Frequency Drift Compensation in Chirp Pulse-Based Distributed Acoustic Sensing

Abstract

Aspects of the present disclosure relate to frequency drift compensation for coded DAS systems that use chirp pulses as probe signals. Our inventive approach estimates timing jitter by correlating the amplitude of the estimated Rayleigh impulse response of each frame with a reference frame, and then realigns each frame by the estimated timing jitter. Since the amount of timing jitter varies within a frame, each frame is divided into blocks where all samples have similar timing jitter, and an overlap-and-reserve method is used to estimate and compensate for timing jitter on a block-by-block, frame-by-frame basis. Allowing the reference frame to be updated periodically allows tracking of slowly changing channels.

Description

Technical Field

[0001] The present disclosure generally relates to distributed fiber optic sensing (DFOS) systems, methods, and structures, and more particularly to frequency drift compensation in chirp pulse-based distributed acoustic sensing (DAS).

Background Art

[0002] Recently, DFOS systems and methods have been used to provide excellent acoustic and / or vibration monitoring of roads, bridges, and buildings. The reliability, robustness, and sensitivity of such systems are generally known to be incomparable to existing conventional systems and methods. Considering such characteristics, further improvement of DFOS / DAS technology combined with new analysis systems and methods would be welcome in the art.

Summary of the Invention

[0003] According to aspects of the present disclosure related to frequency drift compensation of a coded DAS system using chirp pulses as probe signals, an advancement in technology is achieved.

[0004] Frequency drift causes timing jitter in the estimated Rayleigh impulse response obtained by correlating the received Rayleigh backscatter with the transmitted chirp. In any received frame, since adjacent samples have similar timing jitter, this effect can be compensated by dividing each frame into small blocks where all samples are shifted by the same timing jitter, correlating the amplitude profiles between frames, and then readjusting in time.

[0005] The approach of the present invention provides an architecture for estimating and correcting timing jitter using an overlap-and-save architecture with low algorithm complexity, and enables the use of an encoded DAS system with a "cheaper" laser having a larger frequency drift by compensating for the hardware using a DSP.

[0006] From one aspect, the approach of the present invention estimates the timing jitter by correlating the amplitude of the estimated Rayleigh impulse response of each frame with a reference frame, and then readjusts each frame by the estimated timing jitter. Since the amount of timing jitter varies within a frame, each frame is divided into blocks where all samples have similar timing jitter, and the overlap-and-save method is used to estimate and compensate for the timing jitter in block units and frame units. By enabling the periodic update of the reference frame, it becomes possible to track a gradually changing channel.

[0007] From a first aspect, the present disclosure describes a DFOS system, method, and structure for monitoring an outdoor cabinet housing optical fiber equipment, where the cabinet / optical fiber cable contained therein is configured to provide excellent acoustic sensing.

[0008] From a second aspect, the present disclosure describes a DFOS system, method, and structure for monitoring a manhole structure.

[0009] Finally, from yet another perspective, the present disclosure describes a DFOS system, method, and structure using a machine learning-based analysis method using a time-series network.

Brief Description of the Drawings

[0010] A more complete understanding of the present disclosure can be achieved by referring to the accompanying drawings.

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Figure 1(A)

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Figure 2(B)

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MODE FOR CARRYING OUT THE INVENTION

[0017] The following merely illustrates the principles of the present disclosure. Thus, it will be understood that those skilled in the art can devise various configurations that embody the principles of the present disclosure and are included within its spirit and scope, although not explicitly described or illustrated herein.

[0018] Furthermore, all examples and conditional terms described in this specification are intended solely for the educational purpose of helping the reader understand the concepts contributed by the inventors to facilitate the principles and techniques of the present disclosure, and should not be construed as limited to such specifically recited examples and conditions.

[0019] Furthermore, all descriptions of this specification that describe the principles, aspects, and embodiments of the present disclosure, as well as specific examples thereof, are intended to encompass both their structural and functional equivalents. Furthermore, such equivalents are intended to include both currently known equivalents and equivalents developed in the future, i.e., developed elements that perform the same function regardless of structure.

[0020] Thus, for example, it will be understood by those skilled in the art that any block diagram in this specification represents a conceptual diagram of an exemplary circuit that implements the principles of the present disclosure.

[0021] Unless otherwise specified in this specification, the figures constituting the drawings are not drawn to scale.

[0022] As additional background, note that distributed fiber optic sensing (DFOS) is an important and widely used technology for detecting environmental conditions (such as temperature, vibration, acoustically excited vibration, stretch levels, etc.) at any location along an optical fiber cable that is sequentially connected to an interrogator. As is known, modern interrogators are systems that generate an input signal to the fiber, which is then reflected / scattered and the received signal is detected / analyzed. The signal is analyzed and an output is generated that indicates the environmental conditions encountered along the fiber. The signal received in this way can be due to reflections within the fiber such as Raman backscattering, Rayleigh backscattering, Brillouin backscattering. DFOS can also use forward signals that utilize the velocity differences of multiple modes. Without loss of generality, the following description assumes a reflected signal, but the same approach can be equally applied to transmitted signals.

[0023] Figure 1(A) is a schematic diagram of a generalized prior art DFOS system. As is understood, modern DFOS systems include an interrogator that periodically generates optical pulses (or any encoded signal) and injects them into an optical fiber. The injected optical pulse signal is transmitted along the optical fiber.

[0024] At positions along the fiber, a small portion of the signal is reflected and returned to the interrogator. The reflected signal transmits information that the interrogator uses to detect, such as a change in power level indicating mechanical vibration. Although not shown in detail, the interrogator can include an encoded DFOS system that can employ a coherent receiver configuration known in the art as shown in Figure 1(B).

[0025] The reflected signal is converted to the electrical domain and processed within the interrogator. Based on the pulse injection time and the time the signal is detected, the interrogator can determine from which position on the fiber the signal is coming and sense the behavior of each position on the fiber.

[0026] One skilled in the art would understand and recognize that by implementing signal encoding 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, i.e., DAS) and Brillouin scattering-based systems (e.g., Brillouin optical time domain reflectometry, i.e., BOTDR).

[0027] Operationally, the DFOS system is assumed to be a Rayleigh scattering-based system (e.g., distributed acoustic sensing, i.e., DAS) and a Brillouin scattering-based system (e.g., Brillouin optical time domain reflectometry, i.e., BOTDR) with coding implementation. In such coding designs, these systems are likely to be integrated with fiber communication systems due to their low operating power, and the influence of the response time of optical amplifiers also becomes significant.

[0028] In the configuration illustratively shown in the block diagram, it is assumed that an encoded interrogation sequence is digitally generated and modulated onto the sensing laser via digital-to-analog conversion (DAC) and an optical modulator. The modulated interrogation sequence may be amplified to an optimal operating power before being sent into the fiber for interrogation.

[0029] Advantageously, the DFOS operation can also be integrated with communication channels via WDM in the same fiber. In the sensing fiber, the interrogation sequence and the returned sensing signal are optically amplified either via a discrete (EDFA / SOA) or distributed (Raman) scheme. The returned sensing signal is sent to a coherent receiver after amplification and optical band-pass filtering. The coherent receiver detects the optical fields of both polarizations of the signal and down-converts them to four baseband lanes for analog-to-digital conversion (ADC) sampling and digital signal processor (DSP) processing. As would be readily understood and recognized by those skilled in the art, the decoding operation is performed by the DSP to generate the Rayleigh or Brillouin response of the fiber in response to the interrogation, after which the changes in the response are identified and interpreted for sensor readout.

[0030] Continuing to refer to the figure, since the encoded interrogation sequence is generated digitally, the out-of-band signal is also generated digitally and then integrated with the code sequence before the waveform is generated by the DAC. When generated digitally together, the out-of-band signal is generated only outside the period of the code sequence, so that when added together, the amplitude of the integrated waveform is constant.

[0031] In distributed acoustic sensing (DAS), the interrogator launches a probe signal x(t) into the fiber under test (FUT) and estimates its Rayleigh impulse response h(t). The received signal is given by the convolution

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[0032] To measure the time variation of the Rayleigh impulse response caused by acoustic vibrations, the interrogator uses a frame rate T pPeriodically transmit x(t). If Tp is longer than the round-trip propagation time T of the FUT, the received signal will be the result of a series of optical time domain reflectometry (OTDR). rt The spatial resolution of y(t) is

[0033] given by

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[0034] Due to the weak power of Rayleigh backscattering, the achievable reach with DAS is limited. Encoded DAS can increase the signal-to-noise (SNR) ratio achieved by DAS and enable longer FUTs. Instead of emitting pulses, as described above, the probe signal for encoded DAS is a sequence with an autocorrelation function r xx (t) = x(t) * x(t), which is as close as possible to a delta function and whose width is limited only by the bandwidth of x(t). The encoded DAS interrogator correlates the received signal with x(t)

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[0035] A well-known group of sequences with good autocorrelation characteristics is the chirp pulse

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[0036] Here, T c is the chirp duration, a is the chirp rate,

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[0037] When the chirp duration is long (T c >> T), the width of the main lobe of the sinc in Equation (2) is T = 1 / αT c = 1 / B, which is the same spatial resolution as a rectangular pulse of the same bandwidth used in conventional OTDR.

[0038] The chirp pulse has the special property that the correlation function between two chirp pulses x1(t) and x2(t) is only the sinc function in Equation (2) when their chirp rates α1 and α2 match. In this case, their correlation peaks occur where their center frequencies match.

[0039] Figures 2(A) and 2(B) are a pair of graphs showing the influence of frequency shift on the correlation of chirp pulses according to an aspect of the present invention. Figure 2(A) shows the case without frequency shift, and Figure 2(B) shows the case with a frequency shift of 4 MHz. The chirp has a special property that a correlation peak is generated when the instantaneous frequencies of the chirps are aligned. Therefore, frequency modulation of one chirp pulse causes a time shift of the correlation peak (also, since the overlap of the bandwidths decreases, the main lobe of the correlation function spreads slightly).

[0040] An example is shown in Figures 2(A) and 2(B). In Figures 2(A) and 2(B), a chirp pulse with a duration T c = 50 μs and a bandwidth B = 10 MHz (α = 2 × 10 11 s -2 ) is correlated with the same chirp pulse frequency-shifted by Δν = 4 MHz. It is observed that the correlation x2(t)*x1(t) of those chirp pulses is centered around an offset of Δτ = Δν / α = 20 μs. Furthermore, the width of the correlation is inversely proportional to the overlap of the bandwidths and is 6 MHz in this example.

[0041] This property of the chirp pulse, that the center of the correlation function is shifted in time by frequency modulation, is important for chirp pulse-based coded DAS.

[0042] Figure 3 is a schematic diagram showing a standard model of a coded DAS interrogator in which the same laser is used to generate a probe signal x(t) and function as a local oscillator for coherently detecting the Rayleigh backscattering y(t) of a fiber under test (FUT) using a Rayleigh impulse response h(t).

[0043] Consider the standard model of the DAS system shown in Figure 3, where the same laser is used to generate a probe signal and function as a local oscillator (LO) for coherent detection of Rayleigh backscattering. Consider the reflection from the point shown in the FUT of Figure 3. Between the reflected signal and the LO, there is

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[0044] As shown in FIGS. 2(A) and 2(B), the effect of frequency drift is the uncertainty in the time (or position) of the Rayleigh impulse response. Usually, the fiber position z is mapped to the time coordinate of the Rayleigh impulse response h(t)

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[0045] The influence of laser frequency drift on the performance of DAS is understood in an experiment that tested a 50 km long FUT using a chirp pulse with a repetition rate of 1 kHz and a duration T c = 50 μs and a bandwidth B = 10 MHz (spatial resolution

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[0046] Using a specific ("bad") laser results in a very serious frequency drift, and the background noise level becomes higher than the vibration amplitude of the PZT. However, using another ("good") laser, a 67 Hz sine wave can be easily observed.

[0047] Although it is possible to construct a chirp pulse DAS using a "good" laser, it is more expensive. Since the frequency drift only results in a linear transformation of the time / position axis and the laser frequency is stable on a short time scale, similar time / position shifts should occur at adjacent fiber positions. Therefore, digital signal processing (DSP) can be used to correct this time (position) jitter so that a "bad" laser with high phase noise can be used in the DAS.

[0048] Nevertheless, the present disclosure provides a solution for frequency drift compensation for a coded DAS system using a chirp pulse as a probe signal. The frequency drift introduces timing jitter to the estimated Rayleigh impulse response obtained by correlating the received Rayleigh backscattering with the generated chirp.

[0049] In any received frame, since adjacent samples exhibit similar timing jitter, the disclosure of the present invention divides each frame into small blocks where all samples are shifted with the same timing jitter, correlates the amplitude profiles between frames, and then corrects this effect by readjusting them temporally. The solution of the present disclosure provides an architecture for estimating and correcting timing jitter using an overlap-save architecture, and also provides a preferred implementation with low algorithm complexity. The solution of the present disclosure enables coded DAS using a "cheaper" laser with a larger frequency drift by using a DSP to compensate for hardware defects.

[0050] As will be apparent to those skilled in the art, the features of the disclosed solution according to aspects of the present disclosure include (i) estimating timing jitter by correlating the amplitude of the estimated Rayleigh impulse response of each frame with a reference frame, and then (ii) readjusting each frame by the estimated timing jitter.

[0051] Since the amount of timing jitter varies within a frame, in the approach of the present invention, all frames are divided into blocks where all samples exhibit similar timing jitter, and the overlap-save method is used to estimate and correct timing jitter in block units and frame units. In the innovative approach of the present invention, by enabling the periodic update of the reference frame, it becomes possible to track a gradually changing channel.

[0052] FIG. 4 is a schematic diagram showing an exemplary architecture for laser frequency drift compensation in coded DAS according to an aspect of the present invention. As shown in the figure, the architecture shown in FIG. 4 is for chirp pulses using an overlap-save algorithm. Operationally, after correlating the received signal with a known chirp to obtain h j (t)=x(t)*y j (t), the time axis has a duration T b and an overlap period Tov into N b overlapping blocks. With frame j = 0 as a reference, the squared amplitude of each block in subsequent frames

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[0053] To make it more explicit, the complex-valued signal vector received by the probe pulse transmitted in frame j is

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[0054] Next, the initial frame

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[0055] The downstream operations required to estimate the vibration at each point of the FUT are the same as those of the conventional DAS. These operations can include calculating the differential beat product with a predefined gauge length, diversity combining the beat products from different polarizations, frequencies, spatial channels, etc., and finally obtaining the unwrapped phase at all positions of the FUT.

[0056] According to the disclosure of the present invention, the block length T b is the root mean square (r.m.s.) frequency drift σ b over a duration T Δν (T b ) should be chosen to be much smaller than the time resolution T = 1 / B of the chirp, i.e.,

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[0057] Assume that the instantaneous frequency of the laser is ν(t). S νν (f) is the frequency noise on both sides of ν(t). The frequency drift b between two time instances separated by only T

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[0058] The variance

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[0059] Similarly, the repetition period T ov is the maximum value for which compensation may be required

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[0060] In fact, [Number] the Rayleigh impulse response estimated by [Number] gradually changes with time due to polarization rotation, temperature fluctuations, etc. The reference [Number] if held for a long time, ultimately [Number] changes significantly, and [Number] no longer matches the input frame. Therefore, the reference needs to be updated periodically every N u frames. That is, the mN u th timing jitter compensation frame [Number] should be used as a new reference for compensating the frame [Number] from [Number] to.

[0061] Figure 5 is a schematic diagram showing an exemplary procedure for compensating the time shift of each block according to an aspect of the present invention. First, the correlation with a reference is calculated to find an index n j0,b such that r

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[0063] Implementation with Reduced Complexity

[0064] According to the foregoing description, operations with high computational costs are (a) correlating each block

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[0065] Digital correlation

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[0066] It should be noted that the timing jitter can be further reduced by an alternative implementation with a slightly increased computational complexity.

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[0067] ​Two ratios quantized to a given timing jitter accuracy

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[0068] Examples of experimental results

[0069] Experimental results demonstrating the operation of frequency drift compensation according to the present disclosure were obtained. The experimental setup is as shown in Figure 2, using a digital / analog converter (DAC) that drives a Mach-Zehnder I / Q modulator to generate chirp pulses with a repetition rate of 1 kHz, a duration T c = 50 μs, and a bandwidth B = 10 MHz (spatial resolution

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[0070] The FUT consists of a 50 km spool of standard single-mode fiber (SSMF) followed by a piezoelectric transducer (PZT) 12 m in length, and its output is terminated with a 100 m length of fiber. The PZT is driven by a 67 Hz sine wave with a peak-to-peak amplitude of 1.6 rad. It was observed that the distortion caused by the frequency drift was very large, and the resulting background noise level was greater than the vibration amplitude of the PZT.

[0071] To evaluate the timing jitter associated with the laser frequency drift, more than 200 traces of

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[0072] This jitter was characterized and the frequency drift was corrected using the method of the present invention. The block size was T b = 25 μs, and the repetition period was T ovwas set to 2.5 μs. When this method is applied to the above data, that is, the

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[0073] When the technology of the present invention is applied to the performance of DAS using the aforementioned "inexpensive / poor" laser in frequency drift compensation, vibrations of 67 Hz are clearly observed, and the phase spectrum

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[0074] So far, several specific examples have been used to present this disclosure, but those skilled in the art will recognize that the teachings of the present invention are not limited thereto. Therefore, this disclosure should be limited only by the appended claims.

Claims

1. An optical fiber sensor cable, a DAS interrogator that optically communicates with the optical fiber sensor cable, A frequency drift compensation method in a chirp pulse-based distributed acoustic sensing system (DAS) having: The DAS interrogator includes: a seed laser, a coherent receiver, and is configured to: The interrogator generates a probe signal including a chirp pulse at a predetermined frame rate, chirp duration, and chirp throughput rate, and transmits the probe signal to the optical fiber sensor cable, using the coherent receiver to recover Rayleigh backscattering from the optical fiber sensor cable, The seed laser exhibits a non-negligible frequency drift such that the frequency of the local oscillator is different from the frequency at which the chirp pulse was generated, resulting in timing jitter in the Rayleigh impulse response determined from the correlation between the received backscattering and the known chirp, The method includes: correcting timing jitter in the Rayleigh impulse response estimated by dividing each received frame of Rayleigh backscattering into overlapping blocks, where the first received frame is a reference frame, and from this reference frame, the timing jitter of subsequent frames is estimated by correlating the amplitude profile of the subsequent frames with the amplitude profile of the reference frame, re-aligning the frames according to the estimated jitter.

2. The method of claim 1, wherein the timing jitter of the reference frame is periodically updated to track a channel that changes gradually.

3. The method of claim 2, wherein the timing jitter is estimated with an accuracy equal to an integer multiple of signal samples such that time re-alignment corresponds to a barrel shift of the samples in a register.

4. The method of claim 3, wherein the timing jitter is estimated with an accuracy equal to a ratio of signal samples.

5. The method of claim 4, wherein the ratio of signal samples is obtained using a look-up table indexed by the ratio between the peak correlation value and the correlation value of its adjacent peak.

6. The method of claim 5, wherein each block of each frame of the Rayleigh impulse response is time-resampled by convolving with an interpolation filter. ​

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