Patent Publication Number: US-2018044234-A1

Title: Fiber Bragg Gratings in Carbon-Coated Optical Fibers and Techniques for Making Same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of co-pending U.S. patent application Ser. No. 15/144,563, filed on May 2, 2016. 
     U.S. patent application Ser. No. 15/144,563 is a divisional of U.S. patent application Ser. No. 14/169,541, filed on Jan. 31, 2014, which issued as U.S. Pat. No. 9,353,001 on May 31, 2016. 
     U.S. patent application Ser. No. 14/169,541claims the priority benefit of U.S. Provisional Patent Application Serial No. 61/784,347, filed on Mar. 14, 2013, now expired. 
     All of the above applications are owned by the assignee of the present application, and are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to the field of fiber optics, and in particular to fiber gratings and like optical devices for use in harsh environments. 
     Background Art 
     The demand for optical fibers and fiber-based devices has dramatically increased over the past decade. However, in humid or hydrogen-rich environments, fiber-based devices, such as sensors or other devices employing fiber Bragg gratings (FBGs), typically require shielding in order to address mechanical robustness and reliability issues, as well as issues relating to optical functionality. The reliability and optical performance of these devices are of primary importance to designers of optical components and systems. 
     There is thus an ongoing need in the art for FBGs and other fiber-based devices that are capable of reliable performance in harsh environments. 
     SUMMARY OF INVENTION 
     An aspect of the invention is directed to a method for fabricating gratings or like optical devices in a carbon-coated optical fiber. A photosensitive optical fiber is provided having a hermetic carbon coating. Further provided is a laser having a beam output that is configured to inscribe one or more refractive index modulations into the optical fiber through the hermetic carbon layer while leaving the hermetic carbon layer intact. The laser is used to inscribe one or more optical devices into the optical fiber through the hermetic carbon layer. 
     Further aspects of the invention are directed to structures and techniques for mass production of carbon-coated optical devices. In one practice of the invention, optical devices are written into a carbon-coated optical fiber in a post-secondary-coating process. In another practice of the invention, optical devices are written into a fiber on the draw tower during the optical fiber drawing process, subsequent to the application of a carbon coating onto the fiber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  show diagrams, respectively, of a test optical fiber and a grating inscription setup used in an experimental demonstration of the invention. 
         FIGS. 3-6  shows a series of reflection spectra illustrating the results of tests conducted using the fiber and the grating inscription setup shown in  FIGS. 1 and 2 . 
         FIG. 7A  shows an isometric view of a carbon-coated optical fiber configured for use in a post-secondary-coating inscription technique, in accordance with an aspect of the invention. 
         FIG. 7B  shows a cross section of the carbon-coated optical fiber shown in  FIG. 7A  through the plane  7 B- 7 B. 
         FIG. 8  shows a schematic diagram of an automated grating writing system for use with the fiber shown in  FIGS. 7A and 7B . 
         FIG. 9  is a schematic diagram of an automated system according to an aspect of the invention for inscribing gratings into a carbon-coated optical fiber as part of the draw process. 
         FIG. 10  shows a flowchart of a general technique according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the invention are directed to optical devices, such as fiber Bragg gratings (FBGs) and the like, that are fabricated in an optical fiber having a hermetic carbon coating. The structures and techniques described herein can be used, for example, to fabricate FBGs for use as sensors in a harsh environment. In addition, the described structures and techniques can be used to fabricate FBGs displaying improved fatigue resistance. 
     As used herein, the term “harsh environment” generally refers to an environment in which a hermetic carbon coating is useful in protecting an optical device. Such an environment includes, for example: a high-humidity environment, a hydrogen-rich environment; an environment that is both high-humidity and hydrogen-rich; a high-water-content or aqueous environment, such as liquid water; or the like. Generally speaking, hermeticity to water improves mechanical reliability in a harsh environment, and hermeticity to hydrogen improves optical reliability. 
     As used herein, the terms “hermetic carbon coating” and “carbon coating” refer to a coating applied to a fiber surface so as to hermetically seal the portion of the fiber contained within the carbon coating. The carbon coating serves a number of purposes, including, for example: providing the glass fiber surface with a seal against water molecules, including condensed water, water vapor at a wide range of temperatures, and the like; protecting against ingress of hydrogen or deuterium to ˜130° C. or higher, depending on the coating structure; and extending the expected life of a given fiber in a given environment by retarding fatigue and by reducing stress corrosion failures. 
     In one carbon-coating technique, a carbon layer is applied onto the outer surface of an optical fiber in a draw tower as part of the drawing process. The draw tower is equipped with a carbon reactor, through which optical fiber travels after it has been drawn from a preform. Within the carbon reactor, a chemical vapor deposition (CVD) technique is used to deposit a hermetic layer of amorphous carbon at a temperature of 1200° C. onto the outer surface of the optical fiber. A typical thickness for the carbon coating is ˜0.02 μm to ˜0.08 μm. 
     The CVD technique is implemented by causing reactant gases to flow through the carbon reactor in the absence of oxygen. A pyrolytic reaction occurs at the glass surface of the drawn fiber, resulting in thermal deposition of the carbon onto the fiber surface. A minimum thickness of deposited carbon is required in order for the carbon coating to be hermetic. Typically, a polymer coating is then applied over the carbon layer. 
     A carbon-coating technique is described in greater detail, for example, in Lindholm et al., “Low Speed Carbon Deposition Process for Hermetic Optical Fibers,” International Wire and Cable Symposium (1999). 
     For a number of reasons, carbon coatings have not been used in connection with FBGs and like optical devices. First, gratings may not be inscribed into a fiber prior to the application of a carbon coating because the high temperatures required to apply the carbon coating may destroy the gratings. Second, previously, it has generally been believed that gratings cannot be inscribed into a fiber subsequent to the application of a carbon coating, because of the carbon coating&#39;s lack of transparency with respect to a UV laser beam. 
     The present invention is based on the insight that, if a fiber has a sufficiently high degree of photosensitivity, and if the inscribing laser is configured to operate within a suitable power range, it is possible to apply a hermetic carbon coating onto the fiber&#39;s cladding and then subsequently inscribe gratings into the fiber through the carbon coating without breaching the hermetic seal. 
     Generally speaking, the inscription of gratings into carbon-coated optical fibers can employ any of the configurations and techniques that are used for inscribing gratings into optical fibers without carbon coatings. Thus, gratings can be inscribed into a carbon-coated optical fiber using a holographic technique, a phase-mask technique, or the like. Depending upon a particular situation, some adjustments or modifications may be necessary. 
     As discussed below, the inscription of gratings into a carbon-coated optical fiber can be performed as part of the initial draw process or in a separate, post-secondary-coating process. 
     Photosensitivity 
     As noted above, in order for gratings to be written into a fiber through a carbon coating, the fiber must have a suitable degree of photosensitivity. The degree of photosensitivity required in a given situation depends upon a number of factors, including (1) the strength of the gratings to be inscribed and (2) the interaction between the inscribing laser beam and the carbon coating through which the gratings are to be written. 
     According to an aspect of the invention, a suitable degree of photosensitivity in a given fiber is achieved through the use of one or more dopants that are known to create photosensitivity. These dopants include, for example, germanium, as well as fluorine, boron, or the like. Depending upon a particular application, it may be desirable to employ one or more co-dopants in one or more fiber regions in order to arrive at a fiber design having a desired degree of photosensitivity as well as a particular refractive index profile or other desired property. 
     A suitable fiber for a given application of the present invention may be provided in any of a number of different ways. For example, in certain situations it may be possible to employ an already existing fiber design, if the fiber has a suitable degree of photosensitivity. Alternatively, an already existing fiber design may be modified by making suitable adjustments to its photosensitivity. In some situations, it may be necessary or desirable to custom design a suitable optical fiber. The design of such an optical fiber will be understood adequately by a practitioner in the art to include, e.g.: core, trench, clad dimensions and refractive index, as well as the possibility of incorporating other structures, such as multiple cores, polarization-maintaining stress rods, a star-shaped or octagonal cladding, a rectangular core, one or more coatings of various types, and the like. 
     According to a further aspect of the invention, hydrogen loading or deuterium loading may be used to enhance the photosensitivity of the carbon-coated fiber, and thus the reflectivity (i.e., strength) of gratings written into the fiber. As mentioned above, a carbon coating protects against hydrogen ingression to ˜130° C. or higher, depending on the coating structure. Thus, it is possible to load a carbon-coated optical fiber with hydrogen or deuterium, prior to inscription, by placing the fiber in a pressurized chamber containing hydrogen or deuterium at a suitable concentration and maintaining a fiber temperature above ˜130° C. for a suitable amount of time. The parameters for such a conditioning schedule (e.g., time, temperature, and pressure) are dependent upon the required photosensitivity of the fiber, the hermeticity of the carbon coating, and the capability of the secondary coating to survive this exposure. 
     Gratings are subsequently inscribed into the fiber in accordance with the techniques described herein. According to a further aspect of the invention, once the gratings have been inscribed into the fiber, they are then annealed at a temperature of greater than ˜130° C. to allow the excess loaded hydrogen or deuterium to escape from the fiber. 
     Experimental Confirmation 
     Aspects of the invention have been confirmed in a series of experiments, in which gratings were successfully inscribed into a carbon-coated test fiber. The successful outcome of these experiments has led to the development of a number of manufacturing techniques, discussed below, that can be used for mass production of carbon-coated gratings. 
       FIGS. 1 and 2  show diagrams, not drawn to scale, of the test fiber  10  and an inscription station  20 . The test fiber  10  was a single-core fiber comprising a core region  12 , a cladding region of 80 μm in diameter surrounding the core  14 , a hermetic carbon layer  16 , and a protective polyimide coating  18  surrounding the carbon layer  16 . The fiber core region  12  was doped with germanium at a concentration resulting in a relatively high degree of photosensitivity and a numerical aperture of 0.21. The carbon coating  14  was applied during the draw process and had a thickness of approximately 0.029 μm. The polyimide coating  18  was also applied during the draw process, subsequent to the application of the carbon layer, and had a thickness of approximately 15 μm. 
     It is noted that, depending upon a particular application, a carbon coating having a thickness as high as 0.08 μm, or thicker, may be considered. 
     The fiber  10  was loaded into the inscription station  20 , which included means (not shown) for holding and positioning the fiber relative to the output of laser  22  and beam delivery optics  24 . Prior to inscription, the polyimide coating  18  was stripped from the fiber, leaving the carbon coating  16  intact. 
     In the  FIG. 2  inscription station  20 , the laser  22  is implemented using an excimer UV laser manufactured by TuiLaser, which provided a laser beam having a wavelength of 248 nm. A phase mask  26  was used to generate a periodic interference pattern from the UV laser beam, which was used to inscribe a grating  28  into fiber core  12 , or fiber core  12  and part of cladding  14 , depending on the fiber design. It will be appreciated that the present invention can be practiced with other suitable types of lasers, depending upon a given application. 
     It is noted that there is a relatively wide range of wavelengths at which a carbon coating is sufficiently transparent to allow gratings to be written therethrough. These wavelengths include 193 nm, 244 nm, and 248 nm. Longer wavelengths may be employed, such as when using a “cold-writing” technique, a femtosecond laser, or other approaches. In the tests, a number of gratings were inscribed into the carbon-coated test fiber using different inscription parameters. The performance of the inscribed gratings was measured using a setup having a broadband source, a 3-port circulator and an optical spectrum analyzer (OSA).  FIGS. 3-5  show a series of reflection spectra illustrating the test results. Among other things, the reflection spectra confirm that it is possible to inscribe gratings into a carbon-coated optical fiber. In addition, the reflection spectra confirm that, generally speaking, stronger FBGs can be written by longer exposure to the inscribing laser light and/or increasing the concentration of germanium (or other suitable dopant or combination of dopants) in the fiber into which the grating is written. 
       FIG. 3  shows a reflection spectrum  30  generated by an 8 mm grating inscribed into a length of the test fiber, as described above. The TuiLaser excimer laser was used to provide a 40 Hz pulsed laser beam having a wavelength of 248 nm. The laser energy density in the inscription region was ˜25 mJ/cm 2 . The exposure time was 60 seconds, after which time the grating did not change significantly. The laser energy density was chosen because at ˜25 mJ/cm 2  the carbon coating showed no visual damage when inspected under a microscope. Experiments have shown that a power density level of 80 mJ/cm 2  will burn or damage the carbon coating. The exact upper limits for the inscription parameters have not yet been determined, but are expected to be between 25 mJ/cm 2  and 80 mJ/cm 2  for the 248 nm TuiLaser excimer laser. For other wavelengths or types of lasers, the laser energy density ranges may be different. 
     As shown in  FIG. 3 , the 8 mm grating reflection spectrum reaches a peak of ˜−11 dB, which corresponds to a peak reflectivity of ˜8%. 
       FIG. 4  shows the reflection spectrum  40  of an FBG written into a carbon-coated optical fiber by a single laser pulse (i.e., a “one-pulse” inscription process). The setup used was exactly the same as the one used to inscribe the FBG in  FIG. 3 . The one-pulse FBG achieved a peak reflectivity of ˜0.29% (˜−25.5 dB). This result is significant because it implies that the carbon-coated fiber gratings could be fabricated by using a draw tower grating writing technique. 
     The grating illustrated in  FIG. 3  is the stronger of the two, and is suitable for use, for example, in a wavelength division multiplexing (WDM) based interrogator. The weaker one-pulse grating illustrated in  FIG. 4  is suitable for use, for example, in a weak grating interrogator based on time division multiplexing/wavelength division multiplexing (TDM/WDM) techniques. In a TDM/WDM system, the reflectivity of individual FBGs can be as low as −35 dB (i.e., a reflectivity of ˜0.03%). 
       FIG. 5  shows the reflection spectra  50  of a series of four 8 mm gratings written into the carbon-coated test fiber using the same testing setup, but with a different number of laser pulses (one pulse  51 , two pulses  52 , three pulses  53 , and five pulses  54 ). 
     A further experiment was conducted to determine whether the inscription of gratings in the test fiber resulted in damage to the carbon coating not visually observed under a microscope. 
     Six FBGs similar to the one shown in  FIG. 3  were inscribed into the test fiber. Their wavelengths were measured at room temperature after annealing for 64 hours at 85° C. The six FBGs were then placed into a deuterium exposure chamber having a pressure of 4,400 psi pressure and a temperature of 50° C. After 20 days, the FBGs were removed from the chamber and their wavelengths were measured again at room temperature. Deuterium gas was chosen because the setup for creating the above deuterium environment was readily available. It is expected that the same results should be obtained by replacing deuterium gas with hydrogen gas. 
     Compared to the wavelengths before the loading, the wavelength shifts from these FBGs ranged from −12 pm to 0 pm with an average wavelength shift of −7.2 pm. Given a variation in room temperature of ˜0.5° C. and the relatively small wavelength shifts, it can be concluded that deuterium did not enter into the fiber core with those loading conditions. It can also be concluded that the grating writing did not damage the carbon coating. 
       FIG. 6  shows a reflection spectrum  60  for a grating with a length of 10 mm that was written into another carbon-coated fiber, in which the carbon coating thickness was ˜0.056 μm, the cladding diameter was 125 μm, and the numerical aperture was 0.16. The laser conditions were the same as those in the tests of the gratings inscribed into carbon-coated optical fibers, in which the carbon coating had a thickness of ˜0.029 μm. In reflection spectrum  60 , the 0.6% peak reflection  61  is referenced to the cleaved fiber end reflection of 4%. Compared to the results obtained from the 0.029 μm carbon coated fiber, this lower reflectivity is believed to be mainly due to both the less photosensitive fiber and the thicker carbon coating. To verify this, gratings were also written in a fiber with the same glass structure as that of the 0.056 μm thick carbon coated fiber, but a 0.024 μm thick carbon coating. The results showed that 1.7% peak reflectivity was obtained for a 10 mm grating. It is believed that with higher germanium doping in the fiber core and extra photosensitization of the fiber, much higher grating peak reflectivity can be obtained even in the thick carbon coated fiber. 
     Mass Production of Carbon-Coated Optical Fiber Gratings 
     The above encouraging results have given rise to a number of possible scenarios for mass-producing FBGs in carbon-coated optical fibers. In one approach, FBGs are inscribed into a carbon-coated optical fiber in a post-secondary-coating process. In a second approach, FBGs are inscribed into a fiber as part of the draw process, after the application of a carbon coating. Each of these approaches is described in turn. 
     Post-Secondary-Coating Inscription 
     As used herein, the term “post-secondary-coating inscription” refers to an inscription process that is performed subsequent to the application of one or more secondary coatings over a carbon coating layer. A “post-secondary-coating inscription” technique can be performed in a draw tower as part of an optical fiber drawing process, or can be performed in an inscription station after a fiber has been removed from a draw tower. 
       FIG. 7A  shows an isometric view of a carbon-coated optical fiber  70  that is configured for use in a post-secondary-coating inscription technique, in accordance with an aspect of the invention.  FIG. 7B  shows a cross section of the carbon-coated optical fiber shown in  FIG. 7A  through the plane  7 B- 7 B. 
     Fiber  70  comprises a core region  72 , a cladding region  74 , and a hermetic carbon layer  76 . Surrounding the hermetic carbon layer is a protective “write-through” coating  78  that is substantially transparent to ultraviolet light. Such a coating is described in U.S. Pat. No. 5,620,495, which is owned by the assignee of the present application, which is incorporated herein by reference in its entirety. As disclosed therein, the outer coating comprises a polymer with low UV absorption. Such a coating may comprise, for example, a polymer selected from the group consisting of acrylates, aliphatic polyacrylates, silesesquioxanes, alkyl-substituted silicones and vinyl ethers. As used herein, the terms “acrylates” and “aliphatic polyacrylates” include methacrylate, poly(meth)acrylate, urethane acrylate, and the like. 
       FIG. 8  shows a schematic diagram of an automated FBG writing system  80  for use with fiber  70 . System  80  comprises a programmable controller unit  81 , an automated fiber winding system  82 , an automated fiber clamping system  83 , and a UV laser  84 . The beam delivery optics  85  can be based on phase mask or holographic techniques. Fiber  70  is loaded into the automated fiber winding system  82 , and gratings are written into the fiber  70  in a series of inscription cycles. In each cycle, the clamping system  83  clamps a portion of fiber  70  in position relative to the output of UV laser  84 , as shaped by the beam delivery optics  85 . 
     An FBG array fabricated using fiber  70  and grating inscription system  80  maintains pristine fiber strength. Further, as discussed above, since the fiber is carbon-coated, it can be configured for use in humid, hydrogen-rich, or other harsh environments, such as those described above. The “post-secondary-coating” approach allows for greater degrees of manipulation to achieve strict optical spectrum requirements not normally achievable with draw-tower grating writing. 
     Pre-Secondary-Coating Inscription 
     As used herein, the term “pre-secondary-coating inscription” refers to an inscription process that is performed subsequent to the application of a carbon coating layer, but prior to the application of a secondary coating over the carbon coating layer. 
     In an exemplary practice of a pre-secondary-coating inscription technique according to an aspect of the invention, gratings are automatically inscribed into a carbon-coated optical fiber in a draw tower as part of the draw process.  FIG. 9  is a schematic diagram of an automated system  90  according to this aspect of the invention. 
     System  90  comprises a draw tower  91  having a furnace  92  at its top that is configured to receive a fiber preform from which a fiber  93  is drawn. The drawn fiber descends through the following stages: a carbon coating applicator  94 , in which a hermetic carbon coating is applied to the fiber  93 ; a grating inscription stage  95 , in which gratings are inscribed into the fiber; a secondary coating die  96 , in which an outer coating is applied over the carbon coating; and a fiber winding system  97  for collecting the finished fiber and winding it onto a bulk reel. 
     Grating inscription stage  95  comprises a UV laser  951  and laser beam delivery optics  952  that are configured to deliver an inscription laser beam  953  to a mask  954  and its mounting fixtures. Of course, other grating inscription techniques, such as a holographic technique, can also be implemented here. The amount of time required to inscribe each grating is sufficiently short, such that it is possible to inscribe gratings into fiber  93  without having to stop the fiber as it descends through the draw tower  91 . 
     Because the outer coating is applied subsequent to the inscription of gratings into fiber  93 , it is not necessary for the outer coating to be a write-through coating. Thus, the described approach provides flexibility with respect to the outer coatings that can be applied by the secondary coating die  96 . Any of a number of different outer coatings may be used, including, for example, acrylate, polyimide, vinyl ether, silicone, silsesquioxane, epoxy, and even metal. 
     General Technique 
       FIG. 10  is a flowchart illustrating a general technique  100  according to the above-described aspects of the invention. 
     Technique  100  comprises the following steps: 
       101 : Provide a photosensitive optical fiber having a cladding onto which has been applied a hermetic carbon coating. 
       102 : Provide a laser having a beam output that is configured to inscribe one or more refractive index modulations into the optical fiber through the hermetic carbon layer while leaving the hermetic carbon layer intact. 
       103 : Use the laser to inscribe one or more optical devices into the optical fiber through the hermetic carbon layer. 
     Conclusion 
     Fiber Bragg gratings fabricated in carbon-coated fibers in accordance with the present invention have a number of advantages, including: (1) resisting hydrogen ingression below a certain temperature; (2) increasing long-term reliability when the fiber is under stress and in a humid environment. 
     In both post-secondary-coating and pre-secondary-coating inscription of FBGs in carbon-coated optical fibers, another advantage is that the FBG-inscribed carbon-coated optical fiber will have almost the same breaking strength as that of the pristine carbon-coated optical fiber (˜550 kpsi), i.e., prior to inscription of the FBG therein. 
     While the foregoing description includes details that will enable those skilled in the art to practice the invention, it should be recognized that the description is illustrative in nature and that many modifications and variations thereof will be apparent to those skilled in the art having the benefit of these teachings. It is accordingly intended that the invention herein be defined solely by the claims appended hereto and that the claims be interpreted as broadly as permitted by the prior art.