Abstract:
A grating sensor and method for optical interrogation of that sensor uses a lock-in technique to achieve simultaneous measurements of strain (and related temperature) and ultrasonic stress wave signals, as well as other environmental conditions that affect a reflection spectrum of the grating sensor. It achieves this by using a lock-in amplifier or a software demodulator to detect slight shifts in the grating reflection spectrum with high sensitivity and accuracy. A dynamic feedback loop based on the lock-in error signal output retunes the light wavelength of the light source (e.g., a tunable laser) or of a wavelength filter in the reflection path to maintain it relative to a specified reflection point of the grating reflector. The lock-in error signal serves as a measure of temperature/strain changes and of ultrasonic vibrations.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This patent application claims priority under 35 U.S.C. §119(e) from prior U.S. provisional application 60/970,018 filed Sep. 5, 2007. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    This invention was made with government support under SBIR contract number NND07AA04C awarded by the National Aeronautics and Space Administration (NASA). The government has certain rights in the invention. 
     
    
     TECHNICAL FIELD 
       [0003]    The present invention relates to optical measuring and testing of strain, ultrasonic stress waves, and temperature using optical interrogation of a grating sensor, such as tunable laser interrogation of a fiberoptic waveguide with Bragg grating reflector. 
       BACKGROUND ART 
       [0004]    Laser-based fiber Bragg grating (FBG) interrogation techniques have long been used commercially for strain monitoring [Rose, J.,  Ultrasonic Waves in Solid Media,  Cambridge University Press, 1999]. More recently, a number of FBG interrogation techniques have been proposed for ultrasonic wave monitoring and other structural health monitoring applications [W. Prosser, M. Gorman and J. Dorighi, “Extensional and Flexural Waves in a Thin-Walled Graphite/Epoxy Tube”,  J. Compos. Mat.  26:418 (1992); M. Gorman and W. Prosser, “AE Source Orientation by Plate Wave Analysis”,  J. Acoustic Emission  9:283 (1991)]. In a standard FBG interrogation technique, light from a tunable laser is typically tuned to the mid-reflection wavelength of the Bragg grating, and the reflected light is detected by a photodetector, which converts the light signal to an electrical signal. As the sensing grating experiences loads, dynamic strain, or acoustic fields, the Bragg reflection wavelength shifts, causing the reflected intensity from the Bragg grating to change. The DC to low-frequency AC signal corresponds to static and quasistatic strain, while a higher frequency AC signal corresponds to dynamic strain or ultrasonic stress waves. The signal amplitude (sensitivity) is directly proportional to the slope of the Bragg reflection spectrum. 
         [0005]    To date, no single laser-based FBG interrogation technique has been developed that reliably and simultaneously monitors both strain and ultrasonic waves with both high strain resolution and high stress wave sensitivity. High strain resolution and high stress wave sensitivity require that the slope of the Bragg reflectivity spectrum of a fiber Bragg grating be as steep as possible. However, the steeper the FBG slope, the more sensitive the reflection signal to environmental changes. As a result, these laser-based FBG interrogation techniques lack the robustness and sensitivity required for long-term strain and ultrasonic wave detection for damage detection of structures in harsh environments. 
       SUMMARY OF DISCLOSURE 
       [0006]    A grating sensor and a corresponding method for optical interrogation of the grating sensor have been developed which use an AC lock-in technique to measure a variety of environmental conditions with sensitivity and accuracy. A grating reflector is optical coupled to receive light from a light source, such as a wavelength tunable laser, or an intensity modulated broadband light source. The grating reflector is characterized by a reflection spectrum that is dependent upon environmental conditions. In the case of a tunable laser, the coupled light wavelength may be dithered about a selected reflection wavelength of the grating, such as a peak reflection wavelength or along one of the slopes of the spectrum. The grating reflector may be a Bragg grating formed in an optical fiber or may be a surface-relief grating. In any case, the reflected light from the grating is coupled to a photo-detector which produces an electrical signal corresponding to the light intensity that it receives. In the case of a broadband light source, a tunable wavelength filter is positioned in the reflection path in front of the photo-detector to select the wavelength to be received and detected. 
         [0007]    The sensor also includes a lock-in system, which may be either a hardware lock-in amplifier or a corresponding software demodulator of the signal. In either form, the lock-in mechanism picks out the dithering signal at a given reference frequency to get rid of around 90% of the noise, enabling the system to lock-in the wavelength of the laser source or of the wavelength filter at a specified point in the grating&#39;s spectrum. The lock-in mechanism provides feedback to the light source or filter to retune it to the specified reflection wavelength of the grating. For example, a tunable semiconductor laser may be tuned via either temperature or current control, or both. The lock-in error signal also serves as a measure of the environmental conditions to which the sensor is subject. 
         [0008]    In one particular implementation of the invention, in order to achieve both the sensitivity and accuracy required for simultaneous measurements of strain, temperature, and ultrasonic stress wave signals, a laser demodulation technique for FBG interrogation based upon a simple lock-in scheme integrates the laser control electronics and photo-detector signals together in a dynamic feedback loop circuit to provide stable laser wavelength locking to the Bragg wavelength of the FBG sensors. For strain measurements, a sub-microstrain resolution requires picometer wavelength-shift detection sensitivity, which is often obscured by environmental and system noise. To effectively remove the noise contribution, the output signal of the photodetector is fed into a commercial lock-in amplifier, which in turn provides a feedback signal for laser control electronics. This lock-in scheme enables the laser wavelength to be continuously locked to the stable point at the mid-reflection wavelength of the Bragg grating to produce the highest signal-to-noise, providing a direct strain measurement via the generated error signal. Due to the out-of-band noise rejection by the lock-in amplifier, the resulting signal to noise ratio is greatly enhanced, permitting sub-microstrain detection sensitivity. Once the laser is locked, the DC strain signal is very stable, and laser wavelength is highly resistant to environmental noise that tends to move the laser wavelength away from the stable point it is locked to, enabling both improved signal-to-noise strain measurements and reliable AC strain and stress wave detection. 
         [0009]    This lock-in laser-based demodulation technique interrogating optical waveguide Bragg gratings (BG) provides simultaneous, reliable, high resolution, high sensitivity measurements of strain, temperature, stress, acoustic emission, and ultrasonic wave detection. The lock-in scheme integrates the laser control electronics and photo-detector signals together in a dynamic feedback loop circuit to provide stable laser wavelength locking to the Bragg wavelength of the BG sensors. Using this technique, detection of sub-microstrain resolution by the lock-in based FBG interrogation system can be routinely obtained while the system simultaneously monitors ultrasonic stress waves with high sensitivity and reproducibility. Due to the high noise rejection, inherent self-reference, and robust wavelength locking capabilities, the lock-in strain signal output and the laser wavelength tracking are extremely stable and are immune of optical power fluctuations due to system and environmental noise. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic of an experimental setup of an FBG sensor using the lock-in laser-based FBG interrogation technique in accord with the present invention for performing strain and ultrasonic wave measurements. 
           [0011]      FIG. 2  is a graph of lock-in error voltage signal from measuring FBG temperature-induced strain over a 17-hour timeframe using the setup of  FIG. 1 , and, for comparison, a temperature monitoring signal (in degrees Celsius) from a Thorlab temperature controller monitor recorded over the same timeframe. 
           [0012]      FIGS. 3   a  and  3   b  are graphs of FBG sensor dynamic response (in Volts over about 250 μs) to 10-cycle sine tone burst stress waves on a carbon fiber composite plate in the setup of  FIG. 1 , at respective times (a) t=0 and (b) t=17 hours, while the laser is locked to the FBG&#39;s mid-reflection wavelength and the temperature-induced strain data is continuously recorded. 
           [0013]      FIG. 4  is a schematic of an alternative grating sensor using a software lock-in system. 
           [0014]      FIG. 5  is a schematic of an alternative grating sensor using a broadband light source. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    With reference to  FIG. 1 , feasibility of the laser lock-in based FBG interrogation technique of the present invention is demonstrated in the illustrated experimental setup by locking a commercial tunable distributed feedback (DFB) laser to a commercial FBG sensor to simultaneously obtain sub-microstrain resolution and high sensitivity ultrasonic wave detection with high reproducibility and stability over a period of time, from a few hours to a few weeks. 
         [0016]    As seen in  FIG. 1 , a distributed feedback (DFB) tunable semiconductor laser  11  (with associated control electronics) is optically coupled to supply laser light to an FBG sensor  15  via a beamsplitter  13 . Other wavelength tunable light sources or even broadband light sources could be used. Here the FBG sensor  15  is an optical fiber with a Bragg grating formed therein. The optical fiber material may be silica, fluorozirconate glass, a polymer, or any other material that is transparent at the wavelengths of interest and can serve as a waveguide to the grating. The selection of the material may be optimized to be sensitive to the environmental conditions that are desired to be measured. For example, for stress measurement, a softer material (smaller Young&#39;s modulus) would be better. The Bragg grating may be constructed to have a high reflectivity slope in its spectrum for good dynamic range. The grating reflector changes its reflection spectrum, either by a change of reflective index of the material in which the light is guided or by a change in the grating pitch or spacing. Instead of a Bragg grating formed in an optical fiber or other waveguide, a surface-relief grating could be used, where changes in environmental conditions change the grating pitch e.g., by thermal expansion. 
         [0017]    A photodiode  17  or other photodetector is also coupled to receive reflected light from the FBG sensor  15  via the beamsplitter  13 . An electrical signal output is transmitted from the photodiode  17  along the conductive line  19  to a lock-in system  21 . Here a hardware lock-in amplifier system is used. ( FIG. 4  shows a software lock-in system.) The photodiode output is also transmitted along the conductive line  23  to an analog-to-digital converter  25 . The converted digital value of the photodiode output is used as a control input  27  to the lock-in device  21  and is also fed to a computer  29 . The lock-in amplifier&#39;s output  29  provides a feedback signal to control the wavelength of DFB laser  11 . This allows the laser  11  to be continuously tuned to the mid-reflection wavelength of the Bragg grating  15  to produce the highest signal-to-noise ratio for the photodiode  17 . 
         [0018]    To demonstrate the invention, a commercial FBG sensor  15  was bonded to a composite test plate  31 . Light from a DFB laser  11  was locked to the mid-reflection wavelength of the Bragg grating using a Stanford Research SR530 lock-in amplifier  21 . The mid-reflection point is generally the most sensitive for strain and stress measurements. For other measurements, such as the presence of a chemical (via polymer fibers that change reflective index) or an electromagnetic field (via the Faraday effect), the selected lock-in point might be on one slope of the reflector spectrum. For comparison purposes, the ambient temperature was also monitored using a Thorlabs TEC2000 temperature monitor. Over a time period of 17 hours, both the FBG sensor and the temperature sensor were exposed to room temperature fluctuation, and the error signal from the lock-in amplifier  21  and the room temperature data were recorded by a desktop computer  29  using two channels from a National Instrument PCI-6111 data acquisition board and LabView software.  FIG. 2  shows the lock-in error signal  41  and room temperature signal  43  as a function of time. The lock-in signal  41  clearly demonstrated a superior signal-to-noise ratio compared to that of the Thorlabs temperature sensor. The near periodic fluctuation in the recorded signals were due to the room temperature fluctuation cycle with a period between 30 minutes to two hours. 
         [0019]    From time t=0 to t=17 hours, the Thorlabs sensor recorded a temperature shift of 0.713° C. Using the commonly published FBG intrinsic temperature sensitivity value, Δλ B /ΔT=9 pm/° C. [A. Othonos and K. Kalli, “ Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing” , Artech House, Inc., 1999], this temperature shift induces a thermal strain in the FBG which corresponds to a wavelength shift of 6.4 pm. During this time, the lock-in amplifier  21  registered a changed voltage signal ΔV(t)=69.8 mV, with a noise level of approximately 0.7 mV, corresponding to a signal-to-noise ratio (SNR) of 100:1 and a temperature-induced strain resolution of 0.06 με. 
         [0020]    To demonstrate the capability of the lock-in technique for simultaneous measurement of both strain and ultrasonic wave signals, while the laser was still locked to the mid-reflection wavelength of the Bragg grating, ultrasonic waves at 200 kHz were launched into the composite plate  31  using a commercial pulser system from Physical Acoustic Corporation including a PAC S-9208 PZT actuator  33  and a PAC ARB-1410-150 waveform generator  35  with a 10-cycle sine burst signal input. The stress wave signals were collected by the same photodetector  17  that was used to record the thermal strain signal. The digitized data were averaged 100 times, plotted by a LabView program, and saved in the desktop computer  29  for further data analysis.  FIGS. 3   a  and  3   b  compare the detected stress wave signals  45   b  and  45   b  at t=0 and t=17 hours, respectively. The high SNR, reproducible stress wave signal  45   b  after 17 hours of laser locking in  FIG. 3   b  demonstrates that robust, high resolution strain and high sensitivity acoustic wave signals can be simultaneously measured by the same Bragg grating sensor using the laser-based lock-in demodulation technique. 
         [0021]    With reference to  FIG. 4 , a software-based lock-in system for grating sensing applications is shown. As in the hardware lock-in system of  FIG. 1 , a wavelength-tunable light source, such as DFB semiconductor laser  41 , is optically coupled to a grating reflector, such as an optical fiber with a Bragg grating  45  formed therein. A photodetector, such as the photodiode  47 , receives the reflected light from the grating reflector e.g., via a beamsplitter  43 . The photodiode  47  produces a signal output corresponding to the received light intensity which is then converted into digital form  51  using an analog-to-digital converter  49 . 
         [0022]    A software demodulator  53  performs the lock-in function. The basic operating principle behind the software-based lock-in system is similar to its hardware counterpart. The (digitized) modulated AC signals  51  are compared with a reference signal  55  and the resulting demodulated error signal V error  is used as a feedback to the laser or other light source  41  to facilitate tracking of its wavelength output to a preset position of the grating&#39;s changing reflection spectrum. In terms of implementation and performance, the software lock-in system offers a number of advantages over the hardware lock-in amplifier. Since the software performs the actual demodulation process, the lock-in amplifier hardware is unnecessary, resulting in lower costs. The software is scalable to multiple channels with ease, with the same software being used with minimal modification to carry out multi-channel lock-in, so additional channels do not include the cost of additional lock-in amplifiers. The software is easily modified to accommodate different gains, frequency responses and other settings, as well as carry out additional algorithms. 
         [0023]    Software generates a periodic, typically sinusoidal, reference signal (100 Hz to 50 kHz). This is numerically added (59) to V set current , the average operating laser current voltage. The sum is connected to a Digital-to-Analog (D/A) converter  61 , and the analog output drives the current controller  65  of the DFB laser  47 . In addition to demodulating the signal, the software also provides necessary integration, gain, and offset voltage. The resulting error signal V error  is added (57) to V set temp , the operating laser temperature voltage. The sum is connected to a D/A converter  61 , the analog output of which drives the temperature controller  63  of the DFB laser  41 . In this fashion, the error voltage V error  constantly tracks the change in FBG wavelength and corrects the laser  41  accordingly. In addition to locking the laser to the grating  45  the error voltage also accurately measures the environmental parameters that induce change in FBG, including but not limited to temperature, strain, and pressure. 
         [0024]    With reference to  FIG. 5 , a lock-in system for grating sensing applications is shown with a broadband light source  71 . The basic operating principle behind a broadband light source based system is similar to its single wavelength (tunable laser) counterparts (cf.,  FIGS. 1 and 4 ). However, a difference from the tunable laser embodiments lies in the manner of carrying out the modulation and feedback. In this broadband system, the light source  71  is intensity modulated by an external modulator  72 , and the feedback error signal V error  for each channel is fed into its respective tunable wavelength filter  76 , which preferentially transmits the light within its narrow wavelength band. A system with a single broadband source has a scalability and cost advantage in that it can be used for many more wavelength channels compared to the generally more limited tuning range and single wavelength-at-a-time in tunable laser based systems. The broadband source can be used either in a system with a hardware lock-in amplifier as in  FIG. 1 , or with a software system as in  FIG. 4 , but due to advantages of scalability and low cost, the software version is preferred and is shown in  FIG. 5 . 
         [0025]    The broadband light source  71  is intensity modulated by an external modulator  72  and the modulated light is then coupled to a feedback grating sensor  75  e.g., via a beamsplitter  73  and optical fiber  74 . As in the other lock-in system embodiments, the sensor  75  could be either a fiberoptic Bragg reflector or a surface relief grating. The reflected light passes through a tunable wavelength filter  76  and is received by a photodiode detector  77 . The detector  77  produces a signal output that corresponds to the received light intensity of the filter-selected wavelength. The photodiode signal is converted into digital form using an analog-to-digital converter  79 . 
         [0026]    A software demodulator  83  performs the lock-in function. In particular, the (digitized) modulated AC signal  81  is compared with a reference signal  85  and the resulting demodulated error signal V error  is used as feedback to facilitate tracking of the grating sensor&#39;s changing spectrum to a preset position by the tunable wavelength filter  76 . The reference signal  85  is typically a software-generated periodic sinusoidal signal of from 100 Hz to 50 kHz. By comparing the reference signal  85  and the modulated digital signal  81 , the modulated signal  81  is demodulated by the software demodulator  83 . In addition to demodulating the signal, the software provides integration, gain, and offset voltage. The resulting error signal V error  is added,  87 , to a V set     —     wavelength , the nominal operating voltage for the tunable wavelength filter  76 . The sum is supplied to a digital-to-analog converter  91 , the analog output of which governs a wavelength controller  93  that drives the tunable filter  76 . In this fashion, the error voltage V error  continually tracks the change from the grating&#39;s wavelength and corrects the tunable filter  76  accordingly such that center wavelength of the filter  76  is always “in sync” with the grating sensor  75 . 
         [0027]    The reference signal  85  is also numerically added,  89 , to V set     —     intensity , the set voltage for the external intensity modulator  72 . The sum is supplied to a digital-to-analog converter  91  and the analog drives the intensity modulator. 
         [0028]    The broadband system can be readily scaled up to multiple (N) channels by simply including N tunable filters like filter  76 , all optically coupled to the grating sensor  75 . The detected signal for each filter would be locked in separately from the others using its own demodulator feedback and wavelength controller. 
         [0029]    Similar to the single wavelength case, the error voltage V error , in addition to locking the filter  76  to the grating sensor  75 , also accurately measures the environmental parameters that induce changes in the sensor&#39;s grating, including but not limited to temperature, strain, and pressure. 
         [0030]    In addition to strain and ultrasonic wave detection, the technique can be applied to high sensitivity temperature, stress, acoustic emission, corrosion monitoring, pressure sensing, chemical sensing, and electromagnetic field (or current) sensing. Other potential applications include wavelength locker for long-term laser wavelength stability in telecommunication applications.