Abstract:
An optical fiber flaw detection apparatus for detecting white light emitted through a sidewall of an optical fiber and methods of detecting the flaw are disclosed. One embodiment of the apparatus includes a detector located adjacent the fiber and processing circuitry to amplify and convert a signal produced to recognize flaws. A second embodiment includes a plurality of view systems oriented about the fiber and a mechanism to rapid draw fiber through the systems in addition to the detector. Each system is an optical assembly for collecting and directing the light. One method includes the steps of introducing the light into the fiber, detecting the light, and determining on-line the nature of the flaw. A second method includes the steps of heating a preform to a temperature sufficient to draw fiber, drawing fiber, and detecting the emitted light. The draw temperature is sufficient to transmit the light along the fiber.

Description:
This application claims benefit of provisional application No. 60/047,399 filed May 22, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to quality control in the production of optical fiber and the early detection of optical fiber defects. More particularly, the invention relates to a photodetection system and methods for the detection of flaws and surface defects, such as those caused by particles adhering to the surface of an optical fiber during its manufacture. 
     BACKGROUND OF THE INVENTION 
     During the drawing of an optical fiber from a blank, various imperfections may occur. These imperfections include holes in the fiber, inclusions or particles within the fiber, particles on the surface of the fiber, and surface abrasions. The presence of surface abrasions and of particles on the fiber can cause the fiber to break at later stages of manufacture. Thus, quality control is critical during the manufacture of optical fiber both from the point of view of achieving the highest possible manufacturing yield. To this end, a variety of techniques for testing the quality of optical fibers are known. For example, tension screening of fiber may be performed off-line some time after manufacturing is completed. Defects or flaws result in fiber breakage. None of the presently existing techniques addresses the on-line real time detection of surface particles as optical fiber is drawn or manufactured. 
     SUMMARY OF THE PRESENT INVENTION 
     The ability to detect particles on the optical fiber as it is drawn would be beneficial as both a process improvement tool and by providing the information necessary to reduce the number of particle induced breaks occurring later in the fiber optic manufacturing process. By way of example, real time detection of a large increase in the number of flaws might indicate that the furnace refractories are deteriorating and the furnace should be replaced. 
     The present invention recognizes that a fiber optic perform and a drawn fiber including the cladding prior to its coating form excellent waveguides. As such, light permeates substantially the entire cross section of the fiber prior to coating. The majority of the light is guided down the entire length of the fiber unless scattered out along the length of the fiber by an imperfection, such as a hole, surface abrasion, or particle. During the drawing process, optical fiber is filled with light in the draw furnace which is an excellent source of intense white light. As the fiber is drawn, light from the draw furnace is conducted along it and scattered out as a result of imperfections. Because of the speed at which the fiber is moving as it is being drawn, the scattered light will be seen by a detector as a brief flash as the defect rapidly passes the detector. 
     Among the other needs addressed by the present invention is the need for a detector which can detect flashes caused by scattering of light due to imperfections in an optical fiber as the fiber is being drawn. The present invention also provides a detector which can operate effectively at the normal speed at which optical fiber is drawn to identify a flash caused by the presence of a particle on the surface of the optical fiber. Such particles may be referred to herein as particles on glass or POGs. 
     One aspect of the present invention preferably comprises a high speed large area detector placed at one focal point of an elliptical mirror. The optical fiber to be subjected to detection is placed at the other focal point of the mirror. The mirror arrangement provides that 180 degrees of the light scattered from the fiber will be reflected onto the detector. The detection components are preferably shielded to block out stray light and purged with an inert gas to keep their optical surfaces clean. 
     According to another aspect of the invention, an elliptical mirror is used. The fiber is placed at one focal point of the mirror and a fluorescent rod which preferably has a diameter of 1 cm is placed at the other focal point. The rod preferably has polished ends and is oriented parallel to the fiber. One end of the rod is preferably mirrored, while a high speed Silicon detector is attached to the other end. The rod is doped with a fluorescent dye which absorbs visible light, preferably yellow light, and fluoresces in the visible or near IR part of the spectrum. As the light from the fiber hits the rod, fluorescence will occur. Approximately half of the light will fluoresce out of the rod and be lost. One quarter of the light will be guided inside the rod, reflect off the mirrored surface and then be guided back down to the detector, while the remaining quarter of the light will be guided directly to the detector. Fluorescence occurs very fast, on the order of 10 ns, so even very fast flashes may be readily detected. Also, since the rod has a diameter of 1 cm, all of the light scattered from the fiber will hit the rod and the system will be insensitive to fiber movement. Further, it will be recognized that by increasing the length of the rod the time during which a flash will be observed will be increased. Additionally, the rod may be preferentially masked to distinguish between particles and holes, for example. 
     Another aspect of the present invention preferably comprises a two-view system. Each view includes a small area, high-speed, sensitive Silicon detector, two lenses, and a spherical mirror. The lens system makes the view less sensitive to fiber movement. In the preferred embodiment of the invention, lenses are used which allow the system to tolerate fiber movement of 2.5 mm. 
     The lenses are placed on one side of the fiber, and the spherical mirror is placed on the other side of the fiber at its radius of curvature away from the fiber. All the incident light from the fiber is thus reflected back to the detector. Using two views spaced 90 degrees apart increases the likelihood that a flash from a POG will be collected. Also, it is believed that signal differences between the detectors may be used to discriminate particle type and hole presence given the addition of suitable signal processing support to rapidly analyze the signal differences. The system is preferably housed in a box to block out stray light and purged with an inert gas. The housing will preferably be mounted on adjustable x and y stages to properly locate the detector with respect to the fiber. A source of purge gas will be supplied to keep the optical surfaces of the detector contamination free. 
     Another aspect of the invention preferably comprises a three-view system. Each view includes a small area, high-speed, sensitive Silicon detector, three lenses, and a spherical mirror. The lenses are placed on one side of the fiber, and the spherical mirror is placed on the other side of the fiber at its radius of curvature away from the fiber. All the incident light from the fiber is thus reflected back to the detector. Using three views spaced 120 degrees apart eliminates any dead zone in which the fiber can be present without a flash being detected. The use of three views and three lenses permits a detector according to this aspect of the invention to detect flashes at high speeds, such as typical draw speeds used during optical fiber manufacturing, given the proper selection and configuration of parts. Further, while a variety of approaches are described in detail, it will be recognized that multiple view systems may be employed with the number of views determined by the application and considerations such as cost. Various other optical arrangements and detectors may be suitably employed. 
     A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following Detailed Description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an optical fiber draw process and system according to the present invention; 
     FIG. 2A illustrates a top view of a flash detection system comprising an elliptical mirror and a detector according to the present invention; 
     FIG. 2B illustrates a side view of the flash detection system of FIG. 2A; 
     FIG. 3 illustrates a side view of a flash detection system employing an elliptical mirror and a fluorescent rod according to the present invention; 
     FIG. 4 illustrates a top view of a two-view flash-detection system according to the present invention; 
     FIG. 5 is a side view of a portion of the two-view flash detection system as depicted in FIG. 4; 
     FIG. 6 is a three-view flash-detection system according to the present invention; and 
     FIG. 7 illustrates a method of flash detection in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates an optical fiber draw process and system  10  employing optical fiber flaw detection in accordance with the present invention. In the system  10 , an optical fiber  1  is drawn from a draw furnace  11  which preferably heats to the fiber to a temperature of approximately 1900-2000° C. This temperature is sufficiently hot to cause the fiber to glow and to be substantially filled with white light. As the fiber  1  leaves the draw furnace  11 , it passes through a flaw detector  12  which will preferably be constructed in accordance with the principles described further below in conjunction with the discussion of FIGS. 2A-7 although it will be recognized that other optical detection arrangements and housings may be suitably devised consistent with the teachings and claims which follow. The flaw detector  12  is preferably located directly after the draw furnace  11  as at this point in the process substantially all surface abrasions have been eliminated by the high heat of the draw furnace  11 . However, it will be recognized that the flaw detector  12  may also be located at points, such as the points A or B located after the diameter checker  13  or the cooling tube  15  as shown in FIG. 1, or alternatively might be combined with the diameter checker  13  to form a dual function single housing unit if desired. 
     Next, the fiber  1  passes through a diameter checker  13  which checks the diameter of the fiber  1  in a known fashion. The diameter checker  13  may suitably be an interference diameter measurement (“IDM”) device such as those described in U.S. Pat. No. 5,309,221 which is assigned to the assignee of the present invention and which is incorporated by reference herein in its entirety. From the diameter checker  13 , fiber  1  then passes a cooling tube  15 , a primary coater  17 , a first coating curer  19 , a secondary coater  21 , a second coating curer  23  and a tractor  25 . The tractor  25  provides the appropriate force to draw the fiber  1  at the desired process speed. Further details of one existing draw system are described in U.S. Pat. No. 5,443,610 which is assigned to the assignee of the present invention and incorporated by reference herein in its entirety. 
     In addition to the above described components, system  10  also includes a source of purge gas  14  which provides gas to purge the optical surfaces of the flaw detector  11  to prevent such surface from becoming contaminated. System  10  also includes a data acquisition system  20 , a chart recorder  30  and an alarm system  40 . The data acquisition system  20  will preferably include fast digital signal processing circuitry to amplify, convert and process the signal produced by the flaw detector  12 . A PC, workstation or minicomputer including a suitably programmed controller, memory, a display, a keyboard, and printer may suitably be included. By connecting the data acquisition system  20  to the flaw detector  12 , the diameter checker  13  and the tractor  25 , holes can be discriminated from other flaws such as POGs as the IDM can be employed to detect holes in a known fashion, for example, as described in U.S. Pat. No. 5,185,636 which is assigned to the assignee of the present invention and incorporated by reference in its entirety. The data acquisition system  20  can monitor and check the draw speed at the fiber  1 . The chart recorder  30  can be employed to keep a log of the location of various flaws along the optical fiber  1 . Alarm system  40  may be employed to provide appropriate alarms to manufacturing process control personnel or a control processor in response to real time flaw detection. 
     FIG. 2A illustrates a top view of a flash detection system  100  according to one aspect of the present invention. This system  100  may suitably be employed as the flaw detector  12  of FIG.  1 . Detection system  100  comprises an elliptical mirror  102  and a detector  104 . The detector  104  is placed at one focal point of the mirror  102 . An optical fiber  106  is introduced between detector  104  and mirror  102  at the other focal point of the mirror  102 . As seen in FIG. 2A, the optical fiber  106  is traveling down into the page. Light is normally conducted through the fiber  106  without exiting the fiber  106 . An irregularity on the surface or other flaw of the fiber  106 , however, will cause light  112  to escape from the surface. Such an irregularity is most often a particle on glass, or POG, adhering to the surface of the fiber  106 . Light  112  will escape from the fiber  106  at the POG. About 180 degrees of the light  112  will strike the mirror  104  and be reflected onto the detector  104 . 
     FIG. 2B illustrates a side view of the detection system  100 , with a fiber  106  being fed into the detection system  100  from a furnace  114 , and providing additional views of the mirror  102 , the detector  104 , and the fiber  106 . FIG. 2B also illustrates an enclosure or housing  116  in which detector  100  is suitably enclosed to prevent the entry of ambient light. Baffles  118  and  120  provide additional shielding from ambient light at the points where the fiber  106  enters and exits the housing  116 . Additionally, x and y translation stages  130  may be suitably employed to manually or automatically adjust the position of the mirror  102  with respect to the fiber  106 . 
     Detector  100  may be advantageously used during the drawing of fiber  106 . Fiber  106  can be fed into detector  100  as it is drawn from the furnace  114  where that furnace is a draw furnace such as the draw furnace  11  of FIG.  1 . The furnace  114  heats the perform from which fiber  106  is drawn white hot. This heating results in intense white light being conducted within the fiber  106 . 
     Because the fiber  106  and the detector  104  are at the opposite foci of the elliptical mirror  102 , the detector  100  according to the illustrated aspect of the invention is sensitive to movement of the fiber  106 . Movement of the fiber  106  moves the fiber  108  off the focus of the mirror  102 , thereby preventing the mirror  102  from precisely focusing the light escaping from fiber  106  onto the detector  104 . 
     FIG. 3 illustrates a detection system  200  according to another aspect of the present invention. System  200  may also suitably be employed as the flaw detector  12  of FIG.  1 . The detection system  200  includes an elliptical mirror  202 . Fiber  204  is placed at the first focus of the mirror  202 , while a fluorescent rod  208 , preferably one centimeter in diameter, is placed at the second focus of the mirror  202 . A detector  212  is placed at a first end  214  of the rod  208 , while a mirror  216  is placed at the second end  218  of the rod  208 . Rod  208  can suitably be of an acrylic or glass material which is doped with a fluorescent dye. The dye preferably absorbs visible light, such as is emitted by the white hot perform, and fluoresces in the visible or near IR region of the spectrum. 
     Light introduced into one end of the fiber  204  will be transmitted along the length of the fiber  204  until encountering an irregularity  220 . Scattered light  222  will then be emitted from the fiber  204  and collected by the mirror  202 . A substantial amount of scattered light  222  will then be focused onto the rod  204 , which will fluoresce, producing fluorescent light  224 . Approximately one half of the fluorescent light  224  will escape from the rod  208  and be lost. About one quarter of the fluorescent light  224  will be guided up the rod  208 , reflected by the mirror  216 , guided back down by the rod  208 , and onto the detector  218 . An additional one quarter of fluorescent light  224  will be guided directly down rod  208  to the detector  218 . Because of the volume of the rod  208 , the detection system  200  is relatively insensitive to movement of the fiber  204 . A slight loss of focus of light  222  will not prevent the light  222  from striking rod  208  somewhere. Rod  208  will then produce fluorescent light  224  for detection by the detector  218 . 
     By increasing the length of the rod  208 , the observation time of a flash will be increased. Also, the rod may be selectively masked so that it picks up preferential scattering differently. For example, a mask  209  may shield the upper third of the rod  208  from light impinging from a range of angles. It will be recognized that additional masks which are not shown might also be employed. By comparing the signals measured as a flash passes the top third with the signals measured as a flash passes the bottom of the rod  208 , it should be possible to distinguish holes from surface particles and it may be possible to distinguish different types of particles if those particles result in preferential scattering. 
     The detection system  200  may be suitably enclosed in a light-tight enclosure  226  with baffles  228  and  230  at the points where fiber  204  enters and exits the light-tight enclosure  226  in order to prevent the entry of ambient light into light-tight enclosure  226 . Purge gas and adjustable stages may be employed as discussed above. 
     FIG. 4 illustrates a top view of a two-view flash detector  300  according to another aspect of the present invention. Detector  300  may suitably be employed as the flaw detector  12  of FIG.  1 . The detection system  300  includes first and second view systems  302  and  304 , respectively. While a presently preferred optical assembly is described below, it will be recognized that any other suitable optical assembly may be employed to collect and guide light emitted from the sidewalls of the fiber  314 , and direct it to first detector  312 . 
     First view system  302  includes a first spherical mirror  306 , first and second lenses  308  and  310 , and a first detector  312 . Mirror  306  is placed a distance equal to its radius of curvature from fiber  314 . Second lens  310  is placed on the opposite side of the fiber  314  from mirror  306 , in line with mirror  306 , and a distance equal to the focal length of lens  310  from the fiber  314 . First lens  308  is placed on the opposite side of lens  310  from fiber  314 , in line with mirror  306  and lens  310 . First detector  312  is on the opposite side of lens  308  from fiber  314 , in line with mirror  306  and lenses  308  and  310 , at a distance from lens  308  equal to the focal length of the lens  308 . Lenses  308  and  310  are preferably mounted in a lens mount  322 . The lens mount  312  is preferably adapted to fit directly into the housing  326  which encloses the detector  312 . This arrangement shields detector  312  from ambient light which may have entered the detection system  300 . 
     Second view system  304  includes a second spherical mirror  316 , third and fourth lenses  318  and  320 , and a second detector  322 . The second view system  304  is oriented 90 degrees perpendicular to the first view system  302  and is located in the same plane as the first view system  302 . Mirror  316  is located a distance equal to its radius of curvature from the fiber  314 . Fourth lens  320  is placed on the opposite side of the fiber  314  from mirror  316 , in line with mirror  316 , and at a distance from fiber  314  equal to the focal length of the lens  320 . Third lens  318  is placed on the opposite side of lens  320  away from the fiber  314 , in line with the mirror  316  and the lens  320 . Second detector  322  is on the opposite side of lens  318  away from the fiber  314 , in line with mirror  316  and lenses  318  and  320 , at a distance from lens  318  equal to the focal length of the lens  318 . Lenses  318  and  320  are mounted in a lens mount  324 , and the lens mount  322  is preferably adapted to fit directly into a housing  328  which encloses the detector  322 . This arrangement operates to shield the detector  322  from ambient light. 
     The detection system  300  is preferably enclosed in a light-tight enclosure having suitable entry and exit points for fiber  314 , with baffles shielding the entry and exit points. The light-tight enclosure of detection system  300  is not shown, but may be similar to those described above in conjunction with the embodiments of FIGS. 2B and 3. While it is presently preferred to have the view systems  302  and  304  coplanar, it will be recognized that they may also be arranged so as to be displaced from one another and their outputs may then be processed to analyze for preferential scattering to detect holes and distinguish different types of particles. 
     It will be recognized that two view and multiple view systems more generally may provide multiple signals simultaneously as the fiber  314  moves past. With appropriate analysis, it may prove possible to analyze differences in such signals to gain further valuable information. For example, it may prove possible to distinguish a hole from a POG, or one type of POG from another. 
     FIG. 5 illustrates a side view of the view system  302  of the detector  300 . As discussed above, the view  302  includes the mirror  306 , first and second lenses  308  and  310 , and first detector  312 , all oriented in a straight line which intersects the fiber  314 . 
     FIG. 6 provides a top view of a three-view detection system  600 . The detection system  600  may also suitably be used as the flaw detector  12  of FIG.  1 . The detection system  600  includes view systems  602 ,  604  and  606 . It is presently preferred that each of the view systems  602 ,  604 , and  606  be located an equal distance from the fiber  314 . The view system  602  includes spherical mirror  608 , first, second and third lenses  610 ,  612  and  614 , respectively, and a first detector  616 . A concave mirror, with a focal distance, f=25 mm, part number 44351 from Oriel Corp. may suitably be used as the mirror  608 . Suitable choices for the first, second and third lenses  610 ,  612  and  614 , respectively, are Bi-Convex lens, f=25.4 mm, Part number KBX046AR.14, Bi-Convex lens, f=50.2 mm, Part number KBX142AR.14, and Piano-Convex lens, f=6.4 mm, Part number KPX010AR.14, respectively, all from Newport Corporation. A suitable choice for the first detector  616  is Detector/Amplifier Package, Part Number PDA50, from Thorlabs. Mirror  608  is placed at its radius of curvature away from fiber  618 . Third lens  614  is placed in a straight line from mirror  608  and fiber  618 , on the opposite side of fiber  618  from mirror  608 , at a distance from the fiber  618  of the focal length of the lens  614 . Lens  612  is placed in a straight line with mirror  608 , fiber  618  and lens  614 , on the opposite side of lens  614  from fiber  618 . Lens  610  is placed in a straight line with mirror  608 , fiber  618  and lenses  614  and  612 , on the opposite side of lens  612  from lens  614 . Lenses  610  and  612  are preferably spaced 17.8 mm apart. Detector  616  is placed in a straight line with mirror  608 , fiber  618  and lenses  614 ,  612  and  610 , on the opposite side of lens  610  from lens  612 , at a distance from lens  610  of the focal length of lens  610 . Lenses  610 , 612  and  614  are suitably mounted in a lens mount  640  which is preferably adapted to fit directly into a housing  646  for the detector  616 . This arrangement operates to shield the detector  616  from ambient light which may have entered the detection system  600 . 
     View system  604  is oriented in a straight line which intersects the fiber  618 , 60 degrees from the view system  602 , in the same plane as view system  602 . View system  604  includes second spherical mirror  620 , fourth, fifth and sixth lenses  622 ,  624  and  626 , respectively, and a second detector  628 . Mirror  620  is placed at its radius of curvature away from fiber  618 . Sixth lens  626  is placed in a straight line from mirror  620  and fiber  618 , on the opposite side of fiber  618  from mirror  620 , at a distance from fiber  618  of the focal length of lens  626 . Lens  624  is placed in a straight line with mirror  620 , fiber  618  and lens  626 , on the opposite side of lens  626  from fiber  618 . Lens  622  is placed in a straight line with mirror  620 , fiber  618  and lenses  626  and  624 , on the opposite side of lens  626  from lens  624 . Detector  628  is placed in a straight line with mirror  608 , fiber  618  and lenses  626 ,  624  and  622 , on the opposite side of lens  624  from lens  622 , at a distance from lens  622  of the focal length of lens  622 . Lenses  622 ,  626  and  624  are suitably mounted in a lens mount  642  which is preferably adapted to fit directly into a housing  648  for the detector  628 . This arrangement operates to shield the detector  628  from any ambient light which may have entered detection system  600 . 
     View system  606  is oriented in a straight line which intersects the fiber  618 , 60 degrees from view system  602  in the opposite direction from view system  604 , in the same plane as the view systems  602  and  604 . View system  606  includes spherical mirror  628 , seventh, eighth and ninth lenses  630 ,  632  and  634 , respectively, and third detector  636  which are located, mounted and housed similarly to the corresponding components of the view systems  602  and  604 . The parts listed above as suitable in the description of the first view system  602  are also suitably used for the corresponding parts in the second view system  604  and the third view system  606 . 
     Detection system  600  is preferably enclosed in a light-tight enclosure with suitable entry and exit points for fiber  618 , with baffles shielding these entry and exit points. The light-tight enclosure of detection system  600  is not shown, but may be similar to those described above in conjunction with the embodiments of FIGS. 2B and 3. 
     The three-view system  600  eliminates the dead zone to which the two-view detector system  300  of FIG. 3 is subject. Moreover, through an appropriate selection of the components such as those listed above, the three-view system may be constructed so as to maximize the time that a flash is in the field of view, thereby facilitating flash detection at high speeds, such as typical draw speeds. 
     FIG. 7 is a flowchart illustrating a process of flash detection  700  according to the teaching of the present invention. At step  702 , light is introduced into an optical fiber, such as the fiber  1  of FIG. 1 which is filled by light in the draw furnace  11 . At step  704 , the fiber is introduced into and pulled rapidly past a detector, the detector preferably comprising mirrors, lenses, or other optical devices, as well as, detection circuitry for detecting light as the fiber is rapidly moving by a predetermined point. The fiber continues in motion through the detector, enabling the entire fiber to pass through the detector and light escaping at any point along the length of the fiber is detected making the process highly suitable for real time testing during the manufacturing process. As addressed above, the detector elements are preferably contained within a substantially light-tight enclosure. At step  706 , light escaping from the fiber is directed to a desired point within the detector. At step  708 , light falling on the desired point is detected and an electrical signal representative of that light is produced. Finally, in step  710 , the electrical signal is processed and analyzed to defect fiber flaws. A data processing and analysis system, such as the data acquisition system  20  of FIG. 1, may be employed to analyze the signal and to distinguish one type of flaw from another. Records of the detected flaws may be stored in memory, printed on a chart recorder, such as the recorder  30  of FIG. 1, or displayed on a display. 
     While the present invention is disclosed in the context of a presently preferred embodiment, it will be recognized that a wide variety of implementations may be employed by persons of ordinary skill in the art consistent with the above discussion and the claims which follow below.