Patent Document

CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     This patent application is co-pending with two related patent applications entitled FIBER OPTIC PITCH OR ROLL SENSOR Ser. No. 09/983,047 and FIBER OPTIC CURVATURE SENSOR FOR TOWED HYDROPHONE ARRAYS Ser. No. 09/983,048, by the same inventors as this application. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefore. 
     BACKGROUND OF THE INVENTION 
     (1). Field of the Invention 
     This invention relates to a system for the multiplexing and interrogation of fiber optic Bragg grating based sensors. 
     (2). Description of the Prior Art 
     Fiber optic Bragg gratings are periodic refractive index differences written into the core of an optical fiber. They act as reflectors with a very narrow reflected wavelength band, while passing all other wavelengths with little loss. Temperature or strain changes the wavelength at which they reflect. They can be made into sensors for any one of a number of measurands by designing a package that strains the grating in response to changes in the measurand. 
     U.S. Pat. Nos. 5,633,748 to Perez et al.; 4,996,419 to Morey; 5,627,927 to Udd; 5,493,390 to Varasi et al.; and 5,488,475 to Friebele et al. illustrate the use of Bragg gratings as a sensor. All of the sensors in these patents function by using the shift of the Bragg grating reflection wavelength. 
     U.S. Pat. No. 5,564,832 to Ball et al. relates to a birefrigent active fiber laser sensor. While Ball et al. use more than one Bragg grating laser in his sensor, they use each laser singly rather than in a pair. Moreover, each laser is birefringent such that it lases in two separate polarization modes at different frequencies. Ball et al. detect the wavelength difference between these two modes. The use of birefringent sensors means that Ball et al. must arrange the measurand to affect the birefringence. Ball et al. determine the frequency difference between the two birefringent modes by electronically measuring the beat or difference frequency. The present invention does not use lasers which are birefringent nor rely on changes in birefringence. 
     An alternative sensor is the fiber optic Bragg grating laser. Two gratings at matched wavelengths are written into a length of optical fiber which is doped to be an active medium. The most common is an Erbium doped silica glass fiber. When power from a pump laser is injected into the cavity, the structure emits output laser light. If the cavity is short enough, the emission is in a single longitudinal mode. Any measurand which strains the cavity causes the laser emission to shift in wavelength. 
     The difficulty to date has been in developing systems which can both read the wavelength shift, and hence the strain, with great sensitivity, and do so efficiently for multiple sensors. The most sensitive techniques developed have used interferometric means to measure the shift in wavelength. However, these techniques measure only dynamic changes and are incapable of reading absolute values. A device such as the Wavemeter sold by Burleigh Instruments uses an interferometric technique to give both high sensitivity and absolute measurements. However, it does so by changing the path delay in the interferometer, resulting in a slow measurement. Diffraction based spectrum analyzers have limited resolution, 0.1 nm corresponding to 60 microstrains. Fabry-Perot etalon spectrum analyzers have high resolution but read relative wavelength. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object to provide an improved system for interrogating a plurality of fiber optic Bragg grating based sensors. 
     It is a further object of the present invention to provide a system as above which provides efficient measurement of many sensors with absolute measurements, high strain sensitivity, high dynamic range, and fast measurements. 
     The foregoing objects are achieved by the sensor interrogation system of the present invention. 
     In accordance with the present invention, a sensor interrogation system broadly comprises an optical fiber, at least one sensor containing first and second fiber lasers attached to the optical fiber with the first fiber laser being located spectrally at a first wavelength and the second fiber laser being located spectrally at a second wavelength different from the first wavelength, means for causing light to travel down the optical fiber so as to cause each of the fiber lasers to lase at its distinct wavelength and generate a distinct laser signal representative of the distinct wavelength; filter means for receiving the laser signals generated by the first and second lasers and for transmitting the laser signals from the first and second lasers within a wavelength band, and means for receiving the laser signals and for determining the wavelength difference between the fiber lasers. 
     A method for interrogating a sensor system having an optical fiber, at least one sensor containing first and second fiber lasers attached to the optical fiber with the first fiber laser being located spectrally at a first wavelength and the second fiber being located spectrally at a second wavelength broadly comprises the steps of causing light to travel down the optical fiber so as to cause each of the fiber lasers to lase at its distinct wavelength and generate a distinct laser signal representative of the distinct wavelength. transmitting the laser signals generated by the first and second fiber lasers to a filter means, allowing laser signals within a wavelength band to pass through said filter means, providing analyzer means to receive the laser signals passed through the filter means, and determining the wavelength difference between the first and second fiber lasers from the received laser signals. 
     Other details of the sensor interrogation system of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description and the accompanying drawings, wherein like reference numerals depict like elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a sensor used in the system of the present invention; 
     FIG. 2 is a schematic representation of a multiplexed fiber laser sensor system; 
     FIG. 3 is an output trace from a scanning Fabry-Perot spectrum analyzer; and 
     FIG. 4 illustrates an alternative embodiment of a multiplexed fiber laser sensor system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIG. 1 illustrates a sensor to be used in the system  12  of the present invention. The sensor  10  has an optical fiber  14  containing a first optical fiber Bragg grating laser  16  and a second optical fiber Bragg grating laser  18 . The Bragg gratings of each of the lasers  16  and  18  reflects at a different wavelength so that the lasers  16  and  18  emit at different wavelengths. The sensor  10  is designed so that the measurand has a different effect on the two lasers  16  and  18 . In one embodiment of the sensor  10 , one of the lasers  16  and  18  may be sensitive to the measurand while the other of the lasers is insensitive. In a second embodiment of the sensor  10 , each of the lasers  16  and  18  may be sensitive to the measurand but in the opposite direction. The sensor  10  may be used to measure any measurand provided that the sensor structure can be designed which strains the fiber lasers  16  and  18  in the manner just described. 
     As the measurand shifts, the difference in wavelength between the two lasers  16  and  18  changes and the difference can be calibrated to the value of the measurand to provide an absolute measurement. 
     Referring now to FIG. 2, a multiplexed fiber laser sensor system  12  is illustrated. In this system, a single optical fiber  20  contains numerous fiber lasers  22 , two of which form each sensor  24 . Each laser  22  is located spectrally at a different wavelength. 
     The system includes a pump laser  26  which provides pump light at the distinct pump wavelength through a wavelength demultiplexer  28 . The pump light travels down the optical fiber  20  and is absorbed within each fiber laser cavity, causing each laser  22  to lase at its distinct wavelength in a continuous manner. The light from each laser  22  returns down the optical fiber  20 , through the wavelength demultiplexer  28 , through an optional fiber amplifier  30 , to a filter  32 . The filter  32  passes a narrow wavelength band and is tunable to change the band selected. The band is wide enough to pass the laser signals from both lasers  22  comprising a single one of the sensors  24 . All other lasers  22  are blocked or severely attenuated. The signals then pass to a junction  34  where the light is split to two scanning Fabry-Perot spectrum analyzers  36  and  38 . One such device which may be used for each of the analyzers  36  and  38  is the Supercavity device from Newport Corporation of Irvine, Calif. Such devices provide high finesse, thus giving a high ratio of dynamic range to accuracy. 
     A scanning Fabry-Perot spectrum analyzer is characterized by a free spectral range which is the spectral dynamic range over which spectral features can be unambiguously identified. Two laser sensors must emit at wavelengths within one free spectral range of each other if the scanning Fabry-Perot spectrum analyzer is to read the spectral difference accurately. In a typical sensor system, the laser sensors should be separated by a particular spectral distance. This would normally set the requirement for a scanning Fabry-Perot spectrum analyzer with a greater free spectral range. Since the resolution is directly related to the free spectral range, this yields a limitation on the resolution that may be achieved. The present invention however includes a means to measure spectral features which are separated by more than one free spectral range without ambiguity. This effectively extends the dynamic range of the device without sacrificing its resolution. This in turn allows greater resolution in the readout of the sensor. 
     The two scanning Fabry-Perot spectrum analyzers  36  and  38  differ in construction by the gap of the etalon and hence the free spectral range. The first analyzer  36  has a small gap, L 1 , on the order of about 20 microns. Such a device with a finesse of 5000 will have a free spectral range of 60 nanometers. The free spectral range is the spectral range between orders of the interferometer. When two lasers at different wavelengths are injected into the analyzer  36 , an output trace such as that shown in FIG. 3 is provided. One laser  22  in the sensor  24  produces several narrow peaks  40  separated by the free spectral range of the Fabry-Perot for that wavelength. The second laser  22  in the sensor  24  produces another set of peaks  42  with a slightly different spacing. The order number for each peak is given by the equation: 
     
       
           n=L   1 /λ. 
       
     
     where n is the order number, L 1  is the gap of the first analyzer  36 , and λ is the emission wavelength of the laser whose peak is being considered. 
     The free spectral range (FSR) is much greater than the difference in emission wavelength of the two fiber lasers in the sensor  24 . As a result, their peaks appear close together and the peaks share the same order. To perform a measurement, the trace generated by the scanning Fabry-Perot spectrum analyzer  36  is transmitted to a computer  37  where it is digitized and where a computer program analyzes the trace of FIG.  3 . The computer  37  may comprise any suitable computer known in the art. The computer program may be any suitable program for identifying the two peaks  40  and  42  and for determining the spectral spacing of the peaks, Δλ 1 . The computer program can be in any conventional computer language known in the art. 
     Another portion of the light enters the second analyzer  38 . This device has a smaller gap, L 2 , on the order of about 25 mm. As a result, the analyzer  38  has very high resolution but a small free spectral range. The difference in laser emission wavelength of the two lasers  22  in the sensor  24  is so large in contrast to the free spectral range of the analyzer  38 , that adjacent peaks of the two lasers do not have the same order number. The order number of a laser line in this analyzer is given by the equation: 
     
       
           n=L   2 /λ. 
       
     
     where n is the order number, L 2  is the gap of the analyzer  38 , and λ is the emission wavelength of the laser whose peak is being considered. 
     To obtain the spectral difference between the two lasers  22  in a sensor  24  with the resolution of the analyzer  38 , it is necessary to measure the difference between the peaks of the same order. In a typical scanning Fabry-Perot spectrum analyzer, this is not possible because the scan range may not be sufficient that the same order is even displayed for each laser. Furthermore, it is not possible to tell the order number of each line. This invention uses the Δλ 1  information from the analyzer  36  to calculate the order number difference between two selected peaks on the second analyzer  38 . The measured spectral difference between these two peaks can then be corrected for the order number difference to give the true spectral difference between the outputs of the lasers  22  in the sensor  24 . 
     The trace from the analyzer  38  is also transmitted to computer  37  where it is digitized and the aforementioned computer program is used to analyze the trace. The computer program in the computer  37  identifies two adjacent peaks, one corresponding to each of the lasers  22 . The scanning Fabry-Perot spectrum analyzer scan distance corresponding to the first laser is d 1 , while the distance corresponding to the second laser is d 2 . The computer program also identifies the peaks corresponding to the same laser by looking for uniform spectral differences. The scan difference between two adjacent peaks of the same laser is calculated and gives the laser wavelength. This gives the emission wavelength of the first laser λ 1 , and that of the second laser, λ 2 . 
     The emission wavelength of the second laser  22  may also be computed as: 
     
       
         λ 2 ′=λ 1 +Δλ 1 . 
       
     
     The order difference between the two peaks is given by: 
     
       
         Δ n =( d   1 /λ 1 )−( d   2 /λ 2 ′). 
       
     
     It should be noted that λ′ rather than λ 2  has been used in this calculation. The accuracy of Δn depends on the accuracy of the difference between the two wavelengths and using λ 2 ′ is more accurate. 
     The scan distance difference between the two adjacent peaks of the two different lasers is: 
     
       
         Δ d=d   2   −d   1 . 
       
     
     This is now corrected by the order number difference so that the scan distance of two same order peaks are compared: 
     
       
         Δ d′=Δd+Δnλ   2 . 
       
     
     The sensor measurand is proportional to this corrected scan distance difference. Calibration of the sensor will yield the calibration factor. 
     It is noted that the use of the order number correction has allowed the system to compare features in the second analyzer  38  that do not have the same order number. It has thus greatly expanded the dynamic range of the analyzer  38  and allowed it to be configured for finer resolution. 
     An option is to do the entire order number correction using a single scanning Fabry-Perot spectrum analyzer. In the above illustration, λ 2  could have been used instead of λ 2 ′ in the equation for Δn. Since it is available directly from the trace of the second analyzer  38 , the first analyzer  36  is not required. However, to ensure that the order number difference Δn is calculated without error, the scanning Fabry-Perot spectrum analyzer&#39;s cavity must be shortened, limiting its resolution. This option is useful when less resolution is required by the application. It reduces the system components and the cost. 
     An alternative configuration for the system  12  is shown in FIG.  4 . In this system  12 , the returning light is split by an optical coupler  50  into two paths. A tunable narrowband filter  52  is placed in either path. One filter  52  selects the wavelengths of the first laser sensor  22  of the sensor  24  to be selected. The other filter  52  selects the wavelength of the second laser sensor  22  of the sensor  24  to be selected. These are then combined by another coupler  54  and then split to the two analyzers  36  and  38 . This alternative configuration allows a narrower filter because each filter  52  passes one instead of two lasers. This in turn allows the lasers  22  to be placed closer in wavelength and more lasers to be placed on each optical fiber  20 . 
     As can be seen from the foregoing discussion, the system of the present invention achieves very fine strain sensitivity, yet does so with absolute measurements. This level of absolute strain sensitivity exceeds that achieved by other techniques. 
     Many sensors are multiplexed on a single fiber. By achieving high sensitivity, large dynamic range is achieved without requiring the laser sensors to vary too far in wavelength. This allows more sensors to be placed per fiber. 
     The measurement provided by the system of the present invention is fast as compared to alternative absolute measurement techniques. This results because the requirement to scan an optical component by several centimeters is eliminated. The rapid, short distance scanning of the piezo transducers in the scanning Fabry-Perot spectrum analyzer is sufficient. The measurement technique employed herein provides high dynamic range. 
     It should also be noted that common mode effects affecting both lasers of a sensor are eliminated. As an example, temperature may cause a fiber laser sensor to shift. This shift can cause a signal erroneously interpreted as a shift in the measurand. Because both lasers are co-located, they both shift in the same manner with temperature and their difference is approximately temperature insensitive. 
     If desired, the two lasers  22  comprising one of the sensors  24  may also be located on separate optical fibers. When such a configuration is used, after their filters, they would be combined by a single coupler. 
     It should be noted that any sensor configuration which results in the measurand producing a different effect on the two lasers may be used in the system of the present invention. 
     It is apparent that there has been provided in accordance with the present invention a multiplexed fiber laser sensor system which fully satisfies the objects, means, and advantages set forth hereinbefore. While the invention has been described in the context of specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Therefore, it is intended to embrace those alternatives, modifications, and variations which fall within the broad scope of the appended claims.

Technology Category: g