Abstract:
A lithographic process is used to place a marking on a waveguide to indicate optical channels within the waveguide. A photonic component is positioned against the waveguide based on the markings and adjusted until aligned.

Description:
This patent application is a Divisional Application of U.S. patent application Ser. No. 09/741,642, entitled “Markings for Aligning Fiber Optic Bundle”, filed Dec. 19, 2000, now U.S. Pat. No. 6,636,671. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The described invention relates to the field of optics. In particular, the invention relates to alignment and coupling of photonic components. 
   2. Description of Related Art 
   Photonic components propagate light and include waveguides, optical fibers, amplifiers, couplers, splitters, and other devices for carrying light-based signals. A fiber optic bundle has multiple optical fibers for propagating light, and an array waveguide (AWG) has multiple channels for propagating light within. Coupling a fiber optic bundle to an AWG, for example, 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  form a termination block for the fiber optic bundle by sandwiching the optical fibers together within their V-grooves  24 . 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 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  42  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 first be achieved and then 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. 5  shows a flowchart for aligning a photonic component to a waveguide. 
   

   DETAILED DESCRIPTION 
   One of the issues with aligning a photonic component with an array waveguide is that the channels of the array waveguide are not visible. Although the channels have a higher index of refraction than the surrounding substrate, they are not readily distinguishable by the human eye. 
     FIG. 4  shows one embodiment for aligning a fiber optic bundle  140  to an array waveguide (AWG)  142 . Markings  160  are placed on a surface of the waveguide  142  indicative of the channels  145  within the waveguide  142 . The markings  160  are lithographically-defined, i.e., they are placed on the waveguide during the lithographic processing of the waveguide, and they precisely indicate the location of the channels  145 . In one embodiment, an ‘X’ marking may indicate the center of a channel directly beneath it. In another embodiment, markings may indicate the outside boundaries of an interior channel. In one embodiment, markings are placed over the outermost channels of the waveguide  142 , however, the markings can be placed over any of the channels  145 . 
   The markings may be achieved by etching into the substrate of the waveguide, by placing ink on a, e.g., passivation layer, or by any other technique that produces a marking that is visible to the human eye. 
   The fiber optic bundle may have alignment markings  172  of its own. Thus, viewed from the top it would be easy to line up the markings  160  of the waveguide to the alignment markings  172  of the fiber optic bundle  140 . Alternatively, the tops of V-grooves  170  may be used to align the fiber optic bundle  140  to the markings  160  of the waveguide  142 . 
   In another embodiment, marking  190  on the side surface of the waveguide  142  may be used to indicate a depth of the channels  145  within the waveguide  142 . As an example, a marking  190  on the side of the waveguide may be achieved by deposition of a layer of material having a different color than the rest of the substrate. This layer may be limited to the side surfaces of the waveguide  140 . 
     FIG. 5  shows a flowchart for aligning a photonic component to a waveguide. The flowchart starts at block  200  and continues at block  202 , at which the photonic component, such as a fiber optic bundle, is placed against the waveguide. At block  204 , the photonic component is aligned to the lithographically-defined markings on the waveguide. In one embodiment, the markings on the photonic component are used to help with course alignment with the markings on the waveguide. Some type of optical measuring device may be needed to help with fine alignment by optimizing the optical coupling between the photonic component and the waveguide. At block  206 , the photonic component is bonded to the waveguide. In one embodiment, an epoxy having an index of refraction that is substantially similar to the channels of the waveguide and to the optical fibers of the fiber optic bundle is used to help maintain the optical coupling between the waveguide and the photonic component. 
   Thus, a method and apparatus for aligning a photonic component with 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. For example, although a fiber optic bundle aligned to a waveguide was described, the same process can be used for aligning other photonic components, such as aligning two waveguides to each other. The invention is limited only by the scope of the appended claims.