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Title:
SYSTEM FOR SIMULTANEOUS MULTI-POINT DYNAMIC PARAMETER MEASUREMENT IN DISTRIBUTED OPTICAL SENSING, AND METHODS THEREOF
Document Type and Number:
WIPO Patent Application WO/2018/207214
Kind Code:
A1
Abstract:
The present invention discloses a system and method to ascertain simultaneous multipoint dynamic strain or temperature variations in a long length of optical fiber using Brillouin Optical Correlation Domain Analysis (BOCDA) based on external phase modulation. The frequency modulation of pump and probe is achieved by using an optical phase modulator and an Arbitrary Waveform Generator (AWG) which helps in generating multiple independent correlation peaks within fiber under test and hence multiple locations can be examined simultaneously. Mapping of the correlation peak is done by gating the pump and also demonstrate its tunability across the sensing fiber.

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JP2007132811LINEAR ENCODER
Inventors:
SRINIVASAN, Balaji (1/B2, TVA Koil StreetR.A. Puram, Chennai-8, Chennai 600028, 600028, IN)
VENKITESH, Deepa (C2-3-9, Bonn AvenueIIT Madras, Chennai-6, Chennai 600036, 600036, IN)
SOMEPALLI, Bhargav (Department of Electrical Engineering, IIT Madras Chennai-6, Chennai 600036, 600036, IN)
Application Number:
IN2018/050295
Publication Date:
November 15, 2018
Filing Date:
May 11, 2018
Export Citation:
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Assignee:
INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT MADRAS) (The Dean, Intellectual Property Management Cell Centre for Industrial Consultancy & Sponsored Research, Indian Institute of Technology Madras, , Chennai 600 036, Chennai 6, 600036, IN)
International Classes:
G01D5/32; G01K11/32
Domestic Patent References:
WO2014177198A12014-11-06
Foreign References:
US20130308682A12013-11-21
CN104729750A2015-06-24
CN102607621A2012-07-25
Attorney, Agent or Firm:
ARUMBU BOOPALAN, Rajasekaran (Rajasekaran Associates, F4 Brindavan Apartments,,19, Lakeview Road, Brindavan Nagar, Adambakkam, Chennai-8, Chennai 600088, 600088, IN)
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Claims:
We Claim:

1. A method for simultaneously sensing in real time, one or more parameters in an optical fiber by optical distributed sensing comprising the steps of:

passing a narrow band laser output through an external phase modulator, wherein the said phase modulator is driven with a plurality of signals with different frequencies;

splitting the output of the phase modulator into two arms, a first arm and a second arm;

passing the output of the first arm through the first end of the optical fibre;

conditioning the output from the second arm and passing it through the other end of the optical fibre ;

extracting the signals propagating in a direction opposite to the signal in the first arm from the first end of the fibre;

detecting the above extracted optical signals using an optical photodetector ;

analyzing the output of the detector to extract specific modulated signals whose frequency or its harmonics corresponds to the frequency fed to the phase modulator above; and

determining the desired measurable parameter by identifying the changes in the optical frequency or amplitude or both.

2. The method as claimed in claim 1 wherein the optical distributed sensing corresponds to Brillouin distributed sensing and the optical frequency measured is the Brillouin frequency.

3. The method as claimed in claim 1 or 2, further includes the step of tuning the frequency of signal fed to the phase modulator such that the sensing can be carried out independently at multiple locations all along the length of the optical fibre

4. The method as claimed in claim 1, 2, or 3, wherein the said measurable parameter may be strain or temperature, or both, or any other physical parameter such as pressure or force that results in strain or temperature change on the sensing fiber .

5. The method as claimed in claim 1, 2, 3 or 4, wherein the said strain is static or dynamic.

6. The method as claimed in claims 1, 2, 3, 4, or 5 wherein the said first arm is pump, and the second arm is probe.

7. The method as claimed in claims 1, 2, 3, 4, 5, or 6, wherein the conditioning in the second arm consists of modulation of the light intensity with tunable radio frequency signals such that the original light frequency is suppressed. The method as claimed in claims 1, 2, 3, 4, 5, 6, or 7, wherein the said first end is pump end of the optical fibre.

A system for simultaneously sensing in real time, one or more parameters in an optical fiber by distributed sensing, the system comprising atleast of: an external phase modulator through which a narrow band laser output is passed, the said phase modulator is driven with a plurality of signals with different frequencies;

an optical power splitter to split the output of the phase modulator into two arms, a first arm and a second arm, and pass the output of the first arm through the first end of the optical fibre a means to condition the output from the second arm and pass it through the other end of the optical fibre;

an extractor to extract the signals propagating in a direction opposite to the signal in the first arm from the first end of the fibre; an optical photodetector to detect the above extracted optical signals; and

an analyzer to analyze the output of the detector to extract specific modulated signals whose frequency or its harmonics corresponds to the frequency fed to the phase modulator above; and determine the desired measurable parameter by identifying the changes in the optical frequency or amplitude or both.

10. The system as claimed in claim 9 wherein the optical distributed sensing corresponds to Brillouin distributed sensing and the optical frequency measured is the Brillouin frequency.

11. The system as claimed in claim 9 or 10, further includes a tuner to tune the frequency of signal fed to the phase modulator such that the sensing can be carried out independently at multiple locations all along the length of the optical fibre

12. The system as claimed in claim 9, 10, or 11, wherein the said measurable parameter may be strain or temperature, or both, or any other physical parameter such as pressure or force that results in strain or temperature change on the sensing fiber.

13. The system as claimed in claim 9, 10, 11, or 12, wherein the said strain is static or dynamic.

14. The system as claimed in claims 9, 10, 11, 12, or 13, wherein the said first arm is pump, and the second arm is probe.

15. The system as claimed in claims 9, 10, 11, 12, 13, or 14, wherein the conditioning in the second arm consists of modulation of the light intensity with tunable radio frequency signals such that the original light frequency is suppressed.

16. The system as claimed in claims 9, 10, 11, 12, 13, 14, or 15, wherein the said first end is pump end of the optical fibre.

Description:
SYSTEM FOR SIMULTANEOUS MULTI-POINT DYNAMIC PARAMETER MEASUREMENT IN DISTRIBUTED OPTICAL SENSING, AND METHODS THEREOF

FIELD OF THE INVENTION

[0001] The present invention relates to a system and method for detection of measurable physical parameters at multiple locations simultaneously in a long length of optical fiber. Typical applications where it can be used include aircraft health monitoring, structural fatigue evaluation due to seismic man-made activities, dynamic loadings on bridges etc.

BACKGROUND OF THE INVENTION

[0002] Stimulated Brillouin Scattering (SBS) based optical fiber sensors have been used for distributed sensing of parameters like strain and temperature for the past two decades. Due to the use of optical fiber as sensing element, these sensors have advantages like light weight, localized probe, ability to bend over corners and immune to electro ¬ magnetic interference.

[0003] Specifically, Brillouin optical correlation domain analysis (BOCDA) has been widely used for dynamic strain sensing applications requiring sub- meter spatial resolution such as load monitoring of aircraft. In BOCDA, a frequency-modulated or phase- modulated narrow linewidth laser source is split into two to generate the pump and probe lightwaves. These two lightwaves are counter propagated in the fiber under test (FUT) thereby localizing the SBS process through correlation peaks formed at specific periodic locations.

[0004] Conventional frequency-modulated BOCDA sensors employ direct modulation of a narrow linewidth laser source such as a distributed feedback (DFB) laser to generate frequency modulated (FM) pump and probe. This technique is useful to generate only one independent correlation peak within the FUT, thus limiting the measurements to a single location at any given instance. Distributed sensing is achieved by sweeping the same correlation peak along the FUT by varying the modulation frequency ( f m ) . However, several SHM applications require faster measurements (~100 Hz) as well as more number of sensing points. This can be addressed by enabling simultaneous measurements at multiple locations. Although several configurations have been demonstrated to monitor multiple locations such as random access BOCDA and temporal gating BOCDA multiple locations are monitored sequentially in all such configurations, thereby limiting the measurement speed.

[0005] Thus, there is a need in the art for a BOCDA method and system for simultaneous sensing of dynamic strain at different locations. OBJECTS OF THE INVENTION

[0006] Therefore, it is an object of the invention to provide a method for simultaneous sensing of measurable parameters at different locations in an optical fibre.

[0007] It is yet another object of the invention to provide for independently tuning the modulation frequency such that multiple locations all along the length of the fiber are addressed simultaneously and sensing of measurable parameters is carried out at different locations.

[0008] it is an object of the invention to provide a system for simultaneous sensing of measurable parameters at different locations in an optical fibre .

[0009] It is yet another object of the invention to provide a system for independently tuning the modulation frequency such that multiple locations all along the length of the fiber are addressed simultaneously and sensing of measurable parameters is carried out at different locations.

SUMMARY OF THE INVENTION

[0010] To meet the objects of the invention and overcome the disadvantages of the prior art it is disclosed herein, a method for simultaneously sensing in real time, one or more parameters in an optical fiber by optical distributed sensing comprising the steps of:

passing a narrow band laser output through an external phase modulator, wherein the said phase modulator is driven with a plurality of signals with different frequencies;

splitting the output of the phase modulator into two arms, a first arm and a second arm;

passing the output of the first arm through the first end of the optical fibre;

conditioning the output from the second arm and passing it through the other end of the optical fibre ;

extracting the signals propagating in a direction opposite to the signal in the first arm from the first end of the fibre;

detecting the above extracted optical signals using an optical photodetector ;

analyzing the output of the detector to extract specific modulated signals whose frequency or its harmonics corresponds to the frequency fed to the phase modulator above; and

determining the desired measurable parameter by identifying the changes in the optical frequency or amplitude or both. 1] It is disclosed herein a system for simultaneously sensing in real time, one or more parameters in an optical fiber by distributed sensing, the system comprising atleast of: an external phase modulator through which a narrow band laser output is passed, the said phase modulator is driven with a plurality of signals with different frequencies;

an optical splitter to split the output of the phase modulator into two arms, a first arm and a second arm, and pass the output of the first arm through the first end of the optical fibre

a means to condition the output from the second arm and pass it through the other end of the optical fibre ;

an extractor to extract the signals propagating in a direction opposite to the signal in the first arm from the first end of the fibre;

an optical photodetector to detect the above extracted optical signals; and

an analyzer to analyze the output of the detector to extract specific modulated signals whose frequency or its harmonics corresponds to the frequency fed to the phase modulator above; and determine the desired measurable parameter by identifying the changes in the optical frequency or amplitude or both.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 shows (a) Typical frequency spectra of an electrical signal with FM modulation at different center frequencies and (b) corresponding output spectrum of optical phase modulator. (c-d) Independent tunability of correlation peaks by modifying the modulation frequency of one of the FM signals from fm2 to fm3.

[0013] FIG. 2 shows simulated Brillouin gain spectra (BGS) traces at correlation peak locations corresponding to (a) fml = 74 kHz and (b) fm2 = 78 kHz. Strain is simulated at the location corresponding to the correlation peak at fml = 74 kHz. BGS traces obtained through lock-in detection corresponding to 2 fm frequencies.

[0014] FIG. 3 is a schematic representation of the experimental setup and the fiber under test.

[0015] FIG. 4 shows amplified probe trace obtained by pulsing the pump showing two correlation peaks when phase modulator is driven with two FM signals.

[0016] FIG. 5 is a trace showing the independent tunability of the two correlation peaks.

[0017] FIG. 6 shows spectrum recorded at the output of the photo detector when phase modulator is driven with one FM signal with fm of 75 kHz.

[0018] FIG. 7 represents (a) BGS along 1.1 km long fibre under test obtained by varying fm from 71 kHz to 80 kHz and (b) the corresponding Brillouin frequency shift (BFS) as a function of the sensing fiber length.

[0019] FIG. 8 represents BGS traces corresponding to the correlation peaks at (a) 510 m and (b) 1057 m generated using fml = 75 kHz and fm2 = 80.5 kHz respectively when strain is applied on Fiber 2.

[0020] FIG. 9 shows fibre under test to emulate dynamic strain over 100 m long fiber using an optical switch .

[0021] FIG. 10 shows BGS traces of the correlation peak location within (a) Fiber 1 (fml = 75 kHz) and (b) Fiber2 ( fm2 = 80.5 kHz) when Fiber 2 is subjected to dynamic strain with a switching frequency of 1 Hz. The corresponding BFS as a function of time are shown in (c) and (d) . Step size of probe frequency scanning is 3 MHz.

[0022] FIG. 11 shows BGS traces of the correlation peak location within (a) Fiber 1 (fml = 75 kHz) and (b) Fiber2 ( fm2 = 80.5 kHz) when Fiber 2 is subjected to dynamic strain with a switching frequency of 3.3 Hz. The corresponding BFS as a function of time are shown in (c) and (d) . Step size of probe frequency scanning is 10 MHz.

[0023] FIG. 12 represents (a) Amplified probe at a lock-in frequency of 161 kHz at a fixed pump-probe frequency offset of 10.750 GHz, when Fiber 2 is subjected to dynamic strain with a switching frequency of 50 Hz. BGS of the two fibers are shown in (b) for reference. DE TAILED DESCRIPTION OF THE INVENTION

[ 0024 ] The invention and its various embodiments is better understood by reading the description along with the accompanying drawings which appear herein for purpose of illustration only and does not limit the invention in any way.

[ 0025 ] The BOCDA system typically uses direct modulation of a narrow linewidth source with sinusoidal signal to achieve frequency modulated pump and probe, which in turn results in periodic correlations with a correlation at the center of the fiber - referred to as the zeroth order correlation peak. The separation between the adjacent correlations ( d ) is given by

d = a)

2nf m

where c is the speed of light in vacuum, n is the effective index of the fundamental mode in fiber and f m represents the modulation frequency.

[ 0026 ] The location of the correlation peaks (except for the zeroth order peak) can be tuned by changing the modulation frequency f m . The full-width at half maximum of each correlation ( Δζ ) is given by

cAv B

2nnf m Af where Av B is the Brillouin gain bandwidth (~ 30 MHz) and Δ is the frequency deviation 7] Thus, for a given fiber, the location of sensing is solely determined by the modulation frequency, while the spatial resolution (which depends on the width of the correlation) is additionally influenced by the frequency deviation. In order to have unambiguous and reliable sensing in a given length of FUT, the modulation frequency is chosen such that only one correlation peak exists within the FUT thereby monitoring only one location. One of the two lightwaves - pump or probe, is delayed relative to the other such that the correlation peak generated within the FUT corresponds to non-zeroth interaction and hence can be tuned across the FUT for distributed sensing. In order to monitor multiple locations simultaneously, multiple correlation peaks have to be generated within the FUT. This would require optical modulation with signals that have different f m values, each of which would uniquely determine a sensing location. Additionally, each of these correlation peaks have to be tuned independently in order to monitor the user-specified locations. Direct modulation of laser is not amenable to the above constraints due to the mixing of the different FM signals in a semiconductor laser . [0028] It is disclosed herein a method and system to generate the requisite sinusoidal FM signals with unique sets of f m and Δ in the electrical domain using an arbitrary waveform generator and embed these features in the optical domain through an external phase modulator (PM) .

[0029] Referring to FIG.1(a) to FIG. (d), an optical phase modulator when driven by an electrical signal generates multiple side- bands with a spectral content similar to that of the driving electrical signal. The typical structure of the frequency spectrum of the sinusoidal FM drive signal and that at the output of the phase modulator are shown in FIGs. 1(a) and 1(b) respectively.

[0030] The drive signal is comprised of multiple FM signals at distinct center frequencies and with different f m frequencies in the electrical domain as is shown in FIG. 1(a) . The frequency deviation f decides the strength of the side bands for each FM set. If the FM sets are within the bandwidth of the phase modulator, the optical output of the same, as shown in FIG.1(b) is expected to have optical carrier (at f c ) and multiple FM signals on both sides of the optical carrier with same f m frequencies as in the electrical domain. When such an optical signal is used to generate the pump and the probe in a Brillouin sensing experiment, each of these multiple FM signals generates a corresponding correlation peak whose location and width are determined by the respective f m and Δ " values. The location of each correlation peak can be tuned independent of the other by modifying the corresponding FM signal in the electrical domain as shown in FIGs. 1(c) and 1 (d) .

[0031] Driving the phase modulator with two FM signals with modulation frequencies f ml and f m2 (as shown in FIG.l(c)) generates two correlation peaks at locations given by Eq. (1) . When the modulation frequency of one of the FM signals is modified from fm2 to f m3 , the location of the corresponding correlation peak alone can be changed as shown in FIG.1 (d) .

[0032] Thus, by driving the phase modulator using multiple FM signals and with a careful choice of f m values, multiple independently-addressable correlation peaks can be generated at specific locations across the sensing fiber, thereby enabling the ability to monitor multiple locations simultaneously. Even though the illustrations shown in FIG.l indicate distinct center frequencies for different FM signals, the difference between the center frequencies do not influence the measurement result; they could in fact be identical in an experiment .

Simulation results

[0033] One of the key challenges in monitoring multiple locations simultaneously is the extraction of the strain information from multiple sensing locations without significant cross-talk. Simulations are performed to verify the dependence of BGS from each correlation peak with that of the other correlation peaks in the FUT . The methodology followed in B. Somepalli, D. Venkitesh, and B. Srinivasan, "Spatial mapping of correlation profile in Brillouin optical correlation domain analysis," Meas. Sci. Technol. 28(4), 045202 (2017) is extended to simulate the SBS interaction over 1 km long fiber and estimate the amplified probe when the two lightwaves - pump and probe are modulated with multiple sinusoidal FM signals. Under undepleted pump approximation, SBS interaction between pump and probe is modeled using the steady-state propagation equations. The amplified probe power is computed using the pump power and SBS gain which depends on the local BFS and the instantaneous frequency offset between pump and probe. 4] The time step size considered is 5 ns which corresponds to space step size of 1 m. The BFS of the fiber is considered as 10.800 GHz. The pump and probe are considered to be modulated with two sinusoidal FM signals centered at 6 GHz with f m frequencies 74 kHz and 78 kHz and Δ of 2 GHz each. The probe is delayed by 70 με relative to the pump. This generates two correlation peaks at 450 m and 800 m as per Eq. (1) . The spatial resolution, given by the width of correlation (Eq. (2)), is nearly 6 m each. The frequency offset between pump and probe is varied from 10.700 GHz to 10.900 GHz. A strain perturbation equivalent to an increase in BFS of 10 MHz is simulated at the correlation peak location which corresponds to an f m frequency of 74 kHz. The BGS traces are obtained by simulating lock-in detection at the corresponding 2 f m frequencies sequentially. The BGS at the two correlation peak locations obtained through simulations in the presence and absence of strain are shown in FIG.2.

[0035] In the absence of strain, the BFS at the two locations is 10.800 GHz. In the presence of strain, the peak of the BGS at the correlation peak location corresponding to a modulation frequency of 74 kHz is shifted to 10.81 GHz while the other peak corresponding to a modulation frequency of 78 kHz has not shifted. This conveys that the BGS of each correlation peak location is independent on the BGS of the other correlation peak locations.

[0036] The invention is further described by taking an exemplary case, the example provided herein is for the purpose of describing the invention in detail and does not limit the invention.

Example

[0037] Referring to FIG.3, a narrowband laser (linewidth 25 kHz) at a wavelength of ~1560 nm is used as a light source. The output of the laser is modulated using an external phase modulator, which is driven by the sum of two sinusoidal FM signals generated from an arbitrary waveform generator. The two FM signals are centered at 6 GHz with a frequency deviation ( Δ/ ) of 2 GHz each. The modulation frequencies of the two FM signals are varied between 71 kHz and 80 kHz, which corresponds to a measurement range and spatial resolution of about 1.3km and 6m respectively according to Eqs . (1) and (2) . The output of the phase modulator is filtered using a bandpass filter to extract the frequency modulated optical signals which are subsequently split into pump and probe waves. The pump light wave after amplification is launched from one end of the FUT consisting of a 1km long fiber (Fiber 1) followed by a 100 m long fiber (Fiber 2) . The probe light wave on the other arm is passed through 14 km long delay fiber so that the correlation peak generated within the FUT corresponds to non-zeroth order interaction. The delayed probe is amplified, frequency shifted by the Brillouin frequency ( f B ) using an electro-optic modulator (EOM) in carrier suppressed configuration and is launched from the other end of the FUT. 8] The frequency modulated pump and probe interact in the FUT and generate multiple correlation peaks at locations determined by the carefully chosen f m frequencies. The amplified probe is filtered using a fiber Bragg grating to extract the Brillouin Stokes component and is detected using a 45 MHz photo receiver. Lock-in detection at 2f m frequency is performed using an electrical spectrum analyzer in zero-span mode. Results and Discussion

Spatial Mapping of Multiple Correlation Peaks

[0039] It is one of the key objects of the invention to generate multiple correlation peaks in the FUT simultaneously. In order to verify the generation and independent control of multiple correlation peaks, initial tests are carried out with only Fiber 1 (~lkm) as the FUT. The procedure disclosed in B. Somepalli, D. Venkitesh, and B. Srinivasan, "Spatial mapping of correlation profile in Brillouin optical correlation domain analysis," Meas. Sci. Technol. 28(4), 045202 (2017) is followed in order to spatially map the correlation profiles to confirm their actual locations. In order to enable spatial mapping, the pump is modulated with a narrow pulse train (50 ns pulse width, 11 με period) using an EOM and observe the amplified probe on an oscilloscope. The phase modulator is driven with two sinusoidal FM signals with modulation frequencies ( f m ) of 84 kHz and 94 kHz respectively and Δ of 500 MHz each, which corresponds to a measurement range and spatial resolution of 1.1 km and 21 m respectively according to Eqs . (1) and (2) . The amplified probe trace observed on an oscilloscope for 10.800 GHz frequency offset between pump and probe is shown in FIG.4. The time axis in the plot is translated to corresponding distances using the time of flight of pump.

[0040] The trace contains two distinct peaks indicating that two correlation peaks are generated due to the two FM signals at locations determined by the respective modulation frequencies { f ml and f m2 ) . The width of the correlation features are 40 m and 34 m respectively which have been verified independently through simulations. In order to demonstrate the independent tunability of the two correlation peaks, the modulation frequency ( m2 ) of one of the FM signals is varied from 84 kHz to 94 kHz while that of the other is unchanged. The amplified probe traces obtained are shown in FIG.5 (a) The location of correlation peak corresponding to the varying modulation frequency f m2 alone has changed while the one due to the fixed modulation frequency f ml remain unchanged. This was also checked by keeping the modulation frequency m 2 fixed and varying the other modulation frequency fmi as shown in FIG.5(b) . As expected, the width of each correlation feature was observed to change from 40 m to 34 m with change in f m from 84 kHz to 94 kHz. Both the results of FIG.5 clearly demonstrate that multiple correlation peaks generated through external phase modulation-based BOCDA can be tuned independently .

Sensing of static strain from multiple correlation peaks

[0041] In order to detect static strain variations using phase modulation-based BOCDA, Fiber 2 (~100 m) is added to the FUT consisting of Fiber 1 (~1 km) . FM signals with modulation frequencies between 70 kHz and 80 kHz are used to ensure that only one correlation peak is generated within the FUT due to each of the FM signals. Varying the modulation frequency from 71 kHz to 79 kHz sweeps the correlation peak across the Fiber 1 and with a modulation frequency closer to 80 kHz, the other correlation peak is localized within Fiber 2. In order to obtain the BGS and estimate the BFS along the FUT, the phase modulator is initially driven by one FM signal with f of 2 GHz. The spectrum of the amplified probe after photo detection with a modulation frequency of 75 kHz is shown in FIG.6.

[0042] The spectrum consists of distinct peaks at a frequency of f m and its harmonics. The frequency offset between pump and probe is varied from 10.701 GHz to 10.900 GHz in steps of 1 MHz and the corresponding BGS is captured using the ESA in zero- span mode locked to 2f m frequency. The efficacy of choosing 2 f m frequency for lock-in detection as opposed to other harmonics was experimentally verified through independent experiments. The BGS is acquired for different f m values which sweeps the correlation peak along the 1.1 km long FUT. The BGS traces and the corresponding BFS obtained from peak detection at different locations are shown in FIG.7.

[0043] The BFS of Fiber 1 was nearly 10.798 GHz while that of Fiber 2 is slightly lower (-10.793 GHz) . The BFS of these fibers are measured independently through Brillouin optical time domain analysis measurements which are in good agreement with these values. It was then proceeded to drive the phase modulator with two sinusoidal FM signals centered at 6 GHz with f m frequencies of 75 kHz and 80.5 kHz such that correlation peaks are generated in the 1 km fiber and 100 m fiber respectively. Fiber 2 was wound across two posts mounted on translational stages and static strain was applied by moving one of the stages. The BGS traces obtained by locking ESA to the corresponding 2f m frequencies in zero-span mode are shown in Fig. 8.

[0044] Due to strain in Fiber 2, the BGS of the correlation peak location within Fiber 1 has not shifted while that in Fiber 2 has shifted by 10.110.8 MHz which corresponds to a strain perturbation of 202116 μ . Such a strain perturbation on Fiber 2 is verified independently using a fiber Bragg grating wound across the two posts. These experiments validate the embodiment of the invention of phase modulation-based BOCDA to measure strain at independent locations in the fiber; the BGS obtained from each correlation peak is dependent only on the strain applied at that location. Due to the presence of multiple frequency modulated signals in the system, there could be an increase in beat noise when compared to a system with single correlation peak. However, the lock-in detection process still allows the precise extraction of BGS from multiple locations as shown in FIGs. 8(a) and 8(b) .

Sensing of dynamic strain from multiple correlation peaks [0045] In order to demonstrate the capability of phase modulation-based BOCDA in detecting dynamic strain variations at multiple locations, dynamic strain was emulated in Fiber 2 by switching the optical path between two fibers whose Brillouin frequencies differ by 40 MHz approximately. The FUT is shown in FIG.9 where an optical switch was used to emulate dynamic strain in the 100 m long fiber.

[0046] Similar to the static strain measurements, two sinusoidal FM signals were used with f m frequencies of 75 kHz and 80.5 kHz and Af of 2 GHz each such that two correlation peaks are generated - one each in the 1 km fiber and the 100 m fiber. The optical path was switched between the two 100 m fibers, every 500 ms (switching frequency = 1 Hz) . The frequency offset between pump and probe is varied from 10.701 GHz to 10.900 GHz in steps of 3 MHz; this process took about 210 ms for each BGS measurement, limited only by the sweeping time of the probe frequency in the experimental setup. FIG.10 shows the BGS traces and the corresponding BFS as a function of time obtained through lock-in detection at the corresponding 2f m frequencies.

[0047] The BGS of the correlation peak location within Fiber 1 is observed to remain unchanged as seen from FIG.10 (a) and the corresponding BFS remains constant as a function of time (FIG.10(c)). On the other hand, the BGS of the correlation peak generated within Fiber 2 is found to be switching periodically as seen from Fig. 10(b) and the corresponding BFS is in good agreement with the square wave fitting with a switching frequency of 1 Hz (FIG.lO(d)) . The standard deviation in the estimated BFS is nearly 1.2 MHz. This demonstrates the multi-point dynamic strain sensing capability of the phase modulation-based BOCDA technique.

[0048] It was then proceeded to switch the optical path, every 150 ms (switching frequency = 3.3 Hz) . As described earlier, a scan over 200 MHz with a step size of 3 MHz in the probe frequency requires a minimum of 210 ms . In order to resolve a dynamic strain at 3.3 Hz, the step size was increased to 10 MHz resulting in a BGS measurement time of 65 ms . The BGS traces and the corresponding BFS are shown in FIG.11.

[0049] The BGS traces obtained are as predicted and the BFS of the correlation peak location within Fiber 2 is in good agreement with the expected square wave fitting. The standard deviation in the estimated BFS is nearly 2 MHz.

[0050] In the above experiments, the maximum rate of BFS variations that can be detected is limited by the finite sweeping time of the frequency offset between the pump and the probe. However, in situations where measurement of the absolute amplitude of strain is not very critical, perturbations at higher rates can be detected by measuring the intensity variations of the amplified probe at a specific pump-probe frequency offset. In order to demonstrate this, the optical path is switched between the two 100 m fibers every 10 ms (switching frequency = 50 Hz) and the frequency offset between the pump and the probe is fixed at 10.750 GHz. The amplified probe at a lock-in frequency of 161 kHz is monitored as a function of time and is shown in FIG.12 (a) .

[0051] The amplitude variations shown in FIG.12 (a) resemble a square wave with a frequency of 50 Hz which closely matches the frequency of dynamic strain variations. The extinction of the trace is nearly 3 dB, consistent with the change in Brillouin gain corresponding to the BGS shift as seen from the BGS traces in FIG.12(b) . The rate of dynamic strain variations detected is limited by the switching speed of the optical switch used. Thus, the phase modulation-based BOCDA technique is suitable to detect dynamic strain variations at multiple locations .

[0052] In all the above experiments, an electrical spectrum analyzer in the zero span mode was used as a single channel lock-in amplifier where the two correlation peaks are monitored sequentially by only changing the lock-in frequency. This can be further extended to simultaneous monitoring of multiple locations by using a multi-channel lock-in amplifier, with each channel being locked to the corresponding 2f m frequency. In the instant example, the measurement range was 1.1 km and the spatial resolution was 6 m as decided by the choice of f m and Af . The technique can further be extended to smaller FUT and sensing with better spatial resolution by choosing appropriate FM parameters. For instance, a measurement range of 10 m with a spatial resolution of 2 cm can be achieved with an f m in the range of 10 MHz and Af of 5 GHz.

[0053] Another key aspect of the disclosed method is that it is scalable and can be used to monitor the strain in multiple locations by generating multiple correlation peaks with appropriate sets of f m and Af . In a typical BOCDA implementation, the highest measured frequency of dynamic strain is limited by the sampling rate of the receiver. In case of conventional BOCDA, simultaneous measurement of dynamic strain from two different sensing points would require a proportionately higher sampling rate. In contrast, the external phase modulation- based BOCDA provides a pathway to scale the number of sensing points while maintaining the original sampling rate, thereby preserving the maximum detectable frequency of dynamic strain at each sensing point.

[0054] A novel method and system for multi-point sensing of dynamic strain variations using external phase modulation-based BOCDA which provides a clear pathway for monitoring multiple locations simultaneously is disclosed. Multiple frequency modulations are generated with appropriate f m and Δ values in the electrical domain, which are further transferred to the pump and the probe using external phase modulation. The BGS from multiple correlation peak locations are shown to be independent in the detection of strain variations through simulations, which are subsequently validated through controlled experiments. Two correlation peaks each 6 m wide are generated within the 1.1 km long FUT and the static strain variations at the two correlation peak locations are detected independently through lock- in detection at a frequency corresponding to twice the modulation frequency. It is also shown herein the detection of dynamic BFS variations at 3.3 Hz, only limited by the 65 ms sweep time of the probe frequency. By fixing the frequency offset between the pump and the probe, the capability of the testing apparatus to detect BFS variations at a rate of 50 Hz (limited by the speed of the optical switch used) is shown. Thus it is shown conclusively that the external phase modulation-based BOCDA technique is a viable solution for monitoring dynamic strain variations at multiple locations in a fiber simultaneously .