Abstract:
A system and method for measuring stress exerted on an optical fiber including providing an optical fiber that includes a fiber optic sensor, and exposing the optical fiber and the fiber optic sensor to various stresses associated with the process by moving the optical fiber and the fiber optic sensor through the process to be measured. The system and method further includes transmitting a source light through the optical fiber as the optical fiber and the fiber optic sensor are exposed to various stresses, receiving a return light signal from the fiber optic sensor as the optical fiber and the fiber optic sensor are exposed to various stresses, and comparing the source light signal to the return light signal for determining the stress exerted on the optical fiber.

Description:
This application is a divisional application of U.S. Ser. No. 09/829,783 filed on Apr. 10, 2001 U.S. Pat. No. 6,366,711 which is a divisional application of and claims priority to and the benefit of U.S. Ser. No. 09/407,579, filed Sep. 28, 1999, U.S. Pat. No. 6,314,214. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for measuring stress exerted on an optical fiber, and in particular to a method for measuring stresses exerted on an optical fiber during manufacturing of the fiber or of a cabled fiber. 
     2. Technical Background 
     Numerous forms of fiber optic sensors have been developed to monitor parameters in various systems and processes, including the Fabry-Perot Interferometer, the Bragg Grating, the Mach-Zehnder Interferometer, and the Michelson Interferometer, to name but a few. These fiber optic sensors are used in a wide variety of applications, including use as strain gauges, dynamic pressure sensors, bearing condition sensors, non-contact proximity sensors, and temperature sensors. In each of these applications, the fiber optic sensor is fixedly attached to the system to be monitored, and usually is encased within a housing or rigid structure that is fixedly attached to the system to communicate parameter changes in the system to the fiber optic sensor. 
     As strain gauges, fiber optic sensors have been used to monitor dynamic strain. In such applications, the fiber optic sensor is imbedded within a material that is attached to a component of a structure such that the strain within the component may be monitored. Applications of fiber optic strain gauges have typically included civil structures such as dams, buildings, and bridges. 
     As dynamic pressure sensors, fiber optic sensors have been used in a variety of applications including the monitoring of performance of internal combustion engines, as well as monitoring the performance of compressors and pumps. When used to monitor the performance of an internal combustion engine, the fiber optic sensor is typically placed within a housing mated with a cylinder of the engine. The housing typically has a metal diaphragm that is attached to one end of the fiber optic sensor. Pressures exerted on the diaphragm are transferred to the fiber optic sensor, thereby changing the overall length of the sensor and allowing measurement of continuous real-time in cylinder pressures permitting improved engine control, providing preventive maintenance data, and predictive emissions monitoring. When used to monitor the performance of compressors and pumps, the fiber optic sensor is imbedded within an aluminum alloy rod, or similar metal, by an encasing process. The aluminum rod encasing the fiber optic sensor is then placed within a metal housing having a diaphragm similar to that described above in relation to engine monitoring. By placing the diaphragm in contact with the fluid being transferred by the compressor and/or pump, measurements of cavitation, flow instability, and surge detection are possible, thereby reducing the risk of catastrophic mechanical failure. 
     As bearing condition sensors, fiber optic sensors are used to monitor the condition of bearing or rotor imbalance. Typically, the fiber optic sensor is encased within a housing that includes a deformable diaphragm. The fiber optic sensor is in contact with the diaphragm which is, in turn, in contact with the outer race of a bearing, thereby allowing for the transfer of any vibrations between the associated bearings and the outer race to the fiber optic sensor. 
     In non-contact proximity sensors, fiber optic sensors are used to measure shaft vibration, rotor thrust position, shaft rotational speed, as well as rotor imbalance and misalignment. In these applications, the fiber optic sensor is encased within a steel rod having a magnet attached to an end thereof. The steel rod encasing the optical fiber and the magnet are positioned within a stationary housing. The housing is then located such that the magnet is in close proximity to the rotating shaft to be monitored. Imbalances in the shaft cause the magnet to move which motion is transferred to the optical sensor for monitoring of the position or condition of the shaft. 
     As temperature sensors, fiber optic sensors are typically inserted into areas desired to be monitored, or imbedded into cast parts, thereby allowing the direct measurement of temperatures therein. 
     Typically, fiber optic sensors have been used to monitor systems that allow for stationary or fixed placement of the sensor within the system. The construction of these sensors have made it difficult if not impossible to monitor processes, systems, or machines that require the optical fiber and the associated fiber optic sensor to be moved throughout the system being monitored. Further, these systems typically require the fiber optic sensor to be cast within a part or structure to be monitored, or placed within a housing that is attached directly to the system to be monitored, thereby adding to the size and cost associated with the monitor system. 
     The manufacturing procedures and processing of optical fibers and fiber optic cables are numerous and varied. Many of these processes include placing a stress on the optical fiber or fibers being processed. These stresses when applied over time, however short, result in sub-critical growth of the pre-existing flaws located within the optical fibers, thereby decreasing the overall strength of the optical fiber. In certain applications, it is important that the optical fiber, or bundle of fibers, has sufficient strength to withstand loads place thereon without damaging the optical fiber or overall fiber optic cable. As a result, reliability models are created to estimate the strength of the fiber and the associated fiber optic cables after the processing and manufacturing. Reliability models for optical fibers are based on three things: the size distribution of flaws or cracks within the fiber; fatigue crack growth parameters; and the stress-time profile which the fiber experiences during processing. High-stress processing events may result in degradation of the fiber strength. Until now, direct measurements of the stresses exerted on an optical fiber during high-speed processing has not been possible, and, as a result, the stress-time profile of optical fiber has been an assumed quantity. 
     The ability to collect real-time measurements of the stresses exerted on an optical fiber during processing and cable manufacturing would be valuable for reliability analysis and modeling, process and equipment design, trouble-shooting of manufacturing lines, as well as fiber and cable installation. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is to provide a method and system for measuring stress exerted on an optical fiber including providing an optical fiber that includes a fiber optic sensor, and exposing the optical fiber and the fiber optic sensor to various stresses associated with a process by moving the optical fiber and the fiber optic sensor through the process to be measured. The method and system further include transmitting a source light signal through the optical fiber as the optical fiber and the fiber optic sensor are exposed to the various stresses, receiving a return light signal from the fiber optic sensor as the optical fiber and the fiber optic sensor are exposed to the various stresses, and comparing the source light signal to the return light signal for determining the stresses exerted on the optical fiber. 
     Another aspect of the present invention is to provide a method and a system for measuring stress exerted on an optical fiber including providing a light source emitting a light having a predetermined frequency, providing a first photo detector coupled to the light source that produces a first electrical signal proportional to the light, providing a first adjustable amplifier coupled to the first photo detector which amplifies a first electrical signal therefrom, and providing the optical sensor coupled to the light source by the optical fiber to transmit at least a portion of the light to the optical sensor. The method and system further includes providing a second photo detector coupled to the optical sensor that detects at least a portion of the light reflected from the optical sensor, and produces a second electrical signal proportional thereto, providing a second adjustable amplifier coupled to the second photo detector that amplifies the second electrical signal therefrom, providing a comparator coupled to the second photo detector that compares the first and second electrical signals to provide a signal representative of the relationship between the first and second signals. In one embodiment, a microcontroller is coupled to the comparator and generates a plurality of trigger signals at a fixed frequency, each initiating a modulated cycle and further that generates a control signal in response to detecting a predetermined transition between the first and second output voltages. The method and system further includes providing a modulator coupled to the light source and the microcontroller to modulate the light source in a periodic manner to provide pulses in response to receiving a trigger signal from the microcontroller. A counter is coupled to the microcontroller and begins counting the periodic pulses in response to receiving a trigger signal, and ends counting in response to receiving a control signal from the microcontroller to generate a count value. The optical fiber and the fiber optic sensor is exposed to various stresses associated with a process by moving the optical fiber and the fiber optic sensor through the process to be measured, and the microcontroller computes the stresses exerted on the optical fiber in response to receiving the count value for each modulation cycle. 
     Yet another aspect of the present invention is to provide a method for determining the amount of stress exerted on an optical fiber that utilizes a source of light signals, a detector for detecting a return light signal, a comparator for comparing the emitted light signal to the return light signal, and a microcontroller for calculating the stress exerted on the optical fiber. The system includes providing an optical fiber, with a fiber optic sensor. The optical fiber and the fiber optic sensor is exposed to various stresses associated with a process by moving the optical fiber and the associated fiber optic sensors through the process to be measured, while monitoring the detected light signals for calculating the stress exerted on the optical fiber during the process. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings. 
     It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a manufacturing process for an optical fiber employing the method and sensing system of the present invention; 
     FIG. 2 is multiple cross-sectional views showing an optical fiber and the steps to construct a Fabry-Perot Interferometer; 
     FIG. 3 is an electrical circuit diagram in block and schematic form of the system of the present invention; 
     FIG. 4 is a schematic view of a coloring die and a corresponding stress-time graph obtained from the system shown in FIG. 3; and 
     FIG. 5 is a side view of a frictional trigger generator employed in the system of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, a sensing system  10  embodying the present invention is employed in the processing of an optical fiber  12  having a fiber optic sensor  14  imbedded therein. An electrical circuit  16  is connected through fiber coupler  56  to optical fiber  12  for monitoring a process, system, or machine  18  treating the optical fiber  12 . 
     The optical fiber  12  can be a single-mode optical fiber that includes a core  20  (FIG.  2 ), a cladding  22 , and a polymeric outer coating  24 . A single-mode optical fiber, as designated herein, is optical fiber that propagates only two mutually orthogonal polarization modes of the HE11 mode of light, at a chosen signal wavelength. Further, while a Corning SMF28™ fiber has proven effective, other types and classifications of optical fibers may be used. The optical fiber  12  is preferably provided on a conically-shaped spool  25  (FIG. 1) to allow free, unrestricted unspooling and delivery of the optical fiber  12  into the process or system  18  to be monitored, however, optical fiber  12  can be provided in any form that allows free, unrestricted delivery. 
     The most preferred fiber optic sensor  14  utilized in the present invention is a Fabry-Perot Interferometer  26  (FIG.  2 ). While the most preferred fiber optic sensor  14  is the Fabry-Perot Interferometer  26 , the described signal processing system may also be applied to other sensors, such as Michelson and Mach-Zehnder Interferometer sensors. In addition, the optical power can be provided to sensors monitored in transmission as well as in reflection such as in a fiber Bragg gratings, and others as known in the art. The sensing system  10  employs the measurement of the relative phase shift within a Fabry-Perot interferometer  26 . It is assumed that the measurement of interest, the stress exerted on the optical fiber  12  and the embedded Fabry-Perot Interferometer  26 , affects the optical phase shift in the Fabry-Perot Interferometer  26  and that the value of the strain is related to the phase shift through a calibration factor. Suitable mathematical computations associated with calculating the phase shift and the strains associated therewith are disclosed, for example, in U.S. Pat. No. 5,557,406. 
     The Fabry-Perot Interferometer  26  as used in the present system  10  is constructed or disclosed in FIG. 2 by first removing the polymeric outer coating  24  from about a portion of optical fiber  12 , as shown in step  1 . The optical fiber  12  is then cut forming a first cleaved end  28  and a second cleaved end  29 , using, for example, a Fujikura model 30SF splicer  25  as shown in step  2 . A first dielectric mirror  30  of titanium dioxide or similar compound is then fabricated by vacuum deposition, or a similar method, on cleaved end  28 , as shown in step  3 . Second cleaved end  29  is then spliced onto first dielectric mirror  30  by way of electric arc fusion, or a similar method, as shown in step  4 . Optical fiber  12  is then re-cut a distance L from first dielectric mirror  30 , thereby forming a third cleaved end  32  and a fourth cleaved end  33 , as shown in step  5 . A second dielectric mirror  34  of titanium dioxide or similar compound is formed on third cleaved end  32 , shown in step  6 . Fourth cleaved end  33  is then spliced onto second dielectric mirror  34 , thereby reforming optical fiber  12  into a continuous length of fiber and forming the Fabry-Perot Interferometer  26  therein, as shown in step  7 . Distance L can be formed so as to be between about 0.5 millimeters (mm) and 10 meters. Preferably, distance L is between 5 and 25 millimeters. Most preferably, distance L is approximately 12 millimeters. Optical fiber  12 , including interferometer  26  is then re-coated with outer coating  24  along the area from which outer coating  24  was previously removed, as shown in step  8 . The adhesion between the dielectric mirrors  30  and  34  and the optical fiber  12  is sufficient to withstand the stresses exerted on the optical fiber  12  and the associated fiber optic sensor  14  as the optical fiber  12  travels through the process or system  18 . 
     As an alternative to dielectric mirrors, Bragg gratings can be used to form the Fabry-Perot cavity. One would write the Bragg gratings at the appropriate spacing, L. In order to achieve broadband reflection encompassing the wavelength of a light signal, for example, from a laser, the gratings would be chirped. 
     The electrical circuit  16  (FIG. 1) includes a semiconductor laser diode or light emitting device  36  coupled to an end of fiber  12  and which emits a source light signal  44  at a predetermined frequency or set of frequencies. A first photo detector  38  is coupled to source  36  to convert at least a portion of the source light signal  44  into an electrical signal. A second photo detector  40  is coupled through coupler  56  to optical fiber  12  to detect a return light signal  46  and provide a second electrical signal representative thereof. A comparator  42  is coupled to detectors  38  and  40  to compare the first and second electrical signals. Microcontroller  48  is coupled to comparator  42  and analyzes the signal therefrom to calculate the stress being exerted upon the optical fiber  12  and the associated fiber optic sensor  14 . Circuit  16  has an output terminal  50  for providing a signal to a data acquisition system  52  such as an oscilloscope or an appropriate processor that allows the operator to monitor the output of the system, in real time. A detailed description of the circuit  16  is now presented with reference to FIG.  3 . 
     In FIG. 3, a system clock  64  generates and sends a clock pulse to a function generator  54  and the microcontroller  48 . The function generator  54  produces an appropriate current periodic waveform in response to the clock pulse for modulating the source light signal  44  from the laser diode  36  through a modulation cycle. The signal generated by the function generation  54  is converted to a digital signal by a analog to digital converter  55  and combined with the signal from the microcontroller  48  by a variable gain amplifier  57 . The gain of amplifier  57  is controlled by the signal applied to a gain control input from microcontroller  48 . The resultant output signal from the amplifier  57  is then coupled to a laser driver amplifier  59 , which in turn, provides the source signal to laser diode  36 . The first photo detector  38  receives a portion of the laser output power from the laser diode  36  and generates an electrical signal proportional to the optical power received by first photo detector  38 . An optical isolator (not shown) may optionally be provided to prevent reflected light from destabilizing the laser output. The light propagated in the optical fiber  12  is then split by a fiber coupler  56  to provide optical power to the fiber optic sensor  14  located upstream of the processing area  18 . 
     A portion of the source light signal  44  is reflected by the fiber optic sensor  14  as a function of stress on the optical fiber  12  creating a return light signal  46  that is routed through the fiber coupler  56  to the second photo detector  40 , which converts the return light signal  46  into an electrical signal proportional to the optical energy reflected from the sensor  14 . 
     The electrical signal from the second photo detector  40  serves as the input to a second variable gain operational amplifier  59  which has one or more gain stages controlled by a signal applied to an input terminal  49  from microcontroller  48 . The output signal from amplifier  59  is provided as one input to comparator  42 . The signal from first photo detector  38  is routed to an operational amplifier  60  which provides an output signal that serves as the other input to the comparator  48 . The gain of amplifier  59  is adjusted so that the two comparator input signals cross one or more times during a modulation cycle, so that each time a crossing occurs, the output of the comparator  48  changes from a logic “low” to logic “high” state and vice versa. At the time a modulation cycle begins, a first control signal is sent by the microcontroller  48  to a counter  62  to initiate the counting of clock pulses generated internally or received from the system clock generator  64  via conductor  63 . Starting at a preprogrammed time after the beginning of each modulation cycle, the microcontroller  48  monitors the output signal from the comparator  42  to determine when a selected transition occurs in the output signal of the comparator  42 . When such a change is identified, the microcontroller  48  sends a second control signal to the counter  62  to stop counting. The accumulated count during the time from the initiation of the cycle to the end of the cycle is then provided to the microcontroller/microprocessor  48 , where it is further processed. The microprocessor  48  may process the accumulated count by baseline subtraction to remove common or DC components and then multiplication by a calibration factor as described in the above identified &#39;406 patent to produce a digital output whose value equals that of the stress exerted on the optical fiber  12  at any given time. 
     As the optical fiber  12  is run through a process or system  18  which places stresses upon optical fiber  12  and the fiber optic sensor  14 , physical stress exerted on optical fiber  12  results in a change of length of optical fiber  12  including that section which makes up the fiber optic sensor  14 . As discussed above, this change in length results in a relative phase shift of the light signals  44  and  46  within the fiber optic sensor  14  which is converted into a strain reading which, in turn, is then converted into real-time stress readings, thereby allowing for accurate determination of the stress-time profile exerted on the optical fiber. 
     Sensing system  10  can be used to effectively monitor those processes and systems associated with high speed processing of optical fiber and fiber optic cable manufacturing, as well as the installation of each. These applications of the present sensing system  10  include measuring the unload rate during proof testing of an optical fiber, thereby ensuring that the optical fiber meets industry strength standards and requirements. In addition, sensing system  10  can be used in determining the stress transfer of varying belt and capstan designs, as well as developing stress profiles that occur during coloring, ribboning and stranding processes. Further, sensing system  10  can be used to troubleshoot processes associated with manufacturing fiber optic cables such as finding worn bearings as well as high stress points within the process. The processes as listed above are in no way intended to be limiting on the applicability of the present invention and are provided as merely exemplary applications. 
     By way of one example, as seen in FIG. 4, the present invention could be used to determine the amount of stress exerted on optical fiber  12  when processed through a coloring die  66 . As known in the art, coloring dies are used to apply a colorant  69 , usually provided in the form of a UV uncured acrylate, to an optical fiber. As shown in the corresponding graph of FIG. 4, the coloring die  66  exerts a stress on optical fiber  12  due to a frictional force between optical fiber  12  and coloring die  66 . As the optical fiber  12  travels in a direction  68  through coloring die  66 , the frictional forces acting upon the integral Fabry-Perot Interferometer  26  results in a change in length thereto, thereby allowing for the calculation of the stress exerted on the optical fiber  12  as described above. 
     The generated stress versus time profile as seen in FIG.  4  and allows for accurate determination of the stress exerted on the optical fiber  12  when it passes through coloring die  66 . The stress-time profile can then be used for purposes such as troubleshooting the associated optical fiber production line, optimizing die tension, calculating the stress history of the optical fiber to develop accurate optical fiber strength reliability models, as well as numerous applications. 
     Another aspect of the present sensor system  10  is to provide a friction trigger  68  (FIGS. 1 and 5) located at a fixed position relative to the movement of the optical fiber  12  as optical fiber  12  travels through the process or machine  18  to be monitored. Friction trigger  68  includes a first body  70  having an inner face  71 , and an opposed second body  72  having an inner face  73 . A cloth cover  74  is secured to inner face  71  of first body  70  and to the inner face  73  of the second body  72 . A compression spring  76  holds the cloth covers  74  secured to the first body  72  and second body  74  in close contact. In operation, frictional trigger  68  is positioned about optical fiber  12  (extending into and out of the place of drawing FIG. 5) so as to provide a slight frictional force therebetween. When the fiber optic sensor  14  passes through friction trigger  68  a slight increase in the stress exerted on the optical fiber  12  occurs, thereby providing the user with a point in time from which to begin reading the stress forces as encountered by fiber optic sensor  14  as it travels through process or machine  18 . By knowing the location of the friction trigger, the speed which the optical fiber  12  travels through the process or machine  18 , and the distance between the components within the process or machine  18  that will exert a stress on the fiber optic sensor  14 , the operator can determine the stress exerted on the optical fiber  12  by each component of the process or machine  18 . The description of the frictional trigger  68  as provided above is in no way intended to limit the configuration of the trigger device or its function in the process. 
     It will be apparent to those skilled in the art that various modifications and adaptations can be made to the present invention without departing from the spirit and scope of this invention. Thus, it is intended that the present invention, provided they come within the scope of the appended claims and their equivalents.