Abstract:
An optical fiber is interfaced with an optical device formed on a substrate. The substrate includes a groove under and behind an interface between the optical fiber and the optical device. Provision of such a groove allows the substrate to be used for alignment and support of the optical fiber, while reducing fusion loss and improving durability of the interface. Steps for facilitating alignment may be provided in the substrate. Solder may be used to further improve durability of the interfaced structure.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a method of fusion pigtailing an optical fiber to an integrated optical device and structures formed thereby. 
     The term pigtailing generically refers to interfacing an optical fiber with an optical device, e.g., a light source, a detector, a waveguide, etc. Current techniques for securing this interface include bonding, welding, and fusing the optical fiber and the optical device. 
     In current fusion pigtailing processes, a laser, e.g., a CO 2  laser, is used as a heat source to fuse an optical fiber to a device. This process is sufficient for a small number of ports on a substrate. However, the mechanical strength of the fused joints decreases with increasing chip size. 
     This decrease in mechanical strength is due to the formation of imperfectly melted joints. The imperfect melting arises when a substrate on which the device is formed has a low thermal conductivity. This low thermal conductivity results in poor diffusion of heat in the depth direction of the substrate. This poor diffusion leads to a large temperature difference between the clad surface layer and the lower part of the core of the fused joint. The mechanical strength may be improved by increasing the power of the laser used for fusing the fiber and the device. 
     However, since the fiber and the device are being fused together, rather than using another material to secure the interface, power should be kept to the minimal level required for this fusion process. An increase in power also results in an increase in optical loss. This increase in optical loss arises from the higher temperature associated with an increase in power, which may deform the shape of the fiber and/or the device. 
     Thus, for current fusing methods, if the laser energy is high enough to provide strong fusion, fusion loss is increased. However, if the laser energy is low enough to avoid fusion loss, strong fusion cannot be achieved. 
     Attempts to create strong fusion without increasing the energy level include providing a heater under the waveguide for preheating or reducing the thickness of the substrate having the waveguide at the interface. However, current solutions involve an interface at which no portion of the substrate is below the fiber. This requires the use of additional elements, such as a fiber carrier, which then must be aligned with and attached to the substrate. Usually, the use of a fiber carrier will also involve using a reinforcing plate. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is therefore directed to a method of fusion pigtailing an optical fiber and an optical device, a structure to be fused to an optical fiber, and the structure formed thereby, which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art. 
     These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, aspects and advantages will be described with reference to the drawings, in which 
     FIGS. 1A-1D illustrate the fusion pigtailing process in accordance with the present invention. FIGS. 1C and 1D also illustrate resultant structures, before and after the optical fiber is interfaced, respectively, in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The process of fusion pigtailing an optical fiber in accordance with the present invention is shown in FIGS. 1A-1D. In particular, FIG. 1A shows a device, here a waveguide, on a substrate before processing in accordance with the present invention. FIGS. 1B and 1C show the modifications made to the structure in FIG.  1 A. FIG. 1C is the resultant structure for receiving an optical fiber. FIG. 1D shows the resultant structure with an optical fiber mounted thereon. 
     FIG. 1A illustrates a substrate  10 , e.g. a silica chip, with an optical device, e.g., a waveguide  12 , formed thereon. An overclad layer  14  is preferably formed on the waveguide  12 . Steps are then formed in the substrate  10  by dicing the structure in FIG. 1A to create the structure of FIG.  1 B. Alternatively, the steps could be formed using other techniques, e.g., ultrasonic machining. As shown in FIG. 1B, these steps are constructed from a first indentation  16 , a second indentation  18 , and a third indentation  20 . 
     All of the indentations formed in the substrate  10  are lower than the waveguide  12 . As shown in the example of FIG. 1B, the first indentation  16  is the deepest of the three, the second indentation  18  is the shallowest of the three, with the third indentation  20  having a depth between the depths of the other two indentations. 
     The first and second indentations  16 ,  18  together form an alignment structure for the fiber to be mounted thereon, with the relative difference in the depths of the first and second indentations being determined in accordance with the thickness of the housing of an optical fiber to be mounted thereon. In other words, the indentations  16 ,  18  serve to align an end face of the optical fiber with the waveguide  12 , as shown in FIG.  1 D and discussed in connection therewith. 
     The third indentation  20  is formed to allow creation of a groove  22  as shown in FIG.  1 C. Thus, the depth of the third indentation  20  is determined in accordance with a desired thickness of a portion  11  of the substrate  10  over the groove  22  and a desired depth of the groove  22  under this portion  11 . The groove  22  may be formed, for example, with a wire-saw or with ultrasonic machining. 
     The desired thickness of the portion  11  is determined in accordance with the thermal conductivity of the substrate material. The thickness of the portion  11  is preferably small enough to allow substantially uniform heating over the portion  11  at the fusing temperature. If the portion  11  is too thick, the same problems will arise as if the groove was not provided at all. If the portion  11  is too thin, the temperature on the portion  11  may become too high, and result in damage to the waveguide. 
     The other physical parameters of the groove also need to be considered in order to achieve the desired uniform heating along the joint. The depth of the groove  22  should be great enough to allow sufficient insulation of the portion  11  such that the desired substantially uniform heating of the portion  11  may be realized. The length of the groove  22  also influences the temperature effects of the groove  22 . The optimum length of the groove  22  depends on the thermal characteristics of the substrate  10 , the thickness of the portion  11 , and the required strength of the substrate. 
     For example, when the substrate  10  is silica, the thickness of the portion  11  is preferably between 85 microns and 500 microns, the depth of the groove is preferably between 100 and 200 microns, and the length of the groove is preferably up to 400 microns. More specifically for example, the portion  11  having a thickness of 140 microns, a depth of 120 microns and a length of 300 microns is sufficient to reduce the heat capacity difference between a silica substrate and the optical fiber to be mounted thereon, while not weakening the silica substrate. 
     Thus, FIG. 1C illustrates the structure including the waveguide  12  to which an optical fiber is to be mounted. The mounting of the optical fiber may be performed using laser fusion just at the fusion temperature, as currently performed. By reducing the thickness of the area to be fused with the optical fiber, the difference in temperature due to the bad diffusion of heat in the depth direction is reduced, thereby improving the joint. 
     FIG. 1D illustrates the pigtailed structure in which an optical fiber  30  in a housing  32  is secured to the waveguide  12  at an end face  36  of the optical fiber  30 . As shown in FIG. 1D, an adhesive  40 , such as a polymer or mineral glue, preferably is provided to insure the alignment of the fiber is maintained. This adhesive  40  preferably is provided at a location removed from the fused joint and over at least a portion of both the first and second indentations  16 ,  18  provided for alignment. 
     In accordance with the present invention, by providing a horizontal groove in a substrate below a device to be fused with an optical fiber, fused joints having an improved mechanical strength, e.g., greater than 5N, and low optical loss, e.g., less than 0.2 dB/interface, on a large chip size, e.g., greater than 3 mm, may be realized. 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the present invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility without undue experimentation. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.