Abstract:
An optical sensor in which acceleration, acoustic velocity, or displacement (vibration) causes a corresponding shift in the center wavelength of the sensor output. The sensor can be coupled to a high-speed interferometric interrogator through an unbalanced fiber interferometer. The unbalanced interferometer functions to translate optical wavelength shift into phase shift, which is easily demodulated by the interrogator. A method of measuring acceleration uses the sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 12/253,161, filed on Oct. 16, 2008 and now issued as U.S. Pat. No. 7,999,946, which claims priority of provisional Application No. 60/999,246 filed on Oct. 16, 2007. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to sensors and methods for acquiring acceleration and/or velocity data using fiber optics. Specifically, the invention relates to grating sensors with ultra narrow band gratings, combined with interferometric wavelength-to-phase conversion and low noise interferometric interrogation. 
     BACKGROUND OF THE INVENTION 
     There are many applications that require a device to measure the dynamic acceleration or acoustic velocity signal at a given location. Examples include: the seismic exploration/monitoring of oilfields, seismic monitoring for earthquakes, structural integrity monitoring, and health monitoring of vibrating equipment/machinery acoustic monitoring in marine environments (e.g., SONAR). For decades, such monitoring has been almost exclusively performed using electronic-based sensors such as piezoelectric sensors and magnet/coil sensors. These sensors typically generate a voltage output that is proportional to the intensity of the applied vibratory motion (displacement, velocity or acceleration). Because the generated voltage levels are relatively weak (i.e., low level), electronics are required for amplification, signal conditioning, filtering, and in most cases digitization/multiplexing. These electronics must be located very close to the sensor to limit the introduction of noise into the system. Thus, the electronics must be designed to operate in the local environment (temperature/vibration/humidity/shock) where the sensor is placed. 
     Recently, the use of fiber optic sensors has become more prevalent for sensing applications, particularly in those applications where the sensors must be placed in harsh environments, which seriously affects the performance/reliability of the associated electronics. Fiber optic sensors have an advantage in that they require no electronics at or near the sensor. In fiber optic sensors, light is sent through the optical fiber from a remote location (in a benign environment). The measurand causes a change in the optical transmissive property of the fiber which is then detected as a change in the received light signal at the remote electronics. 
     Fiber optic sensors generally fall into two categories, those designed for making high speed dynamic measurements, and those designed for low speed, relatively static measurements. Examples of dynamic sensors include hydrophones, geophones, and acoustic velocity sensors, where the signal varies at a rate of 1 Hz and above. Examples of low speed (static) sensors include temperature, hydrostatic pressure, and structural strain, where the rate of signal change may be on the order of minutes or hours. This invention relates primarily to dynamic measurements of acceleration, acoustic velocity, and vibration using fiber optic sensors. Historically, such sensors have been more costly than the legacy electronic versions because they are difficult to manufacture, require complicated and expensive equipment for even limited automated assembly, and involve significant amounts of skilled touch labor to produce. Although fiber Bragg grating (FBG) accelerometers are currently available, they incorporate spectroscopic interrogation, which limits the sensitivity to about 1 mg. However, many applications require sensitivities on the order of 30-50 ng. Fiber laser devices have also been used for sensing. However, they are expensive and tend to be unstable. The invention endeavors to solve these problems and more to provide extremely high sensitivity acceleration measurements suitable for a wide range of applications requiring sensors in environments in which electronics often cannot survive. 
     SUMMARY OF INVENTION 
     To solve these and other problems, and in view of its purposes, the present invention provides fiber optic sensors with a level of performance several orders of magnitude higher than is otherwise achievable using prior technologies. The FBG sensor is packaged as a “particle motion sensor,” such that acceleration, acoustic velocity, or displacement (vibration) cause a corresponding shift in the center wavelength of the FBG reflection (or transmission) spectrum. The sensor can be coupled to a high-speed interferometric interrogator through an unbalanced fiber interferometer. The unbalanced interferometer functions to translate the FBG wavelength shift into a phase shift, which is easily demodulated by the interrogator, i.e., the wavelength shift of an FBG sensor is detected by utilizing the inherent wavelength dependence of an unbalanced fiber interferometer. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
         FIG. 1  is a block diagram of a particle motion sensing system in accordance with an embodiment of the invention; 
         FIG. 2A  is a cross-sectional view of a sensor suitable for use in the system of  FIG. 1 ; 
         FIGS. 2B and 2C  show details of the circular hinge, with  FIG. 2C  illustrating a cross-section taken along the line  2 C- 2 C of  FIG. 2B ; 
         FIGS. 3A and 3B  show two embodiments of the narrow linewidth grating; 
         FIG. 4  is a transmission spectrum of a phase shifted grating; 
         FIG. 5  is a block diagram of the source optics; 
         FIG. 6  is a schematic of the receive optics; 
         FIG. 7  is a diagram of an embodiment of the ASE filter; 
         FIG. 8  is a block diagram of a closed loop interferometric interrogator; 
         FIG. 9  is a block diagram of a WDM/TDM multiplexed system; 
         FIG. 10  is a block diagram of the source optics of a WDM/TDM multiplexed system; and 
         FIG. 11  is a block diagram of the receive optics of a WDM/TDM multiplexed system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and devices are omitted so as to not obscure the description of the present invention with unnecessary detail. 
     A particle motion sensing system  10  according to one embodiment of the present invention is shown in  FIG. 1 . The particle motion sensing system  10  includes a transducer or sensor  100 , source optics  200 , receive optics  300 , an interferometric interrogator  400  and signal processing/recording electronics  500 . 
     Although a number of different configurations of the sensor  100  may be employed,  FIG. 2A  shows an exemplary embodiment for use with narrow band gratings. Sensor  100  includes a housing  110 , an optical fiber  130 , a proof mass  150 , and a pretension spring  170 . The optical fiber  130  has a free region  132  in which a grating  135  is inscribed. The optical fiber  130  is attached at one end to the housing  110  by means of a first anchor  120  and at the other end to the proof mass  150  by means of a second anchor  160 . The optical fiber  130  may be attached to the first anchor  120  and the second anchor  160  by bonding or any other suitable method for preventing the optical fiber  130  from slipping relative to either the first anchor  120  or the second anchor  160 . Both the first anchor  120  and/or the second anchor  160  may be round spool-shaped structures forming a capstan to help secure the optical fiber  130  to it with the friction therebetween caused by wrapping the optical fiber  130  around the outer diameter of the first anchor  120  or second anchor  160 . The proof mass  150  is suspended from the housing  110  by means of a suspension member  180 , a clamping ring  140 , standoffs  145 , and screws  147 . 
     Motion of the sensor  100  is identical to motion of the housing  110 . Motion of the sensor  100  along a direction  112  results in motion of the housing  110  relative to the proof mass  150 . Relative motion between the housing  110  and the proof mass  150  is constrained to occur only in the direction  112  by the suspension member  180 . Relative motion between the housing  110  and the proof mass  150  along direction  112  is controlled by the optical fiber  130  and the pretension spring  170 . Pretension spring  170  controls the quiescent tension on the optical fiber  130  in conjunction with the mass of the proof mass  150 . The force applied between the housing  110  and the proof mass  150  by the pretension spring  170  is controlled by a flexible cantilever  175  and an adjustment screw  177 . The flexible cantilever  175  is permanently attached at one end to the housing  110 . 
     Referring to  FIGS. 2B and 2C , the suspension member  180  comprises one or more flexible circular membranes or diaphragms fabricated by stamping or forming a flat stock of ductile metal to form a series of concentric waves  185 . These waves  185  allow the central region  182  of suspension member  180  to move with little resistance along direction  112  relative to outer portion  183  of suspension member  180  while ensuring central portion  182  and outer portion  183  of suspension member  180  remain parallel when the proof mass  150  is sandwiched between a pair of suspension members  180 . Thus, for small amplitude motions, motion of the proof mass  150  is allowed along direction  112 , but resisted in all other directions, including rotational motions. 
     Referring to  FIG. 3A , the grating  135  is created by fabricating two FBGs  1050 , each of which is a periodic change of the refractive index of the glass core  133  of the optical fiber  130 , by means of a laser, a phase mask, an interferometer or other methods well known to practitioners in the art. The two FBGs are separated by a small space  1060  on the order of 100 microns. Alternatively, as shown in  FIG. 3B , the grating  135 ′ can be fabricated as a single grating comprising two halves  1065  and  1070  which are shifted in phase relative to one another, for example by it radians. The resulting phase-shifted grating has a typical transmission spectrum  1005  shown in  FIG. 4 . The significant features of the transmission spectrum  1005  are a central peak  1000 , two stop bands  1010  and two pass bands  1020 . Typical values for the spectrum  1005  are a peak transmission width of 0.4 pm, a stop band  1010  depth of &gt;40 dB, stop band  1010  width of about 800 pm and near 100% transmission in the pass bands  1020 . 
     Relative motion between the housing  110  and the proof mass  150  changes the longitudinal strain within the free region  132  of optical fiber  130  between the first anchor  120  and the second anchor  160 . Changes in the longitudinal strain within the optical fiber  130  cause a proportional shift of the peak wavelength of the reflection or transmission spectrum of the grating  135 . 
     Referring to  FIG. 5 , the source optics  200  include a broadband optical source  210 , prefilters  220  and an optical amplifier  230 . In the exemplary embodiment of the invention, the broadband optical source  210  is a Superluminescent Light Emitting Diode (SLED). However, any suitable optical source with a bandwidth of at least approximately 1 nm may be used, such as an Amplified Spontaneous Emission (ASE) source, Light Emitting Diode (LED), etc. The source should provide an intensity of at least 0.4 mW/nm into an optical fiber and have a spectral output at least 1 nm wide. The output of the broadband optical source  210  is connected to the input of the prefilters  220  through an optical fiber  215 . The prefilters  220  may comprise one or more band pass optical filters, each of which has a passband of about 1 nm. Examples of such a filter are a Dense Wavelength Division Multiplexer (DWDM) or an Optical Add Drop Multiplexer (OADM), both of which are well known to those practiced in the art of telecommunication and sensing optics. The output of the prefilters  220  is connected to the input of the optical amplifier  230  through optical fiber  225 . The optical amplifier  230  can be any suitable means for providing optical gain. Examples of appropriate optical amplifiers are Erbium-Doped Fiber Amplifiers (EDFAs) and Semiconductor Optical Amplifiers (SOAs), both of which are well known to those practiced in the art of telecommunication and sensing optics. The output of the optical amplifier  230  is connected to the input of the sensor  100  through an optical fiber  235 . 
     Referring to  FIG. 6 , the receive optics  300  include an Amplified Spontaneous Emission (ASE) filter  305  and a mismatched path interferometer  310 . The output of the sensor  100  is connected to the input of the ASE filter  305  through an optical fiber  302 . The ASE filter  305  is a bandpass filter used to minimize the intensity of amplified spontaneous emission from the optical amplifier  230  that is outside the stop band  1010  of the grating  135 . The ASE filter  305  preferably has a very narrow transmission passband. An example of an appropriate ASE filter  305  is a 50 GHz OADM. 
     Details of ASE filter  305  are shown in  FIG. 7 . ASE filter  305  includes an optical circulator  303  and an FBG  304 . The optical circulator  303  is a passive optical device well known within the field of telecommunications that passes light from a first port  309  to second port  308 , but not vice versa. It also passes light from second port  308  to third port  311 , but not vice versa. It also does not pass light from third port  311  to first port  309 . In other words, light can only circulate in and out of the circulator  303  in one direction. Connected to output power of the circulator  303  is the FBG  304 . The FBG  304  has a high peak reflectivity (&gt;80%) and a full width half maximum bandwidth of about 300 pm. Such devices are well known to those who practice in the art. The distal lead of FBG  304  remains unconnected. 
     Referring again to  FIG. 6 , the mismatched path interferometer  310  includes a 2×2 optical coupler  320 , a phase modulator  330 , an optical delay line  340  and two mirrors  350 . The input leg  307  of the 2×2 optical coupler  320  is connected to the output of the ASE filter  305 . The 2×2 optical coupler  320  divides the input light with half going to each of its output leads  325  and  337 . One output lead  325  is connected to the phase modulator  330 , which is connected to mirror  350  through optical fiber  335 . The phase modulator  330  is used to impose a known phase to the light traveling within a leg  370  of the mismatched path interferometer  310 . The other output lead  337  of the 2×2 optical coupler  320  is connected to the optical delay line  340 , which is connected to mirror  350  through optical fiber  345 . The physical length difference between the leg  370  and a leg  380  of the mismatched path interferometer  310  is non-zero, and is preferably in the range of approximately 1-5 meters. 
     The mismatched pathlength interferometer  310  converts the changing peak wavelength in the central peak  1000  of the light transmitted from the sensor  100  into a change in phase angle of the light traversing the two legs  370  and  380 . The conversion of the peak wavelength to phase is on the order of 2 rad/pm, and increases with larger differences in length between the two legs  370  and  380 . 
     After the light passes through the mismatched pathlength interferometer  310 , it travels by means of output fiber  355  to the interferometric interrogator  400 . The function of the interferometric interrogator  400  is to measure the change in the phase angle difference between the two legs  370  and  380  of the mismatched pathlength interferometer  310  over time. A number of approaches have been used for interferometric interrogation, such as heterodyne demodulation and homodyne demodulation. For example, the Optiphase OPD-4000 is a suitable demodulator. It applies a sinusoidal modulation waveform to the phase modulator  330 . An example frequency for the modulation waveform is 20 kHz, well above the planned maximum operational frequency of the system—about 150 Hz. The resultant modulated optical waveform that arrives at the interferometric demodulator  400  is converted to an electrical signal, digitized and downconverted within the interferometric demodulator  400 . 
       FIG. 8  illustrates a low noise method of measuring the phase angle difference between the two legs  370  and  380  of the mismatched pathlength interferometer  310  over time using a closed loop interferometric interrogator  400 . A stable, low noise local oscillator  460  provides a modulation waveform such as a sine wave. A bias amplifier  470  adjusts the amplitude of the output of the local oscillator  460  to be applied to the phase modulator  330 . Ideally, a π/2 radian phase shift is applied to the phase modulator  330  to ensure that the mismatched pathlength interferometer  310  operates within a roughly linear range of its transfer function. 
     The interference signal from the mismatched pathlength interferometer  310  travels along optical fiber  411  and illuminates photodetector  410 . The purpose of photodetector  410  is to convert light into an electrical current. A number of suitable devices are available for photodetector  410 . The exemplary embodiment utilizes an ETX-100, manufactured by JDS Uniphase. The electrical output of the photodetector  410  is connected to a very low noise, high gain preamplifier  420 . The output of the preamplifier  420  is connected to an Automatic Gain Control (AGC)  430 . The AGC  430  enables continuous correction for changes in optical intensity levels throughout the system. The output of AGC  430  is mixed with the signal from the local oscillator  460  within an analog multiplier  440 . The purpose of the analog multiplier  440  is to provide a pair of signals equal to the sum and difference of the AGC  430  output and local oscillator  460 . The output of the analog multiplier  440  is connected to the input of a low pass filter  450 . For a 150 Hz maximum frequency range system, the cutoff frequency of the low pass filter  450  would be around 500 Hz. The cutoff frequency of the low pass filter is well below the sum frequency of the output of the analog multiplier  440 . This ensures only the low frequency difference signal from the analog multiplier  440  is passed. The combination of local oscillator  460 , analog multiplier  440  and low pass filter  450  functions as a synchronous detector. The output signal from the low pass filter  450  is passed along to a high gain amplifier  455 . The output of the high gain amplifier  455  is connected to the input of the variable gain output driver amplifier  495  which provides a voltage output proportional to the phase angle difference between the two legs  370  and  380  of the mismatched pathlength interferometer  310  over time. The output voltage of the amplifier  495  is also proportional to the amplitude of the acceleration experienced by the sensor  100 . 
     The output of the bias amplifier  470  is added to the output of the high gain amplifier  455  in a summing amplifier  480 . The output of the summing amplifier is connected to the input of a modulator driver amplifier  490 . The output  491  of the modulator driver amplifier  490  is applied to electrical input  331  of the phase modulator  330  within the mismatched pathlength interferometer  310  ( FIG. 6 ). 
     The negative overall loop gain of the interferometric interrogator  400  acts to provide negative feedback to the phase modulator  330  which is equal and opposite to the optical phase angle difference between the two legs  370  and  380  of the mismatched pathlength interferometer  310 . This nulling action serves to maintain operation of the mismatched pathlength interferometer  310  within the linear range of its transfer function. 
     The operation of the particle motion sensing system  10  is therefore governed by the following scale factor equation:
 
SF system =SF sensor *SF FBG *SF interferometer  
 
Where the overall system scale factor SF system  is the product of the sensor scale factor SF sensor , typically 1,000 microstrain/g, the FBG scale factor SFfbg, typically 1.2 pm/microstrain, and the interferometer scale factor SF interferometer , typically about 3 Rad/pm. These typical values result in an overall system scale factor of 2,988 rad/g (69.5 dB:Rad/g). The dominant noise source in these types of systems is the Relative Intensity Noise (RIN) caused by the extreme filtering of the broadband optical source  210  by the FBG  135 . This results in a phase noise floor of about −80 dB:rad/VHz. Therefore, the resulting noise floor would be −80 dB-69.5 dB=−149.5 dB:g/VHz. For normalized detection within a 1 Hz bandwidth, this provides a minimum detectable acceleration of −149.5 dB:g or about 33 ng, which is typical performance for electronic, moving coil-type geophones, but about 10,000 times better resolution than FBG accelerometers that employ typical, or spectroscopic-type interrogation.
 
     Practical systems frequently require a number of sensors to be combined and processed with a single set of electronics. Mutiplexing multiple sensors is easily accomplished with interferometric FBG acceleration sensing. One such embodiment is a hybrid Wavelength Division Multiplexing (WDM)/Time Division Multiplexing (TDM) multiplexed system such as that shown in  FIG. 9 , which is simplified for a four sensor system. It will be recognized that the same principles apply to larger arrays of sensors. 
     An embodiment of a WDM/TDM multiplexed system  2000  is shown in  FIG. 9 . This system includes source optics  2100 , which is shown in greater detail in  FIG. 10 . The output of a broadband optical source  2110  is connected to the input of an optical switch  2113  via an optical fiber  2112 . Semiconductor Optical Amplifiers (SOAs) are typical devices suitable for high extinction ratio optical switching. Suitable devices are manufactured by companies such as Inphenix and Kamelian. The optical switch  2113  creates a series of pulses needed for interrogation. Dense Wavelength Division Multiplexer (DWDM)  2115  divides the light along multiple fibers  2120 , each with a different central wavelength, typically separated by about 0.8 nm. Along each of the fibers  2120  is added a different fiber optic delay line  2116 ,  2117 ,  2118 , and  2119 , typically 50 to 100 m. The four different wavelengths of light travelling through the delay lines  2116  through  2119  are passed through a second DWDM  2135 , which recombines all four wavelengths and outputs them together along optical fiber  2125  to an optical amplifier  2130 . The output of the optical amplifier  2130  passes through optical fiber  2170 . 
     Referring back to  FIG. 9 , the output of the source optics  2100  passes through optical fiber  2170  to the sensor array  2150 . The sensor array  2150  consists of a series of sensors and filters in a ladder configuration with one downlink optical fiber and one uplink optical fiber. Light travelling from optical fiber  2170  continues along downlink optical fiber  2175  to OADM  2200 . OADM  2200  acts to filter out a narrow (on the order of 1 nm wide) wavelength band of light for the first sensor and passes the remainder of the light for the remaining sensors. The “drop” leg of OADM  2200  is connected to the input of a sensor  2210 . The output of sensor  2210  is connected to the “add” leg of OADM  2250 . The “pass” leg of OADM  2250  is connected to the uplink fiber  2255 . The light from the sensor  2210  thus passes along the uplink optical fiber  2255  to the receive optics  2260 . 
     The light from the “pass” leg of OADM  2200  is connected to the input of OADM  2220 . OADMs  2200 ,  2220 ,  2320  and  2340  have different add wavelengths. OADMs  2200 ,  2220 ,  2320  and  2340  have different pass wavelengths. The “drop” leg of OADM  2220  is connected to a sensor  2230 . The output of sensor  2230  is connected to the “add” leg of OADM  2240 . The “pass” leg of OADM  2240  is connected to the input leg of OADM  2250 . The “pass” leg of OADM  2220  is connected to the input leg of OADM  2320 . The “drop” leg of OADM  2320  is connected to the input of a sensor  2325 . The output of sensor  2325  is connected to the “add” leg of OADM  2350 . The “pass” leg of OADM  2350  is connected to the input leg of OADM  2240 . The “pass” leg of OADM  2320  is connected to the input leg of OADM  2340 . The “drop” leg of OADM  2340  is connected to the input of sensor  2425 . The output of sensor  2425  is connected to the “add” leg of OADM  2450 . The “pass” leg of OADM  2450  is connected to the input leg of OADM  2350 . The “pass” leg of OADM  2340  and the input leg of OADM  2450  remain unconnected. 
     Referring to  FIG. 11 , the uplink optical fiber  2255  is connected to the input of DWDM  2400 . DWDM  2400  divides the light into four bands, one for each of the sensors  2210 ,  2230 ,  2325  and  2425 . Each output leg of the DWDM  2400  is connected to a respective one of four ASE filters  2410 ,  2420 ,  2430  and  2440 . The ASE filters are identical to ASE filter  305 . The outputs of the ASE filters  2410 ,  2420 ,  2430  and  2440  are connected to the four inputs of DWDM  2460 , which recombines the wavelengths onto a single fiber  2465 . Fiber  2465  is connected to the mismatched pathlength interferometer  2470 . The output of the mismatched pathlength interferometer  2470  is connected to a fiber  2265 . 
     Referring again to  FIG. 9 , fiber  2265  is connected to TDM demodulator  2300 . A number of different TDM demodulators are available, such as the ERS-5100 manufactured by Optiphase, Inc., Van Nuys, Calif. The TDM demodulator  2300  controls the optical switch  2113 , which provides light pulses to each of the sensors  2210 ,  2230 ,  2325  and  2425  that are separated in time such that each sensor can be interrogated separately by the same TDM demodulator  2300 . The TDM demodulator  2300  also controls the amplitude and phase of the phase modulator within the mismatched pathlength interferometer  2470 , which is identical to the mismatched pathlength interferometer  310  used for a single sensor  100 . The output of the TDM demodulator  2300  is a digital representation of the output of each of the sensors  2210 ,  2230 ,  2325  and  2425  and is input to the signal processing/recording electronics  2500  for further filtering, averaging, storage and display. 
     In general, it will be recognized that the above-described invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the disclosure. Thus, it is understood that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.