Laser frequency drift compensation in forward distributed acoustic sensing

Abstract

To cover the acoustic band with reduced processing speed while allowing for laser frequency drift, we disclose a forward phase method using a conventional narrow-linewidth CW laser. A narrow-linewidth CW laser is used at the transmitter side to launch its power into an optical fiber. At the receiver side, another narrow-linewidth CW laser is used to coherently detect the received signal. The detected signal contains both X and Y polarizations, each with in-phase and quadrature phase to represent a "complex" channel, and is connected to the input of an ADC. Subsequent signal processing at the ADC input extracts the phase change in the acoustic band.

Description

[Technical Field]

[0001] This application relates generally to distributed acoustic sensing (DAS) systems, methods, structures, and related techniques. More specifically, this application relates to laser frequency drift compensation in forward distributed DAS. [Background technology]

[0002] Forward methods for detecting phase changes accumulated along the length of an optical fiber have long intrigued communications and optical sensor engineers, and this interest has spawned numerous applications. In one such application, a transmitter sends an unmodulated signal from a continuous-wave (CW) laser into the optical fiber, and a coherent receiver extracts the optical phase changes. These phase changes reflect activity accumulated along the entire length of the optical fiber between the transmitter and receiver. They were initially proposed for seismic wave detection using undersea cables as part of undersea seismology research, which required subhertz detection capabilities. However, laser phase noise is reflected in the extracted signal, making it difficult to distinguish from environmental disturbances. Summary of the Invention

[0003] An advancement in the art is made by embodiments of the present disclosure directed to a forward phase method using a conventional narrow-linewidth CW laser to cover the acoustic band at reduced processing speed while tolerating laser frequency drift. According to embodiments of the present disclosure, a narrow-linewidth CW laser is used to launch its power into an optical fiber at the transmitter side. At the receiver side, another narrow-linewidth CW laser is used to coherently detect the received signal. The detected signal contains both X and Y polarizations, each with in-phase and quadrature phase to represent a "complex" channel, and is connected to the input of an ADC. Subsequent signal processing at the ADC input extracts the phase changes in the acoustic band.

[0004] In contrast to the prior art, the system and method of the present invention uses a circuit configured to perform phase difference averaging to track slow frequency drift. This average value is fed to another stage with a fixed, preset step. This second stage uses a phase from the sum of a preset value and the value output from the first stage to generate a frequency-shifted carrier signal to downconvert the input. The adjustment from the first stage includes the laser frequency drift, so the output of the second stage is free of laser drift. [Brief explanation of the drawings]

[0005] [Figure 1(A)] FIG. 1 is a schematic diagram illustrating an exemplary prior art uncoded DFOS system. [Figure 1(B)] FIG. 1 is a schematic diagram illustrating an exemplary prior art coded DFOS system.

[0006] [Figure 2] FIG. 1 is a schematic block diagram illustrating an exemplary system architecture with a CW laser on the Tx side and a CW laser feeding a coherent detector on the receiver / interrogator side, according to an embodiment of the present disclosure.

[0007] [Figure 3] FIG. 1 is a schematic block diagram illustrating an exemplary top-level digital signal processor (DSP) circuit including polarization combining, frequency drift tracking circuitry (branch 1), and phase extraction circuitry (branch 2), according to an embodiment of the present disclosure.

[0008] [Figure 4] 1 is a schematic diagram illustrating an example complex signal rotation for aligning two polarizations in the same direction, according to an embodiment of the present disclosure. FIG.

[0009] [Figure 5] FIG. 4 is a schematic diagram illustrating an example frequency drift tracking in branch 1 of FIG. 3 according to an embodiment of the present disclosure.

[0010] [Figure 6] FIG. 1 is a schematic diagram illustrating an exemplary phase tracking circuit according to aspects of the present disclosure.

[0011] [Figure 7] FIG. 6 is a schematic diagram illustrating an example circuit of a frequency shifter of branch 1 of FIG. 5, according to an embodiment of the present disclosure.

[0012] [Figure 8] 4 is a schematic circuit for frequency generation in branch 2 of FIG. 3 according to an embodiment of the present disclosure.

[0013] [Figure 9] 1 is a schematic circuit for phase extraction according to an aspect of the present disclosure.

[0014] [Figure 10] FIG. 1 is a schematic diagram illustrating an alternative exemplary arrangement of a DSP according to an aspect of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] Unless otherwise specified herein, the figures comprising the drawings are not drawn to scale.

[0020] As some additional background, note that a distributed fiber optic sensing system interconnects an optoelectronic integrator to an optical fiber (or cable), transforming the fiber into an array of sensors distributed along the fiber. In effect, the fiber becomes the sensor, and the interrogator generates / injects laser light energy into the fiber to sense / detect events along the fiber.

[0021] As those skilled in the art will understand and appreciate, DFOS technology can be deployed to continuously monitor vehicle movement, human traffic, drilling activity, seismic activity, temperature, structural integrity, liquid and gas leaks, and many other conditions and activities. It is used worldwide to monitor power plants, communication networks, railroads, roads, bridges, borders, critical infrastructure, onshore and offshore power lines and pipelines, and downhole applications in oil, gas, and enhanced geothermal power generation. Advantageously, distributed fiber optic sensing is not constrained by line of sight or remote power access and, depending on the system configuration, can be deployed over continuous lengths of more than 30 miles, with sensing / detection possible at every point along that length. Therefore, the cost per sensing point over long distances is typically incomparable to competing technologies.

[0022] Distributed fiber optic sensing measures changes in the "backscatter" of light that occurs within an optical sensing fiber when the fiber encounters environmental changes, including vibration, strain, or temperature change events. As previously mentioned, the optical sensing fiber acts as a sensor along its entire length, providing real-time information about the physical and environmental surroundings and the integrity and security of the fiber. Furthermore, distributed fiber optic sensing data pinpoints the precise location of events and conditions occurring on or near the sensing fiber.

[0023] A schematic diagram illustrating the generalized arrangement and operation of a distributed optical fiber sensing system that may advantageously include artificial intelligence / machine learning (AI / ML) analysis is illustratively shown in Figure 1(A). Referring to Figure 1(A), it can be seen that the optical sensing fiber is connected to an interrogator. Although not shown in detail, the interrogator can include a coded DFOS system that can employ a coherent receiver arrangement known in the art, such as that shown in Figure 1(B).

[0024] As is well known, a modern interrogator is a system that generates an input signal into an optical sensing fiber and detects and analyzes the reflected / backscattered signal that is then received. The received signal is analyzed and an output is generated that is indicative of the environmental conditions encountered along the fiber. The received backscattered signal may be due to reflections within the fiber, such as Raman backscattering, Rayleigh backscattering, or Brillouin backscattering.

[0025] As will be appreciated, modern DFOS systems include an interrogator that periodically generates optical pulses (or any coded signal) and launches them into an optical sensing fiber, which transmits the optical pulse signal along the optical fiber.

[0026] At each location along the fiber, a small portion of the signal is backscattered / reflected and transmitted back and forth to the interrogator, where it is received. The backscattered / reflected signal carries information that the interrogator uses to detect, such as changes in power level that indicate mechanical vibrations.

[0027] The received backscattered signal is converted to the electrical domain and processed within the interrogator. Based on the time of pulse incidence and the time the received signal is detected, the interrogator can determine from which location along the optical sensing fiber the received signal returned, thereby sensing activity at each location along the optical sensing fiber. Classification methods may also be used to detect and locate events or other environmental conditions, including acoustic and / or vibration and / or heat, along the optical sensing fiber.

[0028] This additional background is provided to introduce distributed acoustic sensing. When DAS techniques are used, the receiver / interrogator is located at the far end of the transmitter-receiver configuration.

[0029] Systems, methods, and structures according to aspects of the present disclosure are further shown and described below.

[0030] FIG. 2 is a schematic block diagram illustrating an exemplary system architecture with a CW laser on the Tx side and a CW laser feeding a coherent detector on the receiver / interrogator side, according to an embodiment of the present disclosure.

[0031] As shown in the example, a transmitter launches an optical signal from a narrow linewidth laser into an optical fiber. After passing through a length of optical fiber and cascaded amplifiers, the optical signal reaches a receiver / interrogator that uses another narrow linewidth laser as a local oscillator (LO) for coherent detection. The output of the coherent receiver contains X and Y polarizations, each with in-phase and quadrature phase components, resulting in a complex signal:

number

[0032] Although the two narrow-linewidth lasers are initially tuned to produce the same wavelength, temperature, mechanical vibration, and other factors can cause each to drift in frequency independently, resulting in signals input to the DSP ranging in the tens of megahertz (MHz) range. This frequency drift is typically gradual, ranging from sub-hertz to tens of hertz. The present invention focuses on DSPs and methods for eliminating laser frequency drift.

[0033] FIG. 3 is a schematic block diagram illustrating an exemplary top-level digital signal processor (DSP) circuit including polarization combining, frequency drift tracking circuitry (branch 1), and phase extraction circuitry (branch 2) according to an embodiment of the present disclosure.

[0034] The X and Y polarizations from the ADC are first combined into a single signal by rotating the less powerful polarization to the more powerful polarization. The average signal of the X polarization is

number

number

number

number

number

number

[0035] In practical cases, the power of the two polarizations may change dynamically, so instead of using the instantaneous average, we use the average of the "rotation signal" as a reference and rotate both polarizations in that direction.If both polarizations always have high SNR, we can simplify the above procedure by first selecting one polarization (e.g., X) and then rotating the other polarization to always have the same average direction.

[0036] The composite signal is first input to a branch that performs frequency drift tracking (branch 1 in Figure 3). In one embodiment, frequency tracking includes a frequency shifter that converts the signal to a low frequency near DC, followed by a low-pass filter (LPF). Phase tracking is then used to determine the average phase difference every two samples.

[0037] FIG. 5 is a schematic diagram illustrating an example frequency drift tracking in branch 1 of FIG. 3 according to an embodiment of the present disclosure.

[0038] FIG. 6 is a schematic diagram illustrating an exemplary phase tracking circuit according to an embodiment of the present disclosure.

[0039] The phase tracking circuit shown in FIG. 6 first converts the complex signal to an angle, then calculates the phase difference every two samples and converts it to the range of (-π, +π]. The phase differences are averaged and the result is provided to branch 2. In one embodiment, the frequency shifter uses preset coefficients from a memory to multiply the input signal using a complex multiplier, as shown in FIG. 7. FIG. 7 is a schematic diagram illustrating an exemplary circuit of the frequency shifter of branch 1 of FIG. 5 in accordance with aspects of the present disclosure.

[0040] The phase step of the coefficients in memory is -2πf / S, where f is the frequency to shift and S is the sampling rate of the ADC. For the frequency f, we can use max(gcd(S,f)), where "gcd" is the "greatest common divisor" and |f-f0| is as low as possible within the allowed phase tracking range to reduce memory usage.

[0041] FIG. 8 is a schematic circuit for frequency generation in branch 2 of FIG. 3 according to an embodiment of the present disclosure. As shown in FIG. 8, the output from branch 1 is connected to the frequency generator in branch 2 to generate a frequency that includes both a fixed offset and laser frequency drift. The output from the frequency shifter reflects the sensing results from the environment of the entire fiber path, excluding the laser drift. A low-pass filter (LPF) removes high-frequency noise and retains only acoustic frequencies. Finally, a phase extractor obtains phase information that is linear with respect to environmental disturbances.

[0042] Figure 8 shows the circuit for generating the frequency shift coefficient for branch 2. The sum of a fixed set value from the processor interface ("phase step" in the diagram) and the average value from branch 1 is connected to an accumulator, which is further fed into a function generator exp(j * <input> ), where <input> is the output of the accumulator. This function generator is implemented using the CORDIC (Coordinate Rotation Digital Computing) algorithm, and cos( <input> ) and sin( <input> )

[0043] To reduce processing complexity, the input to the phase extractor is downsampled to a low sampling rate that covers the Nyquist band of the acoustic signal. An example of a phase extractor architecture is shown in Figure 9, which is a schematic circuit for phase extraction according to an embodiment of the present disclosure.

[0044] First, the angle of the complex signal is calculated, and then the unwrapping module restricts the phase difference between every two samples to within the range of (-π, +π). The unwrapped phase is connected to a phase-locked loop (PLL) to output the acoustic signal. Note that the PLL can also be implemented using a low-pass filter.

[0045] As described above, the optical signal input to the receiving interrogator is CW light carrying the disturbance along the entire fiber, which is coherently detected using a local narrow-linewidth laser to generate a complex output in two polarizations.

[0046] The two polarizations are first combined by rotating them in the same direction.

[0047] The polarization-combined complex signal uses the first branch to track the frequency difference and drift of the laser. A frequency shifter is used to move the signal closer to DC and calculate the angle for each sample. The phase difference is then calculated every two samples. The phase difference is averaged and updated in real time to reflect frequency drift. The average value is output to the second branch.

[0048] The second branch uses a preset fixed step and the input from branch 1 as input to an accumulator. The accumulator generates a phase signal, which is converted to a complex signal and used as a frequency shift coefficient for each input sample. The input signal is multiplied by a dynamic coefficient to remove frequency drift. A phase extraction module then calculates real-time phase information that reflects the detected environmental activity.

[0049] As mentioned above, Figure 1 shows an exemplary system architecture in which the present invention can be applied. A transmitter launches an optical signal from a narrow linewidth laser into a fiber. After passing through the entire fiber and cascaded amplifiers, the signal reaches a receiver / interrogator, which uses another narrow linewidth laser as a local oscillator (LO) for coherent detection. The output from the coherent receiver contains X and Y polarizations, each with in-phase and quadrature phases, resulting in a complex signal:

number

[0050] As shown in FIG. 10, an alternative exemplary configuration is provided in which a frequency drift tracking branch is implemented in parallel with the phase extraction branch. This exemplary architecture configuration includes a "branch 1" that adds a frequency drift estimation function. The estimate is represented by an "updated phase step" that reflects the residual frequency offset from the fixed "frequency shift" factor. This estimate is sent to a second branch (branch 2 in FIG. 10) to generate a dynamic frequency shift coefficient that can remove the laser frequency drift. The required modifications are the frequency tracking function in branch 1 and the dynamic frequency generator in branch 2.

[0051] While the present disclosure has been presented above using some 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 method for laser frequency drift compensation in forward distributed acoustic sensing, comprising: receiving an optical signal with a coherent optical receiver; coherently detecting the optical signal to produce a complex output having two polarizations X and Y; performing frequency drift tracking on the complex output; performing phase extraction on the frequency drift tracked complex output; The method, wherein the optical signal is a distributed optical fiber sensing optical signal.

2. The method of claim 1 , wherein the phase extraction determines real-time phase information indicative of detected environmental activity.

3. The method of claim 2 , further comprising combining the two polarizations by rotating them in the same direction.

4. The method of claim 3 , wherein the frequency drift tracking tracks a frequency difference and drift of a first laser that generated the optical signal.

5. The method of claim 4 , wherein the frequency drift tracking determines a phase difference every two samples and averages them to determine the frequency drift.

Images