Specialized drawing equipment and method for fiber grating fabrication

Abstract

This invention proposes a special fiber drawing equipment and method for fabricating fiber gratings. The equipment includes a fiber drawing furnace and an ultraviolet exposure system arranged sequentially from high to low. An optical fiber preform is melted into bare fiber in the drawing furnace, and the ultraviolet exposure system inscribes the grating onto the bare fiber. A sensitization system is also provided, positioned between the drawing furnace and the ultraviolet exposure system. The sensitization system includes an oxyhydrogen torch for forming an oxyhydrogen flame, which is directed at the grating inscription area in the bare fiber to enhance its sensitivity. This invention, by adding a sensitization system between the drawing furnace and the grating inscription system, utilizes the combustion of hydrogen and oxygen to form an oxyhydrogen flame to sensitize the freshly melted bare fiber, thereby improving the photosensitivity of the grating inscription area and achieving the fabrication of highly reflective fiber gratings.

Description

Technical Field

[0001] This invention belongs to the field of fiber grating production technology, specifically relating to a special fiber drawing equipment and method for fiber grating fabrication. Background Technology

[0002] The traditional method for fabricating fiber gratings involves utilizing the absorption characteristics of germanium doped in the fiber at ultraviolet wavelengths to achieve a photoinduced refractive index change in the fiber core. The intensity of this photoinduced refractive index change directly determines the reflectivity of the fiber grating for a specific wavelength (from one part per million to 99.9%). Because ordinary optical fibers have inherently low doping concentrations, their photosensitivity is weak. In direct fabrication, a single-pulse modulation intensity can only result in a reflectivity of one part per million to one part per ten thousand, while a multi-pulse modulation intensity can only result in a reflectivity of a few percent.

[0003] To improve the reflectivity of fiber Bragg gratings, the current main method involves loading the optical fiber with hydrogen. This utilizes the hydrogen to form a special lattice structure with germanium, oxygen, and silicon in the fiber core, thereby increasing photosensitivity and enabling the fabrication of high-reflectivity fiber Bragg gratings. Hydrogen loading requires a special environment; the optical fiber must be placed in a high-concentration (99.9% pure hydrogen) environment at 100 atmospheres for more than 24 hours in a closed system to allow hydrogen infiltration. However, this technology cannot be used in the dynamic fabrication process of fiber drawing towers.

[0004] In existing fiber grating fabrication processes using drawing towers, the photosensitivity of the fiber itself is mainly improved by heavily doping the preform, forming highly photosensitive fibers with high germanium doping or germanium-boron co-doping, thereby increasing the reflectivity of the dynamically fabricated fiber gratings. However, these fibers still differ from ordinary fibers in performance, exhibiting issues such as high loss, mismatched numerical aperture, and non-uniform fiber dispersion.

[0005] The current fiber drawing towers used for fiber grating fabrication have the following drawbacks: First, the reflectivity of fiber gratings fabricated from ordinary fiber single pulses cannot be improved (less than one-thousandth to one ten-thousandth); second, in the exposure process, the effective writing range of the pulsed laser emitted by the ultraviolet laser after being focused by the cylindrical lens is within tens of micrometers near the focal point, while during the fiber drawing process, the fiber will jitter across its entire circumference with an amplitude of hundreds of micrometers, leading to large differences in grating reflectivity and even grating leakage; in addition, special photosensitive fibers have different fiber performance, which greatly limits the application scenarios. Summary of the Invention

[0006] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention proposes a special wire drawing equipment and method for fiber grating fabrication.

[0007] To achieve the above objectives, according to one aspect of the present invention, a special fiber drawing equipment for fiber grating fabrication is provided, comprising a fiber drawing furnace and an ultraviolet exposure system arranged sequentially from high to low; the fiber preform is melted into bare fiber in the fiber drawing furnace, and the grating is inscribed on the bare fiber by the ultraviolet exposure system;

[0008] It also features a sensitivity enhancement system, located between the wire drawing furnace and the ultraviolet exposure system;

[0009] The sensitization system includes an oxyhydrogen torch for forming an oxyhydrogen flame, which is then directed at the grating writing area in the bare fiber to sensitize it.

[0010] According to the above scheme, the hydrogen-oxygen torch consists of two sets, symmetrically distributed with the bare fiber as the axis;

[0011] Several hydrogen-oxygen torches are arranged in a chain along the fiber drawing direction within each group; the total length of the chain of hydrogen-oxygen torches is adapted to the fiber drawing speed to ensure the time for sensitization treatment.

[0012] According to the above scheme, the sensitization system also includes two sets of synchronously moving mechanisms, each corresponding to one of the two sets of hydrogen-oxygen torches;

[0013] The hydrogen-oxygen torch is fixed to the moving end of the moving mechanism and is used to perform sensitization processing on the grating writing area in a follow-up manner.

[0014] According to the above scheme, the moving mechanism moves in a reciprocating motion, and each group of hydrogen-oxygen torches consists of one torch; or

[0015] The moving mechanism moves in a cyclic manner, with at least two hydrogen-oxygen torches in each group, distributed at the moving end of the moving mechanism, sequentially following each other to perform sensitization processing on the grating writing area with spacing.

[0016] According to the above scheme, the hydrogen-oxygen torch includes a central nozzle and an annular nozzle surrounding the central nozzle, and the central nozzle is equipped with a wick for ignition.

[0017] The sensitization system also includes a tubular furnace, which consists of hydrogen and oxygen pipelines. The hydrogen pipeline is connected to the central nozzle, and the oxygen pipeline is connected to the annular nozzle.

[0018] According to the above scheme, the ultraviolet exposure system includes a laser, a beam-shrinking system and a phase mask arranged sequentially along the laser optical path; the laser emitted by the laser is shrunken by the beam-shrinking system and then forms interference fringes perpendicular to the direction of the bare fiber through the phase mask, which are then processed on the grating writing area after being sensitized to form a grating.

[0019] Beam shrinking systems are used to reduce the size of the laser spot while simultaneously compressing the laser's energy density by a factor of two.

[0020] According to the above scheme, the beam shrinking system includes two-stage beam shrinking optical paths, which are used to reduce the size of the laser spot in the height direction. The size of the laser beam in the height direction after beam shrinking by the beam shrinking system is 0.01 to 0.005 times the original size.

[0021] According to the above scheme, the ultraviolet exposure system also includes: an adjustable slit and / or an adjustable apodized aperture disposed between the laser and the beam-shrinking system;

[0022] Adjustable slits are used for grating length control;

[0023] Adjustable apodized apertures are used for spectral shape control of grating reflection spectra.

[0024] According to the above scheme, it also includes at least one of the following structures:

[0025] The cooling pipe is located between the sensitization system and the ultraviolet exposure system to cool the bare fiber after sensitization.

[0026] The first laser diameter measurement system, located between the sensitization system and the ultraviolet exposure system, is used to monitor the cladding diameter of the bare fiber.

[0027] An optical fiber coating and curing system is used to coat a protective layer onto bare optical fibers etched with gratings and to cure the coated protective layer.

[0028] The cable winding system, located at the end of the entire equipment, is used to rewind the final optical fiber.

[0029] According to another aspect of the present invention, a special fiber drawing method for fabricating fiber gratings is provided, wherein a bare fiber that has just been melted into filaments is subjected to sensitization treatment by aligning a grating writing area in the bare fiber with an oxyhydrogen flame, a grating is written on the sensitized grating writing area, and then subsequent processing is performed; wherein the bare fiber contains germanium.

[0030] Following the above method, the sensitization treatment time is 5~30 min, and the temperature of the oxyhydrogen flame is 1000℃~1500℃.

[0031] Using the above method, for high-density gratings, a chain of oxyhydrogen flames arranged along the drawing direction is used to perform whole-segment sensitization treatment on bare fibers. The length of the bare fibers being treated is matched with the drawing speed to ensure the sensitization treatment time.

[0032] For low-density gratings, an oxyhydrogen flame that moves synchronously along the drawing direction is used to perform follow-up sensitization processing on the grating writing area. The oxyhydrogen flame moves at the same speed as the drawing speed, and the distance and speed of the oxyhydrogen flame follow are matched to ensure the time for sensitization processing.

[0033] The specific steps for writing the grating according to the above method include:

[0034] The laser beam is adjusted to reduce the size of the laser spot in the height direction, while simultaneously compressing the laser energy density by a factor of two.

[0035] Interference fringes perpendicular to the direction of the bare fiber are formed by using a phase mask and then processed on the grating writing area after sensitization treatment to form a grating;

[0036] Add at least one of the following steps before beam shortening adjustment:

[0037] An adjustable slit is added to control the length of the grating;

[0038] An adjustable apodized aperture is added to control the spectral shape of the grating reflection spectrum.

[0039] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0040] 1. Without altering the optical and geometric performance parameters of the optical fiber itself, a sensitization system is added between the fiber drawing furnace and the grating writing system. This system utilizes an oxyhydrogen flame formed by the combustion of hydrogen and oxygen to sensitize the freshly fused bare fiber. This allows for simultaneous sensitization during fiber drawing and grating writing. This invention enables the online fabrication of high-reflectivity fiber gratings, simplifies the fabrication process, and improves the fabrication efficiency of high-reflectivity fiber gratings.

[0041] 2. The oxyhydrogen flame was adapted to different grating spacings to improve the synchronization of drawing and sensitization.

[0042] 3. In the grating writing process, the laser energy density is increased by beam shrinking. High-energy-density parallel light is used for grating writing, thereby achieving a large-size, uniform energy density exposure window, improving the accuracy of grating writing, realizing an online fabrication scheme for fiber gratings with zero grating leakage, and achieving high consistency with reflectivity difference <1dB. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of an equipment structure provided in an embodiment of the present invention.

[0044] Figure 2 This is a schematic diagram of a sensitivity enhancement system structure provided in an embodiment of the present invention.

[0045] Figure 3 yes Figure 2 A detailed local diagram.

[0046] Figure 4 This is a schematic diagram of another sensitivity enhancement system structure provided in an embodiment of the present invention.

[0047] Figure 5 yes Figure 2 A partial cross-sectional view.

[0048] Figure 6 This is a schematic diagram of an ultraviolet exposure system provided in an embodiment of the present invention.

[0049] Figure 7 This is a schematic diagram of the spectrum of a fiber Bragg grating obtained using existing technology.

[0050] Figure 8 This is a schematic diagram of the reflectivity fluctuation of fiber Bragg gratings obtained using existing technology.

[0051] Figure 9 This is a schematic diagram of the spectrum of a fiber optic grating obtained using an embodiment of the present invention.

[0052] Figure 10 This is a schematic diagram of the reflectivity fluctuation of a fiber Bragg grating obtained using an embodiment of the present invention.

[0053] Figure 11 This is a schematic diagram of lattice defects.

[0054] In the picture:

[0055] 1. Optical fiber preform; 2. Drawing furnace; 3. Sensitization system; 4. Cooling tube; 5. First laser diameter measurement system; 6. Ultraviolet exposure system; 7. Optical fiber coating system; 8. UV curing lamp; 9. Second laser diameter measurement system; 10. Take-up and lay-up system; 11. Bare fiber;

[0056] 3-1. Hydrogen-oxygen torch; 3-1-1. Central nozzle; 3-1-2. Annular nozzle; 3-2. Tube furnace; 3-2-1. Hydrogen pipeline; 3-2-2. Oxygen pipeline;

[0057] 6-1. Laser; 6-2. Adjustable slit; 6-3. Adjustable apodized aperture; 6-4. First cylindrical lens; 6-5. Second cylindrical lens; 6-6. Third cylindrical lens; 6-7. Fourth cylindrical lens; 6-8. Phase mask. Detailed Implementation

[0058] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0059] In this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0060] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0061] like Figure 1 As shown, according to one aspect of the present invention, a special drawing equipment for fiber grating fabrication is provided, comprising a drawing furnace 2, a sensitization system 3, a cooling tube 4, a first laser diameter measurement system 5, an ultraviolet exposure system 6, an optical fiber coating system 7, a UV curing lamp 8, a second laser diameter measurement system 9, and a take-up and laying system 10 arranged sequentially from high to low.

[0062] It should be noted that the cooling pipe 4, the first laser diameter measurement system 5, the fiber coating system 7, the UV curing lamp 8, the second laser diameter measurement system 9, and the take-up and cabling system 10 are all optional additional units and can be implemented by conventional technical means. They are not the core inventive points of this invention, and only their placement and function are described here. The cooling pipe 4 is placed after the sensitization system 3 to cool the sensitized bare fiber; the first laser diameter measurement system 5 is placed after the cooling pipe 4 to monitor the cladding diameter of the bare fiber; the fiber coating system 7 is placed after the UV exposure system to coat the bare fiber with the grating etched with a protective layer; the UV curing lamp 8 is placed after the fiber coating system 7 to cure the coated protective layer; the second laser diameter measurement system 9 is placed after the UV curing lamp to monitor the thickness of the protective layer (the diameter of the fiber after adding the protective layer minus the diameter of the bare fiber measured by the first laser diameter measurement system 5 is twice the thickness of the protective layer); the take-up and cabling system 10 is placed at the end of the entire equipment to rewind the finally obtained fiber.

[0063] It should be emphasized that the sensitization system 3 is a new unit in this invention, which is used to sensitize the bare fiber after the optical fiber preform 1 is melted into bare fiber by the drawing furnace 2, and then the ultraviolet exposure system 6 writes the grating on the bare fiber.

[0064] Specifically, the sensitization system 3 includes an oxyhydrogen torch for forming an oxyhydrogen flame, which is directed at the grating writing area in the bare fiber to enhance its sensitization. The sensitization principle is as follows: the photosensitivity of optical fiber refers to the temporary or permanent change in the physical or chemical properties of a material under the influence of external light. It has been found that germanium-doped silica optical fibers undergo changes in refractive index, absorption spectrum, internal stress, and other characteristics under 193nm and 240nm ultraviolet light irradiation. This indicates that germanium-doped silica optical fibers possess certain ultraviolet photosensitivity characteristics, primarily due to... Figure 11 The absorption bands at 193nm and 240nm are caused by the germanium oxygen deficient center (GODC) in the fiber. Therefore, the photosensitivity of the optical fiber is directly related to the concentration of germanium oxygen deficient centers in the fiber material, and the two are approximately proportional. Therefore, this invention mainly utilizes an oxyhydrogen flame to perform a brushing treatment on the fiber segment to be exposed (i.e., the grating writing area), causing a reduction reaction in the fiber material, increasing the concentration of germanium oxygen deficient centers, and rapidly forming more germanium oxygen deficient centers (GODC) in the fiber core, thereby significantly improving the photosensitivity of the optical fiber.

[0065] In some embodiments, to achieve the sensitization effect (more than 10 times increase in photosensitivity), the sensitization treatment time is 5 to 30 minutes, and the temperature of the oxyhydrogen flame is 1000°C to 1500°C.

[0066] In this embodiment, the hydrogen-oxygen torch includes two sets, symmetrically distributed around the bare fiber, so as to heat the bare fiber simultaneously from both sides.

[0067] This embodiment provides different forms of hydrogen-oxygen torches for different types of gratings.

[0068] For high-density gratings, such as all-fiber grating array structures, in some embodiments, several oxyhydrogen torches within each group are arranged in a chain along the fiber drawing direction, i.e., continuously arranged on both sides along the fiber drawing direction, such as... Figure 2 As shown. The specific structure of the sensitization system is as follows. Figure 2 , Figure 3 and Figure 5As shown, the apparatus includes a tube furnace 3-2 and an oxyhydrogen torch 3-1. The oxyhydrogen torch 3-1 includes a central nozzle 3-1-1 and an annular nozzle 3-1-2 surrounding the central nozzle. The central nozzle 3-1-1 is equipped with a wick for ignition. The tube furnace 3-2 consists of a hydrogen pipeline and an oxygen pipeline. The hydrogen pipeline is connected to the central nozzle, and the oxygen pipeline is connected to the annular nozzle. The hydrogen and oxygen pipelines each include a main pipeline and a distribution chamber. Gas is introduced from the main pipeline through the distribution chamber to the corresponding nozzle of the oxyhydrogen torch 3-1. Oxygen is ejected from the annular nozzle 3-1-2, surrounding the hydrogen and ensuring thorough mixing. After ignition, the flame is continuously ejected in a chain-like manner, sensitizing the bare fiber as a whole and resulting in higher treatment consistency. The total length of the chain-type oxyhydrogen torch is matched with the drawing speed of the bare fiber to ensure the time for sensitization treatment; by controlling the flow rate of hydrogen and oxygen, the temperature of the oxyhydrogen flame can reach 1000℃~1500℃.

[0069] For low-density gratings, such as dot gratings, in some embodiments, such as Figure 4 As shown, the sensitization system includes a tube furnace 3-2, two sets of synchronously moving mechanisms, and oxyhydrogen torches 3-1. The oxyhydrogen torches 3-1 are fixed to the moving end of the moving mechanisms and are used to sensitize the grating writing area in a following manner. Following means continuously moving along the wire drawing direction, aligned with a specific grating writing area. The moving mechanism can move reciprocally, with one oxyhydrogen torch 3-1 in each set. After sensitizing one grating writing area, the oxyhydrogen torch is turned off and quickly returns to the top of the sensitization system to ignite and follow the next grating writing area. Alternatively, the moving mechanism can move cyclically around a circular path, with at least two oxyhydrogen torches 3-1 distributed at the moving end of the moving mechanism, sequentially sensitizing spaced grating writing areas in a following manner. During the sensitization process, the moving speed of the control mechanism is the same as the wire drawing speed. One of the oxyhydrogen torches 3-1 follows the grating writing area for sensitization starting from the top of the control mechanism. After the sensitization process is complete, the oxyhydrogen torch 3-1 moves to the bottom of the control mechanism and returns to the top along the outer circular path. The spacing of the grating writing areas, the distance between adjacent oxyhydrogen torches 3-1, and the moving speed of the control mechanism are matched to ensure synchronization between the oxyhydrogen torches 3-1 and the grating writing areas. For example, if the spacing of the grating writing areas is the same as the distance between the oxyhydrogen torches 3-1, the moving speed of the control mechanism remains the same as the wire drawing speed.

[0070] See Figure 6As a further optimization, the ultraviolet exposure system of this embodiment includes a laser 6-1, a beam-shrinking system, and a phase mask 6-8 arranged sequentially along the laser optical path. The laser emitted by the laser 6-1 is beam-shrinked by the beam-shrinking optical path and then forms interference fringes perpendicular to the direction of the bare fiber through the phase mask 6-8. These fringes are then processed on the grating writing area of ​​the bare fiber 11 after sensitization treatment to form a grating.

[0071] The beam-shrinking system reduces the size of the laser spot and simultaneously compresses the laser's energy density. Unlike existing technologies that use a single cylindrical lens for beam focusing, which results in a strong focusing process where the energy density increases and then decreases in the horizontal direction, this presents significant variations in exposure energy when the fiber optic cable moves horizontally. In contrast, this invention employs a two-stage beam-shrinking energy density compression scheme, producing a parallel beam with consistent energy density in the horizontal direction. Even with horizontal fiber movement, the exposure energy remains uniform.

[0072] In this embodiment, the beam-shrinking system includes two stages of beam-shrinking optical paths, each used to reduce the size of the laser spot in the height direction. Using staged beam-shrinking reduces the performance pressure on individual lenses, minimizes spot distortion caused by minor errors in lens precision and refractive index uniformity, and allows for better control of the beam-shrinking ratio, enabling fine-tuning. Furthermore, small apertures can be added to each stage of the beam-shrinking system to achieve multiple spatial filters, resulting in high-quality beam output. Depending on the optical performance of the beam-shrinking optical path, single-stage or three-stage or higher beam-shrinking optical paths can also be configured. The laser beam in the height direction, shrunken by the beam-shrinking system, is 0.01 to 0.005 times its original size. (Continue to see...) Figure 6 In some embodiments, the first-stage beam-shrinking optical path includes a first cylindrical lens 6-4 and a second cylindrical lens 6-5, and the second-stage beam-shrinking optical path includes a third cylindrical lens 6-6 and a fourth cylindrical lens 6-7.

[0073] Further optimized, continue as Figure 6 As shown, the ultraviolet exposure system also includes an adjustable slit 6-2 and / or an adjustable apodized aperture 6-3 disposed between the laser 6-1 and the beam-shrinking system. The adjustable slit 6-2 is used for grating length control; the adjustable apodized aperture 6-3 is used for spectral shape control of the grating reflection spectrum.

[0074] In this embodiment, laser 6-1 is a 193nm excimer laser, and phase mask 6-8 is a 193nm phase mask.

[0075] According to another aspect of the present invention, this embodiment provides a special fiber drawing method for the fabrication of fiber gratings. The bare fiber that has just been melted into filaments is subjected to sensitization treatment by oxyhydrogen flame directed at the grating writing area in the bare fiber. The grating is then written on the sensitized grating writing area and further processing is performed.

[0076] The sensitization treatment time is 5~30 min, and the temperature of the oxyhydrogen flame is 1000℃~1500℃.

[0077] For high-density gratings, an oxyhydrogen flame arranged in a chain along the drawing direction is used to perform whole-segment sensitization treatment on the bare fiber. The length of the bare fiber being treated is matched with the drawing speed to ensure the sensitization treatment time.

[0078] For low-density gratings, an oxyhydrogen flame that moves synchronously along the drawing direction is used to perform sensitization processing on the grating writing area in a following manner. The following speed of the oxyhydrogen flame is the same as the drawing speed, and the following distance of the oxyhydrogen flame is adapted to the speed to ensure the sensitization processing time. Specifically, based on the principle that speed multiplied by time equals distance, the following distance of the oxyhydrogen flame is set according to the following speed to ensure the sensitization processing time; or, to ensure that the sensitization processing time and the following distance of the oxyhydrogen flame are within a reasonable range, the following speed and the drawing speed are adjusted.

[0079] To further optimize, the specific steps for writing the grating include:

[0080] The laser beam is adjusted to reduce the size of the laser spot in the height direction, while simultaneously compressing the laser energy density by a factor of two.

[0081] Interference fringes perpendicular to the direction of the bare fiber are formed by using a phase mask and then processed on the grating writing area after sensitization treatment to form a grating.

[0082] In addition, at least one of the following steps may be added before beam contraction adjustment:

[0083] An adjustable slit is added to control the length of the grating;

[0084] An adjustable apodized aperture is added to control the spectral shape of the grating reflection spectrum.

[0085] The following two specific examples further illustrate the present invention.

[0086] Example 1:

[0087] like Figure 1 As shown, the optical fiber preform 1 is clamped above the drawing furnace 2, and is melted into fibers at a high temperature of 2000°C inside the drawing furnace 2. Under the traction of the take-up and laying system 10, the molten bare fiber continues to extend downwards, passing through... Figure 2 , Figure 3 and Figure 5The sensitization system 3 shown controls the flame temperature between 1000 and 1500°C by controlling the flow rates of hydrogen and oxygen. The fiber drawing speed is between 0.1 and 20 m / min, and the entire sensitization system 3 is 5 meters long. Taking sensitization for 5 minutes as an example, by controlling the drawing speed at 1 m / min, the optical fiber can be continuously sensitized in an oxyhydrogen flame environment for 5 minutes. After passing through the sensitization system 3, the optical fiber still has a high temperature of approximately 1000-1500°C, so it needs to be rapidly cooled by the cooling tube 4. Then, the diameter of the bare optical fiber is monitored by the first laser diameter measurement system 5, and feedback is provided in real time. After all system parameters are stable, the fiber grating is fabricated by the ultraviolet exposure system 6.

[0088] Taking a center wavelength of 1550nm, a 3dB bandwidth of 5nm, and a reflectivity of 0.1% as an example, with a grating length of 10mm and a grating spacing of 1 meter, and based on a drawing speed of 1m / min, laser 6-1 is a 193nm excimer laser. Figure 6 As shown, the parameters given to laser 6-1 are: exposure delay of 1 minute, outputting a beam with energy of 100mJ and spot size of H (=24mm) every minute. A single-pulse laser with a diameter of L (=12mm) outputs a diameter of H (=24mm) after passing through an adjustable slit 6-2. A single-pulse laser with a diameter of L (=10mm) is output as H (=2mm) after passing through the first beam-shrinking optical path. A single-pulse laser with a diameter of L (=10mm) outputs a diameter of H (=0.2mm) after passing through the second beam-shrinking optical path. A single-pulse laser with a wavelength of L (=10 mm) illuminates a phase mask with a grating spacing of 1072.1 nm (0th-order diffraction efficiency at 193 nm wavelength <3%, mask chirp 3 nm / cm), creating interference fringes with a period of 536.05 nm. These fringes are then fabricated onto a sensitized optical fiber, achieving grating writing. H represents the height direction, i.e., the fiber drawing direction; L represents the length direction, i.e., the grating writing direction. The bare fiber with the grating written on it then enters the fiber coating system 7 and UV curing lamp 8 for coating curing. After coating, the fiber is then passed through a second laser diameter measurement system 9 for diameter monitoring, and finally rewound into a disc using a take-up and lay-up system 10.

[0089] refer to Figure 7 and Figure 8 , Figure 7 The horizontal axis represents the center wavelength (in nm). Figure 7 The vertical axis represents power (in dBm). Figure 8 The horizontal axis represents the fiber length (in km). Figure 8The ordinate represents the scattering intensity (in dB). Fiber Bragg gratings fabricated using conventional drawing towers exhibit poor reflectance spectral flatness for individual gratings. Within the entire 5nm bandwidth, the reflectance varies significantly (above 3dB) for each wavelength and is low (below 0.01%). For the entire fiber Bragg grating array, the reflectance of each grating varies (the grating reflectance is distributed differentially from 0.01% to 0.002%, with differences exceeding 5dB).

[0090] refer to Figure 9 and Figure 10 , Figure 9 The horizontal axis represents the center wavelength (in nm). Figure 9 The vertical axis represents power (in dBm). Figure 10 The horizontal axis represents the fiber length (in km). Figure 10 The ordinate represents the scattering intensity (in dB). The fiber gratings fabricated online using a special drawing tower of this invention address the issue of low photosensitivity in conventional germanium-doped fibers by utilizing a sensitization system. An oxyhydrogen flame is employed to achieve online enhancement of the fiber core's photosensitivity. For individual gratings, the spectrum exhibits high reflectance flatness and high reflectivity (above 0.1%). Simultaneously, a UV exposure system with a beam-shrinking scheme achieves high energy density consistency in the processing beam, resolving processing deviations caused by fiber jitter due to the drawing tower itself. For the entire fiber grating array, the reflectivity of each grating exhibits good consistency (the difference in grating reflectivity is within 1 dB).

[0091] Example 2

[0092] Fabrication of point-type fiber Bragg gratings, such as Figure 1 As shown, the optical fiber preform 1 is clamped above the drawing furnace 2, and is melted into fibers at a high temperature of 2000°C inside the drawing furnace 2. Under the traction of the take-up and laying system 10, the molten bare optical fiber continues to extend downwards, passing through... Figure 5The sensitization system 3 shown controls the flame temperature between 1000 and 1500°C by controlling the flow rates of hydrogen and oxygen. The wire drawing speed is between 0.1 and 20 m / min, and the entire sensitization system 3 is 5 meters long. Taking sensitization for 20 minutes as an example, by controlling the wire drawing speed at 0.25 m / min, the optical fiber can be continuously sensitized for 20 minutes in an oxyhydrogen flame environment. With the assistance of the moving structure, the hydrogen-oxygen torches can be independently controlled to move along their respective slide rails in a following manner. The slide rails are segmented with a length of 2.5m. Based on the fiber drawing speed of 0.25m / min fed back by the system, the upper left torch is controlled to move along the corresponding slide rail from top to bottom at a speed of 0.25m / min to perform local fiber sensitization treatment. This movement along the slide rail takes 10 minutes. Immediately afterward, the lower left torch moves along the slide rail from top to bottom at a speed of 0.25m / min to perform a second sensitization treatment on the locally sensitized fiber treated by the upper left torch for 10 minutes. At this point, the next grating writing area to be sensitized, with a spacing of 2.5m, is located at the upper right torch. The upper right torch moves along the corresponding slide rail from top to bottom at a speed of 0.25m / min to complete the local sensitization of the grating writing area. Then, the lower right torch continues the second sensitization treatment for 10 minutes... This cycle of sensitization is repeated to achieve sensitization treatment of grating writing areas with different spacings. After completing the sensitization process, each torch is extinguished and quickly moved upwards to its initial position to perform subsequent local fiber optic sensitization processing.

[0093] After passing through the sensitization system 3, the optical fiber still maintains a high temperature of approximately 1000-1500℃, thus requiring rapid cooling via the cooling tube 4. It then passes through the first laser diameter measurement system 5 to monitor the bare fiber diameter and provide real-time feedback control. Once all system parameters are stable, the fiber grating is fabricated using the ultraviolet exposure system 6. Taking a center wavelength of 1550nm, a 3dB bandwidth of 0.2nm, and a reflectivity of 10% as an example, the grating length is 10mm, the grating spacing is 2.5 meters, and the fiber drawing speed is 0.25m / min. Laser 6-1 is a 193nm excimer laser. (See also...) Figure 6 The parameters given to laser 6-1 are: exposure delay of 10 minutes, outputting a beam with energy of 250 mJ and spot size of H (=24 mm) every 10 minutes. A single-pulse laser with a diameter of L (=12mm) outputs a diameter of H (=24mm) after passing through an adjustable slit 6-2. A single-pulse laser with a diameter of L (=10mm) is output as H (=2mm) after passing through the first beam-shrinking optical path. A single-pulse laser with a diameter of L (=10mm) outputs a diameter of H (=0.2mm) after passing through the second beam-shrinking optical path. A single-pulse laser with an L (=10 mm) is used to illuminate a phase mask with a grating spacing of 1072.1 nm (the 0th order diffraction efficiency at a wavelength of 193 nm is <3%, and the mask is a uniform mask), resulting in interference fringes with a period of 536.05 nm. These fringes are then fabricated on an optical fiber that has undergone sensitization treatment to achieve grating writing.

[0094] The bare optical fiber with the grating written into it then enters the optical fiber coating system 7 and UV curing lamp 8 for coating curing. After coating, the optical fiber passes through a second laser diameter measurement system 9 for diameter monitoring, and finally is rewound into a disc by the take-up and lay-up system 10.

[0095] The following details the differences in the photosensitivity of optical fibers caused by different hydrogen-oxygen flame temperatures and sensitization processing times in the sensitization system.

[0096] Table 1

[0097]

[0098] As shown in Table 1, without sensitization treatment, the refractive index change of the optical fiber after exposure to a 193nm excimer laser under the same conditions was 0.0002. After 30 minutes of sensitization treatment, the refractive index change after exposure to the same 193nm excimer laser under the same conditions was 0.00225, and the photosensitivity was 11.25 times higher than before, achieving a significant improvement in optical fiber photosensitivity. By comparing the sensitization treatment time from 0 to 35 minutes, it was found that the photosensitivity of the optical fiber increased with time, reaching saturation at around 30 minutes.

[0099] Table 2

[0100]

[0101] As can be seen from Table 2, when the sensitization time is the same, the photosensitivity of the optical fiber increases with the increase of the temperature of the oxyhydrogen flame from 0 to 2000℃. However, at 2000℃, the optical fiber softens, becomes brittle, and is prone to breakage, which does not meet the requirements of practical applications.

[0102] This invention adds a sensitization system between the fiber drawing furnace and the grating writing system. Utilizing an oxyhydrogen flame formed by the combustion of hydrogen and oxygen, the bare fiber that has just been molten is sensitized, improving the photosensitivity of the grating writing area and thus enabling the fabrication of highly reflective fiber gratings. In the grating writing stage, this invention increases the laser energy density through beam-shrinking processing, thereby achieving a large-size, uniform energy density exposure window and improving the accuracy of grating writing.

[0103] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0104] It should be noted that, depending on the implementation needs, the various steps / components described in this application can be broken down into more steps / components, or two or more steps / components or parts of the operation of steps / components can be combined into new steps / components to achieve the purpose of this invention.

[0105] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A special fiber drawing equipment for fiber grating fabrication, comprising a fiber drawing furnace and an ultraviolet exposure system arranged sequentially from high to low; an optical fiber preform is melted into bare fiber in the fiber drawing furnace, and a grating is inscribed on the bare fiber by the ultraviolet exposure system; characterized in that: It also features a sensitivity enhancement system, located between the wire drawing furnace and the ultraviolet exposure system; The sensitization system includes an oxyhydrogen torch for forming an oxyhydrogen flame, which is directed at the grating writing area in the bare fiber to perform sensitization treatment; the bare fiber contains germanium; the specific method of sensitization treatment is as follows: For high-density gratings, an oxyhydrogen flame arranged in a chain along the drawing direction is used to perform whole-segment sensitization treatment on the bare fiber. The length of the bare fiber being treated is matched with the drawing speed to ensure the sensitization treatment time. For low-density gratings, an oxyhydrogen flame that moves synchronously along the drawing direction is used to perform follow-up sensitization processing on the grating writing area. The speed at which the oxyhydrogen flame follows is the same as the drawing speed, and the distance and speed of the oxyhydrogen flame following are matched to ensure the time for sensitization processing. The hydrogen-oxygen torch consists of two sets, symmetrically distributed around the bare fiber as an axis. Several hydrogen-oxygen torches are arranged in a chain along the fiber drawing direction within each group; the total length of the chain of hydrogen-oxygen torches is adapted to the fiber drawing speed to ensure the time for sensitization treatment.

2. The special fiber drawing equipment for fiber grating fabrication according to claim 1, characterized in that: The sensitization system also includes two sets of synchronously moving mechanisms, each corresponding to one of the two sets of hydrogen-oxygen torches; The hydrogen-oxygen torch is fixed to the moving end of the moving mechanism and is used to perform sensitization processing on the grating writing area in a follow-up manner.

3. The special fiber drawing equipment for fiber grating fabrication according to claim 2, characterized in that: The moving mechanism moves in a reciprocating motion, with one hydrogen-oxygen torch in each group; or The moving mechanism moves in a cyclic manner, with at least two hydrogen-oxygen torches in each group, distributed at the moving end of the moving mechanism, sequentially following each other to perform sensitization processing on the grating writing area with spacing.

4. The special fiber drawing equipment for fiber grating fabrication according to any one of claims 1 to 3, characterized in that: The hydrogen-oxygen torch includes a central nozzle and an annular nozzle surrounding the central nozzle, with a wick for ignition in the central nozzle. The sensitization system also includes a tubular furnace, which consists of hydrogen and oxygen pipelines. The hydrogen pipeline is connected to the central nozzle, and the oxygen pipeline is connected to the annular nozzle.

5. The special fiber drawing equipment for fiber grating fabrication according to claim 1, characterized in that: The ultraviolet exposure system includes a laser, a beam-shrinking system, and a phase mask arranged sequentially along the laser beam path. The laser emitted by the laser is shrunken by the beam-shrinking system and then passes through the phase mask to form interference fringes perpendicular to the direction of the bare fiber. These fringes are then processed on the grating writing area, which has been sensitized, to form a grating. Beam shrinking systems are used to reduce the size of the laser spot in the height direction, while simultaneously compressing the laser's energy density by a factor of two.

6. The special fiber drawing equipment for fiber grating fabrication according to claim 5, characterized in that: The beam-shrinking system comprises two stages of beam-shrinking optical paths. The laser beam shrunken by the beam-shrinking system has a height dimension that is 0.01 to 0.005 times its original dimension; and / or The ultraviolet exposure system also includes: an adjustable slit and / or an adjustable apodized aperture disposed between the laser and the beam-shrinking system; the adjustable slit is used for grating length control; the adjustable apodized aperture is used for spectral shape control of the grating reflection spectrum.

7. The special fiber drawing equipment for fiber grating fabrication according to claim 5, characterized in that: The special fiber drawing equipment further includes at least one of the following: a cooling tube, a first laser diameter measurement system, an optical fiber coating and curing system, and a take-up and lay-up system. The cooling tube is located between the sensitization system and the ultraviolet exposure system to cool the sensitized bare fiber. The first laser diameter measurement system is located between the sensitization system and the ultraviolet exposure system to monitor the cladding diameter of the bare fiber. The optical fiber coating and curing system is used to coat the bare fiber with the grating and to cure the coated protective layer. The take-up and lay-up system is located at the end of the entire equipment to rewind the final optical fiber.

8. A special fiber drawing method for fabricating fiber gratings, characterized in that: For the bare fiber that has just been melted into filament, a hydrogen-oxygen flame is used to sensitize the grating writing area in the bare fiber. A grating is written on the sensitized grating writing area, and then subsequent processing is performed. The bare fiber contains germanium. The specific methods of sensitization processing are as follows: For high-density gratings, an oxyhydrogen flame arranged in a chain along the drawing direction is used to perform whole-segment sensitization treatment on the bare fiber. The length of the bare fiber being treated is matched with the drawing speed to ensure the sensitization treatment time. For low-density gratings, an oxyhydrogen flame that moves synchronously along the drawing direction is used to perform follow-up sensitization processing on the grating writing area. The speed at which the oxyhydrogen flame follows the drawing is the same as the drawing speed, and the distance and speed of the oxyhydrogen flame following the drawing are matched to ensure the time for sensitization processing.

9. The special fiber drawing method for fiber grating fabrication according to claim 8, characterized in that: The sensitization treatment time is 5~30 min, and the temperature of the oxyhydrogen flame is 1000℃~1500℃.

10. The special fiber drawing method for fiber grating fabrication according to claim 8, characterized in that: The specific steps for writing the grating include: The laser beam is adjusted to reduce the size of the laser spot in the height direction, while simultaneously compressing the laser energy density by a factor of two. Interference fringes perpendicular to the direction of the bare fiber are formed by using a phase mask and then processed on the grating writing area after sensitization treatment to form a grating; Add at least one of the following steps before beam shortening adjustment: An adjustable slit is added to control the length of the grating; An adjustable apodized aperture is added to control the spectral shape of the grating reflection spectrum.

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