Data transmission-tolerant distributed acoustic sensing using chirped pulses with time-domain roll-off

Abstract

Aspects of the present disclosure describe distributed optical fiber sensing / distributed acoustic sensing systems, methods, and structures that advantageously use chirped interrogation pulses (sensing signals) that exhibit a smooth amplitude profile created by adding an out-of-band chirp to the leading and trailing edges of the sensing signal. The addition of leading and trailing out-of-band signals to the chirped pulse sensing signal to create a "smooth" amplitude profile facilitates the coexistence of sensing and communication signals on the same optical fiber.

Description

[Technical Field]

[0001] The present disclosure generally relates to distributed optical fiber sensing (DFOS) systems, methods, and structures that utilize chirped pulses with time-domain roll-off. [Background technology]

[0002] Recently, DFOS systems and methods have been employed to provide superior acoustic and / or vibration monitoring of roads, bridges, and buildings. The reliability, robustness, and sensitivity of such systems are generally recognized to be unparalleled compared to existing conventional systems and methods. More recently, distributed acoustic sensing (DAS) using chirped pulses has gained popularity due to its ability to increase the signal-to-noise ratio (SNR) of the returning backscattered signal without sacrificing spatial resolution or increasing the peak power of the interrogation signal. Given the importance of chirped pulse technology to DFOS / DAS, its improvement is a welcome addition to the art. Summary of the Invention

[0003] An advancement in the art is made in accordance with aspects of the present disclosure relating to DFOS / DAS systems, methods, and structures that use chirped interrogation pulses (sensing signals).

[0004] In contrast to the prior art, the sensing signal according to the present disclosure exhibits a smooth amplitude profile that is produced by adding an out-of-band chirp to the leading and trailing edges of the sensing signal.

[0005] In the present disclosure, the inventive feature of adding leading-edge and trailing-edge out-of-band signals to a chirped pulse sensing signal to create a "smooth" amplitude profile advantageously facilitates coexistence of sensing and communication signals on the same optical fiber. [Brief explanation of the drawings]

[0006] A more complete understanding of the present disclosure may be realized by reference to the accompanying drawings.

[0007] [Figure 1(A)] FIG. 1 is a schematic diagram illustrating a DFOS system according to an embodiment of the present disclosure.

[0008] [Figure 1(B)] FIG. 1 is a schematic diagram illustrating a coded constant amplitude DFOS system with out-of-band signal generation, according to an aspect of the present disclosure.

[0009] [Figure 2(A)] The time and frequency domain representations of the sensing signal in a conventional N-fold frequency diversity chirp pulse DAS, where each chirp CPi has duration Tc and bandwidth αTc. The amplitude profile is rectangular with duration NTc, and the signal has a bandwidth of NαTc. [Figure 2(B)] 10A-10C are time-domain and frequency-domain representations of a sensing signal where an out-of-band signal is inserted before and after a conventional sensing signal to produce a smooth amplitude profile. The signal of duration Trt at its rising and falling edges must be outside the bandwidth occupied by CP1 through CPN, and although the resulting signal has a longer duration and wider bandwidth, XPM on the communication signal is reduced, allowing a higher peak power Psens to be emitted as the interrogation (sensing) signal without adversely affecting the communication signal, according to aspects of the present disclosure.

[0010] [Figure 3(A)]Figure 1 shows the improvement in nonlinear tolerance by utilizing a smooth amplitude profile by adding an out-of-band chirp to frequency diversity chirp pulse interrogation (sensing) (N=20, Tc=10μs, B=10MHz, Tp=10.5ms). The graph shows the relationship between the post-FEC BER measured by a real-time coherent transponder and the peak power Psens of the sensing signal for various rise times Trt. The XPM nonlinearity caused by the sensing signal causes cycle slips and burst errors, which are not corrected by the coherent transponder's FEC. Increasing Trt increases the maximum allowable Psens at which the post-FEC BER is zero. [Figure 3(B)] 10 is a graph illustrating the improvement in nonlinear tolerance by utilizing a smooth amplitude profile by adding an out-of-band chirp to frequency diversity chirp pulse interrogation (sensing) (N=20, Tc=10 μs, B=10 MHZ, Tp=10.5 ms), showing the maximum allowed Psens versus Trt according to an embodiment of the present disclosure.

[0011] [Figure 4] FIG. 1 is a schematic diagram illustrating an experimental setup for evaluating systems and methods according to aspects of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] The following is merely illustrative of the principles of the present disclosure, and it will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the present disclosure and are included within its spirit and scope.

[0013] Furthermore, all examples and conditional language set forth herein are intended to be for educational purposes only to aid the reader in understanding the concepts contributed by the inventors to further the principles and techniques of the present disclosure, and should not be construed as being limited to such specifically recited examples and conditions.

[0014] Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, such equivalents are intended to include both currently known equivalents as well as equivalents developed in the future, i.e., elements developed that perform the same function, regardless of structure.

[0015] Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure.

[0016] Unless otherwise specified herein, the figures comprising the drawings are not drawn to scale.

[0017] As additional background, we begin by noting that distributed fiber optic sensing (DFOS) is an important and widely used technology for detecting environmental conditions (e.g., temperature, vibration, acoustic excitation, and strain levels) anywhere along a fiber optic cable that is in turn connected to an interrogator. As known, a modern interrogator is a system that generates an input signal into the fiber, detects and analyzes the reflected / scattered, and then received signal. The signal is analyzed, and an output is generated that indicates the environmental conditions encountered along the fiber. Such received signals can result from reflections within the fiber, such as Raman backscattering, Rayleigh backscattering, and Brillion backscattering. DFOS can also use forward signals that exploit the velocity differences of multiple modes. Without loss of generality, the following discussion assumes reflected signals, but the same approach is equally applicable to forward signals.

[0018] Figure 1(A) is a schematic diagram of a generalized prior art DFOS system. As will be appreciated, modern DFOS systems include an interrogator that periodically generates optical pulses (or any coded signal) and injects them into an optical fiber. The injected optical pulse signal is transmitted along the optical fiber.

[0019] At locations along the fiber, a small portion of the signal is reflected back to the interrogator. The reflected signal carries information that the interrogator uses to detect, for example, changes in power level indicative of mechanical vibrations. Although not shown in detail, the interrogator can include a coded DFOS system that can employ a coherent receiver configuration known in the art, such as that shown in Figure 1(B).

[0020] The reflected signal is converted to the electrical domain and processed within the interrogator. Based on the time of pulse injection and the time the signal is detected, the interrogator can determine which location on the fiber the signal is coming from and sense the activity at each location on the fiber.

[0021] Those skilled in the art will understand and appreciate that by implementing signal coding on the interrogation signal, more optical power can be transmitted into the fiber, thereby advantageously improving the signal-to-noise ratio (SNR) of Rayleigh scattering-based systems (e.g., distributed acoustic sensing, or DAS) and Brillouin scattering-based systems (e.g., Brillouin optical time-domain reflectometry, or BOTDR).

[0022] As currently practiced in many modern implementations, a dedicated fiber is allocated to the DFOS system in a fiber optic cable, physically separated from existing optical communication signals carried on different fibers. However, given the exponential growth in bandwidth demand, it is becoming increasingly difficult to economically operate and maintain optical fiber solely for DFOS operations. As a result, there is growing interest in integrating communication and sensing systems on a common fiber that is part of a larger multi-fiber cable.

[0023] Operationally, DFOS systems are envisioned as Rayleigh scattering-based systems (e.g., distributed acoustic sensing, or DAS) and Brillouin scattering-based systems (e.g., Brillouin optical time-domain reflectometry, or BOTDR) with coded implementations. Such coded designs make these systems more likely to be integrated with fiber communication systems, as they operate at lower power and are more sensitive to the response time of optical amplifiers.

[0024] The exemplary arrangement shown in the block diagram assumes that the coded interrogation sequence is generated digitally and modulated onto the sensing laser via a digital-to-analog converter (DAC) and an optical modulator. The modulated interrogation sequence may be amplified to an optimal operating power before being sent down the fiber for interrogation.

[0025] Advantageously, DFOS / DAS operations can also be integrated together with communication channels within the same fiber.

[0026] As mentioned above, distributed acoustic sensing (DAS) using chirped pulses has recently gained popularity because it can increase the signal-to-noise ratio (SNR) of the returning backscatter without sacrificing spatial resolution or increasing the peak power of the sensing signal. Such chirped pulse DAS is also known as "time-gated orthogonal frequency-domain reflectometry" (TGD-OFDR).

[0027] The principle of preserving spatial resolution is sens , duration T c , and bandwidth B=αT c (γ is the chirp coefficient)

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[0028] Chirped-pulse DAS has achieved a reach of approximately 171 km without repeaters, and recent field test results employing this technology have shown reaches of over 1,000 for all-Raman amplified links.

[0029] One problem with using conventional chirped pulses with a rectangular envelope, as described by p(t), is that the abrupt change in signal amplitude at the beginning and end of the pulse induces a large nonlinear penalty in the copropagating communication channel. This is similar to how conventional on-off keying (OOK) signals impose a large cross-phase modulation (XPM) penalty on coherent communication signals. Because the bandwidth of the sensing signal is much narrower than that of the communication signal, chromatic dispersion can be temporarily ignored when considering the nonlinear effects of the sensing signal on the communication signal.

[0030] Each effective nonlinear length is L eff N span We assume a link with 10 identical fiber spans. We also assume that the sensing channel is adjacent to the communication channel of interest, thus ignoring "walk-off." Due to Kerr nonlinearity, the sensing signal is distorted relative to the communication signal.

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[0031] Thus, at the beginning and end of the chirp sensing pulse, the nonlinear phase varies from 0 to γN over the time scale ∼1 / B. span L eff P sens The carrier phase error (BER) varies rapidly from 0 to 1, which is too fast for the carrier phase recovery (CPR) of a coherent receiver to track. This can cause cycle slips, which can lead to burst errors, and if the interleaver used in the forward error correction (FEC) is not long enough, the burst errors may not be sufficiently randomized, resulting in a non-zero post-FEC bit error rate (BER), even if the pre-FEC BER is below a threshold.

[0032] In summary, XPM imposed by a sensing signal on a communication channel can cause the communication channel to "stall" (non-zero post-FEC BER).

[0033] One way to make the sensing channel coexist with the communication channel is to use P until the post-FEC of the communication channel becomes zero. sens The goal is to reduce P sens Reducing , also reduces the SNR of the Rayleigh backscatter received by the sensing transponder.

[0034] In accordance with aspects of the present invention, the systems and methods of the present invention use a modified sensing signal with a smooth amplitude profile to reduce deleterious nonlinear effects on co-propagating communication signals by adding leading and trailing edges around the sensing signal and filling them with an out-of-band chirp.

[0035] A feature of the present invention is the addition of leading-edge and trailing-edge out-of-band signals to the chirped pulse sensing signal, which ensures that its amplitude profile is "smooth," reducing the nonlinear penalty imposed on the communication signal and facilitating the coexistence of sensing and communication signals on the same optical fiber.

[0036] Another way to mitigate the nonlinear penalty in data transmission is to use a sensing signal with a smooth amplitude profile.

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[0037] According to an embodiment of the present disclosure, a sensing signal such as that shown in FIG. 2B is employed, and a typical conventional signal is shown in FIG. 2A. N-fold frequency diversity is assumed to be used, and each CP i is the center frequency f i Note that the chirp pulse is of the form p(t) modulated by CP i is the overall repetition rate T that is greater than the round-trip propagation time of the fiber under test (FUT). p The amplitude profile of the emitted signal is rectangular and has a duration of NT c The frequency domain representation of the signal is also shown.

[0038] T rt The amplitude profile is smooth before CP1 and after CP so that the rise and fall times are given by N The sensing signal is modified by inserting an out-of-band signal after the

[0039] Figure 2(A) and Figure 2(B) are representations in the time and frequency domains. Figure 2(A) shows the time domain and frequency domain representations of each chirp CP. i is duration Tc and bandwidth αT c is a sensing signal in a conventional N-fold frequency diversity chirped pulse DAS with a duration of N. c The signal is NαT c Figure 2(B) shows the time and frequency domain representation of the sensing signal where the out-of-band signal is inserted before and after the conventional sensing signal to produce a smooth amplitude profile. The durations of the rising and falling edges are T rt The signal is transmitted from CP1 to CP N Therefore, in accordance with aspects of the present disclosure, a higher peak power P can be achieved without adversely affecting the communication signal, since the resulting signal has a longer duration and wider bandwidth but reduces XPM on the communication signal. sens can be emitted as an interrogation (sensing) signal.

[0040] Figure 2(B) shows an example structure where the out-of-band amplitude profile is a raised cosine. Other example configurations are possible, such as a ramp function. The only requirement is that φ NL The function needs to be smooth so that the rise and fall times of the sensing signals CP1 to CP2 change slowly. N must be outside the bandwidth of

[0041] In Figure 2(B), f0,f -1 ,···,f N+1 Out-of-band chirp CP0,CP centered at -1 ,···,CP N+1 A particular configuration using

[0047] is shown. Other out-of-band signals can be used provided their amplitude is a smooth function as shown by the contour curve.

[0042] Advantageously, our method was recently tested on a 1,000 km fiber link using all-Raman amplification, where the sensing signal was co-propagated with 50 × 200 Gb / s DP-16QAM data channels. The increase in nonlinear tolerance was significant, first measuring the post-FEC BER of a real-time coherent transponder 50 GHz away from the sensing signal.

[0043] Figures 3(A) and 3(B) show the results of frequency diversity chirp pulse interrogation (sensing) using out-of-band chirps (N=20, T c =10μs, B=10MHZ, T p 3(A) shows the improvement of nonlinear tolerance by utilizing a smooth amplitude profile by adding a 100 ms delay (=10.5 ms). Here, Fig. 3(A) shows the post-FEC BER measured by a real-time coherent transponder versus the peak power P sens The relationship between various rise times T rt It is shown that the XPM nonlinearity caused by the sensing signal causes cycle slips and burst errors, which are not corrected by the FEC of the coherent transponder, and rt Increasing , the maximum allowable P at which the post-FEC BER is zero sens FIG. 3(B) shows the T rt Maximum allowable P for sens Shows.

[0044] As can be seen from Figure 2(B), P sens If P is large, the burst errors caused by cycle slips will not be corrected by the FEC, resulting in a non-zero post-FEC BER. sens There exists a threshold at which the post-FEC BER becomes zero as the peak power P sens is T rt In Figure 2(B), T rt from 0 to 60 μs (3T c (equal to P sens,maxIt can be seen that the gain increases from -2dBm to +3dBm. Experimental equipment

[0045] As mentioned earlier, the use of communication fiber optic infrastructure for distributed sensing is gaining increasing attention among service providers as it promotes public safety and smarter cities while enabling new revenue streams for communication service providers / operators. Coexistence of data transmission and sensing has been demonstrated.

[0046] In particular, distributed acoustic sensing (DAS) based on Rayleigh backscattering phase optical time-domain reflectometry (φ-OTDR) enables applications such as traffic monitoring, intrusion detection, and earthquake monitoring. To date, DAS has mostly been implemented over tens of kilometers of optical fiber due to limitations in the optical signal-to-noise ratio (OSNR). It is desirable to extend the reach of DAS to be comparable to long-distance data transmission. This would reduce the number of sensing transponders required in a given geographic area and enable early warning of offshore earthquakes over submarine cables.

[0047] According to an aspect of the present invention, we employ all-Raman amplification without an in-line isolator and use frequency-diversity chirped-pulse DAS (FD-CP-DAS) with correlated detection and diversity combining. Here, we report the first DAS results over more than 1,000 km of standard single-mode fiber (SSMF) using an all-counter-pumped Raman amplification scheme.

[0048] The experimental setup is shown schematically in Figure 4. As shown in this figure, in the transmitter, the sensing signal is generated by passing a 1550.112 nm low-phase-noise NKT X15 fiber laser through a Mach-Zehnder I / Q modulator (MZM) driven by an arbitrary waveform generator (AWG) operating at 1 GSa / s. The generated sensing signal consists of 20-fold frequency-diversity chirped pulses (CP), each with a bandwidth B = 10 MHz (spatial resolution z ≈ 10 m) and a duration T = 10 ns.

[0049] Twenty CPs centered at f are fired consecutively, with their mutual frequency spacing Δf = B. The repetition period was set to T = 10.5 ms to allow an interrogation distance of over 1,000 km.

[0050] The theoretical OSNR gain over a single-frequency, non-chirp DAS at the same spatial resolution is 33 dB, of which 20 dB comes from correlated detection and 13 dB comes from frequency diversity.To reduce cross-phase modulation (XPM) distortion in communication signals (such as the XPM effect observed in the co-propagation of conventional on-off keyed and coherent signals), an out-of-band chirp is used to insert additional rising and falling edges of duration T, creating a smooth amplitude profile.

[0051] For the data channels, we used a real-time 32 GHz DP-16QAM coherent transponder supporting a data rate of 200 Gb / s per channel. Co-propagation of 50 dense wavelength-division multiplexed (DWDM) data channels from 191.75 THz to 194.25 THz was emulated using a noise load, and an amplified spontaneous emission (ASE) noise source was amplified by a two-stage EDFA with an intermediate wavelength selective switch (WSS), which simultaneously equalized the transmit spectrum and combined the emulated WDM adjacent 200 Gb / s channel under test (CUT) with the sensing signal.

[0052] The link consists of 13 spans of standard single-mode fiber (SSMF). The first 10 spans are lab fiber. Spans 11 and 12 consist of two loops of 79.8 km of field fiber from the previously reported North Dallas pool. Span 13 consists of two more field fiber loopbacks, 5.5 km and 9.3 km long, followed by two 10.1 km spans of lab fiber. Full Raman amplification was used to allow Rayleigh backreflections to return to the transponder. Backward pumping was used for spans 1–10 (average span length: 79.7 km, average span loss: 16.5 dB) and span 13. Forward and backward Raman pumping was used for high-loss spans 11 and 12, with losses of 19.9 dB and 20.9 dB, respectively. The power of the Raman pump (1426 nm–1466 nm) was adjusted to equalize the loss of the previous span while maximizing gain flatness.

[0053] The WDM signal was transmitted into the link at +14 dBm (3 dBm / ch), and the sensing signal was transmitted at 14.2 dBm (peak power: +2.2 dBm).

[0054] The received Rayleigh backscattered light passed through an EDFA preamplifier, followed by a 0.1 nm optical bandpass filter (OBPF), and then another EDFA preamplifier. The resulting signal was detected using a conventional coherent receiver consisting of a dual-polarization optical hybrid, a balanced photodiode (BPD), a 110 MHz electrical low-pass filter (ELPF), and sampling and digitization by a digital sampling oscilloscope (DSO) operating at 250 MSa / s. The DSO was operated in sequential capture mode, with the frame rate trigger provided by the AWG. N = 2,000 frames were acquired per dataset, each containing 200 μs of data (100 M samples per dataset). Oscillations up to the Nyquist frequency of 50 Hz were observed with a frequency resolution of 1 / NT = 0.05 Hz. For the data channel, the pre-FEC (pre-forward error correction) bit-error rate (BER) and post-FEC (post-forward error correction) bit-error rate (BER) were measured by the real-time transponder receiver. Experimental results

[0055] First, we measured the Rayleigh impulse response of each sensing channel using correlation detection. The approximate power profile of the link can be observed as spikes due to reflections at the connectors. Stronger reflections were observed at the SC-PC connector than at the FC-APC connector.

[0056] To reduce "spatial leakage," differential beat products are calculated at twice the minimum gauge time set by the bandwidth of each CP, i.e., τ = 2 / R = 0.2 μs. After retiming, the beat products are optimally combined using rotational vector summation to generate the complex value ζ[n,m], where n and m are the distance and frame index, respectively. Taking the unwrap angle at each fiber position n yields the estimated strain-induced optical phase shift at that position.

[0057] To demonstrate spatial resolution, 1.5-m and 12-m long piezoelectric transducers (PZTs) were inserted near the end of span 10. The PZTs were driven with 16-Hz and 27-Hz sine waves, respectively. No spatial crosstalk was observed, and phase noise was nearly flat from 10 to 48 Hz, with an increase at low frequencies due to laser phase noise. The phase noise power spectral density (PSD) floor in the flat region from 15 Hz to the Nyquist frequency R / 2 was estimated in rad Hz. By acquiring data centered at various points within the link, a sensitivity vs. distance sweep with 1-km resolution was obtained. The large phase noise PSD observed in spans 11 and 12 and the first half of span 13 was caused by real-world vibrations.

[0058] In this experiment, we conducted the first DAS experiment over 1,000 km using FD-CP-DAS with correlation detection and diversity combining. A fully Raman-amplified scheme enabled Rayleigh backscattering to return to the sensing transponder. The sensing signal coexisted with 10 Tb / s data transmission, emulating a real-time DP-16QAM transponder and noise load. After FEC encoding, all data channels achieved zero BER, and a sensing performance of ~100 pHz was achieved over a 20-meter gauge length. The DAS also successfully recovered the waveforms of real-world vibration events.

[0059] While the present disclosure has been presented using certain specific examples, those skilled in the art will recognize that the present teachings are not so limited. Accordingly, the present disclosure should be limited only by the scope of the claims appended hereto.

Claims

1. 1. A distributed fiber optic sensing (DFOS) / distributed acoustic sensing (DAS) system, comprising: an optical fiber sensor cable; a DFOS / DAS interrogator in optical communication with the fiber optic sensor cable; the DFOS / DAS interrogator is configured to generate a probe signal, launch the generated probe signal into the fiber optic sensor cable, and receive a Rayleigh backscatter signal from the fiber optic sensor cable; The distributed fiber optic sensing (DFOS) system is characterized in that the probe signal has a leading edge and a trailing edge added to it before launch, the probe signal including the leading edge and the trailing edge exhibits a smooth amplitude profile, and the probe signal includes a chirped pulse.

2. The system of claim 1 , wherein the probe signal comprises a concatenation of chirped pulses, each chirped pulse being centered at a different center frequency.

3. The system of claim 2 , wherein the leading and trailing edges are chirp pulses having center frequencies outside the frequency of the concatenated chirp pulses.

4. The system of claim 3 , wherein the fiber optic sensor cables simultaneously carry communication traffic.

5. The system of claim 4 , wherein the communication traffic is transmitted as wavelength division multiplexed (WDM) optical signals on the fiber optic sensor cable.

6. The system of claim 2 , wherein the leading and trailing edges exhibit a raised cosine amplitude profile.

7. The system of claim 2 , wherein the leading and trailing edges exhibit a ramp function amplitude profile.

8. The probe signal exhibits N-fold frequency diversity, with each chirp pulse (CP i ) is the center frequency f 1 3. The system of claim 2, wherein the chirped pulse is of the form p(t) modulated by

9. The CP i is an overall repetition rate T that is greater than the round trip propagation time of the fiber optic sensor cable. p 9. The system of claim 8, wherein the first and second laser beams are fired sequentially in time.

10. 1. A method for performing distributed fiber optic sensing (DFOS) / distributed acoustic sensing (DAS), comprising: providing an optical sensor fiber; providing a DFOS / DAS interrogator in optical communication with the optical sensor fiber, the DFOS / DAS interrogator configured to generate optical probe pulses, couple the generated optical probe pulses into the optical sensor fiber, and receive backscattered signals from the optical sensor fiber; providing an intelligent analyzer configured to analyze the backscattered signal and determine from the backscattered signal an environmental condition occurring at a location along the optical sensor fiber; A method in which a leading edge and a trailing edge are added to the optical probe pulse before it is introduced into the optical sensor fiber, the optical probe pulse including the leading edge and the trailing edge exhibiting a smooth amplitude profile, and the optical probe pulse includes a chirped pulse.

11. The method of claim 10 , wherein the optical probe pulse comprises a concatenation of chirped pulses, each chirped pulse being centered at a different center frequency.

12. 12. The method of claim 11, wherein the leading and trailing edges are chirp pulses having center frequencies outside the frequency of the concatenated chirp pulses.

13. The method of claim 12 , wherein the optical sensor fibers simultaneously carry communication traffic.

14. The method of claim 13 , wherein the communication traffic is transmitted as a wavelength division multiplexed (WDM) optical signal in the optical sensor fiber.

15. The method of claim 11 , wherein the leading and trailing edges exhibit a raised cosine amplitude profile.

16. The method of claim 11 , wherein the leading and trailing edges exhibit a ramp function amplitude profile.

17. The optical probe pulse exhibits N-fold frequency diversity, with each chirp pulse (CP i ) is the center frequency f 1 12. The method of claim 11, wherein the chirped pulse is of the form p(t) modulated by

18. The CP i is the overall repetition rate T that is greater than the round trip propagation time of the optical sensor fiber. p 18. The method of claim 17, wherein the laser beams are fired sequentially in time.

19. The method of claim 14 , wherein the communication traffic and the optical probe pulses are multiplexed into the optical sensor fiber.

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