Orbital angular momentum interferometer based on double-resonance spiral long-period fiber grating and application

Abstract

The invention provides an orbital angular momentum interferometer based on a double-resonance spiral long-period fiber grating and application, and belongs to the field of optical fiber sensing and optical communication passive devices. According to the invention, the double-resonance spiral long-period fiber grating is used as an OAM mode converter, the wavelength of the laser is secondarily regulated and controlled to match left and right resonance peaks, phase superposition of conjugate OAM beam pairs is realized, and secondary sensitization of the OAM interferometer is realized based on a single mode converter. The OAM interferometer is applied to the field of temperature and strain sensing, high-sensitivity detection of temperature variation and strain variation is achieved by combining a double-phase matrix decoupling algorithm, and the problem of cross sensitivity is effectively solved. The invention provides an all-fiber integrated and environment interference resistant innovative solution for high-sensitivity OAM optical fiber sensing, and is suitable for the fields of industrial monitoring, environment sensing and optical communication.

Description

Technical Field

[0001] The present invention relates to the field of fiber optic sensing and optical communication passive devices, and particularly relates to an orbital angular momentum (OAM) interferometer based on a dual-resonant spiral long-period fiber grating (LPFG) and its applications. Background Art

[0002] OAM beams with phase singularities and helical phase fronts (where represents the topological charge number, represents the azimuth angle) have been widely studied and have shown important application values in the fields of optical communication, quantum information, high-resolution imaging, etc. In recent years, OAM interferometers constructed by combining OAM beams with traditional optical interferometers have received extensive attention and have achieved various applications in the field of high-precision measurement.

[0003] The core mechanism of the OAM interferometer is to introduce a pair of conjugate OAM beams to achieve a new type of phase demodulation technology. Its innovation lies in transferring the phase information demodulation from the traditional spatial domain or frequency domain to the azimuth angle domain, significantly improving the measurement efficiency and accuracy. However, existing OAM interferometers mostly use bulk optical elements to generate OAM beams in free space, resulting in problems such as large system volume, high alignment accuracy requirements, and poor environmental stability, which limit their applications in integrated systems.

[0004] To solve the above problems, researchers have been committed to developing OAM interferometer design schemes based on fiber optic devices. By using all-fiber devices to replace traditional bulk optical elements to generate pairs of conjugate OAM beams. This all-fiber OAM interferometer not only significantly reduces the system volume, but also has advantages such as low insertion loss, high mode conversion efficiency, and natural compatibility with fiber optic systems. However, existing schemes still need to be regulated by auxiliary devices such as polarization controllers and pressure plates, which substantially increases the complexity and instability of the interferometer. In addition, the generally existing polarization-dependent characteristics of existing OAM interferometers also greatly limit the application expansion in complex environments.

[0005] Based on this, the present invention proposes an OAM interferometer based on a dual-resonant spiral LPFG, which directly excites a helical phase front in the optical fiber through the spiral LPFG without any auxiliary devices, and has better stability, flexibility, and integration. Summary of the Invention

[0006] In view of the above problems existing in the prior art, the present invention provides an OAM interferometer based on a dual-resonant spiral LPFG and its applications in temperature and strain sensing. The OAM interferometer of the present invention utilizes the special mode dispersion characteristics of the dual-resonant spiral LPFG near the dispersion turning point (DTP), causing the left and right resonant peaks to shift in opposite directions under external perturbations. By secondarily adjusting the laser wavelength to match the left and right resonant peaks, the phase superposition of the conjugate OAM beam pair is achieved, and further, the secondary sensitization of the OAM interferometer is realized based on a single mode converter. This OAM interferometer is applied to the fields of temperature and strain sensing. Combining with the dual-phase matrix decoupling algorithm, it realizes the highly sensitive detection of the temperature change amount and the strain change amount, effectively solves the cross-sensitivity problem, and provides an all-fiber integrated and anti-environmental interference innovative solution for high-sensitivity OAM fiber sensing, which is applicable to the fields of industrial monitoring, environmental sensing, and optical communication.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows: An orbital angular momentum interferometer based on a dual-resonant spiral long-period fiber grating, comprising: A tunable laser, which is used to sequentially output spherical wave lasers with two different wavelengths; A first fiber coupler, which is used to distribute the spherical wave laser of each wavelength output by the tunable laser to a reference arm and a sensing arm; A sensing arm, which is composed of a single-mode fiber and is used to measure sensing information; A dual-resonant spiral LPFG, which is integrated in the reference arm and excites OAM +1 mode and OAM -1 mode at the left and right resonant wavelengths respectively; A second fiber coupler, which is used to couple the light beams generated by the sensing arm and the reference arm. After coupling, interference occurs to form spiral interference fringes; An optical fiber collimator, which is used to collimate the light beam into parallel light; An interference detection module, which is used to record the mode field and the spiral interference fringes formed by the second fiber coupler and calculate the change amount of the external parameter based on the phase change of the spiral interference fringe image.

[0008] Furthermore, the left and right resonant wavelengths of the dual-resonant spiral LPFG are respectively and ; the and are respectively two resonant wavelengths corresponding to the same grating period of the dual-resonant spiral LPFG, and the grating period is greater than the period value corresponding to the dispersion turning point of the dual-resonant spiral LPFG.

[0009] Further, the two wavelengths of the tunable laser respectively correspond to the left resonant wavelength of the dual-resonant spiral LPFG and the right resonant wavelength .

[0010] Further, = 1269 nm, = 1439 nm.

[0011] Further, the splitting ratio of the first fiber coupler is 50:50, and the spherical wave laser is evenly distributed to the reference arm and the sensing arm.

[0012] Further, the interference detection module includes an infrared CCD camera and a computer; the infrared CCD camera is used to record the mode field and the spiral interference fringe image; the computer analyzes the adjacent two frames of spiral interference fringe images taken at high frequency, extracts the phase change information, and calculates the change amount of the external parameter based on the dual-phase matrix decoupling algorithm.

[0013] An application of a dual-resonant spiral long-period fiber grating-based orbital angular momentum interferometer in temperature and strain sensing, including: Step 1: The tunable laser outputs a spherical wave laser with a wavelength of . After being equally divided into two beams by the first fiber coupler, the two beams respectively enter the reference arm where the dual-resonant spiral LPFG is located and the sensing arm where the single-mode fiber is located; the vortex beam generated by the reference arm and the beam carrying temperature, strain and other information in the sensing arm are coupled by the second fiber coupler and then interfere to form a spiral interference fringe; the fiber collimator collimates the beam into a parallel light, the infrared CCD camera records the mode field and the spiral interference fringe image, and the computer analyzes the spiral interference fringe image and extracts its phase change ; Step 2: The tunable laser outputs a spherical wave laser with a wavelength of . Repeat the process of Step 1 to obtain the phase change ; Step 3: The computer calculates the temperature change amount or the strain change amount based on and using the dual-phase matrix decoupling algorithm.

[0014] Further, the method for calculating the temperature change amount or the strain change amount based on and using the dual-phase matrix decoupling algorithm is as follows: The linear relationship between the phase change and the external perturbation is: (1) (2) The decoupling formula is as follows: (3) (4) Wherein, and respectively represent the temperature change and the strain change, and respectively represent the temperature sensitivity coefficients at the left and right resonance wavelengths, and respectively represent the strain sensitivity coefficients at the left and right resonance wavelengths.

[0015] Furthermore, the obtaining methods of the , and , are as follows: When an external perturbation (such as the refractive index change caused by temperature or strain) acts on the sensing arm, additional phase changes will occur in the interference fringes of the OAM interferometer , and its expression can be represented as: (5) Wherein, and respectively represent the refractive index change and the refractive index of the sensing area, and respectively represent the change in the fiber length and the original length of the sensing area; When temperature acts on the sensing fiber alone, the first and second terms in Equation (5) respectively correspond to the thermo-optic effect and the thermal expansion effect of the single-mode fiber. Usually, the value of the first term is much larger than the second term. Therefore, the phase change caused by the thermal expansion effect is ignored, and the phase change caused by temperature can be represented as: (6) Derive Equation (6) to obtain the calculation formula for temperature sensitivity: (7) In the formula, is the temperature change, is the fiber length of the sensing area, and respectively represent the thermo-optic coefficient and the refractive index of the sensing fiber, is the laser wavelength used. When takes or , the temperature sensitivities of the OAM interferometer at the left and right resonance wavelengths can be calculated respectively according to Equation (7) and 。

[0016] Similarly, when the strain acts on the sensing optical fiber alone, the phase change caused by it can be expressed as: (8) Deriving Equation (8), the calculation formula for the strain sensitivity is obtained: (9) In the formula, represents the applied strain, is the optical fiber length of the sensing region, and respectively represent the refractive index and Poisson's ratio of the sensing optical fiber, and represent the two photoelastic coefficients of the sensing optical fiber, represents the laser wavelength used. When takes or , according to Equation (9), the strain sensitivities of the OAM interferometer at the left and right resonant wavelengths can be calculated respectively and 。

[0017] Furthermore, = 1269 nm, = 1439 nm, = 0.6 m, = 8.22×10 -6 , = 1.452, = 0.17, = 0.116, = 0.258, and the calculated results are = 35.457 rad / °C, = -31.269 rad / °C, = 3.430 rad / με, = -3.024 rad / με.

[0018] Advantages of the present invention: The OAM interferometer of the present invention utilizes the special mode dispersion characteristics of the dual-resonant spiral LPFG near the DTP, causing the left and right resonant peaks to shift in opposite directions under external perturbations. By secondarily adjusting the laser wavelength to match the left and right resonant peaks, the phase superposition of the conjugate OAM beam pair is achieved, and further, secondary sensitization of the OAM interferometer is realized based on a single mode converter. This OAM interferometer is applied to the fields of temperature and strain sensing. Combining with the dual-phase matrix decoupling algorithm, high-sensitivity detection of temperature change and strain change is achieved, and the cross-sensitivity problem is effectively solved. Description of the Drawings

[0019] Figure 1 For the few-mode fiber LP 01 and LP 11 coupled phase matching curve.

[0020] Figure 2 Is the transmission spectrum of the dual-resonant spiral LPFG.

[0021] Figure 3 Is a schematic structural diagram of an OAM interferometer based on a dual-resonant spiral LPFG provided by an embodiment of the present invention.

[0022] Figure 4 Is a schematic diagram of the OAM interferometer temperature measurement experiment.

[0023] Figure 5 Is a schematic diagram of the relationship between the phase and temperature of the OAM interferometer at the left and right resonant peaks of the dual-resonant spiral LPFG.

[0024] Figure 6 Is a schematic diagram of the OAM interferometer strain measurement experiment.

[0025] Figure 7 Is a schematic diagram of the relationship between the phase and strain of the OAM interferometer at the left and right resonant peaks of the dual-resonant spiral LPFG.

[0026] In the figure: 1 Tunable laser; 2 First fiber coupler; 3 Sensing fiber; 4 Dual-resonant spiral long-period fiber grating; 5 Second fiber coupler; 6 Fiber collimator; 7 Infrared CCD camera; 8 Computer; 9 Thermostat; 10 Magnet base; 11 Electrically controlled displacement stage. Specific embodiments

[0027] The object of the present invention is to provide an OAM interferometer based on a dual-resonant spiral LPFG and its application in temperature and strain sensing, which can realize high-sensitivity detection of temperature and strain.

[0028] To make the above objects, features and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] Based on the femtosecond laser direct writing technology, a helically symmetric refractive index modulation structure is induced in the few-mode fiber to prepare a few-mode spiral LPFG. The femtosecond laser has the characteristics of "cold processing" and will not damage the fiber structure.

[0030] To achieve the coupling between fiber modes, it is necessary to solve the relationship between the grating period and the resonant wavelength according to the phase matching condition. In the few-mode spiral LPFG, LP 01 and LP 11 The phase matching conditions corresponding to the modes are: (10) Wherein, is the grating period, represents the resonant wavelength, and are the effective refractive indices of the LP 01 mode and the LP 11 mode respectively. Based on the phase matching condition, a phase matching curve is plotted, which shows a parabolic non-monotonic change trend, as shown in Figure 1 . When the grating period is 609.533 μm, the few-mode helical LPFG operates at the DTP. Near the DTP, a grating period of 614 μm is selected, and this period corresponds to two resonant wavelengths ( = 1269 nm, = 1439 nm), that is, double resonant peaks are generated.

[0031] According to the fiber mode dispersion characteristics and the LPFG sensing theory, when the double-resonant LPFG operates near the DTP, its resonant wavelengths have extremely high response sensitivity to external perturbations, and its left and right resonant peaks shift in opposite directions. In the present invention, a double-resonant helical LPFG is used as an OAM mode converter. By secondarily adjusting the wavelength of the laser to match its left and right resonant peaks, the phase superposition of the excited conjugate OAM beam pair is further achieved, so as to realize the purpose of secondarily sensitizing the OAM interferometer with a single mode converter. The transmission spectrum of the double-resonant helical LPFG is as shown in Figure 2 .

[0032] As shown in Figure 3 , an embodiment of the present invention provides an OAM interferometer based on a double-resonant helical LPFG, which mainly includes a tunable laser 1, a first fiber coupler 2, a sensing fiber 3, a double-resonant helical LPFG 4, a second fiber coupler 5, a fiber collimator 6, an infrared CCD camera 7, and a computer 8.

[0033] The spherical wave laser output by the tunable laser 1 is equally divided into two beams by the first fiber coupler 2 and then enters the reference arm where the double-resonant helical LPFG 4 is located and the sensing arm where the sensing fiber 3 is located respectively; the vortex beam generated by the reference arm interferes with the beam carrying information such as temperature and strain in the sensing arm after being coupled by the second fiber coupler 5 to form spiral interference fringes; the fiber collimator 6 collimates the beam into parallel light, the infrared CCD camera 7 records the mode field and the spiral interference fringe image, and the computer 8 analyzes the adjacent two frames of spiral interference fringe images taken at high frequency, extracts the phase change information, and calculates the temperature change amount or strain change amount based on the double-phase matrix decoupling algorithm.

[0034] The thermo-optic coefficient of the sensing fiber 3 is 8.22×10 -6, with a refractive index of 1.452, a Poisson's ratio of 0.17, the photoelastic coefficients of the sensing optical fiber are 0.116 and 0.258 respectively, and the length of the optical fiber in the sensing area is 0.6 m in the experiment.

[0035] As Figure 4 shown, it is the temperature measurement experiment of the OAM interferometer. The sensing optical fiber 3 is placed in the constant temperature oven 9 for the temperature measurement experiment. The experimental steps of temperature measurement are as follows: Adjust the laser wavelength of the tunable laser 1 to = 1269 nm, and record the initial spiral interference fringes; Increase the temperature from 0 °C to 50 °C in steps of 2 °C, record the spiral interference fringe images at each temperature point through the infrared CCD camera 7, and the computer 8 analyzes the spiral interference fringe images to extract their phase changes ; Switch the laser wavelength of the tunable laser 1 to = 1439 nm, repeat the above steps to obtain the phase change ; Calculate the actual temperature sensitivities and at the left and right resonant wavelengths respectively through linear fitting.

[0036] Experimental results: The actual temperature sensitivities and at the left and right resonant wavelengths are 37.120 rad / °C and -32.977 rad / °C respectively, and the linearity R 2 is greater than 0.999. The relationship between the OAM interferometer phase and temperature at the left and right resonant peaks of the double-resonant spiral LPFG is as Figure 5 shown.

[0037] As Figure 6 shown, it is the strain measurement experiment of the OAM interferometer. One end of the sensing optical fiber 3 is fixed to the magnet base 10, and the other end is fixed to the electric control displacement stage 11. By precisely controlling the electric control displacement stage 11, an axial strain in the range of 0 με to 100 με can be applied to the sensing optical fiber 3. The experimental steps of strain measurement are as follows: Adjust the laser wavelength of the tunable laser 1 to = 1269 nm, and record the initial spiral interference fringes; Apply a strain of 0 με to 100 με in steps of 5 με, record the spiral interference fringe images at each strain point through the infrared CCD camera 7, and the computer 8 analyzes the spiral interference fringe images to extract their phase changes ; Switch the laser wavelength of the tunable laser 1 to = 1439 nm, repeat the above steps to obtain the phase change ; Calculate the actual strain sensitivities at the left and right resonant wavelengths respectively by linear fitting and .

[0038] Experimental results: The actual strain sensitivities at the left and right resonant wavelengths and are 3.469 rad / με and -3.058 rad / με respectively, and the linearity R 2 is greater than 0.998. The relationship between the OAM interferometer phase and strain at the left and right resonant peaks of the dual-resonant spiral LPFG is as Figure 7 shown.

[0039] An OAM interferometer based on a dual-resonant spiral LPFG and its application in temperature and strain sensing proposed by the present invention. The OAM interferometer has different response sensitivities to temperature and strain changes and is linearly correlated, which provides a theoretical basis for using the dual-phase matrix decoupling algorithm to solve the cross-sensitivity problem.

[0040] During actual measurement, it is necessary to calibrate the initial phases and at the left and right resonant wavelengths and respectively in advance. When the external parameters change, according to the phase change amounts and at the left and right resonant wavelengths, the temperature change amount or the strain change amount can be calculated by using the decoupling formulas (3) to (4).

[0041] Although the above embodiments have described the technical solutions of the present invention in detail, they are only some preferred embodiments of the present invention rather than all implementation manners. Based on this embodiment, without departing from the design spirit and principles of the present invention, other embodiments obtained through non-creative labor, as well as any modifications, equivalent replacements or improvements made thereto, all fall within the protection scope of the claims of the present invention.

Claims

1. Orbital angular momentum interferometer based on a dual-resonant helical long-period fiber grating, characterized in that, Including: A tunable laser for sequentially outputting spherical wave lasers of two different wavelengths; A first fiber optic coupler for distributing the spherical wave laser of each wavelength output by the tunable laser to a reference arm and a sensing arm; A sensing arm composed of a single-mode fiber for measuring sensing information; A dual-resonant spiral long-period fiber grating is integrated into the reference arm to excite OAM at the left and right resonant wavelengths respectively. +1 Mode and OAM -1 model; A second fiber optic coupler for coupling the light beams generated by the sensing arm and the reference arm. After coupling, interference occurs to form spiral interference fringes; A fiber optic collimator for collimating the light beam into a parallel light; An interference detection module for recording the mode field and the spiral interference fringes formed by the second fiber optic coupler and calculating the change amount of the external parameter based on the phase change of the spiral interference fringe image.

2. The orbital angular momentum interferometer based on a dual-resonant helical long-period fiber grating according to claim 1, wherein The left and right resonant wavelengths of the double-resonant helical long-period fiber grating are respectively and ; the and are respectively two resonant wavelengths corresponding to the same grating period of the double-resonant helical long-period fiber grating, and the grating period is greater than the period value corresponding to the dispersion inflection point of the double-resonant helical long-period fiber grating.

3. The orbital angular momentum interferometer based on a dual-resonant helical long-period fiber grating according to claim 1 or 2, characterized in that, The two wavelengths of the tunable laser respectively correspond to the left resonant wavelength and the right resonant wavelength of the double-resonant spiral long-period fiber grating.

4. The orbital angular momentum interferometer based on a dual-resonant helical long-period fiber grating according to claim 3, wherein =1269 nm, =1439 nm.

5. The orbital angular momentum interferometer based on a double-resonant spiral long-period fiber grating according to claim 1, characterized in that: The splitting ratio of the first fiber optic coupler is 50:

50.

6. The orbital angular momentum interferometer based on a dual-resonant spiral long-period fiber grating according to claim 1, characterized in that The interference detection module includes an infrared CCD camera and a computer; the infrared CCD camera is used for recording the mode field and the spiral interference fringe image; the computer analyzes two adjacent frames of spiral interference fringe images taken at high frequency, extracts the phase change information thereof and calculates the change amount of the external parameter.

7. Application of an orbital angular momentum interferometer based on a dual-resonant spiral long-period fiber grating, characterized in that, Including the following steps: Step 1: The output wavelength of the tunable laser is The spherical wave laser is split into two beams of equal power by the first fiber coupler and then enters the reference arm where the double-resonant spiral long-period fiber grating is located and the sensing arm where the single-mode fiber is located respectively; the vortex beam generated by the reference arm and the beam carrying temperature, strain and other information in the sensing arm interfere with each other after being coupled by the second fiber coupler, forming spiral interference fringes; the fiber collimator collimates the beam into parallel light, the infrared CCD camera records the mode field and the spiral interference fringe image, and the computer analyzes the spiral interference fringe image to extract its phase change ; Step 2: The tunable laser output wavelength is Repeat the process of step 1 to obtain the phase change of spherical wave laser. ; Step 3: Computer and The temperature change or strain change is calculated based on the dual-phase matrix decoupling algorithm.

8. The application of the orbital angular momentum interferometer based on the dual-resonant spiral long-period fiber grating according to claim 7, characterized in that, In step 3, according to and The temperature change or strain change is calculated based on the dual-phase matrix decoupling algorithm. The specific method is as follows: The linear relationship between the phase change and the external disturbance is: （1） （2） The decoupling formula is: （3） （4） Among them, and represent the temperature change amount and the strain change amount respectively, and represent the temperature sensitivity coefficients at the left and right resonant wavelengths respectively, and represent the strain sensitivity coefficients at the left and right resonant wavelengths respectively.

9. The application of the orbital angular momentum interferometer based on a dual-resonant helical long-period fiber grating according to claim 8, characterized in that, The said , and , are obtained as follows: When temperature acts alone on the sensing optical fiber, the phase change caused by it is expressed as: （5） Deriving Equation (5) to obtain the calculation formula for the temperature sensitivity: （6） In the formula, is the temperature change, is the optical fiber length of the sensing area, and respectively represent the thermo-optic coefficient and the refractive index of the sensing optical fiber, is the laser wavelength used. When takes or , the temperature sensitivities and of the interferometer at the left and right resonant wavelengths are calculated according to formula (6); When strain acts alone on the sensing optical fiber, the phase change caused by it is expressed as: （7） Deriving Equation (8) to obtain the calculation formula for the strain sensitivity: （8） Wherein, represents the applied strain, is the fiber length of the sensing area, and respectively represent the refractive index and Poisson's ratio of the sensing fiber, and represent the two photoelastic coefficients of the sensing fiber, represents the laser wavelength used. When takes or , the strain sensitivities and of the interferometer at the left and right resonant wavelengths are calculated according to formula (8) respectively.

10. The application of the orbital angular momentum interferometer based on the dual-resonant helical long-period fiber grating according to claim 9, wherein, = 1269 nm, = 1439 nm, = 0.6 m, = 8.22×10 -6 , = 1.452, = 0.17, = 0.116, = 0.258, calculated to obtain = 35.457 rad / °C, = -31.269 rad / °C, = 3.430 rad / με, = -3.024 rad / με.

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