A laser phase noise suppression system and method in distributed acoustic sensing

Abstract

The application discloses a laser phase noise suppression system and method in distributed acoustic sensing. In the system, distributed acoustic sensing based on a Sagnac interferometer is realized, and phase noise caused by laser line width is suppressed by using a weak measurement technology, and the detection precision of acoustic sensing is improved. In the system, a first light splitting element and a fiber beam splitter are used to form a structure of the Sagnac interferometer, and the same optical path of the first polarized light and the second polarized light is ensured. By means of the same transmission optical path of the structure, the phase noise caused by the laser line width in the first backscattering Rayleigh scattering light and the second backscattering Rayleigh scattering light can be simply extracted as a whole phase, and the light intensity detection of the weak measurement technology can effectively eliminate the phase noise in the final phase change calculation. Further, the influence of the phase noise caused by the laser line width on acoustic signal sensing is solved.

Description

Technical Field

[0001] This application relates to the field of distributed acoustic sensing technology, and in particular to a laser phase noise suppression system and method in distributed acoustic sensing. Background Technology

[0002] Distributed acoustic sensing technology can monitor environmental vibrations and sound waves in real time over long distances via optical fibers. Currently, this technology has significant application value in fields such as oil and gas pipeline monitoring, earthquake early warning, perimeter security, and intelligent transportation. Compared to traditional point sensors, distributed acoustic sensing technology utilizes optical fibers as a continuous sensing medium, enabling spatially continuous and highly sensitive acoustic signal detection, while also possessing advantages such as resistance to electromagnetic interference, corrosion resistance, and long-distance monitoring.

[0003] In distributed acoustic sensing technology, external vibration information is extracted by analyzing the phase change of backscattered Rayleigh light. Distributed acoustic sensing technology has become a research hotspot due to its high sensitivity and wide dynamic range. However, random phase noise caused by the large linewidth of lasers can easily couple directly into the sensing signal, leading to phase demodulation deviations and affecting detection performance. Furthermore, distributed acoustic sensing systems using ultra-narrow linewidth lasers are costly. How to balance the detection performance and economy of distributed acoustic sensing systems is a pressing issue that needs to be addressed. Summary of the Invention

[0004] To address the aforementioned issues, this application provides a laser phase noise suppression system and method for distributed acoustic sensing, aiming to suppress phase noise generated by the large linewidth of the laser during the application of distributed acoustic sensing technology at a low cost, thereby improving detection performance.

[0005] The embodiments of this application disclose the following technical solutions:

[0006] The first aspect of this application provides a laser phase noise suppression system in distributed acoustic sensing. The system includes:

[0007] The system comprises a laser source, an acousto-optic modulator, a first beam splitter, an optical fiber beam splitter, a sensing fiber, a weak measurement component, and a processor; the first beam splitter is a polarization beam splitter; the weak measurement component includes: a pre-selection module, a second beam splitter, a first post-selection module, a second post-selection module, a first photodetector, and a second photodetector.

[0008] The pre-selection module and the acousto-optic modulator are respectively used to modulate light into a pre-selection state and to perform acousto-optic modulation on light.

[0009] The first beam splitter is used to split the beam transmitted by the acousto-optic modulator into a first polarized light and a second polarized light with mutually perpendicular polarization directions. The first polarized light and the second polarized light have the same optical path when they reach the first port and the second port of the fiber optic beam splitter.

[0010] The fiber optic beam splitter is used to transmit the first polarized light and the second polarized light, which are combined at the third port, to the sensing fiber. The first polarized light and the second polarized light each generate a first backscattered Rayleigh light and a second backscattered Rayleigh light within the sensing fiber. When the sensing fiber is placed in an acoustic field, the phases of the first backscattered Rayleigh light and the second backscattered Rayleigh light change. The fiber optic beam splitter is also used to receive the first backscattered Rayleigh light and the second backscattered Rayleigh light from the third port, and then split them into two paths through the first port and the second port before transmitting them to the first beam splitting element.

[0011] The first beam splitter is also used to select the polarization direction of the two beams received from the first port and the second port, and then transmit them to the second beam splitter.

[0012] The second beam splitter is used to split the beam transmitted by the first beam splitter into a first beam and a second beam, and transmit them to the first post-selection module and the second post-selection module respectively.

[0013] The first post-selection module is used to project the first beam into a first post-selection state; the second post-selection module is used to project the second beam into a second post-selection state.

[0014] The first photodetector is used to detect the first light intensity of the light beam output by the first post-selection module; the second photodetector is used to detect the second light intensity of the light beam output by the second post-selection module.

[0015] The processor is used to calculate the phase change of the light field caused by the sound field based on the first light intensity and the second light intensity.

[0016] In an optional implementation of the first aspect, both the first and second post-selection states are within a preset orthogonal range to the first selection state; the first post-selection angle of the first post-selection state and the second post-selection angle of the second post-selection state are opposites of each other.

[0017] In an alternative implementation of the first aspect, the pre-selection module includes: a polarizer and a half-wave plate;

[0018] The first post-selection module includes: a half-wave plate, a quarter-wave plate, and a polarizer;

[0019] The second post-selection module includes: a half-wave plate, a quarter-wave plate, and a polarizer.

[0020] In an optional implementation of the first aspect, the pre-selection module and the acousto-optic modulator are sequentially disposed in the optical path from the laser source to the first surface of the first beam splitter;

[0021] The first port and the second port of the fiber optic beam splitter correspond to the second surface and the third surface of the first beam splitting element, respectively.

[0022] Specifically, the first beam splitter transmits the beam to the second beam splitter through the fourth surface.

[0023] In an optional implementation of the first aspect, the system further includes: a first fiber coupler, a second fiber coupler, a first fiber, and a second fiber;

[0024] The first fiber optic coupler is disposed in the optical path between the first port and the second surface of the first beam splitter; the second fiber optic coupler is disposed in the optical path between the second port and the third surface of the first beam splitter; the two ends of the first fiber are respectively connected to the first port and the first fiber optic coupler; the two ends of the second fiber are respectively connected to the second port and the second fiber optic coupler.

[0025] After the first back Rayleigh scattered light and the second back Rayleigh scattered light converge at the third port, a first portion of the converged beam exits from the first port, travels from the first optical fiber to the first optical fiber coupler, and is transmitted from the first optical fiber coupler to the second surface of the first beam splitter; a second portion of the converged beam exits from the second port, travels from the second optical fiber to the second optical fiber coupler, and is transmitted from the second optical fiber coupler to the third surface of the first beam splitter.

[0026] In an optional implementation of the first aspect, the first beam-splitting element is a polarizing beam-splitting prism;

[0027] The beam-splitting surface of the polarizing beam splitter is used to reflect vertically polarized light and transmit horizontally polarized light; the first polarized light is vertically polarized light and the second polarized light is horizontally polarized light.

[0028] The first beam splitter is specifically used to transmit horizontally polarized light in the first part of the converging beam through the beam splitter surface, and to reflect vertically polarized light in the second part of the converging beam through the beam splitter surface.

[0029] In an optional implementation of the first aspect, the processor is specifically used for:

[0030] The system acquires the first light intensity detected by the first photodetector and the second light intensity detected by the second photodetector; and acquires the total light intensity detected by the first photodetector and the second photodetector when the sensing fiber is not placed in the sound field.

[0031] The phase change of the light field induced by the sound field is calculated based on the first light intensity, the second light intensity, the total light intensity, and either the first post-selection angle or the second post-selection angle.

[0032] In an optional implementation of the first aspect, the processor specifically calculates the phase change of the optical field induced by the acoustic field using the following formula:

[0033] ;

[0034] In the formula, I8 represents the first light intensity, I9 represents the second light intensity, and I0sin 2 ε represents the total light intensity, and ε represents the first post-selection angle. This represents the phase change of the optical field induced by the sound field; wherein, the The calculation formula is:

[0035] ;

[0036] In the formula, θ V (t) represents the first phase change caused by the sound field acting on the first polarized light, θ H (t) represents the second phase change caused by the sound field acting on the second polarized light.

[0037] In an alternative implementation of the first aspect, the system further includes: a signal generator; the signal generator is connected to the acousto-optic modulator; and the acousto-optic modulator is driven by the signal generator.

[0038] A second aspect of this application provides a laser phase noise suppression method in distributed acoustic sensing, which is applied to the system described in any implementation of the first aspect, the method comprising:

[0039] The sensing fiber of the system is placed in the sound field;

[0040] The laser source is turned on, and the pre-selection module and the acousto-optic modulator are used to modulate the light into a pre-selection state and to perform acousto-optic modulation on the light, respectively.

[0041] The first beam splitter splits the beam transmitted by the acousto-optic modulator into a first polarized light and a second polarized light with mutually perpendicular polarization directions. The first polarized light and the second polarized light have the same optical path when they reach the first port and the second port of the fiber optic beam splitter.

[0042] The fiber optic beam splitter transmits the first polarized light and the second polarized light, which are combined at the third port, to the sensing fiber. The first polarized light and the second polarized light each generate a first backscattered Rayleigh light and a second backscattered Rayleigh light within the sensing fiber. When the sensing fiber is placed in an acoustic field, the phases of the first backscattered Rayleigh light and the second backscattered Rayleigh light change. The fiber optic beam splitter receives the first backscattered Rayleigh light and the second backscattered Rayleigh light from the third port, splits them into two paths through the first port and the second port, and then transmits them to the first beam splitter element.

[0043] The first beam splitter selects the polarization direction of the two beams received from the first port and the second port, and then transmits them to the second beam splitter.

[0044] The second beam splitter splits the beam transmitted by the first beam splitter into a first beam and a second beam, and transmits them to the first post-selection module and the second post-selection module respectively. The first post-selection module projects the first beam into a first post-selection state, and the second post-selection module projects the second beam into a second post-selection state.

[0045] The first photodetector detects the first light intensity of the beam output by the first post-selection module, and the second photodetector detects the second light intensity of the beam output by the second post-selection module.

[0046] The processor calculates the phase change of the light field caused by the sound field based on the first light intensity and the second light intensity.

[0047] Compared with the prior art, this application has the following beneficial effects:

[0048] The laser phase noise suppression system in distributed acoustic sensing provided in this application embodiment realizes distributed acoustic sensing based on a Sagnac interferometer and utilizes weak measurement techniques to suppress phase noise caused by laser linewidth, balancing the detection performance and economy of distributed acoustic sensing and improving the detection accuracy. In this system, a first beam splitter and an optical fiber beam splitter are used to construct the structure of the Sagnac interferometer, ensuring that the first polarized light and the second polarized light have the same optical path. Taking advantage of the characteristic of the same transmission optical path of this structure, the phase noise caused by laser linewidth in the first and second back-scattered Rayleigh light can be easily extracted as a whole phase, and the light intensity detection using weak measurement techniques can effectively eliminate phase noise during the final phase change calculation. Thus, the influence of phase noise caused by laser linewidth on acoustic signal sensing is mitigated. Attached Figure Description

[0049] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0050] Figure 1 A schematic diagram of a laser phase noise suppression system in a distributed acoustic sensing system provided in this application embodiment;

[0051] Figure 2 for Figure 1 The light field diagram of the system shown;

[0052] Figure 3 This is a flowchart illustrating a laser phase noise suppression method in a distributed acoustic sensing system, as provided in an embodiment of this application. Detailed Implementation

[0053] In distributed acoustic sensing technology, optical fibers are primarily used as the sensing medium to detect acoustic signals. Lasers are generally used as the light source in distributed acoustic sensing systems; however, lasers with large linewidths can easily introduce additional phase noise into the detection of acoustic signals, affecting the accuracy of the detection. While using ultra-narrow linewidth lasers can reduce phase noise, it increases the system cost.

[0054] Through research, the inventors proposed a laser phase noise suppression system and method for distributed acoustic sensing. In this system, two beams of polarized light with mutually perpendicular polarization directions are transmitted to the sensing fiber via a first beam splitter and an optical fiber beam splitter. The first beam splitter and the optical fiber beam splitter together form the basic structure of a Sagnac interferometer, ensuring that the optical path lengths of the first and second polarized light beams from the first beam splitter to the optical fiber beam splitter are the same. Consequently, the phase noise caused by the laser linewidth acts on the backscattered Rayleigh light with the same effect. By applying weak measurement techniques, the phase change caused by the sound field acting on the sensing fiber can be captured simply and conveniently, and the influence of phase noise caused by the laser linewidth on the detection results can be eliminated. This system ensures the most critical detection performance, improving detection accuracy, while also meeting economic requirements.

[0055] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0056] Figure 1 This is a schematic diagram of a laser phase noise suppression system in a distributed acoustic sensing system provided in an embodiment of this application. Figure 1 As shown in the embodiment of this application, the laser phase noise suppression system in distributed acoustic sensing includes:

[0057] The system comprises a laser source 01, an acousto-optic modulator 02, a first beam splitter 03, an optical fiber beam splitter 04, a sensing fiber 05, a processor 07, a signal generator 12, and a weak measurement component. Specifically, the laser source 01 can be a laser for emitting a laser beam. The selection of the laser source 01 depends on the actual detection requirements. The first beam splitter 03 is a polarizing beam splitter (PBS), such as a polarizing beam splitter prism. In the system provided in this embodiment, the first beam splitter 03 serves to split the laser beam and filter the polarization direction. The optical fiber beam splitter 04 provides bidirectional transmission functions for beam splitting and combining. The signal generator 12 can be a signal generator with arbitrary waveforms.

[0058] In a laser phase noise suppression system within a distributed acoustic sensing system, the weak measurement components include: a pre-selection module 08, a second beam splitter 10, a first post-selection module 11a, a second post-selection module 11b, a first photodetector 06a, and a second photodetector 06b. In an optional implementation, the pre-selection module 08 includes a polarizer and a half-wave plate, with the half-wave plate positioned closer to the input of the acousto-optic modulator 02 than the polarizer. Optionally, the first post-selection module 11a includes a half-wave plate, a quarter-wave plate, and a polarizer, arranged sequentially along the laser beam propagation direction. Optionally, the second post-selection module 11b includes a half-wave plate, a quarter-wave plate, and a polarizer, arranged sequentially along the laser beam propagation direction.

[0059] exist Figure 1The four surfaces of the first beam splitter 03 are shown using lines of different colors. For easy distinction, they are referred to as the first surface S31, the second surface S32, the third surface S33, and the fourth surface S34, illustrated by the four-colored edge lines of the first beam splitter 03. Specifically, the black line on the edge of the first beam splitter 03 represents the first surface S31, the orange line represents the second surface S32, the green line represents the third surface S33, and the gray line represents the fourth surface S34. Figure 1 The red lines inside the first beam splitter 03 also indicate the beam splitting surface of the first beam splitter 03.

[0060] The following describes the position and connection relationship of each device in the optical path.

[0061] like Figure 1 As shown, the preselection module 08 and the acousto-optic modulator 02 are sequentially arranged in the optical path between the output end of the laser source 01 and the first surface S31 of the first beam splitter 03. The signal generator 12 is connected to the acousto-optic modulator 02; the acousto-optic modulator 02 is driven by the signal generator 12. The main purpose of the signal generator 12 is to drive the acousto-optic modulator 02. The fiber optic beam splitter 04 has three ports: a first port p41, a second port p42, and a third port p43. The first port p41 and the second port p42 can be located on the same side of the fiber optic beam splitter 04 and have similar functions. The first port p41 and the second port p42 correspond to the second surface S32 and the third surface S33 of the first beam splitter 03, respectively, in terms of optical transmission. The third port p43 of the fiber optic beam splitter 04 is connected to the sensing fiber 05. When the sensing fiber 05 is placed in the sound field, the system is used to sense the acoustic signal in the sound field. The third port p43 of the fiber optic beam splitter 04 can be directly connected to the sensing fiber 05. The first port p41 and the second port p42 receive two polarized beams from the first beam splitter 03 through fiber optic couplers and other optical fiber equipment. For example, Figure 1 Two fiber optic couplers are shown: a first fiber optic coupler 09a disposed between the second surface S32 of the first beam splitter 03 and the first port p41 of the fiber optic beam splitter 04; and a second fiber optic coupler 09b disposed between the third surface S33 of the first beam splitter 03 and the second port p42 of the fiber optic beam splitter 04. The first port p41 and the first fiber optic coupler 09a are connected via a first optical fiber, and the second port p42 and the second fiber optic coupler 09b are connected via a second optical fiber.

[0062] The fourth surface S34 of the first beam splitter 03 is opposite to the second beam splitter 10, which can also be in the form of a beam splitter prism. Figure 1The red lines inside the second beam splitter 10 indicate its beam splitting surface. A first post-selection module 11a and a second post-selection module 11b are disposed on both sides of the second beam splitter 10. The first post-selection module 11a is optically connected to the first photodetector 06a, allowing the first photodetector 06a to detect light output from the first post-selection module 11a. Similarly, the second post-selection module 11b is optically connected to the second photodetector 06b, allowing the second photodetector 06b to detect light output from the second post-selection module 11b.

[0063] The first photodetector 06a and the second photodetector 06b are both connected to the processor 07. The light detected by the first photodetector 06a and the second photodetector 06b is converted into an electrical signal inside the detector and then transmitted to the processor 07. The processor 07 determines the intensity of the detected light based on the received electrical signal and further performs the phase change calculation.

[0064] To facilitate understanding of the specific operating principles of the system, the working methods of each component in the system are described below.

[0065] In this embodiment of the application, in the laser phase noise suppression system of the distributed acoustic sensing, the laser source 01 emits a laser beam. The pre-selection module 08 and the acousto-optic modulator 02 located on one side of the output end of the laser source 01 are used to modulate the received laser into a pre-selection state and to perform acousto-optic modulation on the light, respectively.

[0066] The first surface S31 of the first beam splitter 03 is adjacent to the output end of the acousto-optic modulator 02. The first beam splitter 03 receives the light beam output by the acousto-optic modulator 02 through its first surface S31, splitting the beam into first polarized light and second polarized light with mutually perpendicular polarization directions. As an example, based on the polarization selection capability of the first beam splitter 03, the first beam splitter 03 reflects the first polarized light through the splitter surface to the second surface S32 and transmits the second polarized light through the third surface S33. Exemplarily, the first polarized light is vertically polarized light, and the second polarized light is horizontally polarized light.

[0067] The first polarized light and the second polarized light have the same optical path length when they are output from the first beam splitter 03 and reach the first port p41 and the second port p42 of the fiber optic beam splitter 04. Furthermore, based on the structural design of the fiber optic beam splitter 04, the first polarized light and the second polarized light have the same optical path length when they reach the inside of the fiber optic beam splitter 04, the same optical path length when they reach the third port p43, and even the same optical path length when they are transmitted to the sensing fiber 05.

[0068] The fiber optic beam splitter 04 has two transmission directions. The transmission direction from the first port p41 and the second port p42 to the third port p43 is called the first direction; the transmission direction from the third port p43 to the first port p41 and the second port p42 is called the second transmission direction.

[0069] In the first transmission direction of the fiber optic beam splitter 04, the fiber optic beam splitter 04 transmits the first and second polarized light from the combined beam at the third port p43 to the sensing fiber 05. The sensing fiber 05 is much longer than the first fiber located at the first port p41 and the second fiber located at the second port p42. When there is a need for acoustic sensing, the sensing fiber 05 in the system can be placed in the sound field.

[0070] When the sensing fiber 05 is placed in an external environment, the first polarized light and the second polarized light each generate a first backscattered Rayleigh light and a second backscattered Rayleigh light within the sensing fiber 05. The first backscattered Rayleigh light is the backscattered Rayleigh light generated by the first polarized light within the sensing fiber 05; the second backscattered Rayleigh light is the backscattered Rayleigh light generated by the second polarized light within the sensing fiber 05. Under the influence of the acoustic signal in the sound field, both the first and second backscattered Rayleigh lights undergo a certain degree of phase change. To achieve acoustic sensing, the ultimate goal of this application's technical solution is to detect the intensity of the optical signal and ultimately demodulate the phase change.

[0071] In this application's technical solution, first polarized light and second polarized light are transmitted to the sensing fiber 05 in the sound field via a first beam splitter 03 and an optical fiber beam splitter 04. The first beam splitter 03 and the optical fiber beam splitter 04 together form the basic structure of the Sagnac interferometer, ensuring that the two optical paths of the first and second polarized light from the first beam splitter 03 to the optical fiber beam splitter 04 are identical. Furthermore, even if the laser source 01 has a large linewidth, the resulting phase noise can still affect the first and second backscattered Rayleigh beams with the same effect. In subsequent demodulation, this phase noise can be effectively eliminated as a whole phase, thereby achieving the purpose of suppressing phase noise. The specific representations of the light field and intensity will be explained in detail later using formulas, and will not be elaborated here.

[0072] In the second transmission direction of the fiber optic beam splitter 04, the fiber optic beam splitter 04 is also used to receive the first backscattered Rayleigh light and the second backscattered Rayleigh light from the third port p43, and then split them into two paths through the first port p41 and the second port p42 before transmitting them to the first beam splitting element 03. The light split to the first port p41 is called the first part of the beam collected by the third port p43, and the other light split to the second port p42 is called the second part of the beam collected by the third port p43. The first part includes a portion of the first backscattered Rayleigh light and a portion of the second backscattered Rayleigh light, and the second part includes another portion of the first backscattered Rayleigh light and another portion of the second backscattered Rayleigh light.

[0073] Since the polarization direction of the first backscattered Rayleigh light is vertical and the polarization direction of the second backscattered Rayleigh light is horizontal, the light received by the second surface S32 of the first beam splitter 03 includes both horizontal and vertical polarization directions, and the light received by its third surface S33 also includes both horizontal and vertical polarization directions. The first beam splitter 03 is also used to select the polarization direction of the two beams received from the first port p41 and the second port p42 before transmitting them to the second beam splitter 10.

[0074] As illustrated in the example above, the beam-splitting surface of the first beam-splitting element 03 is used to reflect vertically polarized light and transmit horizontally polarized light. Therefore, when the first beam-splitting element 03 transmits a beam to the second beam-splitting element 10, it specifically transmits horizontally polarized light from the first portion of the collected beam (from the second surface S32) through the beam-splitting surface, and reflects vertically polarized light from the second portion of the collected beam (from the third surface S33) through the beam-splitting surface.

[0075] The second beam splitter 10 splits the beam transmitted by the first beam splitter 03 into two beams, referred to as the first beam and the second beam for ease of description. The second beam splitter 10 transmits the first beam to the first post-selection module 11a and the second beam to the second post-selection module 11b. The first post-selection module 11a projects the first beam onto a first post-selection state; the second post-selection module 11b projects the second beam onto a second post-selection state. The first photodetector 06a detects the first intensity of the beam output by the first post-selection module 11a, and the second photodetector 06b detects the second intensity of the beam output by the second post-selection module 11b. Finally, the processor 07 calculates the phase change of the light field induced by the sound field based on the first and second intensities.

[0076] To make it easier to understand, the following will be combined with Figure 2The expression illustrates the optical field at each stage and location in the laser phase noise suppression system of the distributed acoustic sensing proposed in this application. Figure 2 for Figure 1 The light field diagram of the system shown.

[0077] like Figure 2 As shown, the light field output by laser source 01 is represented as E1, and its expression is:

[0078] ;

[0079] In the formula, E0 is the initial amplitude of the laser source, e is the base of the natural logarithm function, i is the imaginary unit, and w represents the optical frequency. The phase noise generated by the linewidth of the laser source follows a Wiener process. That is, the object that the technical solution of this application needs to suppress.

[0080] After light enters the preselection module 08, it undergoes a preselection process and is modulated into a preselection state. The light field is represented as follows: .

[0081] In the formula, |i> represents the pre-selection state, and its expression is:

[0082] ;

[0083] Where |H> represents horizontally polarized light and |V> represents vertically polarized light. This expression shows that the pre-selection process involves dual modulation of both horizontal and vertical polarization.

[0084] In the expression for E2, B(t) is the matrix representing the change in the optical field caused by the half-wave plate in the pre-selection module 08, and its expression is:

[0085] ;

[0086] Among them, R H (t) represents the ratio of the horizontally polarized light separated by the first beam-splitting element 03 to the total light intensity before beam splitting, R V (t) represents the ratio of the vertically polarized light intensity of the first beam splitter 03 to the total light intensity before beam splitting.

[0087] Signal generator 12 can drive acousto-optic modulator 02 at Ω AOM Modulation, where Ω AOM The expression is: In this expression, w0 is the frequency shift generated by the acousto-optic modulator 02, and A(t) represents the waveform pulse generated by the signal generator 12.

[0088] After passing through the acousto-optic modulator 02, the light field can be represented as Figure 2 E4, as shown, can be expressed as: .

[0089] The optical field of the first polarized light entering the first fiber coupler 09a after passing through the first beam splitter 03 is denoted as E6, and the optical field of the second polarized light entering the second fiber coupler 09b after passing through the first beam splitter 03 is denoted as E5. The expressions for E6 and E5 are as follows:

[0090] ;

[0091] ;

[0092] The first polarized light, denoted by E6, and the second polarized light, denoted by E5, reach the fiber optic beam splitter 04 after traveling the same optical path. The optical field expression after beam combining by the fiber optic beam splitter 04 is: E7 = E5 + E6.

[0093] When the acoustic signal in the sound field acts on the sensing fiber 05, the light field of the first backscattered Rayleigh light generated by the first polarized light is: The optical field of the second backscattered Rayleigh light generated by the second polarized light is expressed as follows: The expressions for both are as follows:

[0094] ;

[0095] ;

[0096] In the above formula, R(t) represents the backscattering rate of the backscattered Rayleigh light at position z on the sensing fiber 05, e -2αz It is the attenuation coefficient of the optical fiber transmission distance z, θ V (t) represents the first phase change produced by the acoustic signal acting on the first polarized light perpendicular to the polarization direction, θ H (t) represents the second phase change produced by the acoustic signal acting on the second polarized light in the horizontal polarization direction. From above and As can be seen from the expression, the two light fields have the same phase noise effect, that is, both have Phase noise.

[0097] The first and second backscattered Rayleigh beams converge within the fiber optic beam splitter 04, forming a converged beam. This converged beam is then split into two parts through the two ports of the fiber optic beam splitter 04, which are transmitted to the first fiber optic coupler 09a and the second fiber optic coupler 09b, respectively. The beam transmitted from the first fiber optic coupler 09a to the first beam splitter 03 contains both horizontal and vertical polarization directions; the beam transmitted from the second fiber optic coupler 09b to the first beam splitter 03 also contains both horizontal and vertical polarization directions.

[0098] The first beam splitter 03 selects the polarization direction of the light beams received from the first fiber coupler 09a and the second fiber coupler 09b, and then transmits them to the second beam splitter 10. Simultaneously, vertically polarized light entering the first beam splitter 03 from the first fiber coupler 09a and horizontally polarized light entering the first beam splitter 03 from the second fiber coupler 09b will not enter the second beam splitter 10, i.e., they will not be selected.

[0099] In this application, the optical field before entering the second beam splitter 10 is represented as follows:

[0100] ;

[0101] According to the above introduction and The expression above can be used to... The expression is further integrated into:

[0102]

[0103] θ in the above formula can be V (t)- θ H Let (t) be set as Then the above formula can also be expressed as:

[0104]

[0105] As can be seen from the above formula, before entering the second beam-splitting element 10 in the weak measurement component, Still exists in the light field In the expression, before the subsequent calculation and demodulation of the light field phase change caused by the sound field, the photodetector collects the light intensity rather than the light field, using Euler's formula e ix The operation = cos(x) + i sin(x) takes the imaginary part. It can be considered as part of x in Euler's formula, because cos 2 (x) + sin 2 (x)=1, and then in the calculation It can be treated as a global phase culling, and therefore, the calculated phase solution will not be affected. The impact.

[0106] After passing through the second beam splitter 10, the first beam split by the second beam splitter 10 (i.e., as shown in the image) Figure 2 The beam reflected by the second beam splitter 10 shown is projected by the first post-selection module 11a onto the first post-selection state |f1>, and the second beam split by the second beam splitter 10 (i.e., as shown) Figure 2 The beam transmitted by the second beam splitter 10 shown is projected by the second post-selection module 11b onto the second post-selection state |f2>. Both the first post-selection state |f1> and the second post-selection state |f2> are within a preset orthogonal range to the first selection state |i> of the first selection module 08. Here, the preset orthogonal range can be understood as a non-absolute 90°. The preset orthogonal range can be set floating based on 90°, for example, the range of 80° to 100° is the preset orthogonal range.

[0107] The expressions for |f1> and |f2> are:

[0108] ;

[0109] ;

[0110] In the formula, ε represents the first post-selection angle of the first post-selection state, and -ε represents the second post-selection angle of the second post-selection state. It can be seen that the first post-selection angle ε of the first post-selection state and the second post-selection angle -ε of the second post-selection state are opposites of each other.

[0111] After the first post-selection process of the first post-selection module 11a, the light field of the output beam is E8. After the second post-selection process of the second post-selection module 11b, the light field of the output beam is E9, as expressed below:

[0112] ;

[0113] ;

[0114] Finally, the light intensity detected by the first photodetector 06a (i.e., the first light intensity) is denoted as I8, and the light intensity detected by the second photodetector 06b (i.e., the second light intensity) is denoted as I9. Their expressions are as follows:

[0115] ;

[0116] ;

[0117] In the formula, I0 represents the light intensity that the system can detect without weak measurement components.

[0118] In this application's technical solution, since both the first photodetector 06a and the second photodetector 06b are connected to the processor 07, the processor 07 can acquire the first light intensity I8 detected by the first photodetector 06a and the second light intensity I9 detected by the second photodetector 06b; and acquire the total light intensity detected when the sensing fiber 05 is not placed in the sound field. Based on the first light intensity I8, the second light intensity I9, and the total light intensity The phase change of the optical field induced by the acoustic field is calculated by taking either the first post-selection angle ε or the second post-selection angle -ε. The calculation formula is as follows:

[0119] ;

[0120] By substituting the known quantities into the formula above, we can obtain the answer. The value represents the phase change of the light field induced by the sound field. The expression was introduced earlier: .

[0121] Based on the laser phase noise suppression system in distributed acoustic sensing described in the foregoing embodiments, this application also proposes a laser phase noise suppression method in distributed acoustic sensing. This method is specifically applied to the aforementioned laser phase noise suppression system in distributed acoustic sensing. See also... Figure 3 The process of this method includes:

[0122] S41. Place the sensing fiber of the laser phase noise suppression system in the distributed acoustic sensing in the sound field.

[0123] To perform distributed acoustic sensing, the method in this application embodiment requires placing the sensing optical fiber in the system within the sound field.

[0124] S42. Turn on the laser source. The pre-selection module and the acousto-optic modulator are used to modulate the light into a pre-selection state and to perform acousto-optic modulation on the light, respectively.

[0125] Since the distributed acoustic sensing in the system uses a laser as the light source and senses changes in the phase of the sound field through backscattered Rayleigh light generated by the polarized beam transmitted in the sensing fiber, the entire system can only operate after the laser source is turned on. This step requires turning on the laser source and then modulating the beam through the pre-selection module and acousto-optic modulation module located at the front end of the system.

[0126] It should be noted that the execution order of S41 and S42 is not limited in this application. For example, the laser source could be turned on first, and then the sensing fiber in the system could be placed in the sound field. Alternatively, the two steps could be performed simultaneously.

[0127] S43. The first beam splitter splits the beam transmitted by the acousto-optic modulator into a first polarized light and a second polarized light with mutually perpendicular polarization directions. The first polarized light and the second polarized light have the same optical path when they reach the first port and the second port of the fiber optic beam splitter.

[0128] This step mainly describes how the first beam splitter in the system, as a polarization beam splitter, selects the direction of beam polarization. The first beam splitter can transmit horizontally polarized light and reflect vertically polarized light. The vertically polarized light can be considered as the first polarized light, and the horizontally polarized light can be considered as the second polarized light.

[0129] S44. The first polarized light and the second polarized light combined at the third port are transmitted to the sensing fiber by the fiber beam splitter. The first polarized light and the second polarized light respectively generate a first back Rayleigh scattered light and a second back Rayleigh scattered light in the sensing fiber. When the sensing fiber is placed in an acoustic field, the phase of the first back Rayleigh scattered light and the second back Rayleigh scattered light changes. The fiber beam splitter receives the first back Rayleigh scattered light and the second back Rayleigh scattered light from the third port, and transmits them to the first beam splitting element after splitting them into two paths through the first port and the second port.

[0130] This step mainly describes the effect of the acoustic field on the first and second polarized light in the sensing fiber after the fiber optic beam splitter transmits the first and second polarized light to the sensing fiber. Specifically, the first and second polarized light generate first and second backscattered Rayleigh light in the optical fiber path, which propagate along the opposite direction to the first and second polarized light, and finally reach the first beam splitter.

[0131] S45. The first beam splitter selects the polarization direction of the two beams received from the first port and the second port, and then transmits them to the second beam splitter.

[0132] Since the beam received by the first beam splitter from the first port contains both horizontal and vertical polarization directions, and the beam received from the second port also contains both horizontal and vertical polarization directions, and since the first beam splitter receives beams from the first and second ports through two different surfaces, it can select the polarization direction of the beams transmitted from the two surfaces based on its polarization selection capability. In other words, not all received beams will be transmitted to the second beam splitter.

[0133] S46. The beam transmitted by the first beam splitter is divided into a first beam and a second beam by the second beam splitter, and transmitted to the first post-selection module and the second post-selection module respectively. The first post-selection module projects the first beam onto the first post-selection state, and the second post-selection module projects the second beam onto the second post-selection state.

[0134] This step S46 focuses on describing the two post-selection processes performed by the system's weak measurement components.

[0135] S47. The first photodetector detects the first light intensity of the beam output by the first post-selection module, and the second photodetector detects the second light intensity of the beam output by the second post-selection module.

[0136] S48. The processor calculates the phase change of the light field caused by the sound field based on the first light intensity and the second light intensity.

[0137] In this application, the specific calculation method for the optical field phase change mentioned in this step can be understood by referring to the formulas introduced above. Since the system was already described above, the calculation method can be further explained by referring to the formulas introduced above. Figures 1 to 2 The technical solution has been described in considerable detail, so the method described in this embodiment will not be repeated here. For specific implementation details of some method steps, please refer to the preceding description and introduction of the system embodiment.

[0138] The laser phase noise suppression method in distributed acoustic sensing provided in this application is based on a system implementing a Sagnac interferometer-based distributed acoustic sensing system. It utilizes weak measurement techniques applicable to the weak measurement components within the system to suppress phase noise caused by the laser linewidth, balancing the detection performance and economy of distributed acoustic sensing, and improving the detection accuracy. In this method, the first beam splitter and fiber optic beam splitter in the system are used to construct the Sagnac interferometer structure, ensuring that the first and second polarized lights have the same optical path. Taking advantage of this characteristic of identical transmission optical path, the phase noise caused by the laser linewidth in the first and second back-scattered Rayleigh lights can be easily extracted as a whole phase. Furthermore, the intensity detection using weak measurement techniques can effectively eliminate phase noise during the final phase change calculation. Thus, the impact of phase noise caused by the laser linewidth on acoustic signal sensing is mitigated.

[0139] It should be noted that the various embodiments in this specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, for the method embodiments, since they are basically similar to the system embodiments, the description is relatively simple, and the relevant parts can be referred to the description of the system embodiments. The above description is only one specific implementation of this application, but the protection scope of this application is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the protection scope of this application. Therefore, the protection scope of this application should be determined by the protection scope of the claims.

Claims

1. A laser phase noise suppression system in distributed acoustic sensing, characterized in that, include: Laser source, acousto-optic modulator, first beam splitter, fiber optic beam splitter, sensing fiber, weak measurement components and processor; The first beam splitter is a polarization beam splitter; The weak measurement component includes: a front selection module, a second beam splitter, a first back selection module, a second back selection module, a first photodetector, and a second photodetector; The pre-selection module and the acousto-optic modulator are respectively used to modulate light into a pre-selection state and to perform acousto-optic modulation on light. The first beam splitter is used to split the beam transmitted by the acousto-optic modulator into a first polarized light and a second polarized light with mutually perpendicular polarization directions. The first polarized light and the second polarized light have the same optical path when they reach the first port and the second port of the fiber optic beam splitter. The fiber optic beam splitter is used to transmit the first polarized light and the second polarized light, which are combined at the third port, to the sensing fiber. The first polarized light and the second polarized light each generate a first backscattered Rayleigh light and a second backscattered Rayleigh light within the sensing fiber. When the sensing fiber is placed in an acoustic field, the phases of the first backscattered Rayleigh light and the second backscattered Rayleigh light change. The fiber optic beam splitter is also used to receive the first backscattered Rayleigh light and the second backscattered Rayleigh light from the third port, and then split them into two paths through the first port and the second port before transmitting them to the first beam splitting element. The first beam splitter is also used to select the polarization direction of the two beams received from the first port and the second port, and then transmit them to the second beam splitter. The second beam splitter is used to split the beam transmitted by the first beam splitter into a first beam and a second beam, and transmit them to the first post-selection module and the second post-selection module respectively. The first post-selection module is used to project the first beam into a first post-selection state; the second post-selection module is used to project the second beam into a second post-selection state. The first photodetector is used to detect the first light intensity of the light beam output by the first post-selection module; the second photodetector is used to detect the second light intensity of the light beam output by the second post-selection module. The processor is used to calculate the phase change of the light field caused by the sound field based on the first light intensity and the second light intensity.

2. The system according to claim 1, characterized in that, Both the first and second post-selection states are within a preset orthogonal range to the first selection state; the first post-selection angle of the first post-selection state and the second post-selection angle of the second post-selection state are opposites of each other.

3. The system according to claim 1, characterized in that, The pre-selection module includes: a polarizer and a half-wave plate; The first post-selection module includes: a half-wave plate, a quarter-wave plate, and a polarizer; The second post-selection module includes: a half-wave plate, a quarter-wave plate, and a polarizer.

4. The system according to claim 1, characterized in that, The pre-selection module and the acousto-optic modulator are sequentially arranged in the optical path from the laser source to the first surface of the first beam splitter; The first port and the second port of the fiber optic beam splitter correspond to the second surface and the third surface of the first beam splitting element, respectively. Specifically, the first beam splitter transmits the beam to the second beam splitter through the fourth surface.

5. The system according to claim 4, characterized in that, The system further includes: a first fiber optic coupler, a second fiber optic coupler, a first fiber optic cable, and a second fiber optic cable; The first fiber optic coupler is disposed in the optical path between the first port and the second surface of the first beam splitter; the second fiber optic coupler is disposed in the optical path between the second port and the third surface of the first beam splitter; the two ends of the first fiber are respectively connected to the first port and the first fiber optic coupler; the two ends of the second fiber are respectively connected to the second port and the second fiber optic coupler. After the first back Rayleigh scattered light and the second back Rayleigh scattered light converge at the third port, a first portion of the converged beam exits from the first port, travels from the first optical fiber to the first optical fiber coupler, and is transmitted from the first optical fiber coupler to the second surface of the first beam splitter; a second portion of the converged beam exits from the second port, travels from the second optical fiber to the second optical fiber coupler, and is transmitted from the second optical fiber coupler to the third surface of the first beam splitter.

6. The system according to claim 5, characterized in that, The first beam-splitting element is a polarizing beam-splitting prism; The beam-splitting surface of the polarizing beam splitter is used to reflect vertically polarized light and transmit horizontally polarized light; the first polarized light is vertically polarized light and the second polarized light is horizontally polarized light. The first beam splitter is specifically used to transmit horizontally polarized light in the first part of the converging beam through the beam splitter surface, and to reflect vertically polarized light in the second part of the converging beam through the beam splitter surface.

7. The system according to claim 2, characterized in that, The processor is specifically used for: The system acquires the first light intensity detected by the first photodetector and the second light intensity detected by the second photodetector; and acquires the total light intensity detected by the first photodetector and the second photodetector when the sensing fiber is not placed in the sound field. The phase change of the light field induced by the sound field is calculated based on the first light intensity, the second light intensity, the total light intensity, and either the first post-selection angle or the second post-selection angle.

8. The system according to claim 7, characterized in that, The processor specifically calculates the phase change of the light field induced by the sound field using the following formula: ； In the formula, I 8 represents the first light intensity. I 9 represents the second light intensity. I 0 sin 2 ε This represents the total light intensity. ε Indicates the first subsequent selection angle. ( t ) represents the phase change of the optical field induced by the sound field; wherein, the ( t The formula for calculating ) is: ； In the formula, θ V ( t () represents the first phase change caused by the sound field acting on the first polarized light. θ H ( t ) represents the second phase change amount produced by the sound field acting on the second polarized light.

9. The system according to claim 1, characterized in that, The system further includes: a signal generator; the signal generator is connected to the acousto-optic modulator; the acousto-optic modulator is driven by the signal generator.

10. A method for suppressing laser phase noise in distributed acoustic sensing, characterized in that, The method, applied to the system of any one of claims 1-9, comprises: The sensing fiber of the system is placed in the sound field; The laser source is turned on, and the pre-selection module and the acousto-optic modulator are used to modulate the light into a pre-selection state and to perform acousto-optic modulation on the light, respectively. The first beam splitter splits the beam transmitted by the acousto-optic modulator into a first polarized light and a second polarized light with mutually perpendicular polarization directions. The first polarized light and the second polarized light have the same optical path when they reach the first port and the second port of the fiber optic beam splitter. The fiber optic beam splitter transmits the first polarized light and the second polarized light, which are combined at the third port, to the sensing fiber. The first polarized light and the second polarized light each generate a first backscattered Rayleigh light and a second backscattered Rayleigh light within the sensing fiber. When the sensing fiber is placed in an acoustic field, the phases of the first backscattered Rayleigh light and the second backscattered Rayleigh light change. The fiber optic beam splitter receives the first backscattered Rayleigh light and the second backscattered Rayleigh light from the third port, splits them into two paths through the first port and the second port, and then transmits them to the first beam splitter element. The first beam splitter selects the polarization direction of the two beams received from the first port and the second port, and then transmits them to the second beam splitter. The second beam splitter splits the beam transmitted by the first beam splitter into a first beam and a second beam, and transmits them to the first post-selection module and the second post-selection module respectively. The first post-selection module projects the first beam into a first post-selection state, and the second post-selection module projects the second beam into a second post-selection state. The first photodetector detects the first light intensity of the beam output by the first post-selection module, and the second photodetector detects the second light intensity of the beam output by the second post-selection module. The processor calculates the phase change of the light field caused by the sound field based on the first light intensity and the second light intensity.

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