Abstract:
Embodiments of the present invention are directed to a compact and stable fiber optic collimator that takes light from one or more optical fibers and generates one or more beams of collimated light at an increased specified diameter. The collimator is configured for easy assembly and for simple and precise adjustment. In one embodiment, a fiber optic collimator comprises a shuttle plug including a cavity for receiving an optical fiber having an optical fiber tip to emit a light through the shuttle plug. A collimator body has a collimator bore to receive the shuttle plug and constrain the shuttle plug in the collimator bore to be movable in an axial direction along an axis of the collimator bore. A collimating lens is mounted to the collimator body and disposed generally opposite from the optical fiber tip to receive a light beam from the optical fiber tip expanding in size toward the collimating lens.

Description:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The invention described herein was made in the performance of work under Contract No. F33657-01-C-4165. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to transmission of fiber-optic signals and, more particularly, to a collimator for transforming the output from one or more optical fibers into one or more parallel optical beams. 
     Many fiber-optic devices r that the output from one or more optical fibers be converted into collimated beams. A fiber optic collimator takes light from a optical fiber and generates a beam of collimated light at an increased specified diameter. The collimator desirably maintains alignment between the fiber and the collimating lens When two or more optical fibers are coupled to the same collimator (referred to as expanded beam coupling), high precision alignment of the respective fibers is necessary. Previous fiber optic collimators tend to be clumsy, employ complex mechanisms, and are difficult to assemble and adjust for focus and alignment. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the present invention are directed to a compact and stable fiber optic collimator that takes light from one or more optical fibers and generates one or more beams of collimated light at an increased specified diameter. The optical fibers are connected to a single collimator body having precise collimator bores for receiving and aligning the optical fibers relative to the respective collimating lenses. Each lens is configured to be adjustable along its plane for aliment with respect to the tip of the corresponding optical fiber. The optical fiber is configured to be adjustable along an axis of the collimator bore to focus the light beam to obtain the desired wavefront quality. After a first set of optical fiber and collimating lens are aligned, the other sets of optical fiber and collimating lens may be aligned with respect to the first set The collimator is designed to achieve a substantially athermal configuration. The collimator is stable over a specific soak temperature range and maintains alignment through adverse vibration. The collimator is configured for easy assembly and for simple and precise adjustment. 
     In accordance with an aspect of the present invention, a fiber optic collimator comprises a shuttle plug including a cavity for receiving an optical fiber having an optical fiber tip to emit a light through the shuttle plug. A collimator body has a collimator bore to receive the shuttle plug and constrain the shuttle plug in the collimator bore to be movable in an axial direction along an axis of the collimator bore. A collimating lens is mounted to the collimator body and disposed generally opposite from the optical fiber tip to receive a light beam from the optical fiber tip expanding in size toward the collimating lens. The collimating lens is constrained to be movable in a transverse plane normal to the axial direction. The shuttle plug is configured to be movable in the axial direction to adjust a position of the optical fiber tip with respect to the collimating lens and the collimating lens is configured to be movable in the transverse plane to align the collimating lens with respect to the optical fiber tip. 
     In some embodiments, the shuttle plug includes a fiber optic ferrule to attach the optical fiber and position the optical fiber tip within the shuttle plug. The shuttle plug includes a pin keyway and the collimator body includes a rotation alignment pin configured to engage the pin keyway to prevent rotation of the shuttle plug with respect to the collimator body. The fiber optic ferrule is rotationally aligned with respect to the pin keyway for desired polarization of the light beam from the optical fiber. The fiber optic ferrule is connected to the shuttle plug by an adhesive introduced into adhesive tack bond holes in the shuttle plug at two axial locations along the fiber optic ferrule (e.g., six adhesive tack bond holes at two axial locations, 120 degrees apart). The shuttle plug includes an undercut diameter intermediate region between two end regions, and wherein the two end regions each include machined flats to reduce surface contact with the bore of the collimator body (e.g., three 120 degrees opposed machined flats). The shuttle plug is connected to the collimator body by an adhesive introduced into adhesive tack bond holes in the collimator body at two axial locations along the shuttle plug (e.g., six adhesive tack bond 
     holes at two axial locations, 120 degrees apart). 
     In specific embodiments, a lens cell has a seat to receive the collimating lens. The lens cell is configured to mount the collimating lens to the collimator body to permit adjustment in the transverse plane normal to the axial direction to align the collimating lens with respect to the optical fiber tip. The lens cell is connected to the collimator body by an adhesive introduced into adhesive tack bond holes in the collimator body distributed around the lens cell. The lens cell is attached to the collimator body by a plurality of cell clamps (which is desirable in severe environments). The collimator body includes a plurality of raised pads (e.g., three pads) which are coplanar and parallel to the axis of the collimator bore. The raised pads are configured to interface with a mating piece to which the collimator body is to be connected. The collimator body may include a plurality of collimator bores to receive a plurality of shuttle plugs, and the collimator body is configured to mount a plurality of collimating lenses each for alignment and focus with respect to a corresponding one of the plurality of shuttle plugs. 
     In accordance with another aspect of the invention, a fiber optic collimator comprises a shuttle plug including a cavity for receiving an optical fiber having an optical fiber tip to emit a light through the shuttle plug. A collimator body has a collimator bore to receive the shuttle plug and constrain the shuttle plug in the collimator bore to be movable in an axial direction along an axis of the collimator bore. A lens cell has a seat to receive a collimating lens, and is configured to mount the collimating lens to the collimator body generally opposite from the optical fiber tip to receive a light beam from the optical fiber tip expanding in size toward the collimating lens and to permit adjustment of the collimating lens in a transverse plane normal to the axial direction to align the collimating lens with respect to the optical fiber tip. 
     In some embodiments, the lens cell is connected to the collimator body by an adhesive introduced into adhesive tack bond holes in the collimator body distributed around the lens cell. The lens cell is attached to the collimator body by a plurality of cell clamps. 
     In accordance with another aspect of the invention, a method of mounting an optical fiber and a collimating lens to a collimator body comprises mounting an optical fiber to a shuttle plug, the optical fiber having an optical fiber tip to emit a light through the shuttle plug; and sliding the shuttle plug into a collimator bore of the collimator body configured to receive the shuttle plug and constrain the shuttle plug in the collimator bore to be movable in an axial direction along an axis of the collimator bore. A collimating lens is mounted to the collimator body to be disposed generally opposite from the optical fiber tip to receive a light beam from the optical fiber tip expanding in size toward the collimating lens. The shuttle plug is moved in the axial direction to adjust a position of the optical fiber tip with respect to the collimating lens. 
     In some embodiments, mounting the collimating lens comprises placing the collimating lens in a seat of a lens cell; coupling the lens cell to the collimator body to permit adjustment in a transverse plane normal to the axial direction; moving the lens cell with respect to the collimator body in the transverse plane to align the collimating lens with respect to the optical fiber tip; and attaching the lens cell to the collimator body after the collimating lens is aligned with respect to the optical fiber tip. Moving the lens cell comprises connecting the lens cell to two linear stages configured to move the lens cell in two orthogonal directions along the transverse plane. Moving the shuttle plug comprises coupling a focus tooling member with the shuttle plug by supporting a focus tooling rod using a focus tooling clamp temporarily coupled to the collimator body. The focus tooling member is connected to a linear stage configured to move the shuttle plug in the axial direction to focus the optical fiber tip with respect to the collimating lens. The method further comprises attaching the shuttle plug to the collimator body and removing the focus tooling rod and the focus tooling clamp after focusing the optical fiber tip with respect to the collimating lens. 
     In specific embodiments, mounting the optical fiber comprises coupling the optical fiber to a fiber optic ferrule and attaching the fiber optic ferrule to the shuttle plug to position the optical fiber tip within the shuttle plug. The shuttle plug includes a pin keyway, and the fiber optic ferrule is rotationally aligned with respect to the pin keyway for desired polarization of the light beam from the optical fiber. A rotation alignment pin is inserted through a portion of the collimator body to engage the pin keyway to prevent rotation of the shuttle plug with respect to the collimator body. The collimator body includes a plurality of raised pads (e.g., three raised pads) which are coplanar and parallel to the axis of the collimator bore. The method further comprises interfacing the raised pads with a mating piece to which the collimator body is to be connected. The method further comprises providing a pin extending from two raised pads of the collimator body to two oversized pin holes in the mating piece; wet-pinning the pin to the mating piece by introducing an adhesive into the oversized pin holes; and attaching the collimator body to the mating piece by a plurality of screws. This process mitigates against drilling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially cut-away perspective view of a fiber optic collimator according to an embodiment of the present invention; 
     FIG. 2 is another partially cut-away perspective view of the fiber optic collimator of FIG. 1 illustrating the shuttle plug; 
     FIG. 3 is another perspective view of the fiber optic collimator of FIG. 1 illustrating the focus tooling clamp; 
     FIG. 4 is another perspective view of the fiber optic collimator of FIG. 1 illustrating the lens cells and clamps; 
     FIG. 4A is a partially cut-away perspective view of a lens cell of the fiber optic collimator FIG. 1; 
     FIG. 5 is another perspective view of the fiber optic collimator of FIG. 1 illustrating the raised pads; and 
     FIG. 6 is a cross-sectional view illustrating attachment of the fiber optic collimator of FIG. 1 to a mating piece. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a fiber optic collimator  10  having a collimator body  12 . An optical fiber  14  is coupled to one side of the collimator body  12 , while a collimating lens  16  is coupled to another side of the body  12  to generate a beam of collimated light from the light of the optical fiber  14  at an increased specified diameter. 
     The polished end of the optical fiber  14  is connected to a fiber optic ferrule  18  which is disposed in a cavity of a shuttle plug  20  that is inserted into the collimator body  12 . The tip  22  of the optical fiber  14  emits a light beam that expands to the desired diameter as it reaches the collimating lens  16 . The fiber optic ferrule  18  is typically made of a glass or ceramic. Prior to inserting the shuttle plug  20  into the collimator body  12 , the ferrule  18  is inserted into the shuttle plug  20  and rotationally aligned with respect to a pin keyway  26  for optimum polarization of the light beam. The alignment with respect to the pin keyway  26  ensures that the optical fiber  14  is oriented properly during assembly of the shuttle plug  20  with the collimator body  12 . To secure the ferrule  18  to the shuttle plug  20 , two sets of adhesive tack bond holes  30 ,  32  are disposed near the two ends of the ferrule  18 . In the specific embodiment, three bond holes  30  are angularly spaced by about 120 degrees and three bond holes  32  are angularly spaced by about 120 degrees, and each bond hole is about 0.040 inch in diameter for adequate bonding under most circumstances. A suitable adhesive, such as 3M 2216, is introduced into the bond holes  30 ,  32  to secure the ferrule  18 . Of course, other suitable ways of securing the ferrule  18  may be used in different embodiments. 
     As seen in FIGS. 1 and 2, the shuttle plug  20  has an outer diameter that is sized to slide within a precision bore in the collimator body  12 . The shuttle plug  20  serves to provide easy assembly and precise alignment and stability of the optical fiber  14  with respect to the collimating lens  16 . The shuttle plug  20  is made as long as possible (or necessary) given the constraints of the collimator body  12  to minimize the pointing error of the plug  20  (and optical fiber  14 ) with respect to the collimator body  12 . To ensure high precision of the interface between the shuttle plug  20  and the core of the body  12 , the shuttle plug  20  desirably includes an undercut diameter intermediate region  40  between the two end regions. The two end regions each include machined flats  42 ,  44 , respectively, to limit the surface contact with the precision bore of the body  12 . In the specific embodiment shown, the end regions each have three machined flats  42 ,  44 , evenly distributed angularly so as to allow three surfaces, spaced by about 120 degrees, to come in contact with the precision bore. Of course, other ways of limiting surface contact between the shuttle plug  20  and the bore of the collimator body  12  may be used in alternate embodiments. 
     The collimator body  12  includes two sets of adhesive tack bond holes  46 ,  48  that are disposed near the two ends of the shuttle plug  20  for securing the plug  20 . In the specific embodiment shown, three bond holes  46  are angularly spaced by about 120 degrees and three bond holes  48  are angularly spaced by about 120 degrees (aligned to the three lobed, raised diameters at both ends), and each bond hole is about 0.050 inch in diameter for adequate bonding under most circumstances. When the shuttle plug  20  is ready to be secured to the collimator body  12  after alignment with respect to the lens  16 , a suitable adhesive, such as 3M 2216, is introduced into the bond holes  46 ,  48 . Of course, other suitable ways of securing the shuttle plug  20  may be used in alternate embodiments. The collimator body  12  desirably includes bleed holes  49  for bleeding air. 
     Coupled to the shuttle plug  20  is a focus tooling rod  50  which is used to focus the optical fiber  14  in the axial direction along the axis of the collimator bore which coincides with the axis of the shuttle plug  20 , as illustrated in FIGS. 1-3. A focus tooling clamp  52  is connected to both the focus tooling rod  50  and the collimator body  12 , and can be tightened against the rod  50  using a clamp screw  54 . The focus tooling rod  50  extends into a threaded cavity  58  inside the shuttle plug  20 . 
     After sliding the assembly of the shuttle plug  20  and the focus tooling rod  50  into the precision bore of the collimator body  12  and before aligning and securing the plug  20  to the body  12 , a rotation alignment pin  60  is pressed into the body  12  to engage the pin keyway  26  of the shuttle plug  20  with the plug  20  roughly in place. The length of the rotation alignment pin  60  is such that it will be set to the correct depth when the outer end is flush with the body  12 . A counter bore  62  around the outer end of the alignment pin  60  allows for disassembly. The rotation alignment pin  60  prevents rotation of the shuttle plug  20  with respect to the collimator body  12 . After the shuttle plug  20  is secured to the body  12 , the focus tooling clamp  52  is tack bonded to the collimator body  12  with the optical fiber  14  and focus tooling rod  50  protruding therethrough, as best seen in FIG.  3 . The attachment of the focus tooling clamp  52  to the body  12  desirably is temporary, so that it may be detached from the body  12  subsequently. It is understood that other ways of attaching the focus tooling clamp  52  to the body  12  may be used. 
     The collimating lens  16  is mounted to the collimator body  12  using a lens cell  60 , as shown in FIGS. 1,  2 ,  4 A, and  4 . The lens  16  (which may be a doublet) is tack bonded into a precision bore of the lens cell  60  via a plurality of glue holes  64  (sec FIG.  4 A). In a specific embodiment, there are eight equally spaced glue holes  64  extending into the lens bore. It is understood there are other ways to attach the lens. In some embodiments, the glue holes  64  are angled less than 90 degrees from the cell registration seat in which the lens  16  is positioned, thereby causing the adhesive to pull the lens  16  onto the seat as it cures, depending on the adhesive shrinkage during cure. A plurality of adhesive tack bond holes  67  and (if desired) an additional plurality of cell clamps  66  are used to attach the lens cell  60  to the collimator body  12 . FIG. 4 shows either tack bond holes  67  and four cell clamps  66  for each cell  60 . 
     One way to align the lens  16  (and the lens cell  60 ) and the optical fiber  14  (and the shuttle plug  20 ) with respect to the collimator body  12  is by using three single axis, micron resolution stages. With the collimator body  12  held in place by an alignment fixture, one stage (z-stage  70 ) holds the focus tooling rod  50  (in the z or axial direction of the bore) while the other two stages (x-stage  72  and y-stage  74 ) orthogonally locate the lens cell  60  on the precision machined body surface or transverse plane normal to the shuttle plug bore (in the x and y directions). This adjustment aligns the optical center of the collimating lens  16  with respect to the optical fiber tip  22 . The lens cell  60  conveniently has four threaded holes  68  (two in the x-direction and two in the y-direction) for case of attachment to the linear stages. By iteratively moving the lens cell  60  in the x and y directions using the x and y stages  72 ,  74  and moving the focus tooling rod  50  in the z direction using the z stage  70 , the positions of the optical fiber  14  and the lens  16  can be obtained with proper focus and alignment to achieve the desired optical pointing and wavefront. The adjustments typically fall within about ±1.0 mm. During the alignment process, the lens cell  60  can be held against the collimator body  12  using a compression spring mechanism or other suitable temporary attachment mechanisms. After alignment, the lens cell  60  is tack bonded to the collimator body  12  via a plurality of bond holes  67 . Once the adhesive is cured, the four clamps  66  can be used to further secure the lens cell  60  in place, if desired. Of course, other ways of securing the lens cell  60  may be used in alternate embodiments. 
     After alignment of the lens cell  60 , the x-stage  72  and y-stage  74  and temporary attachment mechanism are removed and the wavefront is confirmed and adjusted if necessary via the focus stage  70  prior to bonding the shuttle plug  20  in place. As seen in FIG. 3, the focus tooling clamp  52  contains a simple wrap-around flexure provided by a necked down section that grabs the focus tooling rod  50 , preventing movement of the shuttle plug  20  during cure. Because the collimator body  12  is held in place by the alignment fixture, the entire assembly can be tested both in air and in vacuum prior to bonding the shuttle plug  20  to the body  12 . The focus tool clamp  52  also allows the collimator  10  to be removed from the test setup for tack bonding elsewhere, if desired. Once the collimator  10  is optically verified for quality and alignment of the wavefront beam after the bonding process, it is removed from the alignment fixture and the focus tooling rod  50  and clamp  52  are removed from the collimator body  12 . 
     FIG. 5 shows that the collimator body  12  includes three raised pads  80  which are precision machined to be coplanar and parallel to the center line of the shuttle plug  20 . The three raised pads  80  provide mating surfaces for coupling the collimator  10  to a mating piece, which may be an interferometry apparatus, lithography apparatus, or the like. 
     FIG. 6 illustrates attachment of the fiber optic collimator body  12  to a mating piece  84  using the raised pads  80 . The collimator body  12  includes a pressed pin  86 , an attachment screw  88 , and a washer  89  associated with two of the raised pads  80 . The attachment screw  88  is disposed through an oversized screw aperture  90  in the body  12 . The mating piece  84  includes an oversized pin hole  92  for receiving an end portion of the pin  86 , and a threaded aperture  94  for receiving the attachment screw  88 . After inserting the end portion of the pin  86  into the pin hole  92  and aligning the collimator  10  to the mating piece  84 , an epoxy or the like is back-filled to wet-pin the pin  86  to form a stable interface between the collimator body  12  and the mating piece  84  with no match drilling or adjustment mechanism to change over time. The attachment screw  88  is then tightened to further secure the connection between the two members. 
     All non-optical (optomechanical) components may be manufactured out of either a metal or a glass depending on the required thermal stability. For most applications, Invar  36  (a low thermal expansion alloy) is adequate for the collimator body  12  and shuttle plug  20  while a close matching metal is desirable for the lens  16 , Stainless steel, such as 41.6, are a close match to BK7 and other common glass materials. Ultra low expansion (ULE) glass and Zerodur typically would only be considered for the optomechanical components in cases where thermal stability is extremely tight (in the nanometer range for milli-Kelvin changes). 
     The collimator is designed to achieve an athermal structure as much as possible by selecting materials with the desired thermal properties and selecting the proper locations of attachment and coupling. For example, the piano interface of the lens cell with the collimator body is nearly in-plane or coplanar with the rear edge of the optic (i.e., the lens) within the lens cell. This allows the lens and the lens cell material to expand/contract axially due to thermal effects without affecting the alignment greatly, even though there is a mismatch in thermal properties between the collimator which is typically Invar and the lens cell which can be stainless steel. Thus, the selection of the interface location allows for a desirable athermalized effect. Another example is the use of two axial locations ( 30 ,  32 ) for attaching the fiber optic ferrule to the shuttle plug, and the use of two axial locations ( 46 ,  48 ) for attaching the shuttle plug to the collimator body. One of the axial locations ( 32 ) for attaching the fiber optic ferrule is approximately coplanar with one of the axial locations ( 46 ) for attaching the shuttle plug (see FIG.  1 ). 
     The length to diameter ratio of the glue holes or tack holes is typically equal to or greater than about 2. The glue holes or tack holes each typically include a countersunk to facilitate introduction of adhesives from different angles. 
     The collimator is stable over a specific soak temperature range and maintains alignment through adverse vibration. The collimator is configured for easy assembly and for simple and precise adjustment. The collimator may accommodate a single beam or multiple beams (e.g., two beams in the embodiment shown in FIGS.  1 - 5 ). For multiple beams, a plurality of optical fibers are connected to a single collimator body having precise collimator bores for receiving and aligning the shuttle plugs containing the optical fibers relative to the respective collimating lenses. After a first set of optical fiber and collimating lens are aligned, the other sets of optical fiber and collimating lens may be aligned with respect to the first set. This simplifies the alignment procedure and increases accuracy. 
     The above-described arrangements of apparatus and methods are merely illustrative of applications of the principles of this invention and many other embodiments and modifications may be made without departing from the spirit and scope of the invention as defined in the claims. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.