Abstract:
Methods and arrangements for providing compact optical paths between an optical connector, such as an MTRJ connector, and optoelectronic chips contained in TO cans. The methods and arrangements employed for creating the optical paths include the use of fibers that are not prone to fail from bending stresses, or waveguides.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to a method for providing an optical path in the optical subassemblies used in optoelectronic transceivers. 
   BACKGROUND OF THE INVENTION 
   The optoelectronics industry has made a recent decision to reduce the size of the optical connectors previously used, e.g. the SC connector, to roughly half the size, with corresponding reductions in the size of the transceiver module which mate to the connector. Such a transceiver would mate to an optical connector carrying two fibers, one which carried the outgoing signals from the transmitter, and the other which carried the incoming signals to the receiver. The connectors and transceivers made in accordance with this decision are called “Small Form Factor” (SFF) components. The space allotted to the optoelectronic and supporting chips, circuits and electrical connections, along with the mating optoelectronic interconnection, is typically only about 13 mm×49.5 mm in the plane, and 9.8 mm high. This requirement puts stringent constraints on the design of the transceiver, particularly because the components packaged within this small space must meet the operating specifications while low fabrication costs are maintained. 
   Two major complicating factors in the development of transceivers are the facts that:
         no one optical connector has been chosen as the SFF standard. Major contenders for such connectors include the LC, the MTRJ, the VF45, and the SCDC. For a discussion of the various connectors used with SFF (see J. M. Trewhella, “Performance Comparison of Small Form Factor Fiber Optic Connectors”, Proceedings of 49th ECTC, pp. 398–407, 1999) and   most of the optoelectronic chips (laser and receiver) used in the industry are hermetically sealed in TO cans. The use of these cans presents extra packaging challenges because they are large and awkward on the SFF scale, and are typically made with very wide tolerances.       

   The fabrication of a transceiver which mates to the LC connector has been described previously (W. Hogan, “A Novel Low-Cost Small-Form-Factor Transceiver Module”, Proc. 50 th  ECTC, pp. 725–732, 2000). However, the packaging problem associated with the LC connector is simpler with than that of any other connector, primarily because of the LC&#39;s relatively wide fiber-to-fiber spacing (transmitter fiber to receiver fiber) in the connector, i.e. 6.25 mm, which permits the TO cans to be positioned side by side. No such solution is viable for the MTRJ connector, because that connector has a fiber—fiber spacing of only 750 micrometer, while the width of a TO46 can is about 5.4 mm. Clearly, an optical space transform must be made for the MTRJ connector in order to change the 750 micrometer lateral spacing into a spacing compatible with the use of TO cans. 
   In the past, several optoelectronic devices have been built with waveguides constructed by embedding standard optical fibers in the devices, as well as with non-fiber waveguides specially constructed on the device itself. Either technique for waveguide construction may be used for fabricating a transceiver which mates to an MTRJ connector. In particular, a silicon substrate positioned in the transceiver may be used as the carrier of an “optical coupler” (OC) which serves to interconnect the fibers in a cable terminated by an MTRJ connector to the optoelectronic chips (laser and photodetector). This optical coupler supports the waveguides which are used for the optical interconnect. 
   In particular, V-shaped grooves are precision-etched in silicon to carry two optical fibers using known silicon-optical bench (SiOB) techniques, one each for the transmitter and receiver functions, while larger grooves are simultaneously etched to carry the 750 micrometer diameter pins needed for interfacing to the MTRJ connector. The fibers are anchored to the Si carrier by means of epoxy; the ends of the assembly are then polished to provide the finish required by the fiber faces. 
   However, because of severe space limitations, the required space transform must be confined to a very short longitudinal distance, which consequently requires that the fibers be subjected to a very small bend radius. Small fiber bend radii are usually unacceptable, however, since extensive studies have shown fiber failure under conditions of small bend radius because of stress-induced cracking (see V. Annovazzi-Lodi et al, “Statistical Analysis of Fiber Failures under Bending-Stress Fatigue”, J. Lightwave Tech., vol. 15, pp. 288–293, 1997). 
   In view of the foregoing, a need has been recognized, inter alia, in connection with overcoming the shortcomings and disadvantages encountered in connection with the conventional arrangements discussed above. 
   SUMMARY OF THE INVENTION 
   In accordance with at least one presently preferred embodiment of the present invention, novel features based on silicon-optical bench (SiOB) technology are contemplated in order to achieve reliable, low-cost optical couplers suitable for use with an MTRJ connector. Particularly, whereas the anchoring of fibers in a SiOB is known, the present invention broadly contemplates the imposition of bends in each fiber, and in a manner that avoids the risk of fiber failure. Also contemplated herein is the use of waveguides instead of fibers. 
   At least one embodiment of the present invention provides an apparatus for guiding at least one optical path for an optoelectronic transceiver, the apparatus comprising: an input interface; an output interface; and at least one bent element being disposed between the input and output interfaces; the at least one bent element being adapted to provide at least one optical path; the at least one bent element being adapted to avoid premature mechanical failure. 
   Further, at least one embodiment of the present invention provides a method of forming apparatus for guiding at least one optical path for an optoelectronic transceiver, the method comprising the steps of: providing an input interface; providing an output interface; disposing at least one bent element between the input and output interfaces; adapting the at least one bent element to provide at least one optical path; and adapting the at least one bent element to avoid premature mechanical failure. 
   For a better understanding of the present invention, together with other and further features and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, and the scope of the invention will be pointed out in the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1   a  and  1   b  illustrate side and frontal views, respectively, of an optical coupler; 
       FIG. 2  illustrates a waveguide formed via a wet HF etch; and 
       FIG. 3  illustrates a comparison of an improved waveguide profile with both an ideal profile and that contemplated by  FIG. 2 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Some of the terminology utilized throughout this disclosure, as well as some pertinent background concepts, are described in detail in commonly assigned and copending U.S. patent application Ser. No. 09/348,955, filed Jul. 7, 1999, entitled “Small Form Factor Optoelectronic Transceivers”, and which is hereby fully incorporated by reference as if set forth in its entirety herein. 
     FIGS. 1   a  and  1   b  illustrate an optical coupler ( 90 ) and convey two different concepts according to an embodiment of the present invention. Particularly, as discussed below, the components indicated at  106 ,  108  may be considered to be either fibers or waveguides. 
   As shown in  FIGS. 1   a  and  1   b , V-shaped grooves  92 ,  94 , are preferably precision etched in a silicon base  95  to carry two fibers  106 ,  108 . The etching is preferably undertaken using known SiOB techniques. One fiber each is preferably provided for the transmitter and receiver functions, while larger grooves  96 ,  98  are simultaneously etched in the base  95  and interfacing cover  99  to carry the pins  110 ,  112  (such as typically used for interfacing to the MTRJ connector). 
   Preferably, the precision V grooves ( 92 ,  94 ,  96 ,  98 ) are etched only in narrow plateaus  102 ,  104  in the base of the OC, so that each fiber is anchored only at each end, and is free to take an S-shaped curve between the anchor points. To achieve this shape, the following procedure can be employed:
         1. Using an appropriately configured jig, the fibers  106 ,  108 , with the buffer fully intact, are aligned with their anchoring grooves in the base of the OC.   2. The (preferably) Si cover  99  is clamped to the base  95 , thereby temporarily but firmly holding the fibers  106 ,  108  in place.   3. The assembly is baked in an oven in air at 1000–1100° C. for about 15 min.   4. The protruding portions of the fibers  106 ,  108  are cut if necessary, then the faces of the OC  90  are polished.   5. The metal MTRJ guide pins  110 ,  112  are inserted and epoxied in place. If desired, the fibers  106 ,  108  may also be potted in epoxy.       

   With the exception of the last step, the above operations may preferably be performed in batch mode in order to reduce the cost. 
   A major innovation is represented by step 3 above. Baking accomplishes several goals:
                The stress in the fiber is relieved, so the short bend radii (e.g., 2.5 mm) are achieved under strain-free conditions.              The buffer layer is burned off, leaving clean, bare fiber.              The fibers are anchored to the base and cover by the well known SiO2 sintering technique used for wafer—wafer bonding.              The cover and base of the OC are simultaneously sintered together.   

   It is possible to position the two TO cans containing the laser and receiver chips side by side, adjacent fiber anchor  104 , if an optical coupler such as that described above is used. Of course, a lens should be positioned between the end of each fiber and its corresponding TO can in order to transfer the light efficiently between the optoelectronic chip in the can and the fiber end. 
   Another embodiment of the present invention, also conveyed by the illustration in  FIG. 1 , is also contemplated, particularly, involving the use of waveguides as components  104 ,  106  instead of fibers. 
   Again, a silicon substrate is preferably used with anisotropically etched grooves for the alignment pins for the MTRJ connector. However, the fibers discussed previously are replaced by multimode waveguides ( 106 ,  108 ) having the same S shape shown in  FIG. 1 , but which are processed in parallel over the entire wafer (e.g., a 200 mm diameter silicon wafer). If waveguides are used, very small bending radii should be attainable (less than the 2.5 mm previously considered for fibers) so that the optical coupler could be shortened, thereby conserving space. 
   It can be recognized that the difficulty in using a waveguide-based optical coupler lies in finding a reliable, inexpensive fabrication technique which will give low-loss waveguides with reproducible properties, and which will ensure that the waveguides are well aligned to the alignment pins. Another recognizable problem is in finding a path for development of a waveguide-based optical coupler in a short time, which means that technology which is easily accessible must be used. 
   Various approaches have been used in the past to fabricate multimode waveguides, but these approaches have been found to have drawbacks:
         1. Polymer waveguides have been delineated by standard semiconductor processing techniques. See, for example, the polyimide waveguides made by L. Robitaille et al, “Design and fabrication of low-loss polymer waveguide components for on-chip optical interconnection”, IEEE Phot. Tech. Lett., vol. 8 pp. 1657–1649, 1996. However, with this technique, it is difficult to fabricate good low-loss waveguides which are more than about 10 micrometer thick and hence suitable for multimode, and it is usually difficult to obtain such waveguides with good sidewall profiles.   2. Molded polymer waveguides have been contemplated (L. J. Norton et al, “Optobus I: A production parallel fiber optical interconnect”, Proc. 47th ECTC, pp. 204–209, 1997). The precision molding contemplated therein is very demanding and, it is recognized, would take considerable development effort.   3. Silica waveguides have been fabricated by flame hydrolysis with subsequent reactive-ion etching (RIE) for patterning. However, it has been recognized that multimode-waveguide fabrication is so difficult and expensive that although multimode waveguides have been fabricated in the past, it has been found that such fabrication is generally not economical.   4. A new class of materials known as inorganic-organic photopolymers (ORMOCERs) has recently been used to make waveguides, both single and multimode. M. E. Robertson et al, “O/e-MCM Packaging with new, patternable dielectric and optical materials”, Proc. 48th ECTC, pp. 1413–1421, 1998. Fairly vertical walls are achieved because of the photosensitivity of ORMOCERs, and loss values at 850 nm of 0.2 db/cm have been claimed for multimode waveguides. In addition, an MT type connector has been developed for use with such waveguides. This technology may have great promise, however, it is still in the development stage and at this point its practicality is far from assured.       

   In contrast, in accordance with at least one presently preferred embodiment of the present invention, a waveguide is preferably constructed of a glass core surrounded by polymer cladding. This waveguide structure is fabricated by very straightforward, standard semiconductor-processing techniques. 
   A key feature of this method is the fabrication of the glass core not by building up the glass, e.g., by flame hydrolysis, but by incorporating a monolithic piece of thin glass into the OC. Such glass has recently been developed and marketed in response to demands by the LCD technology by Schott. Schott D263T glass can be purchased in sheets of 50 micrometer thickness, with standard lateral dimensions of 280 mm×415 mm, has excellent transmission and a refractive index of about 1.53. If thinner waveguides are desired, the glass could be ground and polished after step 3 of the procedure outlined below. While such thin glass can of course be shattered if dropped, its handling is usually surprisingly easy. 
   Some presently contemplated process steps for fabrication of this version of the optical coupler are:
         1. The V grooves  96 ,  98  ( FIG. 1   a ) for the alignment pins of the MTRJ connector are first etched into the silicon wafer, assumed to be 200 mm in diameter. This step is preferably identical to that previously described for the embedded-fiber OC, except that fiducial marks positioned near the wafer edge are etched simultaneously with the V grooves; these fiducial marks are used for alignment in subsequent masking operations.   2. The thin glass piece is spin coated with a transparent, low-index adhesive to a thickness of 2–5 micrometer, then positioned so the adhesive is in intimate contact with the silicon wafer. This procedure is best carried out in vacuum to avoid entrapment of air.   3. The adhesive is cured, thereby firmly bonding the glass to the silicon. The adhesive layer not only performs the bonding function, but also serves as the cladding on the underside (silicon side) of the waveguide when the coupler is completed.   4. An etch-masking layer is deposited on the glass in such a fashion that the fiducial marks in the silicon are still visible.   5. A photoresist layer is spin coated over the etch-masking layer. A photomask is aligned to the fiducial marks in the silicon wafer, and the photoresist is exposed and developed, then the etch-masking layer is delineated so that a replica of the desired waveguide geometry is produced (see  FIG. 1 ).   6. The waveguide is etched out of the glass sheet with the aid of the patterned etch-masking layer.   7. All photoresist and etch-masking material is removed.   8. Low-refractive-index UV epoxy is flowed over the entire surface in order to form the side and upper cladding layer.   9. A photomask is placed over the wafer for exposure in a standard mask aligner. The mask is open only in the areas immediately surrounding the delineated waveguides. The UV epoxy is exposed, then the uncured epoxy in the regions far from the waveguides is removed with solvent.   10. The optical couplers are diced from the wafer using a standard dicing saw. (Each wafer should yield more than 500 optical couplers). The individual couplers are stacked together for simultaneous polishing of the waveguide faces.       

   In connection with the above procedure, small-bend radii and hence short couplers with low bending loss are possible because the difference in refractive index between the core and cladding can be made large. Furthermore, the difficult problem of delineating both the waveguides  106 ,  108  and the V grooves  96 ,  98  for the pins is solved in a straightforward manner. If a conventional process is used to delineate the waveguides which requires photoresist to be applied after the V grooves are etched, severe nonuniformities in photoresist thickness would be encountered due to streaking effects. On the other hand, if, in a conventional process, the waveguides are delineated before etching the V grooves, the waveguides and their undercladding could be damaged by exposure to the caustic action of the silicon etchant. 
   The adhesive used in step 2 above could be a thermally cured epoxy, a polyimide, or perhaps the same UV epoxy used in step 8 above. One candidate for the latter might be Luxtrak 4039 from Ablestik, which has a refractive index of 1.455, or alternatively OG134 from Epotek, which has a refractive index of 1.416. The UV epoxy could be cured in a mask aligner in both steps. 
   An important step in the process described above is the etching of the waveguide, since vertical walls are desired. The preferred procedure would be RIE which is known to give vertical walls. While RIE is notoriously slow for glass, if C 2 F 6  is used as the reactive gas in this procedure etch rates of 1–2 micrometer/min can be achieved. In this case, photoresist itself could be a good candidate for an etch mask without the use of a separate etch-stop layer, but then a thick photoresist layer would be required because the etch mask would also be eroded away by the RIE. Such a photoresist layer can be made from Dupont&#39;s Riston. 
   On the other hand, if for some reason the RIE process is found to be too expensive because of excessive RIE times, the RIE could be combined with a previous isotropic wet etch in warm HF to give fairly straight walls (but not as straight as a pure RIE process). In this case the initial HF etch will undercut the wet-etch-stop and form an undesirable concave waveguide boundary, as illustrated in  FIG. 2 . (The photoresist is indicated at  150  while the etch profile is indicated, in dotted lines, at  152 .) 
   However, if the waveguide is also furnished with an RIE etch-masking layer, and the wet etching is followed by an RIE procedure which removes, for instance, only 10 micrometer vertical depth of the excess “tails” of the glass waveguide, the profile is improved as shown in  FIG. 3 . Particularly,  FIG. 3  shows the improved profile at  154 , as contrasted with an “ideal” profile ( 156 ) and the undesirably concave profile ( 152 ) from  FIG. 2 . Thus, the use of both wet etch and RIE yields a “compromise” waveguide profile ( 154 ) at a reasonable cost. 
   It is to be understood that, while coupling to an MTRJ connector is emphasized in the present disclosure, various concepts discussed herein may also be applied to other connector types as well. 
   If not otherwise stated herein, it is to be assumed that all patents, patent applications, patent publications and other publications (including web-based publications) mentioned and cited herein are hereby fully incorporated by reference herein as if set forth in their entirety herein. 
   Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention.