Patent Publication Number: US-9429704-B2

Title: Index matched grating inscription

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application Ser. No. 61/767,270, filed on 2013 Feb. 21, having the title “Index Matched Grating Inscription,” which is incorporated by reference in its entirety as if expressly set forth herein. 
    
    
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates generally to optical fibers and, more particularly, to optical fiber gratings inscription. 
     2. Description of Related Art 
     In optical fibers, gratings are often inscribed by exposing photosensitive regions of an optical fiber, such as the core, to actinic radiation, such as ultraviolet (UV) light. Specifically, an interference pattern is generated from the actinic radiation by, for example, using an interferogram or other known technique. 
     Due to the cylindrical shape of the optical fiber, when UV light irradiates the optical fiber from a direction that is transverse to the cylindrical axis, the curvature of the optical fiber causes focusing of the UV light. This phenomenon, known as lensing, results in undesirable intensity variations within the optical fiber. 
     Given this drawback, there is room for improvement for gratings inscription techniques. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIGS. 1A and 1B  are schematic diagrams showing a lensing phenomenon. 
         FIGS. 2A and 2B  are schematic diagrams showing scattering effects caused by a defect on the surface of an optical fiber. 
         FIGS. 3A and 3B  are schematic diagrams showing one embodiment of a system that mitigates the lensing phenomenon. 
         FIGS. 4A and 4B  are schematic diagrams showing another embodiment of a system that mitigates the lensing phenomenon. 
         FIGS. 5A and 5B  are schematic diagrams showing one embodiment of a system that mitigates the scattering caused by surface defects. 
         FIGS. 6A and 6B  are schematic diagrams showing one embodiment of a system with index-matching material. 
         FIG. 7  is a schematic diagram showing another embodiment of a system with index-matching material and a reel to reel fiber feed for making long gratings. 
         FIG. 8  is a schematic diagram showing one embodiment of an inscription system with index-matching material. 
         FIG. 9  is a schematic diagram showing another embodiment of an inscription system with index-matching material. 
         FIG. 10  is a schematic diagram showing yet another embodiment of an inscription system with index-matching material. 
         FIGS. 11A and 11B  are schematic diagrams showing one embodiment of a pulley-based system with index-matching material. 
         FIGS. 12A and 12B  are schematic diagrams showing another embodiment of a pulley-based system with index-matching material. 
         FIGS. 13A and 13B  are schematic diagrams showing yet another embodiment of a pulley-based system with index-matching material. 
         FIG. 14  is a schematic diagram showing one embodiment of a vessel with index-matching material situated within a groove. 
         FIG. 15  is a schematic diagram showing another embodiment of a vessel with index-matching material. 
         FIG. 16  is a schematic diagram showing yet another embodiment of a vessel with index-matching material. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Gratings are often inscribed onto an optical fiber by exposing photosensitive regions of the optical fiber, such as the core, to an interference pattern of actinic radiation (e.g., ultraviolet (UV) light). As shown in  FIG. 1B , the optical fiber is often cylindrical in shape. Thus, when an actinic beam irradiates the optical fiber from a direction that is transverse to the cylindrical axis, such as that shown in  FIGS. 1A and 1B , the curvature of the optical fiber causes focusing of the UV light, as shown in  FIG. 1B . This phenomenon, known as lensing, results in undesirable intensity variations within the optical fiber. For example, as shown in  FIG. 1B , when an actinic beam  24  irradiates a multi-core fiber  14 , depending on the configuration of the cores within the multi-core fiber  14 , the actinic beam  24  may irradiate some cores  18  but wholly avoid other cores  19  due to the lensing effect. As one can appreciate, lensing can be even more complicated in fibers that do not have a circular cross section. 
     The drawbacks associated with the lensing phenomenon are exacerbated when the optical fiber  14  has a surface defect  45 , such as that shown in  FIGS. 2A and 2B . As shown in  FIGS. 2A and 2B , the surface defect  45  causes the actinic beam  24  to scatter at the point of defect  45 , thereby causing additional imperfections during the inscription process. 
     Applications that are sensitive to lensing and scattering include: (a) gratings with tilted planes, where the grating planes become distorted by the lensing, so that the planes are no longer straight; (b) fibers with non-uniform surfaces, where non-uniformities include intentional diameter variations, frosting of the fiber surface through chemical etching, non-circular fibers, and coatings with imperfections; (c) fibers with internal microstructures, where microstructures exacerbate the lensing effect; (d) twisted fibers with offset cores, where inherent variations exists in inscription strength due to the variation in position of one or more offset cores; (e) tapered fibers that often have abrupt decreases in outer diameter; (f) fibers with large cores (e.g., multimode or higher-order mode (HOM) fibers), where asymmetric grating profiles can exist; and (g) multi-core fibers, both untwisted and twisted along their axes; and (h) a host of other lensing-sensitive scenarios. In such applications, the actinic beam is distorted by the non-uniformities, microstructures, or other fiber properties. This distortion results in imperfect gratings and, in some cases, no gratings at all. Thus, as one can appreciate, mitigation of lensing and scattering can vastly improve the inscription process. 
     Lai, et al., in “Micro-channels in conventional single-mode fibers,” Opt. Lett. 31, 2559-2561 (2006) (“Lai”), discloses tightly-focused beams that are directed at fibers that are immersed in oil. While the Lai scheme is appropriate for inscription techniques that rely on non-linear processes at a tight focus, the Lai scheme is not appropriate for inscription schemes that use interferograms. In particular, the Lai scheme is inappropriate for inscription schemes that: (a) employ linear, highly-photosensitive media; (b) use phase masks; or (c) are intended to write very long gratings (e.g., greater than 1000 periods), especially for gratings that are so long that the fiber is translated with respect to the interferogram by, for example, a reel-to-reel fiber-grating inscription system. Also, to the extent that the femtosecond laser pulses of Lai inscribe index changes in silica when the focus is tight, the Lai inscription process: (a) must be point-by-point; (b) is slow; and (c) requires scanning of the writing laser, thereby limiting the length of the grating. In other words, the Lai technique does not allow for significant translation of the fiber during the inscription process and, in particular, does not allow for very long displacements of the fiber (e.g., displacements more than ten (10) centimeters (cm)). Furthermore, any stray light that arises from the many reflections that come from the index-matching apparatus of Lai will have little effect on the refractive index in Lai due to their low intensities. Conversely, techniques that use interferograms and highly-photosensitive media, such as those shown in  FIGS. 3A through 16 , typically require management or elimination of stray light in order to reduce improper exposure. As such, the embodiments shown in  FIGS. 3A through 16  permit inscription of gratings that are simply unachievable by the Lai technique. 
     Putnam, et al., in “Method and apparatus for forming a Bragg grating with high intensity light,” U.S. Pat. No. 6,249,624 (“Putnam”) discloses a grating inscription approach that avoids ablations to the surface of an optical fiber with the use of dual high-intensity beams. Specifically, Putnam teaches inscription without the use of a phase mask, relying on interference patterns that are created at the intersection of two high-intensity beams. To the extent that surface-ablation problems arise only in the context of high-intensity beams, integrating a phase mask (or any other type of interferometer) into Putnam&#39;s index-matching interface medium would defeat the principle of operation for Putnam&#39;s dual-high-intensity-beam configuration. In other words, one skilled in the art will normally avoid integrating a phase mask with Putnam&#39;s dual-beam interferometer. This is because a phase mask that produces the same interference pattern as the dual-beam would nullify the interference pattern created by Putnam&#39;s dual-beam configuration. Alternatively, if one uses a phase mask that creates a different interference pattern than Putnam&#39;s dual-beam configuration, then the resulting interference pattern would be a complex convolution of the dual-beam interference and the phase-mask interference. As such, one employing Putnam&#39;s dual-beam configuration would not use a phase mask. 
     Unlike Lai or Putnam, the disclosed embodiments provide systems and methods for mitigating lensing and scattering by surrounding an optical fiber with an index-matching material, where the disclosed configurations permit the use of masks or interferometers that are integrated into vessels that hold the index-matching material. In other words, unlike Putnam, where the interference pattern is generated external to the vessel, the disclosed interferometers are integrated within the vessel itself. 
     Also unlike Lai and Putnam the disclosed embodiments allow for long lengths of fiber to be pulled through the index-matching material without any detriment from degradation or depletion of the index-matching material as the inscription process unfolds. Furthermore, the embodiments shown herein reduce the possibility of introducing imperfections into the fiber and/or its coating as the fiber is pulled into the index-matching material or after the fiber exits the index-matching material. 
     The breadth of the inventive concept can only be appreciated with a proper understanding of refractive index principles. Specifically, the lensing phenomenon occurs because light refracts at the interface of two media when the two media have different indices of refraction. This is because light propagates through different media at different speeds, and the speed of propagation is dependent on each medium&#39;s index of refraction. Thus, given the curvature of the fiber and the difference in refractive indices between the air and the fiber, the actinic beam that irradiates a fiber will refract at the air-fiber interface, thereby resulting in the lensing phenomenon. 
     Also, the degree of refraction is dependent on the degree of difference between the refractive indices of the two interfacing media. Thus, if the indices are substantially different, then the degree of refraction is greater. Conversely, if the indices are substantially similar, then there is less refraction. As such, to the extent that the refractive indices of two interfacing materials can be perfectly matched, refraction can be wholly eliminated. 
     The phrase “index-matching material” can mean something different based on context. In other words, index-matching materials may differ based on a desired tolerance for refraction in a particular application. For example, for applications that require absolutely no refraction, index-matching materials will be those that have precisely the same index of refraction. Conversely, for applications that can tolerate some degree of refraction, the index-matching materials will simply be those that reduce the refraction below that tolerance level. It is also possible for the fiber to have a coating whose index is not the same as the fiber cladding or core. In this case the index-matching material may be matching to for example, the coating, the cladding, the average of the two, or some other desired combination of these indices. In general the refractive index of the index-matching material should be set so that the intensity variation inside the fiber is less than it would be if the fiber were surrounded by air. When this is the case we refer to the material as an index-matching material or as substantially index matched or simply index matched to the fiber. Examples of different index-matching materials are provided below with reference to  FIGS. 3A through 16 . Suffice it to say that one having skill in the art will appreciate the degree of tolerance needed for a particular application. As such, this disclosure expressly defines “index-matching material” to be one that reduces an index difference to the extent necessary for its corresponding application. 
     With this understanding of refractive indices, the disclosed systems and methods mitigate the lensing phenomenon by surrounding an optical fiber with an index-matching material. The index-matching material has a refractive index that is sufficient to reduce intensity variations in the actinic radiation within the optical fiber. Depending on the acceptable tolerance, the reduction in intensity variations may range from nominal to substantial. The index-matching material is held in a vessel with an integrated interferometer. The vessels can be configured in any manner, so long as the vessels permit the optical fiber to be surrounded by the index-matching material while the gratings are being written to the optical fiber and have a mask (e.g., phase mask, amplitude mask, etc.) integrated into the vessel itself. 
     Turning from this coarse description, reference is now made to  FIGS. 3A through 16 , which provide a finer description of various embodiments for mitigating lensing and scattering. It should be appreciated that, while several embodiments are described in connection with these drawings, there is no intent to limit the disclosure to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents. 
       FIGS. 3A and 3B  are schematic diagrams showing one embodiment of a system that mitigates the lensing phenomenon. Specifically,  FIG. 3A  shows a side view of a vessel  22  holding an index-matching material  20   a , while  FIG. 3B  shows a cross-sectional view of the vessel  22 . For clarity, the direction of the actinic beam is different between  FIG. 3A  and  FIG. 3B . However, it should be appreciated that the actinic beam can be introduced at any angle that is non-parallel to the direction of the fiber. 
     As shown in  FIG. 3A , the vessel  22  that holds the index-matching material  20   a  comprises a passage for an optical fiber  14 , which is shown here as a multi-core fiber  14 . This passage permits the optical fiber  14  to be surrounded by the index-matching material  20   a  when the optical fiber  14  is situated in the passage. Therefore, any actinic radiation  24  that passes through the optical fiber  14  necessarily passes through the index-matching material  20   a . To the extent that the index-matching material  20   a  has the same index of refraction as the optical fiber  14 , the actinic radiation  24  experiences no refraction at the boundary of the optical fiber  14  and the index-matching material  20   a . For the multi-core fiber  14  of  FIG. 3B , the index-matching material  20   a  causes the actinic radiation  24  to pass through the cross-section of the multi-core fiber  14  without lensing, thereby uniformly irradiating all of the cores within the multi-core fiber  14 . 
       FIGS. 4A and 4B  are schematic diagrams showing another embodiment of a system that mitigates the lensing phenomenon. Unlike  FIGS. 3A and 3B , which show an index-matching material  20   a  that precisely matches the index of the optical fiber  14  and therefore eliminates refraction at the boundary, the embodiments of  FIGS. 4A and 4B  show an index-matching material  20   b  that substantially matches the refractive index of the optical fiber  14 , thereby only reducing refraction at the boundary, rather than wholly eliminating the refraction. Thus, in the embodiment of  FIGS. 4A and 4B , the cores do not experience uniform irradiation. However, unlike the fiber of  FIGS. 1A and 1B , where several cores  19  wholly avoid irradiation due to lensing, the cores in  FIG. 4B  are irradiated by the actinic beam  24  due to the reduced difference in the refractive indices. As one can appreciate, if the goal is to simply irradiate all of the cores, then the index-matching material need not perfectly match the refractive index of the optical fiber, with only a sufficient reduction in refraction being needed to accomplish the irradiation of all of the cores. 
       FIGS. 5A and 5B  are schematic diagrams showing one embodiment of a system that mitigates the scattering caused by surface defects  45 , such as those shown in  FIGS. 2A and 2B . In the embodiment of  FIGS. 5A and 5B , the index-matching material  20   c  is a liquid, such as water, oil, or other suitable liquid that has a refractive index that substantially matches the index of the optical fiber or the coating of the fiber if it has a coating. As such, when the optical fiber is surrounded by the liquid index-matching material  20   c , the liquid index-matching material  20   c  fills the surface defect, thereby ameliorating the scattering that may be caused by the defect  45 . The degree of scatter mitigation is dependent on the degree to which the refractive indices match. Thus, a precise match in the refractive indices can result in elimination of scatter due to surface defects, while a substantial match results in reduction of scatter. 
       FIGS. 6A and 6B  are schematic diagrams showing one embodiment of a system where a vessel  62  holds a liquid index-matching material  20   c  within the vessel  62  via capillary action. Specifically,  FIG. 6A  shows a side view, while  FIG. 6B  shows a cross-sectional view. As shown in  FIG. 6A , the vessel  62  comprises a front plate  64   a  and a back plate  64   b , which for some embodiments are quartz plates. The two plates  64   a ,  64   b  (collectively,  64 ) are separated by a gap that is sufficiently small to allow the liquid index-matching material  20   c  to be maintained within the gap via capillary forces. As shown in  FIG. 6B , the gap between the two plates  64  is maintained by a spacer  66 . 
       FIG. 7  is a schematic diagram showing another embodiment of an index-matching system, which permits a reel-to-reel fiber-feed system. As discussed with reference to  FIGS. 6A and 6B , if the liquid index-matching material  20   c  is maintained within the vessel  62  via capillary action, then the optical fiber  14  can be directed through the gap using a reel-to-reel fiber-feed system  78 . This type of reel-to-reel system permits inscription of multiple gratings as the optical fiber  14  passes through the gap. As one can imagine, the system of  FIG. 7  is susceptible to depletion of liquid index-matching material  20   c  from the vessel  62  if the liquid index-matching material  20   c  adheres to the optical fiber  14  as the optical fiber  14  passes through the gap. One way to mitigate the depletion of liquid index-matching material  20   c  is by replenishing the liquid index-matching material  20   c , as shown with reference to  FIG. 15 . 
       FIG. 15  is a schematic diagram showing a cross-section of one embodiment of a vessel  1502  that permits replenishment of liquid index-matching material  1560  as an optical fiber  1516  passes through the vessel  1502 . Specifically, the vessel  1502  is similar to a coating vessel by Lindholm in “Systems and methods for coating optical fiber,” U.S. Pat. No. 6,958,096. Thus, one will appreciate that the replenishment system can be implemented on a draw tower, such that the draw tower incorporates the vessel holding the liquid index-matching material. For such embodiments, it should be understood that the liquid index-matching material has at least two properties, namely, being curable to form a coating and being able to match an index of refraction. As shown in  FIG. 15 , the vessel  1502  comprises an open cup  1530  that is positioned over a chamber  1532 . It will be seen that the upper portion of the cup  1530  forms an upper vessel opening  1514 . The cup  1530  and chamber  1532  are connected to each other by an entrance aperture  1534 . At the bottom of the chamber  1532 , opposite the entrance aperture  1534 , there is provided an exit aperture  1536 . 
     The cup  1530 , entrance aperture  1534 , chamber  1532 , and exit aperture  1536  together define a liquid pathway  1538  along which a fiber  1516  to be surrounded by the liquid travels into, through, and out of the vessel  1502 . As illustrated by arrow  1554 , liquid index-matching material  1560  is pumped into the chamber  1532  through the input port  1522 . A suitable input fitting  1550 , such as a nipple, has been mounted into port  1522 . The entrance aperture  1534  is dimensioned so that when a fiber  1516  travels down the liquid pathway  1538 , there is sufficient clearance at the entrance aperture  1534  around the fiber  1516  to allow liquid index-matching material  1560  to flow upward into the cup  1530 . As illustrated by arrow  1562 , excess liquid index-matching material  1560  drains out of the cup  1530  through the drain port  1524 . As shown in  FIG. 15 , a suitable output fitting  1552  is mounted into the output port  1524 . The excess liquid index-matching material  1560  may be recirculated for pumping back into the input port  1522 . 
     The entrance aperture  1534  is implemented using an entrance die assembly  1540  that is mounted into a first opening  1542  formed in the vessel  1502  between the cup  1530  and the chamber  1532 . The exit aperture  1536  is implemented using a shaping die assembly  1544  that is mounted into a second opening  1546  at the bottom of the chamber  1532  leading to the exterior of the vessel  1502 . It should be noted that it would also be possible to form the entrance and exit apertures  1534 ,  1536  directly into the vessel  1502  without the use of die assemblies  1540 ,  1544 . However, die assemblies  1540 ,  1544  are useful for a number of reasons. First, they provide flexibility, as they allow different sizes of apertures  1534 ,  1536  to be used, as desired. In addition, using removable die assemblies provides access to the interior of the vessel  1502 , which facilitates cleaning or other maintenance operations. 
     As illustrated by arrow  1556 , a certain amount of liquid index-matching material  1560  flows downward, out through exit aperture  1536 , and around fiber  1516 , where it forms a liquid  1520 . Arrows  1558  illustrate the counter-flow of liquid index-matching material  1560  up through entrance aperture  1534 , and around fiber  1516 , into cup  1530 . The fill level of the cup  1530  may vary, depending upon a number of parameters, including the dimensions of the various elements of the vessel  1502 , the viscosity of the liquid index-matching material  1560  used, and the pressure at which the liquid index-matching material  1560  is introduced into the chamber  1532 . 
     To expose the optical fiber  1516  to an actinic beam  24 , the vessel  1502  comprises a beam conduit  1596  having a phase mask  1598  at the inward end of the conduit  1596 . Thus, as the optical fiber  1516  travels through the vessel  1502 , that optical fiber  1516  can be inscribed by the incoming actinic beam  24  from which an interferogram is generated by the phase mask  1598 . It should be appreciated that for embodiments with a replenishment system, such as that shown in  FIG. 15 , the interference pattern can be generated using mechanisms other than a phase mask. For instance, the beam conduit could accommodate two beams travelling with an angle between them and forming an interferogram inside the vessel. 
     The liquid index-matching material  1560  contained in the chamber  1532  has a predetermined induced pressure above atmospheric pressure. The appropriate pressurization of the chamber  1532  is accomplished by choosing a diameter for the entrance aperture  1534  such that when liquid index-matching material  1560  is pumped into the chamber  1532 , there is sufficient resistance to flow at the entrance aperture  1534  to allow a desired hydrostatic pressure to build up within the chamber  1532 . It should be noted that, although the liquid index-matching material  1560  contained in the chamber  1532  is pressurized, it has been found that turbulence in the liquid index-matching material  1560  in the cup  1530  does not exceed manageable levels. 
     It should be noted that a relatively large entrance aperture  1534  may be used, and thus with a relatively small increase in the hydrostatic pressure of the liquid index-matching material  1560  contained in the chamber  1532 . A large entrance aperture  1534  may be desirable to allow the fiber  1516  to pass freely through the aperture  1534 , to minimize turbulence in the liquid index-matching material  1560  and to avoid any centering issues that may arise in connection with a smaller entrance aperture  1534 . 
     By replenishing the liquid index-matching material  1560  as the optical fiber  1516  passes through the vessel  1502 , one can make sure that the optical fiber  1516  is surrounded by the liquid index-matching material  1560  during grating inscription. Furthermore, by using the vessel  1502  of  FIG. 15 , it may be possible to inscribe gratings as the optical fiber  1516  is being drawn. 
     We note that a replenishment system similar to that of  FIG. 15  may be used in any of the other embodiments of this invention. 
     Another replenishment system suitable for vessels holding index matching liquid by capillary action employs a syringe or dropper to introduce index matching liquid into the vessel. Excess index matching liquid may exit the vessel by gravity. 
     Another approach to ameliorating the depletion of liquid index-matching material  20   c  is shown with reference to  FIG. 16 . Specifically,  FIG. 16  comprises an upright vessel  1602  with four side walls  1604  and a bottom  1610 , which together form a container for liquid index-matching material  20   c . The bottom  1610  comprises phase mask grooves  1612 , and the side walls  1604  comprise an entrance hole  1606  and an exit hole  1608  through which an optical fiber  14  passes. In the embodiment of  FIG. 16 , actinic radiation  24  is introduced through the phase mask grooves  1612  on the bottom  1610 . The diffracting beams after the mask are not shown for clarity. The liquid index-matching material  20   c  can be replenished by simply filling the vessel  1602  when the liquid level drops below a predefined threshold. If the vessel also has a top wall  1620 , then the liquid may be held in place by the vacuum force above the liquid. For embodiments with a top wall, one may also devise a mechanism  1630 ,  1640  to introduce or replenish liquid index-matching material into the vessel. Note that although  FIG. 16  shows the phase mask sitting horizontally, it is also possible for the phase mask to sit vertically. As in  FIGS. 8 through 10  the groves may face in or out of the vessel and may have a cover slip to maintain their index variation. 
       FIG. 8  is a schematic diagram showing one embodiment of a vessel  82  with an integrated phase mask  81   a . As shown in  FIG. 8 , the vessel  82  comprises a back plate  83  and a phase mask  81   a , which are separated from each other by a gap  85  that holds liquid index-matching material  20   c , preferably by capillary action. In some embodiments, both the phase mask  81   a  and the back plate  83  have beveled edges  86  that facilitate movement of the optical fiber  14  within the gap  85  without substantial damage to the optical fiber  14  or its coating if one is present. In the embodiment of  FIG. 8 , the phase mask  81   a  comprises teeth (or grooves)  87  that face away from the gap  85 . Preferably, the phase mask  81   a  should be thin so that: (a) the teeth  87  on the phase mask  81   a  are close to the optical fiber  14 ; (b) there is minimal attenuation or distortion due to the thickness of the phase mask  81   a ; (c) loss of beam coherence is minimized; and (d) the length of fiber that is exposed by only one rather than both the plus-1 and minus-1 orders of the phase mask is minimized. As such, in some embodiments, the thickness of the phase mask  81   a  is between approximately 0.1 mm and approximately 5 mm. 
     One way of bringing the teeth  87  closer to the optical fiber  14  is by facing the teeth  87  toward the gap  85 , as shown in the embodiment of  FIG. 9 . Specifically,  FIG. 9  is a schematic diagram showing an embodiment of a vessel  92  with an integrated phase mask  81   b , where the teeth of the phase mask  81   b  face toward the gap  85  and are in contact with the liquid index-matching material  20   c . By facing the teeth toward the gap  85  ( FIG. 9 ), as opposed to away from the gap  85  ( FIG. 8 ), the distance between the teeth and the optical fiber  14  is further reduced. However, one can appreciate that in order for the configuration of  FIG. 9  to function properly, there should be an index difference between the phase mask  81   b  and the liquid index-matching material  20   c.    
       FIG. 10  shows a schematic diagram with an embodiment of a vessel  1002  with an integrated phase mask  81   c , where the teeth of the phase mask  81   c  face toward the gap  85 , but where the teeth are isolated from the liquid index-matching material  20   c  by a thin UV-transparent cover plate  1004 . As shown in  FIG. 10 , the liquid index-matching material  20   c  is situated between the cover plate  1004  and the back plate. The phase mask  81   c  may be slightly offset to prevent accidental leakage of the liquid index-matching material  20   c  into the teeth of the phase mask  81   c . Additionally, for some embodiments, an antireflective coating may be applied to the cover plate  1004 . Moreover, the back plate or the front plate (or both) may be angled in such a way to direct back reflections away from the fiber. 
     Turning to the phase mask itself, it is also possible to bond the cover plate to the phase mask thus sealing the grooves (or teeth) entirely. By sealing the grooves, the sealed phase mask can then be immersed in a liquid index-matching material and still maintain its ability to generate an interferogram because the liquid index-matching material is no longer able to fill the grooves of the phase mask. 
     In an alternative embodiment, the sealed phase mask can be manufactured by forming cavities or other refractive index modulations beneath the surface of the plate. These cavities or other refractive index modulations can be formed using, e.g., femtosecond IR laser pulses that can penetrate the surface of the plate and affect predefined regions within the plate. Such methods result in the surfaces of the phase plate remaining largely undisturbed while the index non-uniformities (e.g., grooves, voids, or other index variations) of the phase mask are generated below the surface of the phase plate, thereby creating a sealed phase mask. In general, the sealed phase mask will generate an interferogram from an input beam even when placed in a material that has the same refractive index as the outer surface of the sealed phase mask. 
       FIGS. 11A and 11B  are schematic diagrams showing one embodiment of a pulley-based system with index-matching material. Specifically,  FIG. 11A  shows a side view of a vessel  1102  with actinic radiation  24  entering from the top of the vessel  1102 , while  FIG. 11B  shows a front view of the vessel  1102  with actinic radiation  24  being introduced from the side of the vessel  1102 . In the embodiments of  FIGS. 11A and 11B , liquid index-matching material  20   c  is held within the vessel  1102 , and an optical fiber  14  passes through the vessel  1102  via a pulley system  30   a ,  30   b  (collectively,  30 ). Note that the purpose of the pulley system is simply to change the direction of the fiber axis so that it may be fed into the vessel. As one skilled in the art will appreciate, different embodiments may employ different numbers of pulleys. For other embodiments, rods or clamps may be used in lieu of pulleys. For the embodiment of  FIGS. 11A and 11B , an interferogram-generating mechanism  1150  is located external to the vessel  1102 . Thus, as the optical fiber  14  passes through the vessel  1102 , an interferogram generated by the actinic radiation  24  inscribes gratings on the fiber. 
       FIGS. 12A and 12B  are schematic diagrams showing another embodiment of a pulley-based system with index-matching material. Unlike the embodiment of  FIGS. 11A and 11B , the embodiment of  FIGS. 12A and 12B  comprise a phase mask  1240  that is integrated into the vessel  1202  by situating the phase mask  1240  within the vessel  1202 , thereby bringing the phase mask  1240  closer to the optical fiber  14  that is being inscribed. To the extent that the pulleys  30  are described with reference to  FIGS. 11A and 11B , further discussion of the pulley system is omitted here. For embodiments in which the phase mask  1240  is submerged, it is preferable to use a sealed phase mask, as described above. Again, the sealed phase mask may be fabricated by adhering a cover plate above the grooves or, alternatively, by creating the grooves or other index modulations below the surface of a plate. 
       FIGS. 13A and 13B  are schematic diagrams showing yet another embodiment of a pulley-based system with index-matching material. Unlike the embodiments of  FIGS. 11A, 11B, 12A, and 12B , the embodiment of  FIG. 13A  shows a phase mask  1350  that forms a part of the vessel wall. By directly integrating the phase mask  1350  into the vessel  1302 , the embodiment of  FIG. 13A  functions similar to the embodiments of  FIGS. 8 through 10 . To the extent that the inscription mechanism has been described with reference to  FIGS. 8 through 10 , and to the extent that the pulley system has been described with reference to  FIGS. 11A, 11B, 12A, and 12B , further discussion of those mechanisms is omitted with reference to  FIG. 13A . 
       FIG. 13B  shows a side view of the system in  FIG. 13A , but with the actinic beam being introduced from the side. As one can appreciate, for the embodiment of  FIG. 13B , the phase mask  1350  should be integrated into the side wall, rather than on the top panel. 
       FIG. 14  is a schematic diagram showing a cross-sectional view of one embodiment of a vessel  1402  with index-matching material  124  situated within a groove  122  in the vessel  1402 . By matching the refractive index of the vessel  1402  and the index-matching material  124  to the refractive index of the optical fiber  14 , one can judiciously reduce or remove reflection or refraction at the boundary between the optical fiber  14  and the index-matching material  124 , and at the boundary between the index-matching material  124  and the vessel  1402 . 
     We also note that while  FIGS. 3 through 16  show actinic beams passing through interferometers, it is also possible to pass the actinic beam through an amplitude mask. In such a mask the actinic beam is blocked for certain portions of the mask in order to form a spatial modulation in the beam. This spatial modulation may imprint gratings in the fiber core or cores just as the interferogram from a phase mask may inscribe gratings. For instance if the period of the grating is very long compared to the actinic beam wavelength an amplitude mask may be used to imprint a long period grating in a fiber core. Thus the period of the actinic radiation may be less than one micron, while the period of the long period grating (LPG) may be larger than 100 microns. In such a case an amplitude mask may be more effective for grating inscription, however the methods taught herein for reducing the intensity variations inside the fiber will still be an improvement to previous methods of imprinting LPGs. 
     It is also possible that a nonuniform or slowly varying exposure is desired in the core or cores of a fiber. Such slowly varying or nonuniform exposures can for instance be used to apodize a Bragg grating or otherwise modify the propagation of light in a given core. In this case as well it will also be useful to reduce the intensity variation across the fiber cross section especially when there is an offset core, a very large core, twisted cores or in general more than one core. 
     At this point, it is worthwhile to note that the index-matching material should preferably have a refractive index that reduces the distortion of the combined coating and the fiber interfaces by the largest amount. Alternatively, the index-matching material should reduce the overall distortion to something less than the distortion caused by the fiber being surrounded by air. In practice, this means that the rays representing the actinic beam are as close to parallel as possible or, similarly, that the intensity profile of the beam is as uniform as possible throughout the fiber. Thus, the intensity variation (across the cross-section of the fiber) should be less than it would be without the index-matching material. Alternatively, the variation of the intensity contrast of the actinic interferogram should be less than it would be without the index-matching material. 
     In practice, if the refractive index of a coating is different from the fiber cladding, then one may choose an index-matching material with a refractive index equal to the average of the two refractive indices of the coating and the cladding. Alternatively, one may choose an index-matching material with a refractive index that is equal to the refractive index of the fiber cladding. In yet another alternative, the index-matching material may be chosen to match either the coating or the cladding, depending on which of the two is thicker, or depending on which of the two causes a greater refraction of the actinic beam. 
     We also note that it is possible for some controlled amount of the liquid index-matching material to remain on the fiber after the vessel. In such a case, and depending on the material selected for the liquid index-matching material, the liquid index-matching material may be further cured into a fiber coating. For such a vessel where the liquid index-matching material slowly depletes from the vessel as it is coated onto the fiber, it would be desirable to replenish the index-matching material within the vessel. 
     It should also be noted that in several of these embodiments, it may be desirable to provide a process to clean the fiber either before or after the vessel or both. In a case where the fiber leaves the vessel with index-matching material, this material can be cleaned using, for example, a pair (or series) of absorbent pads with solvent. In other embodiments, the optical fiber can be pulled through a vessel of cleaning material (e.g., solvent, etc.), and possibly ultrasonic or thermal treatment, much like the vessel that holds the index-matching material. 
     Although exemplary embodiments have been shown and described, it will be clear to those of ordinary skill in the art that a number of changes, modifications, or alterations to the disclosure as described may be made. For example, while a phase mask is shown as a particular embodiment of an interferometer, it should be appreciated that other interference-generating mechanisms that can be integrated into the vessel are contemplated within the term interferometer. Additionally, it should be appreciated that grating inscription can be accomplished by a variety of mechanisms, such as, for example, by using direct write systems, amplitude mask systems, ultra-short-pulse lasers, etc. That is to say, the phase mask appearing in the varying embodiments can be replaced by an amplitude mask. Furthermore, while liquids, such as water, are disclosed for the index-matching material, it should be appreciated that the index-matching material need not be limited to water, or even liquids, and may include gels or other solids. Thus, it should be appreciated that any material can be used for the index-matching material, as long as that material sufficiently reduces refraction at the boundaries of two media. Moreover, the liquid index-matching material may be a material that wets the surface of the fiber, which may be useful for surrounding any defects in the surface of the fiber. Alternatively, the liquid index-matching material may be a material that does not wet the surface at all, thereby allowing the fiber to be pulled through the index-matching material without leaving the index-matching material on the surface of the optical fiber. Additionally, while UV radiation is recited as one form of actinic radiation, it should be appreciated that, depending on the material, actinic radiation can be any type of radiation that causes a change in the material. Thus, depending on the material, actinic radiation can encompass infrared light or even visible light. All such changes, modifications, and alterations should therefore be seen as within the scope of the disclosure.