Panda-type polarization-maintaining optical fiber and preparation method therefor

Abstract

The present invention belongs to the technical field of optical fibers. Disclosed are a panda-type polarization-maintaining optical fiber and a preparation method therefor. Stress regions of the panda-type polarization-maintaining optical fiber provided in the present invention are symmetrically spaced on two sides of a fiber core, and an inner cladding layer wraps around the entire stress region. In combination with fluorine-doped viscosity matching of the fiber core, the diameters of the stress regions and a gradual design of the stress regions, the grinding performance and mechanical performance of the optical fiber can be improved, so that the optical fiber has a better birefringence performance, full-temperature performance, reliability and deformation resistance. In addition, in the present invention, a polarization-maintaining preform is prepared by means of a PCVD preform process, the fiber core, the inner cladding layer and an outer cladding layer are integrally formed, and the process can achieve complete wrapping of the stress regions by the inner cladding layer. The present invention can greatly simplify the preparation process of panda-type polarization-maintaining optical fibers, and can improve the performance of the optical fibers.

Description

A panda-type polarization-maintaining optical fiber and its fabrication method Technical Field

[0001] This invention belongs to the field of optical fiber technology, and more specifically, relates to a panda-type polarization-maintaining optical fiber and its preparation method. Background Technology

[0002] Polarization-maintaining fiber is widely used in aerospace, aviation, marine, industrial manufacturing, and communications. In interferometric fiber optic sensors based on optical coherence detection, polarization-maintaining fiber ensures that the linear polarization direction remains unchanged, improving the coherence signal-to-noise ratio and enabling high-precision measurement of physical quantities. As a special type of fiber, polarization-maintaining fiber is mainly used in sensors such as fiber optic gyroscopes and fiber optic hydrophones, as well as in fiber optic communication systems such as dense wavelength division multiplexing (DWDM) and erbium-doped fiber amplifiers (EDFA). It is a special type of fiber with broad application value.

[0003] However, existing panda-type polarization-maintaining fibers mainly have the following problems: (1) poor polishing performance; (2) poor all-temperature performance; (3) the stability or resistance to deformation of the fiber needs to be improved; and (4) uneven stress distribution inside the fiber. Existing methods for preparing panda-type polarization-maintaining fibers mainly suffer from complex preparation processes or poor practical operability.

[0004] For example, the technical solution disclosed in patent CN104445912A includes: fabricating an optical fiber core rod and polarization-maintaining stress rods; symmetrically fixing two polarization-maintaining stress rods on both sides of the core rod according to stress distribution requirements; depositing an outer cladding layer on the core rod and polarization-maintaining stress rods using an external deposition method to obtain a preform porous body; and sintering the preform porous body to obtain a polarization-maintaining optical fiber preform with a circular cross-section. The above method has poor practical operability and obvious defects. The two stress rods are symmetrically fixed on both sides of the core rod by a clamp for deposition, and then the rod shrinks during sintering, causing the stress rod positions to change. Therefore, it is difficult to ensure symmetry in actual operation, resulting in uneven stress distribution in the optical fiber.

[0005] For example, the technical solution disclosed in patent CN112305664A includes: firstly, preparing a core rod using an in-tube vapor deposition method, and then preparing an inner cladding and a outer cladding layer using an external vapor deposition method to obtain a polarization-maintaining master rod. The above method has complex procedures, and the core rod, inner cladding layer, and outer cladding layer use different preparation processes, which can lead to a large difference in stress at the interface between the core rod and the inner cladding layer.

[0006] For example, the technical solutions disclosed in patents CN102910812B and CN112327405A are similar, mainly including: using PCVD process to prepare polarization-maintaining core rod and two boron-doped stress rods, melting the polarization-maintaining core rod with a quartz sleeve at high temperature and drawing it thin to obtain polarization-maintaining mother rod, drilling holes in the polarization-maintaining mother rod, embedding the boron-doped stress rods into the holes to form a polarization-maintaining fiber preform, and drawing the polarization-maintaining fiber preform into a polarization-maintaining fiber in a molten state. The above method is a common and widely used method in China, but it has several drawbacks, including: 1. After PCVD fabrication of the core rod, it needs to be stretched using a sleeve to obtain the polarization-maintaining master rod, making the fabrication process complex; 2. The high-temperature melting and stretching process introduces additional thermal stress, and the magnitude of the axial thermal stress varies, leading to uneven stress in the master rod. This stress affects the material's refractive index, resulting in poor consistency of the optical parameters of the stretched fiber; 3. The core rod is sleeved, but there is actually a gap between the core rod and the inner hole of the sleeve. During the stretching process, it is impossible to guarantee the absolute center position of the core rod, resulting in large geometric deviations in the concentricity of the fiber core and cladding; 4. Due to the large cross-sectional area of ​​the sleeve and the small area of ​​the core rod, the width of the inner cladding deposited during core rod fabrication is limited; 5. The parameters of the polarization-maintaining master rod obtained by the sleeve stretching method are affected by the sleeve parameters. The roundness and wall thickness of the sleeve itself will affect the geometric parameters of the polarization-maintaining master rod. Currently, the domestic sleeve production technology is not yet mature, and in most cases, the sleeves rely on imports, resulting in increased costs and poor controllability of raw materials; 6. The stretching preparation process can easily cause secondary contamination of the core rod, leading to problems with attenuation and strength. Summary of the Invention

[0007] This invention provides a panda-type polarization-maintaining fiber and its fabrication method, thereby solving the problems of complex fabrication processes and insufficient fiber performance in the prior art for panda-type polarization-maintaining fibers.

[0008] In a first aspect, the present invention provides a panda-type polarization-maintaining optical fiber, comprising: a fiber core, an inner cladding, stress regions, and an outer cladding; the stress regions are symmetrically distributed on both sides of the fiber core, and the inner cladding encloses the entire stress region.

[0009] Preferably, the diameter D3 of the stress zone is in the range of [0.25D4, 0.3D4], where D4 is the diameter of the outer cladding layer.

[0010] Preferably, the stress zone comprises a flat layer and a gradient layer from the inside to the outside.

[0011] Preferably, the thickness of the gradient layer on one side is 0.1D3.

[0012] Preferably, the viscosity of the stress zone is less than the viscosity of the fiber core, the viscosity of the fiber core is less than the viscosity of the inner cladding, and the viscosity of the inner cladding is less than the viscosity of the outer cladding.

[0013] Preferably, the inner cladding is a fluorine-doped inner cladding, and the relative refractive index Δ2 of the inner cladding is [-0.2%, -0.1%]; the fiber core is a fluorine-germanium co-doped fiber core, and the viscosity of the fiber core is adjusted by the amount of fluorine doping, and the relative refractive index contribution of fluorine in the fiber core is Δ∈[-0.15%, -0.05%].

[0014] Secondly, the present invention provides a method for fabricating the panda-type polarization-maintaining optical fiber as described above, comprising the following steps:

[0015] Using the PCVD large rod process, an inner cladding layer and a core layer are sequentially deposited inside a liner. The deposited liner is then melted and sintered to obtain a polarization-maintaining master rod. The inner cladding layer of the polarization-maintaining master rod encapsulates the entire stress zone. The lathe resonant cavity inner hole size of the PCVD large rod process is greater than or equal to 45mm, and the high-frequency system power supply of PCVD is greater than or equal to 10KW.

[0016] A pair of symmetrical circular holes are longitudinally machined on both sides of the center line of the polarization-maintaining master rod. The center lines of the circular holes are parallel to the center line of the polarization-maintaining master rod and are located in the same plane.

[0017] Two stress rods are prepared, and the stress rods are then polished and etched.

[0018] Two processed stress rods are respectively embedded into the two circular holes on both sides of the polarization-maintaining mother rod to obtain a panda-shaped polarization-maintaining fiber preform.

[0019] The panda-shaped polarization-maintaining fiber preform is drawn into fibers in a molten state to obtain a panda-shaped polarization-maintaining fiber.

[0020] Preferably, during the deposition of the inner cladding, a mixed gas of silicon tetrachloride, oxygen and hexafluoroethane is introduced into the liner to react and generate fluorine-doped silicon dioxide which is deposited in the liner.

[0021] During the deposition of the inner cladding, the flow rate of silicon tetrachloride is 100-1000 sccm, the flow rate of oxygen is 1000-3000 sccm, the flow rate of hexafluoroethane is 10-100 sccm, the temperature is 1000-1500℃, and the number of deposition layers is 19000-20000.

[0022] Preferably, during the deposition of the core layer, a mixed gas of silicon tetrachloride, germanium tetrachloride, oxygen and hexafluoroethane is introduced into the liner tube, and the reaction generates fluorine-doped silicon dioxide and germanium dioxide which are deposited in the liner tube.

[0023] During the deposition of the core layer, the flow rate of silicon tetrachloride is 100-1000 sccm, the flow rate of germanium tetrachloride is 2-100 sccm, the flow rate of oxygen is 300-3000 sccm, the flow rate of hexafluoroethane is 2-100 sccm, the temperature is 1000-1500℃, and the number of deposition layers is 500-4800.

[0024] Preferably, when shrinking the deposited liner, the pressure inside the tube is adjusted to a slightly positive or slightly negative pressure, with a pressure range of -50 mBar to +20 mBar, the temperature is adjusted to 1600-2000℃, the moving speed of the heating furnace is adjusted to 20-100 mm / min, and the shrinkage of each layer of the liner is controlled to be 1-5 mm; during firing, the pressure inside the tube is adjusted to a slightly negative pressure, with a pressure range of -950 mBar to 0 mBar, the temperature is adjusted to 1800-2200℃, and the moving speed of the heating furnace is adjusted to 10-40 mm / min.

[0025] One or more technical solutions provided in this invention have at least the following technical effects or advantages:

[0026] In terms of fiber design, the inner cladding of conventional panda-type polarization-maintaining fibers is relatively narrow, only covering the inner edge of the stress zone, not the entire stress zone. In contrast, the inner cladding of this invention is extremely wide, completely enclosing the stress zone. Furthermore, the stress zone, from the inside out, includes a planarization layer and a gradient layer (i.e., a gradient design is used at the edge of the stress zone). Therefore, it reduces the viscosity difference between the outer edge of the stress zone and the inner cladding interface, mitigating the tendency for cracking at the edge of the stress zone during fiber polishing and improving the fiber polishing performance.

[0027] Furthermore, considering that the full-temperature performance of polarization-maintaining fiber is a crucial factor limiting gyroscope accuracy when used in gyroscope applications, and that poor full-temperature performance of polarization-maintaining fiber is mainly due to compressive or tensile stress on the fiber core caused by material expansion, the polarization-maintaining fiber provided by this invention includes an extremely wide fluorine-doped inner cladding. Since the coefficient of thermal expansion of SiO2 is 0.5 × 10⁻⁶, this invention provides a polarization-maintaining fiber with an extremely wide fluorine-doped inner cladding. -6 At / ℃, the coefficient of thermal expansion of fluorine-doped glass is approximately 0.3×10⁻⁶. -6 / ℃, therefore, this invention increases the effective area of ​​the fluorine-doped inner cladding by increasing the diameter of the fluorine-doped inner cladding, which can reduce the expansion coefficient of the fiber cladding, thereby reducing the tensile or compressive stress on the fiber core caused by temperature changes and improving the full-temperature performance of the fiber. At the same time, fluorine-doped quartz glass exhibits higher performance in terms of strength and elastic modulus than pure quartz glass. The improvement in strength and elastic modulus means that fluorine-doped quartz glass can maintain better shape stability when subjected to external forces and has a higher resistance to deformation.

[0028] Conventional polarization-maintaining optical fibers have a stress zone composed of borosilicate glass with very low viscosity, while the cladding is made of pure silicon glass with very high viscosity. This significant viscosity difference at the stress zone interface leads to high stress at the interface, making the fiber susceptible to external stresses such as fiber polishing, which can cause cracking at the stress zone interface and affect fiber reliability. This invention optimizes the fiber viscosity matching design by using a fluorine-doped, extremely wide inner cladding to encapsulate the entire stress zone. This reduces the viscosity of the outer cladding at the stress zone boundary, thereby reducing the viscosity difference between the inner and outer sides of the stress zone boundary, thus optimizing the fiber stress zone interface stress and improving fiber reliability.

[0029] Simultaneously, the use of an ultra-wide fluorine-doped inner cladding can effectively increase the area ratio of the fluorine-doped inner cladding to the entire cladding, thereby reducing the overall fiber viscosity. The reduction in overall fiber viscosity means that the fiber can be drawn at a lower furnace temperature, which can reduce the diffusion of boron elements in the stress zone at high temperatures, thus reducing fiber attenuation changes and effectively improving attenuation uniformity.

[0030] In terms of optical fiber fabrication, this invention first prepares a polarization-maintaining master rod using a PCVD large rod process (e.g., PCVD-R47 process) with a lathe resonant cavity inner hole size greater than or equal to 45mm and a high-frequency system power supply greater than or equal to 10KW. The obtained polarization-maintaining master rod has an extremely wide fluorine-doped inner cladding, which can achieve the inner cladding to wrap the entire stress region. Two stress rods are then prepared. A pair of circular holes are then machined in the polarization-maintaining master rod. After that, the two stress rods, which have been polished and etched, are respectively embedded into the circular holes on both sides of the polarization-maintaining master rod to obtain a panda-shaped polarization-maintaining fiber preform. Finally, the panda-shaped polarization-maintaining fiber preform is drawn into fibers in a molten state to obtain a panda-shaped polarization-maintaining fiber. Taking PCVD-R47 as an example, this invention fully leverages the advantages of the PCVD-R47 process. Based on the size characteristics of the resonant cavity bore used in PCVD-R47, the core (corresponding to the core layer in the panda-type polarization-maintaining fiber preform), inner cladding, and outer cladding (corresponding to the sleeve in the panda-type polarization-maintaining fiber preform) of the panda-type polarization-maintaining fiber prepared by this invention are integrally formed. Compared with the existing conventional panda-type polarization-maintaining fiber preparation method, which requires sleeve-fitting and stretching processes to prepare the polarization-maintaining master rod, this invention can greatly simplify the preparation process of the polarization-maintaining master rod, and thus greatly simplify the preparation process of the panda-type polarization-maintaining fiber. Furthermore, since this invention eliminates the need for sleeve-fitting and stretching processes, it avoids the core rod alignment problem during the sleeve-fitting process, thus preventing the problem of excessive core-cladding concentricity from the source. At the same time, it can avoid the introduction of impurities during the stretching process. This invention addresses the issue of excessive fiber attenuation by avoiding secondary contamination of the core rod during the stretching preparation process. Since it does not involve a high-temperature melting and stretching process, it does not introduce additional thermal stress, ensuring uniform stress in the master rod and improving the consistency of fiber optical parameters. Utilizing the PCVD-R47 process, the deposition efficiency of the inner cladding and core layers can reach 100%, allowing for precise control of the core rod profile and ensuring consistency with the calculated theoretical values, thus improving fiber optical performance. Compared to conventional fabrication methods, the PCVD-R47 process results in a larger area ratio before and after melting and shrinking, which is more conducive to controlling the non-circularity of the core and inner cladding layers. The non-circularity of both the drawn fiber core and inner cladding layers can be less than 0.5%. Compared to the sleeve stretching method, this invention avoids the influence of sleeve parameters (including the sleeve's own roundness and wall thickness) on the geometric parameters of the polarization-maintaining master rod.

[0031] Furthermore, the polarization-maintaining master rod prepared by the present invention through the PCVD-R47 process can obtain an extremely wide inner cladding, which can ensure that the stress zone is completely wrapped inside the inner cladding. Combined with the design of the stress zone diameter, the gradient design of the stress zone edge position, and the design of fluorine doping in the inner core and fiber viscosity matching, the relevant performance of the panda-type polarization-maintaining fiber can be improved. It can improve the polishing performance and mechanical properties of the fiber, giving the fiber better birefringence performance, full-temperature performance, reliability and resistance to deformation. It can avoid or improve the uneven stress distribution inside the fiber caused by large viscosity differences, and effectively improve axial consistency. Attached Figure Description

[0032] Figure 1 is a schematic diagram of the radial structure of a panda-type polarization-maintaining optical fiber provided in Embodiment 1 of the present invention.

[0033] Figure 2 is a refractive index profile of a panda-type polarization-maintaining fiber provided in Embodiment 1 of the present invention.

[0034] Figure 3 is a correlation diagram of the stress zone diameter, birefringence coefficient, and breaking force of a panda-type polarization-maintaining optical fiber provided in Embodiment 1 of the present invention.

[0035] Figure 4 is a correlation diagram of the stress zone gradient layer thickness of a panda-type polarization-maintaining optical fiber provided in Embodiment 1 of the present invention with the birefringence coefficient and polishing performance.

[0036] Figure 5 is a process flow diagram of a panda-type polarization-maintaining optical fiber fabrication method provided in Embodiment 2 of the present invention.

[0037] Among them, 1-fiber core, 2-inner cladding, 3-stress zone, 4-outer cladding, 5-inner coating, and 6-outer coating. Embodiments of the present invention

[0038] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0039] Example 1:

[0040] Example 1 provides a panda-type polarization-maintaining optical fiber, as shown in Figures 1 and 2. It mainly includes a core 1, an inner cladding 2, stress regions 3, and an outer cladding 4. Furthermore, the outer cladding 4 may be coated with an inner coating layer 5 and an outer coating layer 6. The core 1 is located at the center of the inner cladding 2. The stress regions 3 are symmetrically distributed on both sides of the core 1, and are entirely enclosed within the inner cladding 2. The stress regions 3, from the inside out, successively include a planarization layer and a graded layer.

[0041] The diameter of the fiber core 1 is D1 ∈ [5.5, 7] μm, the diameter of the outer cladding layer 4 is D4 ∈ [40, 125] μm, the distance d from the inner edge of the stress zone 3 to the outer edge of the fiber core 1 is ∈ [1, 4] μm, the diameter of the stress zone 3 is D3 ∈ [0.25D4, 0.3D4], and the diameter of the inner cladding layer 2 is D2 ∈ [D1+2D3+2d, D4]. The diameter of the inner coating layer 5 is denoted as D5, and the diameter of the outer coating layer 6 is denoted as D6, which can be set according to application requirements.

[0042] The diameter of the stress zone 3 needs to be controlled within a certain range compared to the diameter of the outer cladding layer 4. Too small a diameter will result in insufficient birefringence performance of the optical fiber, while too large a diameter will lead to poor mechanical strength. Referring to Figure 3, the birefringence coefficient of the optical fiber increases with the increase of the stress zone diameter. When the stress zone diameter is less than 0.25D4, the birefringence coefficient increases rapidly with increasing stress zone diameter; when the stress zone diameter is greater than 0.25D4, the birefringence coefficient tends to flatten with increasing stress zone diameter. The breaking force of the optical fiber decreases with increasing stress zone diameter. When the stress zone diameter is less than 0.3D4, the breaking force decreases slowly with increasing stress zone diameter; when the stress zone diameter is greater than 0.3D4, the breaking force decreases rapidly with increasing stress zone diameter. Therefore, considering both the birefringence performance and mechanical strength, when the diameter D3 of the stress zone 3 is within the range of [0.25D4, 0.3D4], both the birefringence performance and mechanical strength of the optical fiber can reach ideal values, with a birefringence reaching 7.72 × 10⁻⁶. -4 The above-mentioned fiber breaking force can reach 4.12 GPa or higher.

[0043] Referring to Figures 1 and 2, the inner cladding 2 is a fluorine-doped inner cladding, the relative refractive index Δ2 of the inner cladding 2 is [-0.2%, -0.1%], and the viscosity of the inner cladding at 1950℃ is 5.8~6.1 Pas.

[0044] The fiber core 1 in this invention employs a viscosity-matching design, requiring its viscosity to be lower than that of the inner cladding. According to the optical waveguide design, the relative refractive index Δ1 of the fiber core 1 is [0.55%, 0.75%]. Conventional optical fibers use a pure germanium-doped design, with a viscosity of 6.1~6.25 Pas at 1950℃, meaning the core viscosity is higher than the inner cladding viscosity. However, the fiber core 1 in this invention uses a fluorine-germanium co-doped design. While doping the core with germanium, the viscosity is adjusted by adding a certain amount of fluorine. When the fluorine doping Δ ∈ [-0.15%, -0.05%] and the corresponding germanium doping Δ ∈ [0.6%, 0.9%], the viscosity of the fiber core 1 at 1950℃ is 5.6~5.75 Pas, making the core viscosity lower than the inner cladding viscosity. Here, core Δ1 = germanium doping Δ + fluorine doping Δ.

[0045] The stress zone 3 is a boron-doped stress zone. Boron doping in the stress zone directly affects the birefringence performance of the optical fiber. A boron content of less than 15% leads to insufficient birefringence and weak polarization capability; a boron content of more than 25% leads to high internal stress and poor mechanical properties. In the stress zone 3 of this invention, the boron content of B2O3 is ∈ [15%, 25%], and the viscosity at 1950℃ is 4.7~5.2 Pas. Cracks in the fiber grinding stress zone are mainly caused by high stress at the boundary of the stress zone. Therefore, the stress zone 3 adopts a special design, consisting of a flattened layer and a gradient layer from the inside out. The gradient layer design at the edge of the stress zone effectively reduces the stress at the edge position.

[0046] Referring to Figure 4, the single-sided thickness of the graded layer in the stress zone affects both the probability of fiber polishing cracking and the fiber's birefringence performance. When the single-sided thickness diameter of the graded layer is less than 0.1D³, the fiber's birefringence coefficient decreases slowly with increasing graded layer thickness; when the single-sided thickness of the graded layer is greater than 0.1D³, the fiber's birefringence coefficient decreases rapidly with increasing graded layer thickness; and the probability of fiber polishing cracking decreases rapidly with increasing graded layer thickness, reaching 0% when the single-sided thickness of the graded layer is greater than 0.1D³. Therefore, controlling the single-sided thickness of the graded layer at 0.1D³ is ideal, ensuring that the fiber's birefringence coefficient reaches 7.72 × 10⁻⁶. -4 Meanwhile, the probability of grinding and cracking is 0%.

[0047] The outer cladding layer 4 has a relative refractive index Δ4 of 0% and a viscosity of 6.35 Pas at 1950℃.

[0048] By controlling the core viscosity matching, inner cladding diameter, stress zone diameter, and single-sided thickness of the graded layer in the stress zone, the fiber birefringence performance can be guaranteed to reach 7.72 × 10⁻⁶. -4 The fiber breaking force can reach 4.12 Gpa, and the probability of fiber grinding crack is 0%.

[0049] Example 2:

[0050] The present invention determines the size of the inner cladding based on the optical fiber geometry design described in Example 1, and then determines the number of deposition layers of the inner cladding to ensure that the radius of the inner cladding is greater than the distance from the outer edge of the stress region to the fiber core, so that the stress region is completely wrapped in the inner cladding.

[0051] Example 2 provides a method for fabricating panda-type polarization-maintaining fiber as described in Example 1, as shown in Figure 5, which mainly includes the following steps:

[0052] (1) Mandrel manufacturing and stress rod manufacturing.

[0053] Using the PCVD large rod process, an inner cladding layer and a core layer are sequentially deposited inside the liner. The deposited liner is then melted, sintered, and compacted to obtain a polarization-maintaining master rod. The inner cladding layer of the polarization-maintaining master rod encapsulates the entire stress zone.

[0054] Among them, the inner hole size of the lathe resonant cavity using the PCVD large bar process is greater than or equal to 45mm, and the power supply of the PCVD high-frequency system is greater than or equal to 10KW.

[0055] For example, a polarization-maintaining master rod (i.e., a core rod) can be prepared using the PCVD-R47 process, and two stress rods can be prepared using either the PCVD-R47 process or the PCVD-R31 process.

[0056] In this invention, the manufacturing order of the mandrel and stress rod is not limited; they can be manufactured simultaneously, or the manufacturing order can be adjusted according to the preparation conditions.

[0057] During the deposition of the inner cladding layer, a mixed gas of silicon tetrachloride, oxygen, and hexafluoroethane is introduced into the liner, reacting to generate fluorine-doped silicon dioxide which is deposited in the liner. The flow rate of silicon tetrachloride is 100-1000 sccm, the flow rate of oxygen is 1000-3000 sccm, the flow rate of hexafluoroethane is 10-100 sccm, the temperature is 1000-1500℃, and the number of deposition layers is 19000-20000.

[0058] During the deposition of the core layer, a mixed gas of silicon tetrachloride, germanium tetrachloride, oxygen, and hexafluoroethane is introduced into the liner. The reaction produces fluorine-doped silicon dioxide and germanium dioxide, which are then deposited in the liner. The flow rate of silicon tetrachloride is 100-1000 sccm, the flow rate of germanium tetrachloride is 2-100 sccm, the flow rate of oxygen is 300-3000 sccm, the flow rate of hexafluoroethane is 2-100 sccm, the temperature is 1000-1500℃, and the number of deposition layers is 500-4800.

[0059] When shrinking the deposited liner, adjust the pressure inside the tube to a slightly positive or slightly negative pressure, with a pressure range of -50 mBar to +20 mBar, adjust the temperature to 1600-2000℃, adjust the moving speed of the heating furnace to 20-100 mm / min, and control the shrinkage of each layer of the liner (i.e., the change in the thickness of the mandrel) to 1-5 mm.

[0060] When the molten mandrel is fired, the pressure inside the tube is adjusted to a slight negative pressure, with a pressure range of -950 mbar to 0 mbar, the temperature is adjusted to 1800-2200℃, and the moving speed of the heating furnace is adjusted to 10-40 mm / min.

[0061] The outer diameter of the polarization-maintaining mother rod obtained after sintering is 40-45mm, the diameter of the inner cladding is 30-40mm, and the diameter of the core layer is 1.9-4.5mm.

[0062] In addition, before depositing the inner cladding in the liner, the process may further include: introducing a mixture of hexafluoroethane and oxygen into the liner to polish it.

[0063] In addition, after obtaining the polarization-maintaining master rod and before machining the round hole, the process may include annealing the master rod to eliminate residual stress.

[0064] (2) Stress bar grinding and corrosion.

[0065] Specifically, after the stress bar is prepared, it needs to be polished and etched.

[0066] The process from rod to fiber is a proportional reduction process. The size of the stress rod after polishing directly determines the size of the stress zone of the drawn optical fiber. The outer diameter of the stress rod after polishing is 13.5-19.6 mm. The outer diameter of the gradient layer of the stress rod is the same as the outer diameter of the stress rod after polishing. The diameter of the flat layer of the stress rod is 12.15-17.64 mm.

[0067] Among them, after grinding, the stress bar is treated with hydrofluoric acid to remove impurities introduced during the grinding and machining process by corrosion. The corrosion time is controlled between 20 and 40 minutes.

[0068] (3) Drilling holes in the mandrel.

[0069] Specifically, a pair of symmetrical circular holes are longitudinally machined on both sides of the center line of the polarization-maintaining master rod, with the center lines of the circular holes parallel to and located in the same plane as the center line of the polarization-maintaining master rod.

[0070] The present invention determines the diameter of the stress bar, the distance from the edge of the stress zone to the fiber core, and the processing position of the circular hole according to the optical fiber geometry design described in Example 1. By controlling the deviation of the two circular holes, the symmetry of the stress zone can be ensured, thereby ensuring the uniformity of the stress distribution in the optical fiber.

[0071] The process from rod to fiber is a proportional reduction. The center position of the hole determines the geometric position of the fiber stress zone. The distance from the hole center to the center of the polarization-maintaining rod is 11.0-12.1 mm. Furthermore, the hole diameter must be controlled within a certain range relative to the outer diameter of the stress rod after grinding. This ensures that the geometric deviation of the fiber stress zone is controlled within 1 μm, guaranteeing the geometric symmetry of the stress zone. The single-sided gap must be controlled within 0.2 mm, meaning the hole diameter must be controlled within 0-0.4 mm relative to the outer diameter of the stress rod after grinding.

[0072] It should be noted that the processing order of (2) stress rod grinding and etching and (3) mandrel drilling in this invention is not limited and can be carried out simultaneously. The processing order can also be adjusted according to the preparation conditions. The above order is only for illustrative purposes. The processing of both can be carried out after the manufacture of the mandrel and stress rod and before assembly.

[0073] (4) Combination.

[0074] Two treated stress rods are respectively embedded into the two circular holes on both sides of the polarization-maintaining mother rod to obtain a panda-shaped polarization-maintaining fiber preform.

[0075] (5) Wire drawing.

[0076] The panda-shaped polarization-maintaining fiber preform is drawn into fibers in a molten state to obtain a panda-shaped polarization-maintaining fiber.

[0077] In addition, the present invention may also include:

[0078] (6) Coating.

[0079] An inner coating layer and an outer coating layer are sequentially coated on the outer cladding of the panda-shaped polarization-maintaining fiber.

[0080] In this invention, the core, inner cladding, and outer cladding are integrally formed to directly obtain a polarization-maintaining master rod. It can also obtain an extremely wide fluorine-doped inner cladding that completely encapsulates the stress zone. Specifically, it can be achieved using the R47 rod process of plasma chemical vapor deposition (PCVD) (i.e., PCVD-R47 process).

[0081] The resonant cavity of PCVD-R47 has an inner diameter of 47mm, and the outer diameter of the rod after deposition and sintering can reach 45mm, while the inner cladding diameter can reach 40mm. The inner cladding can completely cover the stress zone. In contrast, the resonant cavity of conventional PCVD-R31 has an inner diameter of 31mm, and the outer diameter of the rod after deposition and sintering can only reach a maximum of 25mm. The inner cladding diameter is only 21mm, and the inner cladding only partially overlaps with the stress zone, making it impossible for the inner cladding to completely cover the stress zone.

[0082] The PCVD-R47 uses a 10kW high-frequency system power supply, allowing for the full reaction of gases such as silicon tetrachloride, germanium tetrachloride, oxygen, and hexafluoroethane introduced during the inner cladding and core deposition process. This results in a reaction efficiency of up to 100%, and more precise control over the core profile, ensuring consistency with the theoretical design value. In contrast, the conventional PCVD-R31 uses a 6kW high-frequency system power supply, making it difficult to achieve 100% gas reaction efficiency during deposition. This leads to a discrepancy between the core profile and the theoretical design value, which can affect optical performance.

[0083] This invention fully leverages the advantages of the PCVD-R47 process, including:

[0084] (1) The outer diameter of the core rod after deposition and sintering can reach 45mm, and the required polarization maintaining master rod can be obtained directly. Compared with the conventional PCVD-R31 process, there is no need for sleeve and stretching processes, the process control is simple, and the fiber production efficiency is higher.

[0085] (2) No need for the core rod to perform the stretching process, which can avoid the problem of core rod alignment during the core rod sleeve process, and avoid the problem of excessive fiber core-pack concentricity from the source. At the same time, it can avoid the problem of excessive fiber attenuation caused by the introduction of impurities during the stretching process.

[0086] (3) The diameter of the inner cladding after deposition and sintering can reach 40 mm, which can meet the design requirements of optical fiber. The inner cladding wraps the entire stress zone, improving the full-temperature performance and polishing performance of optical fiber.

[0087] (4) The deposition efficiency of the inner cladding and core layer can reach 100%, and the core rod profile can be precisely controlled to ensure that the core rod profile is consistent with the calculated theoretical value, thereby improving the optical performance of the fiber.

[0088] (5) The diameters of the conventional PCVD-R31 rods before and after fusion shrinkage sintering are 31mm and 25mm, respectively, and the area ratio after fusion shrinkage is 65% compared to before fusion shrinkage; while the diameters of the PCVD-R47 rods before and after fusion shrinkage sintering are 47mm and 40~45mm, respectively, and the area ratio after shrinkage is 72.4%~91.6% compared to before fusion shrinkage. The larger area ratio before and after fusion shrinkage is more conducive to the control of the non-circularity of the preform core layer and the inner cladding (the non-circularity of the preform core layer is less than 1%, and the non-circularity of the inner cladding is less than 0.5%), resulting in better non-circularity of the drawn optical fiber core and the inner cladding.

[0089] The preparation process and process control of this invention are simple, and it can improve production efficiency and product quality.

[0090] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A panda-type polarization-maintaining optical fiber, characterized in that, include: Core, inner cladding, stress zone, and outer cladding; The stress zones are symmetrically distributed on both sides of the fiber core, and the inner cladding covers the entire stress zone.

2. The panda-type polarization-maintaining optical fiber according to claim 1, characterized in that, The diameter of the stress zone, D3, is in the range of [0.25D4, 0.3D4], where D4 is the diameter of the outer cladding layer.

3. The panda-type polarization-maintaining optical fiber according to claim 1, characterized in that, The stress zone consists of a flat layer and a gradient layer from the inside out.

4. The panda-type polarization-maintaining optical fiber according to claim 3, characterized in that, The thickness of the gradient layer on one side is 0.1D3.

5. The panda-type polarization-maintaining optical fiber according to claim 1, characterized in that, The viscosity of the stress zone is less than the viscosity of the fiber core, the viscosity of the fiber core is less than the viscosity of the inner cladding, and the viscosity of the inner cladding is less than the viscosity of the outer cladding.

6. The panda-type polarization-maintaining optical fiber according to claim 1, characterized in that, The inner cladding is a fluorine-doped inner cladding, and the relative refractive index Δ2 of the inner cladding is [-0.2%, -0.1%]; the fiber core is a fluorine-germanium co-doped fiber core, and the viscosity of the fiber core is adjusted by the amount of fluorine doping, and the relative refractive index contribution of fluorine in the fiber core is Δ∈[-0.15%, -0.05%].

7. A method for fabricating a panda-type polarization-maintaining optical fiber as described in any one of claims 1-6, characterized in that, Includes the following steps: Using the PCVD large rod process, an inner cladding layer and a core layer are sequentially deposited inside a liner. The deposited liner is then melted and sintered to obtain a polarization-maintaining master rod. The inner cladding layer of the polarization-maintaining master rod encapsulates the entire stress zone. The lathe resonant cavity inner hole size of the PCVD large rod process is greater than or equal to 45mm, and the high-frequency system power supply of PCVD is greater than or equal to 10KW. A pair of symmetrical circular holes are longitudinally machined on both sides of the center line of the polarization-maintaining master rod. The center lines of the circular holes are parallel to the center line of the polarization-maintaining master rod and are located in the same plane. Two stress rods are prepared, and the stress rods are then polished and etched. Two processed stress rods are respectively embedded into the two circular holes on both sides of the polarization-maintaining mother rod to obtain a panda-shaped polarization-maintaining fiber preform. The panda-shaped polarization-maintaining fiber preform is drawn into fibers in a molten state to obtain a panda-shaped polarization-maintaining fiber.

8. The method for fabricating panda-type polarization-maintaining optical fiber according to claim 7, characterized in that, During the deposition of the inner cladding, a mixed gas of silicon tetrachloride, oxygen and hexafluoroethane is introduced into the liner, and the reaction generates fluorine-doped silicon dioxide which is deposited in the liner. During the deposition of the inner cladding, the flow rate of silicon tetrachloride is 100-1000 sccm, the flow rate of oxygen is 1000-3000 sccm, the flow rate of hexafluoroethane is 10-100 sccm, the temperature is 1000-1500℃, and the number of deposition layers is 19000-20000.

9. The method for fabricating panda-type polarization-maintaining optical fiber according to claim 7, characterized in that, During the deposition of the core layer, a mixed gas of silicon tetrachloride, germanium tetrachloride, oxygen and hexafluoroethane is introduced into the liner tube, and the reaction generates fluorine-doped silicon dioxide and germanium dioxide which are deposited in the liner tube. During the deposition of the core layer, the flow rate of silicon tetrachloride is 100-1000 sccm, the flow rate of germanium tetrachloride is 2-100 sccm, the flow rate of oxygen is 300-3000 sccm, the flow rate of hexafluoroethane is 2-100 sccm, the temperature is 1000-1500℃, and the number of deposition layers is 500-4800.

10. The method for fabricating panda-type polarization-maintaining optical fiber according to claim 7, characterized in that, When shrinking the deposited liner, adjust the pressure inside the tube to a slightly positive or slightly negative pressure, with a pressure range of -50 mBar to +20 mBar, adjust the temperature to 1600-2000℃, adjust the moving speed of the heating furnace to 20-100 mm / min, and control the shrinkage of each layer of the liner to 1-5 mm; when firing, adjust the pressure inside the tube to a slightly negative pressure, with a pressure range of -950 mBar to 0 mBar, adjust the temperature to 1800-2200℃, and adjust the moving speed of the heating furnace to 10-40 mm / min.