Fiber Bragg grating, its preparation method and preparation device

Abstract

The present invention provides an optical fiber grating, a preparation method thereof and a preparation device. The preparation method includes: obtaining an optical fiber preform, melting and drawing it to obtain a bare optical fiber; using the phase mask method to perform grating inscription on the bare optical fiber in a nitrogen atmosphere to obtain a bare optical fiber grating; then performing coating, curing and grating calibration to obtain a calibrated optical fiber grating, keeping it at 45-60 °C for 16-24 h for post-heat treatment, and then cooling it to room temperature at a cooling rate of 1-3 °C / min to obtain the optical fiber grating. The present invention uses a specific optical fiber grating structure, effectively improving the grating reflectivity, enabling it to have both good photosensitive characteristics and a long transmission distance for the optical fiber; the grating inscription process is carried out in a nitrogen atmosphere during the preparation process, greatly improving the stability of the on-line optical fiber grating inscription, and also adding a special post-heat treatment process, which can effectively eliminate the residual internal stress in the optical fiber and improve the side-mode suppression ratio of the optical fiber grating.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber Bragg gratings, and in particular, to a fiber Bragg grating, a preparation method thereof, and a preparation device therefor. Background Art

[0002] Fiber Bragg gratings are one of the most representative and promising fiber passive devices at present, with advantages such as low insertion loss and high extinction ratio, and are widely used in fiber lasers, fiber amplifiers, fiber filters, fiber sensors, and fiber communication systems. In the field of fiber sensing, the advantages of fiber Bragg gratings are particularly prominent, and large-scale multiplexing can form a long-distance multi-parameter fiber Bragg grating sensing network.

[0003] In the prior art, although there are methods for in-line writing of fiber Bragg gratings, there are no specific solutions for the type of preform applicable, how to maintain the stability of the writing environment, how to maintain the accuracy of fiber writing, and how to perform grating calibration. Moreover, the current fiber Bragg grating products have high residual internal stress and low side mode suppression ratio. Summary of the Invention

[0004] The main object of the present invention is to provide a fiber Bragg grating, a preparation method thereof, and a preparation device therefor, so as to solve the problems of poor in-line writing stability, high residual internal stress, and low side mode suppression ratio of fiber Bragg gratings in the prior art.

[0005] To achieve the above object, according to one aspect of the present invention, a method for preparing an optical fiber grating is provided. The optical fiber grating sequentially includes a core layer, an inner cladding layer, a depressed layer, an outer cladding layer, and a coating layer from the inside out. The preparation method includes the following steps: Step S1, use the VAD process to sequentially prepare the core layer preform and the inner cladding layer preform, then use the MCVD process to prepare the depressed layer preform, and finally use the OVD process to prepare the outer cladding layer preform to obtain an optical fiber preform; Step S2, melt and draw the optical fiber preform to obtain a bare optical fiber: Step S3, adopt the phase mask method to perform grating inscription on the bare optical fiber in a nitrogen atmosphere to obtain a bare optical fiber grating; Step S4, sequentially coat and cure the bare optical fiber grating to obtain a coated optical fiber grating with a coating layer on the surface; Step S5, perform grating calibration on the coated optical fiber grating to obtain a calibrated optical fiber grating; Step S6, keep the calibrated optical fiber grating at 45-60 °C for 16-24 h for post-heat treatment, and then cool it to room temperature at a cooling rate of 1-3 °C / min to obtain an optical fiber grating; wherein, the material of the core layer is silica doped with germanium ions, the doping molar concentration of germanium ions is 1-12 mol%, and the relative refractive index difference Δn1 is 0.43-1%; the material of the inner cladding layer is pure silica, and the relative refractive index difference Δn2 is 0; the material of the depressed layer is silica doped with fluoride ions, the doping molar concentration of fluoride ions is 0.1-6 mol%, and the relative refractive index difference Δn3 is -0.1% to 0; the material of the outer cladding layer is pure silica, and the relative refractive index difference Δn4 is 0.

[0006] Further, the diameter of the optical fiber preform is 20-80 mm, preferably 30-40 mm; preferably, in the optical fiber grating, the radius of the core layer is 4-5 μm, and / or the thickness of the inner cladding layer is 7.5-12 μm, and / or the thickness of the depressed layer is 8-13.5 μm, and / or the thickness of the outer cladding layer is 62-63 μm.

[0007] Further, in Step S2, the melting temperature is 1800-2200 °C, and the melting atmosphere is helium and / or argon; preferably, the volume percentage content of helium in the melting atmosphere is 0-40%, the oxygen volume concentration <50 ppm, and the gas flow rate is 15-50 L / min; preferably, in Step S2, the drawing speed is 5-25 m / min, and the drawing speed deviation <1 m / min; more preferably, the tension of the bare optical fiber during the drawing process is 0-20 g.

[0008] Further, in Step S3, the gas temperature of the nitrogen atmosphere is 70-120 °C; preferably, the grating inscription length of the bare optical fiber grating is 10-20 mm.

[0009] Further, in step S3, an excimer laser is used for grating writing, the light source is 193 nm, 244 nm, 248 nm or 308 nm, the optical pulse energy is 100-200 mJ / Pulse, the voltage is 18-22 KV, and the exposure time interval is h / V, where h is the grating pitch and V is the drawing speed.

[0010] Further, in step S4, the material of the coating layer is one or more of acrylic resin, modified acrylic resin, silicone resin and polyimide.

[0011] According to another aspect of the present invention, there is provided an optical fiber grating obtained by the preparation method of the present invention; preferably, the optical fiber grating sequentially includes a core layer, an inner cladding layer, a depressed layer, an outer cladding layer and a coating layer from the inside to the outside, wherein the material of the core layer is silica doped with germanium ions, the molar doping concentration of germanium ions is 1-12 mol%, and the relative refractive index difference Δn1 is 0.43-1%; the material of the inner cladding layer is pure silica, and the relative refractive index difference Δn2 is 0; the material of the depressed layer is silica doped with germanium ions, the molar doping concentration of germanium ions is 0.1-6 mol%, and the relative refractive index difference Δn3 is -0.1% to 0; the material of the outer cladding layer is pure silica, and the relative refractive index difference Δn4 is 0.

[0012] According to another aspect of the present invention, there is provided an optical fiber grating preparation apparatus, which is prepared by using the above preparation method of the present invention. According to the preparation process of the optical fiber grating, the preparation apparatus includes: a drawing unit, which includes a rod feeder and a drawing furnace, the rod feeder is arranged above the drawing furnace, and the drawing unit is used for melting and drawing an optical fiber preform to obtain a bare optical fiber; a grating writing unit, which includes a nitrogen gas pipe and a grating writing module, the grating writing module is arranged below the nitrogen gas pipe, and the grating writing unit is used for grating writing on the bare optical fiber by using the phase mask method to obtain a bare optical fiber grating; a coating unit, which is used for coating the bare optical fiber grating; a curing unit, which is used for curing the coated bare optical fiber grating to obtain a coated optical fiber grating; a grating calibration unit, which includes a grating calibration module, and the grating calibration unit is used for grating calibration of the coated optical fiber grating to obtain a calibrated optical fiber grating; a wire winding unit, which is used for winding and forming the calibrated optical fiber grating; and a post-heat treatment unit, which is used for post-heat treatment of the wound and formed calibrated optical fiber grating to obtain an optical fiber grating.

[0013] Further, in the grating writing unit, the grating writing module is arranged at 5-50 mm below the lower opening of the nitrogen gas pipe; preferably, the grating writing unit further includes a primary wire diameter gauge, which is arranged below the grating writing module and is used for measuring the diameter of the bare optical fiber grating; preferably, the grating calibration unit further includes a secondary wire diameter gauge, which is arranged above the grating calibration module and is used for measuring the diameter of the coated optical fiber grating.

[0014] Further, the grating calibration method of the fiber grating includes: in the grating calibration module, when the coated fiber grating runs to a length of L + nh + d, grating calibration is performed; where n is an integer greater than or equal to 0, L is the length of the take-up unit, h is the grating interval, and d is the distance between the grating writing module and the coating unit.

[0015] The present invention uses a specific fiber grating structure, effectively improving the grating reflectivity, enabling it to have both good photosensitive characteristics and a long transmission distance for the optical fiber. Grating writing under a nitrogen atmosphere can greatly improve the stability of on-line fiber grating writing, reducing the precision requirements of the demodulation instrument during the use of the fiber grating; at the same time, it avoids the generation of silanol groups and reduces the optical fiber attenuation. In addition, the present invention also adds a special post-heat treatment process, which can effectively eliminate the residual internal stress in the optical fiber and improve the side-mode suppression ratio of the fiber grating. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0017] Figure 1 shows the refractive index profile of a fiber grating according to an embodiment of the present invention; and

[0018] Figure 2 shows a schematic diagram of a fiber grating preparation device according to an embodiment of the present invention.

[0019] Wherein, the above-mentioned accompanying drawings include the following reference numerals:

[0020] 1, drawing unit; 11, rod feeder; 12, drawing furnace; 2, grating writing unit; 21, nitrogen pipe; 22, grating writing module; 23, primary wire diameter gauge; 3, coating unit; 4, curing unit; 5, grating calibration unit; 51, grating calibration module; 52, secondary wire diameter gauge; 6, take-up unit; 7, post-heat treatment unit; a, optical fiber preform. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with the embodiments.

[0022] It should be noted that in the present invention, unless otherwise clearly specified and defined, the first feature being "on" or "under" the second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, the first feature being "above", "over" and "on top of" the second feature may mean that the first feature is directly above or obliquely above the second feature, or simply indicates that the first feature has a higher horizontal height than the second feature. The first feature being "under", "beneath" and "underneath" the second feature may mean that the first feature is directly below or obliquely below the second feature, or simply indicates that the first feature has a lower horizontal height than the second feature.

[0023] Term Explanation:

[0024] VAD: Axial Vapor Deposition.

[0025] MCVD: Modified Chemical Vapor Deposition.

[0026] OVD: Outside Vapor Deposition.

[0027] Phase Mask Method: A resolution enhancement technique that uses the intensity and phase of light to form an image and obtain higher resolution.

[0028] Side Mode Suppression Ratio: The ratio of the intensity of the main mode to the maximum intensity of the side modes, which is an important indicator of the longitudinal mode performance. The key is to reduce the intensity of the side modes and their noise. In a grating, it is shown that reducing the internal stress after grating writing can effectively reduce the side mode noise and improve the side mode suppression ratio.

[0029] As described in the background art of the present invention, there are problems in the prior art such as poor online writing stability, high residual internal stress, and low side mode suppression ratio of fiber gratings. To solve the above problems, in a typical embodiment of the present invention, a method for preparing a fiber grating is provided. The fiber grating sequentially includes a core layer, an inner cladding layer, a depressed layer, an outer cladding layer, and a coating layer from the inside out. The preparation method includes the following steps: Step S1, use the VAD process to sequentially prepare the core preform layer and the inner cladding preform layer, then use the MCVD process to prepare the depressed layer preform layer, and finally use the OVD process to prepare the outer cladding preform layer to obtain a fiber preform; Step S2, melt and draw the fiber preform to obtain a bare fiber; Step S3, use the phase mask method to perform grating writing on the bare fiber in a nitrogen atmosphere to obtain a bare fiber grating; Step S4, sequentially coat and cure the bare fiber grating to obtain a coated fiber grating with a coating layer on the surface; Step S5, perform grating calibration on the coated fiber grating to obtain a calibrated fiber grating; Step S6, keep the calibrated fiber grating at 45 - 60 °C for 16 - 24 h for post-heat treatment, and then cool it to room temperature (20 - 30 °C) at a cooling rate of 1 - 3 °C / min to obtain the fiber grating.

[0030] Among them, asFigure 1 As shown, the material of the core layer is silicon dioxide doped with germanium ions, the doping molar concentration of germanium ions is 1-12 mol%, and the relative refractive index difference Δn1 is 0.43-1%; the material of the inner cladding is pure silicon dioxide, and the relative refractive index difference Δn2 is 0; the material of the depressed layer is silicon dioxide doped with fluoride ions, the doping molar concentration of fluoride ions is 0.1-6 mol%, and the relative refractive index difference Δn3 is -0.1% - 0; the material of the outer cladding is pure silicon dioxide, and the relative refractive index difference Δn4 is 0.

[0031] The fiber grating of the present invention adopts the above special design because the inventor unexpectedly found during the research process that on the one hand, when germanium ions are doped in the optical fiber, at the same power, the grating reflectivity will increase significantly; on the other hand, the doping concentration of germanium ions cannot be lower than 1 mol%, otherwise it will affect the photosensitivity characteristics of the optical fiber, but it cannot be higher than 12 mol% either, otherwise the absorption of light by germanium ions is too strong, which will seriously affect the transmission distance of the optical fiber. The doping of F element is mainly to improve the anti-bending characteristics of the fiber grating for long-distance use after preparation, ensuring that the central wavelength of the fiber grating will not drift due to the bending of the optical fiber during use. In addition, the fiber grating written with germanium-doped optical fiber can also improve the temperature resistance, and the maximum working temperature can reach 500 °C.

[0032] Specifically, the present invention first uses the VAD process to prepare the core layer preform and the inner cladding preform in sequence, then uses the MCVD process to prepare the depressed layer preform, and finally uses the OVD process to prepare the outer cladding preform, obtaining an optical fiber preform for melting and drawing. In the molten state, the preform is transformed from a solid state to an intermediate state of solid and liquid, and at the same time, drawing is carried out to obtain a bare optical fiber: then the phase mask method is adopted to carry out grating writing on the bare optical fiber in a nitrogen atmosphere to obtain a bare fiber grating.

[0033] It should be noted that the writing of the in-line fiber grating requires a stable environment. The present invention adopts a nitrogen pipe because the inventor unexpectedly found during the research process that using this method can keep the temperature and humidity of the writing environment within a controllable range. On the other hand, it can avoid the generation of silicon hydroxyl groups and reduce the attenuation of the optical fiber. At high temperatures, silicon dioxide and water in the optical fiber may react and simultaneously generate Si-O-H (silicon hydroxyl groups). The harm of silicon hydroxyl groups is that there is strong absorption in the 1383 nm band, which will cause a significant increase in the attenuation of the 1383 nm band; secondly, although deuterium elements can be used to replace the hydrogen elements in the silicon hydroxyl groups, the formed Si-O-D has a different molecular weight from the original structure, and the difference in molecular weight will form a refractive index change, resulting in an increase in Rayleigh scattering and an increase in the attenuation of the optical fiber. At the same time, writing in a nitrogen environment can also reduce the attachment of water molecules on the surface of the optical fiber and improve the subsequent coating effect.

[0034] After inscription, the bare fiber grating is coated and cured in sequence to obtain a coated fiber grating with a coating layer having high temperature resistance and corrosion prevention on the surface; then, according to the preparation requirements, the coated fiber grating is calibrated to obtain a calibrated fiber grating; finally, the calibrated fiber grating is kept at 45-60 °C for 16-24 h for post-heat treatment, and then cooled to room temperature at a cooling rate of 1-3 °C / min to obtain a fiber grating. Since internal stress will be formed during the fiber drawing process, affecting the uniformity and periodicity of the formed fiber grating, after adopting the above post-heat treatment process in the present invention, the residual internal stress can be effectively eliminated, and the side mode suppression ratio of the fiber grating can be improved. The side mode suppression ratio of the fiber grating after post-heat treatment is greater than 10 dB.

[0035] In order to make the settings of the materials, refractive indices and thicknesses of each layer of the fiber grating more matched, in a preferred embodiment, the diameter of the fiber preform is 20-80 mm, preferably 30-40 mm, so that the fiber preparation better conforms to the existing production equipment and usage requirements. Preferably, as Figure 1 shown, in the fiber grating, the radius r1 of the core layer is 4-5 μm, and / or the thickness r2 - r1 of the inner cladding is 7.5-12 μm, and / or the thickness of the depressed layer r3 - r2 is 8-13.5 μm, and / or the thickness of the outer cladding r4 - r3 is 62-63 μm; the above thickness limitations of each layer are also more universal, facilitating the obtaining of fibers with various size standards.

[0036] In a preferred embodiment, in step S2, the melting temperature is 1800-2200 °C, so as to further improve the stability of the melting and drawing process. The melting environment is filled with a protective gas, and the melting atmosphere is helium and / or argon; preferably, the volume percentage content of helium in the melting atmosphere is 0-40%, the oxygen volume concentration < 50 ppm, and the gas flow rate is 15-50 L / min, which can further homogenize the fiber microstructure and reduce the fiber loss caused by impurity generation.

[0037] To eliminate the internal stress in the fiber as much as possible, preferably, in step S2, the drawing speed is 5-25 m / min, and the drawing speed deviation < 1 m / min; more preferably, the tension of the bare fiber during the drawing process is 0-20 g, and the total cladding deviation is 125 ± 2 μm, so that the fluctuation range of the fiber during the drawing process is within a circle with a radius not greater than 5 μm.

[0038] In a preferred embodiment, in step S3, the gas temperature of the nitrogen atmosphere is 70-120 °C; preferably, the grating inscription length of the bare fiber grating is 10-20 mm, which can further make the online inscription environment of the fiber grating more stable.

[0039] Specifically, in a preferred embodiment, in step S3, an excimer laser is used for grating writing. The light source is 193 nm, 244 nm, 248 nm or 308 nm, the optical pulse energy is 100 - 200 mJ / Pulse, the voltage is 18 - 22 KV, and the exposure time interval is h / V, where h is the grating pitch and V is the drawing speed.

[0040] For further improving the comprehensive properties such as flexibility, bendability, and stretchability of the fiber grating, in a preferred embodiment, in step S4, according to different requirements, the material of the coating layer is one or more of acrylic resin, modified acrylic resin, silicone resin, and polyimide, and their long-term working range limits are (-70 °C, 60 °C), (-40 °C, 180 °C), (0 °C, 250 °C), (0 °C, 350 °C) respectively. The specific selection is adjusted according to the working temperature of the specific working environment. The working range of the formed fiber grating can generally be stabilized at -70 °C to 350 °C. The curing method can adopt photocuring or thermal curing, and the coating method can be single-layer coating or multi-layer coating.

[0041] In another typical embodiment of the present invention, a fiber grating is also provided, which is obtained by the preparation method of the present invention; preferably, the fiber grating sequentially includes a core layer, an inner cladding layer, a depressed layer, an outer cladding layer, and a coating layer from the inside out. Among them, the material of the core layer is silica doped with germanium metal ions, the molar doping concentration of germanium ions is 1 - 12 mol%, and the relative refractive index difference Δn1 is 0.43 - 1%; the material of the inner cladding layer is pure silica, and the relative refractive index difference Δn2 is 0; the material of the depressed layer is silica doped with germanium metal ions, the molar doping concentration of germanium ions is 0.1 - 6 mol%, and the relative refractive index difference Δn3 is -0.1% - 0; the material of the outer cladding layer is pure silica, and the relative refractive index difference Δn4 is 0.

[0042] The above fiber grating uses a specific fiber grating structure, effectively improving the grating reflectivity, enabling it to have both good photosensitive characteristics and a long transmission distance for the optical fiber. The grating writing process is carried out in a nitrogen atmosphere, greatly improving the stability of on-line fiber grating writing. Additionally, a special post-heat treatment process is added, which can effectively eliminate the residual internal stress in the optical fiber and improve the side mode suppression ratio of the fiber grating.

[0043] In another typical embodiment of the present invention, a preparation device for a fiber grating is also provided, which is prepared using the preparation method of the present invention. According to the preparation process of the fiber grating, such as Figure 2As shown in the figure, the preparation device includes: a wire drawing unit 1, which includes a rod feeder 11 and a wire drawing furnace 12. The rod feeder 11 is arranged above the wire drawing furnace 12. The wire drawing unit 1 is used for melting and drawing a fiber preform to obtain a bare optical fiber; a grating inscription unit 2, which includes a nitrogen gas pipe 21 and a grating inscription module 22. The grating inscription module 22 is arranged below the nitrogen gas pipe 21. The grating inscription unit 2 is used for inscribing a grating on the bare optical fiber by the phase mask method to obtain a bare optical fiber grating; a coating unit 3, which is used for coating the bare optical fiber grating; a curing unit 4, which is used for curing the coated bare optical fiber grating to obtain a coated optical fiber grating; a grating calibration unit 5, which includes a grating calibration module 51. The grating calibration unit 5 is used for calibrating the coated optical fiber grating to obtain a calibrated optical fiber grating; a wire winding unit 6, which is used for winding and forming the calibrated optical fiber grating; a post-heat treatment unit 7, which is used for performing post-heat treatment on the wound and formed calibrated optical fiber grating to obtain an optical fiber grating.

[0044] In the specific preparation process, the fiber preform a is sent into the wire drawing furnace 12 by the rod feeder 11, melted at high temperature and drawn into a glass fiber therein, and then discharged from the lower part of the furnace. The gap between the preform and the side wall where it is located should be between 5 and 15 mm to ensure the stability of heat transfer. Then the bare optical fiber first enters a nitrogen gas pipe 21 with a length of 3 to 6 m. Nitrogen gas with a purity of at least 99.99% is used, and the nitrogen gas flows from top to bottom in a circulating manner, with gas inlet at the upper port and gas outlet at the lower port. The gas temperature is stable at 70 to 120 °C. At the lower port of the nitrogen gas pipe 21, the grating inscription module 22 is used to inscribe an optical fiber grating by the phase mask method. The obtained bare optical fiber grating successively enters the coating unit 3 and the curing unit 4, so as to prepare a coating layer on the surface of the bare optical fiber grating. After obtaining the coated optical fiber grating, it enters the grating calibration module 51 for grating calibration. The obtained calibrated optical fiber grating is wound and formed by the wire winding unit 6. The wound and formed calibrated optical fiber grating enters the post-heat treatment unit 7, is kept at a temperature of 45 to 60 °C for 16 to 24 h for post-heat treatment, and then is cooled to room temperature at a cooling rate of 1 to 3 °C / min to obtain an optical fiber grating.

[0045] The optical fiber grating preparation device of the present invention is reasonably arranged, can dynamically and continuously prepare optical fiber gratings online with high production efficiency; moreover, by using the phase mask method, the grating wavelength can be accurately controlled, and high-quality optical fiber gratings can be prepared. Inscribing the grating after the nitrogen gas pipe 21 can greatly improve the stability of online optical fiber grating inscription, reduce the precision requirements of the demodulation instrument during the use of the optical fiber grating; at the same time, it can avoid the generation of silanol groups and reduce the optical fiber attenuation. In addition, a special post-heat treatment unit 7 is added, which can effectively eliminate the residual internal stress in the optical fiber and improve the side mode suppression ratio of the optical fiber grating.

[0046] In a preferred embodiment, in the fiber grating writing unit 2, the fiber grating writing module 22 is arranged at a position 5 - 50 mm below the lower opening of the nitrogen gas pipe 21, which can further improve the stability of on-line fiber grating writing; preferably, the fiber grating writing unit 2 further includes a primary wire diameter gauge 23, which is arranged below the fiber grating writing module 22 and is used for measuring the diameter of the bare fiber grating; preferably, the fiber grating calibration unit 5 further includes a secondary wire diameter gauge 52, which is arranged above the fiber grating calibration module 51 and is used for measuring the diameter of the coated fiber grating, so as to more precisely control the quality of the fiber grating.

[0047] During the use of the fiber grating, it is necessary to calibrate the position of the grating. In a preferred embodiment, the fiber grating calibration method includes: in the fiber grating calibration module 51, when the coated fiber grating runs to a length of L + nh + d, grating calibration is performed (the starting position of length counting can be adjusted according to actual production and product requirements), where n is an integer greater than or equal to 0, L is the length of the take-up unit 6, h is the grating interval, and d is the distance between the fiber grating writing module 22 and the coating unit 3. The length is based on the counting of the take-up machine, and the metering error of the take-up machine is within ±0.0005 m. When the length of the take-up machine is L, this is used as the reference distance for calculation. At this time, the fiber grating writing module 22 and the fiber grating calibration module 51 start to work. Exemplarily, the positions of the fiber gratings in the optical fiber are L, L + h, L + 2h,..., L + nh. Correspondingly, when the optical fiber runs to L + d, L + h + d, L + 2h + d,..., L + nh + d, position calibration starts. The present invention adopts an on-line calibration method, which can clearly mark the specific positions of the fiber gratings, further improve the accuracy of grating calibration, and improve the quality of the fiber gratings.

[0048] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0049] Unless otherwise specified, the following embodiments of the present invention all use the preparation device as Figure 2 shown for fiber grating preparation.

[0050] Example 1

[0051] Step S1, use the VAD process to sequentially prepare the core preform layer and the inner cladding preform layer, then use the MCVD process to prepare the depressed preform layer, and finally use the OVD process to prepare the outer cladding preform layer to obtain an optical fiber preform with a diameter of 35 mm;

[0052] Step S2, the optical fiber preform is fed into the fiber drawing furnace by a rod feeder, where it is melted at a high temperature and drawn into glass fibers, and then discharged from the lower part of the furnace to obtain a bare optical fiber. Among them, the melting temperature is 2000 °C, the melting atmosphere is a mixture of helium and argon, the volume percentage content of helium is 30%, the oxygen volume concentration < 50 ppm, and the gas flow rate is 30 L / min; the fiber drawing speed is 15 m / min, and the deviation of the fiber drawing speed < 1 m / min; during the fiber drawing process, the tension of the bare optical fiber is 10 g, the typical value of the cladding diameter is 125 μm, and the deviation is ±1.5 μm;

[0053] Step S3, the bare optical fiber first enters a nitrogen gas pipe with a length of 5 m. Nitrogen gas with a purity of at least 99.99% is used, and the nitrogen gas flows from top to bottom in a circulating manner, with gas inlet at the upper port and gas outlet at the lower port. The gas temperature is stable at 100 °C. At 25 mm below the lower port of the nitrogen gas pipe, a fiber Bragg grating is inscribed on the bare optical fiber using the phase mask method with a grating inscription module to obtain a bare optical fiber grating, and the grating inscription length is 15 mm. Among them, an excimer laser is used for grating inscription, the light source is 193 nm, the light pulse energy is 150 mJ / Pulse, the voltage is 20 KV, and the exposure time interval is h / V, where h is the grating pitch and V is the fiber drawing speed;

[0054] Step S4, the bare optical fiber grating enters the coating unit and the curing unit in sequence, so as to prepare a coating layer on the surface of the bare optical fiber grating. The material of the coating layer is acrylic resin to obtain a coated fiber Bragg grating;

[0055] Step S5, the coated fiber Bragg grating enters the grating calibration module for grating calibration to obtain a calibrated fiber Bragg grating. The calibration method is as follows: the positions of the fiber Bragg gratings in the optical fiber are L, L + h, L + 2h,..., L + nh. Correspondingly, when the optical fiber runs to L + d, L + h + d, L + 2h + d,..., L + nh + d, position calibration starts.

[0056] Step S6, the calibrated fiber Bragg grating is wound and formed through a take-up unit, and then enters the post-heat treatment unit, where it is kept at 55 °C for 24 h, and then cooled to room temperature at a cooling rate of 3 °C / min to obtain a fiber Bragg grating.

[0057] Among them, the material of the core layer of the fiber Bragg grating is silica doped with germanium ions, the doping molar concentration of germanium ions is 6 mol%, the relative refractive index difference Δn1 is 0.75%, and the radius is 4.5 μm; the material of the inner cladding is pure silica, the relative refractive index difference Δn2 is 0, and the thickness is 10 μm; the material of the depressed layer is silica doped with fluoride ions, the doping molar concentration of fluoride ions is 1 mol%, the relative refractive index difference Δn3 is -0.05%, and the thickness is 11 μm; the material of the outer cladding is pure silica, the relative refractive index difference Δn4 is 0, and the thickness is 62.5 μm.

[0058] Example 2

[0059] Step S1, the core layer preform and the inner cladding preform are sequentially prepared using the VAD process, then the depressed layer preform is prepared using the MCVD process, and finally the outer cladding preform is prepared using the OVD process to obtain an optical fiber preform with a diameter of 40 mm;

[0060] Step S2, the optical fiber preform is fed into a drawing furnace by a rod feeder, melted at a high temperature therein and drawn into a glass fiber, and then discharged from the lower part to obtain a bare optical fiber; among them, the melting temperature is 2000 °C, the melting atmosphere is a mixture of helium and argon, the volume percentage content of helium is 30%, the oxygen volume concentration < 50 ppm, and the gas flow rate is 30 L / min; the drawing speed is 15 m / min, and the drawing speed deviation < 1 m / min; the tension of the bare optical fiber during the drawing process is 15 g, the typical value of the cladding diameter is 125 μm, and the deviation is ±2 μm;

[0061] Step S3, the bare optical fiber first enters a nitrogen gas pipe with a length of 5 m. Nitrogen with a purity of at least 99.99% is used, and the nitrogen flows from top to bottom in a circulating manner, with gas inlet at the upper port and gas outlet at the lower port. The gas temperature is stabilized at 100 °C. At 30 mm below the lower port of the nitrogen gas pipe, the fiber Bragg grating is written using the phase mask method with a grating writing module to obtain a bare fiber Bragg grating, and the grating writing length is 12 mm; among them, an excimer laser is used for grating writing, the light source is 248 nm, the light pulse energy is 120 mJ / Pulse, the voltage is 25 KV, and the exposure time interval is h / V, where h is the grating pitch and V is the drawing speed;

[0062] Step S4, the bare fiber Bragg grating sequentially enters a coating unit and a curing unit, so as to prepare a coating layer on the surface of the bare fiber Bragg grating. The material of the coating layer is acrylic resin to obtain a coated fiber Bragg grating;

[0063] Step S5, the coated fiber Bragg grating enters the grating calibration module 51 for grating calibration to obtain a calibrated fiber Bragg grating; the calibration method is as follows: the positions of the fiber Bragg gratings in the optical fiber are L, L + h, L + 2h,..., L + nh, and correspondingly, when the optical fiber runs to L + d, L + h + d, L + 2h + d,..., L + nh + d, position calibration starts.

[0064] Step S6, the calibrated fiber Bragg grating is wound and formed through a take-up unit, and then enters a post-heat treatment unit, is kept at 50 °C for 18 h, and then cooled to room temperature at a cooling rate of 1.5 °C / min to obtain a fiber Bragg grating.

[0065] Among them, the material of the core layer of the fiber grating is silica doped with germanium ions. The doping molar concentration of germanium ions is 8 mol%, the relative refractive index difference Δn1 is 0.85%, and the radius is 4.5 μm; the material of the inner cladding is pure silica, the relative refractive index difference Δn2 is 0, and the thickness is 10 μm; the material of the depressed layer is silica doped with fluoride ions. The doping molar concentration of fluoride ions is 2 mol%, the relative refractive index difference Δn3 is -0.1%, and the thickness is 11 μm; the material of the outer cladding is pure silica, the relative refractive index difference Δn4 is 0, and the thickness is 62.5 μm.

[0066] Example 3

[0067] Step S1, use the VAD process to sequentially prepare the core layer preform and the inner cladding preform, then use the MCVD process to prepare the depressed layer preform, and finally use the OVD process to prepare the outer cladding preform to obtain an optical fiber preform with a diameter of 30 mm;

[0068] Step S2, the optical fiber preform is fed into a drawing furnace by a rod feeder, melted at a high temperature and drawn into a glass fiber, and then discharged from the lower part to obtain a bare optical fiber; among them, the melting temperature is 1800 °C, the melting atmosphere is a mixture of helium and argon, the volume percentage content of helium is 10%, the oxygen volume concentration < 50 ppm, and the gas flow rate is 15 L / min; the drawing speed is 5 m / min, and the drawing speed deviation < 1 m / min; the tension of the bare optical fiber during the drawing process is 10 g, the typical value of the cladding diameter is 125 μm, and the deviation is ±1.5 μm;

[0069] Step S3, the bare optical fiber first enters a nitrogen gas pipe with a length of 5 m. Use nitrogen with a purity of at least 99.99%. The nitrogen flows from top to bottom in a circulating manner, with the upper inlet and the lower outlet. The gas temperature is stable at 70 °C. At 25 mm below the lower outlet of the nitrogen gas pipe, use a grating writing module to write the fiber grating by the phase mask method to obtain a bare fiber grating, and the grating writing length is 10 mm; among them, use an excimer laser to write the grating, the light source is 193 nm, the light pulse energy is 100 mJ / Pulse, the voltage is 18 KV, and the exposure time interval is h / V, where h is the grating pitch and V is the drawing speed;

[0070] Step S4, the bare fiber grating sequentially enters a coating unit and a curing unit, so as to prepare a coating layer on the surface of the bare fiber grating. The material of the coating layer is acrylic resin to obtain a coated fiber grating;

[0071] Step S5, the coated fiber grating enters the grating calibration module for grating calibration to obtain the calibrated fiber grating. The calibration method is as follows: the positions of the fiber gratings in the optical fiber are L, L+h, L+2h, …, L+nh. Correspondingly, when the optical fiber runs to L+d, L+h+d, L+2h+d, …, L+nh+d, position calibration starts.

[0072] Step S6, the calibrated fiber grating is wound and formed by the wire winding unit, and then enters the post-heat treatment unit, where it is kept at 45°C for 24 h, and then cooled to room temperature at a cooling rate of 1°C / min to obtain the fiber grating.

[0073] Among them, the material of the core layer of the fiber grating is silica doped with germanium ions, the doping molar concentration of germanium ions is 1 mol%, the relative refractive index difference Δn1 is 0.43%, and the radius is 4 μm; the material of the inner cladding is pure silica, the relative refractive index difference Δn2 is 0, and the thickness is 7.5 μm; the material of the depressed layer is silica doped with fluoride ions, the doping molar concentration of fluoride ions is 0.1 mol%, the relative refractive index difference Δn3 is 0, and the thickness is 8 μm; the material of the outer cladding is pure silica, the relative refractive index difference Δn4 is 0, and the thickness is 62 μm.

[0074] Example 4

[0075] Step S1, the core layer preform and the inner cladding preform are sequentially prepared using the VAD process, then the depressed layer preform is prepared using the MCVD process, and finally the outer cladding preform is prepared using the OVD process to obtain an optical fiber preform with a diameter of 80 mm;

[0076] Step S2, the optical fiber preform is fed into the drawing furnace by the rod feeder, melted at a high temperature and drawn into a glass fiber, and then discharged from the lower part to obtain a bare optical fiber. Among them, the melting temperature is 2200°C, the melting atmosphere is a mixture of helium and argon, the volume percentage content of helium is 40%, the oxygen volume concentration <50 ppm, and the gas flow rate is 50 L / min; the drawing speed is 25 m / min, and the drawing speed deviation <1 m / min; the tension of the bare optical fiber during the drawing process is 20 g, the typical value of the cladding diameter is 125 μm, and the deviation is ±1.5 μm;

[0077] Step S3: The bare optical fiber first enters a nitrogen gas pipe with a length of 5 m. Nitrogen with a purity of at least 99.99% is used, and the nitrogen flows from top to bottom in a circulating manner, with gas inlet at the upper port and gas outlet at the lower port. The gas temperature is stabilized at 120 °C. At 25 mm below the lower port of the nitrogen gas pipe, a fiber grating is inscribed on the bare optical fiber using the phase mask method with a grating inscription module to obtain a bare optical fiber grating, and the grating inscription length is 20 mm. Among them, an excimer laser is used for grating inscription, the light source is 193 nm, the optical pulse energy is 200 mJ / Pulse, the voltage is 22 KV, and the exposure time interval is h / V, where h is the grating pitch and V is the drawing speed.

[0078] Step S4: The bare optical fiber grating sequentially enters a coating unit and a curing unit, so as to prepare a coating layer on the surface of the bare optical fiber grating. The material of the coating layer is acrylic resin to obtain a coated optical fiber grating.

[0079] Step S5: The coated optical fiber grating enters a grating calibration module for grating calibration to obtain a calibrated optical fiber grating. The calibration method is as follows: The positions of the fiber gratings in the optical fiber are L, L + h, L + 2h, …, L + nh. Correspondingly, when the optical fiber runs to L + d, L + h + d, L + 2h + d, …, L + nh + d, position calibration starts.

[0080] Step S6: The calibrated optical fiber grating is wound and formed through a take-up unit, and then enters a post-heat treatment unit, where it is kept at 60 °C for 16 h, and then cooled to room temperature at a cooling rate of 3 °C / min to obtain an optical fiber grating.

[0081] Among them, the material of the core layer of the optical fiber grating is silica doped with germanium ions, the doping molar concentration of germanium ions is 12 mol%, the relative refractive index difference Δn1 is 1%, and the radius is 5 μm; the material of the inner cladding is pure silica, the relative refractive index difference Δn2 is 0, and the thickness is 12 μm; the material of the depressed layer is silica doped with fluoride ions, the doping molar concentration of fluoride ions is 6 mol%, the relative refractive index difference Δn3 is -0.1%, and the thickness is 13.5 μm; the material of the outer cladding is pure silica, the relative refractive index difference Δn4 is 0, and the thickness is 63 μm.

[0082] The test results of the residual internal stress and side mode suppression ratio of the finished optical fiber gratings in Examples 1 to 4 are shown in Table 1.

[0083] Test method:

[0084] Residual internal stress: The magnitude of the internal stress is obtained according to the magnitude of the optical path difference formed by birefringence caused by stress (calculated based on interference light).

[0085] Side mode suppression ratio: The reflection spectrum is obtained according to the Rayleigh scattering principle, and the side mode suppression ratio is obtained through optimization calculation.

[0086] Table 1

[0087] Residual internal stress MPa Side mold suppression ratio dB Example 1 20 12 Example 2 23 11 Example 3 25 10 Example 4 24 10

[0088] As can be seen from the above, the fiber grating of the present invention uses a specific fiber grating structure, effectively improving the grating reflectivity, enabling it to have both good photosensitive characteristics and a long transmission distance for the optical fiber. The grating inscription process is carried out in a nitrogen atmosphere, greatly improving the stability of on-line fiber grating inscription. Additionally, a special post-heat treatment process is added, which can effectively eliminate the residual internal stress in the optical fiber and improve the side mode suppression ratio of the fiber grating.

[0089] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a fiber grating, characterized in that, The fiber Bragg grating sequentially includes a core layer, an inner cladding layer, a depressed layer, an outer cladding layer, and a coating layer from inside to outside. The preparation method includes the following steps: Step S1: Use the VAD process to sequentially prepare a core layer preform and an inner cladding layer preform, then use the MCVD process to prepare a depressed layer preform, and finally use the OVD process to prepare an outer cladding layer preform to obtain an optical fiber preform; Step S2: Melt and draw the optical fiber preform to obtain a bare optical fiber; Step S3: Adopt the phase mask method to perform grating inscription on the bare optical fiber in a nitrogen atmosphere to obtain a bare optical fiber grating; the gas temperature of the nitrogen atmosphere is 70 - 120 °C; Step S4: Coat and cure the bare optical fiber grating in sequence to obtain a coated optical fiber grating with the coating layer on its surface; Step S5: Calibrate the coated optical fiber grating to obtain a calibrated optical fiber grating; Step S6: Keep the calibrated optical fiber grating at 45 - 60 °C for 16 - 24 h for post - heat treatment, and then cool it to room temperature at a cooling rate of 1 - 3 °C / min to obtain the fiber Bragg grating; Among them, the material of the core layer is silica doped with germanium ions, the doping molar concentration of the germanium ions is 1 - 12 mol%, and the relative refractive index difference Δn1 is 0.43 - 1%; the material of the inner cladding layer is pure silica, and the relative refractive index difference Δn2 is 0; the material of the depressed layer is silica doped with fluoride ions, the doping molar concentration of the fluoride ions is 0.1 - 6 mol%, and the relative refractive index difference Δn3 is - 0.1% - 0; the material of the outer cladding layer is pure silica, and the relative refractive index difference Δn4 is 0.

2. The preparation method according to claim 1, characterized in that, The diameter of the optical fiber preform is 20 - 80 mm.

3. The preparation method according to claim 2, characterized in that, The diameter of the optical fiber preform is 30 - 40 mm.

4. The preparation method according to claim 1, characterized in that, In the fiber Bragg grating, the radius of the core layer is 4 - 5 μm, and / or the thickness of the inner cladding layer is 7.5 - 12 μm, and / or the thickness of the depressed layer is 8 - 13.5 μm, and / or the thickness of the outer cladding layer is 62 - 63 μm.

5. The preparation method according to claim 1 or 2, characterized in that, In Step S2, the melting temperature is 1800 - 2200 °C, and the melting atmosphere is helium and / or argon.

6. The preparation method according to claim 5, characterized in that, In Step S2, the volume percentage content of helium in the melting atmosphere is 0 - 40%, the oxygen volume concentration < 50 ppm, and the gas flow rate is 15 - 50 L / min.

7. The preparation method according to claim 1 or 2, characterized in that, In Step S2, the drawing speed is 5 - 25 m / min, and the drawing speed deviation < 1 m / min.

8. The preparation method according to claim 1 or 2, characterized in that, In Step S2, the tension of the bare optical fiber during the drawing process is 0 - 20 g.

9. The preparation method according to claim 1 or 2, characterized in that, In Step S3, the grating inscription length of the bare optical fiber grating is 10 - 20 mm.

10. The preparation method according to claim 1 or 2, characterized in that, In Step S3, use an excimer laser for the grating inscription, the light source is 193 nm, 244 nm, 248 nm, or 308 nm, the light pulse energy is 100 - 200 mJ / Pulse, the voltage is 18 - 22 KV, and the exposure time interval is h / V, where h is the grating pitch and V is the drawing speed.

11. The preparation method according to claim 1 or 2, characterized in that, In the step S4, the material of the coating layer is one or more of acrylic resin, modified acrylic resin, silicone resin, and polyimide.

12. An optical fiber grating, characterized in that, Obtained by the preparation method according to any one of claims 1 to 11.

13. A preparation device for an optical fiber grating, characterized in that, Prepared by using the preparation method according to any one of claims 1 to 11. According to the preparation process of the fiber grating, the preparation device includes: A wire drawing unit (1), which includes a rod feeder (11) and a wire drawing furnace (12). The rod feeder (11) is arranged above the wire drawing furnace (12). The wire drawing unit (1) is used for melting and drawing the optical fiber preform to obtain a bare optical fiber. A grating writing unit (2), which includes a nitrogen gas pipe (21) and a grating writing module (22). The grating writing module (22) is arranged below the nitrogen gas pipe (21). The grating writing unit (2) is used for grating writing on the bare optical fiber by the phase mask method to obtain a bare fiber grating. A coating unit (3), which is used for coating the bare fiber grating. A curing unit (4), which is used for curing the coated bare fiber grating to obtain a coated fiber grating. A grating calibration unit (5), which includes a grating calibration module (51). The grating calibration unit (5) is used for grating calibration of the coated fiber grating to obtain a calibrated fiber grating. A wire winding unit (6), which is used for winding and forming the calibrated fiber grating. A post heat treatment unit (7), which is used for post heat treatment of the calibrated fiber grating after winding and forming to obtain the fiber grating.

14. The preparation device according to claim 13, characterized in that, In the grating writing unit (2), the grating writing module (22) is arranged at a position 5 to 50 mm below the lower opening of the nitrogen gas pipe (21).

15. The production device according to claim 13, characterized in that, The grating writing unit (2) further includes a primary wire diameter gauge (23), which is arranged below the grating writing module (22) and is used for measuring the diameter of the bare fiber grating.

16. The preparation device according to claim 13, characterized in that, The grating calibration unit (5) further includes a secondary wire diameter gauge (52), which is arranged above the grating calibration module (51) and is used for measuring the diameter of the coated fiber grating.

17. The preparation device according to claim 13, characterized in that, The grating calibration method of the fiber grating includes: in the grating calibration module (51), when the coated fiber grating runs to a length of L + nh + d, the grating calibration is performed; where n is an integer greater than or equal to 0, L is the length of the wire winding unit (6), h is the grating pitch, and d is the distance between the grating writing module (22) and the coating unit (3).

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