Method and apparatus for monitoring optical fibers during payout

Abstract: The turns of an optical fiber paid out from a canister are counted, and the signature of the optical fiber is measured, by monitoring the electromagnetic field of the charge on the optical fiber as it moves past an antenna. The optical fiber follows a helical path as it is paid out. A stationary antenna located transversely proximate the payout region detects a periodic repeating variation of the bound and/or free electrical charge associated with the optical fiber, as the optical fiber periodically moves closer to and farther from the antenna along the helical path. Each period of the variation corresponds to one turn of optical fiber paid out from the canister.

Description: BACKGROUND OF THE INVENTION This invention relates to the measurement of optical fiber characteristics during payout of the optical fiber from a support, and, more particularly, to the measurement of the electromagnetic field of the optical fiber during payout. Optical fibers are strands of glass fiber processed so that light transmitted therethrough is subject to total internal reflection. A large fraction of the incident intensity of light directed into the fiber is received at the other end of the fiber, even though the fiber may be hundreds of meters long. Optical fibers have shown great promise in communications applications, because a high density of information may be carried along the fiber and because the quality of the signal is less subject to external interferences of various types than are electrical signals carried on metallic wires. Moreover, the glass fibers are light in weight and made from a highly plentiful substance, silicon dioxide. Glass fibers are fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, and processing the preform to a fiber. The optical fiber is coated with a polymer layer termed a buffer to protect the glass from scratching or other damage. As an example of the dimensions, in one configuration the diameter of the glass optical fiber is about 125 micrometers, and the diameter of the fiber plus the polymer buffer is about 250 micrometers (approximately 0.010 inches). For such very fine fibers, the handling of the optical fiber to avoid damage that might reduce its light transmission properties becomes an important consideration. The fibers are typically wound onto a cylindrical or truncated conical bobbin with many turns adjacent to each other in a side-by-side fashion. After one layer is complete, another layer of fiber is laid on top of the first layer, and so on. An adhesive is applied to the layers to hold them in place on the canister, and to control the payout of the optical fiber to avoid snarls and breaks that might result from payout of multiple turns at once. The final assembly of the bobbin and the wound layers of fiber is termed a canister, and the mass of wound fiber is termed the fiber pack. When the optical fiber is later to be used, the fiber is paid out from the canister in a direction parallel to the axis of the cylinder. E. Edwards et al in Proceedings of SPIE - The International Society for Optical Engineering , vol. 842, pages 113-120 (1987) describe an automated environmental simulation model for analyzing wound fiber optic bobbins. The model automatically controls, collects, processes and analyzes optical loss data while measuring temperature and humidity. This allows the extent of static fatigue caused by stress corrosion in the presence of moisture to be assessed. The environmental conditions for optical fibers wound for various applications are simulated in order to understand the interrelationships between wound fiber parameters including spool composition/design, winding tension, adhesives and fiber cable design. One basic problem during the payout of the optical fiber from the canister is the measurement of the amount of optical fiber that has been paid out. The optical fiber may be paid out at rates as high as several hundred meters per second. Mechanical gauges are not practical because contact of anything to the fiber may damage it. Various types of optical sensors have been tried. These gauges utilize light sources and detectors that detect the optical fiber operating on the principle of interruption of the light beam, reflection, or refraction. These gauges are often bulky, require considerable power, are sensitive to stray light and reflections, and may be inaccurate if gauge placement and calibration are not extremely precise. DE-A-2914406 describes an electrostatic method of determining the speed and extent of payout of an electrically insulating fiber. The fiber (for example, a textile fiber) is electrostatically charged to a predetermined surface charge density as it is paid out. The fiber is then discharged onto a discharge electrode. The resulting discharge current is proportional to the linear payout rate of the fiber, and the total charge collected at the discharge electrode is proportioned to the total amount of fiber paid out. There exists a need for an approach for accurately measuring an optical fiber as it is paid out from a canister at rates as high as several hundred meters per second. The apparatus used to do the measurement should ideally consume little power, be accurate, and be compact. The present invention fulfils this need, and further provides related advantages. The present invention provides a light weight, compact, low power consumption approach for measuring an optical fiber as it is paid out from a canister. Two particularly preferred applications of the approach are the counting of turns of the optical fiber as the turns are paid out, to give a measurement of the total amount of optical fiber paid out and the rate of payout, and the recording of the signature of the optical fiber. The apparatus is not sensitive to small placement errors. It is highly accurate, even when slightly misplaced from its intended position. The approach may be used both during optical fiber canister payout tests, and during actual field use of the optical fiber. Specifically, the present invention provides a process for measuring the amount of an optical fiber as it is paid out from a support, comprising the steps of: providing a canister comprising the optical fiber wound in turns on the support; paying out the optical fiber from the support; and measuring the electromagnetic field produced by the optical fiber as it is paid out. The present invention further provides an apparatus to measure the amount of an optical fiber as it is paid out axially from a support along an optical fiber payout path, the apparatus comprising: a canister comprising the optical fiber wound in turns on the support; and an electromagnetic field antenna mounted near the end of the canister from which the optical fiber is paid out and proximate to the canister axis. The optical fiber is a dielectric, but carries an electrical charge as it is paid out from the support, which is typically a canister upon which a fiber pack is wound. The charge probably originates as the adhesive bonds are broken during payout and from friction, but its origin is not critical. Because the optical fiber does not conduct the electrical charge readily, the charge is stationary for at least a short time after payout. Payout of the optical fiber from the support normally involves side-to-side or transverse movement of the optical fiber, as well as longitudinal movement of the optical fiber. In the case of payout of the optical fiber from a canister, the primary component of the movement of the optical fiber during payout is in the longitudinal direction parallel to the cylindrical axis of the canister. However, since the optical fiber is being unwound from the support generally parallel to the axis of the support, the payout necessarily involves a helical pattern involving transverse motion of the optical fiber in the region adjacent the canister, where the optical fiber has just left the canister. Further away from the canister, the transverse component of the motion of the optical fiber essentially disappears, and there is only the longitudinal motion. In one approach to measuring the optical fiber as it is paid out, an electromagnetic field antenna is placed transversely proximate the region of the paid out optical fiber where the path of the optical fiber has a component of periodically varying transverse motion. The position of the optical fiber periodically moves closer to, and farther from, the antenna as payout continues due to the helical path and associated lateral movement. The electromagnetic field of the electrical charge on the optical fiber as measured at the antenna therefore varies periodically. One period of the variation corresponds to the payout of one turn of the optical fiber. The number of turns paid out from the support is measured by counting the periodic variations in the signal received by the antenna. The properties or characteristics of the optical fiber that may be measured by this approach are, of course, not limited to the counting of turns. For example, the entire pattern of the electromagnetic signal over large numbers of turns constituting multiple layers of optical fiber may be measured as the "signature" of the optical fiber. The signature is dependent upon the physical characteristics of the fiber pack such as the quantity of adhesive used to hold the turns in place. Measurement of the signature aids in developing improved adhesives and adhesive application systems. To conduct such measurements of the optical fiber, the electromagnetic field antenna is placed transversely proximate the region of the optical fiber where its motion has a lateral component. The antenna preferably is disposed off-axis, that is, not placed along the centerline of the optical fiber canister, as there is no relative transverse movement of the optical fiber with respect to the centerline location. The antenna also is preferably not placed at a location that moves with the same period as the helical motion of the fiber, so that there would be no relative transverse motion between the optical fiber and the antenna. In the most preferred approach, the antenna is a straight wire antenna whose axis is parallel to the payout direction. The antenna is mounted stationary on a support outside the helical path, to detect the field of the charge on the optical fiber as the transverse position of the optical fiber varies periodically. The measurement of the electromagnetic field produced by the charge on the optical fiber depends only upon the transverse motion of the optical fiber, and not upon exact placement of the antenna. The present approach is highly versatile, and can utilize virtually any configuration of antenna. The straight wire antenna is satisfactory for measurement of the periodic signal variation to count turns as they are paid out and measurement of the overall signature of the optical fiber. The approach can be used in stationary testing of canister payout, and also in a moving vehicle from which the optical fiber is paid out. In the latter case, the antenna signal can be sent down the optical fiber to a control center, where the counting instrumentation is located. Other features and advantages of the invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic side view of the payout of an optical fiber from a canister, the placement of an antenna to count turns, and associated instrumentation; Figure 2 is a diagrammatic end view of the payout of optical fiber; and Figure 3 is a measured graph of the induced current in the antenna as a function of time during payout of a few turns of the optical fiber; and Figure 4 is a measured graph of the induced current in the antenna as a function of time during payout of many turns of the optical fiber. DETAILED DESCRIPTION OF THE INVENTION In accordance with the invention, a process for measuring an optical fiber as it is paid out from a canister comprises the steps of providing a canister having an optical fiber pack wound thereupon, the optical fiber to be paid out axially from the canister along a payout path so that the optical fiber follows a helical pattern with a transverse component of motion along the portion of the payout path; and placing an electromagnetic field antenna transversely proximate the portion of the payout path where the optical fiber has the transverse component of motion, to detect changes in the electromagnetic field produced by the optical fiber as the optical fiber is paid out. The invention also includes an apparatus for accomplishing the measurements. In accordance with this aspect of the invention, apparatus for measuring an optical fiber as it is paid out axially from a support along an optical fiber path comprises an electromagnetic field antenna located transversely proximate the portion of the optical fiber path where the optical fiber has a transverse component of motion during payout. In one preferred embodiment of the invention, the turns of optical fiber are counted as they are paid out. In accordance with this embodiment, a process for counting the turns of an optical fiber paid out from a canister comprises the steps of providing a generally cylindrical canister having a plurality of turns of an optical fiber wound thereupon, the optical fiber to be paid out axially from a payout end of the canister; placing an electromagnetic field antenna proximate the portion of the payout path where the optical fiber assumes a helical pattern during payout; paying out the optical fiber; measuring the electromagnetic signal detected by the electromagnetic field antenna as the optical fiber is paid out; and counting the number of periodic fluctuations in the signal measured by the electromagnetic field antenna as the optical fiber is paid out, each fluctuation being associated with the payout of one turn of the optical fiber. In another preferred embodiment, the long term pattern of the measured signal is recorded and reviewed to assess the signature of the optical fiber. Figures 1 and 2 illustrate a preferred apparatus 10 that is used to measure an optical fiber 12 during payout, to count the number of turns of optical fiber that are paid out. The optical fiber 12 is wound onto a bobbin 14, which may be considered a support for the optical fiber 12. There are a plurality of turns in each layer on the bobbin 14, and a plurality of layers overlying each other. Together, the turns and layers of optical fiber 12 are termed a fiber pack 16. The bobbin 14 and fiber pack 16 are collectively termed a canister 18. The bobbin 14 has a generally cylindrical shape, but preferably has a slight taper from one end to the other of less than 5 degrees, with the cylindrical diameter decreasing along that length. That is, the bobbin is preferably a truncated cone with a very small conical angle. The bobbin 14 and canister 18 can be described as having an axis 20 along the cylindrical or conical axis of the bobbin 14. When the optical fiber 12 is to be paid out or unwound from the canister 18, it is removed from the canister 18 in a payout direction 22 that is generally parallel to the axis 20, and from the end of the canister 18 with the smaller cylindrical diameter. The payout direction 22 is also termed the longitudinal direction, and any direction perpendicular to the payout direction is termed the transverse direction 24. Payout is accomplished by pulling the optical fiber 12 from the canister 18 or, alternatively, holding a free end of the optical fiber stationary and moving the canister 18 away from the stationary end. In the preferred approach, the canister is mounted on a mandrel 25 that does not turn, but simply holds the canister in the proper attitude for payout. The canister 18 does not turn relative to the mandrel 25. At a large longitudinal distance from the canister 18, the optical fiber 12 therefore assumes a generally straight line, as indicated at numeral 26. Closer to the canister 18, in a region longitudinally adjacent to the canister 18 and indicated by the numeral 28, the path of the optical fiber 12 is not a straight line. Instead, as illustrated in Figure 1, there is a transverse component to the motion of the optical fiber 12. Alternatively stated, the optical fiber 12 follows a helical path as it pays out from the canister 18. The transverse component and helical path occur because the point 30 at which the optical fiber 12 is peeled from the fiber pack 16 moves around the circumference of the cylindrical or conical bobbin 14. The optical fiber 12 must therefore move radially or in the transverse direction 24 just after it is paid out, to reach the line defined in the region 26. (This motion and optical fiber path may be visualized as similar to that where a length of thread is pulled from a spool along the spool axis, producing a helical pattern near the spool.) It has been determined that the optical fiber 12 carries a net electrical charge with it as it is paid out from the canister 18. The electrical charge may be either a free or bound electrical charge, and may be positive or negative. In any event the electric charge does not move any significant distance along the length of the optical fiber during the fraction of a second after payout that the optical fiber 12 follows a helical pattern in the longitudinally adjacent region 28, because of the short time and because the glass optical fiber is not a good conductor of electricity. The origin of the electric charge is not of concern to the operability of the present invention, and the fact of its presence is sufficient. It is believed that the electric charge arises, at least in part, because of the breaking of adhesive bonds at the peel point and the friction created during payout. The electrical charge of the optical fiber produces an electromagnetic field in the neighborhood of the optical fiber as it moves. (The field is termed generally an "electromagnetic" field, even though the magnetic component may be small or zero, and is intended to encompass an electrostatic field.) The magnitude of the electromagnetic field generally falls inversely with some function of distance in the transverse direction from the optical fiber. Because of the helical pattern followed by the optical fiber, the functional variation is likely to be complex in nature, but the exact functional form is not important for the practice of the preferred embodiments of the invention. The magnitude of the electromagnetic field at the antenna 32 varies because the transverse distance from the antenna to the optical fiber undergoing helical motion varies. According to the present invention, an electromagnetic field antenna 32 is mounted at a location transversely proximate to the longitudinally adjacent region 28. That is, the antenna 32 is mounted to the side of the region where the optical fiber 12 is moving in a helical pattern with a transverse component of motion. In the canister configuration of Figure 1, the antenna is mounted just to the side of the canister 18 and near the end of the canister 18 from which the optical fiber 12 is paid out. The antenna 32 is preferably a straight length of electrically conducting wire 34 that is oriented generally parallel to the payout direction 22. The wire 34 is mounted in a nonconducting holder 36. The antenna 32 senses the magnitude of the electromagnetic field at the antenna as an induced electric current signal in the antenna. The signal of the antenna is conducted to an amplifier and signal recorder 38, and then optionally may be displayed on an oscilloscope or strip chart plotter 40. In one preferred embodiment, the antenna 32 includes a straight piece of 1.0 mm (0.040 inch) diameter, 5.1 cm (2.0 inch) long copper wire mounted in a BNC coaxial insulator holder with the long dimension of the antenna parallel to the axis of the bobbin. The amplifier and signal recorder is a model RTP 650A made by KYOWA Electronic Instruments Corp. A number of variations on this basic approach are possible. One is to mount the antenna 32 at another transverse location, as on the end of the canister 18, as indicated by the antenna 32', or on the end of the mandrel 25. Mounting the antenna outside the outer diameter of the canister 18 is preferred, as this minimizes the chances of interference between the optical fiber and the antenna, and yields the largest variation in signal during measurements. The invention is operable with the antenna inside the envelope of the optical fiber, however. Another approach is to use an antenna shape other than a straight wire, such as a loop. Another alternative is to place the recorder 38 and plotter 40 at the other end of the optical fiber 12, and to transmit the signal of the antenna 32, preamplified if necessary, through the optical fiber 12. Where the present invention is used in relation to optical fiber testing in a stationary facility, the antenna signal would be provided directly to the recorder 38 in the manner pictured. However, if the invention is used in conjunction with payout of the optical fiber from a canister in a moving vehicle where weight of the vehicle is critical, it could be preferable to mount the recorder 38 at the fixed end of the optical fiber 12 and to send the antenna signal through the optical fiber to the recorder 38. One use of the present invention is to count the turns of the optical fiber 12 as they are paid out from the canister 18. The payout turn count is a direct indication of the rate of payout and of the total payout during a period, since the length of optical fiber paid out is a function of the number of turns times the length of optical fiber per turn. The length of optical fiber per turn is a function of the geometry of the canister, and is roughly equal to the local diameter of the turn prior to payout times the constant pi (3.14159...). Figure 3 is a tracing of the output of a strip chart plotter 40, made during payout of the optical fiber 12 from the canister 18 at a rate of about 200 meters per second and using the preferred construction of the apparatus 10 as previously described. The figure exhibits a well defined periodic variation of the induced current of the antenna as a function of time. The maximum values such as at numeral 42 correspond to the point in the payout of the helix that the optical fiber is physically closest to the antenna, and the other values correspond to points where the optical fiber is further from the antenna. Each of the maximum values 42 corresponds to the payout of one turn, and the time between occurrence of the maximum values corresponds to the time required for payout of one turn. As can be seen in Figure 3, the peaks are clearly defined. The number of turns that are paid out is obtained by manually counting the number of peaks, or by using a conventional peak discriminating and counting electrical circuit. The clear definition of the peaks makes the counting quite easy. An important feature of the present invention is that the ability to count the payout of turns is not sensitive to the exact placement of the antenna 32. The antenna can be moved to various locations, or even misoriented with respect to the canister, and sufficient information is still obtained to count the peaks. By comparison, optical methods that rely, for example, on the breaking, reflection, or refraction of a light beam by the optical fiber as it is paid out are highly sensitive to exact positioning and calibration, are sensitive to stray light and reflections, and also utilize more equipment. Optical methods are generally too cumbersome and inexact to use in field operations, even if they can be made to work in a laboratory. The present apparatus, in contrast, is rugged, light in weight, and tolerant of placement errors. Another use of the invention is to measure and record the electromagnetic signature of the optical fiber during payout. Figure 4 is a graph of the induced current signal measured by the antenna as a function of time, for the payout of six entire layers of the optical fiber. (Figure 3, by comparison, may be viewed as an enlargement of a very small portion of Figure 4.) The form of Figure 4 is not regular, but appears to exhibit a variation wherein some entire layers have a pattern different from other layers. It is believed that this variation relates to several factors, including the quality of the adhesive application. The signature of payout such as shown in Figure 4 can be used to assess the uniformity of adhesive application to improve manufacturing operations. Thus, the present invention provides a highly useful approach to monitoring characteristics of optical fibers as they are paid out from a support. Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications may be made without departing from the scope of the invention as defined by the appended claims.

Claims: A process for measuring the amount of an optical fiber (12) as it is paid out from a support (14), comprising the steps of: providing a canister (18) comprising the optical fiber (12) wound in turns on the support (14); paying out the optical fiber (12) from the support (14); and measuring the electromagnetic field produced by the optical fiber (12) as it is paid out. A process according to claim 1, wherein the optical fiber (12) is paid out parallel to the axis (20) of the canister (18). A process according to claim 2, wherein the step of measuring comprises the step of detecting the electromagnetic field with an antenna (32) positioned proximate to a portion of the payout path where the optical fiber (12) has a component of motion transverse to the axis (20) of the canister (18). A process according to claim 3, wherein the antenna (32) is a straight wire antenna disposed parallel to the canister axis (20). A process according to claim 3 or 4, wherein the antenna (32) is positioned proximate to a portion of the payout path where the optical fiber (12) assumes a helical pattern during payout. A process according to claim 5, wherein the antenna (32) is mounted stationary outside the envelope of the helical pattern. A process according to claim 5, where the antenna (32) is mounted stationary inside the envelope of the helical pattern. A process according to any preceding claim, wherein the number of periodic variations in the electromagnetic field is counted as a measure of the number of turns of optical fiber (12) that are paid out from the support (14). A process according to any preceding claim wherein the step of measuring comprises measuring the entire pattern of the electromagnetic field over a number of turns corresponding to multiple layers of the optical fiber wound on the support. An apparatus to measure the amount of an optical fiber (12) as it is paid out axially from a support (14) along an optical fiber payout path, the apparatus comprising: a canister (18) comprising the optical fiber (12) wound in turns on the support (14); and an electromagnetic field antenna (32) mounted near the end of the canister (18) from which the optical fiber (12) is paid out and proximate to the canister axis (20). An apparatus according to claim 10, wherein the antenna (32) is a straight wire antenna extending parallel to the canister axis (20). An apparatus according to claim 10 or 11 further comprising means for counting the number of periodic variations in the field detected by the electromagnetic field antenna (32).

Inventors: BAILEY WILBUR M, BAILEY, WILBUR M.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1990
Main classification: G01M11
IPC codes: G02B6, G01P3, G01M11