FIG. 1 are shown inverted with the substrates  12 ,  22  on the top. Each of the substrates are silica having a thickness of approximately 500μ. The devices typically includes a 10μ buffer or undercladding layer adjacent the substrate, a core patterned to include optical waveguides (shown as patterns  11 ,  29  in FIG. 1) of silica with a dopant such as boron, germanium, or the like to provide an index of refraction at 1550 nm of about 1.44 to about 1.55. An overcladding layer of silica having a different but conventional dopant mixture providing an index of refraction of approximately 1% greater than that of the undercladding and of, for example, from about 1.5 to about 1.6 covers the waveguides  11 ,  29  and also has a thickness of approximately 10μ. The substrate and layers of undercladding, overcladding, and the waveguides are patterned utilizing conventional photolithographic and masking processes employed for manufacturing planar optical components, such as optical devices  10  and  20  in FIG. 1. Typically multiple devices are fabricated on a single wafer during mass production, which wafers are subsequently diced into the individual chips containing devices, such as devices  10  and  20 . The edges of the chips with the exposed waveguides have surfaces which typically are flat within 1μ from edge to edge.  
         [0015]    In order to couple the optical devices  10  and  20  employing the method of the present invention, one of the chips, such as device  10 , is mounted in a fixed clamp shown in block form in FIG. 1 as element  16 , while the other device, such as switch  20 , is mounted in an XYZ micro-adjustment stage shown as element  26  in FIG. 1. The fixed clamp and adjustable stage can be conventional devices widely used in the manufacture of optical devices and are suitably mounted to a work station. The clamp and stage hold the end  13  of device  10  in a direction facing end  23  of device  20  for alignment prior to the mechanical and optical coupling of the two devices  10  and  20 . An active alignment system is employed to assure precise alignment of mating waveguides. For such purpose, an input signal at 1550 nm is injected from a signal source  24 , such as a laser, to an input waveguide  14  of splitter  10  which divides the input signal to four outputs, as seen schematically in FIG. 1, for splitter  10 . Switch  20  has four pass-through waveguides and four transversely extending waveguides, as shown schematically in the figure, for selectively coupling signals on any of the inputs to a selected output. For the purpose of actively aligning the components  10  and  20 , a detector  28  is coupled to the outermost waveguides  21 ,  25  of switch  20  by optical fibers  27  such that the signals on each of the waveguides  21 ,  25  can be detected by detector  28 . The devices  10  and  20  are aligned for maximum signal transmission in a switching state which allows the pass through of signals to detector  28 .  
         [0016]    Initially, the devices are aligned with a gap G 1  (FIG. 1) of approximately 5μ to allow movement of device  20  with respect to device  10  while monitoring signals by detector  28  until the maximum signal transmission is detected. Device  20  can be moved in the X, Y, and Z planes, as shown in the diagram of FIG. 1, individually or collectively to provide translation and/or rotation such as in a direction indicated by arrow A of FIG. 1 of device  20  with respect to fixed device  10  until the desired alignment is achieved. Once the maximum signal transmission has been detected, clamp  26  moves device  20  in a direction along its longitudinal axis (X shown in FIG. 1) for joining the two devices  10  and  20 , such that the ends  13  and  23  are in abutment.  
         [0017]    Next, the permanent mechanical coupling of the devices is accomplished. In the preferred embodiment, the substrates  12  and  22  of devices  10  and  20  are fused in areas adjacent to but not contacting any of the waveguides  11  and  21 . In a preferred embodiment of the invention, the mechanical joint for a silica substrate is made by fusion with a CO 2  laser  30 , such as a Synrad 25 watt laser operating at 10 to 15 watts. The beam is controlled by a shutter  32  to provide pulses of radiation directed to a converging lens  34  having a focal length of about 127 mm and spaced from substrates  12  and  22  a distance to focus the elliptical laser beam to a spot  35  of approximately 100μ diameter onto the junction  33  of the substrates. As seen in FIG. 2, the mechanical junction is provided by focusing the laser beam onto the substrate layer of the devices  10  and  20  at positions  35  on one edge,  38  on an opposite edge, and  37  in the center between adjacent waveguides. The shutter  32  is a commercially available model which is controlled to pulse the laser on-time for two to three seconds for several pulses to provide fusion of the silica substrates to a depth of 100μ to 200μ. The laser beam is directed at an acute angle a of, in the preferred embodiment, of about 45° to an axis normal to the plane of the devices  10 ,  20 . In any embodiment angle α is an acute angle. Although three areas of fusion are shown for chips of about 5 mm by 35 m additional areas of fusion could be employed if necessary with larger chip sizes.  
         [0018]    The now optically aligned and mechanically coupled optical devices  10  and  20  are removed from clamps  16  and  26  and flipped over and reclamped (FIG. 3) to present the optical waveguides to the same laser beam. The laser beam is similarly controlled and aligned with respect to the combined devices  10 ,  20  (typically by the utilization of a microscope and a visible guiding laser beam) onto the abutting, aligned waveguides of devices  10  and  20 . The pitch between adjacent waveguides is approximately 350μ such that the spot width of 100μ when aligned with each waveguide, fuses the adjacent waveguides from component  10  to component  20  while maintaining isolation between adjacent waveguides. Fusion of the aligned respective waveguides at locations  40 ,  42 ,  44 , and  46  (FIG. 3) in the example illustrated is achieved using the same laser  30  and application of power therefrom through shutter  32  and lens  34  as described above with respect to FIG. 2. Since one waveguide core is fused to the other waveguide core by the heat generated by the focused laser beam, there is substantially no back reflection at the joint which might otherwise result from a mismatch of index of refraction between the waveguides. Further, by providing the fusion process shown in FIG. 3, different size waveguides can be coupled to one another with the fusion zones coupling the waveguides and blending any slight differences that may exist in the indices of refraction.  
         [0019]    By coupling adjacent planar optical devices as described with references to FIGS.  1 - 3 , the resulting complex optical component need not be hermetically sealed but only conventionally packaged for receiving input and output optical fibers. Although the example given is shown for joining two components, such as a splitter  10  and switch  20 , respectively, additional components, such as a wave division multiplexer  50  (shown in phantom lines in FIG. 3) can similarly be aligned and joined to switch  20  utilizing the processing steps shown in FIGS.  1 - 3  and the resultant component can be tested by the operation of switch  20  and the application of different frequency input signals to test the overall operation of the resultant multiple component device. Thus, the joining of components utilizing the processes illustrated in FIGS.  1 - 3  can be repeated for additional components to form an XY array of multiple components, limited only by the mechanical structure of the substrate thickness employed. Further, if desired, more than two components can be aligned and mechanically and optically coupled at the same time, although it is preferred to join two together and subsequently test the combination before additional components are optically coupled to make certain that, as multiple chip components are constructed, at each stage the resultant component passes quality control standards.  
         [0020]    In the event a silicon substrate is employed as opposed to a silica substrate, in place of the step illustrated in FIG. 2, a bonding adhesive, such as MCA-95 available from Corning Incorporated, is applied to the junction of substrates  12  and  22  of devices  10  and  20  utilizing a syringe to provide coupling of the substrates by adhesive bonding of the substrates. Subsequently, the optical waveguides are coupled by fusion utilizing the same process as described in connection with FIG. 3.  
         [0021]    As yet another alternative, the alignment and coupling of substrate processes illustrated in FIGS. 1 and 2 can be employed to mechanically join aligned complex optical components. The optical waveguides then can be coupled utilizing an optical bonding adhesive in place of fusion. An optical adhesive which may be used is, for example, UV3000 available from Nippon Telephone and Telegraph which has an index of refraction of approximately 1.49 once polymerized. The optical adhesive is applied to the facing ends of the waveguides utilizing a syringe.  
         [0022]    Thus, the system of the present invention provides both the mechanical and optical coupling of planar optical devices and accommodates different material substrates to form a complex optical device. It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.