Abstract:
A device and method for aligning a fiber optic bundle with an array waveguide uses pins that partially extend into both the fiber optic bundle and the array waveguide to achieve course alignment. In one embodiment, the pins are inserted into holes formed by V-grooves in the fiber optic bundle. Finely aligning the fiber optic bundle and the array waveguide may be done by manual adjustment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is related to: 
   U.S. patent application Ser. No. 09/738,799, filed Dec. 15, 2000, entitled “Alignment of Fiber Optic Bundle to Array Waveguide Using an Epoxy,” and 
   U.S. Pat. No. 6,628.865, filed Dec. 15, 2000 and issued Sep. 30, 2003, entitled “Alignment of Optical Fibers to an Etched Array Waveguids.” 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The described invention relates to the field of optics. In particular, the invention relates to coupling a fiber optic bundle to a planar photonic structure, such as an array waveguide (“AWG”). 
   2. Description of Related Art 
   Fiber optic bundles and AWGs are both used for propagating light. A fiber optic bundle has multiple optical fibers for propagating light, and an AWG has multiple channels for propagating light within. Coupling a fiber optic bundle to an AWG, however, is not easy. Manual alignment requires detecting and maximizing light connectivity between the fiber optic bundle and the AWG. Once a good connection is obtained, permanently fixing the alignment is required. 
     FIG. 1A  shows a prior art fiber optic bundle  10 . The fiber optic bundle  10  comprises multiple optical fibers  12  sandwiched between two retainers  16  and  18 . The retainers are substrates made of silicon, for example, that are appropriately masked with a suitable etch mask. Thereafter, symmetrically spaced unmasked areas of the substrate are exposed to a chosen anisotropic etchant, such as hot KOH or ethylenediamine. This etchant preferentially attacks a chosen (100) crystallographic plane of the silicon substrate and preferentially etches in a vertical direction until V-shaped grooves (“V-grooves”) are attained. Upon completion of these V-shaped grooves, optical fibers are placed in the grooves and come to rest in alignment with the center of the V-grooves between the retainers  16  and  18 . 
     FIG. 1B  shows a prior art single retainer without the optical fibers. The two retainers  16  and  18  sandwich the optical fibers together as a termination block for the fiber optic bundle. The termination block maintains the spacing between the optical fibers and allows for easily handling the fiber optic bundle. The ends of the optical fibers  22  are typically polished after being set in the termination block. 
     FIG. 2  shows a prior art example of an AWG. The AWG comprises multiple channels  30  running through the AWG. The AWG may comprise a glass, silicon, oxide or polymer substrate. The channels are made of materials having a slightly higher index of refraction than the rest of the AWG. AWGs and fiber optic bundles may be made with various numbers of channels. 
     FIG. 3  shows a side view of a fiber optic bundle being aligned to an AWG  42 . The optical fibers of the fiber optic bundle and the channels of the AWG have identical spacings and number. A dotted line  45  shows the channels in the AWG. An epoxy  50  is used to hold the termination block  40  of the fiber optic bundle to the AWG  42 , but alignment must be maintained. It is difficult to achieve alignment, i.e., photonically couple the optical fibers to the AWG channels, and then to epoxy without losing alignment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows a prior art fiber optic bundle 
       FIG. 1B  shows a prior art single retainer without the optical fibers. 
       FIG. 2  shows a prior art example of an AWG. 
       FIG. 3  shows a side view of a fiber optic bundle being aligned to an AWG. 
       FIG. 4  shows a first embodiment for aligning a fiber optic bundle to an AWG. 
       FIG. 5A  shows a second embodiment for aligning a fiber optic bundle wit h an AWG using pins (or dowels/rods). 
       FIG. 5B  shows an AWG corresponding to the fiber optic bundle of FIG.  5 A. 
       FIG. 6  shows a cross section of a fiber optic block and an AWG joined with pins  200  to perform a coarse alignment. 
       FIG. 7  shows a third embodiment for aligning optical fibers to an AWG. 
       FIG. 8  shows a side view of the retainer placed over the etched AWG having channels within. 
       FIG. 9  shows another embodiment in which the AWG is etched a predetermined depth below the AWG surface before etching the V-grooves. 
   

   DETAILED DESCRIPTION 
   There are several ways of improving alignment between a fiber optic bundle and an AWG. In some cases, quick coarse alignment is followed up with finely aligning the fiber optic bundle and AWG afterwards. 
     FIG. 4  shows a first embodiment for aligning a fiber optic bundle to an AWG. In this embodiment, the AWG  142  is mounted to a base  110 . The fiber optic bundle&#39;s termination head  140  is also mounted to the base  110  via a high viscosity epoxy  120 . In one embodiment, a spacer  122  attached to the base  110  may be used to reduce the thickness of epoxy  120  employed. 
   Typical epoxies such as that used in prior art  FIG. 3  shrink when cured. This post-bond shrinkage is not a problem in the prior art  FIG. 3  since it pulls the termination head  40  closer to the AWG  42 . However, if the epoxy of  FIG. 4  shrinks, alignment of the fiber optic bundle with the AWG will suffer, as the termination block  140  is pulled toward the base  110 . 
   An epoxy having a silicate content of over 70% by volume has been found to reduce shrinkage. Additionally, the high silicate content makes the epoxy very viscous allowing for manual alignment being maintained after being achieved. Thus, alignment of the termination head  140  and the AWG  142  can be achieved without significant post-bond shrinkage as the epoxy is cured by heat or other methods. 
   Raising the silicate content of the epoxy to up to 90% by volume reduces the post-bond shrinkage even more. However, as the silicate content is increased, the shear strength of the bond is reduced, so a balancing between post-bond shrinkage and shear strength should be performed. 
   The alignment method using the high viscosity epoxy described provides a robust bond area for achieving and maintaining alignment between the fiber optic bundle and the AWG. Additionally, a gel having a refractive index matching the optical fibers and the AWG channels may be dispensed between the fiber optic bundle and the AWG. This helps to prevent light from scattering at an air gap between the fiber optic bundle and the AWG. 
     FIG. 5A  shows a second embodiment for aligning a fiber optic bundle with an AWG using pins (or dowels/rods). In one embodiment, the termination head is made with optical fibers filling all of the grooves except for a groove at each end. The ends of the optical fibers are then polished, as usual. Pins  200  can then be inserted into the open grooves in the termination block of the fiber optic bundle. 
     FIG. 5B  shows an AWG corresponding to the fiber optic bundle of FIG.  5 A. The AWG has recesses  202 . In one embodiment, the AWG recesses are initially filled with materials different from the rest of the AWG. This allows selective etching to form the recesses  202 . However, other methods of making the recesses are possible. 
   The pins  200  of the fiber optic bundle fit snugly into the recesses  202  of the AWG to provide coarse alignment. Additional manual adjustment to more finely align the fiber optic bundle to the AWG may be performed. 
     FIG. 6  shows a cross section of a fiber optic block and an AWG joined with pins  200  to perform a coarse alignment. A gel can be dispensed between the fiber optic bundle and the AWG to provide better photonic coupling, and an epoxy is used to permanently fix the alignment. 
     FIG. 7  shows a third embodiment for aligning optical fibers to an AWG. In this embodiment only one retainer  300  is used in the termination block of the fiber optic bundle, and the optical fibers are attached into the one retainer  300 . V-grooves are etched into the AWG&#39;s substrate in the same way that the retainer was etched, however the V-grooves on the AWG extend only a predetermined distance across the AWG from an edge of the AWG. 
   The one retainer  300  is placed over the V-grooves on the AWG  320  to sandwich the optical fibers between the retainer  300  and the AWG  320 . The optical fibers come to rest within the V-grooves of the AWG  320 . The ends of the optical fibers  322  are butted up against the ends of the AWG&#39;s V-grooves  324 . 
   The interlocking compatibility between the retainer  300  and the V-grooves of the AWG  320  provide for quick coarse alignment of the optical fibers with the channels  350  of the AWG. Manual adjustment may then be performed to more finely align the optical fibers with the AWG. 
     FIG. 8  shows a side view of the retainer  300  placed over the etched AWG  320  having channels  350  within.  FIG. 9  shows another embodiment in which the AWG  325  is etched a predetermined depth below the AWG surface before etching the V-grooves. This allows a better coupling to channels  355  that are deeper below the AWG surface. 
   In one embodiment, over-etching the AWG provides for a better ability to manually align the optical fibers and the AWG afterwards. As previously described, gel or epoxy having a refractive index matching the optical fibers and the channels of the AWG can be dispensed between the retainer and AWG. 
   Thus, a device and method of aligning optical fibers in a fiber optic bundle to a waveguide is disclosed. However, the specific embodiments and methods described herein are merely illustrative. Numerous modifications in form and detail may be made without departing from the scope of the invention as claimed below. Rather, the invention is limited only by the scope of the appended claims.