Preparation method of a rotationally distributed groove type torsional sensor and torsional sensor

Abstract

The present invention discloses a preparation method of a rotationally distributed groove type torsion sensor and a torsion sensor. The preparation method comprises: S1: clamping two ends of a single-mode optical fiber; S2: etching a group of groove modulation areas on the single-mode optical fiber; S3: fixing one end of the single-mode optical fiber, and twisting the other end of the single-mode optical fiber along its axis by a preset angle; S4: etching a group of groove modulation areas on the single-mode optical fiber after one end is rotated; S5: repeating steps S3 and S4 until the total angle of rotation of the single-mode optical fiber reaches a preset total angle, and the etching of the last group of groove modulation areas is completed; S6: reversely twisting one end of the twisted single-mode optical fiber along its axis to restore it to the initial state, and obtaining a torsion sensor with grooves showing a spiral distribution. The present invention destroys the circular symmetry of the optical fiber to the greatest extent by increasing the distribution surface of the grooves, thereby increasing the introduced linear birefringence; at the same time, by gradually increasing the torsional force, more residual stress is frozen in the optical fiber, thereby increasing the introduced circular birefringence.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber sensors, and in particular to a preparation method of a rotating distributed groove type torsion sensor and a torsion sensor. Background Art

[0002] Torsion is a very important mechanical parameter for structural health monitoring in various engineering applications. On the one hand, macroscopic torsion monitoring is performed on structural engineering such as bridges, buildings, tunnels and pipelines, with the aim of detecting anomalies in advance and providing early warning, structural health monitoring and maintenance recommendations to prevent accidents. On the other hand, real-time micro-torsion monitoring is performed on equipment such as robot joint structure tracking and biomedical minimally invasive instruments to give corresponding operating instructions. Therefore, the research on fiber optic torsion sensors has become a hot topic. Among them, the research on fiber Bragg gratings (FBGs) is relatively mature, but their sensitivity to the external environment is generally low. Fiber optic interferometers have good sensing performance, but they have the disadvantages of complex manufacturing and inconvenient structure.

[0003] Long-period fiber grating (LPFG) has become a hot topic of current concern due to its advantages such as easy manufacturing, low back reflection, low insertion loss, polarization independence, high sensitivity and larger stopband width than Bragg fiber grating. Typical optical sensing physical quantities include strain, temperature, bending and torsion, etc., which are widely used in bridge, road and tunnel structural health monitoring. LPFG has a promising application and advantage in torsion measurement due to its low cost, easy manufacturing, high sensitivity, stable spectral response and low insertion loss. The LPFG-based torsion sensor is buried on the axis of the torsion axis of the object to be measured. By measuring the change of the resonant wavelength, the size of the twist rate can be directly obtained. When the twist rate is large, the changes in the resonant wavelength and the loss peak amplitude can be measured simultaneously, and the mutual correction can be made to improve the measurement accuracy. However, when the twist rate is small, the amplitude change of the loss peak will be inconspicuous or irregular, and the measurement accuracy can only be improved by the change of the resonant wavelength, and the change of the resonant wavelength usually represents the sensitivity of the torsion sensor.

[0004] When the LPFG-based torsion sensor is subjected to a certain torsion, the greater the shift of the resonant wavelength, the higher the sensitivity. The shift of the sensor's resonant wavelength depends on the product of the difference between the change in the effective refractive index of the core and the change in the effective refractive index of the cladding and the change in the grating period. Since the change in the grating period is so small that it can be ignored when torsion occurs, the core is located at the center axis of the optical fiber and is subject to little torsion, and the change in the effective refractive index is small. The shift of the resonant wavelength mainly depends on the change in the effective refractive index of the cladding. The change in the effective refractive index of the cladding is caused by elliptical birefringence. The greater the elliptical birefringence, the greater the change in the effective refractive index. Elliptical birefringence is a synthesis of linear birefringence and circular birefringence. Some teams have conducted research on improving the sensitivity of LPFG torsion sensors from three perspectives: increasing the linear birefringence of the optical fiber cladding, circular birefringence, and directly increasing the elliptical birefringence.

[0005] At present, the first idea to improve the sensitivity of LPFG torsion sensor is to increase the linear birefringence of the cladding and then increase the synthetic elliptical birefringence to achieve the sensitivity enhancement effect, which is generally achieved by introducing linear birefringence through asymmetric refractive index modulation of the optical fiber. For example, a scheme has proposed to introduce linear birefringence by segmented grooving on different reference planes to make LPFG. The torsion sensing sensitivity of this scheme can reach 0.3nm / rad / m, but this idea fails to effectively introduce circular birefringence. The second idea is to increase the circular birefringence of the cladding and then increase the synthetic elliptical birefringence to achieve the sensitivity enhancement effect, which is generally achieved by twisting the optical fiber. For example, a scheme has proposed to use a fiber fusion machine to twist the optical fiber to prepare a spiral LPFG torsion sensor. The torsion sensitivity of this scheme can reach 0.085nm / rad / m, but this scheme fails to effectively introduce linear birefringence. The third idea is to simultaneously introduce circular birefringence and linear birefringence to enhance the torsion elliptical birefringence and achieve the effect of increased sensitivity. For example, one scheme proposes twisting the optical fiber three times and then side-polishing it. After releasing the twist, a side spiral polishing structure is formed. The sensitivity of this structure can reach up to 0.199nm / rad / m. Another scheme proposes an intermittent spiral structure with step-by-step twisting, which has a maximum sensitivity of 0.15nm / rad / m.

[0006] However, the above schemes for improving the sensitivity of the LPFG torsion sensor, although they achieve the simultaneous introduction of linear birefringence and circular birefringence, do not introduce enough circular birefringence because they only use a fixed torsion angle, and do not introduce enough linear birefringence because there are not enough asymmetric modulation surfaces of the refractive index. At this time, the applicant found that if the distribution surface of the grooves can be further increased to destroy the circular symmetry of the optical fiber to the greatest extent, the introduced linear birefringence can be further increased; at the same time, if the torsion force can be gradually increased, more residual stress can be frozen in the optical fiber to increase the introduced circular birefringence, which can further increase the elliptical birefringence synthesized by torsion, and further increase the sensitivity of the LPFG torsion sensor. Therefore, how to design an LPFG torsion sensor that improves the introduced linear birefringence and circular birefringence is a technical problem that needs to be solved urgently. Summary of the invention

[0007] In view of the deficiencies of the above-mentioned prior art, the technical problem to be solved by the present invention is: how to provide a method for preparing a rotationally distributed groove-type torsion sensor, by increasing the distribution surface of the grooves to destroy the circular symmetry of the optical fiber to the greatest extent, thereby increasing the introduced linear birefringence; at the same time, by gradually increasing the torsional force, more residual stress is frozen in the optical fiber, increasing the introduced circular birefringence, and then increasing the elliptical birefringence synthesized by the linear birefringence and the circular birefringence during torsion, so that the drift of the resonant wavelength during torsion is increased, thereby improving the sensitivity of the torsion sensor.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0009] A method for preparing a rotating distributed slot torsion sensor, comprising:

[0010] S1: Get a single-mode optical fiber and clamp both ends of the single-mode optical fiber;

[0011] S2: etching a group of groove modulation regions on the single-mode optical fiber; a group of groove modulation regions includes a plurality of grooves with a spacing of Λ;

[0012] S3: Fix one end of the single-mode optical fiber and twist the other end of the single-mode optical fiber along its axis by a preset angle θ;

[0013] S4: etching a group of groove modulation regions on the single-mode optical fiber after one end is rotated;

[0014] S5: repeating steps S3 and S4 until the total angle of rotation of the single-mode optical fiber reaches a preset total angle, and the etching of the last group of groove modulation areas is completed;

[0015] S6: Twist one end of the twisted single-mode optical fiber in the opposite direction along its axis to restore it to its initial state, thereby obtaining a torsion sensor having a plurality of grooves in a spiral distribution.

[0016] Preferably, in step S2, each group of groove modulation areas has two or four grooves.

[0017] Preferably, in step S3, when each group of groove modulation areas has two grooves, the preset angle θ is 60°;

[0018] When each group of groove modulation areas has four grooves, the preset angle θ is 90°.

[0019] Preferably, in step S5, when each group of groove modulation areas has two grooves, the preset total angle is 840°;

[0020] When each group of groove modulation areas has four grooves, the preset total angle is 630°.

[0021] Preferably, in step S6, in the torsion sensor, except for the first group of groove modulation areas, adjacent grooves in each group of groove modulation areas have a rotation angle C twisted along the axis direction of the torsion sensor. Λ .

[0022] Preferably, in step S6, in two adjacent groups of groove modulation areas, the rotation angle C between adjacent grooves in the latter group of groove modulation areas is Λ,i Greater than the rotation angle C between adjacent grooves in the previous group of groove modulation areas Λ,i-1 .

[0023] Preferably, in step S6, after the twisted end of the single-mode optical fiber is reversely twisted along its axis to restore to an initial state, a portion of the residual torsional force is solidified in the torsion sensor;

[0024] The calculation formula of residual torsional force is expressed as:

[0025] τ 0 =2β / L 0 ;

[0026] Where: τ 0 represents the residual torsional force; β represents the initial torsional rate; L 0 Indicates the distance between the two ends of a single-mode optical fiber.

[0027] Preferably, in step S6, the torsion direction of the torsion sensor is determined by monitoring the moving direction of the resonant wavelength of the torsion sensor.

[0028] Preferably, in step S6, the magnitude of the torsional force is measured by calculating the spectral displacement value of the torsion sensor.

[0029] The invention also discloses a rotating distributed slot type torsion sensor, which is a torsion sensor prepared based on the preparation method of the rotating distributed slot type torsion sensor of the invention.

[0030] Compared with the prior art, the preparation method of the rotating distributed slot type torsion sensor and the torsion sensor of the present invention have the following beneficial effects:

[0031] The applicant has found that when the LPFG torsion sensor is subjected to a certain torsion, the greater the shift of the resonant wavelength, the higher the sensitivity. The shift of the resonant wavelength depends on the product of the difference between the change in the effective refractive index of the core and the change in the effective refractive index of the cladding and the change in the grating period. When torsion occurs, the change in the grating period is small enough to be ignored, the core is located at the center axis of the optical fiber and is subject to little torsion, the change in the effective refractive index is small, and the shift of the resonant wavelength mainly depends on the change in the effective refractive index of the cladding. The change in the effective refractive index of the cladding is caused by elliptical birefringence. The greater the elliptical birefringence, the greater the change in the effective refractive index of the cladding, and the elliptical birefringence is synthesized by linear birefringence and circular birefringence. Therefore, increasing the introduced linear birefringence and circular birefringence can increase the elliptical birefringence, so that the drift of the resonant wavelength during torsion increases, thereby improving the sensitivity of the torsion sensor.

[0032] In view of the above discovery of the applicant, when preparing a torsion sensor, the present invention rotates the single-mode optical fiber while etching grooves, so that the final torsion sensor has several grooves with a spiral distribution. Such spirally distributed grooves can destroy the circular symmetry of the optical fiber to the greatest extent by increasing the distribution surface of the grooves, and then perform asymmetric refractive index modulation at different azimuth angles of the torsion sensor, thereby increasing the introduced linear birefringence. At the same time, in the preparation process of the torsion sensor of the present invention, the torsion angle becomes larger and larger in the later stage of production, so that the residual stress frozen in the optical fiber is also increasing. The increase in the residual torsion force will gradually accelerate the rotation of the polarization axis of the circular birefringence, and the gradual acceleration of the rotation of the polarization axis means that the circular birefringence increases. In summary, the present invention increases the elliptical birefringence by increasing the introduced linear birefringence and circular birefringence, so that the drift of the resonant wavelength during torsion increases, thereby improving the sensitivity of the torsion sensor. And the left-handed and right-handed of the elliptical birefringence of the present invention are determined by clockwise torsion and counterclockwise torsion, so the displacement direction of the resonant wavelength is related to the torsion direction, which enables the LPFG torsion sensor of the present invention to measure the direction of the applied torsion force while improving the torsion sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to make the purpose, technical solution and advantages of the invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0034] Figure 1 A logic block diagram of a method for preparing a rotating distributed slot torsion sensor;

[0035] Figure 2Microscopic photos and structural schematics of the RD-LPFG production process: (a) Etching the first set of double grooves; (b) Twisting the right end 60° to etch the second set of double grooves; (c) Twisting the right end 60° again to etch the third set of double grooves; (d) Schematic diagram of the RD-LPFG structure.

[0036] Figure 3 Schematic diagram of the structure of VR-LPFG.

[0037] Figure 4 Schematic diagram of asymmetric refractive index distribution in optical fiber: (a) traditional LPFG is modulated on one side; (b) RD-LPFG is modulated in different azimuth directions.

[0038] Figure 5 These are the RD-LPFG spectra with Λ being 480μm, 470μm, and 460μm respectively.

[0039] Figure 6 Experimental setup for torsion measurement.

[0040] Figure 7 Sensing spectra of RD-LPFG torsion sensing test: (a) RD-LPFG1; (b) RD-LPFG2; (c) RD-LPFG3; (d) fitting diagram of the relationship between torsion rate and resonant wavelength.

[0041] Figure 8 Experimental setup for temperature sensing tests.

[0042] Fig. 9 Temperature sensing experimental data of RD-LPFG: (a) RD-LPFG1 spectrum; (b) RD-LPFG2 spectrum; (c) RD-LPFG3 spectrum; (d) fitting diagram of the relationship between temperature and resonant wavelength.

[0043] Fig.10 Experimental setup for strain sensing tests.

[0044] Fig.11 The strain sensing experimental results of RD-LPFG1: (a) RD-LPFG1 spectrum; (b) fitting diagram of the relationship between strain and resonant wavelength. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but only represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the scope of protection of the present invention.

[0046] The following is a further detailed description through specific implementation methods:

[0047] Embodiment 1:

[0048] This embodiment discloses a method for preparing a rotating distributed slot type torsion sensor.

[0049] like Figure 1 As shown, the preparation method of the rotating distributed slot type torsion sensor comprises:

[0050] S1: Get a single-mode optical fiber and clamp both ends of the single-mode optical fiber;

[0051] In this embodiment, the coating layer is stripped off the middle part of the single-mode optical fiber (SMF28, Corning), and the two sides of the stripped area are respectively fixed on the rotating clamping tables at the left and right ends of the system. One end of the optical fiber is connected to a supercontinuum light source (SLS, NKT, SuperkCompact), and the other end is connected to an optical spectrum analyzer (OSA, AQ6370D, YOKOGAWA), which can monitor the resonant wavelength position and depth in real time.

[0052] S2: etching a group of groove modulation regions on the single-mode optical fiber; a group of groove modulation regions includes a plurality of grooves with a spacing of Λ;

[0053] In this embodiment, at high frequency CO 2 The laser (MC-EB, GD Hans Yueming laser) control software sets several laser etching paths with intervals of Λ, the laser power is set to 33% of the total power, and the frequency is 35kHz. The part of the optical fiber where the coating is stripped is aligned with the focus of the laser, and the high-frequency CO 2 The laser etches several grooves on the optical fiber under the control of a computer.

[0054] S3: Fix one end of the single-mode optical fiber and twist the other end of the single-mode optical fiber along its axis by a preset angle θ;

[0055] S4: etching a group of groove modulation regions on the single-mode optical fiber after one end is rotated;

[0056] S5: repeating steps S3 and S4 until the total angle of rotation of the single-mode optical fiber reaches a preset total angle, and the etching of the last group of groove modulation areas is completed;

[0057] S6: Twist one end of the twisted single-mode optical fiber in the opposite direction along its axis to restore it to its initial state, thereby obtaining a (long-period fiber grating) torsion sensor having a plurality of grooves in a spiral distribution.

[0058] Among them, single-mode optical fiber is a ready-made raw material. The concept of long-period fiber grating is to make grooves one by one on the optical fiber to form a refractive index modulation area, and periodically couple light into the cladding through the grooves. After all the groove modulation areas are etched in the single-mode optical fiber, it is called a long-period fiber grating torsion sensor.

[0059] In this embodiment, the torsion direction of the torsion sensor is determined by monitoring the direction of movement of the resonant wavelength (the direction of clockwise and counterclockwise torsion is to see whether the spectrum is red-shifted or blue-shifted, such as Figure 7 The magnitude of the torsion force is measured by calculating the spectral shift value of the torsion sensor (for every 2.18 rad / m applied, the spectrum shifts by an average of 0.42nm each time. In other words, if I see a 0.42nm shift in the spectrum, I know that 2.18 rad / m is applied).

[0060] The applicant has found that when the LPFG torsion sensor is subjected to a certain torsion, the greater the shift of the resonant wavelength, the higher the sensitivity. The shift of the resonant wavelength depends on the product of the difference between the change in the effective refractive index of the core and the change in the effective refractive index of the cladding and the change in the grating period. When torsion occurs, the change in the grating period is small enough to be ignored, the core is located at the center axis of the optical fiber and is subject to little torsion, the change in the effective refractive index is small, and the shift of the resonant wavelength mainly depends on the change in the effective refractive index of the cladding. The change in the effective refractive index of the cladding is caused by elliptical birefringence. The greater the elliptical birefringence, the greater the change in the effective refractive index of the cladding, and the elliptical birefringence is synthesized by linear birefringence and circular birefringence. Therefore, increasing the introduced linear birefringence and circular birefringence can increase the elliptical birefringence, so that the drift of the resonant wavelength during torsion increases, thereby improving the sensitivity of the torsion sensor.

[0061] In view of the above discovery of the applicant, when preparing a torsion sensor, the present invention rotates the single-mode optical fiber while etching grooves, so that the final torsion sensor has several grooves with a spiral distribution. Such spirally distributed grooves can destroy the circular symmetry of the optical fiber to the greatest extent by increasing the distribution surface of the grooves, and then perform asymmetric refractive index modulation at different azimuth angles of the torsion sensor, thereby increasing the introduced linear birefringence. At the same time, in the preparation process of the torsion sensor of the present invention, the torsion angle becomes larger and larger in the later stage of production, so that the residual stress frozen in the optical fiber is also increasing. The increase in the residual torsion force will gradually accelerate the rotation of the polarization axis of the circular birefringence, and the gradual acceleration of the rotation of the polarization axis means that the circular birefringence increases. In summary, the present invention increases the elliptical birefringence by increasing the introduced linear birefringence and circular birefringence, so that the drift of the resonant wavelength during torsion increases, thereby improving the sensitivity of the torsion sensor. And the left-handed and right-handed of the elliptical birefringence of the present invention are determined by clockwise torsion and counterclockwise torsion, so the displacement direction of the resonant wavelength is related to the torsion direction, which enables the LPFG torsion sensor of the present invention to measure the direction of the applied torsion force while improving the torsion sensitivity.

[0062] The experimental results show that when the period of the torsion sensor is 480μm, the wavelength torsion sensitivity is the highest, which is 0.42nm / rad / m, and the wavelength has different drift directions when twisted clockwise and counterclockwise. At the same time, the temperature sensitivity and strain sensitivity of the torsion sensor were tested, and its temperature sensitivity was 76pm / ℃ and the strain sensitivity was 1.3pm / με. In summary, the sensitivity of the torsion sensor designed by the present invention is more than ten times higher than that of the conventional LPFG, and it has a direction recognition function, which provides a new solution for engineering applications with high-sensitivity torsion sensors.

[0063] In order to better introduce the technical solution of the present invention, this embodiment is described through the following parts.

[0064] 1. Groove Modulation Area

[0065] In this embodiment, each group of groove modulation zones has two or four grooves. In theory, any number of grooves can be selected for the groove modulation zone, but two and four are the most stable ones we found during the production process.

[0066] Specifically, the torsion sensor corresponding to each groove modulation area having two grooves is defined as RD-LPFG; the torsion sensor corresponding to each groove modulation area having four grooves is defined as VR-LPFG.

[0067] like Figure 2As shown, when each group of groove modulation areas has two grooves, the preset angle θ is 60° (the cross-section of the optical fiber is 360°, the preset angle θ is 60°, and it can be distributed on 6 surfaces); the preset total angle is 840° (theoretically, the larger the preset total angle, the higher the sensitivity, but in actual production, the angle is too large, the mechanical strength decreases, and the optical fiber will be melted due to laser radiation).

[0068] like Figure 3 As shown in the figure, when each group of groove modulation areas has four grooves, the preset angle θ is 90° (the cross section of the optical fiber is 360°, the preset angle θ is 60°, and it can be distributed on four surfaces). The preset total angle is 630° (theoretically, the larger the preset total angle, the higher the sensitivity, but in actual production, the angle is too large, the mechanical strength decreases, and the optical fiber will be broken due to laser radiation).

[0069] 2. The rotation angle of the groove in the groove modulation area

[0070] In this embodiment, in the torsion sensor, except for the first group of groove modulation areas, adjacent grooves in each group of groove modulation areas have a rotation angle C twisted along the axis direction of the torsion sensor. Λ ; In two adjacent groups of groove modulation areas, the rotation angle C between adjacent grooves in the latter group of groove modulation areas Λ,i Greater than the rotation angle C between adjacent grooves in the previous group of groove modulation areas Λ,i-1 .

[0071] In this embodiment, an example is taken in which each group of groove modulation areas has four grooves: the single-mode optical fiber is rotated a total of 7 times (ie, the preset total angle is 630°) to obtain 8 groups of groove modulation areas, with a total of 32 grooves. The rotation angle between each groove in the first group of grooves (the 1st to the 4th) is 0°; the rotation angle between each groove in the second group of grooves (the 5th to the 8th) is 2.15°; the rotation angle between each groove in the third group of grooves (the 9th to the 12th) is 4.25°; the rotation angle between each groove in the fourth group of grooves (the 13th to the 16th) is 6.4°; the rotation angle between each groove in the fifth group of grooves (the 17th to the 20th) is 8.55°; the rotation angle between each groove in the sixth group of grooves (the 21st to the 24th) is 10.7°; the rotation angle between each groove in the seventh group of grooves (the 25th to the 28th) is 12.85°; the rotation angle between each groove in the eighth group of grooves (the 29th to the 32nd) is 14.25°.

[0072] 3. Basic Principle of Torsion Sensor Sensitization

[0073] When the specific wavelength light transmitted in the sensor core meets the phase matching condition of the long-period fiber grating, the core mode couples with the cladding mode propagating in the same direction, and the resonant wavelength can be expressed as:

[0074]

[0075] Where: m) represents the resonant wavelength; Λ represents the spacing, i.e., the grating period; denote the effective refractive index of the core mode and cladding mode respectively;

[0076] The resonant wavelength shift caused by torsion is expressed as:

[0077]

[0078] Where: Δλ (m) Indicates the amount of resonant wavelength shift; Respectively represent the changes in the effective refractive index of the core and cladding modes under torsion; ΔΛ is the change in the grating period. Because the change in the grating period is so small that it can be ignored when torsion occurs, the core is located at the center axis of the optical fiber and is subject to little torsion, the change in the effective refractive index is small, and the shift of the resonant wavelength mainly depends on the change in the effective refractive index of the cladding. The change in the effective refractive index of the cladding is mainly affected by the elliptical birefringence. The larger the elliptical birefringence, the greater the change in the effective refractive index. Elliptical birefringence is a composite of linear birefringence and circular birefringence, where linear birefringence is caused by asymmetric refractive index modulation and circular birefringence is caused by optical fiber torsion. Therefore, how to increase the synthesized elliptical birefringence during torsion is the key to improving the sensitivity of the torsion sensor.

[0079] 4. Increase circular birefringence

[0080] In this embodiment, the optical fiber is twisted multiple times and high frequency CO 2 Laser pulse exposure, after making the torsion sensor, release the torsion, and after returning the fiber to its initial state, RD-LPFG (i.e., each group of groove modulation areas has two grooves) is obtained. The final release of the torsion during the production process releases most of the torsion force, but there is still a part of the torsion force that is solidified in the fiber due to the laser radiation. The torsion force solidified in the fiber is defined as the residual torsion force, which can be expressed as:

[0081] τ 0 =2β / L 0 ;

[0082] Where: τ 0 represents the residual torsional force; β represents the initial torsional rate; L 0 Indicates the distance between the two ends of the measuring device.

[0083] The residual torsion force will introduce circular birefringence into the optical fiber. In addition, during the manufacturing process of the sensor of this embodiment, the torsion angle is small at the initial stage of manufacturing, and the residual torsion force introduced is small. The torsion angle becomes larger and larger in the later stage of manufacturing, and the residual torsion force will also continue to increase. The increase of the residual torsion force will gradually accelerate the rotation of the polarization axis of the circular birefringence. The gradual acceleration of the rotation of the polarization axis means that the circular birefringence increases.

[0084] 5. Increase Linear Birefringence

[0085] In this embodiment, the RD-LPFG is produced by a step-by-step twisting process. The gradual increase in the twist rate will cause a certain rotation distribution angle between the two grooves in each group of double groove modulation areas, and a rotation distribution angle will also be formed between each group of double grooves. The rotation distribution angle will gradually increase due to the continuous increase in the twist angle during processing. The rotation angle C between the two grooves in each group of double groove modulation areas Λ , and the torsion rate It is proportional to the interval Λ between the two grooves in each set of double-groove modulation areas.

[0086]

[0087] For traditional LPFG, CO 2 The laser performs asymmetric refractive index modulation on one side. The schematic diagram of asymmetric refractive index distribution in the optical fiber is shown in Figure 4 (a) The RD-LPFG proposed in the present invention performs asymmetric refractive index modulation at different azimuth angles. The schematic diagram of the asymmetric refractive index distribution in the optical fiber is shown in FIG. Figure 4 (b), which will increase the introduced linear birefringence.

[0088] 6. Elliptical birefringence when torsion is increased

[0089] In this embodiment, when the optical fiber is twisted, the circular birefringence introduced by the gradually increasing residual twisting force interacts with the linear birefringence introduced by the increased groove distribution surface to form elliptical birefringence. The synthetic elliptical birefringence during twisting is expressed as:

[0090]

[0091] Where: represents the elliptical birefringence vector introduced by the torsion; G represents the photoelastic coefficient; Indicates the torsion rate.

[0092] The elliptical birefringence induced by torsion is proportional to the torsion rate. The larger the torsion rate, the more intense the elliptical birefringence synthesized by the increased linear birefringence and circular birefringence in the optical fiber, and the greater the distance the RD-LPFG wavelength moves. In addition, the left-handed and right-handed elliptical birefringence are determined by clockwise and counterclockwise torsion, so the displacement direction of the resonant wavelength is related to the torsion direction. Therefore, RD-LPFG can not only improve torsion sensitivity, but also measure the direction of applied torsion force.

[0093] VII. Experimental Description

[0094] In this experiment, in order to study the effect of period Λ on the resonance peak depth and resonance wavelength of the fiber Bragg grating, a series of experimental tests were conducted. When the value of Λ is in the range of 400μm-500μm, the light of a specific wavelength in the core is efficiently coupled to the cladding, and the resonance peak is good. When it exceeds this range, too little light in the core is coupled to the cladding, the resonance peak depth is too small or even disappears, or the resonance peak does not appear in the bandwidth range of the light source spectrometer. When the RD-LPFG samples are subsequently made and the torsional sensing tests and sensitivity improvement studies are conducted, three typical Λ values ​​with good coupling are selected, namely Λ=480μm (RD-LPFG1), Λ=470μm (RD-LPFG2), and Λ=460μm (RD-LPFG3). Their spectra are shown in Figure 2. Figure 5 shown.

[0095] 1. Torsion measurement

[0096] Sensor torsion measurement experimental device Figure 6 As shown, the left and right ends of the RD-LPFG are clamped in the optical fiber rotation fixture. In order to obtain more accurate experimental measurement data, the sensing test light source uses an amplified spontaneous emission light source (ASE, NB-FP, HY). The optical signal emitted from the ASE is transmitted to the OSA through the RD-LPFG. The OSA is used to monitor the change of the resonant wavelength. In the torsion measurement, the torsion rate is defined as follows:

[0097]

[0098] Wherein: θ represents the torsion angle, and L represents the distance between the two clamps, which is 24 cm in this embodiment.

[0099] The torsion sensing test was performed on RD-LPFG1, RD-LPFG2, and RD-LPFG3 respectively. The right rotating fixture was first twisted clockwise from 0 rad / m to -13.08 rad / m, then returned to 0 rad / m, and then twisted counterclockwise from 0 rad / m to 13.08 rad / m, with each increment of 2.18 rad / m. The experimental test sensing spectra of the three probes are shown in Figure 2. Figure 7 (a)-(c) show. The relationship between the different torsion rates of the three sensors and the corresponding resonant wavelengths is fitted to Figure 7 (d), the torsional sensitivity can be obtained.

[0100] according to Figure 7 The experimental spectra of the three sensors are compared in Table 1, and the grating period, the direction of the resonant wavelength shift when twisted clockwise, the direction of the resonant wavelength shift when twisted counterclockwise, the amount of resonant wavelength shift and the sensitivity are listed. It can be seen that when twisted clockwise, the resonant wavelength is blue-shifted; when twisted counterclockwise, the resonant wavelength is red-shifted, that is, the twisting direction can be determined by the direction of the resonant wavelength shift. This is because the clockwise and counterclockwise twisting directions determine the left-handed or right-handed direction of the elliptical birefringence, which is consistent with theoretical expectations. At the same time, the larger the grating period, the higher the torsion sensing sensitivity. The RD-LPFG1 with a grating period of 480μm has a torsion sensitivity of 0.42nm / rad / m.

[0101] Table 1 Comparison of torsion test performance of three RD-LPFG sensors

[0102]

[0103] 2. Temperature measurement

[0104] Three RD-LPFGs were placed on the temperature sensing test platform. Figure 8 As shown. The platform temperature is controlled to change from 30℃ to 120℃, increasing by 10℃ each time. As the temperature increases, the resonant wavelengths of RD-LPFG1, RD-LPFG2, and RD-LPFG3 are as follows: Fig. 9 (a)-(c), the relationship between the corresponding resonant wavelength at different temperatures is fitted as follows: Fig. 9 (d). As the temperature increases, the resonant wavelength redshifts. Through linear fitting, it is found that the temperature sensitivity of RD-LPFG1 is 76pm / ℃, the temperature sensitivity of RD-LPFG2 is 82pm / ℃, and the temperature sensitivity of RD-LPFG3 is 89pm / ℃. The temperature sensitivity of RD-LPFG increases with the decrease of grating period. By comparison, it is found that the temperature sensitivity of RD-LPFG is higher than that of ordinary LPFG.

[0105] By comparing the torsion and temperature data, it is found that the RD-LPFG1 with the largest period has the highest torsion sensitivity, the smallest temperature crosstalk effect, and the greatest application potential. However, in actual applications, micro-displacements of engineering structures often occur, and there is axial strain crosstalk. It is also necessary to test the strain performance of RD-LPFG1.

[0106] 3. Strain measurement

[0107] Clamp RD-LPFG1 on the displacement table of the strain sensor test experimental device. Fig.10As shown in Figure 2, the device settings are increased from 100με to 700με by 100με each time. The sensitivity obtained by the change of the resonant wavelength and the linear fitting is shown in Figure 2. Fig.11 As shown in the figure, the resonance wavelength red-shifts with the increase of strain, and the strain sensitivity is calculated to be 1.3pm / με. The strain sensitivity is similar to that of ordinary long-period fiber gratings.

[0108] 4. Discussion

[0109] The rotating distributed groove structure proposed in the present invention realizes further sensitivity enhancement of the LPFG torsion sensor. When the detection range of the sensor is -13.08-13.08rad / m, the torsion sensitivity of the sensor reaches up to 0.42nm / rad / m. By increasing the modulation surface of the asymmetric refractive index to destroy the circular symmetry of the optical fiber, the introduced linear birefringence can be increased, and at the same time, the unilateral torsion force is gradually increased, so that the residual torsion force is fixed in the optical fiber to increase the circular birefringence, which can increase the synthesized elliptical birefringence during torsion, increase the drift of the resonant wavelength during torsion, and further improve the torsion sensitivity of the sensor.

[0110] In the experiment, it was found that the change of the period Λ of RD-LPFG can lead to different torsion sensitivity. This is because the reduction of the period will affect the asymmetric refractive index modulation area and the residual torsion force fixed in the optical fiber, resulting in a decrease in elliptical birefringence, thereby affecting the change of the effective refractive index of the cladding during torsion, which has a certain impact on the torsion sensitivity. In addition, the temperature sensitivity of the structure was tested, and it was found that the temperature sensitivity is higher than that of ordinary long-period fiber gratings, and there is a certain temperature crosstalk. The temperature sensitivity usually depends on the thermo-optical coefficient of the core and cladding. In this structure, the residual torsion force introduces circular birefringence, and the temperature is also related to the size of the circular birefringence in the structure. The strain sensitivity of the structure was also tested, and it was found that the strain sensitivity of the structure is similar to that of ordinary long-period fiber gratings, and the influence of axial strain on torsion measurement can be ignored.

[0111] The relevant research on LPFG torsion sensor sensitivity enhancement in recent years was investigated, and the torsion sensitivity and manufacturing method were compared with the present invention. The results are shown in Table 2.

[0112] Table 2 Performance comparison of LPFG torsional sensing enhancement scheme

[0113]

[0114]

[0115] References:

[0116] [1]WANG Y-P,CHEN J-P,RAO Y-J.Torsion characteristics of long-periodfiber gratings induced by high-frequency CO2 laser pulses[J].Journal of theOptical Society of America B,2005,22(6).

[0117] [2]LIU W,DUAN S,DU H,et al.A new ultra long-period fiber grating formeasuring torsional characteristics[J].Journal of Modern Optics,2019,66(11):1215-8.

[0118] [3]SUN C,GENG T,HE J,et al.High Sensitive Directional Torsion SensorBased on a Segmented Long-Period Fiber Grating[J].IEEE Photonics TechnologyLetters,2017,29(24):2179-82.

[0119] [4]WANG Y.Review of long period fiber gratings written by CO2 laser[J].Journal of Applied Physics,2010,108(8).

[0120] [5]JIN X,XU X,LV M,et al.Analysis and Comparative Study ofIntermittent-Spiral Long Period Fiber Grating[J].IEEE Photonics TechnologyLetters,2022,34(8):440-3.

[0121] [6]SUN C,WANG R,JIN

[0122] Compared with other LPFG torsion sensors, the rotation distributed slot sensor proposed in the present invention can achieve higher torsion sensitivity. This embodiment proposes and verifies that adding an asymmetric refractive index modulation surface and combining a gradually increasing residual torsion force can increase torsion sensitivity. This provides a new idea for improving the detection sensitivity of LPFG torsion sensors.

[0123] Embodiment 2:

[0124] This embodiment discloses a rotating distributed slot type torsion sensor.

[0125] A rotating distributed slot type torsion sensor is prepared by the method for preparing the rotating distributed slot type torsion sensor described in embodiment one.

[0126] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. A method for preparing a rotating distributed slot torsion sensor, characterized in that: include: S1: Get a single-mode optical fiber and clamp both ends of the single-mode optical fiber; S2: etching a group of groove modulation regions on the single-mode optical fiber; a group of groove modulation regions includes a plurality of grooves with a spacing of Λ; S3: Fix one end of the single-mode optical fiber and twist the other end of the single-mode optical fiber along its axis by a preset angle θ; S4: etching a group of groove modulation regions on the single-mode optical fiber after one end is rotated; S5: repeating steps S3 and S4 until the total angle of rotation of the single-mode optical fiber reaches a preset total angle, and the etching of the last group of groove modulation areas is completed; When preparing the torsion sensor, the single-mode optical fiber is rotated while etching grooves, so that the torsion sensor has a plurality of grooves with a spiral distribution. The spirally distributed grooves destroy the circular symmetry of the optical fiber to the greatest extent by increasing the distribution surface of the grooves, and perform asymmetric refractive index modulation at different azimuth angles of the torsion sensor. In the process of preparing the torsion sensor, the torsion angle becomes larger and larger, which makes the residual stress frozen in the optical fiber increase continuously. The increase of residual torsion force will gradually accelerate the rotation of the polarization axis of circular birefringence. The rotation of the polarization axis gradually accelerates, which increases the circular birefringence. S6: Twist one end of the twisted single-mode optical fiber in the opposite direction along its axis to restore it to its initial state, thereby obtaining a torsion sensor having a plurality of grooves in a spiral distribution.

2. The method for preparing the rotating distributed slot torsion sensor according to claim 1, characterized in that: In step S2, each group of groove modulation areas has two or four grooves.

3. The method for preparing the rotating distributed slot torsion sensor according to claim 2, characterized in that: In step S3, when each group of groove modulation areas has two grooves, the preset angle θ is 60°; When each group of groove modulation areas has four grooves, the preset angle θ is 90°.

4. The method for preparing the rotating distributed slot torsion sensor according to claim 2, characterized in that: In step S5, when each group of groove modulation areas has two grooves, the preset total angle is 840°; When each group of groove modulation areas has four grooves, the preset total angle is 630°.

5. The method for preparing the rotating distributed slot torsion sensor according to claim 1, characterized in that: In step S6, in the torsion sensor, except for the first group of groove modulation areas, adjacent grooves in each group of groove modulation areas have a rotation angle C twisted along the axis direction of the torsion sensor. Λ .

6. The method for preparing the rotating distributed slot torsion sensor according to claim 5, characterized in that: In step S6, in two adjacent groups of groove modulation areas, the rotation angle C between adjacent grooves in the latter group of groove modulation areas is Λ,i Greater than the rotation angle C between adjacent grooves in the previous group of groove modulation areas Λ,i-1 .

7. The method for preparing the rotating distributed slot torsion sensor according to claim 1, characterized in that: In step S6, after the twisted end of the single-mode optical fiber is reversely twisted along its axis to restore to the initial state, a portion of the residual torsional force is solidified in the torsion sensor; The calculation formula of residual torsional force is expressed as: τ0=2β / L0; Where: τ0 represents the residual torsional force; β represents the initial torsional rate; L0 represents the distance between the two ends of the single-mode optical fiber.

8. The method for preparing the rotating distributed slot torsion sensor according to claim 1, characterized in that: In step S6, the torsion direction of the torsion sensor is determined by monitoring the moving direction of the resonant wavelength of the torsion sensor.

9. The method for preparing the rotating distributed slot torsion sensor according to claim 1, characterized in that: In step S6, the magnitude of the torsion force is measured by calculating the spectral displacement value of the torsion sensor.

10. A rotating distributed slot torsion sensor, characterized in that: The torsion sensor is prepared based on the preparation method of the rotating distributed groove torsion sensor described in claim 1.

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