Abstract:
Methods for stripping an optical fiber coating using blue or blue-violet radiation are disclosed. The method includes irradiating a portion of the coating with at least one radiation beam having a processing wavelength in the range of 400 nm to 460 nm for which the coating is substantially transparent. The intensity of the radiation beam exceeds the optical-damage threshold of the coating, and thereby a damaged coating portion that absorbs radiation at the processing wavelength is formed. The damaged coating portion is then irradiated with the radiation beam having an intensity below the optical-damage threshold to cause the damaged coating portion to absorb the radiation and to subsequently heat up and disintegrate to expose a section of the central glass portion of the optical fiber.

Description:
PRIORITY APPLICATION 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 62/076,503, filed on Nov. 7, 2014, the content of which is relied upon and incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    The present disclosure relates to optical fibers, and in particular to methods for stripping the coating of an optical fiber using a blue or blue-violet radiation. 
       BACKGROUND 
       [0003]    Optical fibers are a type of optical waveguide that include a core, a cladding that surrounds the core, and a protective coating that surrounds the cladding. The protective coating is typically made of a polymer (e.g., a UV-cured acrylate), while the core and cladding are typically made of glass. 
         [0004]    Optical fibers are used in a variety of applications that require terminating an end of the fiber, e.g., with an optical fiber connector. This requires among other things stripping away a portion of the protective coating to leave a bare portion of the optical fiber. 
         [0005]    One method of stripping the coating from an optical fiber uses a tool that mechanically contacts the optical fiber. While mechanical stripping tools can be made compact for field deployment and can be low cost, they tend to damage and weaken the resulting bare fiber section. 
         [0006]    To avoid such damage, non-contact stripping techniques have been developed. These include hot-nitrogen-jet, plasma, and laser-based techniques. Laser-based stripping techniques have the advantage of providing precise coating removal without mechanical damage. One type of laser-based stripping is performed using a CO 2  laser, wherein the coating absorbs the infrared wavelength of the CO 2  laser beam. The absorption causes the coating to heat up and then disintegrate. Unfortunately, the equipment for laser-based stripping tools has to date not been amenable to field deployment because of limitations in cost, size, weight and power consumption. 
       SUMMARY 
       [0007]    An aspect of the disclosure is a method of stripping a coating from an optical fiber, wherein the coating has an optical-damage threshold and surrounds a central glass portion of the optical fiber. The method includes: a) irradiating a portion of the coating with at least one beam of radiation having a wavelength at which the coating is substantially transparent and an intensity that exceeds the optical-damage threshold of the coating to form a damaged coating portion that absorbs radiation at the wavelength of the at least one beam of radiation; and b) irradiating the damaged coating portion with the at least one beam of radiation to cause the damaged coating to absorb a portion of the radiation and to subsequently heat up and disintegrate to expose a section of the central glass portion. 
         [0008]    Another aspect of the disclosure is a method of processing an optical fiber having a central glass portion defined by a core and a cladding and having a coating surrounding the glass portion that is substantially transparent to a processing wavelength and that has an optical-damage threshold intensity at the processing wavelength. The method includes: a) irradiating a portion of the coating with at least one first radiation beam having the processing wavelength, wherein the processing wavelength is either 405 nm or 445 nm, and having an intensity greater than the optical-damage threshold intensity to create a damaged coating portion; and b) irradiating the damaged coating portion with at least one second radiation beam having the processing wavelength of either 405 nm or 445 nm and an intensity that is less than the optical-damage threshold intensity. 
         [0009]    Another aspect of the disclosure is the method as described above, including forming the at least one second radiation beam from the at least one first radiation beam by defocusing the at least one first radiation beam. 
         [0010]    Another aspect of the disclosure is a method of processing an optical fiber. The method includes: a) movably holding the optical fiber, wherein the optical fiber has a central glass portion defined by a core and a cladding and has a coating surrounding the glass portion and having a diameter, the coating being substantially transparent to a processing wavelength and having an optical-damage threshold intensity at the processing wavelength; b) forming at least one first focused radiation beam having the processing wavelength and having a depth of focus, a spot size that is less than the diameter of the coating within the depth of focus, and an intensity greater than the optical-damage threshold intensity within at least a portion of the depth of focus; c) arranging the optical fiber within the depth of focus and irradiating a portion of the coating with the at least one focused radiation beam to create a damaged coating portion; d) defocusing the at least one focused radiation beam to define at least one defocused radiation beam having an intensity that is less than the optical-damage threshold intensity; and e) irradiating the damaged coating portion with the at least one defocused radiation beam that irradiates the entire diameter of the coating. 
         [0011]    Additional features and advantages are set forth in the Detailed Description that follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings. It is to be understood that both the foregoing general description and the following Detailed Description are merely exemplary and are intended to provide an overview or framework to understand the nature and character of the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments and together with the Detailed Description serve to explain principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following Detailed Description, taken in conjunction with the accompanying Figures, in which: 
           [0013]      FIG. 1  is a front elevated view of an example optical fiber to be processed using the systems and methods disclosed herein, illustrating the arrangement of the core, cladding and coating; 
           [0014]      FIG. 2A  is a schematic diagram of an example optical system used to carry out the non-contact stripping methods for stripping a portion of the coating disclosed herein; 
           [0015]      FIG. 2B  is similar to  FIG. 2A  and further includes the optical fiber being processed while the optical system is in a focused configuration; 
           [0016]      FIG. 2C  shows an example housing for the optical system and shows length, width and height dimensions, which in an example are selected such that the optical system is compact, e.g., small enough to be handheld; 
           [0017]      FIG. 3  is a close-up cross-sectional view of the optical fiber illustrating how collimated radiation is concentrated within the coating layer and brought to a loose focus just beyond the back side of the outer surface of the coating; 
           [0018]      FIG. 4  is similar to  FIG. 2B  and shows the optical fiber being processed while the optical system is in a defocused configuration; 
           [0019]      FIG. 5  is similar to  FIG. 1  and shows a portion of the coating stripped from the optical fiber and the resulting stripped-fiber section or bare-fiber section; 
           [0020]      FIGS. 6A through 6D  are schematic diagrams that illustrate example optical system configurations for performing the non-contact stripping methods disclosed herein; 
           [0021]      FIG. 7  is similar to  FIG. 4  and illustrates an embodiment of the optical system that includes a radiation-recycling optical system adjacent the back side of the optical fiber surface and configured to receive radiation that passes through or around the coating and direct it back to the coating; 
           [0022]      FIG. 8A  is a front elevated view of an example optical fiber holder used to hold the optical fiber and also to rotate and translate the optical fiber; 
           [0023]      FIG. 8B  is similar to  FIG. 8A  and illustrates an example embodiment where the optical fiber holder is configured to hold the optical fiber under tension; 
           [0024]      FIG. 9  is similar to  FIG. 4  and illustrates an example embodiment wherein the optical system includes a polarizer to define a p-polarized radiation beam; 
           [0025]      FIGS. 10A and 10B  are schematic diagrams of an example optical system that includes a scanning element for scanning the radiation beam along the length of the optical fiber as well as laterally across the optical fiber. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure. 
         [0027]    Cartesian coordinates are shown in some of the Figures for the sake of reference and are not intended to be limiting as to direction or orientation. 
         [0028]      FIG. 1  is a front elevated view of an example optical fiber  10  to be processed using the systems and methods disclosed herein. The optical fiber  10  has a centerline  12 , a core region (“core”)  20  arranged along the centerline, an annular cladding region (“cladding”)  30  that surrounds the core, and an annular coating  50  that surrounds the cladding and defines an outer surface  52  of the optical fiber. The core  20  and cladding  30  are typically made of glass while coating  50  is typically made of a polymer, such as acrylate. In an example, coating  50  is generally transparent to visible radiation, down to wavelengths of about 200 nm. The coating  50  has an optical-damage threshold, which is also referred to herein as an optical-damage threshold intensity since the optical-damage threshold can be measured in units of intensity (i.e., optical power per unit area). 
         [0029]    The optical fiber  10  has a diameter dF that is a measure of the outside diameter of coating  50 . The diameter dF may be, for example, nominally 250 μm, or 0.25 mm. The optical fiber  10  can have any reasonable diameter dF consistent with single-mode or multimode operation. 
         [0030]    The core  20  and cladding  30  define a central glass portion  40  of optical fiber  10 , with coating  50  surrounding the central glass portion. The central glass portion  40  without coating  50  is referred to herein as a “bare section” or “stripped section”  42  of optical fiber  10  (see, e.g.,  FIG. 5 ). 
         [0031]      FIG. 2A  is a schematic diagram of an example optical system  100  used to carry out the non-contact stripping methods disclosed herein. The optical system  100  has an optical axis A 1 , which runs in the z-direction. The optical system  100  includes a radiation source  110  along optical axis A 1  that emits radiation  112  at a divergence angle θ measured relative to the optical axis. The radiation  112  has a wavelength λ, which is referred to herein as the “processing wavelength.” In an example, processing wavelength λ is in the wavelength range from 380 nm to 490 nm and in another example is in the wavelength range from 400 nm to 460 nm. 
         [0032]    In another example, the processing wavelength λ is in the range from 300 nm to 1100 nm. In an example, radiation source  110  comprises a diode laser that emits radiation  112  at a processing wavelength within the 300 nm to 1100 nm range. 
         [0033]    In an example, coating  50  is substantially transparent to radiation  112  having processing wavelength λ. Here, the term “substantially transparent” with respect to coating  50  and processing wavelength λ means that the amount of absorption in the coating at the processing wavelength is not sufficient to heat the (undamaged) coating to the point where the coating is removed from central glass portion  40 . 
         [0034]    In an example, radiation source  110  includes or consists of a 405-nm blue-violet diode laser (i.e., λ=405 nm), which is widely used in Blu-ray Disc™ players. Such diode lasers operate in single mode and thus emit a Gaussian beam having excellent beam quality. Another example radiation source  110  includes or consists of a blue diode laser that emits radiation at a nominal processing wavelength λ=445 nm. In an example, radiation source  110  is operably attached to a portable power supply  116 , which in an example is or includes one or more batteries. 
         [0035]    Generally speaking, radiation source  110  can be any radiation source that emits radiation  112  at a nominal processing wavelength λ of either 405 nm or 445 nm, and that provides sufficient optical power to carry out the optical fiber stripping methods disclosed herein while allowing optical system  100  to be sufficiently compact to be readily deployed and used in the field. 
         [0036]    The optical system  100  also includes a focusing optical system  120  arranged along optical axis A 1  optically downstream from radiation source  110 . The focusing optical system  120  includes one or more optical elements  122  configured to receive radiation  112  from radiation source  110  and form therefrom a radiation beam  112 B. In an example, radiation beam  112 B has a focus position  124  along axis A 1  and an associated convergence angle θ′ measured relative to optical axis A 1 . The one or more optical elements  122  can include lenses, mirrors, apertures, gratings, fibers, beamsplitters, filters, attenuators, polarizers, etc. In an example, the one or more optical elements  122  consist of a single refractive element, such as an aspheric lens element or aspheric mirror element. In an example, the single refractive element includes an antireflection coating. An example material for a single aspheric refractive element is ECO-550 glass, which can be molded into a suitable aspheric shape. In an example, at least one of the one or more optical elements  122  of focusing optical system  120  is movable (e.g., axially movable, as indicated by arrow AR) to adjust focus position  124  and convergence angle θ′. 
         [0037]    The optical system  100  of  FIG. 2A  shows a single optical element  122  by way of example, wherein the single optical element resides a distance d 1  away from radiation source  110  and a distance d 2  from focus position  124 . The radiation beam  112 B constitutes a focused radiation beam  112 BF that forms a focus spot  124 S in the x-y plane at focus position  124 , the focus spot having a diameter dS at focus position  124 . In an example, diameter dS of focus spot  124 S is in the range from 5 μm to 20 μm. In an example, focus spot  124 S resides within a depth of focus Δz associated with focused radiation beam  112 BF, where focus position  124  is at the center of the depth of focus. The diameter dS of focus spot  124 S can be well approximated by the size of the aberration-free airy disk, i.e., dS≈1.22λ/NA, where NA is the numerical aperture and is given by NA=n·sin θ′, where n=1 for air in most instances. For a diameter dS of 20 μm and a processing wavelength λ=405 nm, the numerical aperture NA≈0.025. The clear aperture CA is the diameter of focusing optical system  120  (e.g., of optical element  122 ) and in an example is in the range 2 mm≦CA≦12 mm. 
         [0038]      FIG. 2B  is similar to  FIG. 2A , and includes optical fiber  10  arranged substantially at focus position  124  (e.g., within the depth of focus Δz), with centerline  12  of the optical fiber arranged along the x-direction. Because optical fiber  10  has a relatively small diameter (e.g., 250 microns or 0.25 mm), the optical fiber acts as a strong cylindrical lens.  FIG. 3  is a close-up cross-sectional view of optical fiber  10  showing collimated radiation rays  112  (or “radiation beam  112 ” or simply “radiation  112 ”) incident upon the optical fiber on a front side  52 F of outer surface  52 . The radiation rays  112  are focused by the curved outer surface  52  and are concentrated within coating  50  and brought to a loose focus  126  just beyond a back side  52 B of the outer surface. 
         [0039]    For a convergent or focused radiation beam  112 BF such as is shown in  FIG. 2B , radiation rays  112  are even more concentrated, and loose focus  126  can be formed within coating  50 . The greater the concentration of radiation rays  112 , the greater the intensity of radiation. In an example, the radiation intensity in radiation beam  112 B at back side  52 B of optical fiber  10  is about three times greater than that at front side  52 F. In an example, the intensity of focused radiation beam  112 BF over at least a portion of the depth of focus Az exceeds the optical-damage threshold intensity for coating  50 . 
         [0040]    In an example illustrated in  FIG. 2C , optical system  100  is contained within an interior  105  of a housing  104 . In an example, housing  104  is sized to be compact, e.g., small enough to be handheld. The housing  104  is shown by way of example as being generally rectangular and having a length L, a width W, a height H and a volume V=L×W×H. Example dimensions for housing  104  are 50 mm≦L≦150 mm; 30 mm≦W≦75 mm; and 12 mm≦H≦25 mm. An example range of the volume V of housing  104  is 15 cm 3 ≦V≦60 cm 3 . 
         [0041]    A blue-violet diode laser has an output power of about 700 mW and an operating wavelength of λ=405 nm. It is noted here that coating  50  is transparent to this wavelength of radiation. However, as discussed above, coating  50  has an optical-damage threshold (which can be expressed in units of optical intensity) that is typically no greater than about 2 MW/cm 2  at 405 nm or 455 nm. An intensity that exceeds this damage threshold can be generated by optical system  100  by making the focus spot diameter dS (see  FIG. 1 ) about 20 microns. Moreover, nonlinear effects in coating  50  can increase the absorption and raise the temperature of the coating, and reduce the optical-damage threshold intensity of the coating. 
         [0042]    Thus, with continuing reference to  FIG. 2B , radiation beam  112 B is formed as a focused radiation beam  112 BF such that radiation  112  that irradiates coating  50  of optical fiber  10  has an intensity that exceeds the optical-damage threshold of the coating. This results in localized damage of coating  50 , i.e., a damaged coating portion  50 D. The configuration of optical system  100  shown in  FIG. 2B  wherein radiation beam  112 B forms a focused radiation beam  112 BF having the focus position  124 , which is within or very close to coating  50  is referred to as the “focused configuration.” In an example, the focused configuration is defined by optical fiber  10  being within the depth of focus Δz, with focused radiation beam  112 BF having an intensity that exceeds the optical-damage threshold of coating  50  over at least a portion of the depth of focus, and in one example over the entire depth of focus. 
         [0043]    Once damaged coating portion  50 D is formed, then with reference to  FIG. 4 , optical system  100  can be adjusted to have a “defocused configuration,” wherein radiation beam  112 B constitutes a defocused radiation beam  112 BD that is less concentrated than focused radiation beam  112 BF so that it irradiates the entire diameter dF of optical fiber  10 . In an example, this adjustment of optical system  100  is accomplished by changing the distance d 1  between radiation source  110  and focusing optical system  120 . This change in distance d 1  can be accomplished by axially moving focusing optical system  120 , or at least one of the one or more optical elements  112  therein, or by moving radiation source  110 . In an example, the defocused configuration is defined by optical fiber  10  no longer being within the depth of focus Δz. Also in an example, the intensity of defocused radiation beam  112 BD at the location of optical fiber  10  is less than the optical-damage threshold intensity of coating  50 . 
         [0044]    The damaged coating portion  50 D has an increased absorption at the operating wavelength λ of radiation  112 . This causes a rapid increase in temperature upon continued irradiation, which leads to the burning and decomposition of the material of coating  50 . In an example, focused laser beam  112 BF of the focused configuration is used to form damaged coating portion  50 D of a select size, and then defocused laser beam  112 BD of the defocused configuration is scanned over the damaged coating portion to remove it from central glass portion  40  of optical fiber  10 . This exposes a bare section or stripped section  42 , as shown in  FIG. 5 . In an example, multiple scan passes of radiation beam  112 B (in either the focused or defocused form  112 BF or  112 BD) can be used to remove any residual material of coating  50 . 
         [0045]    In an example, the stripping process can include adjusting optical system  100  back and forth between the focused and defocused configurations, and moving radiation beam  112 B for each of these configurations relative to coating  50  as needed to carry out the stripping operation. 
         [0046]    In another example, optical system  100  is operated only in the defocused configuration with a longer dwell time of defocused radiation beam  112 BD on coating  50  to define damaged coating portion  50 D, after which scanning of the (defocused) radiation beam relative to optical fiber  10  to remove the damaged portion of the coating is initiated. 
         [0047]      FIGS. 6A and 6B  illustrate example embodiments wherein coating  50  of optical fiber  10  is irradiated by multiple (e.g., two) optical systems  100 , denoted  100   a  and  100   b . In  FIG. 6A , the two optical systems  100   a  and  100   b  are shown in the defocused configuration. In  FIG. 6B , optical system  100   a  is in the defocused configuration, while optical system  100   b  is in the focused configuration. In the example of  FIG. 6B , optical system  100   b  is used to provide the higher intensity of radiation  112  at coating  50  to more quickly form damaged coating portion  50 D in the coating, while optical system  100   a  provides the lower intensity but greater spatial coverage to remove the damaged coating portion.  FIG. 6C  is similar to  FIG. 6B  and shows the two optical systems  100   a  and  100   b  having respective optical axes A 1   a  and A 1   b , which are arranged so that optical fiber  10  is irradiated by the respective radiation beams  112 Ba and  112 Bb at a right angle to each other. 
         [0048]    In other embodiments, the two or more optical systems  100  can also be arranged so that their respective optical axes define convenient irradiation angles.  FIG. 6D  illustrates an example that utilizes three optical systems  100 , denoted  100   a ,  100   b  and  100   c , with the three optical systems having respective optical axes A 1   a , A 1   b , A 1   c  and respective radiation beams  112 Ba,  112 Bb and  112 Bc. The optical axes A 1   a , A 1   b  and A 1   c  are arranged about 120 degrees apart and intersect substantially at optical fiber  10 . In other embodiments using multiple optical systems  100 , the angular separations of the optical systems need not be the same. 
         [0049]    In an example, one or more of the multiple optical systems  100  can operated so that focused radiation beam  112 BF is scanned over a portion of optical fiber  10  as discussed above to define damaged coating portion  50 D and then to define the exposed central glass portion  40  of the optical fiber as bare section or stripped section  42 , as shown in  FIG. 5 . 
         [0050]      FIG. 7  illustrates another example that shows a single optical system  100  as described above and that further includes a radiation-recycling optical system  130  that includes one or more optical elements  132 . The radiation-recycling optical system  130  is arranged on the opposite side of optical fiber  10  to focusing optical system  120 , i.e., adjacent back side  52 B of outer surface  52  of coating  50 . The radiation-recycling optical system  130  is configured to receive radiation  112  from radiation beam  112 B that passes through or around optical fiber  10  and direct the radiation back to coating  50 . 
         [0051]    In an example, the one or more optical elements  132  of radiation-recycling optical system  130  includes a converging mirror, such as a curved mirror or a retro-reflecting mirror (e.g., a retro-reflecting cube). Configurations that employ radiation-recycling optical system  130  make efficient use of radiation  112  emitted from radiation source  110 . In an example, at least one of the optical systems  100  in a multi-optical-system configuration uses radiation-recycling optical system  130 , while in another example, each of the optical systems in a multi-optical-system configuration includes its own radiation-recycling optical system. 
         [0052]      FIG. 8A  illustrates an embodiment wherein optical fiber  10  is held by an optical fiber holder  180 . In an example, the optical fiber holder  180  is configured to rotate optical fiber  10  about its centerline  12 , and can also optionally axially translate the optical fiber, i.e., along the optical fiber centerline. This allows for radiation beam  112 B to irradiate different portions of coating  50  about the circumference of outer surface  52  for a given x-position, as well as to irradiate different portions of the coating along the length of the optical fiber (i.e., the x-direction).  FIG. 8B  is similar to  FIG. 8A  and illustrates an example wherein optical fiber  10  is held at both ends by optical fiber holder  180  to provide a degree of axial tension. 
         [0053]    In an example, coating  50  is processed in a clean condition. In another example, coating  50  can be provided with at least one material that decreases the damage threshold of the coating or that otherwise facilitates the removal of the coating when the coating is irradiated by radiation beam  112 B. In an example, the material provided to coating  50  can be what is normally considered a contaminant, such as oil, dirt, debris, etc., that increases the absorption of radiation  112 . In an example, the contaminants can be provided simply by way of a person touching coating  50  with his or her hands. 
         [0054]    In an example embodiment illustrated in  FIG. 9 , one of the optical elements  122  of focusing optical system  120  is or includes a polarizer  122 P that defines a polarization p for radiation beam  112 B. In an example, polarization p is the p-polarization, which is perpendicular to the length (x-direction) of optical fiber  10  to optimize transmission of radiation  112  into coating  50 . 
         [0055]    In another example illustrated in  FIGS. 10A and 10B , one of the optical elements  122  of focusing optical system  120  is or includes a scanning element  122 S that allows for radiation beam  112 B to scan over coating  50 .  FIG. 10B  shows an example scanning element  122 S in the form of a scanning mirror  122 SM that is configured to direct radiation beam  112 B in the x-direction, i.e., to scan the radiation beam along a select length of optical fiber  10  (e.g., by rotation of the scanning element about the y-axis). The scanning element  122 S can also be optionally scanned in the y-direction (e.g., by rotation of the scanning element about the x-axis) in the case wherein radiation beam  112 B is a tightly focused radiation beam  112 BF and does not cover the entire diameter of outer surface  52  of coating  50  in the lateral direction (i.e., the y-direction). The embodiment of optical system  100  of  FIGS. 10A and 10B  can be used in combination with moving optical fiber  10 , i.e., performing at least one of axial translation and a rotation of the optical fiber using, for example, optical fiber holders  180  of  FIGS. 8A and 8B . 
         [0056]    It will be apparent to those skilled in the art that various modifications to the preferred embodiments of the disclosure as described herein can be made without departing from the spirit or scope of the disclosure as defined in the appended claims. Thus, the disclosure covers the modifications and variations provided they come within the scope of the appended claims and the equivalents thereto.