Laser frequency drift compensation in forward-dispersive 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 generally relates to distributed acoustic sensing (DAS) systems, methods, structures, and related technologies. More specifically, this application relates to laser frequency drift compensation in forward distributed DAS.

Background Art

[0002] The forward method of detecting phase changes accumulated along the length of an optical fiber has long interested communication and optical sensor engineers, and that interest has led to numerous applications. In one such application, a transmitter sends an unmodulated signal from a continuous wave (CW) laser into an optical fiber, and a coherent receiver extracts the optical phase change. Such phase changes reflect the activity accumulated along the entire length of the optical fiber between the transmitter and the receiver, and were originally proposed for seismic wave detection using undersea cables as part of research on undersea earthquakes that required sub-hertz detection capabilities. However, since laser phase noise is reflected in the extracted signal, it is difficult to distinguish from environmental disturbances.

Summary of the Invention

[0003] Advances in technology are made by aspects of the present disclosure directed to a forward phase method that uses a conventional narrow linewidth CW laser to cover the acoustic band while tolerating laser frequency drift and reducing processing speed. According to aspects of the present disclosure, a narrow linewidth CW laser is used to inject its power into an optical fiber on the transmitter side. On the receiver side, another narrow linewidth CW laser is used to coherently detect the received signal. The detected signal includes both the X polarization and the Y polarization, each having an in-phase and quadrature phase to represent a "complex" channel, and is connected to the input of an ADC. By signal processing following the ADC input, the phase change in the acoustic band is extracted.

[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 low-speed frequency drift. This average value is fed to another stage with a preset fixed step. This second stage uses the phase from the sum of a preset value and the value output from the first stage to generate a frequency-shifted carrier signal, converting the input to a lower frequency. Since the adjustment from the first stage includes laser frequency drift, the laser drift is removed at the output of the second stage. [Brief explanation of the drawing]

[0005] [Figure 1(A)] This is a schematic diagram illustrating an exemplary prior art unencoded DFOS system. [Figure 1(B)] This is a schematic diagram illustrating an exemplary prior art encoded DFOS system.

[0006] [Figure 2] This schematic block diagram shows an exemplary system architecture according to an aspect of the present disclosure, which includes a CW laser on the Tx side and a CW laser on the receiver / interrogator side that supplies power to a coherent detector.

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

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

[0009] [Figure 5] This is a schematic diagram illustrating an exemplary frequency drift trace at branch 1 in Figure 3, according to an aspect of the present disclosure.

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

[0011] [Figure 7] This is a schematic diagram showing an exemplary circuit of the frequency shifter of branch 1 in Figure 5 according to an aspect of the present disclosure.

[0012] [Figure 8] This is a schematic circuit for frequency generation at branch 2 in Figure 3, according to an aspect of the present disclosure.

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

[0014] [Figure 10] This is a schematic diagram illustrating an exemplary alternative configuration of a DSP according to the embodiments of this disclosure. [Modes for carrying out the invention]

[0015] The following are merely illustrative examples of the principles of this disclosure. Those skilled in the art will therefore understand that various configurations, not expressly described or illustrated herein, can be devised to embody the principles of this disclosure and fall within their spirit and scope.

[0016] Furthermore, all examples and conditional terms described herein are intended solely for educational purposes to help readers understand the concepts to which the inventors have contributed to advance the principles and art of this disclosure, and should be construed as not being limited to such specifically listed examples and conditions.

[0017] Furthermore, all descriptions in 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. Further, 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.

[0018] 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.

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

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

[0021] As will be understood and recognized by those skilled in the art, DFOS technology can be deployed to continuously monitor vehicle movement, human traffic, excavation activities, seismic activities, temperature, structural integrity, leaks of liquids and gases, and many other conditions and activities. This is used worldwide to monitor power plants, communication networks, railways, roads, bridges, borders, critical infrastructure, on - and offshore power lines and pipelines, and downhole applications in oil, gas, and enhanced geothermal power generation. Advantageously, distributed fiber optic sensing is not restricted by line of sight or remote power access and, depending on the system configuration, can be deployed in continuous lengths exceeding 30 miles, with sensing / detection possible at all points along that length. Thus, the cost per sensing point over long distances is usually not comparable to competing technologies.

[0022] Distributed optical fiber sensing measures changes in the "backscattering" of light that occur within an optical sensing fiber when the optical sensing fiber encounters environmental changes including events of vibration, strain, or temperature change. As described above, the optical sensing fiber functions as a sensor over its entire length, providing real-time information regarding the physical / environmental surroundings and the integrity / security of the fiber. Further, distributed optical fiber sensing data identifies the exact location of events and conditions occurring along or near the sensing fiber.

[0023] A schematic diagram illustrating a 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 FIG. 1(A). Referring to FIG. 1(A), it can be seen that an optical sensing fiber is connected to an interrogator. Although not shown in detail, the interrogator can include an encoded DFOS system that can employ a coherent receiver arrangement known in the art as shown in FIG. 1(B).

[0024] As is well known, modern interrogators are systems that generate input signals to an optical sensing fiber, detect / analyze the reflected / backscattered and then received signals. The received signals are analyzed to generate an output indicative of the environmental conditions encountered along the fiber. The received backscattered signals may be due to reflections within the fiber such as Raman backscattering, Rayleigh backscattering, Brillouin backscattering, etc.

[0025] As is recognized, modern DFOS systems include an interrogator that periodically generates optical pulses (or any encoded signal) and directs them into an optical sensing fiber. The incident optical pulse signals are transmitted along the optical fiber.

[0022] Along the fiber, a small portion of the signal is backscattered / reflected, 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 levels indicating mechanical vibrations.

[0027] The received backscattered signal is converted into the electrical domain and processed within the interrogator. Based on the pulse incidence time and the time the received signal was detected, the interrogator determines the location from which the received signal returned along the optical sensing fiber, and as a result, can sense activity at each location along the optical sensing fiber. A classification method may be used to further 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 explanation is for introducing distributed acoustic sensing. When DAS technology is used, the receiver / interrogator is located at the far end of the transmitter-receiver configuration.

[0029] The systems, methods, and structures described herein will be further shown and explained below.

[0030] Figure 2 is a schematic block diagram showing an exemplary system architecture according to an aspect of the present disclosure, which includes a CW laser on the Tx side and a CW laser on the receiver / interrogator side that supplies power to a coherent detector.

[0031] As illustrated, the transmitter emits an optical signal from a narrow-linewidth laser into an optical fiber. The optical signal, having passed through the entire length of the optical fiber and through a cascaded amplifier, 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-polarization and Y-polarization, each in phase and quadrature-phase, as a single complex signal.

number

[0032] Two narrow-linewidth lasers are initially tuned to produce the same wavelength, but due to temperature, mechanical vibrations, and other factors, their frequencies drift independently, resulting in signals input to the DSP ranging from tens of megahertz (MHz). This frequency drift is typically gradual, ranging from sub-hertz to tens of hertz. This invention focuses on a DSP and a method for eliminating laser frequency drift.

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

[0034] The X-polarization and Y-polarization signals from the ADC are first combined into a single signal by rotating the lower-power polarization to the higher-power polarization. As the average signal of the X-polarization...

number

number

number

number

number

number

[0035] In real-world cases, the power of the two polarizations can change dynamically, so instead of using an instantaneous average, the average of the "rotating signal" is used as a reference, and both polarizations are rotated in that direction. If both polarizations always have a high SNR, the above procedure can be simplified by first selecting one polarization (e.g., X) and rotating the other polarization so that it always averages in the same direction.

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

[0037] Figure 5 is a schematic diagram illustrating an exemplary frequency drift trace at branch 1 in Figure 3, according to an aspect of this disclosure.

[0038] Figure 6 is a schematic diagram showing an exemplary phase tracking circuit according to an aspect of the present disclosure.

[0039] The phase tracking circuit shown in Figure 6 first converts the complex signal to an angle, then calculates the phase difference for every two samples and converts it to a range (-π, +π). The phase difference is averaged, and the result is fed to branch 2. In one embodiment, the frequency shifter uses a preset coefficient from memory to multiply the input signal using a complex multiplier, as shown in Figure 7. Figure 7 is a schematic diagram showing an exemplary circuit of the frequency shifter of branch 1 in Figure 5 according to an aspect of the present disclosure.

[0040] The phase step of the coefficients in memory is -2πf / S, where f is the shift frequency and S is the ADC sampling rate. The frequency f can be max(gcd(S,f)), where "gcd" is the "greatest common divisor", and |f-f0| is a value low enough to fall within the acceptable phase tracking range in order to reduce memory usage.

[0041] Figure 8 shows a schematic circuit for frequency generation at branch 2 in Figure 3, according to an aspect of this disclosure. As shown in Figure 8, the output from branch 1 is connected to the frequency generator at branch 2, which generates 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 throughout the fiber path, excluding laser drift. A low-pass filter (LPF) removes high-frequency noise, retaining only acoustic frequencies. Finally, a phase extractor obtains phase information that is linear to environmental disturbances.

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

[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 the phase extractor architecture is shown in Figure 9. Figure 9 is a schematic circuit for phase extraction according to an aspect of this 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), and the acoustic signal is output. 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 disturbances along the entire fiber. This signal is coherently detected using a localized narrow-linewidth laser, generating a complex output with two polarizations.

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

[0047] The polarization-composite complex signal uses a first bifurcation that tracks the laser frequency difference and drift. A frequency shifter is used to bring the signal closer to DC and calculate the angle for each sample. Then, the phase difference is calculated for every two samples. The phase difference is averaged and updated in real time to reflect the frequency drift. The average value is output to a second bifurcation.

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

[0049] As previously mentioned, Figure 1 shows an exemplary system architecture to which the present invention is applied. The transmitter emits an optical signal from a narrow-linewidth laser into a fiber. The signal, having passed through the entire fiber and through a cascade amplifier, reaches a receiver / interrogator, which uses another narrow-linewidth laser as a local oscillator (LO) for coherent detection. The output from the coherent receiver includes X-polarization and Y-polarization, each in phase and quadrature-phase, as a single complex signal.

number

[0050] As illustrated in Figure 10, an alternative exemplary configuration is provided with a frequency drift tracking branch that runs in parallel with the phase extraction branch. This exemplary architectural configuration includes “Branch 1” with added functionality for estimating frequency drift. The estimate is represented by an “updated phase step” that reflects the residual frequency offset from a fixed “frequency shift” element. This estimate is sent to a second branch (Branch 2 in Figure 10) where a dynamic frequency shift coefficient can be generated to remove the laser frequency drift. The necessary modifications here are the frequency tracking function in Branch 1 and the dynamic frequency generator in Branch 2.

[0051] While the disclosure has been presented using several specific examples, those skilled in the art will recognize that the teachings are not limited in this way. Therefore, the disclosure should be limited only by the claims appended to this specification.

Claims

1. A method for compensating for laser frequency drift in forward-dispersive acoustic sensing, Receiving an optical signal with a coherent optical receiver, The optical signal is to be coherently detected such that a complex output with two polarizations X and Y is generated, Perform frequency drift tracing on the aforementioned complex output, This includes performing phase extraction on a frequency-drift-tracked complex output, The optical signal is a distributed optical fiber sensing optical signal, in this method.

2. The method according to claim 1, wherein the phase extraction determines real-time phase information indicating detected environmental activity.

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

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

5. The method according to claim 4, wherein the frequency drift tracking determines the phase difference for every two samples and determines the frequency drift by averaging.

Images