Abstract:
A method and apparatus for a fiber optic measuring system is disclosed which works on the speed of propagation principle. The device can be utilized to determine the position of a disturbance or electronic source along the length of an optical path.

Description:
BACKGROUND OF THE INVENTION 
     The method and device embodying the present invention relates to optic measuring systems, and in particular those systems using fiber optic Sagnac interferometric techniques. 
     Optic measurement techniques are increasingly in demand due to the advantages such as light weight and low maintenance costs. Once an optical path is established, as by means of a fiber optic cable, it is desirable to have the ability to pinpoint any disturbances occurring along the path of optical light propagation. Time domain reflectometry has been used to make position measurements in certain limited situations, but it is cumbersome and unsuitable in many instances. It is especially cumbersome and disadvantageous for use in specific applications involving the Sagnac interferometer. 
     SUMMARY OF THE INVENTION 
     In Sagnac interferometric devices it is critical to know the lengths of the optical paths. Before being placed into service, the devices must be calibrated. The length of a fiber optic sensing loop in a fiber optic sensor must be known with sufficient certainty to permit the most accurate calibration. 
     In a typical Sagnac sensor, such as that described in a patent application entitled &#34;Fiber Optic Sensor&#34;, on behalf of Eric Udd, et al., Ser. No. 917,390, which issued on Nov. 29, 1988 as U.S. Pat. No. 4,787,741, a phase modulator is an important part of the device. A phase modulator is normally placed at one end of the sensing loop. The exact location of the phase modulator is important in order to calibrate the system to accurately detect environmental disturbances. The environmental disturbances typically act in a nonsevere manner upon the fiber optic sensing coil in the form of temperature, pressure and strain. To detect these environmental disturbances and the slight affect they have on the sensing fiber, the system dimensions must be ascertained to the highest degree of accuracy. 
     The fiber optic measuring system of the present invention may be used to determine the position of this type of phase modulator within the Sagnac interferometer to a high degree of accuracy. In particular, it may be used to determine the overall length of the entire fiber optic coil, inclusive of the sensing and detection elements. It can also determine the distance from the phase modulator to the central beamsplitter as well as the distance from the phase modulator to its symmetry point on the fiber optic coil. 
     In addition, the fiber optic measuring system of the present invention can measure the key parameters of an unknown single mode fiber coil. The device herein is ideally suited to perform length and dispersion measurements under fixed or varying environmental conditions. The device herein is a convenient means for defining the key performance parameters of the fiber optic coil and phase modulator in the Sagnac interferometer configuration. Also, the device can be used as a new class of fiber optic sensors by using its optical pathlength measuring capability to determine the magnitude of the environmental effect which induced the change in pathlength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing summary of the invention and other advantages of the invention will be described in more detail below taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a schematic of a generalized Sagnac fiber optic system which will illustrate the workings of the device of the present invention; 
     FIG. 2 is a graphical illustration of the timing sequence of the phase pulses used in the present invention with dimensions referable to those illustrated in FIG. 1; 
     FIG. 3 is a timing and relative phase graph illustrating the time of arrival of the clockwise and counterclockwise phase pulses, and the resulting relative phase separation between them, as utilized in the Sagnac fiber optic system of FIG. 1. 
     FIG. 4 is a graphical illustration of the utilization of the present invention in the Sagnac fiber optic system of FIG. 1, operating in the repetitive pulse train mode at a frequency of repetition less than the characteristic frequency; 
     FIG. 5 is a graphical illustration of the utilization of the present invention in the Sagnac fiber optic system of FIG. 1, operating in the repetitive pulse train mode at a frequency of repetition greater than the characteristic frequency; 
     FIG. 6 is a timing diagram illustrating the utilization of the present invention in the Sagnac fiber optic system of FIG. 1, operating in the repetitive pulse train mode at a frequency of repetition equal to the characteristic frequency; and, 
     FIG. 7 is a schematic of a generalized Sagnac fiber optic system as shown in FIG. 1, with a multicolor switchable light source. 
     FIG. 8 is a schematic of a generalized Sagnac fiber optic system as shown in FIG. 1, illustrating automatic computer operation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a schematic of a generalized Sagnac fiber optic system of the present invention is shown. Light source 11 is fiber optically connected to a first intensity maintenance element 13. First intensity maintenance element 13 is fiber optically connected to a first beam splitter 15. First beam splitter 15 is also fiber optically connected to a detector 17. Detector 17 is electrically connected to an oscilloscope 19. 
     First beam splitter 15 is also fiber optically connected to a polarizer 21. Polarizer 21 is fiber optically connected to a second beam splitter 23. Opposite the fiber optic connection to polarizer 21, second beam splitter 23 has one fiber optic connection to a second intensity maintenance element 25, and another fiber optic connection to a third intensity maintenance element 27. Intensity maintenance elements 25 and 27 are fiber optically connected by an elongate optical fiber 29 arranged generally in the shape of a loop. 
     A phase modulator 31 is situated along the elongate optical fiber 29. A distance L1 is illustrated, and is the product of twice the distance in the counterclockwise direction from phase modulator 31 to the midpoint of the loop formed by elongate optical fiber 29. If the loop formed by elongate optical fiber 29 is considered to have a top half and a bottom half, L1 represents the distance required to travel from pulse generator 33, located on one half of the loop to the point where its mirror image would be located on the other half of the loop. Phase modulator 31 is electrically connected to a pulse generator 33. Pulse generator 33 is electrically connected to a frequency generator 35. 
     Light source 11, such as a light emitting diode (LED) or superradiant diode is used to supply a steady reliable source of light to first intensity maintenance element 13. First intensity maintenance element 13 is used to scramble any residual polarization in the light beam emanating from light source 11. The purpose of the intensity maintenance units is to allow the usage of single mode optical fiber without signal fading alternatively polarization preserving fiber may be used throughout the device. After passing through by intensity maintenance element 13, the light beam from light source 11 enters beam splitter 15. Beam splitter 15 is of the input/output type. A light beam propagating away from light source 11 will pass through beam splitter 15 with almost no light from the light source 11 reflecting back onto detector 17. Detector 17 will be enabled to receive light propagating from polarizer 21 toward beam splitter 15. 
     The polarizer 21 is for the purpose of insuring that reciprocity conditions are met for light travelling in both directions. After light from light source 11 propagates through polarizer 21, it is split into counterpropagating beams by second beam splitter 23. The light beam is split into a clockwise propagating beam and a counterclockwise propagating beam. The clockwise propagating beam propagates through second intensity maintenance element 25, optical fiber 29, phase modulator 31, and third intensity maintenance element 27. The counterclockwise propagating beam propagates through third intensity maintenance element 27, phase modulator 31, optical fiber 29, and second intensity maintenance element 25. Phase modulator 31 should be considered as occupying various points along the length of optical fiber 29. The distances L1 and L2 represent distances which can be varied in terms of their absolute magnitudes. The principles by which the method and device of the present invention operates apply to all magnitudes. The actual positioning of phase modulator in the loop formed by optical fiber 29 is arbitrary. 
     Frequency generator 35 triggers pulse generator 33. Pulse generator 33 drives phase modulator 31 to impress a phase change over a short time interval on the counterpropagating light beams within optical fiber 29. The two counterpropagating light beams recombine on second beam splitter 23. The recombined light beam carries an amplitude modulated signal resulting from the phase difference experienced by the clockwise and counterclockwise counterpropagating beams which formed the recombined light beam. The recombined light beam propagates back to detector 17 through polarizer 21 and first beam splitter 15. Oscilloscope 19 displays the electronic activity within detector 17 by means of an electrical connection therewith. The oscilloscope 19 may have a sweep triggerable by frequency generator 35 to synchronize and therefore enable the best view of the time varying changes occurring within detector 19. 
     The theory of operation of the method and apparatus of the present invention is best illustrated by considering the case of a single pulse impressed upon the counterpropagating beams by phase modulator 31. When the pulse is produced, it is impressed upon both counterpropagating light beams simultaneously. Phase modulator 31 is offset from the exact center of the fiber coil by one half of the distance L1, or (L1)/2. If the two counterpropagating light beams are out of phase, phase pulse impressed by the phase modulator 31 is converted to an amplitude pulse when the clockwise and counterclockwise light beams recombine upon second beam splitter 23. The pulse is transmitted to detector 17 and is readily observable on oscilloscope 19. 
     Referring to FIG. 2, a graphical illustration of the timing sequence of the phase pulses emitted by phase modulator 31 is shown. The occurrence of the events shown in FIG. 2 are referenced to events occurring at detector 17 where the separation in optical path between the central beam splitter 23 and detector 17 is neglected. For exactness the optical path L3 between the central beam splitter 23 and detector 17 should be added to L 1  and L 2  to represent the total optical pathlength difference from the phase modulator 31 to the detector 17. At a time equal to T 0 , phase modulator 31 emits a pulse. Referring to FIG. 1, the pulse combines with both the clockwise and counterclockwise propagating beams of light. The clockwise propagating beam of light with the phase modulated pulse impressed upon it must only travel a distance L 2  before reaching second beam splitter 23. At second beamsplitter 23, the phase modulated clockwise beam of light combines with the non phase modulated counterclockwise propagating beam of light, both to be interferometrically combined then transmitted to detector 17. Sufficient time has not yet elapsed for the portion of the counterclockwise beam of light which has been phase modulated to reach second beam splitter 23. This is why the phase modulated clockwise beam of light will initially combine with a counterclockwise beam of light which has not yet been phase modulated. 
     The combined clockwise and counterclockwise beam arrives at the detector 17 with a first pulse due to the modulation of the clockwise beam of light within optical fiber 29. Referring to FIG. 2, this event is represented on the graph as a unit pulse occurring at time T 1 . Referring back to FIG. 1, when the clockwise phase impressed pulse was travelling toward and combining upon second beam splitter 23, the counterclockwise phase impressed pulse was travelling past the midpoint of and around optical fiber 29. The unit pulse due to the phase impressed counterclockwise light beam does not reach detector 17 until some time after the unit pulse due to the phase impressed clockwise light beam. Referring to FIG. 2, this is represented on the graph as a unit pulse occurring at time T 2 . 
     As shown in FIG. 2, the time before the first pulse arrives at the detector is equal to (L 2  +L 3 )n/c, where L 2  is the distance from phase modulator 31 to second beam splitter 23 as was shown in FIG. 1, L 3  is the distance from the second beam splitter 23 to the detector 17, n is the refractive index of the medium in which the counterpropagating light beams travel, and c is the speed of light. Similarly the time of arrival of the second pulse at the detector is equal to (L 1  +L 2  +L 3 )n/c, where L 1  is twice the distance from phase modulator 31 to the midpoint of optical fiber 29 as was shown in FIG. 1, L 3  is the distance from the second beam splitter 23 to the detector 17, n is the refractive index of the medium in which the counterpropagating light beams travel, and c is the speed of light. Note in FIG. 2 that the quantities of time (L 2  +L 3 )n/c and (L 1  +L 2  +L 3 )n/c do not overlap unless L 1  equals zero representing the special case of the phase modulator 31 located at the midpoint of the fiber loop 29, and are the distances between each of the time quantities T 0 , T 1 , and T 2 . 
     FIG. 3 is a timing and relative phase graph illustrating the time of arrival of the clockwise and counterclockwise phase pulses, and the resulting relative phase separation between them. T 0  is the time of the initialization of the pulse, T 1  is the time of arrival of the clockwise pulse and T 2  is the time of arrival of the counterclockwise pulse. The relative magnitudes of the times shown in FIG. 3 illustrate a typical case wherein the phase modulator 31 of FIG. 1 is offset from the center of the fiber loop 29. 
     Measuring the time difference between the initiation and arrival of the clockwise and counterclockwise pulses is one way to utilize the method and device of the present invention. A more accurate means to measure the distance L 1  of FIG. 1 is to use a pulse train at a variable repetition rate. The repetition rate is adjusted until the successive pulses null each other out. This nulling out occurs when the frequency of pulsation is such that the n+1th clockwise pulse arrives at second beam splitter 23 at the same time as the nth counterclockwise pulse for the case where the phase modulator 31 is closer to the central beam splitter 23 along the counterclockwise path of the optical fiber 29. The situation is reversed if the phase modulator 31 is closer to the central beam splitter 23 along the clockwise path of the optical fiber 29. If both the clockwise and counterclockwise pulses combine upon beam splitter 31 simultaneously, the phases will cancel each other to form the nulled out condition. As the clockwise and counterclockwise beams proceed to detector 17, this nulling effect is transmitted with them. Therefore detector 17 can detect the nulling and facilitate its display on oscilloscope 19. 
     For a fiber optic Sagnac system as shown in FIG. 1, the frequency necessary for the n+1 clockwise pulse to combine with and cancel the nth counterclockwise pulse is referred to as the characteristic frequency of the system. This frequency will be a function of L 1  and L 2 , and thus measure the placement of the phase modulator 31 along optical fiber 29 of FIG. 1. 
     A repetition frequency for the pulsing of phase modulator 31 which is less than the characteristic frequency will result in the reception of both the clockwise and the counterclockwise pulse resulting from a single pulse triggering at detector 17 before receipt of the next one of the next pair of pulses from the next triggering. This is illustrated in FIG. 4. The first line in FIG. 4 illustrates a rate of repetition of the firing of phase modulator 31 which is less than the characteristic frequency for the particular Sagnac fiber optic configuration. The second line illustrates the pairs of pulses received at detector 17 which relate to a single phase modulator 31 pulse. This results in a non continuous, or spaced, condition with respect to each set of received pulses. Both of the pulses from a single phase modulator 31 activation are received before any one of the next pair are received. 
     A repetition frequency for the pulsing of phase modulator 31 which is greater than the characteristic frequency will result in the reception of the clockwise pulse of the next pair of pulses before the counterclockwise pulse of the last pair of pulses is received at detector 17. This is illustrated in FIG. 5. The first line in FIG. 5 illustrates a rate of repetition of the firing of phase modulator 31 which is greater than the characteristic frequency for the particular Sagnac fiber optic configuration. The second line illustrates the pairs of pulses received at detector 17 which relate to a single phase modulator 31 pulse. A repetition frequency for the pulsing of phase modulator 31 which is greater than the characteristic frequency will result in an overlap condition with respect to each set of received pulses. 
     If the repetition frequency for the pulsing of phase modulator 31 is equal to the characteristic frequency, the second of each pair of pulses received at detector 17 will be cancelled by the first of the next pair of pulses. Under these conditions, the only pulses which will not be cancelled will be the very first pulse of the first pair and the last pulse of the last pair, as received at detector 17. This is illustrated in FIG. 6. FIG. 6 illustrates a series of time frames from 1 to N, where N represents the Nth or last time frame. The first line in FIG. 6 illustrates the periodic appearance at detector 17 of the clockwise pulse produced by phase modulator 31. The second line in FIG. 6 illustrates the periodic appearance at detector 17 of the counterclockwise pulse produced by phase modulator 31. The third line in FIG. 6 illustrates the combined relative appearance of both of the pulses at detector 17. The third line is representative of what would appear on the screen of oscilloscope 19. Since the second pulse of the last pair of pulses and the first of the next pair of pulses combine to cancel each other out, the screen of oscilloscope 19 will show the presence of no pulses when the frequency of phase modulator 31 is set to the characteristic frequency of the particular Sagnac fiber optic configuration. 
     Measurement of the frequency of repetition of the firing of phase modulator 17 is much more easily determined and can be determined to a greater degree of accuracy than measurement of exacting time increments by themselves. Both in the case of a direct time measurement as well as a frequency measurement, the time necessary for propagation of the pulses through the other fiber optic elements shown in FIG. 1 should be taken to account. This includes elements such as the polarizer 21, intensity maintenance elements 25 and 27, and the first and second beam splitters 15 and 23. 
     For example, consider a Sagnac fiber optic configuration similar to FIG. 1 having an L 2  of approximately 1 kilometer and an L 1  of approximately 550 meters. Using the relationship for the characteristic frequency F c  =c/(L 1 )n with c=3×10 8  m/s, L 1  =550 m, and n (refractive index of the fiber)=1.45, the estimated characteristic frequency would be 376 kilohertz. This corresponds to a time delay of t d  =(L 1 )n/c which equals 2.66 microseconds. The actual characteristic obtained was 367.14 kilohertz. The oscilloscope 19 used was accurate to about 10 hertz. Using the above equations, with a frequency in kilohertz accurate to a hundredth of a kilohertz, and more accurate values of the speed of light and index of refraction would yield a resolution of L 1  and L 2  accurate to about 1 centimeter. Further accuracy can be expected by performing the nulling out process using synchronous demodulation techniques. 
     Since the frequency is a direct measurement of the length of the optical fiber 29 of FIG. 1, this technique may be used to monitor environmental effects known to change the length of optical fiber 29, such as pressure, temperature, and strain. 
     It is also possible to measure other parameters of the optical fiber 29 such as fiber dispersion by utilizing the techniques herein and employing more than one light source. This arrangement is illustrated in FIG. 7. FIG. 7. is a schematic of a generalized Sagnac fiber optic system similar to that shown in FIG. 1, with a multicolor switchable light source. In FIG. 7 a light source 37 outputs light of a first wavelength and a light source 39 outputs light of a second wavelength. Output from light sources 37 and 39 are combined in a light coupler 41 before passing through first intensity maintenance element 13 and first beam splitter 15. After reaching intensity maintenance element 13, the light passes through the system in a manner similar to that shown in FIG. 1. However, detector 17 is replaced by a pair of detectors, 43 and 45 and fiber beamsplitter 44. Detector 43 detects light from light source 37, which is of one frequency, while detector 45 detects light from light source 39, which is of another frequency. In FIG. 7, detectors 43 and 45 employ dispersive elements to aid in discerning the differences between light which emanated from light sources 37 and 39. 
     The characteristic frequency of phase modulator 31 pulses necessary to null out the signal is a function of the dispersive properties of the fiber. In general, the characteristic frequencies necessary to null out the signal at detectors 43 and 45 will differ because they correspond to the differing wavelengths of light sources 37 and 39. These frequencies are compared and differenced by a comparator 46, which uses the frequency generator 35 to null out the signal on detectors 43 and 45 sequentially. The resulting difference frequency 48 is the output of the system that measures dispersion. 
     An alternative analagous approach would be to sweep single light source through a known wavelength region and track the frequency difference to determine the dispersive properties of the optical fiber. Alternatively, if the fiber dispersion characteristics are accurately known, the wavelength of the light source could be measured. 
     It is also possible to automatically measure the length of the optical fiber 29 by employing an iterative computer control scheme utilizing the techniques herein. This arrangement is illustrated in FIG. 8. FIG. 8 is a schematic of a generalized Sagnac fiber optic system similar to that shown in FIG. 1, but the oscilloscope with which the fundamental frequency is visually determined is replaced. 
     Detector 17 is, instead, connected to a synchronous demodulator 47. Synchronous demodulator 47 is both controllably and informationally connected to a computer 49. Computer 49 is also connected to pulse generator 33. Computer 49 controls the timing of the activation of pulse generator 33. Computer 49 controllably triggers the synchronization frequency of synchronous demodulator 47. Computer 49 also receives information from synchronous demodulator 47 concerning the nulling out of the pulses originating from pulse generator 33. Typically computer 49 may be programmable to iteratively generate a series of prespecified frequencies in its search for the fundamental frequency of the fiber optic system. Using the estimation technique described above, an operator or an automatic software algorithm could enter the estimate into computer 49, and the computer 49 then could automatically compute a frequency limit on either side of the computed frequency. Computer 49 could then proceed to generate a steadily increasing frequency beginning at the lower limit and continuing towards an upper limit until either the fundamental frequency is ascertained, or the upper limit is reached, or until the upper frequency capability of the computer is reached. 
     The foregoing disclosure and description of the invention are illustrative and explanitory thereof, and various changes in the optical circuitry and devices used therein, as well as in the details of the illustrated construction may be made without departing from the spirit and scope of the invention.