CN115373067A

CN115373067A - Manufacturing method of ultra-long period fiber grating based on period fiber core offset

Info

Publication number: CN115373067A
Application number: CN202210383323.8A
Authority: CN
Inventors: 张珊珊; 董跨; 尹吉喆; 杨星月
Original assignee: Tianjin Polytechnic University
Current assignee: Tianjin Polytechnic University
Priority date: 2022-04-15
Filing date: 2022-04-15
Publication date: 2022-11-22

Abstract

The invention belongs to the field of design and manufacture of optical fiber devices, and relates to a manufacturing method of an ultra-long period fiber grating based on periodic fiber core offset. According to the invention, the grating manufacturing platform is built, the super-continuum spectrum light source is used as an input light source, the semi-automatic optical fiber processing platform is used for manufacturing the grating and monitoring the transmission spectrum in real time, the influence of each structural parameter on the transmission characteristic is analyzed, and the super-long period optical fiber grating which has the identification directivity and can realize a larger bending range is manufactured. In the range of 0-7.041m ^‑1 Within the curvature range of (A), the bending wavelength sensitivities in the directions of 0 DEG, 90 DEG, 180 DEG and 270 DEG are respectively-1.830 nm/m ^‑1 、‑0.583nm/m ^‑1 、‑1.674nm/m ^‑1 、‑0.479nm/m ^‑1 The amplitude sensitivity is respectively-1.633 dB/m ^‑1 、‑1.314dB/m ^‑1 、‑1.833dB/m ^‑1 、‑1.478dB/m ^‑1 The method has high bending sensitivity, can effectively identify the direction, has important value for vector bending measurement, and overcomes the limitations of low design and manufacturing repeatability and complex operation of the traditional optical fiber device.

Description

Manufacturing method of ultra-long period fiber grating based on period fiber core offset

Technical Field

The invention belongs to the field of optical fiber sensing, and relates to a manufacturing method of an ultra-long period fiber grating based on period fiber core offset.

Background

The fiber grating is one of the most rapidly developed fiber passive devices in recent decades, and the application of the sensor is particularly important along with the rapid development of the information era. The appearance of the optical fiber and the optical fiber sensor thereof provides new power for industrial development and simultaneously promotes the development speed of the information communication field. The fiber grating is an optical fiber structure formed by periodically modulating the refractive index of an optical fiber by a certain method, and has the advantages of small volume, low return loss, light weight, simple manufacture, flexible and changeable structure and the like. In recent years, fiber gratings are often used for measuring sensing parameters such as temperature, strain, refractive index, and bending, and are also widely used in fiber grating sensors in the communication fields such as fiber amplifiers, wavelength division multiplexers, and fiber dispersion compensation. Bao et al have studied a depressed clad Fiber comprising a Fiber Bragg Grating (FBG) with core-shift positioning, with bend sensitivity up to-124.17 dB/mm ^-1 The specially positioned FBGs have a symmetrical geometry and therefore have a certain dependency in identifying the bending direction. Wang et al manufactured a two-dimensional bending sensor based on three-core fiber, on which two different Long Period Gratings (LPG) were engraved, and the maximum bending sensitivity was 3.324nm/m ^-1 . However, the device fabrication of fiber gratings is complex and requires expensive high power lasers such as UV, femtosecond and CO ₂ Laser, etc., the structure of the optical fiber is generally susceptible to changes in the external environment. Meanwhile, some bending sensors based on directional couplers have been developed. JR Guzman-Sepulveda et al studied a Two-Core Fiber (TCF) design for use as a directional coupler for a Fiber from 0-0.2653m ^-1 Has a small curvature range of-137.8763 nm/m ^-1 The coupling parameters change with the change of the bending curvature, the spectrum changes correspondingly, although the sensitivity is very high, a relatively long optical fiber (e.g., E.E.)20 cm) which limits the practical application. Furthermore, according to the waveguide coupling mechanism, precise alignment between the optical fibers is required to achieve perfect phase matching. In recent years, modal interferometers have attracted much attention as alternatives due to the advantages of simple manufacture and flexible assembly. Zhang et al invented a MZI-based vector bending sensor, which is a biconical photonic crystal fiber structure composed of fusion splicing methods, and due to the asymmetry of the dual cores, two opposite bending directions can be distinguished, and the maximum bending sensitivity is 18.29nm/m ^-1 And-18.13 nm/m ^-1 However, the photonic crystal fiber is expensive and has no general applicability, and the offset structure can only identify the positive and negative directions and is generally fragile. H.Gong et al propose a curvature sensor based on hollow-core photonic crystal fiber interferometer at 0-9.9m ^-1 Has a large range of 0.232nm/m ^-1 But the structure exhibits relatively low bending sensitivity.

Based on the above background, the present invention provides an optical Fiber manufacturing method with simple operation and high repeatability to implement a periodic cascade offset Fiber core on a Single Mode Fiber (SMF) to manufacture a new Ultra-Long Period Fiber Grating (ULPFG). Aiming at different periods, different Period lengths and different Fiber Core offsets, a simulation model of a Core-Offset Ultra-Long periodic Fiber Grating (CO-ULPFG) based on periodic Fiber Core Offset is established by adopting a Beam Propagation Method (BPM), and a foundation is laid for experiments. Through simulation results, the parameters are finally set to be Λ =2mm, d =3 μm and N =3, and the main limitation of the traditional optical fiber bending sensor is overcome.

Disclosure of Invention

The invention aims to provide an optical fiber manufacturing method which is simple to operate and has high repeatability, so that the ULPFG is miniaturized, the manufacturing cost is low, the repeatability is high, and the method is applied to large-range curvature and vector bending measurement. The invention firstly adopts the light beam propagation method CO-ULPFG to carry out theoretical simulation analysis, researches the influence of the grating offset, the period length and the period number on the transmission spectrum, and lays a foundation for experimental manufacture. Then, a grating manufacturing platform is built, a super-continuum spectrum light source is used as an input light source, grating manufacturing is carried out by utilizing a semi-automatic optical fiber processing platform, and meanwhile, a spectrometer is connected for real-time monitoring of a transmission spectrum; then, analyzing the influence of each structural parameter on the transmission characteristic by changing the manufacturing parameters, thereby manufacturing the CO-ULPFG with higher bending sensitivity; and then the manufactured CO-ULPFG is used for sensing and measuring the temperature and vector bending to obtain the bending sensitivity and the temperature sensitivity in different directions.

The method comprises the following specific steps:

step 1: the method is characterized in that a light beam propagation method is adopted to carry out theoretical simulation analysis on the Ultra-Long Period Fiber Grating (CO-ULPFG) based on Period Fiber Core Offset, the influence of Grating Offset, period length and Period number on a transmission spectrum is researched, and a foundation is laid for experimental manufacture;

and 2, step: selecting a pre-compiled program, setting an expected offset, fixing two sections of optical fibers with coating layers removed at two ends of a large-diameter optical fiber welding system (Laser Direct Structure, LDS 2.5) and carrying out alignment correction;

and step 3: taking a section of standard Single Mode Fiber (SMF), wherein one end of the SMF is connected with a super-continuum spectrum light source, and the other end of the SMF is connected to a spectrum analyzer through a sensing joint and is used for monitoring a transmission spectrum in real time;

and 4, step 4: removing the coating layer from the middle part of the SMF about 1cm to expose bare fiber, placing the bare fiber part below an LDS2.5 electrode, fixing the left end and the right end of the SMF on an optical fiber holder, automatically calculating corresponding tension through an optical fiber diameter computer to keep the optical fiber in a straight state, pasting a label at one end to enable the direction of the label to be vertical downwards, and recording the deviation direction in the label direction during measurement;

and 5: the cutting tool head is controlled by a computer to be adjusted out, so that the cutting tool head falls at a position which is away from the optical fiber, the cutting tool backing plate is adjusted out, so that the cutting tool backing plate rises to a position which is just attached to the optical fiber, then the optical fiber is quickly cut after a cutting button on a computer screen is clicked, the cutting tool head is lifted as soon as possible at the moment, the influence of residual ultrasonic vibration on the flatness of the cutting end face of the optical fiber is avoided, and finally the cutting tool head and the cutting tool backing plate are respectively adjusted back to the original positions to finish a cutting process;

and 6: welding the two sections of cut optical fibers by simultaneous discharge of three electrodes to obtain a dislocation structure, and observing the change of the spectrum shape by a spectrometer;

and 7: horizontally moving the optical fiber by a distance Lambda of one period through a three-dimensional displacement platform, and repeating the step 5 and the step 6 until a better spectrum shape can be observed on a spectrometer, namely completing the manufacture of the CO-ULPFG;

and step 8: fixing the period lambada of the grating, adjusting the offset d of the optical fiber in the fusion procedure, respectively manufacturing CO-ULPFG with the same number of cascading dislocations, the same period and four different offsets by repeating the steps 2 to 7, and analyzing the influence of the offset change on the transmission characteristic of the CO-ULPFG;

and step 9: fixing the offset d of the optical fiber, adjusting the moving distance of the displacement platform, respectively manufacturing CO-ULPFGs with the same dislocation number, the same offset and three different periods by repeating the steps 2 to 7, and analyzing the influence of the grating period change on the transmission characteristic;

step 10: selecting a sample CO-ULPFG with the lambda =2mm, d =3 μm and N =3, and measuring the temperature sensing characteristic of the sample CO-ULPFG, wherein in an experiment, a grating is placed in a temperature control box, one end of the grating is fixed by an optical fiber holder, and a 10g weight is hung at the other end of the grating so as to avoid the influence of the bending of the optical fiber on a transmission spectrum, gradually change the temperature and record the change condition of the spectrum;

step 11: measuring the strain sensing characteristic of the CO-ULPFG used in the step 10, in an experiment, fixing two ends of an optical fiber by using an optical fiber holder, keeping a weight in a straightening state, applying different strains by controlling the movement of the optical fiber holder, and recording the change condition of a spectrum;

step 12: and tracking the drift and amplitude change of the resonant wavelength through the spectral change graphs at different temperatures obtained in the step 10 and the spectral change graphs in different bending directions obtained in the step 11, and calculating the temperature sensitivity and the bending sensitivity to realize temperature measurement and bending vector measurement.

Drawings

FIG. 1 is a schematic diagram of CO-ULPFG, FIG. 1 (a) is a schematic diagram of CO-ULPFG structure, and FIG. 1 (b) is a schematic diagram of CO-ULPFG dislocation direction;

FIG. 2 is a schematic diagram of a CO-ULPFG experimental setup;

FIG. 3 is a graph of the evolution of the transmission spectrum of CO-ULPFG with increasing number of cycles;

FIG. 4 is a graph of the effect of grating period length variation on the transmission characteristics of CO-ULPFG, where FIG. 4 (a) is a plot of transmission spectrum variation with different grating period lengths and FIG. 4 (b) is a linear fit of resonant wavelength variation to grating period length variation;

FIG. 5 is a graph of the effect of grating shift change on CO-ULPFG transmission characteristics, where FIG. 5 (a) is a plot of transmission spectrum as a function of different shifts and FIG. 5 (b) is a linear fit of resonant wavelength change to shift;

FIG. 6 is a graph of temperature sensing characteristic measurements for CO-ULPFG, where FIG. 6 (a) reflects resonance peak variation with temperature, FIG. 6 (b) is a fitted linear curve of resonance peak center wavelength versus temperature, and FIG. 6 (c) is a linearly fitted curve of amplitude versus temperature;

fig. 7 is a graph of the bending sensing characteristic measurement results of CO-ULPFG, in which fig. 7 (a) reflects the change of the bending sensing characteristic in the 0 ° direction, fig. 7 (b) reflects the change of the bending sensing characteristic in the 90 ° direction, fig. 7 (c) reflects the change of the bending sensing characteristic in the 180 ° direction, and fig. 7 (d) reflects the change of the bending sensing characteristic in the 270 ° direction;

fig. 8 is a fitted curve of the change in transmission spectrum of CO-ULPFG, in which fig. 8 (a) reflects a fitted curve of the change in resonance wavelength with the change in curvature, and fig. 8 (b) is a fitted curve of the change in amplitude with the change in curvature.

The specific implementation mode is as follows:

the invention provides a manufacturing method of a Fiber Grating (CO-ULPFG) based on a periodic Fiber Core Offset Ultra-Long Period, the structural schematic diagram of the CO-ULPFG is shown in figure 1, wherein figure 1 (a) is the structural schematic diagram of the CO-ULPFG, Λ in the figure is the Grating Period, and d represents the Offset distance of the Fiber Core, namely the Offset. The four directions of the calibration grating are respectively 0 °, 90 °, 180 ° and 270 °, and the left view is shown in fig. 1 (b).

The CO-ULPFG is subjected to theoretical simulation analysis by adopting a beam propagation method, the influence of offset, grating period length and period number on a transmission spectrum is researched, and a foundation is laid for experimental manufacture. A3D simulation model of the CO-ULPFG is constructed, the grating period is 3mm, the offset is 2.0 mu m, and the number of dislocation structures is N =3. When the input light source initially enters the grating, all the energy is concentrated in the core, and no energy is lost. When light reaches the first dislocation structure, energy in the fiber core is partially excited into the cladding, the rest energy is continuously remained in the fiber core for transmission, partial energy in the cladding is coupled back into the fiber core at the second dislocation structure, partial energy is coupled out at the third dislocation structure, and the process verifies the coupling principle of the grating, namely the energy in the fiber core is periodically coupled out and back. When the phase matching condition is satisfied, the strongest resonance is formed with the cladding mode, and thus a grating transmission spectrum is formed. With the increase of the number of the dislocation structures, the resonance peak value becomes deeper gradually, and the bandwidth becomes narrower gradually. When three dislocation structures are adopted, a resonance peak with an extinction ratio of 21.3dB appears near 1240nm, and the resonance peak can be used for sensing measurement. Different offsets can cause the coupling mode of the grating to change, thereby affecting the spectral characteristics of the grating. For CO-ULPFG, the basic parameters of the optical fiber are kept unchanged, the period of the grating is set to be 2mm, the shift amount of the grating is selected to be 2.0 μm, 2.5 μm, 3.0 μm and 3.5 μm, the change of the transmission spectrum is simulated and observed respectively, the resonance peak value is changed from 1160nm to 1320nm along with the increase of the shift amount of the grating, and the four shift amounts are fitted with the central wavelength of the resonance peak value to obtain the linear relation. The offset of the fixed grating is 3.0 μm, the grating grids of the fixed grating are set to be 1mm, 2mm and 3mm for simulation, and the central wavelength of the resonance peak value is observed to generate red shift along with the increase of the period, so that a better linear relation is shown.

A schematic diagram of a CO-ULPFG manufacturing setup is shown in fig. 2. The device comprises three parts: a Super continuous Spectrum light source (SCS) with the wavelength range of 400 nm-1700 nm, an Optical Spectrum Analyzer (OSA) with the resolution of 0.02nm and a semi-automatic Optical fiber processing platform (LDS 2.5) integrating the functions of cutting, splicing and moving the Optical fiber. The semi-automatic processing platform integrating the cutting system and the welding system is used for completing grating cutting and welding operation, and the whole platform is controlled by a computer. In the cutting system part, the real-time monitoring is realized through a computer, the cutting system uses ultrasonic vibration to replace a traditional blade, the cutting method does not need to be directly contacted with the optical fiber, and the end face of the cut optical fiber is smoother. In the welding system part, a common optical fiber welding program is selected, a new dislocation welding program is compiled, the discharge intensity is 400bits, the discharge time is 2000ms so as to be suitable for the welding of a dislocation structure, and the deviation is selected within the range of 2.0-3.5 mu m to carry out experiments by combining parameters obtained by simulation.

Fig. 3 shows the change in the grating spectrum with Λ =2mm and d =3.0 μm depending on the number N of the dislocation structures. When only one dislocation structure is arranged, the spectrum has about 3dB loss and no coupling peak, when two dislocation structures are arranged, a coupling peak of about 5dB appears in the spectrum, and when three dislocation structures are arranged, a coupling resonance peak of about 26.22dB is formed at the wavelength of 1231.6 nm. In summary, when the three dislocation structures are periodically arranged on the optical fiber axis, strong modulation occurs, so as to form strong coupling, and experimental results show that the size of the grating is only 8mm, which is more compact than the ULPFG manufactured by a laser.

The variation of the transmission spectrum with the period length of the grating is shown in fig. 4 (a), and by combining simulation analysis, experimental parameters select grating period lengths of 1mm, 2mm and 3mm respectively, fixed offset is 3.0 μm, parameters of other instruments are unchanged, grating samples with period lengths of 1mm, 2mm and 3mm are manufactured for research, the transmission spectrum is observed, and the position of the center wavelength is tracked and recorded. As shown in fig. 4 (b), as the grating period is gradually increased, the resonance peak shifts to the long wavelength direction and shows a better fitting characteristic.

The influence relationship of the change of the offset on the transmission characteristic of the CO-ULPFG is shown in FIG. 5, the condition that the transmission spectrum curve changes along with the offset is shown in 5 (a), and the influence of different offsets on the spectrum curve is analyzed. In the experiment, samples with a fixed grating period length of 2mm and offsets of 2.0 μm, 2.5 μm, 3.0 μm, and 3.5 μm CO-ULPFG were prepared, where the number of dislocation structures of the sample with an offset of 2.0 μm was N =5 and the number of dislocation structures of the sample with an offset of 2.5 μm, 3.0 μm, and 3.5 μm was N =3. The experimental result shows that the resonance peak shifts to the long wavelength direction along with the increase of the offset. Different offsets have different coupling constants, the larger the offset is, the stronger the coupling capability is, the different numbers of dislocation structures required for grating formation are also different, tracking recording is carried out on the central wavelengths of the resonance peaks with different offsets, and the linear fitting curve is shown in fig. 5 (b).

Temperature testing was performed on CO-ULPFG with period Λ =2mm, d =3.0 μm, N =3. In the experiment, the grating was placed in a temperature controlled box with one end fixed with a fiber holder and the other end hanging a 10g weight to avoid the effect of fiber bending on the transmission spectrum. The temperature change range is 30-90 ℃, the change condition of the recorded spectrum is shown in fig. 6 (a), the wavelength of the resonance peak is red shifted due to the temperature rise, the peak wave is transmitted by Gu Bianshen, and the temperature and the wavelength and the amplitude of the resonance peak show better linear relation through linear fitting. As shown in FIGS. 6 (b) and 6 (c), the wavelength sensitivity and the amplitude sensitivity were measured at 80 pm/deg.C and 0.014 dB/deg.C, respectively. The sensitivity of this sample is at the same level as other ULPFGs based on SMF.

The bending test was performed on a CO-ULPFG with a period Λ =2mm, d =3.0 μm, N =3, and first, the bending characteristic of the grating in the 0 ° direction was tested. The curvature increases gradually each time the mobile station is pushed 2mm inwards. After recording a set of data, the grating is rotated by an angular step of 90 ° and the process is repeated. Fig. 7 (a) - (d) show the transmission spectrum variation with respect to 0 °, 90 °, 180 ° and 270 ° bending curvatures, respectively. The CO-ULPFG was measured over a curvature range of 0-7.041m-1, corresponding to the directions of the four gratings 0 °, 90 °, 180 ° and 270 °, respectively, forThe evolution of the transmission spectrum of the bending curvature is shown in fig. 7. FIG. 7 (a) shows the attenuation of the blue shift and peak attenuation of the resonant wavelength when the CO-ULPFG is bent in the 0 ° direction, with total fluctuations of 11.364nm and 12.701dB, corresponding sensitivities of-1.834 nm/m, respectively ^-1 And-1.633 dB/m ^-1 . Fig. 7 (b) shows the evolution of the transmission spectrum bent in the direction of 90 °. The resonant wavelength and the peak value attenuation change differently along with the increase of the curvature, the total fluctuation of the resonant wavelength and the peak value attenuation are-4.230 nm and-9.234 dB respectively, and the corresponding sensitivity is-0.583 nm/m ^-1 And-1.612 dB/m ^-1 . Fig. 7 (c) shows the evolution of the transmission spectrum with respect to the bending curvature along the 180 ° bending direction. The total fluctuation and sensitivity of resonance wavelength and peak attenuation are-11.573 nm, -12.754dB, -1.286nm/m respectively ^-1 And-1.676 dB/m ^-1 . Fig. 7 (d) shows the transmission spectral response bent in the 270 ° direction. Their total fluctuation was 0.479nm and 10.139dB, and the corresponding sensitivities were-1.834 nm/m respectively ^-1 And-1.633 dB/m ^-1 . FIG. 8 is a graph of the resonance peak wavelength and amplitude variation as the grating is bent in different directions. The curve fitted to the change in the resonance peak wavelength with curvature is shown in fig. 8 (a), and the curve fitted to the change in the amplitude with curvature is shown in fig. 8 (b).

The invention provides a manufacturing method of an ultra-long period fiber grating based on period fiber core offset, which is researched by adopting a method combining theory and experiment. The results show that the experiments can achieve the expected effect. The influence of the grating offset, the period length and the number of periods on the transmission spectrum is studied. In the range of 0-7.041m ^-1 Within the curvature range of (1), the bending wavelength sensitivity in the directions of 0 DEG, 90 DEG, 180 DEG and 270 DEG is-1.830 nm/m ^-1 、-0.583nm/m ^-1 、 -1.674nm/m ^-1 、-0.479nm/m ^-1 The amplitude sensitivity is-1.633 dB/m ^-1 、-1.314dB/m ^-1 、 -1.833dB/m ^-1 、-1.478dB/m ^-1 And the measurement of the vector bending characteristic in a larger range is realized.

Claims

1. A manufacturing method of an ultra-long period fiber grating based on period fiber core offset comprises the following specific steps:

step 1: the method comprises the following steps of carrying out theoretical simulation analysis on a Core-Offset Ultra-Long periodic Fiber Grating (CO-ULPFG) based on periodic Fiber Core Offset by adopting a light beam propagation method, researching the influence of Grating Offset, period length and Period number on a transmission spectrum, and laying a foundation for experimental manufacture;

step 2: selecting a pre-compiled program, setting an expected offset, fixing two sections of optical fibers with coating layers stripped at two ends of a semi-automatic optical fiber processing platform (Laser Direct Structuring, LDS 2.5) with integrated optical fiber cutting, splicing and moving functions, and carrying out alignment correction;

and 4, step 4: removing a coating layer from the middle part of the SMF about 1cm, exposing bare fibers, placing the bare fibers below an LDS2.5 electrode, fixing the left end and the right end of the SMF on an optical fiber holder, automatically calculating corresponding tension through an optical fiber diameter computer to keep the optical fibers in a straight state, pasting a label at one end to enable the direction of the label to be vertical to the lower side, and recording the offset direction in the direction of the label during measurement;

step 6: welding the two sections of cut optical fibers by simultaneous discharge of three electrodes to obtain a dislocation structure, and observing the change of the spectrum shape by a spectrometer;

and 8: fixing the period lambada of the grating, adjusting the offset d of the optical fiber in the fusion procedure, respectively manufacturing CO-ULPFG with the same number of cascading dislocations, the same period and four different offsets by repeating the steps 2 to 7, and analyzing the influence of the offset change on the transmission characteristic of the CO-ULPFG;

and step 9: fixing the offset d of the optical fiber, adjusting the moving distance of the displacement platform, respectively manufacturing three CO-ULPFGs with the same dislocation number, the same offset and different period lengths by repeating the steps 2 to 7, and analyzing the influence of the change of the grating period length on the transmission characteristic of the grating;

step 10: selecting a CO-ULPFG sample with the thickness of Λ =2mm and d =3 μm, measuring the temperature sensing characteristic of the sample, and placing a grating in a temperature control box in an experiment, wherein one end of the grating is fixed by an optical fiber holder, and the other end of the grating is hung with a 10g weight to avoid the influence of the bending of the optical fiber on a transmission spectrum, gradually changing the temperature and recording the change condition of the spectrum;

step 12: tracking the drift and amplitude change of the resonant wavelength through the spectral change graphs at different temperatures obtained in the step 10 and the spectral change graphs in different bending directions obtained in the step 11, and calculating the temperature sensitivity and the bending sensitivity to realize temperature measurement and bending vector measurement.

2. The method according to claim 1, wherein in step 5, a semi-automatic fiber processing platform is used that integrates fiber cutting, fusion splicing and moving functions, the operation of the whole platform is controlled by a computer, in the cutting system, the required tension can be automatically applied to the fiber according to the fiber diameter, so that the fiber is straightened, the flatness of the cut end face is ensured, the cutting process and quality can be monitored by a camera above the electrode and displayed on a computer screen in real time, the cutting system of the platform utilizes ultrasonic vibration to rapidly cut the fiber instead of traditional blade cutting, the cutting method does not need to directly contact with the fiber, and the cut fiber end face is smoother.

3. The method according to claim 1, wherein in the fusion process of fabricating the dislocation structure in step 6, a processing procedure of the dislocation structure is established in a computer, the discharge intensity is 400bits, the arc discharge duration is 2000ms, the fiber offset can be adjusted between 2.0 and 3.5 μm according to the offset of the fabricated dislocation structure, the moving precision of the three-dimensional displacement platform is 0.1 μm, and the fiber holder is fixed thereon to drive the fiber to move together.

4. The method of claim 1 wherein the period Λ of the CO-ULPFG formed in step 7 is greater than 1mm, the sizes and shapes of the cascaded offset structures are substantially the same, the offset is at a magnitude of 2 μm ± 0.1 μm, and a distinct transmission peak can be formed after 3 offset structures.

5. The method of claim 1, wherein in steps 10 and 11, the temperature sensitivity obtained by testing CO-ULPFG with Λ =2mm, d =3 μm is 80pm/° c, 0.014dB/° c; when the CO-ULPFG is subjected to bending test towards the 0-degree direction, the wavelength sensitivity and the amplitude sensitivity are respectively-1.830 nm/m ^-1 And-1.633 dB/m ^-1 (ii) a When the test is performed by bending towards the 90-degree direction, the wavelength sensitivity and the amplitude sensitivity are respectively-0.583 nm/m ^-1 And-1.314 dB/m ^-1 (ii) a When the probe is bent to the direction of 180 degrees, the wavelength sensitivity and the amplitude sensitivity are respectively-1.674 nm/m ^-1 And-1.833 dB/m ^-1 (ii) a When bent in the direction of 270 degrees, the wave length is very strongSensitivity and amplitude sensitivity are respectively-0.479 nm/m ^-1 And-1.478 dB/m ^-1 。