Abstract:
Two optical wavelengths are used to interrogate a fiber optic Fabry-Perot sensor having a moveable diaphragm that changes the width of a gap between two reflective surfaces. By picking the right operating point for the gap, the power output for one wavelength increases as the gap width changes and the power for the other wavelength decreases. A ratio of the difference of the two powers over the sum of the two powers is formed to generate a detected signal independent of power and phase fluctuations in a fiber between signal sources and sensor and between sensor and detector. This ratio, which is called the visibility, has a response proportional to the pressure of acoustic disturbances that move the diaphragm. The push-pull sensor can be used with both TDM and CW fan-out array architectures.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    Applicant claims priority for this application based upon U.S. Provisional Application Ser. No. 61/134,509, filed Jul. 10, 2008. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]    This invention relates generally to a Fabry Perot interferometer for use in a fiber optic sensor array to sense changes in acoustic pressure. This invention relates particularly to a two-wavelength Fabry-Perot sensor system for sensing acoustic pressure. 
         [0003]    The use of Fabry-Perot sensors in fiber optic acoustic sensor arrays has been proposed many times. However, difficulties are encountered in implementing such sensor arrays with Fabry-Perot interferometers. Fiber optic sensor systems have power fluctuations associated with source lasers, time dependent polarization effects in the fiber, and other disturbances between the source laser and sensors and between sensors and photodiode detectors. 
       SUMMARY OF THE INVENTION  
       [0004]    This invention provides a two-wavelength Fabry-Perot interferometric sensor system that overcomes problems caused by optical power fluctuations in prior interferometric acoustic sensor systems. 
         [0005]    A fiber optic sensor system according to the present invention comprises a first coherent optical signal source that produces an optical signal of wavelength λ A , a first optical fiber arranged to receive the optical signal of wavelength λ A , a second coherent optical signal source that produces an optical signal of wavelength λ B , a second optical fiber arranged to receive the optical signal of wavelength λ B , an input wavelength division multiplexer arranged to receive the optical signals of λ A  and λ B  from the first and second optical fibers, respectively, a signal transmission optical fiber arranged to receive optical signals of both wavelengths λ A  and λ B  from the wavelength division multiplexer, an array of two-wavelength Fabry-Perot interferometric sensors coupled to the signal transmission optical fiber and arranged to receive optical signals of wavelengths λ A  and λ B  therefrom, the array of two-wavelength Fabry-Perot interferometric sensors being arranged to operate in a push-pull mode to produce interferometer output signals in response to an acoustic pressure wave incident upon the array of two-wavelength Fabry-Perot interferometric sensors, a detector array coupled to the signal transmission optical fiber to produce electrical signals in response to the interferometer output signals, and a signal processor for processing the electrical signals from the detector array to indicate pressure from the acoustic pressure wave. 
         [0006]    A fiber optic sensor array according to the present invention may further comprise an optical on-off switch arranged to control transmission of optical signals through the signal transmission optical fiber. 
         [0007]    The array of two-wavelength Fabry-Perot interferometric sensors may comprise a time division multiplexed architecture connected to the optical switch wherein a plurality of optical couplers couple a corresponding plurality of two wavelength Fabry-Perot interferometric sensors to the signal transmission fiber. and an output wavelength division multiplexer arranged to divide optical signals output from the plurality of Fabry-Perot interferometric sensors into a first output signal having wavelength λ A  and a second output signal having wavelength λ B . 
         [0008]    The array of two-wavelength Fabry-Perot interferometric sensors may alternatively comprise a fan-out architecture of a plurality of two wavelength Fabry-Perot interferometric sensors coupled to the input wavelength division multiplexer to receive optical signals of wavelength λ A  and λ B  therefrom, and a photodetector array coupled to the fan-out architecture such that each two-wavelength Fabry-Perot interferometric sensor therein has an output coupled to a first corresponding photodetector arranged to detect signals of wavelength λ A  and to a corresponding second photodetector arranged to detect signals of wavelength λ B . 
         [0009]    The fiber optic sensor array may comprise a Fabry-Perot interferometric sensor formed to include a single mode optical fiber, a ferrule mounted at an end of the single mode optical fiber so as to extend a distance W gap  beyond the end, a diaphragm mounted on the ferrule to form an enclosed region, the diaphragm being arranged to receive an acoustic pressure wave, the diaphragm being movable with respect to the end of the single mode optical fiber to modulate the distance between the diaphragm and end of the single mode optical fiber in response to pressure variations in the acoustic pressure wave, a fluid within the enclosed region; and a multimode optical fiber having an end of a multimode core arranged such that optical signals of wavelength λ A  and λ B  propagating in the single mode optical fiber undergo multiple reflections and produce a diverging light beam comprising interference signals of wavelengths λ A  and λ B  that propagates through the diaphragm and are injected into the end of the multimode core. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]      FIG. 1  illustrates a reflection mode Fabry Perot interferometer formed with an optical fiber; 
           [0011]      FIG. 2  graphically illustrates reflectivity as a function of gap width for the reflection mode Fabry-Perot interferometer of  FIG. 1  for a wavelength of 1550 nm; 
           [0012]      FIG. 3  graphically illustrates reflectivity as a function of gap width for the reflection mode Fabry-Perot interferometer of  FIG. 1  for a first wavelength of 1550 nm and a second wavelength of 1480 nm; 
           [0013]      FIG. 4  graphically illustrates reflectivity as a function of gap width for the reflection mode Fabry-Perot interferometer of  FIG. 1  for a first wavelength of 1550 nm and a second wavelength of 1475.5 nm; 
           [0014]      FIG. 5  graphically illustrates visibility as a function of gap width for the reflection mode Fabry-Perot interferometer of  FIG. 1  for a first wavelength of 1550 nm and a second wavelength of 1480 nm; 
           [0015]      FIG. 6  graphically illustrates visibility slope as a function of gap width for the reflection mode Fabry-Perot interferometer of  FIG. 1  for a first wavelength of 1550 nm and a second wavelength of 1480 nm; 
           [0016]      FIG. 7  graphically illustrates reflectivity as a function of gap width for a reflection mode Fabry-Perot interferometer for two wavelengths operated in a push only mode; 
           [0017]      FIG. 8  graphically illustrates reflectivity ratios as a function of gap width for a reflection mode Fabry-Perot interferometer for two wavelengths operated in a push only mode; 
           [0018]      FIG. 9  graphically illustrates the slope of reflectivity ratios as a function of gap width for a reflection mode Fabry-Perot interferometer for two wavelengths operated in a push only mode; 
           [0019]      FIG. 10  schematically illustrates a time division multiplexed (TDM) two wavelength reflection mode Fabry Perot Sensor array according to the present invention; 
           [0020]      FIG. 11  schematically illustrates a continuous wave (CW) two-wavelength reflection mode fan-out Fabry Perot Sensor array according to the present invention. 
           [0021]      FIG. 12  illustrates a transmission mode Fabry Perot interferometer; 
           [0022]      FIG. 13  schematically illustrates a continuous wave (CW) two wavelength transmission mode Fabry Perot Sensor array according to the present invention that includes a plurality of transmission mode Fabry Perot interferometers according to  FIG. 12 ; 
           [0023]      FIG. 14  graphically illustrates signal transmission as a function of gap width for the transmission mode Fabry-Perot interferometer of  FIG. 9  operated in a push-pull mode; 
           [0024]      FIG. 15  graphically illustrates transmission visibility ratio as a function of gap width for the transmission mode Fabry-Perot interferometer of  FIG. 9  operated in a push-pull mode; and 
           [0025]      FIG. 16  graphically illustrates transmission visibility slope as a function of gap width for the transmission mode Fabry-Perot interferometer of  FIG. 9  operated in a push-pull mode. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]      FIG. 1  illustrates a Fabry Perot interferometer  20  designed for incorporation into a fiber optic sensor array. The Fabry Perot interferometer  20  comprises an optical fiber  22  having a core  24  and a cladding  26  that surrounds the core  24 . Optical fiber normally has a protective jacket (not shown). In the portion of the optical fiber  22  shown in  FIG. 1  the jacket has been removed and replaced with a ferrule  28  that is preferably formed of a hollow glass rod. An end  30  of the ferrule  28  extends a small distance beyond the core  24  and cladding  26  to form a small cavity  32 . A diaphragm  34  is bonded to the outer end  30  of the ferrule  28  such that there is a small gap  36  between the diaphragm  34  and the optical fiber end  38 . The diaphragm  34  may be formed of silica. The gap  36  preferably is filled with a fluid such as oil or other substance that has a good impedance match with water. 
         [0027]    A light wave in the optical fiber  22  exits the optical fiber end  38  and enters the fluid filled gap  36 . On the far side of the gap  36  the diaphragm  34  moves in response to incident acoustic pressure waves in a water environment. Light is reflected from both the end  38  of the optical fiber core  24  and the inner surface  40  of the diaphragm  34  back into the fiber  22 . Reflectivities R 1  and R 2  for the fiber end  38  and the diaphragm surface  40 , respectively, and the gap width Wgap determine how much light goes back into the optical fiber  22 . The reflectivities are characteristics of the optical fiber core  24  and the diaphragm surface  40 . The gap width is a function of the pressure in the acoustic wave incident upon the diaphragm  34 . 
         [0028]    The fiber end  38  and the surface  40  of the diaphragm  34  that faces the fiber tip  38  may have coatings  42  and  44 , respectively formed thereon to enhance the reflectivities. The coatings may comprise either a dielectric or a metal material. The gap width Wgap between the fiber tip  38  and the diaphragm  34  is typically less than 50 microns. If the gap width is too large, the light exiting the fiber tip  38  spreads by diffraction to such an extent that after a double pass through the gap  36  a very small portion of the light can be coupled back into the fiber  22 . For gap widths of 15 microns or less the diffraction spread factor is negligible for single mode fiber propagating light at 1550 nm. 
         [0029]      FIG. 2  graphically illustrates reflectivity as a function of gap width for the reflection mode Fabry-Perot interferometer of  FIG. 1  for a wavelength of 1550 nm. 
         [0030]    Reflected power R fp  for the Fabry-Perot interferometer  10  back into the optical fiber  12  is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0031]    The refractive index of the fluid in the gap is given by n fluid . A plot of reflected power back into the fiber as a function of gap width and mirror reflectivities is shown in  FIG. 2 . The plot shows reflected power minima at multiples of λ/2=583 nm where λ is 1550 nm and the fluid is water having a refractive index n=1.33. Matched mirror reflectivities of 0.30, 0.60, and 0.82 show a pronounced narrowing of the resonance dip as reflectivity goes up. The resonance dip goes to zero only for the case of the matched reflectivities. The finesse associated with the highest reflectivity is 16. 
         [0032]    On each side of the resonance dip the slope or change in reflectivity with change in gap width is a maximum at a reflectivity of about 30%. This is the operating point of the sensor. At this point, small displacements of the diaphragm yield small changes in gap width to produce a maximum change in power going back into the fiber  22 . 
         [0033]    Two Wavelength Push-Pull Method 
         [0034]      FIGS. 3 and 4  show plots of reflected power back into the optical fiber  12  for two wavelengths λ A  and λ B  as functions of gap for both wavelengths. Both plots show two traces that cross one another at a reflected power of about 30%. Comparing  FIGS. 3 and 4  shows that small changes in wavelength yield substantial changes in the desired gap width. Two wavelengths of about 1480 and 1550 nm have been selected. These are common wavelengths that are easily separated from one another by a small, inexpensive wavelength division multiplexer (WDM. 
         [0035]    At a gap width associated with the crossing point, a small change in gap width increases reflected power for one wavelength and decreases by a like amount reflected power for the other wavelength. This push-pull behavior can be used to advantage in forming an acoustic sensor. The difference of the two reflected powers is twice as great as either one alone with a small change in gap width. 
         [0036]    To implement this configuration in a fiber optic sensor array, more factors have to be taken into consideration. A method involving ratios of received powers at the two wavelengths can overcome these difficulties. A visibility function of the reflected powers at the two wavelengths is given by 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    where α is the ratio of optical powers input at the two wavelengths. When α is equal to one, equal amounts of optical power at the two wavelengths are delivered to the Fabry Perot sensor  20 . 
         [0037]      FIG. 5  is a plot of visibility as a function of gap width. The visibility ranges between minus one and plus one with the steepest slope being at the sensor operating point where visibility is about zero. As shown by the plot, a substantial imbalance in the two powers at the two wavelengths has a minimal effect on the visibility. For example, a twenty percent imbalance of incident power at the two wavelengths has a very small effect on the visibility function. 
         [0038]      FIG. 6  is a plot of the slope of the visibility function, or delta visibility, over delta gap width. For a range of motion of 5 nm, the slope is flat to about 3% for α=1.0. A flat slope implies a linear response. In other words, a change in acoustic pressure produces a proportional change in the visibility function. As the range of motion increases, there is more variability in the slope that produces increased harmonic distortion for very large acoustic tones. This is unlike the case of the phase generated carrier in which there is a catastrophic failure. 
         [0039]      FIG. 7  graphically illustrates reflectivity as a function of gap width for a reflection mode Fabry-Perot interferometer for two wavelengths operated in a push only mode.  FIG. 8  graphically illustrates reflectivity ratios as a function of gap width for a reflection mode Fabry-Perot interferometer for two wavelengths operated in a push only mode.  FIG. 9  graphically illustrates the slope of reflectivity ratios as a function of gap width for a reflection mode Fabry-Perot interferometer for two wavelengths operated in a push only mode. 
         [0040]      FIGS. 10 and 11  show examples of two different array architectures for push-pull Fabry Perot sensors.  FIG. 10  shows a time division multiplexed (TDM) architecture  45 , and  FIG. 11  shows a fan-out continuous wave (CW) architecture  47 . 
         [0041]    Referring to  FIG. 10 , a pair of lasers  46  and  48  provides optical signals at wavelengths λ A  and λ B , respectively, to corresponding optical fibers  50  and  52 , respectively. The optical fibers  50  and  52  guide the signals output from the lasers  46  and  48  to a wavelength division multiplexer (WDM)  54 . The WDM  54  inputs the signals from the lasers  46  and  48  into an optical fiber  56  that is arranged to guide the laser signals of wavelengths λ A  and λ B  into an optical on-off switch  58 . Signals output from the switch  58  are input to a fiber optic coupler (or circulator)  60 . The two wavelengths λ A  and λ B  propagate from the coupler  60  in an optical fiber  62  to couplers C 1 -C 3  that couple the optical signals into two-wavelength Fabry Perot sensors  1 - 3  formed as described above with reference to  FIG. 1 . The optical fiber  62  terminates in a Fabry-Perot sensor  4 . Signals output from the Fabry-Perot sensors  1 - 4  propagate back to the optical coupler  60  to be coupled into an optical fiber  64 . The optical fiber  64  guides the sensor output signals to a WDM  66  that is arranged to input sensor output signals of wavelength λ A  into an optical fiber  68  and sensor output signals of wavelength λ B  into an optical fiber  70 . The optical fiber  68  guides sensor output signals of wavelength λ A  to a photodetector  72 , which is also designated as photodetector A, and the optical fiber  70  guides sensor output signals of wavelength λ B  to a photodetector  74 , which is also designated as photodetector B. The photodetectors  72  and  74  produce electrical signals that indicate the intensities of the signals of wavelength λ A  and λ B , respectively, and are processed by a signal processor  76  to determine the pressure in the incident acoustic wave. 
         [0042]    The gating of pulses is produced by the external on-off switch  58  as shown or by turning the lasers  46  and  48  on and off by current modulation. The optical switch  58  can be a semiconductor optical amplifier (SOA) gate or an electro-optic gate. Inexpensive distributive feedback (DFB) lasers such as those used in telecommunications should be adequate for many applications. The coupler ratios for each tap coupler are designed for maximum return signal to the detectors. For N sensors, the factor 1/N 2  governs the amount of light from each sensor incident on the detector. 
         [0043]    The fan-out architecture  47  of  FIG. 11  has no optical gating, which allows for much lower bandwidth operation. This comes at the expense of requiring many more detectors. However, detector arrays arc available that have a small footprint, especially for low bandwidth operation. For N sensors, the factor 1/N governs the amount of light from each sensor incident on the detector. 
         [0044]    Referring to  FIG. 11 , the fan-out architecture  47  includes the lasers  46  and  48  as described above with reference to  FIG. 10 . Optical fibers  78  and  80  guide signals of the two wavelengths λ A  and λ B  respectively, to a 2×2 optical coupler  81  that divides the input signals equally between optical fibers  82  and  83 . Signals in the optical fiber  82  propagate to a 1×2 optical coupler  84  that couples light from the optical fiber  82  into two optical fibers  85  and  86 . Signals in the optical fiber  83  propagate to a 1×2 coupler  88  that couples light from the optical fiber  83  into two optical fibers  89  and  90 . 
         [0045]    The optical fibers  85 ,  86 ,  89  and  90  and provide signals of both wavelengths λ A  and λ B  to corresponding two-wavelength Fabry-Perot sensors  1 - 4 . Signals output from the Fabry-Perot sensors  1 - 4  are coupled into WDMs  92 - 95 , respectively, that are arranged to provide sensor output signals of wavelengths λ A  and λ B  to a photodetector array  100  that has separate photodetectors A and B for signals of wavelengths A and B output from each Fabry-Perot sensor  1 - 4 . A signal processor  102  is connected to the photodetector array  100  to receive electrical signals therefrom. 
         [0046]      FIG. 12  illustrates a second embodiment of a Fabry-Perot interferometer  103  that operates in a transmission mode. The Fabry-Perot interferometer  103  includes the optical fiber  22 , ferrule  28  and fluid filled gap  36  of  FIG. 1 . However, the Fabry-Perot interferometer  103  includes a diaphragm  35  that transmits a portion of the incident light as a diverging light beam to an end  107  of a multimode optical fiber  104 . The multimode optical fiber has a multimode core  105  and a cladding  106  as shown in  FIG. 12 . The where the interference signals are injected into the multimode core  105  for transmission to the photodetector array  100 . The transmission mode Fabry-Perot interferometer  103  has a visibility ratio that may be expressed as 
         [0000]    
       
         
           
             
               
                 
                   VIS 
                   = 
                   
                     
                       
                         T 
                         1 
                       
                       - 
                       
                         T 
                         2 
                       
                     
                     
                       
                         T 
                         1 
                       
                       + 
                       
                         T 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where T 1  and T 2  are the transmissivities of the Fabry-Perot interferometer  103  at the wavelengths λ 1  and λ 2 , respectively. 
         [0047]      FIG. 14  graphically illustrates signal transmission as a function of gap width for the transmission mode Fabry-Perot interferometer of  FIG. 9  operated in a push-pull mode.  FIG. 15  graphically illustrates transmission visibility ratio as a function of gap width for the transmission mode Fabry-Perot interferometer of  FIG. 9  operated in a push-pull mode.  FIG. 16  graphically illustrates transmission visibility slope as a function of gap width for the transmission mode Fabry-Perot interferometer of  FIG. 9  operated in a push-pull mode. 
         [0048]      FIG. 13  shows a transmission mode sensor array  109  that includes a plurality of Fabry-Perot interferometers  5 - 8  formed in accordance with  FIG. 12 . The array  109  includes the lasers  46  and  48  that provide laser light at wavelengths λ A  and λ B , respectively. The optical signal output from the lasers  46  and  48  are input to optical fibers  110  and  112 , respectively, that are each connected to a 2×2 optical coupler  114 . The WMD  114  couples signals of both wavelengths λ A  and λ B  into optical fibers  116  and  118 . The fiber  116  guides the optical signal therein to a 1×2 optical coupler  120  that divides the optical signal between the optical fiber  116  and an optical fiber  122  that provide light to the Fabry-Perot interferometers  5  and  6 , respectively. The optical fiber  118  guides the optical signal therein to a 1×2 optical coupler  124  that divides the optical signal between the optical fiber  118  and an optical fiber  126  that provide light to the Fabry-Perot interferometers  7  and  8 , respectively. 
         [0049]    The Fabry-Perot interferometers  5 - 8  provide interference signals to corresponding WDMs  128 - 131 , respectively. The WDMs divide the signal from each of the Fabry-Perot interferometers  5 - 8  into separate signals according to wavelength to provide separate signals of wavelength λ A  and λ B  for each of the Fabry-Perot interferometers  5 - 8 . The signals output from the WDMs are incident upon photodetectors A and B for each wavelength λ A  and λ B . 
         [0050]    The array  103  of transmission mode Fabry-Perot interferometers  5 - 8  requires fewer couplers than are required for the reflection mode Fabry-Perot interferometer architecture arrays  45  and  47  and therefore provides an increase in detected power.