Abstract:
Various embodiments of free-space fiber waveguide connectors, feed-throughs and GRIN lens assemblies and methods of bonding GRIN lenses in, and aligning waveguide fibers to, such connectors, feed-throughs and assemblies. In one embodiment, a free-space fiber waveguide connector comprises a waveguide fiber mount including a fiber holder, a fiber waveguide, and a bonding agent bonding said fiber waveguide to said fiber holder, a coefficient of expansion of said fiber holder nominally matching a coefficient of expansion of said fiber waveguide and also nominally matching a coefficient of expansion of said bonding agent, said fiber waveguide having a surface oriented at a nonzero angle with respect to an axis of said fiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 12/628,595 filed Dec. 1, 2009, entitled “ENVIRONMENTALLY RUGGED FREE-SPACE FIBER WAVEGUIDE CONNECTOR AND METHOD OF MANUFACTURE THEREOF” which claims the benefit of U.S. Provisional Application Ser. No. 61/252,090, filed by Richard Laughlin on Oct. 15, 2009, entitled “Environmentally Rugged Free-Space Fiber Waveguide Connector and Method of Manufacture Thereof,” both commonly assigned with this application and incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This application is directed, in general, to connectors for optical fiber waveguides and optical component coupling, and more particularly, to free-space fiber waveguide connectors and methods of aligning single-mode and multi-mode optical fiber waveguides using graded refractive index (GRIN) lenses. 
       BACKGROUND 
       [0003]    The need for optical fiber waveguide connectors in optical fiber waveguide communication systems and other applications has long been apparent. Ideally, connectors should present only a minimal loss in the fiber waveguide transmission medium. There are two basic types of connectors: the fiber waveguide to fiber waveguide connector, and the collimated beam free-space connector. Fiber-to-fiber waveguide connectors are the simplest and least expensive; however, they are extremely sensitive to misalignment. To keep losses below a tenth of a decibel (0.1 dB) in a typical single mode fiber-to-fiber waveguide connector, the gap between the fibers and any lateral misalignment must be kept below two microns. Most fiber-to-fiber waveguide connectors in the past have depended on axial alignment of the components in the connector to minimize loss. Typical of these are ferrule-type connectors, which produce the best tolerances. Typical losses for the best of these connectors, single mode, are 0.2 dB mean and 0.3 dB at three standard deviations. A significant problem with these connectors is the contact. Since it is virtually impossible to maintain spacing on the order of one micron, the common practice is to butt the fiber waveguides to one another. However, since such connectors are often deployed in a high vibration environment, such as an aircraft, they tend to degrade over time as a result of vibration-induced spalling of the ends of the fibers. 
         [0004]    One of the solutions to this issue is the free-space collimated connector. Although any lens can be used in such connectors, ball and GRIN lenses are most often used to form a collimated beam. Collimating a beam amounts to trading spatial sensitivity for angular sensitivity. For example, to maintain a 0.2 dB insertion loss, the optical axis of a two millimeter focal length lens must be aligned to within three minutes of arc for a single-mode fiber waveguide. 
         [0005]    The majority of collimated lens connectors align the fiber waveguide to the outside diameter of the lens and the inside diameter of the ferrule. In other words, the outside radii of the fiber waveguide is referenced to the outside radii of the lens; this radially-referenced axis is then used to align the halves together. Unfortunately, this alignment technique requires great precision, otherwise significant losses can result. The most important aspect is controlling the tolerances on the GRIN lens, including both the diameter and angle of its face. Another aspect is aligning the mechanical center of the fiber waveguide precisely with the mechanical center of the lens. Failure to do so can result in significant insertion loss 0.75 to 1.5 dB and variation in the insertion loss. 
       SUMMARY 
       [0006]    One aspect provides a free-space fiber waveguide connector. In one embodiment, the connector includes: (1) an insert having a waveguide fiber bonded in a fiber mount attached proximate one end of the insert and an angle-faced GRIN lens attached proximate an opposing end of the insert and (2) a lens collar attached to the GRIN lens, the one end defining a reference plane and a virtual axis of the GRIN lens perpendicular to the reference plane, the fiber mount adjustable to a reflection from a reflective surface bonded parallel to the reference plane. 
         [0007]    Another aspect provides a free-space fiber waveguide connector. In one embodiment, the connector includes: (1) a first connector-half including: (1a) a transparent body attached to a planar face of a lens holder, the holder defining a reference plane, and (1b) a GRIN lens bounded in the lens holder having a fiber mount at the opposing end of the GRIN lens, the fiber mount adjustable to a virtual axis defined by a reflective surface perpendicular to the virtual axis and (2) a second connector-half including: (2a) a free-space connector having a lens axis alignment and spacing corresponding to the first connector-half. 
         [0008]    Yet another aspect provides a free-space fiber waveguide connector. In one embodiment, the connector includes: (1) a first connector-half including: (1a) a lens holder defining a reference plane and having a face at a nonzero angle with respect thereto and (1b) a GRIN lens bonded in the lens holder and having an adjacent face parallel to the face of the lens holder and a fiber mount at an opposing end thereof, the fiber mount adjustable to a virtual axis defined by a reflective surface perpendicular to the virtual axis and (2) a second connector-half including: (2a) a free-space connector having a lens axis alignment and spacing corresponding to the first connector-half. 
         [0009]    Still another aspect provides a free-space fiber waveguide connector. In one embodiment, the connector includes: (1) a first connector-half including: (1a) an insert mount that defines a reference plane and (1b) an insert having a waveguide fiber bonded in a fiber mount attached proximate one end of the insert and an angle-faced GRIN lens attached proximate an opposing end of the insert and (2) a second connector-half including: (2a) a free-space connector having a lens axis alignment and spacing corresponding to the first connector-half. 
         [0010]    Still yet another aspect provides a free-space fiber waveguide connector. In one embodiment, the connector includes a waveguide fiber mount including a fiber holder, a fiber waveguide, and a bonding agent bonding the fiber waveguide to the fiber holder, a coefficient of expansion of the fiber holder nominally matching a coefficient of expansion of the fiber waveguide and also nominally matching a coefficient of expansion of the bonding agent, the fiber waveguide having a surface oriented at a nonzero angle with respect to an axis of the fiber. 
         [0011]    Another aspect provides a free-space feed-through. In one embodiment, the connector includes: (1) a fiber holder, (2) a waveguide fiber and (3) a glass frit hermetic seal securing the waveguide fiber within the fiber holder, a coefficient of expansion of the fiber holder nominally matching a coefficient of expansion of the fiber waveguide and also nominally matching a coefficient of expansion of the glass frit. 
         [0012]    Yet another aspect provides a method of bonding a GRIN lens. In one embodiment, the method includes: (1) polishing a surface of the GRIN lens and a surface of another material, (2) placing the surfaces adjacent one another and (3) applying a substantial electric field across the surfaces to place the surfaces into opposite ionic states. 
         [0013]    Still another aspect provides a method of aligning a fiber to a virtual axis of a GRIN lens. In one embodiment, the method includes: (1) bonding the GRIN lens to a beveled, transparent plate, the plate defining a reference plane perpendicular to the virtual axis, (2) placing a reference reflector parallel to the reference plane, (3) injecting a signal into the fiber, (4) adjusting a position of the fiber until at least a near-maximum reflected signal is achieved and (5) bonding the fiber at the position. 
         [0014]    Still yet another aspect provides a GRIN lens assembly. In one embodiment, the assembly includes: (1) a GRIN lens having a bevel on an output face thereof, (2) a fiber mount located on an opposing end of the GRIN lens and providing longitudinal and angular references and (3) a waveguide fiber bonded to the fiber mount in an orientation that is perpendicular to the longitudinal reference. 
     
    
     
       BRIEF DESCRIPTION 
         [0015]    Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0016]      FIGS. 1A-D  illustrate various embodiments of techniques for achieving an angled GRIN lens; 
           [0017]      FIGS. 2A and 2B  illustrate an embodiment of a fiber waveguide mount and an embodiment of a technique for achieving the mount; 
           [0018]      FIGS. 3A and 3B  illustrate side and end views of an embodiment of a lens collar; 
           [0019]      FIG. 4  illustrates an embodiment of a collimating insert assembly; 
           [0020]      FIG. 5  illustrates an embodiment of an alignment technique for the collimating insert assembly of  FIG. 4 ; 
           [0021]      FIG. 6  illustrates an embodiment of a bonding of a GRIN lens to a fiber waveguide mount; 
           [0022]      FIG. 7  illustrates an embodiment of the bonding of a fiber waveguide to a fiber waveguide holder; 
           [0023]      FIG. 8  illustrates an embodiment of a fiber waveguide feed-through to a fiber waveguide mount; 
           [0024]      FIG. 9  illustrates end and side views of an embodiment of a fiber waveguide connector; 
           [0025]      FIG. 10  illustrates end and side views of an embodiment of a fiber waveguide connector with a transparent plate; 
           [0026]      FIG. 11  illustrates end and side views of an embodiment of a connector with removable inserts; 
           [0027]      FIG. 12  illustrates side and end views of an embodiment of a linear GRIN lens assembly; 
           [0028]      FIG. 13  illustrates side and end views of an embodiment of an alignment method for a linear GRIN lens assembly; and 
           [0029]      FIG. 14  illustrates end and side views of an embodiment of a fiber connector with a linear GRIN lens assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    As stated above, the majority of collimated lens connectors align the fiber waveguide to the outside diameter of the lens and the inside diameter of the ferrule, which requires great precision, otherwise significant losses can result. 
         [0031]    U.S. Pat. No. 4,637,683, issued to Asawa, recognizes the challenge inherent in achieving this level of precision and teaches an alternative technique in which a reference plane is ground at the end of a fiber waveguide array of a connector half. Asawa uses a reflective coating proximate the reference plane to facilitate the alignment of the fiber waveguide to the optical axis which was perpendicular to the reference plane. While this arrangement addresses the issue of optical insertion loss as a function of alignment, it fails to address prealignment and assembly of individual fiber waveguides or replacement and repair of individual fiber waveguides. Specifically, when the reflective coating is removed the reference plane now becomes the end of the GRIN lens. This introduces two problems. The first is that while the GRIN lenses must remain in contact to maintain alignment, vibration and other movement occurring over time causes a degradation of the interface between the two lenses and an increase in both loss and back reflection. The second is the significant back reflection caused if a gap is induced between the two output ends of the GRIN lenses and the gap has an index of refraction that differs from that of the GRIN lenses. 
         [0032]    Asawa also teaches a connector in which a plurality of fiber waveguides and a lens are aligned to form one half of a connector. A common lens holder contains the half, and the lens holder and the lens are ground to make a coplanar surface. Unfortunately, the individual fiber waveguides cannot be individually assembled and or repaired. In addition the lens surface can not be ground such that back-reflections are reduced or substantially eliminated. 
         [0033]    U.S. Pat. No. 4,509,827, issued to Cowen, et al., teaches a similar alignment technique. Cowen uses a “master reference rod” in a precision bore bushing to adjust the mirror, to find the center of the bore and replaces the “master reference rod” with the actual GRIN lens and adjust it to the reference mirror. This teaching also has limitations. One must change lenses, which is a significant step in a manufacturing operation. There is no reference plane apart from the inside bore. The most significant limitation is the tolerance. In order to work over some reasonable temperature range and to allow the insertion and removal of GRIN lenses. There must be some tolerance greater than several microns. This amount of misalignment will cause a significant insertion loss, greater than 0.3 dB. 
         [0034]    U.S. Pat. No. 6,540,411, issued to Cheng, teaches an optical coupling having an optical fiber waveguide inserted into a fiber waveguide tube, which is mounted in a first sleeve. A GRIN lens is mounted in a second sleeve. The first sleeve is adjusted at abutting ends thereof with a second sleeve to produce a maximum coupling. One of the Cheng&#39;s limitations is the required gap between the fiber waveguide and the lens which results in Fresnel loss and back reflection. 
         [0035]    U.S. Pat. No. 5,809,193, issued to Takakhasi, teaches an angled surface at the fiber waveguide GRIN lens interface, to prevent back reflections. However Takakhasi relies on mechanical tolerances for the alignment of the fiber waveguide core to the optical axis. In addition, Takakhasi does not address the reflections at the opposing, output, end of the GRIN lens. 
         [0036]    U.S. Pat. No. 4,691,985, issued to Shank, teaches a lens body for a connector in which the lens is connected to the fiber waveguide body. Unfortunately Shank fails to teach a reference plane; alignment is by mechanical tolerance. 
         [0037]    For one reason or another, all of the above-described connectors fall short of providing connectivity that is not only suitable for demanding applications but also that remains reliable under conditions in which connectors are now likely to be deployed. More specifically, modern connectors are required to operate as follows: they should be able to stand up to high shock (as in shipboard applications) and high vibration (as in aircraft applications) with a insertion loss of 0.75 dB standard, 0.5 dB enhanced and a 0.2 dB goal; and a return loss (back reflection) of −30 dB standard, −40 dB enhanced and a −60 dB goal. There is also, in some cases, to withstand high temperatures (about 200° C.) while maintaining a hermetic seal. In addition each fiber waveguide must be capable of individual assembly and repair. 
         [0038]    Recently a new requirement has been encountered, that being fiber waveguide feed-throughs and fiber waveguide mounts that do not materially outgas or materially cause stress fractures. Initially epoxy was used as a bonding agent in fiber waveguide feed-throughs and fiber waveguide mounts. Epoxy has two limitations. The first was that it could not stand extreme temperatures. In addition, when applied to hermetically sealed packages it has been found to outgas, contaminating the seal. In response to these shortcomings, a technique of metalizing the fiber waveguide and soldering it into the assembly was developed. Over time it was discovered that this technique resulted in stress fractures when the fiber waveguides were cycled through high temperatures. Today&#39;s requirements therefore frequently call for a high temperature, hermetically sealed, “connectorized” fiber waveguide feed-through. 
         [0039]    A quarter-pitch GRIN lens functions as a collimating lens. The refractive index of the lens material varies radially in such a manner as to expand a very small source of light, emerging from a single-mode fiber waveguide, into a much broader, parallel beam. If a second quarter-pitch GRIN lens is placed adjacent to, and axially aligned with, the first one, the parallel beam is focused down to almost a point focus, for launching into a single-mode fiber waveguide in the second connector half. This approach has the advantage of greatly reducing the requirements for lateral alignment of the fiber waveguides, i.e., the required tolerance for lateral fiber waveguide alignment is greater. However, the use of connector lenses requires extreme precision of angular alignment. For a connector loss of 0.1 dB, the angular alignment tolerance is 0.0003 radians, or approximately one minute of arc ( 1/60 th  of a degree). Therefore, the GRIN lens connector approach trades dimensional alignment tolerance for angular alignment tolerance, giving rise to a need for an accurate and convenient method of angular alignment of the connector lenses. 
         [0040]    One cannot rely on precision manufacture of the GRIN lenses, since not all such lenses are perfect plane cylinders, and losses or partial wastage will inevitably result. In the past, techniques for assuring precision in the connector halves have relied on there being a near-perfect “master” parallel beam, generated either from a perfect connector half or from a separate source. The procedure typically used is to align and orient each manufactured connector half with the parallel beam. This may not always be possible for some lens components, and can still lead to wastage. The fiber waveguide is then positioned and attached to the lens. In theory, any two connector halves that have been matched to the master parallel beam will be perfectly matched to each other. Unfortunately, these prior-art techniques do not reliably work in practice. A new approach is needed for angular alignment of GRIN lens connector halves. 
         [0041]    Reference will now be made to FIGURES wherein like structures will be provided with like reference designations. It is to be understood that the drawings are diagrammatic and schematic representations of certain embodiments of the invention, and are not to be construed as limiting the invention, nor are the drawings necessarily drawn to scale. 
         [0042]      FIG. 1A  shows an angled face GRIN lens  20   a . A conventional GRIN lens  4   a  is shaped, ground or cleaved in various embodiments to produce a beveled face  22   a . To reduce the back reflection due to Fresnel reflections at the GRIN lens air interface, an angle, nominally about 8°, is introduced into the output face of the GRIN lens  4 . 
         [0043]      FIG. 1B  illustrates a GRIN lens  4   b  in a lens holder  6   b . The lens holder  6   b  can hold a single lens  4   b  or a plurality of lenses  4   b  (1 . . . n). After the one or more lenses  4   b  are secured in place, an angled bevel  22   b  is formed, ground or polished in various embodiments. In one embodiment, the bevel  22   b  is between 3° and 8°, depending upon the differential in the index of refraction at the beveled surface. 
         [0044]      FIG. 1C  illustrates another embodiment of the angled face GRIN lens  20   c . Attached to the face opposing the focal plane  3  is a transparent wedged cylinder  24 . In one embodiment the index of refraction, of the wedged cylinder  24  nominally matches the index of refraction of the GRIN lens. This reduces, minimizes or perhaps totally eliminates Fresnel reflections. The Fresnel reflections from the wedge cylinder  24  air interface are now directed at such an angle that they are focused at a point, on the focal plane  3  such that they are displaced from the input point on the focal plane  3 . The index of refraction of the transparent wedge should match the index of refraction of the GRIN lens to with in 3.46% to keep the Fresnel reflection at the interface to less than −35 dB and 0.25% to maintain the reflection at less than −55 dB. 
         [0045]      FIG. 1D  shows an angled face GRIN lens  20   d  where the beveled transparent plate  26  is larger than the GRIN lens. Those skilled in the pertinent art will recognize that the beveled transparent plate  26  can be any shape. In two example alternative embodiments, it is a cuboid or a cylinder with one or more GRIN lenses  4   c  attached. 
         [0046]      FIG. 2A  illustrates a planar fiber waveguide mount  21   a , with a surface  32   a  perpendicular to the fiber waveguide axis to less than 0.03 rad. for a 0.1 dB loss, or 0.06 rad. for a 0.5 dB loss, incurred as a result than a less-than-perfect alignment. In one embodiment the fiber waveguide holder  27   a  is made of glass with substantially the same coefficient of expansion (see,  FIG. 10 ) as that of the fiber waveguide. One skilled in the pertinent art will recognize that the differing coefficients of expansion can cause stress fractures in the fiber waveguide when the assembly is exposed to wide temperature ranges. The fiber waveguide holder  27   a  can be formed from tubing, machined solid glass, molded ceramic or other suitable materials and processes. The fiber waveguide  2  is inserted into the fiber waveguide holder and bonded to the fiber waveguide holder with a bonding agent  30 . In one embodiment, the bonding agent is an epoxy. In another, it is a cement. In still another, it is a glass frit. 
         [0047]      FIG. 2B  illustrates a beveled fiber waveguide mount  21   b , with a beveled surface  32   b  at some angle to the plane perpendicular to the fiber waveguide axis. In one embodiment the angle is 3° to 8°, depending on the differential in the index of refraction between the fiber waveguide and the GRIN lens. 
         [0048]      FIGS. 3A and 3B  show the lens collar  28   a ,  28   b , respectively. The lens collar has an inner diameter  40  such that the angle face GRIN lens  20  can be inserted into the lens collar  28  and bonded. The lens collar  28   a ,  28   b  has an angular reference  38  delineated for alignment purposes. In the illustrated embodiment, this angular reference a flat included in the lens collar  28   a ,  28   b . Those skilled in the pertinent art will recognize that there are a multitude of techniques that can be used to delineate the angular position, including pins and keys. 
         [0049]      FIG. 4  illustrates a collimating insert  34  assembly, for a free-space fiber waveguide connector. The assembly consist of a fiber waveguide mount  21  bonded to an angled face GRIN lens  20 . The angle-faced GRIN lens  20  is inserted and bonded into the lens collar  28 . The lens collar  28  facilitates the mounting of the collimating insert  34  assembly into various devices. In one embodiment, the angle-faced GRIN lens  20  is inserted into a free-space optical connector. The surface  36  of the lens collar  28  defines a reference plane and the optical axis of the collimating insert  34  assembly. 
         [0050]      FIG. 5  illustrates one embodiment of an alignment method for the collimating insert  34  assembly. The collimating insert  34  is temporarily, placed in a calibrated spacer  42 . The calibrated spacer  42  positions the reflective surface  5  parallel to the reference plane  36 , of the collimating insert  34 . The fiber waveguide mount  21  is spaced with a gap  44 , between the fiber waveguide holder reference plane  37  and the focal plane  3  of the GRIN lens  4 . An optical beam is injected into the fiber waveguide  2  from the reflectometer  1 . This signal is transmitted through the fiber waveguide  2  into the angled GRIN lens  20 . The angled GRIN lens  2  collimates the beam, and it is reflected from the reflective surface  5  back into the GRIN lens  4  which focuses it on the focal plane  3  of the GRIN lens  4 . The fiber waveguide mount  21  is adjusted parallel to the insert reference plane  36  for a maximum signal. Those skilled in the pertinent art will recognize this as an autocollimation process. When the collimated optical beam is aligned perpendicular to the reflective surface, and thus the insert reference plane, the fiber waveguide mount  21  is translated along the axis, perpendicular to the insert reference plane  36  and bonded. The collimating insert is then removed from the calibrated spacer  42 . The bonding can be accomplished by a variety of means. In one embodiment it is accomplished by electrostatic or ionic bonding. 
         [0051]      FIG. 6  shows electrostatic bonding or ionic boding of a GRIN lens to a glass surface. The properties of the GRIN lens  4  are effected by diffusion of ions into a molten glass. Those skilled in the art will recognize that various ions, such as Na(+), Li(+) and Ag(+), may be used to dope the glass to alter its index of refraction. The implantation of these ions provides the excess ions, in the GRIN lens material that facilitate the electrostatic or ionic bond  52 . A cylinder of glass, such as the fiber waveguide holder  27  is polished and is brought into contact with a material with excess ions, such as a GRIN lens  4 , that has a polished surface. Upon contact, the Van der Waals forces provide a relatively weak bond. A positive electrode  46  and a negative electrode  48  are placed around the two cylinders, and a voltage is applied. In one embodiment, this voltage varies between 1000 and 2000 volts. The applied field resulting from this voltage, across the interface between the fiber waveguide holder  27  and the GRIN lens  4  causes some of the electrons, from the positive ions to migrate  50  from the GRIN lens  4  into the fiber waveguide holder  27 , thus producing an electrostatic bond. Those skilled in the pertinent art will recognize that this technique may also be used to bond the GRIN lens  4  to the wedge  24  and  26 . 
         [0052]      FIG. 7  illustrates the bonding of the fiber waveguide to the fiber waveguide holder  27 . A fiber waveguide mount material  56  is selected such that it matches the coefficient of expansion of the fiber waveguide such that over the temperature pressure excursions the stress resulting from the differential between the coefficient of expansion of the waveguide fiber  2  and the fiber holder  27  and the bonding agent  30  the stress induced does not exceed one-third of the proof stress P s . The proof stress of a typical fiber is about 100,000 psi. In one embodiment this is a glass tube with the inside bore  58  slightly larger than the outside diameter of the cladding of the fiber waveguide to be mounted. One skilled in the pertinent art understands that the fiber waveguide mount material  56  can take on many forms, including but not limited to: a glass block that is drilled or bored, a piece of ceramic that is drilled or bored, a molded glass or a molded ceramic. 
         [0053]    A fiber waveguide  2  is inserted in the inside bore  58  of the fiber waveguide mount material  56 . A glass frit  31  is placed in the space between the fiber waveguide  2  and the inside wall of the bore  58 . The glass frit  31  is selected such that its coefficient of expansion nominally matches the coefficient of expansion of the glass fiber waveguide. 
         [0054]    The fiber waveguide feed-through  54   a  assembly is then heated to the melting temperature of the frit  31 , nominally 180 C.° to 300 C.°. Those skilled in the pertinent art will recognized that this can be accomplished in an oven, by induction heating or by many other techniques. 
         [0055]      FIG. 8  illustrates converting a fiber waveguide feed-through  54  to a fiber waveguide mount  21 . The fiber waveguide feed-through  54 , is shaped with a planar surface  32  at some angle from the plane perpendicular to the fiber waveguide axis. In one embodiment the shaping is accomplished by polishing, and one angle is about 8°. The angle is a function of the: acceptable back reflection, index of refraction of the fiber waveguide and the index of refraction of the opposing media into which the fiber is radiating. 
         [0056]      FIG. 9  illustrates a connector array  60 . A plurality of GRIN lens  4  are mounted in a lens holder  6  and shaped to form a ground face  7  at some predefined angle, nominally 6° to 8°, to the fiber waveguide holder reference plane  37 . As  FIG. 4  shows, the fiber waveguide holder  27  is positioned to align the fiber waveguide  2  from a reflective surface  5  (not shown) that is parallel to the reference plane  37 . The two halves of the connector  64   a    64   b  along with a precision spacer  7  are assembled to form a free-space connector array  60 . One skilled in the pertinent art will recognize that the lens holder  6  can take on any one of a number of shapes. In one embodiment, it is a truncated cylinder. In another embodiment, it is an extruded cuboid. In various alternative embodiments, multiple GRIN lenses  4  are arranged in circular, linear and staggered arrays. 
         [0057]      FIG. 10  illustrates another embodiment of a connector array  62 . In the embodiment of  FIG. 10 , a transparent plate  26  or a transparent cylinder  24  is attached to a planar face of the lens holder  6 , which is nominally parallel to the reference plane  37 , lens holder  6 . As  FIG. 7  shows, the fiber waveguide holder  27  is positioned to align the fiber waveguide  2  from a reflective surface  5  (not shown) that is parallel to the reference plane  37 . The two halves of the connector  64   a ,  64   b , along with a precision spacer  42 , are assembled to form a free-space connector array  62 . One skilled in the pertinent art will recognize that the lens holder  6  can take on any of a plurality of alternative shapes. In one embodiment it is a truncated cylinder. In another embodiment it is an extruded cuboid. The GRIN lens  4  may be inserted in a circle, a linear array or a staggered array. 
         [0058]      FIG. 11  illustrates a connector  66  with inserts. With the fiber waveguide  2  aligned to the reference plane  36   a , one or more of collimating insert(s)  34 , are inserted into the insert mount  35   a  and  35   b . The insert mount can take on a plurality of shapes. It is circular in one embodiment. However a cuboid or irregular shape will perform the same function. The two connector haves  64   a  and  64   b , containing the insert  34  and the insert mount  35 , are mated together with a calibrated spacer  42 , such that the two reference planes  36  are nominally parallel to one another. Those skilled in the pertinent art will recognize that additional registration in the rotational axes to align the optical axis of each lens. Alignment may be achieved by many structures, such as flats, pins or keys. In addition, the two connector haves  64   a  and  64   b  may also employ rotational registration in addition to X, Y registration. In one embodiment, a common (to free-space connectors) connector housing (not shown) is used to accomplish this function. 
         [0059]      FIG. 12  illustrates a linear GRIN lens assembly  76 . The liner GRIN lens assembly  76  includes GRIN lens  4  with and bevel  22  on its output face and a fiber mount  21  bonded on the opposing end. The GRIN lens assembly  74  is shaped to provide a longitudinal reference and an angular reference. In one embodiment this shaping is a pair of flat surfaces  39  ground at some angle to one another and parallel to the virtual axis of the assembly. 
         [0060]      FIG. 13  illustrates an alignment method for the GRIN lens assembly  76 . The GRIN lens assembly  76  is placed in the alignment base  70 . A alignment plunger  68  forces the GRIN lens assembly  76  firmly into the alignment key  72 , referencing the GRIN lens assembly  76  both longitudinally and angularly. A reflective surface  5  is placed in a plane perpendicular to the planes of the alignment key  72 . The fiber mount  21  is adjusted for maximum return, aligning the waveguide fiber  2  to the virtual axis of the GRIN lens assembly  76 . The fiber mount  21  is then bonded to the GRIN lens  4 . In one embodiment this is electrostatic bonding. 
         [0061]      FIG. 14  illustrates a free-space linear inserts, fiber connector  78 , with GRIN lens assemblies  74 , with reference parallel to a virtual axis. The first half of the connector  64  is composed of a insert holder  35 , that is formed with an alignment key  72  and an alignment plunger  68 , to register the GRIN lens assembly  74  that is placed in the insert holder  35 . The insert holder  35  has a reference plane reference plane  39  that is perpendicular to the alignment key  72  plane(s). 
         [0062]    One skilled in the pertinent art will recognize that there may be a single or a plurality of alignment surfaces and that these surfaces can be formed at various angles. The alignment plunger  64  is a detent spring in one embodiment, but takes on alternative forms in alternative embodiments. 
         [0063]    Those skilled in the pertinent art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments.