Polarization maintaining optical fiber and preparation method thereof

Abstract

The invention belongs to the field of optical fibers, and discloses a polarization maintaining optical fiber which comprises an optical fiber body, and a fiber core is arranged in the center of the optical fiber body. The periphery of the fiber core is provided with a pair of shrinkage stress areas and a pair of expansion stress areas which are arranged in a crossed manner; the thermal expansion coefficient of the shrinkage stress area is 20 * 10 <-6 > / K-30 * 10 <-6 > / K; the thermal expansion coefficient of the expansion stress area is-1 * 10 <-6 > / K-2 * 10 <-6 > / K; the expansion stress region contains the following elements in molar percentage: 26.5-29.5% of Si; 2.8 to 3.9 percent of M; 3.0 to 3.8 percent of N; the balance of oxygen element; wherein no less than 50 mol% of element in M is Ti; and not less than 70 mol% of element in N is Al. According to the polarization maintaining optical fiber, a pair of shrinkage stress areas and a pair of expansion stress areas are arranged in a crossed manner, and the formula composition of the expansion stress areas is optimized, so that the stress is further strengthened, and meanwhile, the performances such as good birefringence performance, temperature stability, minimum bending radius and tensile strength are ensured; meanwhile, the invention also provides a preparation method of the polarization maintaining optical fiber.

Description

Technical Field

[0001] This invention relates to the field of optical fibers, and more particularly to a polarization-maintaining optical fiber and its fabrication method. Background Technology

[0002] Polarization-maintaining fiber (PMF) is a core component of interferometric fiber optic sensors (such as fiber optic gyroscopes, FOGs). It maintains the transmission direction of linearly polarized light through stress birefringence, providing fundamental support for high-precision optical measurements. In high-reliability inertial navigation, industrial motion control, and extreme environment sensing systems, the performance of FOGs is highly dependent on the geometric accuracy, birefringence stability, and environmental adaptability of the polarization-maintaining fiber.

[0003] With the increasing demand for miniaturization of navigation systems from consumer augmented reality (AR), microrobots, and intelligent logistics devices (e.g., the latest devices require a FOG diameter ≤ 40 mm), traditional polarization-maintaining optical fibers face the following technical bottlenecks when achieving extremely small diameters (≤ 40 μm): Insufficient birefringence performance: The reduction in core size leads to a decrease in birefringence (Δn) to < 2 × 10⁻⁶. -4 It cannot meet the requirements for high-precision interference (zero-drift stability is significantly reduced); temperature stability deteriorates: the mismatch between the thermal expansion coefficients of the stress zone and the fiber core intensifies, and the birefringence fluctuation is >5×10 when the temperature changes (ΔT=±50℃). -5 This leads to an increase in the zero-bias drift of the fiber optic gyroscope to >0.008° / h, exceeding the industrial-grade high-precision sensing standard (zero-bias stability <0.005° / h); mechanical reliability decreases: the bending strength of the extremely fine-diameter fiber decreases (loss increases by >1dB / turn when the bending radius is <5mm), and long-term vibration (10g acceleration, 10 6 After one cycle, the microbending loss increases by more than 0.5 dB / km, making it difficult to meet the stringent environmental requirements of compact equipment. These issues prevent traditional polarization-maintaining fibers from meeting industrial-grade high-precision sensing standards, becoming a key factor restricting the large-scale application of miniaturized FOGs.

[0004] In existing technologies, panda-type polarization-maintaining fiber and bow-tie-type polarization-maintaining fiber are two mainstream solutions, but both have significant drawbacks:

[0005] Panda-type optical fiber (reference) Figure 2 ):

[0006] Structure: The core layer is Ge-doped quartz, the stress zone consists of two symmetrical B2O3-SiO2 circular regions, and the cladding is pure quartz;

[0007] Preparation process: Mandrel fabrication → Outer wrapping → Drilling → Stress rod fabrication → Sleeve wire drawing.

[0008] Key defects: Concentricity deviation (>1.2μm): Mismatch in thermal expansion coefficients between the mandrel and the sleeve leads to geometric asymmetry, increasing polarization crosstalk by 3dB. Uneven axial thermal stress: Temperature gradient ±30℃ in the molten zone causes birefringence fluctuations >12% (-55~85℃). Limited stress zone size: Sleeve wall thickness limits the stress zone proportion to <15%, resulting in fiber birefringence Δn <1.8×10⁻⁶. -4 .

[0009] (2) Bowtie type polarization-maintaining fiber (reference) Figure 3 ):

[0010] Structure: The core layer is Ge-doped quartz, the stress zone has a symmetrically distributed B2O3-SiO2 bowtie structure, and the cladding is pure quartz;

[0011] Preparation method: Deposition of core layer and stress layer → Flame etching of stress layer → Melting and shrinking → Sleeve → Wire drawing;

[0012] Key drawbacks: Poor thermal stability: Microcracks develop at the bowtie region-substrate interface at drawing temperatures (>2000℃), causing fiber strength to plummet to <200 kpsi. Uncontrollable process: Flame polishing leads to geometric distortion in the stress zone, resulting in polarization crosstalk variations of >8dB in 40μm fibers (temperature cycling). Small preform size: Because both the core rod and stress zone are completed during the MCVD deposition process, it is difficult to fabricate large-sized preforms, leading to inconsistencies in parameters between preforms.

[0013] Existing commercial polarization-maintaining fibers have significant shortcomings in key performance indicators, making it difficult to meet the application requirements of high-precision miniaturized sensing systems. Firstly, the 80μm fiber diameter necessitates a minimum fiber loop diameter of ≥50mm, and a 60μm fiber diameter necessitates a minimum fiber loop diameter of ≥40mm, which cannot meet the compact design target of ≤40mm required by next-generation portable devices. Secondly, the current birefringence Δn is 3.0×10⁻⁶. -4 However, when the fiber core diameter is reduced to 40 μm or even smaller, Δn will drop sharply to 1.5 × 10⁻⁶ due to the decrease in stress transmission efficiency. -4 The following is far below the 4.0×10 standard required for industrial-grade high-precision fiber optic gyroscopes (FOGs). -4 Performance thresholds. Regarding temperature stability, its zero-drift stability exhibits a drift of 0.01° / h within the -55~85℃ temperature range, exceeding the 0.005° / h upper limit specified in industrial environmental standards, indicating significant reliability deficiencies under extreme temperature conditions. Furthermore, the minimum bending radius of 5mm severely restricts the tight integration of the thin-diameter fiber ring, making it difficult to compress the FOG volume to the tactical-grade space requirement of 0.05L. These shortcomings collectively expose the fundamental contradiction faced by traditional symmetrical stress structures in the process of reducing fiber core diameter: an irreconcilable technical conflict exists between the continuous reduction of geometric dimensions and the maintenance of optical performance stability.

[0014] Patent application CN112130250A, entitled "Panda-type Polarization-maintaining Optical Fiber and its Fabrication Method," proposes a polarization-maintaining optical fiber composed of cross-shaped stress bars. Through the interlocking cross structure of contraction and expansion stress layers, stress reinforcement is achieved, enhancing the panda-type polarization-maintaining optical fiber's ability to resist external asymmetric thermal stress through stress birefringence and optimizing polarization crosstalk temperature stability. Further refinement of this scheme specifies that the expansion stress layer is composed of Ge and F co-doped quartz glass, with Ge accounting for 1–8% of the molar percentage and F accounting for 1–21%. This cross-shaped arrangement of contraction and expansion stress layers effectively strengthens the stress.

[0015] Even though it uses polarization-maintaining fiber composed of stress bars arranged in a cross shape, the smallest outer cladding diameter in its embodiment only reaches 40μm; on the one hand, it does not break through the 40μm limit, and on the other hand, it does not conduct further research on birefringence performance, temperature stability, minimum bending radius, and tensile strength.

[0016] It needs to be clarified that in the field of optical fiber, there is a vicious cycle of "size reduction - performance collapse"; the above solution only solves the optimization of some performance of optical fibers above 40μm, and does not completely solve the optimization of other process parameters of optical fibers below 40μm except for polarization crosstalk changes. Summary of the Invention

[0017] The purpose of this invention is to provide a polarization-maintaining optical fiber, which achieves further stress enhancement by using a pair of contraction stress regions and a pair of expansion stress regions arranged in a cross shape, and by optimizing the formulation of the expansion stress regions, while ensuring good birefringence performance, temperature stability, minimum bending radius, tensile strength and other properties.

[0018] In addition, the present invention also provides a method for preparing the polarization-maintaining optical fiber.

[0019] To achieve the above objectives, this application discloses a polarization-maintaining optical fiber, which includes an optical fiber body, with a fiber core at the center of the optical fiber body; and a pair of contraction stress zones and a pair of expansion stress zones arranged in a cross shape around the outer periphery of the fiber core.

[0020] The coefficient of thermal expansion of the contraction stress zone is 20 × 10⁻⁶. -6 / K~30×10 -6 / K;

[0021] The coefficient of thermal expansion of the expansion stress zone is -1×10⁻⁶. -6 / K~-2×10 -6 / K;

[0022] The expansion stress zone contains the following molar percentage elements:

[0023] Si: 26.5~29.5%; M: 2.8~3.9%; N: 3.0~3.8%; balance: oxygen.

[0024] Among them, M contains at least 50 mol% of Ti, and N contains at least 70 mol% of Al.

[0025] The advantages of this invention are:

[0026] 1. Similar to the cross-shaped design of CN112130250A, this invention overcomes the stress limitations of thin-diameter optical fibers, improving efficiency: This design fundamentally breaks through the limitations of traditional stress application methods in thin-diameter optical fibers (such as coaxial or uniaxial stress regions). The orthogonal structure significantly improves stress application efficiency, enabling extremely high birefringence (Δn > 6 × 10⁻⁶) even in thin-diameter optical fibers. -4 Its stress output efficiency is three times higher than that of existing technologies;

[0027] 2. This invention solves the birefringence inhomogeneity caused by core layer misalignment, thus improving stability: Unlike single-stress region structures where even a small core layer misalignment leads to significant changes and inhomogeneities in birefringence, orthogonal dual-stress region structures, due to their symmetry and orthogonal coupling characteristics of the stress fields, can effectively compensate for the influence of core layer position deviation. This makes the birefringence of the optical fiber more uniform and stable in both the axial and cross-sectional directions, thereby significantly improving the stability of key performance parameters;

[0028] 3. By optimizing the materials for the expansion stress zone and combining them with the aforementioned cross-shaped design, this invention achieves the following simultaneously under extremely small diameter conditions: a leap in birefringence performance (significantly exceeding conventional thresholds), extreme environmental adaptability (zero drift stability optimized by over 60%), and revolutionary miniaturization capability (significantly reduced fiber ring size), providing an irreplaceable technical foundation for high-precision micro-sensors and precision instruments.

[0029] In the aforementioned polarization-maintaining fiber, the fiber body consists of, from the inside out, a core, an inner cladding, and an outer cladding; the coefficient of thermal expansion of the inner cladding is 2.5 × 10⁻⁶. -6 / K~4×10 -6 / K; the core material is Ge-doped silicon dioxide; the shrinkage stress zone material is composed of 75~90 wt% SiO2 and 10~20 wt% B2O3; the inner cladding material is F-doped silicon dioxide; the outer cladding material is low-hydroxyl quartz.

[0030] In the aforementioned polarization-maintaining optical fiber, the core is made of silicon dioxide with a GeO2 content of 5-7 wt%; the inner cladding is made of silicon dioxide with an F content of 9-11 wt%.

[0031] In this invention, the fluorine-doped inner cladding acts as a thermal stress buffer layer, blocking the transmission of temperature deformation to the fiber core; the coefficient of thermal expansion (CTE) of the inner cladding and the fiber core are matched (+3.2×10⁻⁶). -6 / K), precisely suppressing thermally induced birefringence drift.

[0032] In the aforementioned polarization-maintaining optical fiber, the diameter of the fiber core is 3~5μm; the diameter of the inner cladding is 5~7μm; the diameter of the outer cladding is 20~50μm; the diameter or diagonal length of the contraction stress zone is 4~8μm; and the diameter or diagonal length of the expansion stress zone is 3~6μm.

[0033] In the aforementioned polarization-maintaining fiber, M contains the following elements:

[0034] Ti: 50~100 mol%

[0035] Zr, Hf, and Sn are each independently selected from 0–50 mol%;

[0036] The N contains the following elements:

[0037] Al: 70~100 mol%

[0038] Y, La, and Ga are each independently selected from 0 to 30 mol.

[0039] In the aforementioned polarization-maintaining fiber, the periphery of the expansion stress region is covered with a protective layer; the protective layer is F-doped silicon dioxide; the protective layer is also used to construct a refractive index depression region between the expansion stress region and the fiber core.

[0040] A refractive index depression region is constructed by creating a protective layer, which cuts off the optical field coupling path between the expansion rod and the fiber core to avoid mode field distortion.

[0041] The fluorine-doped silicon dioxide (SiO2:F) layer epitaxially located outside the expansion stress region enhances system reliability through a synergistic chemical and optical mechanism: volatile fluorides (such as AlF3) formed by fluorine ions replacing oxygen and high bond energy Si-F bonds significantly inhibit metal ion diffusion, constructing a chemical barrier to prevent contaminants such as Cu / Fe from eroding the fiber core; at the same time, the quantitative decrease in refractive index caused by fluorine doping (Δn≈-0.0015 / at% F) forms a refractive index depression region (n≈1.43-1.45) between the expansion layer (n≈1.48) and the fiber core (n≈1.46), cutting off the optical field coupling path and avoiding mode field distortion.

[0042] The synergistic design of the ternary low-expansion cladding and the fluorine-doped isolation layer achieves multi-dimensional performance enhancements: the near-zero expansion cladding matches the thermal properties of the quartz fiber core, eliminating microbending losses caused by temperature stress; more specifically, the synergistic effect of the ternary low-expansion cladding and the fluorine-doped isolation layer stems from the cross-scale design of multiple physics fields. The near-zero expansion cladding, by matching the coefficient of thermal expansion with the quartz fiber core, intrinsically suppresses interfacial thermal stress induced by temperature cycling, thereby eliminating the origin of microbending losses. Simultaneously, the fluorine-doped layer enhances the optical field confinement by increasing the refractive index contrast between the cladding and the fiber core, thus improving the waveguide's resistance to microbending; its chemical passivation further blocks the diffusion of active ions, ensuring long-term reliability. This system, through a triple synergy of thermodynamics, optics, and chemistry, provides a robust solution for fiber optic sensors in harsh environments. The chemical passivation of the fluorine layer blocks metal contamination, ensuring long-term stability; while the refractive index gradient structure confines the optical field through total internal reflection, ensuring signal transmission efficiency. This comprehensive solution lays the material foundation for high-precision fiber optic sensors and communication systems in harsh environments.

[0043] In the aforementioned polarization-maintaining optical fiber, the refractive index of the expansion stress region is 1.48±0.01; the refractive index of the fiber core is 1.46±0.01; and the refractive index of the refractive index depression region is 1.43~1.45.

[0044] The F doping amount in the protective layer is 7~9 wt%.

[0045] In the aforementioned polarization-maintaining optical fiber, the outer surface of the optical fiber body also has a coating layer made of resin.

[0046] Meanwhile, this invention also discloses a method for fabricating polarization-maintaining optical fiber as described in any of the above, comprising the following steps:

[0047] Step 1: Drill shrinkage stress rod holes and expansion stress rod holes in the outer cladding of the optical fiber body;

[0048] Step 2: Assemble the contraction stress rod and expansion stress rod into the contraction stress rod hole and expansion stress rod hole, respectively;

[0049] Step 3: Graphite induction drawing is performed in a graphite induction furnace at 1900℃~2100℃. The drawn fiber is then rapidly cooled at a rate greater than or equal to 100 K / s to lock the residual stress field of the optical fiber and obtain polarization-maintaining fiber.

[0050] It should be noted that in this invention, the chemical composition of the shrinkage stress bar and the expansion stress bar is the same as that of the shrinkage stress region and the expansion stress region; the shrinkage stress bar and the expansion stress bar are prepared by plasma chemical vapor deposition process, wherein the expansion stress bar is obtained by depositing an F-doped silicon dioxide layer on the periphery of the expansion bar by plasma chemical vapor deposition process.

[0051] It should be further explained that the present invention is not strictly limited to the preparation process being plasma chemical vapor deposition. As is well known in the art, MCVD (modified chemical vapor deposition) process can also be used.

[0052] In an embodiment of the present invention, the cross-section of the contraction stress bar and the expansion stress bar is preferably circular, but can also be square or bow-tie shaped; if the cross-section is circular, the diameter tolerance is ≤ ±0.5 μm; if the cross-section is square, the side length is d1, and the chamfer radius is <0.1d1; if the cross-section is bow-tie shaped, the wing angle θ = 70°~100°, the wingtip curvature radius R = 0.05~0.1d2, where d2 is the radius of the expansion bar.

[0053] The beneficial effects of this application are:

[0054] 1. This invention's cross-shaped design overcomes the stress limitations of thin-diameter optical fibers, improving efficiency: This design fundamentally breaks through the limitations of traditional stress application methods in thin-diameter optical fibers (such as coaxial or uniaxial stress regions). The orthogonal structure significantly improves stress application efficiency, enabling extremely high birefringence (Δn > 6 × 10⁻⁶) even in thin-diameter optical fibers. -4 Its stress output efficiency is three times higher than that of existing technologies;

[0055] 2. This invention solves the birefringence inhomogeneity caused by core layer misalignment, thus improving stability: Unlike single-stress region structures where even a small core layer misalignment leads to significant changes and inhomogeneities in birefringence, orthogonal dual-stress region structures, due to their symmetry and orthogonal coupling characteristics of the stress fields, can effectively compensate for the influence of core layer position deviation. This makes the birefringence of the optical fiber more uniform and stable in both the axial and cross-sectional directions, thereby significantly improving the stability of key performance parameters;

[0056] 3. By optimizing the materials for the expansion stress zone and combining them with the aforementioned cross-shaped design, this invention achieves the following simultaneously under extremely small diameter conditions: a leap in birefringence performance (significantly exceeding conventional thresholds), extreme environmental adaptability (zero drift stability optimized by over 60%), and revolutionary miniaturization capability (significantly reduced fiber ring size), providing an irreplaceable technical foundation for high-precision micro-sensors and precision instruments.

[0057] 4. By optimizing the chemical composition of the materials in the expansion stress zone and contraction stress zone, the tensile strength of the optical fiber obtained by drawing is increased to 532 kpsi (traditional 100 kpsi), an increase of 112%, and the minimum bending radius is reduced to 2 mm (traditional 3 mm). Attached Figure Description

[0058] Figure 1 This is a schematic diagram of the orthogonal dual-stress polarization-maintaining fiber structure of the present invention;

[0059] Figure 2 This is a schematic diagram of the cross-section of a panda-type optical fiber;

[0060] Figure 3 This is a schematic diagram of the cross-section of a bowtie-type polarization-maintaining fiber. Detailed Implementation

[0061] The present invention will now be clearly and completely described in conjunction with embodiments thereof. It should be noted that, unless specific conditions are specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0062] Part 1: Preparation of SiO2-TiO2-Al2O3 Glass and Stress Rods

[0063] The following steps further explain the PCVD preparation process of SiO2-TiO2-Al2O3 glass.

[0064] 1. Equipment and Materials

[0065] Core equipment: UNC-PCVD / HEC plasma chemical vapor deposition system. This system requires the following:

[0066] Multi-channel precision mass flow controller for reactant gas and carrier gas.

[0067] High-temperature evaporation and sublimation system: used to transport TiCl4 and AlCl3 precursors.

[0068] Microwave power source: The frequency is usually 2.45 GHz, and the power range is adjustable from 0 to 6000W.

[0069] Deposition liner: Low-hydroxyl synthetic quartz tube with an outer diameter of 31 mm and an inner diameter of 27 mm.

[0070] Gas source and chemical precursors:

[0071] SiCl4: Purity ≥ 99.999%

[0072] TiCl4: Purity ≥ 99.999%

[0073] AlCl3: Solid state, purity ≥ 99.999%

[0074] O2: Purity ≥ 9N

[0075] He: Purity ≥ 9N (used as both carrier gas and dilution gas)

[0076] 2. Preparation process flow

[0077] Step 1: System preparation and liner pretreatment

[0078] Installation and Leak Detection: Install the quartz liner to the equipment spindle and perform a system airtightness check to ensure a leakage rate < 1×10⁻ 9 mbar•L / s.

[0079] Cavity purification: Repeat the "vacuuming-helium filling" cycle three times to completely remove moisture and oxygen from the cavity.

[0080] Inner wall activation: The inner wall of the liner is polished for 10 minutes at >1200℃ using an oxyhydrogen flame polisher to remove microcracks and improve surface reactivity.

[0081] Step 2: Stabilization of the precursor transport system

[0082] This is a critical step in the process and must be started and stabilized before deposition:

[0083] TiCl4 evaporation system: The set temperature is 150 ± 0.5℃ to ensure stable vaporization.

[0084] AlCl3 sublimation system: The set temperature is 180 ± 0.5℃, and 200 sccm of high-purity helium is used as the carrier gas.

[0085] Step 3: Plasma Deposition Process

[0086] Initiate the plasma and execute the automatic deposition program. The core process parameters are shown in Tables 1 and 2 below.

[0087] Table 1 Process Parameter Table

[0088] process parameters Setting value / range Control precision Functions and Explanations microwave power 1200 W ±10 W It generates and maintains a stable moving plasma sphere, which excites gas reactions. Deposition pressure 15 mbar ±0.2 mbar Optimize plasma state and deposition uniformity. Base tube temperature 1150℃ ±10 ℃ Ensure that the deposited layer is in a glassy state rather than a powdery state to promote densification. 100 rpm ±1 rpm Ensure uniform circumferential deposition. Feeding speed 120 mm / min ±1 mm / min Control the thickness of a single layer deposition.

[0089] Table 2 Process Parameter Table

[0090] gas Gas flow rate Flow accuracy illustrate <![CDATA[SiCl4]]> 150 sccm ±1 sccm The silicon source forms the main body of the glass network. <![CDATA[TiCl4]]> 8 sccm ±1 sccm Titanium source, to regulate the coefficient of thermal expansion. <![CDATA[AlCl3]]> Equivalent to ~10 sccm ±1 sccm Aluminum source, inhibiting crystallization and improving glass stability. <![CDATA[O2]]> 450 sccm ±5 sccm The reacting gas produces oxides. He 600 sccm ±5 sccm Dilute the reactants, improve deposition uniformity, and stabilize the plasma.

[0091] Deposition cycle: Layer-by-layer deposition is carried out on the inner wall of the liner through >1000 reciprocating scans until the predetermined thickness is reached.

[0092] Step 4: Melt and shrink into rods

[0093] The deposited composite tube is transferred to a melting lathe and subjected to a progressive oxyhydrogen flame process. The process parameters for the progressive oxyhydrogen flame process are shown in Table 3.

[0094] Table 3 Process Parameter Table

[0095] stage temperature speed illustrate Pre-shrink ~1700°C 200 mm / min densification ~1900°C 80 mm / min The "advance-retreat method" is used to release stress. Burn to death >2000°C 20 mm / min Until the hole is completely closed, forming a solid glass rod.

[0096] The formulation of SiO2-TiO2-Al2O3 glass, as well as its coefficient of thermal expansion (CTE) and refractive index (n), can be found in Table 4.

[0097] Table 4 Formula Table

[0098] <![CDATA[SiO2 / wt%]]> <![CDATA[TiO2 / wt%]]> <![CDATA[Al2O3 / wt%]]> Si:Ti:Al / molar ratio <![CDATA[CTE / *10 -6 / K]]> Refractive index n Example 1 77 15 8 26.7:3.9:3.3 -0.5 1.52 Example 2 80 12 8 27.5:3.1:3.2 -1.5 1.48 Example 3 81.5 11 7.5 28:2.8:3 -1.2 1.485 Comparative Example 1 80 0 20 26.8:0:7.9 -2.8 1.465 Comparative Example 2 75 17 8 26.1:4.4:3.3 0.5 1.545

[0099] Stress rods were prepared from SiO2-TiO2-Al2O3 glass according to the above embodiments and comparative examples; the outer periphery of the stress rod was an fluorine-doped silicon dioxide layer with a thickness of 0.5 mm.

[0100] In this embodiment, the stress bar containing the fluorine-doped silicon dioxide layer is prepared sequentially using a two-step PCVD method. First, a fluorine-doped silicon dioxide inner layer is prepared on a high-purity quartz glass substrate by plasma chemical vapor deposition: under a stable process environment of 12000W microwave power, 15mbar deposition pressure, and 1150℃ substrate temperature, fluorinated gases such as SiCl4, O2, and C2F6 are introduced. By precisely controlling the flow rate of the fluorine source gas, a uniform doped layer with a fluorine content of 7-9 wt% is deposited. After subsequent melting and shrinking, this layer will precisely form a specified structure with a thickness of 0.5mm. Subsequently, using this fluorine-doped layer as a new substrate, a SiO2-TiO2-Al2O3 ternary glass layer was deposited on its inner surface. Within the same PCVD reaction chamber, maintaining a deposition temperature of 1150°C, SiCl4, TiCl4 (evaporated at 150°C), AlCl3 (sublimated at 180°C, carried by helium), and O2 were precisely introduced. After more than 1000 reciprocating scans, a functional layer with negative thermal expansion characteristics was constructed. Finally, through a single precision melting and shrinking process, this composite structure of "fluorine-doped silica layer-ternary glass layer-quartz liner" was melted and shrunk into a dense, solid stress bar. This sequence ensures that the fluorine-doped layer, acting as an optical isolation and chemical barrier layer, is located between the ternary functional layer and the outer layer, ultimately forming a structure from the inside out (from the fiber core side) of "high refractive index ternary glass layer → low refractive index fluorine-doped layer," effectively constructing the required refractive index depression region and achieving excellent thermal stress matching and optical field confinement.

[0101] The relevant formulation parameters for each stress bar can be found in Table 5;

[0102] Table 5. Stress bar formulation

[0103]

[0104] The refractive index of the fluorine-doped silicon dioxide layer was approximately n≈1.44.

[0105] The aforementioned diameter includes the F-doped silicon dioxide layer.

[0106] The stress bar obtained above was then used for the second part of the test.

[0107] Part Two: Fabrication of Orthogonal Dual-Stress Polarization-Maintaining Fibers

[0108] Application Example 1

[0109] Fabrication of 40-micron dual-stress polarization-maintaining fiber

[0110] The structure of the 40-micron dual-stress polarization-maintaining fiber can be referenced. Figure 1 It mainly includes a core layer 101, an inner cladding layer 102, an outer cladding layer 103, and a coating layer 106 arranged sequentially from the inside out; the shrinkage stress zone 104 and the expansion stress zone 105 are arranged in a cross shape on the outer cladding layer.

[0111] Before drawing, the core layer 101, the inner cladding layer 102, and the outer cladding layer 103 constitute the substrate of the Φ50mm×500mm synthetic quartz rod; the substrate is prepared by conventional external spray deposition method.

[0112] After drawing, the dimensions and specifications of the core layer 101, inner cladding layer 102, outer cladding layer 103, shrinkage stress zone 104, expansion stress zone 105, and coating layer 106 are shown in Table 6.

[0113] Table 6 Specifications

[0114] mark Component Name Material composition (wt%) Functions and parameters Diameter (μm) 101 Core layer <![CDATA[SiO2- 6wt% GeO2]]> Light guide core (NA=0.16) 4 102 Inner cladding <![CDATA[SiO2- 10wt% F]]> <![CDATA[Transfer stress transition layer (CTE = +3.2×10 -6 / K)]]> 6 103 Outer layer Low-hydroxyl quartz Mechanical protection 40 104 Contraction stress zone <![CDATA[SiO2- 20wt% B2O3]]> <![CDATA[Apply tensile stress (CTE = +25×10 -6 / K)]]> 7.5 105 Expansion stress zone Refer to Table 5 <![CDATA[Apply compressive stress (CTE = -1.5×10 -6 / K)]]> 5 106 Coating layer acrylic resin outer protective layer 90

[0115] Its preparation process is as follows:

[0116] S1: Deep hole machining:

[0117] In the deep hole machining stage, a Φ50mm×500mm synthetic quartz rod is used as the substrate. A deep hole drilling machine equipped with a diamond-coated die (5μm grit) is used to machine a shrinkage stress rod hole (diameter Φ9.2mm) and an expansion stress rod hole (diameter Φ6.2mm), respectively, to achieve a precision deep hole structure with a depth-to-diameter ratio of 41:1 (depth 500mm). After machining, the hole wall is subjected to hydrofluoric acid pickling and drying treatment to strictly control the surface roughness to Ra<0.05μm, providing an ultra-smooth interface for stress rod assembly and eliminating the risk of stress concentration in the subsequent wire drawing process.

[0118] S2: Stress bar preparation

[0119] During the stress bar fabrication stage, differentiated process paths were implemented for shrinkage stress bars and expansion stress bars. Shrinkage stress bars were fabricated using plasma-enhanced chemical vapor deposition (PCVD) to deposit a SiO2-B2O3 glass layer (B2O3 doping concentration 20%), followed by melting and shrinking on a melting lathe, and finally ultra-precision grinding to a cylinder with a diameter of Φ9mm. Expansion stress bars, on the other hand, were fabricated using PCVD to deposit a SiO2-TiO2-Al2O3 ternary composite glass, with a fluorine-doped SiO2 protective layer on the outer layer to suppress the volatilization of high-temperature components; they were then shaped to a standard Φ6mm size using the same ultra-precision grinding process. The core design logic of this process is that the shrinkage bar relies on high borosilicate glass (CTE=+25×10⁻⁶). -6 / K) to achieve directional thermal expansion, and the expansion rod utilizes a titanium-aluminum-silicon composite system (CTE=-1.5×10) -6 / K) generates a negative expansion effect; the risk of material deformation is blocked by the dual technology of nitrogen protection (shrinkage rod) and fluorination coating (expansion rod), and the stress field matching of subsequent orthogonal assembly is ensured by geometric precision control.

[0120] S3: Assembly

[0121] During assembly, the polarization-maintaining master rod (50mm in diameter) undergoes ultrasonic cleaning, hydrofluoric acid (HF) etching, and drying to ensure that the deviation of the central angle of its orthogonal holes is ≤0.02°. The simultaneously implanted boron-doped quartz tensile stress rod (9mm in diameter) and titanium-aluminum-silicon glass compressive stress rod (6mm in diameter) are both surface activated through fine grinding, HF etching, and drying to strictly eliminate the risk of interface bubbles. The high-precision assembly of the three components, through coordinated control of diameter tolerances (master rod ±0.1mm / stress rod ±0.2mm) and surface energy optimization, ensures the geometric orthogonality of the stress zone and zero-defect bonding at the interface, establishing a high-precision stress field foundation for the subsequent wire drawing process.

[0122] S4: Graphite Induction Drawing

[0123] In the graphite induction drawing stage, a graphite induction furnace is used as the core equipment, and fiber forming is achieved through multi-level precision control: the furnace temperature gradient is stabilized at 2000℃ by PID control of the graphite heating element, providing a uniform thermal field for the molten preform; the drawing speed is set at 500 m / min, and the servo motor system achieves a speed accuracy of ±0.1%; the cooling stage uses helium vortex tube technology for forced rapid cooling, enabling the fiber surface cooling rate to exceed ≥100 K / s, quickly locking the residual stress field; the fiber diameter is monitored in real time by a dual laser interferometer and dynamically fed back to the traction system, strictly controlling the final diameter within a tolerance range of 40±0.1 μm. This process, through the synergistic effect of precise thermal field control, uniform speed traction, and ultrafast cooling, solidifies the thermodynamic configuration of the orthogonal stress region at the molecular scale, laying the foundation for the optical anisotropy and mechanical reliability of polarization-maintaining fibers.

[0124] The expansion stress rods used in S2 above are stress rod 1, stress rod 2, stress rod 3, stress rod 5, stress rod 6, stress rod 7 and stress rod 8 as shown in Table 2;

[0125] The dual-stress polarization-maintaining fibers prepared in this way are numbered as fiber 1, fiber 2, fiber 3, fiber 5, fiber 6, fiber 7, and fiber 8, respectively.

[0126] Application Example 2

[0127] Fabrication of 25-micron dual-stress polarization-maintaining fiber

[0128] The application is largely the same as in Example 1, and its specifications are shown in Table 7.

[0129] Table 7 Specifications

[0130] mark Component Name Material composition (wt%) Functions and parameters Diameter (μm) 101 Core layer <![CDATA[SiO2- 6wt% GeO2]]> Light guide core (NA=0.16) 4 102 Inner cladding <![CDATA[SiO2- 10wt% F]]> <![CDATA[Transfer stress transition layer (CTE = +3.2×10 -6 / K)]]> 5.5 103 Outer layer Low-hydroxyl quartz Mechanical protection 25 104 Contraction stress zone <![CDATA[SiO2- 20wt% B2O3]]> <![CDATA[Apply tensile stress (CTE = +25×10 -6 / K)]]> 5 105 Expansion Stress Zone Stress rod 4 <![CDATA[Apply compressive stress (CTE = -1.5×10 -6 / K)]]> 3.5 106 Coating layer acrylic resin outer protective layer 60

[0131] In this embodiment, a 50 mm synthetic quartz rod is used as the matrix material. Orthogonally distributed double circular holes (central angle deviation ≤ 0.02°) are precisely machined using a deep hole drilling machine, and shrinkage stress rods and expansion stress rods are implanted in them respectively. The shrinkage stress rod is made of SiO2-20wt% B2O3 glass (diameter 10.0 mm), and the expansion stress rod is made of SiO2-12wt% TiO2-8wt% Al2O3 ternary composite glass (atomic ratio Si:Ti:Al = 27.5:3.1:3.2, diameter 7.0 mm), with a diameter ratio d1 / d2 = 1.5. The expansion rod surface is coated with a 0.5 mm fluorine-doped SiO2 protective layer, forming a 90° orthogonal double stress zone structure. High-speed drawing is performed at 1900℃ and 10⁻³ Pa vacuum. Helium quenching technology (cooling rate 100 K / s) is used to lock the residual stress field, and the fiber diameter accuracy (25±0.1 μm) is controlled in real time using a dual-laser interferometer.

[0132] Optical fiber 4 is obtained through the above process.

[0133] Meanwhile, as a comparison, this invention also provides a conventional panda fiber (40 μm), whose core layer is Ge-doped quartz, the stress region consists of two symmetrical B2O3-SiO2 circular regions, and the cladding is pure quartz. This sample is named conventional panda fiber (40 μm).

[0134] Performance testing:

[0135] Test conditions:

[0136] Fiber optic sample length: 100 m

[0137] Environmental testing standard: GB / T 9771.3-2020 "Optical fibers for communication - Part 3: Characteristics of wavelength-band extended non-dispersion-shifted single-mode optical fibers";

[0138] Test items: birefringence Δn, temperature stability (zero deviation drift, -65~85℃, ° / h), minimum bending radius, tensile strength;

[0139] The test results are shown in Tables 8 and 9;

[0140] Table 8 Test Results

[0141] Performance indicators Traditional Panda fiber (40μm) Fiber 2 Fiber 4 Test methods Birefringence Δn <![CDATA[1.8×10 -4 ]]> <![CDATA[4.05×10 -4 ]]> <![CDATA[6.22×10 -4 ]]> Sagnac Interference Method Temperature stability (zero offset drift, -65~85℃, ° / h) >0.01 0.005 0.002 Zero Deviation Drift Measuring Instrument Minimum bending radius 3 mm 2 mm 1 mm GB / T 15972.47-2021 tensile strength 100 kpsi 240 kpsi 310 kpsi GB / T 15972.31-2021

[0142] Table 9 Test Results

[0143] Performance indicators Fiber 1 Fiber 3 Fiber 5 Fiber 6 Fiber 7 Fiber 8 <![CDATA[Birefringence Δn (× 10 -4 )]]> 3.82 4.8 4.1 4 2.1 4.95 Temperature stability (zero offset drift, -65~85℃, ° / h) 0.007 0 0.005 0.005 0.015 0.006 Minimum bending radius (mm) 2.2 1.8 2 2.1 2.8 2.3 Tensile strength (kpsi) 220 260 235 230 180 210

[0144] Results analysis:

[0145] As can be seen from the results in Table 8, the SiO2-TiO2-Al2O3 glass material of the present invention, when applied to polarization-maintaining optical fibers, has the following advantages:

[0146] (1) Birefringence performance: The measured birefringence Δn of the 40μm dual-stress region polarization-maintaining fiber of this invention reaches 4.05×10 -4 Compared to traditional Panda fiber (1.8×10), -4 The gain is 125%. This breakthrough gain stems from the synergistic effect of the orthogonal dual stress zones: the contraction stress bar (20wt% B2O3) achieves a high positive thermal expansion coefficient (CTE = +25 × 10⁻⁶). -6 / K) applies directional tensile stress to the fiber core, while the expansion stress bar (12wt% TiO2-8wt% Al2O3) utilizes negative thermal expansion characteristics (CTE=-1.5×10) -6 / K) generates reverse compressive stress, and the two together form a strong asymmetric stress field in the core region. Although the stress zone spacing of the 40 μm structure is slightly larger than that of the 25 μm version (resulting in Δn = 6.22 × 10), -4 However, it still achieves birefringence multiplication of traditional structures through material composition optimization.

[0147] (2) Temperature stability performance: Under harsh temperature cycling conditions of -65~85℃, the zero-bias drift of the new 40μm optical fiber is only 0.004° / h, which is 60% lower than that of the traditional structure (>0.01° / h). This advantage is supported by the following three technologies: the negative expansion stress zone (SiO2-TiO2-Al2O3) utilizes the "structural contraction effect" of the titanium oxide octahedron (high-frequency transverse vibration-induced structural contraction) to effectively offset the thermal expansion of the matrix; the fluorine-doped inner cladding (SiO2-10wt% F) acts as a thermal stress buffer layer to block the transmission of temperature deformation to the fiber core; and the inner cladding and fiber core are designed with a matching coefficient of thermal expansion (CTE) of +3.2×10 -6 / K), precisely suppressing thermally induced birefringence drift.

[0148] (3) Enhanced mechanical reliability: The negative expansion characteristics (CTE=-1.5×10) of titanium aluminum silicon glass (SiO2-TiO2-Al2O3) -6 / K) During the fiber cooling process, radial compressive stress (approximately -85 MPa) is applied to the quartz matrix. This residual stress field, together with the low-hydroxyl quartz cladding, enhances reliability: when bent, the compressive layer preferentially offsets the surface tensile stress, reducing the minimum bending radius to 2 mm (33% improvement over the traditional 3 mm fiber); at the same time, combined with the fiber drawing rapid cooling process (≥100 K / s) and deep hole acid etching technology (Ra<0.05 μm), a gradient compressive stress structure that resists crack propagation is formed on the surface, driving the tensile strength to jump to 540 kpsi (an increase of 116%), achieving a generational breakthrough in mechanical properties.

[0149] (4) Enhancement Mechanism Essence: Titanium aluminum silicon glass significantly improves the overall performance of optical fiber through thermo-mechanical coupling effect: its "structural contraction effect" effectively counteracts the positive thermal expansion of the quartz matrix during temperature cycling, suppressing the thermally induced deformation of the fiber core to ≤0.02μm / 100℃, thereby greatly reducing polarization crosstalk (ΔCT optimization range reaches 52%); the synchronously established residual compressive stress field (-85 MPa) forms a mechanical synergy with the outer cladding, preferentially resisting surface tensile stress during bending, achieving a minimum bending radius of 2mm, and simultaneously sealing the propagation path of microcracks. Combined with the rapid cooling process to lock in the high-density structure, a dual breakthrough in temperature stability and mechanical reliability is achieved.

[0150] As can be seen from the results in Table 9, the SiO2-TiO2-Al2O3 glass material of the present invention, when applied to polarization-maintaining optical fibers, has the following advantages:

[0151] By systematically analyzing the performance data of fibers 1 to 8, the regular influence of material composition and structural parameters on fiber characteristics can be clearly observed. Regarding material composition optimization, fibers 1 to 3 demonstrate the significant effect of fine-tuning the composition in the SiO2-TiO2-Al2O3 system: as the TiO2 content was optimized from 15wt% to 12wt% and then to 11wt%, the birefringence coefficient increased from 3.82 × 10⁻⁶. -4 4.05×10 -4 Up to 4.80×10 -4 The stable improvement in birefringence, coupled with a corresponding improvement in temperature stability (0.007→0.005→0.004° / h), confirms that there exists an optimal ratio of titanium and aluminum elements within the 11-15wt% content range, enabling an ideal thermal compensation mechanism through the "structural shrinkage effect." Structural parameter studies show that fiber 4, by increasing the diameter of the expansion rod to 7mm while maintaining the same fluorine-doped layer parameters, successfully increased the birefringence to 6.22×10⁻⁶. -4 The peak value and tensile strength reached 310 kpsi, and the minimum bending radius was optimized to 1.0 mm, proving that moderately increasing the size of the stress zone helps to form a more uniform stress distribution, thereby achieving a simultaneous breakthrough in optical and mechanical properties.

[0152] Experiments on the fluorine-doped layer (fibers 5-6) revealed the precise window requirement for fluorine content: when the fluorine content was adjusted from the baseline of 8 wt% to 7 wt%, the birefringence remained at 4.10 × 10⁻⁶. -4 The birefringence remained at a relatively high level, with temperature stability maintained at 0.005° / h; however, when the fluorine content increased to 9wt%, the birefringence decreased slightly to 4.00×10⁻⁶. -4 This indicates that excessive fluorine doping may weaken the interfacial bonding strength, and this phenomenon determines that 7-9 wt% is the optimal doping range for fluorine. Of particular note is the experimental confirmation of the crucial role of titanium in the material system through the study of comparative groups of fibers 7-8: the titanium-deficient sample (fiber 7) exhibits severe performance degradation, with birefringence plummeting to 2.10 × 10⁻⁶. -4 The temperature stability deteriorated to 0.015° / h; while the titanium-excess sample (fiber 8) maintained a high birefringence (4.95 × 10⁻⁶). -4 However, its temperature stability (0.006° / h) and mechanical properties (210kpsi) were significantly lower than those of the optimized group. This result verifies from both positive and negative perspectives the necessity of titanium content in the range of 2.8-3.9 mol% for maintaining negative thermal expansion characteristics.

[0153] This study established the application criteria of the SiO2-TiO2-Al2O3 material system in polarization-maintaining optical fibers through eight sets of systematic experiments: In terms of material composition, an optimal ratio window exists between 11-15 wt% TiO2 and 7.5-8 wt% Al2O3; in terms of structural design, a 6-7 mm stress bar diameter combined with a 0.5 mm fluorine-doped layer (7-9 wt% F) achieves the best performance balance; and in terms of elemental control, a titanium content below 2.8 mol% cannot form an effective negative expansion effect, while a content above 3.9 mol% introduces the risk of crystallization. These principles collectively constitute the theoretical basis for the design of high-performance polarization-maintaining optical fibers, providing a scientific basis for the development of next-generation fiber optic sensors.

[0154] This study ultimately constructed a multi-layered material synergy mechanism: In a thermodynamic dimension, a stable asymmetric stress field is formed in the fiber core region through the negative expansion of SiO2-TiO2-Al2O3 and the positive expansion of B2O3-SiO2; in an optical dimension, effective optical field confinement is achieved through the refractive index gradient constructed by the fluorine-doped layer (n=1.43-1.45) and the stress region (n=1.48); and in a mechanical dimension, dynamic mechanical equilibrium is formed by the compressive field of the negative expansion stress region and the tension of the cladding. This multi-layered synergistic design lays a solid material foundation for next-generation high-precision fiber optic sensors and space communication systems, marking a significant paradigm shift in special fiber optic materials from empirical optimization to rational design.

Claims

1. A polarization-maintaining optical fiber, characterized in that, It includes an optical fiber body, with a fiber core at the center of the optical fiber body; and a pair of contraction stress zones and a pair of expansion stress zones arranged in a cross shape around the outer periphery of the fiber core. The coefficient of thermal expansion of the contraction stress zone is 20 × 10⁻⁶. -6 / K~30×10 -6 / K; The coefficient of thermal expansion of the expansion stress zone is -1×10⁻⁶. -6 / K~-2×10 -6 / K; The expansion stress zone contains the following molar percentage elements: Si: 26.5~29.5%; M：2.8~3.9%； N: 3.0~3.8%; balance: oxygen. Among them, M contains at least 50 mol% of Ti, and N contains at least 70 mol% of Al.

2. The polarization-maintaining optical fiber according to claim 1, characterized in that, The optical fiber body consists of a core, an inner cladding, and an outer cladding, arranged from the inside out; the coefficient of thermal expansion of the inner cladding is 2.5 × 10⁻⁶. -6 / K~4×10 -6 / K; the core material is Ge-doped silicon dioxide; the shrinkage stress zone material is composed of 75~90 wt% SiO2 and 10~20 wt% B2O3; the inner cladding material is F-doped silicon dioxide; the outer cladding material is low-hydroxyl quartz.

3. The polarization-maintaining optical fiber according to claim 2, characterized in that, The core is made of silicon dioxide with a GeO2 content of 5-7 wt%; the inner cladding is made of silicon dioxide with an F content of 9-11 wt%.

4. The polarization-maintaining optical fiber according to claim 2, characterized in that, The diameter of the fiber core is 3~5μm; the diameter of the inner cladding is 5~7μm; the diameter of the outer cladding is 20~50μm; the diameter or diagonal length of the shrinkage stress zone is 4~8μm; and the diameter or diagonal length of the expansion stress zone is 3~6μm.

5. The polarization-maintaining optical fiber according to claim 1, characterized in that, M contains the following elements: Ti: 50~100 mol% Zr, Hf, and Sn are each independently selected from 0–50 mol%; The N contains the following elements: Al: 70~100 mol% Y, La, and Ga are each independently selected from 0 to 30 mol.

6. The polarization-maintaining optical fiber according to claim 1, characterized in that, The expansion stress zone is surrounded by a protective layer; the protective layer is F-doped silicon dioxide; the protective layer is also used to construct a refractive index depression zone between the expansion stress zone and the fiber core.

7. The polarization-maintaining optical fiber according to claim 6, characterized in that, The refractive index of the expansion stress region is 1.48±0.01; the refractive index of the fiber core is 1.46±0.01; and the refractive index of the refractive index depression region is 1.43~1.

45. The F doping amount in the protective layer is 7~9 wt%.

8. The polarization-maintaining optical fiber according to any one of claims 1 to 7, characterized in that, The outer surface of the optical fiber body also has a coating layer made of resin.

9. A method for fabricating a polarization-maintaining optical fiber as described in any one of claims 1 to 8, characterized in that, Includes the following steps: Step 1: Drill shrinkage stress rod holes and expansion stress rod holes in the outer cladding of the optical fiber body; Step 2: Assemble the contraction stress rod and expansion stress rod into the contraction stress rod hole and expansion stress rod hole, respectively; Step 3: Graphite induction drawing is performed in a graphite induction furnace at 1900℃~2100℃. The drawn fiber is then rapidly cooled at a rate greater than or equal to 100 K / s to lock the residual stress field of the optical fiber and obtain polarization-maintaining fiber.

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