Microwave photonic link strain measurement system based on A-T splitting

Abstract

The invention discloses a microwave photonic link strain measurement system based on A-T splitting, which belongs to the technical field of weak strain monitoring and comprises a broadband light source, the broadband light source is connected with a port I of a circulator, a port II of the circulator is connected with a fiber bragg grating strain sensor, a port III of the circulator is connected with a light input port of an electro-optical modulator, and a port III of the electro-optical modulator is connected with a fiber bragg grating strain sensor. A modulation port of the electro-optical modulator is connected with a first port of the vector network analyzer, a light output port of the electro-optical modulator is connected with a first input end of the optical coupler, a section of linear chirp fiber grating is arranged between the first output end and a second output end of the optical coupler and is connected with the first output end and the second output end of the optical coupler, and the second input end of the optical coupler is connected with the photoelectric detector. The other end of the photoelectric detector is connected with the vector network analyzer; according to the microwave photonic link strain measurement system based on A-T splitting, the problems of low resolution and low sensitivity in the field of micro strain measurement are solved, and the strain measurement system with high sensitivity and high resolution is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of weak strain detection, and in particular to a microwave photonic link strain measurement system based on A-T splitting. Background Art

[0002] Public transportation facilities such as high-speed railways, ships, and airplanes, infrastructure such as railways and cross-sea bridges, and housing buildings, etc., are designed to have a service life of several decades or even over a century. As the service life increases, these facilities will experience problems such as material aging and structural wear, resulting in performance degradation and increased safety hazards. In extreme cases, accidents such as collapses may occur. Therefore, structural health monitoring is crucial to predict and avoid large-scale safety accidents. Commonly used monitoring methods include mechanical measurement methods and electrical measurement methods, but the former has low sensitivity and the latter is easily affected by the environment, and both are difficult to meet the long-term monitoring requirements. In contrast, fiber optic sensing technology has the advantages of small size, low loss, strong anti-electromagnetic interference and chemical corrosion resistance, etc., and is more suitable for the long-term monitoring of large and complex facilities.

[0003] As a new type of optical measurement element, fiber Bragg grating has the advantages of small size, light weight, resistance to harsh environments, easy networking and installation, etc., and can measure physical quantities such as temperature and stress in real time. However, there are many problems with traditional grating demodulation methods. Spectral analyzers are costly, large in size, slow in demodulation, and difficult to achieve a resolution of the picometer level, and cannot meet the requirements of fiber Bragg grating strain sensors. The matching grating method has a small demodulation range, the filter method is expensive and has high requirements for light sources, the interference method is easily affected by the environment, and the imaging spectroscopy method has a complex process and the image is easily distorted. Therefore, there is an urgent need for an efficient, high-resolution, highly sensitive and low-cost demodulation scheme.

[0004] The fiber Bragg grating demodulation scheme based on microwave photonics combines the advantages of the resolution and sensitivity of the optical system, and also utilizes the characteristics of good microwave signal quality, polarization insensitivity and high stability, etc., effectively solving some problems existing in the existing fiber Bragg grating demodulation methods. The present invention combines the Autler-Townes splitting effect with microwave photonics to achieve fiber Bragg grating demodulation. Summary of the Invention

[0005] The object of the present invention is to provide a microwave photonic link strain measurement system based on A-T splitting to solve problems such as system redundancy caused by relying on dispersion accumulation and relying on high-frequency demodulation instruments in traditional microwave photonic fiber Bragg grating demodulation schemes.

[0006] To achieve the above object, the present invention provides a microwave photonic link strain measurement system based on A-T splitting, comprising: a broadband light source (BBS), the broadband light source is connected to port 1 of a circulator (Cir), port 2 of the circulator is connected to a fiber Bragg grating (FBG) strain sensor, port 3 of the circulator is connected to the optical input port of an electro-optical modulator (EOM), the modulation port of the electro-optical modulator is connected to port 1 of a vector network analyzer (VNA), the optical output port of the electro-optical modulator is connected to input end 1 of an optical coupler (OC), a section of linear chirped fiber Bragg grating (LCFBG) is built in and connected between output end 1 and output end 2 of the optical coupler, input end 2 of the optical coupler is connected to a photodetector (PD), and the other end of the photodetector is connected to the vector network analyzer.

[0007] Preferably, the broadband light generated by the broadband light source enters the fiber Bragg grating strain sensor through port 1 of the circulator.

[0008] Preferably, the reflected light of the fiber Bragg grating strain sensor serves as an optical carrier and sequentially enters the electro-optical modulator through port 2 and port 3 of the circulator, and is modulated by the driving signal of port 1 of the vector network analyzer.

[0009] Preferably, the optical coupler is a 2×2 type with two input ports and two output ports, and its splitting ratio is 50:50.

[0010] Preferably, optical fields E1 and E2 are respectively input to input end 1 and input end 2 of the optical coupler. After entering the optical coupler, optical fields E1 and E2 are divided into a clockwise-propagating light E3 and a counterclockwise-propagating light E4. The transmitted light and reflected light of E3 and E4 at the linear chirped fiber Bragg grating respectively. These four beams of light generate the Autler-Townes splitting effect when propagating to the coupler. E3 and E4 experience different time delays to reach the linear chirped fiber Bragg grating. After being processed by the linear chirped fiber Bragg grating, they become optical fields E3 ' and E4 ' with specific phases and time delays. E3 ' is the sum of the light intensities of the two beams of light, including the reflected light of E3 at the linear chirped fiber Bragg grating and the transmitted light of E4 at the linear chirped fiber Bragg grating. E4 ' Similarly, E3' and E4 ' Recombined to form the final output optical field E1 ' and E2 ' .

[0011] Preferably, the expression formula of the optical field E2 ' is as follows:

[0012]

[0013] where r is the reflection coefficient, and the reflection coefficient r is controlled by the reflection wavelength of the FBG and the slope of the descending edge of the linearly chirped fiber grating, t is the transmission coefficient, α is the loss factor, and are phase factors, corresponding to two different paths respectively, and i is the imaginary unit.

[0014] Preferably, the photodetector converts the coupled optical signal into an electrical signal.

[0015] Preferably, the vector network analyzer observes and records the microwave frequency response under different strains.

[0016] Therefore, the present invention adopts the above-mentioned microwave photon link strain measurement system based on A-T splitting, and has the following beneficial effects:

[0017] (1) By introducing the Autler-Townes splitting effect in the microwave photon demodulation system, the resolution and sensitivity of the system are effectively improved;

[0018] (2) The present invention abandons the dispersion module, and the sensitivity can be improved without dispersion accumulation, effectively reducing the bandwidth limitation of the demodulation instrument.

[0019] Next, through the drawings and embodiments, the technical solutions of the present invention will be further described in detail. Brief Description of the Drawings

[0020] Figure 1 is a schematic diagram of a microwave photon link strain measurement system based on A-T splitting of the present invention;

[0021] Figure 2 is a schematic diagram of the Autler-Townes splitting of a microwave photon link strain measurement system based on A-T splitting of the present invention;

[0022] Figure 3 is a graph showing the change of the microwave interference spectrum when the optical path difference is 0.12 m and 1.2 m respectively under different reflection coefficients of the present invention. Detailed Embodiments

[0023] The following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0024] Embodiment

[0025] As Figure 1 shown, the present invention provides a microwave photonic link strain measurement system based on A-T splitting, including a broadband light source (BBS). The broadband light source is connected to port 1 of a circulator (Cir). Port 2 of the circulator is connected to a fiber Bragg grating (FBG) strain sensor. The broadband light generated by the broadband light source enters the fiber Bragg grating strain sensor through port 1 of the circulator. Port 3 of the circulator is connected to the optical input port of an electro-optical modulator (EOM). The modulation port of the electro-optical modulator is connected to port 1 of a vector network analyzer (VNA). The reflected light of the fiber Bragg grating strain sensor enters the electro-optical modulator as an optical carrier through port 2 and port 3 of the circulator in sequence and is modulated by the driving signal of port 1 of the vector network analyzer. The optical output port of the electro-optical modulator is connected to input terminal 1 of an optical coupler (OC). The optical coupler is of 2×2 type with two input ports and two output ports, and its splitting ratio is 50:50. The modulated light enters the optical coupler. A section of linear chirped fiber Bragg grating (LCFBG) is built in and connected between output terminal 1 and output terminal 2 of the optical coupler. The light entering the optical coupler propagates in opposite directions and returns to be coupled at the optical coupler. Input terminal 2 of the optical coupler is connected to a photodetector (PD). The photodetector converts the coupled optical signal into an electrical signal. The other end of the photodetector is connected to the vector network analyzer. Port 2 of the vector network analyzer observes and records the microwave frequency responses under different strains.

[0026] As Figure 2As shown, optical field \(E1\) and optical field \(E2\) are respectively input into input terminal 1 and input terminal 2 of the optical coupler. After the optical fields \(E1\) and \(E2\) enter the optoelectronic coupler, they are divided into the clockwise-propagating light \(E3\) and the counterclockwise-propagating light \(E4\). \(E3\) and \(E4\) experience different time delays and reach the linearly chirped fiber grating. When the carrier is within the total reflection bandwidth of the linearly chirped fiber grating, the system is like a traditional Michelson interferometer. After \(E3\) and \(E4\) reach the linearly chirped fiber grating, they return to the optical coupler along the original path. When the carrier is within the total transmission bandwidth of the linearly chirped fiber grating, after \(E3\) and \(E4\) reach the linearly chirped fiber grating, they respectively pass through the linearly chirped fiber grating and are transmitted back to the optical coupler, and the optical path difference between the two paths is 0, so no interference occurs. When the carrier is at the upper and lower edges of the reflection bandwidth of the linearly chirped fiber grating, after \(E3\) and \(E4\) reach the linearly chirped fiber grating, there are transmitted light and reflected light passing through the linearly chirped fiber grating respectively. At this time, the four beams of light reach the optical coupler and couple, and the common action produces the Autler-Townes splitting effect.

[0027] \(E3\) and \(E4\) experience different time delays and reach the linearly chirped fiber grating. After being processed by the linearly chirped fiber grating, they become optical fields \(E3\) with specific phases and time delays ' and \(E4\) ' , \(E3\) ' is the sum of the optical intensities of the two beams of light, including the reflected light of \(E3\) at the linearly chirped fiber grating and the transmitted light of \(E4\) at the linearly chirped fiber grating. \(E4\) ' Similarly, \(E3\) ' and \(E4\) ' are recombined to form the final output optical fields \(E1\) ' and \(E2\) ' .

[0028] The expression formula of the optical field \(E2\) ' is as follows:

[0029]

[0030] Among them, \(r\) is the reflection coefficient, and the reflection coefficient \(r\) is controlled by the reflection wavelength of the FBG and the slope of the descending edge of the linearly chirped fiber grating. \(t\) is the transmission coefficient, and \(\alpha\) is the loss factor. and are phase factors, corresponding to two different paths respectively, and \(i\) is the imaginary unit.

[0031] It can be seen from the formula that the Autler-Townes splitting effect is composed of the common action of four beams of light. The intensities of the four beams of light \(E1\), \(E2\), \(E3\), and \(E4\) are determined by the reflection coefficient \(r\). As Figure 3 shown, the splitting peaks gradually become narrower as the reflection coefficient \(r\) changes. The smaller the optical path difference, the more obvious the narrowing degree. Therefore, the microstrain suffered by the fiber grating can be demodulated by measuring the drift of the splitting peaks.

[0032] Therefore, the present invention adopts the above-mentioned microwave photonic link strain measurement system based on A-T splitting, solves the problems of low resolution and small sensitivity in the field of micro-strain measurement, and realizes a strain measurement system with high sensitivity and high resolution.

[0033] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that they can still modify or equivalently replace the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A microwave photonic link strain measurement system based on A-T splitting, characterized in that Comprising: A broadband light source, the broadband light source is connected to port 1 of the circulator, port 2 of the circulator is connected to the fiber Bragg grating strain sensor, port 3 of the circulator is connected to the optical input port of the electro-optic modulator, the modulation port of the electro-optic modulator is connected to port 1 of the vector network analyzer, the optical output port of the electro-optic modulator is connected to input terminal 1 of the optical coupler, a section of linearly chirped fiber Bragg grating is built-in and connected between output terminal 1 and output terminal 2 of the optical coupler, input terminal 2 of the optical coupler is connected to the photodetector, and the other end of the photodetector is connected to the vector network analyzer.

2. The microwave photonic link strain measurement system based on A-T splitting according to claim 1, wherein: The broadband light generated by the broadband light source enters the fiber Bragg grating strain sensor through port 1 of the circulator.

3. A microwave photonic link strain measurement system based on A-T splitting according to claim 1, characterized in that: The reflected light of the fiber Bragg grating strain sensor enters the electro-optic modulator successively through port 2 and port 3 of the circulator and is modulated by the driving signal of port 1 of the vector network analyzer.

4. A microwave photonic link strain measurement system based on A-T splitting according to claim 1, characterized in that: The optical coupler is of 2×2 type with two input ports and two output ports, and its splitting ratio is 50:

50.

5. The microwave photonic link strain measurement system based on A-T splitting according to claim 1, wherein: At the input end 1 and input end 2 of the optical coupler, optical fields E1 and E2 are respectively input. After the optical fields E1 and E2 enter the optoelectronic coupler, they are divided into light E3 propagating clockwise and light E4 propagating counterclockwise. The transmitted light and reflected light of E3 and E4 at the linear chirped fiber grating respectively. These four beams of light propagate to the optoelectronic coupler to produce the Autler-Townes splitting effect. E3 and E4 experience different time delays to reach the linear chirped fiber grating. After being processed by the linear chirped fiber grating, they become optical fields E3 ' and E4 ' . E3 ' is the sum of the optical intensities of the two beams of light, including the reflected light of E3 at the linear chirped fiber grating and the transmitted light of E4 at the linear chirped fiber grating. E4 ' Similarly, E3 ' and E4 ' are recombined to form the final output optical fields E1 ' and E2 ' .

6. The microwave photonic link strain measurement system based on A-T splitting according to claim 5, characterized in that, The light field E2 ' has the following representation formula: where r is the reflection coefficient, t is the transmission coefficient, α is the loss factor, and are phase factors corresponding to two different paths respectively, and i is the imaginary unit.

7. A microwave photonic link strain measurement system based on A-T splitting according to claim 1, characterized in that: The photodetector converts the coupled optical signal into an electrical signal.

8. A microwave photonic link strain measurement system based on A-T splitting according to claim 1, characterized in that: The vector network analyzer observes and records the microwave frequency responses under different strains.

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