Patent Publication Number: US-2003231850-A1

Title: Optical fiber array

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims priority from U.S. Provisional Application No. 60/365,680, filed Mar. 18, 2002. 
    
    
     
       BACKGROUND OF INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The invention relates generally to fiber-optic network systems. More specifically, the invention relates to a method and device for aligning glass fibers, such as optical fibers, lensed fibers, and rod lenses.  
       [0004] 2. Background Art  
       [0005] Fiber-optic light-wave technology has found enormous application in long-distance communication. Copper wires and coaxial cables, and even microwave relays and satellites in some cases, are being replaced by fiber-optic systems. Fiber-optic links have several advantages over their metallic-based counterparts. These advantages include lower loss, higher information-carrying capacity, lower cost per channel, and a smaller physical mass. Currently, fiber-optic links carry hundreds of terabits per second over distances greater than 1,000 km. Even though this is orders of magnitude beyond the capacity of metallic links, the demands of global communication are driving the system capacity to double every year. To meet these demands, techniques such as wavelength division multiplexing (WDM) are being used to increase the transmission capacity of the fiber-optic link.  
       [0006] In WDM systems, many optical signals at different wavelengths are combined into a single beam for transmission in a single optical fiber. At the exit of the fiber, a demultiplexer is used to separate the beam by wavelength into independent signals. In communication networks employing transmission formats such as WDM, a cross-connect is needed to selectively route individual optical signals to different destinations. An N×N cross-connect is a switch fabric that can switch a signal from any of N transmission lines to another of the N transmission lines. In optical networks, the majority of the signal routing is still performed electronically. This requires frequent optical-to-electrical and electrical-to-optical signal conversion, which slows down the network. To take full advantage of speed and bandwidth of optical signal transmission, an all optical network is required.  
       [0007] One approach to large-scale optical cross-connect is based on free-space (three-dimensional) micro-optic switching. In micro-optic switching, the optical signal from a channel is re-routed by an array of micro-electronic (MEMS) actuated mirrors or prisms to any of the other output channels and then focused back into the output fiber by an array of collimating lenses. For free-space micro-optic switching (as well as two-dimensional switching), the optical fibers need to be arrayed and aligned with the array of collimating lenses. The challenge in making these large-scale optical switches is how to efficiently align a large number of optical fibers to a large lens array. The current method for aligning fibers to a lens array involves gluing or splicing optical fibers to a substrate with an array of collimating lens. Each fiber-lens alignment can take  6  to  12  minutes, which makes the method unsuitable for large-scale optical switch having several hundred ports. In addition, there is adhesive in the optical path when the fiber is secured to the lens array by gluing, and the athermalization performance of this fiber-lens connection is unknown.  
       [0008] Instead of serially aligning fibers to a lens array, a fiber array can be constructed and mated to a lens array. The fiber array can be populated more quickly with fibers. The general approach is to make the fiber array with V-grooves etched in silicon and stacked in piles. In this approach, any associated lens array would need to match silicon in expansion to achieve athermalization. There is a certain cost and accuracy limit associated with this approach. Furthermore, there are limits for silicon V-groove technology in terms of two-dimensional pitch tolerances that essentially eliminates its application to M×N arrays (where M and N&gt;&gt;1). To improve fiber-lens alignment accuracy, the fiber array can be populated with lensed fibers. A lensed fiber is a monolithic device having an optical fiber terminated with a lens. Lensed fibers allow for improved alignment accuracy because the signal is collimated before exiting the lensed fiber, thus requiring less active fiber-lens alignment. Lastly, lensed fibers also have low insertion loss.  
       SUMMARY OF INVENTION  
       [0009] In one aspect, the invention relates to a fiber array which comprises a substrate made of a soft drillable material and a plurality of holes drilled through the substrate for holding a plurality of glass fibers.  
       [0010] In another aspect, the invention relates to an optical device which comprises a fiber array mated to a lens array, wherein the fiber array comprises a substrate made of a soft drillable material having a low coefficient of thermal expansion and a plurality of holes drilled through the substrate for holding a plurality of glass fibers, and a lens array comprising a material exhibiting a coefficient of thermal expansion that substantially matches the coefficient of thermal expansion of the soft drillable material.  
       [0011] In yet another aspect, the invention relates to a method of making a fiber array which comprises drilling a plurality of holes through a substrate made of a soft drillable material having a low coefficient of thermal expansion, inserting glass fibers into the holes, and securing the glass fibers in place in the holes.  
       [0012] In another aspect, the invention relates to a method of making a fiber array which comprises drilling a plurality of holes through a substrate made of a soft drillable material having a low coefficient of thermal expansion, inserting glass fibers into the holes, securing the glass fibers in place in the holes, and forming a lens on an end of the each of the glass fibers protruding from a face of the substrate.  
       [0013] Other features and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
     [0014]FIG. 1A shows a fiber array according to an embodiment of the invention.  
     [0015]FIG. 1B is a vertical cross-section of the fiber array in FIG. 1A.  
     [0016] FIGS.  2 A- 2 F illustrate a process for populating a fiber array with optical fibers according to one embodiment of the invention.  
     [0017]FIG. 3 shows a fiber array mated with a lens array according to one embodiment of the invention.  
     [0018]FIG. 4A is a schematic of a lensed fiber.  
     [0019]FIG. 4B shows a lensed fiber inserted in a fiber array according to another embodiment of the invention.  
     [0020] FIGS.  5 A- 5 D illustrate a process for populating a fiber array with lensed fibers according to another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0021] The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art, that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in detail in order to not unnecessarily obscure the invention. The features and advantages of the invention may be better understood with reference to the drawings and discussions that follow.  
     [0022] A fiber array consistent with the principles of the invention is inexpensive to manufacture, athermal, and highly accurate in core-to-core positioning and axial alignment of glass fibers, such as optical fibers, lensed fibers, and rod lenses. For illustration purposes, FIG. 1A shows a fiber array  100  according to an embodiment of the invention. The fiber array  100  includes a substrate  102  having a plurality of holes  104  in a predetermined pattern. The holes  104  are receptacles for glass fibers (not shown). The profile of the holes  104  may be tailored to match the profile of the glass fibers and/or to facilitate insertion of the glass fibers into the holes  104 , as will be further described below. The holes  104  are drilled through the substrate  102  using conventional machining techniques, preferably computer-numerical-controlled (CNC) machining techniques, which will allow better control of positioning and dimensions of the holes  104  in the substrate  102 .  
     [0023] The choice of material for the substrate  102  is important because very small diameter holes with significant depth (relative to the diameter) would have to be drilled through the substrate  102 , usually with very small hole-to-hole spacing. Therefore, the substrate material should be selected such that hole-dimensional integrity is maintained throughout the fiber array  100 . The thermal expansion of the substrate material may also have to be matched to fused silica, or other similar glass material, to achieve athermalization. In one embodiment, the substrate  102  is made of a soft drillable material, i.e., a material that can easily be drilled using conventional machining techniques. Examples of soft drillable materials include, but are not limited to, pyrolytic graphite, other machinable graphite grades, and machinable ceramics, such as pyrolytic boron nitride and other machinable boron nitride grades. In one embodiment, the soft drillable material has a coefficient of thermal expansion (CTE) that is similar to or matches that of a lens array to be mated to the fiber array  100 . In one embodiment, the soft drillable material has a low CTE, e.g., ranging from about 0.5×10 −6  to 8.0×10 −6 /° C.  
     [0024] Pyrolytic graphite is advantageous to the demands of a fiber array in at least two respects. First, pyrolytic graphite is a highly anisotropic material in terms of its thermal properties. Along the basal plane, also known as the “ab” plane, pyrolytic graphite has a very low CTE of about 0.5×10 −6 /° C. at room temperature and about 0.68×10 −6 /° C. at 2000° C. This makes pyrolytic graphite inherently athermal and an excellent expansion match to fused silica, should it be important to match the fiber array to a lens array made of fused silica. Perpendicular to the basal plane, also known as the “c” plane, pyrolytic graphite has a CTE of about 6.8×10 −6 /° C. at room temperature and about 8.0×10 −6 /° C. at 2000° C. This also makes pyrolytic graphite an excellent expansion match to a lens array made from photonucleable (or photosensitive) glass, such as Coming SMILE® lens array, or other glass having a higher CTE than fused silica.  
     [0025] Secondly, pyrolytic graphite is relatively soft, making it possible to drill very small diameter holes, e.g., 126-μm holes, to significant depths relative to diameter. Holes have been drilled in pyrolytic graphite using carbide twist drills, although other types of drills can also be used. It has been demonstrated that holes at least ten times deeper than hole diameter can be drilled through the material while maintaining hole dimensional integrity throughout the fiber array. This would not be possible using any low-expansion metal such as invaru. It has also been demonstrated that hole location can be controlled to within a few microns, e.g., within 2 μm of a target position. Because the holes in the fiber array substrate have significant depth relative to diameter, and because the diameters of the holes can be held within a few microns, additional error (e.g., in optical switching performance) due to fiber insertion location and pointing error are minimized. Further, because of the soft nature of pyrolytic graphite, additional detail can be machined into the hole geometry as necessary to allow glass fibers to be easily threaded into the holes.  
     [0026]FIG. 1B shows a vertical cross-section of the fiber array  100 . To facilitate insertion of glass fibers (not shown) into the holes  104 , the holes  104  may be terminated with tapered or flared holes  106 . Alternatively, other enlarged hole geometry, such as a counterbore, may be used in place of the tapered holes  106 . An added advantage of the tapered holes  106  is that adhesive can amass around the portion of the glass fibers in the tapered holes  106 , thereby allowing a robust connection to be formed between the fiber array  100  and the glass fibers. This makes it easier to handle the assembled fiber array.  
     [0027] Populating the fiber array  100  with glass fibers is straightforward. One example of a process of populating the fiber array  100  is illustrated in FIGS.  2 A- 2 F. The process starts by stripping and cleaning optical fiber ends. FIG. 2A shows an optical fiber  200  having a buffered fiber  202  and a stripped fiber  204 . As shown in FIG. 2B, the process next involves filling the holes  104  in the fiber array  100  with epoxy  206  or other suitable thermoset or adhesive material. The holes  104  may be filled with epoxy  206  using a dispensing needle. Next, as shown in FIG. 2C, the optical fiber  200  is inserted into the hole  104 . As shown in FIG. 2D, the optical fiber  200  is threaded through the hole  104  until the buffered fiber  202  fills the tapered hole  106 . Then, the stripped fiber  204  protruding beyond the face  210  of the fiber array  100  is tacked in place, as shown at  208 , with a UV-curable epoxy or other thermoset material to prevent the optical fiber  200  from being pulled out of the hole  104  while the remaining holes  104  in the fiber array  100  are populated with optical fibers.  
     [0028]FIG. 2E shows the fiber array  100  after it has been completely populated with optical fibers  200 . After populating the fiber array  100 , the fiber array  100  is heated to thermally cure the epoxy  206 , thus forming a robust connection between the optical fibers  200  and the fiber array  100 . The next step is to remove the excess stripped fiber  204  protruding beyond the face  210  of the fiber array  100 . One method for removing the excess fiber is by manually snipping the fiber ends off with a wire cutter. Another method for removing the excess fiber is by cutting the fiber ends with a wiresaw. In both of these methods, there is the risk of breaking the fibers. In both of these methods, the entire face  210  of the fiber array  100  would need to be re-potted with epoxy (or other polymer) to cover the exposed ends of the fibers  200 . The ends of the fibers  200  (encapsulated in epoxy) are then lapped and polished to desired flatness, and the face  210  of the fiber array  100  is polished and surface-finished. The amount of polishing required to achieve the desired flatness can be substantial. To reduce the amount of polishing, the excess fiber can be removed by applying a single pulse of a laser near the face  210  of the fiber array  100  to vaporize the fiber ends. The face  210  can then be polished and surface-finished. FIG. 2F shows the final product.  
     [0029] It should be noted that pyrolytic graphite can be flammable. Therefore, when the fiber array  100  is made of pyrolytic graphite or other flammable material, care should be taken to ensure that the fiber array  100  is not damaged while cutting the fiber ends with laser. Various types of lasers can be used in the invention, e.g., CO 2 , Nd-YAG, etc. In an experiment with 10-W CO 2  laser, a sample of pyrolytic graphite was irradiated for approximately 30 seconds with no visible damage to the pyrolytic graphite. The sample became heated, however. This is because of the anisotropic thermal conductivity characteristics of the pyrolytic graphite. This means that the laser can be used to cure the epoxy  206  used to secure the fibers  200  to the fiber array  100  while cutting the fiber ends. In other words, a separate heating step to cure the epoxy  206  may not be needed. It should be noted that a ball (not shown) may form at the end of the fiber  200  if the fiber end does not vaporize quickly. Typically, the size of the ball decreases as the laser power increases. The ball may serve to further lock the fiber  200  in place during the polishing step. The ball may also serve to increase the mode field diameter at the end of the fiber  200 , which could reduce insertion losses when coupling light into the fiber  200 .  
     [0030] The fiber array of the invention is suitable for use in aligning an array of optical fibers with a lens array. The lens array may be one made by molding or by thermal opacification of a photonucleable glass or by other suitable method. Photonucleable glasses are described in, for example, U.S. Pat. Nos. 2,326,012, 2,422,472, 2,515,936, 2,515,938, 2,515,275, 2,515,942 and 2,515,943, the contents of which are incorporated herein by reference. Methods for transforming a photonucleable glass to a lens array are described in, for example, U.S. Pat. Nos. 4,572,611, 4,518,222 and 5,062,877, the contents of which are incorporated herein by reference. One commercial implementation of a method for transforming a photonucleable glass to a lens array is known as SMILE® process. In general, a photonucleable glass is transformed into a lens array by subjecting it to an ultraviolet radiation step, a heat treatment step, and an ion exchange step. When the photonucleable glass is subjected to ultraviolet radiation, the exposed portions undergo shrinkage, exerting a compressive force on the unexposed portions and drawing them into a raised pattern with a smooth curved surface, which can act as a lens.  
     [0031]FIG. 3 shows the fiber array  100  aligned with a lens array  300 . In a specific example, the lens array  300  is formed from a photonucleable glass, such as FOTOFORM® glass, and includes light-transmitting channels  304  and spherical lenses  306 , where the spherical lenses are connected to the light-transmitting channels  304 . The channels  304  and the spherical lenses  306  are the uncrystallized portion of the photonucleable glass. The channels  304  are surrounded by crystallized (or glass-ceramic) matrix  308 . In this example, the fiber array  100  provides individual alignment of optical fibers  200  with corresponding light-transmitting channels  304  and spherical lenses  306  in the lens array  300 . The fiber array  100  eliminates the need to serially align each of the optical fibers  200  with the corresponding light-transmitting channel  304  and spherical lens  306 . Further, the fiber array  100  provides highly accurate core-to-core positioning and axial alignment of the optical fibers  200 .  
     [0032] The invention has been described so far with respect to inserting optical fibers into the drilled holes in a fiber array. However, as previously mentioned, the fiber array of the present invention can also be populated with lensed fibers. In one embodiment of the invention, the geometry of the holes ( 104  in FIG. 1A) in the fiber array ( 100  in FIG. 1A) is modified to accommodate lensed fibers. In the following sections, a lensed fiber will be described first. Then, a fiber array having a modified hole geometry to accommodate the lensed fiber will be described.  
     [0033]FIG. 4A shows a lensed fiber  400  having a planoconvex lens  404  attached to, or formed at, an end of an optical fiber  406 . The optical fiber  406  is a stripped region of a fiber pigtail  408 . FIG. 4B shows a partial cross-section of a fiber array  410  having a drilled hole  412  for receiving the lensed fiber  400 . Typically, the fiber array  410  would include multiple drilled holes, such as drilled hole  412 , arranged in a predetermined pattern. The fiber array  410  may be constructed of pyrolytic graphite or other grades of graphite or other soft drillable materials, as previously discussed. The geometry of the drilled hole  412  includes a small-diameter hole  414  and a large-diameter hole  416 . The small-diameter hole  414  is drilled from the front surface  418   a  of the fiber array substrate  418 , while the large-diameter hole  416  is drilled from the back surface  418   b  of the substrate  418 . A chamfer (or tapered hole or counterbore)  420  is formed at the end of the small-diameter hole  414 .  
     [0034] The diameter of the small-diameter hole  414  is slightly larger than the diameter of the fiber pigtail  408 . This allows the entire length of the fiber pigtail  408  to be inserted into the large-diameter hole  416  through the chamfer  420  and small-diameter hole  414 . The chamfer  420  provides a holding place for the lens  404  and adds precision to lens height control. The neck region  422  behind the lens  404  has a diameter that matches the diameter of the small-diameter hole  414 . This allows the neck region  422  to fit snugly in the chamfer  420 . The neck region  422  is further secured in place using an optical adhesive, such as epoxy. Adhesive  424  is also used to seal the optical fiber  406  in the small-diameter hole  414 . The chamfer  420  aids in accurately positioning the front surface  426  of the lens  404  with respect to the front surface  418   a  of the substrate  418 . If desired, the chamfer  420  can be widened to allow the front surface  426  of the lens  404  to be flush with the front surface  418   a  of the substrate  418 .  
     [0035] Another method for populating a fiber array with lensed fibers includes first populating the fiber array with optical fibers as previously described in FIGS.  2 A- 2 E, and then attaching or forming planoconvex lenses at the ends of the optical fibers. FIG. 5A shows a fiber array  500  populated with optical fibers  502 . The fiber ends  504  protruding from the front surface  506  of the fiber array  500  have been reduced to a desired thickness and flatness, e.g., by lapping and polishing or by use of a laser. At this point, planoconvex lenses can be attached to or formed at the fiber ends  504 . A method of forming planoconvex lenses at the fiber ends  504  includes splicing a length of glass fiber to each of the fiber ends  504  using, for example, a filament or laser. FIG. 5B shows a glass fiber  506  spliced to the fiber ends  504 . The next step is to taper-cut the glass fiber  506  to the desired length. This process involves moving a heat source, such as a filament, along the glass fiber  506  while applying axial tension to the glass fiber  506 . FIG. 5C shows the glass fiber  506  after taper-cutting. The tip  508  of the glass fiber  506  has a small radius of curvature, typically in a range from 5 to 20 μm. This radius of curvature can be enlarged by an additional step, called a melt-back step. The melt-back step involves moving a heat source towards the tip  508 , allowing surface tension to pull the tip  508  into a large sphere. FIG. 5D shows the glass fiber  506  after a melt-back step.  
     [0036] The invention provides one or more advantages. The invention provides a method for constructing a fiber array that is highly accurate and stable in dimensions, i.e., overall surface flatness, hole diameter, and hole-to-hole spacing. The fiber array can be constructed using known technology, e.g., CNC machining, and known and readily-available machinable materials. The fiber array can be deployed with a lens array produced by a variety of methods without the use of optical adhesive in the optical path. The material used in making the fiber array can be expansion-matched to the material used in making the lens array to achieve athermalization. Constructing the fiber array from pyrolytic graphite is advantageous when the lens array is made of fused silica or from a photonucleable glass such as FOTOFORM®. The fiber array can be populated with optical fibers or lensed fibers or rod lenses. Fiber arrays populated with lensed fibers allow for improved alignment accuracy because the signal is collimated before exiting the lensed fiber. For a design that includes lensed fibers, a centering accuracy of ±3 μm is required as compared to the ±1 μm accuracy required for alignment of standard fibers. The fiber array can also be deployed with other optical devices, such as a single large collimating lens.  
     [0037] While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.