Abstract:
A cladding portion (15) of an optical fiber (16) is laser machined by focusing a laser beam (13) that is of an appropriate wavelength to ablate the cladding. When the laser beam completely penetrates through the cladding (15) to impinge on the optical fiber core (18) light is transmitted to the two ends of the fiber. A photodetector (31) is placed in close proximity to one of the ends of the optical fiber (16) with the photodetector output being connected to a laser control device (23). When the light detected by the detector exceeds a threshold, it generates a signal that stops the laser. Even if the cladding is of an unpredicted thickness, the laser beam is not terminated until there has been complete penetration through the cladding, and after complete ablation the laser beam is promptly terminated so as to avoid subsequent damage to the optical fiber.

Description:
TECHNICAL FIELD 
     This invention relates to the fabrication of optical fiber couplers and, more particularly, to methods for controllably removing a cladding portion of an optical fiber. 
     BACKGROUND OF THE INVENTION 
     Primarily because of their low transition loss, optical fibers, which comprise a cylindrical glass core surrounded by a cladding layer, have become the dominant medium for transmitting information in the form of light. In time, it is foreseen that optical fibers will be used for transmitting light over relatively short distances much as metal conductors presently transmit electrically in electronic circuits. As such time, there will be a great need for inexpensive optical couplers that can controllably remove light from, or apply light to, an optical fiber. 
     An important step in the fabrication of such optical couplers or optical fiber taps is the controlled removal of part of the cladding to expose part of the core of the optical fiber. The size, shape, position, and cleanliness of the cladding removal is critical in making the coupler; it is also important that the optical fiber core not be damaged mechanically or optically during the cladding removal and that the fabrication method be reasonably convenient to perform. Various mechanical stripping methods and chemical etching methods have been proposed. These methods are suitable for laboratory purposes, but have not proven amenable to mass production because of the difficulty of obtaining a suitably high yield, that is, the difficulty of obtaining a high proportion of usable devices while avoiding serious damage to the optical fiber. 
     The copending application of Coyle et al., Ser. No. 454,603, filed Dec. 21, 1989, hereby incorporated by reference herein, describes a method for using an excimer laser to remove a cladding portion of an optical fiber by ablation while minimizing damage to the core. The laser produces a beam of ultraviolet light that is absorbed by the cladding but transmitted efficiently by the core so that damage to the core is minimized. A drawback of the invention is that the laser beam is transmitted transversely through the optical fiber core and impinges on the cladding on the side of the optical fiber opposite the laser. It is desired that cladding be removed from only one well defined area, and destruction of the cladding on the opposite side can be quite harmful since it may lead to spurious leakage of optical energy. The Coyle et al. application solves this problem by including on the side of the optical fiber opposite the opening an assembly for containing repair media that can be used to repair such damage. This requirement, of course, further complicates the fabrication of the coupler. 
     It has been recognized that it would be preferable to control the laser machining of the cladding so as to avoid serious damage to the cladding intended to be left intact. The cladding on optical fibers is not always uniform, and so the Coyle et al. technique can sometimes cause more damage than predicted. In sum, there has been a long-felt need for a method to strip controllably a cladding portion of an optical fiber in a manner that is amenable to mass production, is highly reliable, which does not require greater operator skill, and which does not depend on optical fiber uniformity, particularly of the cladding thickness. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the invention, a cladding portion of an optical fiber is laser machined by focusing a laser beam that is of an appropriate wavelength to ablate the cladding in the general manner described in the Coyle et al. application. We have found that when the laser beam completely penetrates through the cladding to impinge upon the optical fiber core, light is transmitted to the two ends of the fiber. Thus, a photodetector is placed in close proximity to one of the ends of the optical fiber with the photodetector output being connected to a laser control device. When the light detected by the detector exceeds a threshold, it generates a signal that stops the laser. Thus, even if the cladding is of an unpredicted thickness, the laser beam is not terminated until there has been complete penetration through the cladding, and after complete ablation the laser beam is promptly terminated so as to avoid subsequent damage to the optical fiber. 
     The laser is preferably an excimer laser that generates light in the form of pules. After penetration of the cladding, light is detected by the detector in the form of pulses. To assure complete removal of all of the cladding intended to be removed, one may delay termination of the laser beam until after a predetermined number of pules have been detected. The initial pulses that are detected are of a lower intensity with the intensity increasing thereafter. Complete machining can alternatively be obtained by terminating the machining only after a predetermined threshold of light intensity has been detected. 
     These and other objects, features and benefits of the invention will be better understood from a consideration of the following detailed description taken in conjunction with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a partially sectioned schematic view of apparatus for making an optical coupler in accordance with an illustrative embodiment of the invention; and 
     FIG. 2 is a graph of light intensity detected by the detector of FIG. 1 versus time. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates schematically apparatus for making an optical fiber coupler, or optical fiber tap, in accordance with an illustrative embodiment of the invention. Some of the dimensions shown are purposely distorted for clarity of exposition. A boring assembly 11 contains a laser 12 for producing a laser beam schematically indicated at 13 for machining an opening in the cladding 15 of an optical fiber 16. The optical fiber comprises a cylindrical glass core 18 surrounded by a cladding 15, typically of a polymer material. A buffer layer 19 of the optical fiber is removed from the region of the optical fiber within which the optical coupler is to be made. 
     The portion of the optical fiber 16 in which the coupler is to be made is enclosed by a housing 20 containing a window or aperture 21 for admitting the lower beam 13. The laser 12 is preferably an excimer laser, which emits light in an ultraviolet band when properly biased by a controller 23. The laser beam 13 is directed successively through an aperture 24 and lenses 25. The aperture 24 defines the outer periphery of the laser beam and is imaged by lenses 25 onto the portion of cladding 15 that is to be removed. The material of the cladding 15 is relatively absorptive of the laser band of frequencies, while the material of the core 18 is relatively unabsorptive of such frequencies. As is described in the Coyle et al. application, the ultraviolet light of an excimer laser breaks down the chemical bonds of the polymeric material from which cladding 15 is made and removes the cladding by ablation. A discussion of the ablative mechanism of excimer lasers is also included in the article, &#34;Excimer Lasers: An Emerging Technology in Material Processing,&#34; by T. A. Znotins et al., Laser Focus/Electro-Optics, May,  1987. The ablative mechanism produces an opening 26 in the cladding 15 to expose a portion of the optical fiber core 18. Aperture 24 in combination with lenses 25 defines both the size and shape of opening 26. 
     After the laser machining step, an optical fiber 28 is inserted in a fiber guide 29 such that one end of fiber 28 is in close proximity to the opening 26. Thereafter, the cavity within housing 20 is filled with a junction medium for aiding optical coupling between optical fiber 16 and optical fiber 28. As is known in the art, this medium may typically be an ultraviolet curable acrylate which may be made to have a different indices of refraction, depending upon whether the coupler is to be used predominantly for extracting optical energy from fiber 16 or for launching optical energy onto fiber 16. 
     In accordance with the invention, a photodetector 31 is located closely adjacent one end of optical fiber 16. The photodetector 31 is connected to controller 23 which controls the operation of laser 12. When the laser beam 13 has completely penetrated cladding 15, light appears at the end of optical fiber 16, which is detected by detector 31. Detector 31 and controller 23 are operated in any of various ways to turn off laser 12 after sufficient machining has been performed to appropriately define opening 26, but before the laser beam can seriously damage the core 18 of the optical fiber or the portion of the cladding 15 opposite the opening 26. 
     The variation of light intensity with time detected by detector 31 is illustrated in FIG. 2. With the laser beam 13 being in the form of pulses, the light intensity detected by the detector 31 will likewise be in the form of pulses 32. As the opening 26 is enlarged with successive pulses, light intensity at the end of the optical fiber becomes progressively larger, as manifested by the increasing height of pulses 32, until it reaches a stable continuous madimum value, as shown. 
     Normally, it is not desirable to turn off the laser after the first pulse of light appears at the optical fiber end because, at that juncture, machining of the opening is not complete. The exact time at which the laser is turned off is preferably determined by experiment. We have found that the controller 23 can easily be designed to count successive pulses of equal value, and also to turn off laser 12 after three successive pulses of nearly equal magnitude (or having magnitude differences of less than a predetermined value) have been counted. In FIG. 2, that would mean that the laser would be turned off after the reception of the seventh pulse. Controller 23 may have analog or digital means to determine either the peak energy or the total energy per pulse to characterize the pulse magnitude using techniques commonly used in the art. 
     Alternatively, the laser could be turned off after the light intensity detected has passed some threshold. For example, if the threshold of conduction of the photodetector were the threshold shown by light intensity 33, or if the controller were designed to respond to an electrical signal corresponding to light intensity 33, then the laser would be turned off after the fifth pulse. As still another alternative, a delay device 34 may be connected between the detector 31 and controller 23, as shown in FIG. 1, to provide a predetermined delay between the actuation of the photodetector 31 and the actuation of controller 23 for turning off the laser 12. 
     Depending upon the materials used for the optical fiber, there may be a trade-off between definition of opening 26 and laser beam damage to be avoided; for example, one may wish to terminate the laser beam before the maximum size of opening 26 has been defined to avoid any possibility of damage to the optical fiber. This, in turn, may dictate the use of a larger aperture 24 for increasing the area of impingement of the laser beam on the optical fiber 16. Alteratively, one may wish to tolerate a certain small degree of damage prior to laser beam termination. 
     We are not sure of the nature of the physical mechanisms involved in launching the light in optical fiber 16 that is detected by detector 31. Since laser beam impingement is perpendicular to the central axis of the optical fiber 16, on would not expect the clearly detectable pulses to appear at the optical fiber ends. Moreover, there is a frequency shift in the light detected. Although the light beam 13 that was used in our experiments was ultraviolet, some of the light at the end of the optical fiber was visible; in fact, it was the observation of the visible light that led us to the invention. Thus, the detector that is used as detector 31 may be any of a number of known photodetectors including those that are responsive to visible light. A cap 35 may be included on the end of the optical fiber opposite the detector 31 to prevent light from other sources from being transmitted by the optical fiber. The inner surface of cap 35 may be made reflective to visible light to increase the amount of light detected by detector 31. 
     From the foregoing, one can appreciate that an important benefit of the invention is that the machining operation can be made to the essentially automatic, without reliance on operator skill for terminating the machining. Regardless of coating thickness non-uniformities or other unpredictable factors, the invention provides for laser beam termination at the proper time. The apparatus shown can be combined with known robotic and manipulating apparatus in a mass-production environment in any of various ways that would be obvious to those skilled in the art. 
     The laser 12 that we have used in our experiments is a Questek Model 2660 laser, which may be operated at a wavelengths of one hundred ninety-three or two hundred forth-eight nanometers. The optical fiber that was used is known as HCS optical fiber, commercially available from the Ensign-Bickford Optics Company, Avon, Connecticut. The beam was first formed by aperture 24 and then imaged through a 4:1 telescope formed by lenses 25. As a consequence, a one millimeter circular aperture formed a circular image of two hundred fifty microns on the optical fiber coating 15. With the laser 12 operated in the pulse mode at one pulse per second, the energy density at the coupler site was in the range of five to ten milli-Joules per square millimeter. The optical fiber core 18 was one millimeter in diameter, the cladding 15 was ten to fifteen microns thick, and buffer layer 19 was two hundred microns thick. The optical fiber core was of glass and the cladding 15 was of a polymeric material. Approximately forty to eighty pulses were required under these circumstances to produce circular holes with diameters of two hundred fifty to six hundred twenty-five microns and elliptical openings of six hundred fifty by three hundred fifty microns. The junction medium was inserted through opening 21 with an opening 34 being used to expel air during the insertion of the coupling medium. 
     It is to be understood that the embodiment described is intended to be merely illustrative. Other lasers such as the carbon dioxide laser could be used for the laser machining or laser ablation, and it is possible that sources of light other than lasers could be used. Other fiber shapes in addition to cylindrical ones may be used. Laser machining on the flat surfaces of square or rectangular cross-section fibers may be particularly advantageous because of the uniformity of ultraviolet radiation striking the surface. Various other embodiments may be made by those skilled in the art without departing from the spirit and scope of the invention.