A carbon-coated fiber grating and a method for manufacturing the same

Abstract

The application belongs to the field of fiber gratings, and specifically discloses a carbon-coated fiber grating and a preparation method thereof. The fiber grating comprises a fiber core, a cladding and a carbon coating layer arranged in sequence from inside to outside along the radial direction of the fiber grating. The fiber core is provided with gratings arranged at intervals along the axial direction of the fiber grating. The cladding comprises a first cladding and a second cladding in sequence from inside to outside along the radial direction of the fiber grating. The refractive index of the first cladding is smaller than the refractive indexes of the fiber core and the second cladding, the thickness of the first cladding is smaller than the thickness of the second cladding, and the single-side thickness of the first cladding along the radial direction of the fiber grating is smaller than 0.7 microns. Through the application, a carbon-coated fiber grating with better grating carving effect and good sealing performance is provided.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of fiber grating, more particularly, relates to a carbon-coated fiber grating and a preparation method thereof. BACKGROUND

[0002] A fiber grating is an optical device that establishes a refractive index periodic distribution (axial distribution or radial distribution) in a certain length of fiber core, only has the effect of mode coupling on specific wavelengths of light, and as an important fiber optical device, has the advantages of anti-electromagnetic interference, low transmission loss, small size, easy multiplexing and easy integration, and is widely used in many fields such as fiber sensing, fiber communication and industrial laser, but has the problems of large hydrogen damage and short service life when used in oil and gas wells. The conventional fiber grating cannot work effectively in the oil and gas well for a long time. The carbon-coated fiber has good hydrogen damage resistance and is very suitable for downhole application.

[0003] At present, the preparation of the carbon-coated fiber grating is to engrave the grating on the carbon-coated fiber, but the ultraviolet and femtosecond laser cannot penetrate the carbon coating layer or the penetration effect is poor, and the grating engraving cannot be completed or only the carbon-coated fiber with a carbon film thickness of 30 nm or less can be engraved with a grating. The hydrogen damage resistance of this type of carbon film is poor, and the grating engraving effect is poor.

[0004] In addition, when the existing technology is used to engrave the grating on the carbon-coated fiber, it is difficult to position the fiber core because the carbon coating layer is not transparent to light. When the fiber core is positioned through the edge of the fiber coating, the grating area often deviates from the fiber core due to insufficient accuracy. Moreover, due to the absorption of the carbon coating layer to the laser, a larger laser power is often required to achieve grating engraving. In addition, the carbon-coated fiber cannot be engraved with ultraviolet light, and can only be engraved with a femtosecond laser. These problems greatly limit the preparation and application of the carbon-coated fiber grating. SUMMARY

[0005] In view of the defects of the prior art, the purpose of the present application is to provide a carbon-coated fiber grating and a preparation method thereof, which aims to solve the problems of inaccurate positioning of the grating area, large required grating power, limited thickness of the carbon coating layer, poor grating engraving effect and poor hydrogen damage resistance of the fiber grating during the preparation and engraving of the existing fiber grating.

[0006] To achieve the above-mentioned purpose, in a first aspect, the present application provides a carbon-coated fiber grating, comprising: a fiber core, a cladding and a carbon coating layer arranged in sequence from inside to outside along the radial direction of the fiber grating.

[0007] The fiber core is provided with gratings arranged at intervals along the axial direction of the fiber grating.

[0008] The cladding comprises, in sequence from inside to outside along the radial direction of the fiber grating, a first cladding and a second cladding; the refractive index of the first cladding is smaller than the refractive indexes of the core and the second cladding, the thickness of the first cladding is smaller than the thickness of the second cladding, and the single-side thickness of the first cladding along the radial direction of the fiber grating is smaller than 0.7 μm.

[0009] It should be noted that a conventional cladding can be provided with an inner cladding and an outer cladding, the refractive index of the inner cladding being smaller than the refractive indexes of the core and the outer cladding to form a refractive index lower limit structure for confining the light transmitted in the fiber in the core transmission; the first cladding in the present application belongs to a newly added cladding with a relatively narrow thickness, which is usually small, and can be a single-side thickness smaller than 0.7 μm, so as to form a refractive index narrow depression design between the core and the second cladding; so that the boundary of the core can be clearly positioned, so as to realize real-time adjustment of the focal point during grating etching according to the core position when etching the grating on the core, realize high-precision grating etching, and improve the precision of grating etching. Preferably, the reaction raw material of the carbon coating layer is an aromatic hydrocarbon and / or a halide of an aromatic hydrocarbon, and the preparation of the carbon coating layer is performed after the completion of grating etching on the core. The specific preparation process can be referred to the introduction of the second aspect below.

[0010] In a possible implementation, the cladding further comprises a third cladding; the third cladding is coated outside the second cladding, and the refractive index of the third cladding is greater than the refractive index of the second cladding.

[0011] It can be understood that the third cladding described above is equivalent to a conventional outer cladding, which usually does not play a substantial role in fiber transmission, and therefore is not a necessary structure. Further, the outer cladding is mostly a deposition liner tube during the preparation of the fiber. If the liner tube is not stripped after the completion of fiber deposition, the liner tube can be retained as the outer cladding of the fiber to play a role of external protection for the fiber.

[0012] In a possible implementation, the relative refractive index difference of the first cladding relative to the second cladding ranges from -0.21% to -0.1%, and the single-side thickness of the first cladding along the radial direction of the fiber grating ranges from 0.15 μm to 0.7 μm.

[0013] In a possible implementation, the relative refractive index difference of the first cladding relative to the second cladding ranges from -0.15% to -0.07%, the single-side thickness of the first cladding along the radial direction of the fiber grating ranges from 0.15 μm to 0.7 μm, and the ratio of the diameter of the second cladding to the diameter of the core is greater than or equal to 2.5.

[0014] In a possible implementation, the relative refractive index difference of the first cladding relative to the third cladding ranges from -0.55% to -0.14%, and the relative refractive index difference of the second cladding relative to the third cladding ranges from -0.4% to -0.07%.

[0015] It should be noted that the refractive index difference of the first cladding relative to the second cladding or relative to the third cladding, the thickness ratio of the first cladding, etc. are set to be able to better achieve the positioning of the core, and those skilled in the art can configure the corresponding refractive index parameters according to actual verification.

[0016] In a possible implementation, the thickness of the carbon coating layer is greater than 30 nm.

[0017] Preferably, the thickness of the carbon coating layer is 30 nm to 120 nm.

[0018] Since the carbon coating layer in the present application is prepared after the grating is prepared, if other suitable reactants are selected to prepare the carbon coating layer, the preparation temperature of the carbon coating layer is reduced, and the damage of the grating caused by the later preparation of the carbon coating layer is avoided, then theoretically the preparation thickness of the carbon coating layer in the present application will no longer be limited.

[0019] In a second aspect, the present application provides a preparation method of a carbon-coated fiber grating, comprising the following steps:

[0020] Preparation of a fiber preform rod; the fiber preform rod comprises a core and a cladding arranged in turn along the radial direction of the fiber preform rod; the cladding comprises a first cladding and a second cladding; the refractive index of the first cladding is less than the refractive index of the core and the second cladding, the thickness of the first cladding is less than the thickness of the second cladding, and the single-side thickness of the first cladding along the radial direction of the fiber grating is less than 0.7 μm;

[0021] Drawing the fiber preform rod to obtain a drawn fiber;

[0022] Etching a grating on the core of the fiber; the position of the core is obtained according to the refractive index change trend of the first cladding relative to the core and the second cladding;

[0023] Using aromatic hydrocarbons and / or halogenated aromatic hydrocarbons as reaction raw materials, a carbon coating layer is prepared on the periphery of the fiber after etching the grating to obtain a carbon-coated fiber grating.

[0024] In a possible implementation, the preparation of the carbon coating layer on the periphery of the fiber after etching the grating comprises:

[0025] Preheating the fiber after etching the grating, and the preheating temperature is in the range of 600℃ to 1000℃;

[0026] Using aromatic hydrocarbons and / or halogenated aromatic hydrocarbons as reaction raw materials, a carbon coating layer is prepared on the periphery of the fiber after etching the grating to obtain a carbon-coated fiber grating.

[0027] In a possible implementation, the sealed gas is introduced when the optical fiber after etching the grating is preheated and when the carbon coating reaction is carried out, so as to prevent air from entering; further preferably, the flow rate of the sealed gas introduced when the optical fiber after etching the grating is preheated ranges from 1 L / min to 10 L / min; the flow rate of the gas of the reaction raw material introduced when the carbon coating reaction is carried out ranges from 0.1 L / min to 1 L / min, and the flow rate of the sealed gas introduced ranges from 0.5 L / min to 5 L / min.

[0028] In a possible implementation, the cladding further comprises a third cladding, the third cladding being wrapped around the second cladding, and the third cladding has a refractive index greater than that of the second cladding.

[0029] The method for preparing the optical fiber preform comprises the following steps:

[0030] The core and the cladding are prepared in the liner tube through a PCVD process, so as to obtain the optical fiber preform.

[0031] In a possible implementation, the relative refractive index difference of the first cladding relative to the second cladding ranges from -0.21% to -0.1%, and the radial single-side thickness of the first cladding along the fiber grating ranges from 0.15 μm to 0.7 μm.

[0032] In a possible implementation, when the cladding further comprises a third cladding, the relative refractive index difference of the first cladding relative to the second cladding ranges from -0.15% to -0.07%, the radial single-side thickness of the first cladding along the fiber grating ranges from 0.15 μm to 0.7 μm, and the ratio of the diameter of the second cladding to the diameter of the core is greater than or equal to 2.5.

[0033] Preferably, the relative refractive index difference of the first cladding relative to the third cladding ranges from -0.55% to -0.14%, and the relative refractive index difference of the second cladding relative to the third cladding ranges from -0.4% to -0.07%.

[0034] Overall, compared with the prior art, the above technical solutions conceived by the present application have the following beneficial effects:

[0035] The application provides a carbon-coated fiber grating and a preparation method thereof. Through the cross-section design of the narrow lower limit cladding close to the periphery of the fiber core, accurate positioning of the fiber core during grating inscription can be realized, and the quality stability of on-line grating inscription during the drawing process is ensured. In the application, the grating is inscribed first and then the carbon coating is performed, so that the absorption effect of the carbon coating layer or even the resin coating layer does not need to be considered during grating inscription, and the grating inscription effect is better (the depth of grating inscription can be deeper, and the uniformity is higher). In addition, the grating is inscribed first and then the carbon coating is performed, so that a relatively thick carbon coating layer can be prepared, and a carbon coating layer with good sealing performance is obtained. Compared with the process of performing carbon coating first and then inscribing the grating, the carbon coating layer in the application scheme will not interfere with the grating inscription process. By using the method, the prepared carbon-coated fiber grating not only has good reflectivity and sensitivity, but also has excellent hydrogen damage resistance, thereby ensuring the service life of the fiber grating in a water vapor and hydrogen-rich environment. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 FIG. 1 is a structural diagram of a carbon-coated fiber grating provided by an embodiment of the application;

[0037] Figure 2 FIG. 2 is a fiber grating array schematic diagram of the carbon-coated fiber grating provided by the embodiment of the application;

[0038] FIG. 3(a) is another structural diagram of the carbon-coated fiber grating provided by the embodiment of the application;

[0039] FIG. 3(b) is still another structural diagram of the carbon-coated fiber grating provided by the embodiment of the application;

[0040] Figure 4 FIG. 4 is a refractive index profile schematic diagram of the fiber core and cladding of the carbon-coated fiber grating provided by the embodiment of the application;

[0041] Figure 5 FIG. 5 is another refractive index profile schematic diagram of the fiber core and cladding of the carbon-coated fiber grating provided by the embodiment of the application;

[0042] Figure 6 FIG. 6 is a flow chart of the preparation method of the carbon-coated fiber grating provided by the embodiment of the application;

[0043] Figure 7 FIG. 7 is an architectural diagram of the preparation device of the carbon-coated fiber grating provided by the embodiment of the application;

[0044] Figure 8 FIG. 8 is a process flow chart of the preparation process of the carbon-coated fiber grating provided by the embodiment of the application;

[0045] In all the drawings, the same reference signs are used to indicate the same elements or structures, wherein: 1 is a core, 2 is a cladding, 3 is a carbon coating layer, 4 is a resin coating layer, 5 is a grating; 21 is a first cladding, 22 is a second cladding, 23 is a third cladding; 100 is a grating etching device, 200 is a preheating furnace, 300 is a carbon coating reaction chamber, 400 is a resin coating device, 500 is a resin curing furnace, 600 is a fiber drawing furnace, and 700 is a fiber preform. DETAILED DESCRIPTION

[0046] In order to make the purposes, technical solutions and advantages of the present application clearer, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.

[0047] The embodiments of the present application are described below in combination with the drawings in the embodiments of the present application.

[0048] Figure 1 is a structural diagram of a carbon-coated fiber grating provided by the embodiments of the present application; as shown in Figure 1 , the carbon-coated fiber grating generally includes, in order from inside to outside along the radial direction of the fiber grating: a core 1, a cladding 2, a carbon coating layer 3, and a resin coating layer 4.

[0049] Further, the above-mentioned core 1 is provided with gratings 5 arranged at intervals along the axial direction of the fiber grating, as shown in Figure 2 . Among them, the above-mentioned grating can be a conventional grating array or a weak grating array, which can be set according to actual needs by those skilled in the art, and the embodiments of the present application do not make any limitation on this.

[0050] Among them, the conventional cladding 2 generally includes an inner cladding and an outer cladding, that is, generally 2 layers of cladding.

[0051] The carbon coating layer 3 is usually a nanoscale carbon film, which can effectively prevent hydrogen in the hydrogen-rich gas from reaching the inside of the fiber when the thickness reaches a certain thickness, and has good hydrogen damage resistance.

[0052] Further preferably, the thickness of the above-mentioned carbon coating layer 3 can be greater than 30 nm, so that the fiber grating has good hydrogen damage resistance.

[0053] The resin coating layer 4 is used to protect the fiber and prevent loss and interference of optical signals, and generally needs to have good adhesion, waterproofness, corrosion resistance and other properties. Commonly used resin coating layer materials include: polyimide or acrylate organic resin.

[0054] It should be noted that when the organic resin is polyimide, it is generally necessary to coat and cure two to three times due to the thin thickness of a single coating. When the organic resin is an acrylate coating, it is generally necessary to coat a low modulus inner layer resin and a high modulus outer layer resin respectively, which is a conventional process and will not be described in detail. In addition, in the above-mentioned scheme provided by the embodiments of the present application, the resin coating layer 4 is replaced by a metal coating layer, which is also a potential optional scheme.

[0055] In some embodiments, when the resin coating layer 4 is a polyimide coating, the diameter of the resin coating layer is 145 μm to 165 μm, and when the resin coating layer 4 is a double-layered acrylate resin, the diameter of the inner coating layer is 180 μm to 210 μm, and the diameter of the outer coating layer is 230 μm to 260 μm.

[0056] Further preferably, in order to improve the existing carbon-coated fiber grating, the embodiments of the present application mainly adopt the following three measures: the first measure is to divide the cladding layer 2 into three cladding layers, and a narrow depressed cladding layer is added next to the outer periphery of the core 2, wherein the depression refers to the change of the refractive index. The second measure is to perform the grating inscription operation on the core before the preparation of the carbon coating layer, that is, to inscribe the grating first and then prepare the carbon coating layer. The third measure is to select aromatic hydrocarbons and their halogenated compounds as the raw materials for the preparation of the carbon coating layer, which is different from the selection of ethyne, ethylene and other raw materials for the existing carbon coating layer.

[0057] The above three measures will be introduced in combination with the accompanying drawings and related descriptions as follows:

[0058] For the first measure, referring to FIG. 3(b), the cladding layer 2 can include a first cladding layer 21 and a second cladding layer (which can be understood as the inner cladding layer in the existing fiber grating) 22; the newly added cladding layer is the first cladding layer 21.

[0059] Referring to FIG. 3(b), the cladding layer 2 can include a first cladding layer 21, a second cladding layer (which can be understood as the inner cladding layer in the existing fiber grating) 22 and a third cladding layer (which can be understood as the outer cladding layer in the existing fiber grating) 23; the newly added cladding layer is the first cladding layer 21.

[0060] Referring to FIG. 3(b), the cladding layer 2 can include a first cladding layer 21, a second cladding layer (which can be understood as the inner cladding layer in the existing fiber grating) 22 and a third cladding layer (which can be understood as the outer cladding layer in the existing fiber grating) 23; the newly added cladding layer is the first cladding layer 21. Figure 4 and Figure 5 wherein, Figure 4 is a pure silicon core fiber cross-sectional view provided by the embodiments of the present application, Figure 5 is a single-mode fiber cross-sectional view provided by the embodiments of the present application, Figure 4 in which the pure silicon of the core 1 and the third cladding layer (the outer cladding layer) 23 are pure silicon, so their refractive indexes are consistent, Figure 5 in which the core 1 is not pure silicon, and the refractive index of the core 1 is greater than that of pure silicon, so Figure 5 the refractive index of the core 1 is greater than that of the third cladding layer (the outer cladding layer) 23.

[0061] Combining Figure 4 And Figure 5 It can be seen that the structure of the core 1 and the cladding 2 can be a pure silicon core design or a conventional single-mode design. Therefore, the scheme of the present application can be applied to different fiber cores, and the design concept of the carbon-coated fiber grating is applicable to different types of optical fibers and can be applied in different carbon-coated fiber gratings. The embodiments of the present application do not make special description here.

[0062] Further, referring to FIG. 3(b), the structure of the cladding 2 is further described. Figure 4 And Figure 5 It can be seen that the refractive index of the first cladding 21 to the third cladding 23 in the cladding 2 increases in turn, and the refractive index of the first cladding 21 is less than the refractive index of the core 1. In combination with FIG. 3 to Figure 5 It can be seen that the thickness of the newly added first cladding 21 is extremely narrow, and the thickness of the cladding 2, even the first cladding 21 in the structure of the core 1 and the cladding 2, is small. In this way, a layer of narrow lower limit cladding is arranged outside the core 1.

[0063] It should be noted that due to the narrow depression design between the fiber core layer and the cladding, the boundary of the core can be clearly positioned, and when the core is close to the objective lens used for grating inscription and far away from the objective lens used for grating inscription, there will be obvious refractive index boundary changes, which can realize effective tracking of the core and accurate positioning of the core. After realizing the accurate positioning of the core, the real-time adjustment of the focal point during grating inscription can be realized according to the position of the core, the high-precision grating area inscription is realized, and the precision of grating etching is improved.

[0064] Further optionally, the thickness of the first cladding 21 along the radial direction of the fiber grating is 0.15 μm to 0.7 μm. When the thickness of the above-mentioned first cladding 21 is too large, it cannot better play the positioning effect of the core 1, and therefore the thickness ratio of the first cladding needs to be relatively small.

[0065] In a more specific embodiment, when the cladding 2 includes the first cladding 21 and the second cladding 22, the relative refractive index difference of the first cladding 21 relative to the second cladding 22 is in the range of -0.21% to -0.1%; the thickness of the first cladding 21 along the radial direction of the fiber grating is in the range of 0.15 μm to 0.7 μm;

[0066] In a more specific embodiment, when the cladding 2 comprises the first cladding 21, the second cladding 22 and the third cladding 23, the relative refractive index difference of the first cladding 21 to the second cladding 22 ranges from -0.15% to -0.07%; the radial single-side thickness of the first cladding 21 to the fiber grating ranges from 0.15 μm to 0.7 μm, and the diameter of the second cladding 22 is 2.5 times or more of the diameter of the core 1; further preferably, the relative refractive index difference of the first cladding 21 to the third cladding 23 ranges from -0.55% to -0.14%, and the relative refractive index difference of the second cladding 22 to the third cladding 23 ranges from -0.4% to -0.07%.

[0067] It should be noted that the processes of fiber preform drawing, on-line grating drawing and carbon coating drawing are generally carried out at the same time (see the description of Figure 7 and Figure 8 ); accordingly, the carbon coating drawing speed is generally fast, while the on-line grating drawing speed is generally slow, and the design of increasing the refractive index depression between the core and the cladding can facilitate the determination of the core boundary during grating drawing, so as to realize fast and accurate positioning of the core during grating drawing, improve the on-line grating drawing speed, and better match the faster carbon coating drawing speed.

[0068] For the second measure, the conventional carbon-coated fiber grating generally etches the core after the preparation of the carbon coating layer. However, as introduced in the background, this way has the following disadvantages: the carbon coating layer cannot be too thick, otherwise the light source cannot penetrate the carbon coating layer or the penetration effect is poor when etching the grating on the core, and accordingly the grating etching cannot be performed or the etching effect is poor (shallow etching depth and / or poor etching uniformity); after the carbon coating layer is thinned, the grating etching can be performed, but the hydrogen damage resistance of the too thin carbon coating layer is poor, and the performance of the fiber is also poor, which cannot be applied to the hydrogen-rich gas environment, and the application will be greatly limited.

[0069] To solve the above problems, the step of grating etching in the embodiments of the present application is controlled to be performed before the preparation of the carbon coating layer, at this time the penetration of the light source of etching the grating through the fiber cladding to the core will no longer be limited by the carbon coating layer, and therefore the penetration effect will be better and more controllable, and the etching depth of the grating can be deeper and the etching uniformity will be better.

[0070] Further preferably, the point spacing of the grating is 1 m to 5 m, and the length of the grating is 3 mm to 10 mm, so that the effective modulation area is greater than 90% of the core area.

[0071] Further preferably, the thickness of the carbon coating layer is greater than 30 nm; the thickness of the carbon coating layer is 30 nm to 120 nm.

[0072] On this basis, when the grating is etched, the carbon coating layer is prepared again at this time, and the subsequent carbon coating layer is no longer limited by the grating etching, and can be flexibly set. The thickness of the carbon coating layer is set to consider the demand for hydrogen damage resistance. Therefore, a carbon-coated fiber grating with better hydrogen damage resistance can be prepared.

[0073] It should be noted that the second measure described above is combined with the third measure, as described below.

[0074] For the third measure, Figure 6 is a flow chart of a preparation method of a carbon-coated fiber grating provided by an embodiment of the present application; as Figure 6 shown, comprising the following steps:

[0075] Step S101, preparing a fiber preform; the fiber preform includes a core and a cladding arranged in turn along the radial direction of the fiber preform; the cladding includes a first cladding, a second cladding and a third cladding; the refractive index of the first cladding to the third cladding increases in turn, the thickness of the first cladding in the first cladding to the third cladding is the smallest, and the single-side thickness of the first cladding along the radial direction of the fiber grating is less than 0.7 μm.

[0076] It should be noted that the core, the first cladding and the second cladding can be prepared in a liner tube by a plasma chemical vapor deposition (PCVD) process to obtain the fiber preform; wherein the liner tube serves as the third cladding; or the core and the cladding can be prepared in a liner tube by a PCVD process, and then the liner tube is removed to obtain the fiber preform.

[0077] Step S102, drawing the fiber preform to obtain a drawn fiber.

[0078] It can be understood that when the fiber preform is drawn into the size of the required fiber, it can be referred to as a fiber at this time.

[0079] Figure 7 is a schematic diagram of a preparation device of a carbon-coated fiber grating provided by an embodiment of the present application; in combination with Figure 7 It can be known that after obtaining the fiber preform, the fiber drawing, grating etching, carbon coating and resin coating are prepared in a process, that is, the fiber drawing, grating etching, carbon coating and resin coating are carried out along the axial direction of the fiber preform, and finally the fiber is collected. The specific process flow can also be referred to in Figure 8As shown, the preform rod is continuously drawn in the axial direction when being drawn, and is sequentially sent to subsequent devices for processing according to the order of wire drawing; the part drawn first (which can be referred to as an optical fiber) is first sent to the grating etching device 100 for grating etching, and the optical fiber after grating etching has been cooled and needs to be sent to the preheating furnace 200 for preheating, and after preheating, is sent to the carbon coating reaction chamber 300 for carbon coating; then, is sent to the resin coating device 400 and the resin curing furnace 500 for resin layer preparation, and then is collected to obtain the prepared carbon-coated fiber grating.

[0080] In step S103, a grating is etched on the fiber core; the position of the fiber core is positioned according to the refractive index variation trend of the first cladding relative to the fiber core and the second cladding.

[0081] It should be noted that the optical fiber preform rod can be drawn into a bare optical fiber in a drawing furnace at about 2000°C; then the bare optical fiber first passes through the positioning window of the femtosecond machining platform during grating etching. The white light source transmits through the fiber core and passes through the dichroic mirror to realize the transmission of the imaging light to the high-speed CCD to realize fiber core imaging; in the above process, due to the narrow depression design between the fiber core layer and the cladding layer, the boundary of the fiber core can be clearly positioned, and when the fiber core is close to the objective lens and far away from the objective lens, there will be obvious boundary changes, realizing effective tracking of the fiber core, and the focal point can be adjusted in real time according to the position of the fiber core.

[0082] Further, after the fiber core positioning is completed, the above grating etching process can refer to the related prior art. In one example, an infrared femtosecond laser can be used to output a laser of a preset wavelength and a Gaussian spot of a preset spot size, the power of the femtosecond laser is precisely controlled through a power attenuator, and then the spot is focused on the fiber core through a long working distance objective lens to realize point-by-point grating etching.

[0083] In step S104, an aromatic hydrocarbon and / or a halide of an aromatic hydrocarbon are used as reaction raw materials to prepare a carbon coating layer on the periphery of the optical fiber after grating etching, to obtain a carbon-coated fiber grating.

[0084] It should be noted that the above steps S103 and S105 are the third measure adopted by the embodiments of the present application, that is, the grating is etched first, and then an aromatic hydrocarbon and / or a halide of an aromatic hydrocarbon are used as reaction raw materials to prepare a carbon coating layer on the periphery of the optical fiber after grating etching.

[0085] Specifically, after the fiber grating is etched, this part of the optical fiber has been cooled, so it needs to be preheated before being sent to the carbon coating device; however, due to the grating etching process, the drawing speed of the optical fiber preform rod needs to be relatively low, and the temperature of the optical fiber decreases rapidly after preheating, and the temperature of the optical fiber after preheating may have dropped to below 800°C, for example, 600°C-800°C.

[0086] The embodiment of the present application selects aromatic hydrocarbon and / or halide of aromatic hydrocarbon as the reaction raw material of the carbon coating layer, and the corresponding carbon coating reaction temperature is 600-800°C, so that the drawing speed can be reduced to realize grating writing before carbon coating, and the temperature (600-800°C) of the preheated grating-written optical fiber in the reaction chamber is sufficient to promote the cracking reaction, while avoiding the influence of the excessively high carbon coating reaction temperature on the grating.

[0087] It should be noted that, generally, the ultraviolet-written grating will gradually start to be erased at 300-500°C or above according to the writing depth, but there will still be a weak grating structure, and the weak grating array is also a common grating type (when a weak grating array needs to be prepared, the reaction temperature of the carbon coating layer provided by the embodiment of the present application is 600-800°C, which is suitable for the preparation of a weak grating after ultraviolet grating writing, so the scheme provided by the embodiment of the present application can be applied to the ultraviolet grating writing mode); further, the femtosecond-written grating will be affected only at 800-1200°C or above, and can normally perform the carbon coating preheating process described in the embodiment of the present application. When the femtosecond-written grating is used in the embodiment of the present application, the preheating temperature of the preheating furnace needs to be reduced as much as possible to below the range of 800-1200°C, for example, the preheating temperature range is 600-1000°C, that is, the preheating process does not damage the etched grating, and the temperature of the optical fiber after preheating entering the carbon coating reaction chamber is reduced to the range of 600-800°C.

[0088] It should be further noted that the use of aromatic hydrocarbon as the raw material in the present application can shorten the reaction process, improve the reaction efficiency, shorten the preparation time of the carbon coating layer, and improve the preparation efficiency of the carbon coating layer.

[0089] In summary, the carbon-coated fiber grating preparation method disclosed by the present application can be divided into four steps: the first step is to prepare a preform rod with a narrow depression profile design, the second step is to perform online grating writing during the drawing process through a grating writing device, the third step is to heat the optical fiber after the second step through preheating and then enter a carbon coating reaction chamber to deposit a carbon film, and the fourth step is to coat and cure a polyimide coating or an acrylate coating.

[0090] The carbon-coated fiber grating prepared by the above method has the following characteristics: the narrow depression profile cladding design of the optical fiber ensures the quality stability of the online grating writing during the drawing process; the carbon coating after grating writing ensures that the absorption of the carbon coating layer or even the resin coating layer does not need to be considered during grating writing, and the grating writing effect is better; the carbon coating after grating writing can prepare a carbon coating layer with a thickness greater than 30nm, and the carbon coating layer with good sealing performance ensures the service life of the optical fiber grating in a water vapor and hydrogen-rich environment.

[0091] To supplement the embodiments of the present application, the following can refer to the relevant data of some embodiments and comparative examples given in Table 1 and Table 2:

[0092] Table 1

[0093]

[0094] It should be noted that the effective modulation area of the fiber grating in each embodiment in the above Table 1 is greater than 90% of the core area, and the reflectivity range of the ultraviolet grating is 0.01%~0.1%, and the reflectivity of the femtosecond grating is between 50%~60%; It should be noted that the effective modulation area of the fiber grating desired in the present application is greater than 90%; The ultraviolet grating is generally an array, and the reflectivity corresponds to a reflectivity range. The smaller the range, the smaller the reflectivity fluctuation, and the higher the grating quality. The femtosecond grating is a single-point grating, and the reflectivity corresponds to an accurate value. The higher the reflectivity, the better the grating performance. It can be seen that the indicators of the carbon-coated fiber grating prepared in the embodiments given in the above Table 1 are within the corresponding indicator range.

[0095] Table 2

[0096]

[0097] Referring to the comparative examples given in the above Table 2, the reflectivity fluctuation range of the ultraviolet grating is 0.0001%~0.1%, or 0.001%~0.1%, and the reflectivity fluctuation range is large, and the grating quality is poor; The reflectivity of the femtosecond grating is less than 50%; In addition, the effective modulation area of the above comparative example 1, comparative example 3 and comparative example 4 accounts for less than 90%; It can be seen that if any of the corresponding cladding, refractive index or diameter range in the embodiments of the present application is not followed, the effective modulation area and / or reflectivity of the fiber grating prepared cannot meet the indicator requirements of the fiber grating.

[0098] The following will take examples 1 and 2 in the above Table 1 as an example for further introduction:

[0099] Example 1:

[0100] The prepared fiber cross-sectional design is shown in Figure 4 The fiber is a single-mode fiber, the core is a pure silicon core, the refractive index is 1.457, and the diameter is 9 μm; The first cladding diameter is 11 μm, and the corresponding refractive index is 1.45; The second cladding diameter is 75 μm, and the corresponding refractive index is 1.452; The outer cladding is pure silicon, and the diameter is 125 μm, and the corresponding refractive index is 1.457; The coating layer is polyimide, and the diameter is 155 μm.

[0101] In combination with Figure 8The preparation process diagram shown, the fiber grating described in example 1 is prepared, the preform is heated and softened in the drawing furnace to draw the optical fiber, and the drawing speed can be 20 m / min. After the optical fiber is drawn, the optical fiber enters the grating writing window, at this time the cladding diameter of the optical fiber is 125 um, the core diameter is 9 um, the core distance is a suitable distance from the phase mask, the laser ultraviolet light is used as the energy source, the square light spot with suitable energy and suitable size is output, the single pulse output mode is used, a pulse output is completed at a preset time interval, the light spot is initially shaped through the adjustable diaphragm, then expanded through two cylindrical lenses in the direction of the fiber drawing, then compressed through the cylindrical lens in the direction perpendicular to the fiber drawing, and then the effective exposure length is 5 mm through the grating control, and the grating point is written through the interference of the ±1 order diffracted light through the phase mask.

[0102] After the grating is written, the optical fiber enters the preheating furnace, the temperature of the preheating furnace can be 900 DEG C, and the optical fiber quickly enters the reaction cavity after leaving the preheating furnace for carbon coating. The carbon coating reaction device is as shown in Figure 7 Due to the low drawing speed, the preheating furnace is filled with inert protective gas, helium, argon or nitrogen, the gas flow is 5 L / min, the preheating furnace 100 is directly connected with the reaction cavity 200, the sealing gas is also one of helium, argon and nitrogen, and the sealing gas also needs to be fully preheated. The sealing gas flow is 1 L / min, the temperature of the optical fiber in the reaction cavity 200 is about 700 DEG C ~ 800 DEG C, the carbon coating raw material is benzene vapor, the flow rate is 0.25 L / min, the cracking reaction occurs in the reaction cavity 200, and the amorphous carbon film is deposited on the surface of the optical fiber cladding. The reaction exhaust gas is discharged from the exhaust pipe.

[0103] The optical fiber with the carbon film continues to be coated and cured with polyimide for single or multiple times, and finally forms a carbon-coated fiber grating array with a polyimide coating layer and an outer diameter of 155 um. The hydrogen damage test is performed on the fiber grating, and the additional loss increases by less than 0.1 dB / km under the test conditions of 200 DEG C, 10 standard atmospheres and 3 days, which indicates that the embodiment has good hydrogen damage resistance. The optical indicators of the fiber grating do not appear obvious attenuation, the grating center wavelength is 1550 nm, the bandwidth is 0.3 nm, and the reflectivity is in the range of 0.01% ~ 0.1%.

[0104] Example 2:

[0105] The optical fiber is a single-mode optical fiber, the core refractive index is 1.462, the diameter is 9 μm, the first cladding diameter is 11 μm, the corresponding refractive index is 1.455, the second cladding diameter is 75 μm, the corresponding refractive index is 1.456, the third cladding is a pure silicon outer cladding layer, the diameter is 125 μm, the corresponding refractive index is 1.457, and the coating layer is a double-layer ultraviolet curing acrylate, the inner coating layer modulus is 0.7 MPa, the diameter is 195 μm, the outer coating layer modulus is 800 MPa, and the diameter is 250 μm.

[0106] In combination Figure 8 The fiber grating described in Example 1 is prepared according to the preparation process shown in the figure, and the preform is heated and softened in a drawing furnace to draw an optical fiber, and the drawing speed can be 120 m / min.

[0107] After the fiber drawing is completed, the fiber enters a grating writing window, the fiber cladding diameter is 125 um, the fiber core diameter is 9 um, the fiber core is at a suitable distance from the phase mask, an infrared femtosecond laser is used as an energy source, an appropriate energy and size of a Gaussian spot is output, passes through a reflector to reach a power attenuator to realize accurate control of the femtosecond laser power, then passes through a dichroic mirror to realize reflection of the preset wavelength light, and then passes through a long working distance objective lens to focus the spot to the fiber core. At this time, the processing light path is fixed on a three-dimensional adjustment table, the image is enlarged through the white light passing through the fiber core through the objective lens, then passes through the dichroic mirror to realize transmission of the imaging light to the high-speed CCD to realize fiber core imaging, the fiber core tracking is realized through a software algorithm, the focal point is adjusted in real time according to the fiber core position, and the femtosecond point-by-point grating writing is realized. The grating point spacing is 2 m, the grating length is 5 mm, and the effective modulation area is more than 95% of the fiber core area.

[0108] After the grating writing is completed, the fiber enters a heating furnace, the heating furnace temperature can be 900 ℃, and the fiber quickly enters a reaction cavity after leaving the heating furnace for carbon coating. The preheating furnace does not need to be filled with inert protective gas, and the preheating furnace is separated from the reaction cavity by about 1 cm to prevent the gas in the furnace from being brought into the reaction cavity by the fiber. The sealing gas is also one of helium, argon and nitrogen, and the sealing gas flow is 1 L / min. The sealing gas also needs to be preheated. The temperature of the fiber in the reaction cavity is about 700 ℃-800 ℃. The carbon coating raw material is benzene vapor, and the flow rate is 0.5 L / min. The cracking reaction occurs in the reaction cavity to generate an amorphous carbon film deposited on the surface of the fiber cladding. The reaction exhaust gas is discharged from the exhaust pipe.

[0109] The fiber with the deposited carbon film continues to be coated with acrylate coating and cured. The curing can be performed in an ultraviolet curing furnace. The above coating and curing process is repeated to complete the curing of the outer coating layer. The coated inner and outer coating layers are high-temperature resistant acrylic resins.

[0110] The fiber grating is tested for hydrogen damage, and the additional loss increases by less than 0.1 dB / km under the test conditions of 150 DEG C, 10 standard atmospheres, and 3 days, indicating that the embodiment has good hydrogen damage resistance. The optical indicators of the fiber grating do not show obvious attenuation, the center wavelength is 1550 nm, the bandwidth is 0.3 nm, and the reflectivity is 55%.

[0111] In summary, the present application can facilitate the core finding process of online grating inscription by designing the refractive index profile of the core and cladding, and can realize high-speed and accurate grating inscription. Then, the grating inscription is performed first in the drawing process, and then the carbon coating process is performed. Compared with the process of coating carbon first and then inscribing the grating, the carbon coating layer does not interfere with the grating inscription process. By using this method, the prepared carbon-coated fiber grating not only has good reflectivity and sensitivity, but also has excellent hydrogen damage resistance. The entire process integrates the strict matching of the fiber refractive index profile design, the grating inscription process, and the carbon coating process, so that the final product, the carbon-coated fiber grating, has good quality and hydrogen damage resistance.

[0112] It should be understood that expressions such as "include" and "may include" used in the present application indicate the presence of disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In the present application, terms such as "include" and / or "have" can be interpreted as indicating the presence of a specific characteristic, number, operation, constituent element, component, or combination thereof, but cannot be interpreted as excluding the presence or addition of one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0113] In addition, in the present application, the expression "and / or" includes any and all combinations of the associated listed terms. For example, the expression "A and / or B" can include A, can include B, or can include both A and B.

[0114] In the description of the embodiments of the present application, it should be noted that unless otherwise clearly specified and limited, the term "connection" should be understood broadly, for example, "connection" can be detachable connection, or can be non-detachable connection; can be direct connection, or can be indirect connection through intermediate medium. Among them, "fixed connection" refers to the relative position relationship after connection is unchanged. "Rotary connection" refers to the relative rotation after connection. "Sliding connection" refers to the relative sliding after connection. The orientation language mentioned in the embodiments of the present application, such as "top", "bottom", "inner", "outer", "left", "right", etc., is only the direction of the drawing, therefore, the orientation language used is to better, more clearly illustrate and understand the embodiments of the present application, and is not indicative or implied that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, therefore, it cannot be understood as a limitation on the embodiments of the present application.

[0115] In addition, in the embodiments of the present application, the mathematical concepts mentioned, such as symmetry, equality, parallel, perpendicular, etc. These limitations are all for the current process level, and are not strictly defined in the mathematical sense, and a small amount of deviation is allowed, such as approximately symmetric, approximately equal, approximately parallel, approximately perpendicular, etc. For example, A is parallel to B, which means that A and B are parallel or approximately parallel, and the included angle between A and B can be between 0 degrees and 10 degrees. A is perpendicular to B, which means that A and B are perpendicular or approximately perpendicular, and the included angle between A and B can be between 80 degrees and 100 degrees.

[0116] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A carbon-coated fiber grating, characterized by, The application relates to a carbon-coated fiber grating. The fiber core, the cladding and the carbon coating layer are arranged in sequence from inside to outside along the radial direction of the fiber grating. The fiber core is provided with etched gratings arranged at intervals along the axial direction of the fiber grating, and the carbon coating layer is prepared on the fiber periphery of the etched gratings. The cladding comprises the first cladding and the second cladding in sequence from inside to outside along the radial direction of the fiber grating.

2. The fiber grating of claim 1, wherein, The third cladding is arranged on the periphery of the second cladding, and the refractive index of the third cladding is greater than that of the second cladding.

3. The fiber grating of claim 1, wherein, The relative refractive index difference of the first cladding relative to the second cladding ranges from -0.21% to -0.1%.

4. The fiber grating of claim 2, wherein, The relative refractive index difference of the first cladding relative to the second cladding ranges from -0.15% to -0.07%, and the ratio of the diameter of the second cladding to the diameter of the fiber core is greater than or equal to 2.

5.

5. The fiber grating of claim 2, wherein, The relative refractive index difference of the first cladding relative to the third cladding ranges from -0.55% to -0.14%, and the relative refractive index difference of the second cladding relative to the third cladding ranges from -0.4% to -0.07%.

6. The fiber grating of claim 1, wherein, The thickness of the carbon coating layer is greater than 30 nm.

7. The fiber grating of claim 6, wherein, The thickness of the carbon coating layer ranges from 30 nm to 120 nm.

8. A method of making a carbon-coated fiber grating, comprising: The application further discloses a preparation method of the carbon-coated fiber grating. The fiber preform rod comprises the fiber core and the cladding arranged in sequence from inside to outside along the radial direction of the fiber preform rod. The fiber preform rod is drawn to obtain a drawn fiber. The etched gratings are arranged on the fiber core, and the position of the fiber core is determined according to the refractive index variation trend of the first cladding relative to the fiber core and the second cladding. The carbon coating layer is prepared on the fiber periphery of the etched gratings by using aromatic hydrocarbons and / or halogenated aromatic hydrocarbons as reaction raw materials, so as to obtain the carbon-coated fiber grating.

9. The method of claim 8, wherein, The carbon coating layer is prepared on the fiber periphery after the etched gratings, which comprises the following steps. The fiber after the etched gratings is preheated, and the preheating temperature ranges from 600 DEG C to 1000 DEG C. The carbon coating reaction is carried out on the preheated fiber by using aromatic hydrocarbons and / or halogenated aromatic hydrocarbons as reaction raw materials, so as to prepare the carbon coating layer on the fiber periphery after the etched gratings. The sealing gas is introduced when the fiber after the etched gratings is preheated and when the carbon coating reaction is carried out, so as to prevent air from entering.

10. The method of claim 9, wherein, The flow rate of the sealing gas introduced when the fiber after the etched gratings is preheated ranges from 1 L / min to 10 L / min. The gas flow rate of the reaction raw materials introduced during the carbon coating reaction ranges from 0.1 L / min to 1 L / min, and the flow rate of the sealing gas introduced ranges from 0.5 L / min to 5 L / min. And / or the cladding further comprises: a third cladding, the third cladding is coated outside the second cladding, the refractive index of the third cladding is greater than the refractive index of the second cladding; The preparation optical fiber preform comprises: The core and the cladding are prepared in the liner tube by a PCVD process to obtain the optical fiber preform.

11. The method of claim 8, wherein, The relative refractive index difference of the first cladding relative to the second cladding ranges from -0.21% to -0.1%, and the radial single-side thickness of the first cladding along the fiber grating ranges from 0.15 μm to 0.7 μm.

12. The method of claim 9, wherein, When the cladding further comprises a third cladding, the relative refractive index difference of the first cladding relative to the second cladding ranges from -0.15% to -0.07%, the radial single-side thickness of the first cladding along the fiber grating ranges from 0.15 μm to 0.7 μm, and the ratio of the diameter of the second cladding to the diameter of the core is greater than or equal to 2.

5.

13. The method of claim 12, wherein, The relative refractive index difference of the first cladding relative to the third cladding ranges from -0.55% to -0.14%, and the relative refractive index difference of the second cladding relative to the third cladding ranges from -0.4% to -0.07%.

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