LPFG torsion sensor preparation method based on rotary single-mode fiber and sensor

Abstract

The invention discloses a preparation method of an LPFG torsion sensor based on a rotary single-mode optical fiber and the sensor. The method comprises the following steps: heating a single-mode optical fiber through oxyhydrogen flame, and driving two ends of the single-mode optical fiber to rotate clockwise and anticlockwise respectively by a rotary clamp; obtaining a rotary single-mode optical fiber after a preset time is reached; etching a first group of groove modulation areas on the rotating single-mode optical fiber; controlling the two ends of the rotating single-mode fiber to rotate by a preset angle at the same time, and etching a second group of groove modulation areas on the rotating single-mode fiber; and controlling the two ends of the rotary single-mode fiber to reversely rotate so as to restore to an initial state, thereby obtaining the LPFG torsion sensor. The rotary single-mode optical fiber is prepared through oxyhydrogen flame heating and rotation, and a large amount of twisting force is cured; and meanwhile, the rotary single-mode fiber is rotated and the groove is etched, so that the introduced linear birefringence and circular birefringence are increased, the elliptical birefringence is further increased, the drift distance of the resonant wavelength during torsion is increased, and the torsion sensitivity of the LPFG torsion sensor is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensors, and in particular to a method for preparing an LPFG torsion sensor based on a rotating single-mode optical fiber and a sensor. Background Art

[0002] In recent years, optical fiber sensors have been widely studied by the academic community. Optical fiber-based sensors mainly include fiber interferometers (MZI), fiber Bragg gratings (FBG), and long-period fiber gratings (LPFG). Among them, LPFG has the advantages of simple structure, easy production, and good repeatability. It is widely used in the measurement of physical parameters such as temperature, refractive index, bending, strain, and torsion.

[0003] Torsion sensors based on LPFG have been widely used in bridge structure health monitoring. When a vehicle passes on one side, the bridge will twist slightly. At this time, the health of the bridge structure can be evaluated by observing the amplitude of the change in the resonant wavelength caused by the twisting. The amplitude of the change in the resonant wavelength represents the torsional sensitivity. At present, the torsional sensitivity based on LPFG needs to be further improved.

[0004] In order to improve the sensitivity of torsion sensing, many research teams have proposed production methods and special structures based on LPFG torsion sensors. For example, one scheme proposes to use a femtosecond laser to make grooves on one side of a single-mode optical fiber to produce LPFG. The torsion sensing sensitivity of this scheme can reach up to 181.7pm (rad / m). One scheme proposes to make segmented grooves on different reference planes to produce segmented-LPFG. This scheme introduces linear birefringence by making grooves on different reference planes of the single-mode optical fiber. The torsion sensing sensitivity can reach up to 0.3nm / (rad / m). However, the production ideas of the above schemes are to modulate the refractive index on one or more sides of the traditional single-mode optical fiber. Although the effect of improving sensitivity is achieved, the torsional force is not effectively introduced into the single-mode optical fiber.

[0005] In order to solve the problem of introducing twisting force, a proposal was made to use a fiber fusion splicer to twist single-mode optical fiber to prepare a spiral LPFG twist sensor, with a twist sensitivity of 0.085nm / (rad / m). A proposal was made to use a Fujik-ura fusion splicer to spirally process single-mode optical fiber to produce H-LPFG, with a twist sensitivity of up to 0.138nm / (rad / m). The above proposals all use a fusion splicer to discharge and solidify the twisting force, and introduce circular birefringence by solidifying the twisting force, thereby achieving the effect of improving sensitivity, but fail to increase linear birefringence.

[0006] To address the problem of increasing linear birefringence, a solution is proposed to twist the traditional single-mode fiber three times and then use CO 2The laser is used for side polishing, and the twist is released after the production is completed to form a side spiral polishing structure. The sensor sensitivity produced by this scheme can reach up to 0.199nm / (rad / m). There is a scheme that proposes an intermittent spiral structure with step-by-step twisting. The sensor sensitivity produced by this scheme can reach up to 0.15nm / (rad / m). There is a scheme that proposes to pre-twist the traditional single-mode optical fiber 540° and then use CO 2 The laser cuts a V-groove to form a spiral-like structure. The sensor produced by this scheme can have a maximum sensitivity of 0.654nm / (rad / m). 2 Laser grooves are used to fix the twisting force and increase the linear birefringence. However, although the above scheme achieves the simultaneous introduction of linear birefringence and twisting force, due to the use of welding machines or CO 2 Lasers, these machines modulate the refractive index of a portion of the optical fiber, and the curing twisting force is not strong enough. 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 an LPFG torsion sensor based on a rotated single-mode optical fiber, wherein the rotated single-mode optical fiber is prepared by heating and rotating with a hydrogen-oxygen flame, thereby solidifying a large amount of twisting force; at the same time, the rotated single-mode optical fiber is rotated and grooves are etched to increase the introduced linear birefringence and circular birefringence, thereby increasing the elliptical birefringence, thereby increasing the drift of the resonant wavelength during torsion, and thus improving the torsion sensitivity of the LPFG torsion sensor.

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

[0009] The invention relates to a method for preparing an LPFG torsion sensor based on a rotating single-mode optical fiber, comprising:

[0010] S1: Obtain a single-mode optical fiber and fix both ends of the single-mode optical fiber by a rotating clamp;

[0011] S2: The single-mode optical fiber is heated by a hydrogen-oxygen flame, and at the same time, the two ends of the single-mode optical fiber are driven to rotate clockwise and counterclockwise respectively by a rotating fixture;

[0012] S3: After the preset time is reached, the hydrogen-oxygen flame heating is stopped and both ends of the single-mode optical fiber are released to obtain a rotated single-mode optical fiber;

[0013] S4: Clamping two ends of the rotated single-mode optical fiber, and etching a first group of groove modulation areas on the rotated single-mode optical fiber; a group of groove modulation areas includes a plurality of grooves;

[0014] S5: controlling both ends of the rotated single-mode optical fiber to rotate at a preset angle simultaneously, and etching a second group of groove modulation areas on the rotated single-mode optical fiber;

[0015] S6: Control the two ends of the rotated single-mode optical fiber to rotate in the opposite direction to restore to the initial state, and obtain an LPFG torsion sensor with a plurality of grooves.

[0016] Preferably, in step S2, the flame width of the hydrogen-oxygen flame is set to 1 cm-2 cm, and the hydrogen flow rate is set to 180 SCCM-200 SCCM.

[0017] Preferably, in step S2, the rotation speed of the single-mode optical fiber driven by the rotating fixture is set to 10000 μm / s-11000 μm / s.

[0018] Preferably, in step S3, the preset time is 4s-6s.

[0019] Preferably, in step S3, there is residual twisting force in the rotated single-mode optical fiber due to solidification by heating with hydrogen-oxygen flame;

[0020] The formula for the residual torsional force is expressed as:

[0021] R 0 =2τ / L 0 ;

[0022] Where: R 0 represents the residual twisting force; τ represents the initial twisting force; L 0 Indicates the distance between the two ends of a single-mode optical fiber when it is clamped.

[0023] Preferably, in step S4, CO 2 The laser etches grooves in the spun single-mode fiber;

[0024] CO 2 The power of the laser was set to 38%-40% and the maximum output power was set to 30w.

[0025] Preferably, in step S4, a group of groove modulation areas includes 15 grooves with a width of 300 μm, and the interval between two grooves is 500 μm.

[0026] Preferably, in step S5, the preset angle is set to 10°-350°.

[0027] Preferably, in step S7, the resonant wavelength of the LPFG torsion sensor is expressed as:

[0028]

[0029] Where: represents the resonant wavelength of the LPFG torsion sensor; represents the initial wavelength; It represents the effective refractive index of the fiber core; represents the effective refractive index of the cladding; Λ represents the grating period; denote the effective refractive indices of the grating core and cladding modes, respectively.

[0030] The present invention also discloses an LPFG torsion sensor, which is an LPFG torsion sensor prepared by the LPFG torsion sensor preparation method based on a rotating single-mode optical fiber in the present invention.

[0031] Compared with the prior art, the LPFG torsion sensor preparation method based on the rotation single-mode optical fiber in the present invention has the following beneficial effects:

[0032] In the process of manufacturing the rotated single-mode optical fiber, the present invention heats the single-mode optical fiber by rapidly twisting it through a hydrogen-oxygen flame, and applies a large amount of helical twisting force to the optical fiber. Although the optical fiber is released back to its original state after the manufacturing is completed, part of the twisting force is released, but most of the twisting force is solidified in the optical fiber due to the high-temperature heating, that is, a large amount of residual twisting force is solidified in the rotated single-mode optical fiber. The residual twisting force will generate a large amount of circular birefringence.

[0033] The present invention prepares an LPFG torsion sensor by etching grooves at different azimuth angles of the rotated single-mode optical fiber on the basis of preparing the rotated single-mode optical fiber. This etching method can 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, thereby increasing the introduced linear birefringence. When the optical fiber is twisted, the increased linear birefringence and the circular birefringence interact to synthesize the elliptical birefringence, that is, the elliptical birefringence is increased, so that the drift of the resonant wavelength during twisting is increased, thereby improving the torsion sensitivity of the subsequent LPFG torsion sensor.

[0034] In summary, the present invention prepares a rotated single-mode optical fiber by heating and rotating with a hydrogen-oxygen flame, solidifies a large amount of twisting force; at the same time, the rotated single-mode optical fiber is rotated and grooves are etched to increase the introduced linear birefringence and circular birefringence, thereby increasing the elliptical birefringence, so that the drift of the resonant wavelength during twisting increases, thereby improving the twist sensitivity of the LPFG twist sensor. At the same time, the twisting direction can be judged by the resonant wavelength shift and change amplitude of the LPFG twist sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] 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:

[0036] Figure 1 It is a schematic diagram of the process of preparing a LPFG torsion sensor based on a rotated single-mode optical fiber;

[0037] Figure 2 (a) is a physical picture of a rotated single-mode optical fiber. Figure 2(b) is a physical picture of R-LPFG;

[0038] Figure 3 (a) is the spectrum of the rotated single-mode fiber. Figure 3 (b) is the spectrum of R-LPFG;

[0039] Figure 4 is the transmission spectrum of multiple R-LPFGs with the same parameters;

[0040] Figure 5 Experimental setup for torsion measurement

[0041] Figure 6 (a) is the clockwise twist spectrum of R-LPFG1. Figure 6 (b) is the counterclockwise twist spectrum of R-LPFG1; Figure 6 (c) is the fitting diagram of the relationship between the clockwise torsion rate and the resonance wavelength of R-LPFG. Figure 6 (d) is the fitting diagram of the relationship between the counterclockwise twist rate and the resonance wavelength of R-LPFG;

[0042] Figure 7 Experimental device for temperature sensing test;

[0043] Figure 8 (a) is the spectrum of R-LPFG1, Figure 8 (b) is the fitting diagram of the relationship between temperature and resonance wavelength. DETAILED DESCRIPTION

[0044] 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.

[0045] The following is a further detailed description through specific implementation methods:

[0046] Embodiment 1:

[0047] This embodiment discloses a method for preparing an LPFG torsion sensor based on a rotated single-mode optical fiber.

[0048] like Figure 1As shown, the method for preparing the LPFG torsion sensor based on the rotation single-mode optical fiber comprises:

[0049] S1: Obtain a single-mode optical fiber (SMF28, Corning), and clamp and fix both ends of the single-mode optical fiber by a rotating fixture (the rotating fixture here is electrically controlled);

[0050] In this embodiment, the coating layer of the single-mode optical fiber needs to be stripped off first.

[0051] S2: The single-mode optical fiber is heated by an oxyhydrogen flame (the optical fiber softens when heated by the oxyhydrogen flame), and at the same time, the two ends of the single-mode optical fiber are driven by a rotating fixture to rotate clockwise and counterclockwise respectively (i.e., one end rotates clockwise and the other end rotates counterclockwise);

[0052] S3: After the preset time is reached, the hydrogen-oxygen flame heating is stopped and both ends of the single-mode optical fiber are released to obtain a rotating single-mode optical fiber;

[0053] In this embodiment, after reaching the preset time T, the rotation of the rotating fixture will apply a large amount of spiral torsional force to the optical fiber, and the appearance of the optical fiber will appear slightly twisted, such as Figure 2 (a) As shown. The left end of the rotated single-mode fiber is connected to a supercontinuum light source (SLS, NKT, Superk Compact), and the right end is connected to an optical spectrum analyzer (OSA, AQ6370D, YOKOGAWA). At this time, the spectrum after light is still displayed as a traditional single-mode fiber, as shown in Figure 3 (a) shown.

[0054] S4: clamping both ends of the rotating single-mode optical fiber by a rotatable optical fiber clamp (the rotatable optical fiber clamp is manually controlled here), and etching a first group of groove modulation areas on the rotating single-mode optical fiber; a group of groove modulation areas includes a plurality of grooves;

[0055] In this embodiment, the rotating single-mode optical fiber is fixed on the laser pulse focus by a rotatable optical fiber clamp (HFR007, THORLABS), the left end of the optical fiber is connected to a supercontinuum light source, and the right end is connected to a spectrum analyzer for real-time monitoring of the wavelength position and resonance peak loss.

[0056] S5: controlling the two ends of the rotating single-mode optical fiber to rotate at a preset angle simultaneously by using a rotatable optical fiber clamp, and etching a second group of groove modulation areas on the rotating single-mode optical fiber;

[0057] S6: Control the two ends of the rotated single-mode optical fiber to rotate in the opposite direction to restore to the initial state, and obtain an LPFG torsion sensor with a plurality of grooves (hereinafter also referred to as R-LPFG torsion sensor or R-LPFG).

[0058] In this embodiment, Figure 2 As shown in (b), the spectrum of the prepared LPFG torsion sensor shows a resonance peak after light is passed through it. The structure of the LPFG torsion sensor is as follows: Figure 3 (b) as shown.

[0059] 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.

[0060] In the process of manufacturing the rotated single-mode optical fiber, the present invention heats the single-mode optical fiber by rapidly twisting it through a hydrogen-oxygen flame, and applies a large amount of helical twisting force to the optical fiber. Although the optical fiber is released back to its original state after the manufacturing is completed, part of the twisting force is released, but most of the twisting force is solidified in the optical fiber due to the high-temperature heating, that is, a large amount of residual twisting force is solidified in the rotated single-mode optical fiber. The residual twisting force will generate a large amount of circular birefringence.

[0061] The present invention prepares an LPFG torsion sensor by etching grooves at different azimuth angles of the rotated single-mode optical fiber on the basis of preparing the rotated single-mode optical fiber. This etching method can 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, thereby increasing the introduced linear birefringence. When the optical fiber is twisted, the increased linear birefringence and the circular birefringence interact to synthesize the elliptical birefringence, that is, the elliptical birefringence is increased, so that the drift of the resonant wavelength during twisting is increased, thereby improving the torsion sensitivity of the subsequent LPFG torsion sensor.

[0062] In summary, the present invention prepares a rotated single-mode optical fiber by heating and rotating with a hydrogen-oxygen flame, solidifies a large amount of twisting force; at the same time, the rotated single-mode optical fiber is rotated and grooves are etched to increase the introduced linear birefringence and circular birefringence, thereby increasing the elliptical birefringence, so that the drift of the resonant wavelength during twisting increases, thereby improving the twist sensitivity of the LPFG twist sensor. At the same time, the twisting direction can be judged by the resonant wavelength shift and change amplitude of the LPFG twist sensor.

[0063] The experimental results show that the torsional sensitivity of the sensor is 0.527nm / rad / m from -8.72rad / m to 0rad / m, and 0.733nm / rad / m from 0rad / m to 8.72rad / m. In addition, the temperature sensitivity of the sensor is 75pm / ℃. The torsional sensitivity of the sensor proposed in the present invention is 20 times higher than that of the conventional LPFG. The sensor provides a new solution in the fields of bridge health monitoring.

[0064] In order to better introduce the technical solution of the present invention, this embodiment is described through the following parts.

[0065] 1. Sensor Manufacturing

[0066] In this embodiment, the flame width of the hydrogen-oxygen flame is set to 1 cm-2 cm, and the hydrogen flow rate is set to 180 SCCM-200 SCCM.

[0067] In this embodiment, the rotation speed of the rotating fixture driving the single-mode optical fiber to rotate is set to 10000 μm / s-11000 μm / s. The stretching speed of the rotating fixture is set to 0 μm / sec.

[0068] In this embodiment, the preset time is 4s-6s. Through experiments, the selection of the preset time T was studied many times, and it was found that when the twisting time exceeded 4s, the spectrum analyzer showed an interference spectrum. This is because the flame heating twisting time was too long, and the optical fiber was significantly twisted, causing the light in the core to be coupled to the cladding, and then returned to the core after being transmitted over a distance, resulting in interference. When the twisting time is less than 4s, the fixed spiral twisting force in the optical fiber is too small, and the torsion sensitivity in the subsequent torsion test experiment is not significantly improved. In the subsequent sensor sample production, heating twisting for 4s was selected.

[0069] In this embodiment, CO 2 Laser (MC-EB, GD Hans Yueming laser) etches grooves on the spun single-mode fiber; CO 2 The power of the laser was set to 38%-40% and the maximum output power was set to 30w.

[0070] In this embodiment, a group of groove modulation areas includes 15 grooves with a width of 300 μm, and the interval between two grooves is 500 μm (Λ=500 μm).

[0071] In this embodiment, the preset angle is set to 10°-350° (preferably 90°). The setting logic of the preset angle is: as long as it is not on the same horizontal line as the original group, it is fine, because not being on the same horizontal line can increase the sensor line birefringence.

[0072] In this embodiment, in order to verify the repeatability of the sensor, multiple R-LPFGs with the same parameters were prepared using the same method. The spectra are shown in Figure 4 The resonance peaks are not consistent in depth, which is due to changes in physical factors such as external temperature or humidity, resulting in CO 2 There is an error in the depth of the laser-etched grooves.

[0073] 2. Sensor Principle

[0074] When light of a specific wavelength is transmitted into the R-LPFG, the traditional phase matching conditions are met, and the core mode couples with the cladding mode in the same transmission direction. According to the mode coupling theory, the resonant wavelength of the R-LPFG can be expressed as:

[0075]

[0076] Where: represents the resonant wavelength of the LPFG torsion sensor; represents the initial wavelength; It represents the effective refractive index of the fiber core; represents the effective refractive index of the cladding; Λ represents the grating period; denote the effective refractive indices of the grating core and cladding modes, respectively.

[0077] From the above formula, we can see that Resonance wavelength In single-mode optical fiber, since the diameter of the cladding is much larger than that of the core, the shear stress on the core is very small relative to the cladding, and the refractive index change affected by the shear stress is extremely small. The change of can be ignored. So the change of the resonant wavelength mainly depends on change. The change is affected by elliptical birefringence. The larger the elliptical birefringence, the greater the change in the resonance wavelength. Elliptical birefringence is a synthesis of linear birefringence and circular birefringence when the optical fiber is twisted. The linear birefringence is caused by asymmetric refractive index modulation, and the circular birefringence is caused by optical fiber twisting.

[0078] In the process of making a single-mode fiber, the single-mode fiber is twisted quickly by heating with a hydrogen-oxygen flame, exerting a large amount of helical twisting force on the fiber. After the production is completed, the fiber is released back to its original state. Although part of the twisting force is released, most of the twisting force is solidified in the fiber due to high temperature heating. The twisting force solidified in the fiber is the residual twisting force, which is expressed as:

[0079] R 0 =2τ / L 0 ;

[0080] Where: R0 represents the residual twisting force; τ represents the initial twisting force; L 0 Indicates the distance between the two ends of a single-mode optical fiber when it is clamped. The initial twisting force τ can be calculated using the following logic: When the preset time is set to 4s, the single-mode optical fiber will rotate 4 circles (defined as one circle in 1s), and the initial twisting force = 360°*4.

[0081] The residual twisting force in the optical fiber will produce a large amount of circular birefringence. The magnitude of the residual twisting force is linearly related to the time of twisting the oxyhydrogen flame fixture. 2 The laser etches grooves at different azimuth angles in the rotating single-mode optical fiber, further destroying the circular symmetry of the optical fiber and increasing the linear birefringence in the optical fiber. When the optical fiber is twisted, the increased circular birefringence interacts with the linear birefringence to synthesize elliptical birefringence. It can be expressed as:

[0082]

[0083] in, is the elliptical birefringence vector, G is the elastic-optical coefficient of the optical fiber, is the distortion rate.

[0084] and When the sensor has no residual distortion or the residual distortion is very small, The left and right rotation of the sensor are determined by clockwise and counterclockwise twisting, and the direction of the resonant wavelength movement is related to the twisting direction. However, due to the rapid twisting of the hydrogen-oxygen flame heating, a large amount of residual twisting force exists in the entire sensor. When light of a certain wavelength enters, it has already been left-handed or right-handed, and applying a twisting force in the opposite direction fails to offset the residual twisting force. The direction of rotation changes. At this time, the resonant wavelength is independent of the twisting direction.

[0085] 3. Experimental Description

[0086] The experimental device for testing the torsional performance of R-LPFG is as follows: Figure 5 As shown, the R-LPFG is fixed in two rotating fixtures, and the distance between the fixtures is 24cm, defined as L. In order to obtain more accurate measurement data, the light source uses an amplified spontaneous emission light source (ASE, NB-FP, HY) with a central band of 1310nm. The optical signal emitted from the ASE is transmitted to the OSA through the R-LPFG. The twist rate is applied to the structure by controlling the fixture, and the spectrum changes are observed and recorded. The twist rate is defined as:

[0087]

[0088] Where θ is the twist angle and L is defined. During the measurement, the twist first changes clockwise from 0 rad / m to 8.72 rad / m. As the twist rate changes, the resonant wavelength redshifts from 1278.8 nm to 1285 nm, as shown in Figure 6 (a). Then it returns to 0 rad / m, and then twists counterclockwise from 0 rad / m to -8.72 rad / m. As the twist rate changes, the resonant wavelength redshifts from 1278.4 nm to 1282.5 nm, as shown in Figure 6 (b) As shown in the figure, it can be found that both clockwise and counterclockwise torsion show red shift, and the amount of movement is different. This is because the sensor is heated by hydrogen-oxygen flame and twisted quickly, and a large amount of residual twisting force is solidified in the sensor, resulting in different elliptical birefringence synthesized in the optical fiber when the sensor is twisted clockwise and counterclockwise. At the same time, the rotation direction of the elliptical birefringence cannot be changed when twisted in the opposite direction. In addition, due to the two important effects of elliptical birefringence on the torsion characteristics of LPFG, one is that the resonant wavelength will be offset by the change of the effective refractive index of the cladding, corresponding to the difference in the resonant wavelength in the transmission spectrum; the other is that the change in the effective refractive index of the cladding will cause the amplitude of the resonant wavelength to change accordingly. As a result, the amplitude of change of the resonant wavelength of the sensor in the clockwise and counterclockwise twisting process is inconsistent. The sensor can judge the torsion direction by combining the difference in the amount of movement and the amplitude of change when twisting clockwise and counterclockwise.

[0089] In order to test the repeatability of the sensor, R-LPFG1, R-LPFG2, R-LPFG3, and R-LPFG4 prepared with the same parameters were subjected to the same torsion test, and the relationship between the different torsion rates of the sensor and the corresponding resonance wavelength was fitted to Figure 6 (c)-(d). As can be seen from the figure, the sensitivity is 0.733nm(rad / m), 0.642nm(rad / m), 0.694nm(rad / m), 0.699nm(rad / m) when twisting clockwise, and 0.477nm(rad / m), 0.444nm(rad / m), 0.527nm(rad / m), 0.467nm(rad / m) when twisting counterclockwise. The slight fluctuation in sensitivity in the repeatability test experiment is within the normal error range. The main factors are the machine processing error during the manufacturing process and the human error in the torsion test process. Since the sensing area has not undergone optical precision processing, it has high mechanical strength and is not easy to damage, so the sensor has good repeatability.

[0090] In actual sensor applications, the external temperature will keep changing. The sensor will usually move its resonant wavelength due to temperature changes. Therefore, the crosstalk problem caused by temperature is a point that cannot be ignored for optical fiber sensors. It is also necessary to test the temperature performance of the sensor. The R-LPFG with the highest torsion sensitivity is placed on the temperature sensing test platform. Figure 7 The platform temperature was controlled to change from 30℃ to 120℃, increasing by 10℃ each time. As the temperature increases, the resonant wavelength of R-LPFG1 is shown in Figure 8 (a), the relationship between the corresponding resonant wavelength at different temperatures is fitted as follows Figure 8 (b). As the temperature increases, the resonant wavelength redshifts, and the temperature sensitivity of R-LPFG1 is 75pm / ℃ through linear fitting. The temperature crosstalk of R-LPFG1 is calculated to be 0.1 (rad / m) / ℃. The temperature sensitivity usually depends on the core and cladding thermo-optic coefficients, but in the sensor of the present invention, the circular birefringence is introduced due to the residual twisting force, and the temperature is also related to the size of the circular birefringence in the structure.

[0091] IV. Conclusion

[0092] The R-LPFG torsion sensor based on rotating single-mode optical fiber proposed in the present invention achieves a further improvement in torsion sensitivity. The sensor has a torsion sensitivity of up to 0.733nm / rad / m in the detection range of -8.72 to 8.72rad / m. First, the single-mode optical fiber is heated and rotated rapidly using a hydrogen-oxygen flame, thereby greatly increasing the residual torsion force in the optical fiber, and a large amount of circular birefringence is introduced through the residual torsion force. Then, CO 2 The laser destroys the circular symmetry of the optical fiber and introduces linear birefringence. This greatly increases the elliptical birefringence synthesized during torsion detection, increases the shift of the resonant wavelength during torsion, and further improves the sensitivity of the torsion sensor.

[0093] In the experiment, multiple sensors were prepared with the same parameters to verify the repeatability of the sensors. Due to the errors in machine processing during the manufacturing process and the human errors in the torsion test process, the torsion sensitivity of the sensors was different. In addition, the movement and change amplitude of the resonance wavelength were different during clockwise and counterclockwise torsion detection. This is because there is a large amount of torsional force in the optical fiber, which causes different changes in the elliptical birefringence in the optical fiber during clockwise and counterclockwise torsion.

[0094] In order to evaluate the performance of the sensor of the present invention, a comparison was made with other references in terms of fabrication method and torsional sensitivity, as shown in Table 1.

[0095] Table 1 Performance comparison between the sensor of the present invention and related sensors

[0096]

[0097]

[0098] References:

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[0102] [4] DONG X, XIE Z, SONG Y, et al. Highly sensitive torsion sensor based on long period fiber grating fabricated by femtosecond laser pulses[J]. Optics & Laser Technology, 2017, 97: 248 - 53.

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[0105] [7]SUN C,WANG R,JIN

[0106] Compared with other torsion sensors, the torsion sensor based on rotating single-mode optical fiber in the present invention can achieve higher torsion sensitivity. The present invention proposes and verifies that the use of hydrogen-oxygen flame to heat and quickly twist single-mode optical fiber can greatly increase the torsional force in the optical fiber, thereby increasing the torsion sensitivity. This provides a new idea for improving the sensitivity of LPFG torsion sensors and a new torsion sensor for bridge structure health detection.

[0107] Embodiment 2:

[0108] This embodiment discloses a LPFG torsion sensor.

[0109] A LPFG torsion sensor is an LPFG torsion sensor prepared according to a method for preparing an LPFG torsion sensor based on a rotating single-mode optical fiber according to an embodiment.

[0110] 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 LPFG torsion sensor based on a rotating single-mode optical fiber, characterized in that: include: S1: Obtain a single-mode optical fiber and fix both ends of the single-mode optical fiber by a rotating clamp; S2: The single-mode optical fiber is heated by a hydrogen-oxygen flame, and at the same time, the two ends of the single-mode optical fiber are driven to rotate clockwise and counterclockwise respectively by a rotating fixture; S3: After the preset time is reached, the hydrogen-oxygen flame heating is stopped and both ends of the single-mode optical fiber are released to obtain a rotated single-mode optical fiber; S4: Clamping two ends of the rotated single-mode optical fiber, and etching a first group of groove modulation areas on the rotated single-mode optical fiber; a group of groove modulation areas includes a plurality of grooves; S5: controlling both ends of the rotated single-mode optical fiber to rotate at a preset angle simultaneously, and etching a second group of groove modulation areas on the rotated single-mode optical fiber; S6: Control the two ends of the rotated single-mode optical fiber to rotate in the opposite direction to restore to the initial state, and obtain an LPFG torsion sensor with a plurality of grooves.

2. The method for preparing a LPFG torsion sensor based on a rotated single-mode optical fiber according to claim 1, characterized in that: In step S2, the flame width of the hydrogen-oxygen flame is set to 1 cm-2 cm, and the hydrogen flow rate is set to 180 SCCM-200 SCCM.

3. The method for preparing a LPFG torsion sensor based on a rotated single-mode optical fiber according to claim 1, characterized in that: In step S2, the rotation speed of the single-mode optical fiber driven by the rotating fixture is set to 10000 μm / s-11000 μm / s.

4. The method for preparing a LPFG torsion sensor based on a rotated single-mode optical fiber according to claim 1, characterized in that: In step S3, the preset time is 4s-6s.

5. The method for preparing a LPFG torsion sensor based on a rotated single-mode optical fiber according to claim 1, characterized in that: In step S3, there is residual twisting force in the rotated single-mode optical fiber solidified by heating with hydrogen-oxygen flame; The formula for the residual torsional force is expressed as: R0=2τ / L0; Where: R0 represents the residual twisting force; τ represents the initial twisting force; L0 represents the distance between the two ends of the single-mode optical fiber when it is clamped.

6. The method for preparing a LPFG torsion sensor based on a rotated single-mode optical fiber according to claim 1, characterized in that: In step S4, a groove is etched on the rotating single-mode optical fiber by a CO2 laser; The power of the CO2 laser was set to 38%-40% and the maximum output power was set to 30w.

7. The method for preparing a LPFG torsion sensor based on a rotated single-mode optical fiber according to claim 1, characterized in that: In step S4, a group of groove modulation areas includes 15 grooves with a width of 300 μm, and the interval between two grooves is 500 μm.

8. The method for preparing a LPFG torsion sensor based on a rotated single-mode optical fiber according to claim 1, characterized in that: In step S5, the preset angle is set to 10°-350°.

9. The method for preparing a LPFG torsion sensor based on a rotated single-mode optical fiber according to claim 1, characterized in that: In step S7, the resonant wavelength of the LPFG torsion sensor is expressed as: Where: represents the resonant wavelength of the LPFG torsion sensor; represents the initial wavelength; It represents the effective refractive index of the fiber core; represents the effective refractive index of the cladding; Λ represents the grating period; denote the effective refractive indices of the grating core and cladding modes, respectively.

10. A LPFG torsion sensor, characterized in that: The LPFG torsion sensor is prepared by the method for preparing an LPFG torsion sensor based on a rotating single-mode optical fiber as described in claim 1.

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