A single crystal polarization maintaining optical fiber based on intrinsic birefringence and a preparation method thereof

Abstract

The application discloses a single-crystal polarization-maintaining optical fiber based on intrinsic birefringence and a preparation method thereof, relates to the technical field of online car-hailing safety monitoring, and discloses the following technical scheme: the single-crystal fiber core of the optical fiber main body is integrally grown from a sapphire, lithium niobate and yttrium vanadate uniaxial crystal, the traditional stress-induced birefringence structure is abandoned, the intrinsic birefringence effect of the crystal is utilized to realize polarization maintenance, the optical fiber axial direction and the optical axis of the crystal form a non-zero included angle of 5°-90°, the fiber core is coated with a low-refractive-index cladding layer, and the optical fiber can be stably operated in an extreme high-temperature environment of 500 DEG C or above for a long time. The single-crystal polarization-maintaining optical fiber prepared by the application has the advantages of strong birefringence stability, excellent high-temperature resistance and high polarization maintenance precision, and is suitable for high-end scenes such as high-temperature sensing, optical transmission in extreme environments, and optoelectronic detection in aerospace.

Description

Technical Field

[0001] This invention relates to the field of lithium battery current collector technology, and in particular to a single-crystal polarization-maintaining optical fiber based on intrinsic birefringence and its fabrication method. Background Technology

[0002] Polarization-maintaining fiber is a special type of optical fiber capable of stably maintaining the linear polarization of light transmission. It is widely used in precision optoelectronic fields such as fiber optic gyroscopes, fiber optic sensing, coherent optical communication, and laser detection. Currently, most commercially available and existing polarization-maintaining fibers are structural polarization-maintaining fibers, which mainly rely on stress birefringence generated by internal stress units and elliptical core structures to achieve polarization maintenance.

[0003] Existing technologies have significant drawbacks: First, the birefringence of traditional stress-type polarization-maintaining fibers is induced by mechanical stress. Under high-temperature environments, the internal stress structure of the fiber is prone to relaxation and release, leading to a significant decrease in the birefringence coefficient, a sharp increase in polarization crosstalk, and rapid failure of polarization-maintaining performance. The maximum stable operating temperature of conventional polarization-maintaining fibers is generally below 500℃, making them unsuitable for extreme high-temperature scenarios such as high-temperature industrial testing, aerospace high-temperature conditions, and geothermal exploration. Second, structural polarization-maintaining fibers rely on complex preform drawing processes, making it difficult to precisely control structural uniformity. The fiber's axial birefringence consistency is poor, and long-term use can easily lead to polarization stability drift. Third, traditional fabrication processes are prone to generating internal defects and residual stress, resulting in poor fiber mechanical properties and thermal stability, and a short service life under extreme environments.

[0004] Single-crystal materials possess excellent high-temperature resistance, optical homogeneity, and structural stability. Uniaxial crystals themselves have natural intrinsic birefringence properties, and polarization can be maintained without relying on artificial stress structures, making them ideal materials for fabricating high-temperature polarization-maintaining optical fibers.

[0005] Therefore, we propose a single-crystal polarization-maintaining fiber based on intrinsic birefringence and its fabrication method. Summary of the Invention

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A single-crystal polarization-maintaining optical fiber based on intrinsic birefringence, comprising: The optical fiber includes an optical fiber body with a cladding layer on its outer side. The optical fiber body is a single-crystal structure integrally grown from a single-axis crystal material. The axial direction of the optical fiber body forms a non-zero angle of 5° to 90° with the optical axis of the single-axis crystal material. The single-axis crystal material is selected from any one of sapphire, lithium niobate, and yttrium vanadate. The optical fiber body is a single-crystal core. The refractive index of the cladding layer is lower than that of the single-crystal core. The material of the cladding layer is a crystal homogeneous with the core, a heterogeneous crystal, or optical glass. It is suitable for extreme high-temperature environments above 500°C.

[0007] A method for fabricating the above-mentioned single-crystal polarization-maintaining optical fiber based on intrinsic birefringence. The preparation of the aforementioned single-crystal polarization-maintaining fiber includes the following steps: Seed crystal and source rod preparation: An optical-grade non-cubic uniaxial crystal is selected to prepare an oriented seed crystal. The axial direction of the seed crystal forms a non-zero angle of 5° to 90° with the optical axis of the crystal. The crystal orientation of the seed crystal is calibrated using an X-ray diffractometer, and the deviation of the seed crystal axial direction from the preset crystal orientation is controlled within ±0.5°. High-purity polycrystalline material of the corresponding material is selected and cut into raw material rods of a preset size. The raw material rods and seed crystals are respectively fixed to ceramic rods with high-temperature ceramic adhesive and dried and cured for later use. Precise seed crystal orientation calibration can control the consistency of the birefringence parameters of the fiber from the source, and the control of minute angular deviations ensures the uniformity of performance of batch products.

[0008] Laser melting clamping: The raw material rod and seed crystal are clamped separately in the lower feed device and upper lifting device of the laser heating base growth equipment, and their centers are adjusted to be coaxial. The furnace cavity and laser reflector are cleaned, a protective atmosphere is introduced, and the laser power is gradually increased to melt the tip of the raw material rod. The center of the raw material rod is adjusted to be at the laser reflection focal point, and the power is further adjusted to form a stable hemispherical molten zone. The melt state is stabilized at a constant temperature. Coaxial adjustment can avoid crystal growth eccentricity and dimensional inhomogeneity. A stable molten zone provides a basis for uniform single crystal growth, and the protective atmosphere can prevent oxidation and impurity doping of the high-temperature melt, ensuring the optical purity of the fiber.

[0009] Directional crystal growth: The seed crystal is slowly lowered to ensure its bottom end is fully wetted and in contact with the molten zone. The seed crystal position is adjusted to the center of the molten zone, and the laser power is adjusted until the seed crystal diameter is stable. The lifting device is then activated, and the necking and shoulder-forming equal-diameter growth processes are completed sequentially. During the growth process, the crystal growth direction is guided by the directional seed crystal to ensure that the angle deviation between the fiber axis and the crystal optical axis is controlled within ±0.5°. Through seed crystal directional guidance, the preset crystal orientation parameters are accurately replicated, ensuring that the intrinsic birefringence characteristics of each fiber segment are uniform and stable.

[0010] In the directional crystal growth step, the pulling speed of the constant diameter growth process is 0.3–1.0 mm / min, and the feed speed of the raw material rod is 0.05–0.2 mm / min. Precisely defining the growth rate avoids crystal defects and lattice distortion caused by excessively fast growth, or coarse grains and low production efficiency caused by excessively slow growth, thus ensuring the integrity and uniformity of the single crystal structure.

[0011] Final extraction: After the fiber has grown to the preset length, the laser power is adjusted to slowly extract the crystal from the molten zone. The laser power is then gradually reduced at a fixed rate until it is turned off to avoid thermal shock that could cause the crystal to crack. The fiber is then removed after the furnace cavity has cooled to room temperature. This step-by-step power reduction effectively alleviates local temperature differences in the crystal, eliminates thermal stress, prevents defects such as cracking and warping in single-crystal fibers, and improves the product yield.

[0012] High-temperature annealing: After the final extraction step, the single-crystal fiber undergoes high-temperature annealing. The grown single-crystal fiber is placed in a high-temperature sintering furnace and annealed in air or an inert atmosphere to eliminate internal stresses from crystal growth. The specific annealing procedure is as follows: the temperature is increased to 800°C at a heating rate of 5°C / min and held for 2 hours; then the temperature is increased to the target annealing temperature at a heating rate of 3°C / min and held at that temperature; then the temperature is slowly reduced to 800°C at a cooling rate of 2°C / min, and finally the fiber is allowed to cool naturally to room temperature in the furnace. This gradient heating and cooling annealing process can completely eliminate residual internal stresses generated during crystal growth, optimize lattice integrity, and further improve the thermal stability and optical uniformity of the fiber.

[0013] Polarization-maintaining axis calibration: After the growth process, an extinction ratio test optical path is constructed, consisting of a linearly polarized laser, fiber coupler, precision rotating fixture, rotatable analyzer, and optical power meter. The fiber under test is clamped onto the precision rotating fixture, the polarization direction of the input linearly polarized light is fixed, and the fiber is rotated unidirectionally to change the relative angle between its polarization-maintaining axis and the input polarization direction. Simultaneously, dynamic mechanical perturbation is applied to the middle section of the fiber to suppress multimode interference noise. When rotated to a certain angle, if the output light power remains stable and reaches its maximum value under the perturbation, it is determined that the input polarization direction is aligned with one of the polarization-maintaining principal axes of the fiber. Further rotation of approximately 90° obtains another orthogonal polarization-maintaining principal axis. The crystal c-axis orientation is determined using an X-ray diffractometer, and the fast and slow axes are distinguished based on the difference in refractive index between the ordinary and extraordinary rays. The extinction ratio is tested and calculated to complete the fiber performance calibration. This testing method can accurately identify the fiber polarization-maintaining principal axis, suppress test noise, accurately obtain fiber polarization performance parameters, and ensure product accuracy.

[0014] The beneficial effects of this invention are: 1. This optical fiber abandons the traditional stress birefringence principle and utilizes the natural intrinsic birefringence effect of uniaxial crystals to achieve polarization preservation. There is no need to set up stress application structures, the crystal structure has extremely strong stability, and the high temperature resistance is greatly improved. By limiting the non-zero angle between the fiber axis and the crystal optical axis, the fiber has stable and efficient birefringence characteristics, ensuring polarization preservation capability. The low refractive index cladding layer can realize total internal reflection transmission of light, ensuring the optical fiber's light guiding performance, while also being suitable for high temperature working environments.

[0015] 2. Excellent high-temperature resistance, suitable for extreme working conditions. This invention uses sapphire, lithium niobate, and yttrium vanadate uniaxial single-crystal materials to fabricate optical fibers. Polarization maintenance is achieved through the intrinsic birefringence of the crystal, eliminating artificial stress structures and completely solving the problems of high-temperature stress relaxation and birefringence failure in traditional polarization-maintaining fibers. It can operate stably in extreme high-temperature environments above 500℃, filling the technological gap in high-temperature polarization-maintaining fibers. It exhibits high polarization maintenance stability and excellent optical performance. By precisely controlling the angle deviation between the fiber axis and the crystal optical axis (within ±0.5°), combined with a single-crystal integrated growth structure, the fiber exhibits good axial birefringence uniformity, low polarization crosstalk, stable extinction ratio, and no structural stress drift issues. Long-term optical performance consistency is excellent, the fabrication process is precise and controllable, and the product yield is high. This invention employs laser-heated substrate directional growth technology, combined with precise pulling and feed rate control, stepped power termination, and gradient annealing processes, effectively eliminating crystal defects, residual stress, and thermal shock damage. The single-crystal fiber has a complete structure, high dimensional accuracy, excellent mechanical and thermal stability, accurate testing and calibration, and controllable product precision. The dedicated extinction ratio test optical path, combined with dynamic perturbation noise reduction technology, can accurately calibrate the fiber polarization-maintaining main axis, distinguish between fast and slow axes, accurately test polarization performance, achieve precise control of product performance, and adapt to high-precision optoelectronic application scenarios. Attached Figure Description

[0016] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a flowchart of a method for fabricating a single-crystal polarization-maintaining optical fiber based on intrinsic birefringence, as proposed in this invention. Detailed Implementation

[0017] The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic diagrams, illustrating only the basic structure of the invention, and therefore only show the components relevant to the invention.

[0018] refer to Figure 1 The present invention will be further described in detail below with reference to specific embodiments. The following embodiments are used to illustrate the present invention, but are not limited to the scope of protection of the present invention. Example 1 This example provides a sapphire-based single-crystal polarization-maintaining optical fiber and its fabrication method. The specific steps are as follows: 1. Seed crystal and source rod preparation: Optical grade sapphire uniaxial crystal was selected to prepare an oriented seed crystal, and the angle between the seed crystal axis and the optical axis of the sapphire crystal was controlled at 45°. The crystal orientation was calibrated using an X-ray diffractometer, and the angle deviation was controlled within ±0.5°. High-purity alumina polycrystalline material was selected and cut into a raw material rod with a diameter of 3 mm and a length of 50 mm. The raw material rod and the seed crystal were fixed to the ceramic rod with high-temperature ceramic adhesive and dried at 80°C for 2 hours for later use.

[0019] 2. Laser melting clamping: Clamp the raw material rod on the lower feed device of the laser heating base growth equipment, and clamp the seed crystal on the upper lifting device, adjusting the coaxiality deviation of the two to ≤0.02mm; clean the furnace cavity and laser reflector, and introduce high-purity argon gas as a protective atmosphere; start the laser, gradually increase the laser power to melt the top of the raw material rod, adjust the center of the raw material rod to the laser focus, stabilize the power to form a hemispherical melting zone, and keep the temperature stable for 3 minutes.

[0020] 3. Directional Crystal Growth: Slowly lower the seed crystal to fully wet the bottom of the seed crystal with the molten zone, calibrate the seed crystal to the center of the molten zone, and fine-tune the laser power to stabilize the seed crystal diameter; turn on the lifting device and complete the necking and shoulder forming processes in sequence to enter the constant diameter growth stage; control the lifting speed to 0.5 mm / min and the raw material rod feed speed to 0.1 mm / min, and monitor the crystal orientation in real time during the growth process to ensure that the deviation between the fiber axis and the crystal optical axis is ≤ ±0.5°.

[0021] 4. Final extraction: After the fiber grows to a preset length of 100mm, slowly increase the laser power and extract the crystal from the molten zone at a uniform speed; adopt a stepped power reduction mode, reduce the power by 5W every 30s until the laser is completely turned off to avoid thermal shock cracking; after the furnace cavity cools naturally to room temperature, take out the single crystal fiber.

[0022] 5. High-temperature annealing treatment: The prepared sapphire single crystal fiber is placed in a high-temperature sintering furnace under an inert argon atmosphere; the temperature is increased to 800℃ at a rate of 5℃ / min and held for 2 hours; then the temperature is increased to the target annealing temperature of 1200℃ at a rate of 3℃ / min and held at a constant temperature for 3 hours; the temperature is decreased to 800℃ at a rate of 2℃ / min, and finally cooled to room temperature with the furnace to eliminate residual stress from growth.

[0023] 6. Polarization-maintaining axis calibration: Construct an extinction ratio test optical path, including a linearly polarized laser, fiber coupler, precision rotating fixture, rotatable analyzer, and optical power meter; clamp and fix the fiber, stabilize the polarization direction of the input linearly polarized light, rotate the fiber unidirectionally, and simultaneously apply a small dynamic mechanical disturbance in the middle section of the fiber to suppress multimode interference noise; lock the position of the stable maximum output power, calibrate the first polarization-maintaining principal axis, and rotate 90° to calibrate the second orthogonal principal axis; determine the crystal c-axis using an X-ray diffractometer, distinguish the fast and slow axes based on the difference in refractive index between the ordinary and extraordinary rays of sapphire, test and calculate the fiber extinction ratio, and complete the product calibration.

[0024] The sapphire single-crystal polarization-maintaining fiber prepared in this embodiment, combined with a low-refractive-index optical glass cladding layer, can work stably for a long time in a high-temperature environment of 600℃, with no significant attenuation of birefringence characteristics, and its extinction ratio stability is better than that of traditional stress-type polarization-maintaining fibers.

[0025] Example 2

[0026] This embodiment provides a lithium niobate-based single-crystal polarization-maintaining optical fiber and its preparation method, which is basically the same as the steps in Embodiment 1, except that: an optical-grade lithium niobate uniaxial crystal is selected to prepare the seed crystal, and the angle between the seed crystal axis and the crystal optical axis is 30°; the constant diameter growth pulling speed is 0.8 mm / min, and the raw material rod feed speed is 0.15 mm / min; the annealing target temperature is 1000℃, and the constant temperature holding time is 2.5 h; the cladding layer uses a heterogeneous low-refractive-index crystal material. The optical fiber of this embodiment can work stably in high-temperature environments above 550℃, has high polarization maintenance accuracy, and is suitable for high-temperature optical fiber sensing scenarios.

[0027] Example 3

[0028] This embodiment provides a yttrium vanadate-based single-crystal polarization-maintaining optical fiber and its preparation method, which is basically the same as the steps in Embodiment 1, except that: the angle between the seed crystal axis and the crystal optical axis is 60°; the constant diameter growth pulling speed is 0.3 mm / min, and the raw material rod feed speed is 0.05 mm / min; the annealing target temperature is 900℃, and the constant temperature holding time is 2 hours; the cladding layer uses a low-refractive-index yttrium vanadate crystal of the same material as the fiber core. The optical fiber in this embodiment has excellent crystal uniformity and high consistency of birefringence parameters, making it suitable for high-precision, high-temperature coherent optical transmission scenarios.

[0029] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A single-crystal polarization-maintaining optical fiber based on intrinsic birefringence, comprising an optical fiber body, characterized in that, The optical fiber body is provided with a cladding layer on the outside. The optical fiber body is a single crystal structure integrally grown from a single-axis crystal material. The axial direction of the optical fiber body forms a non-zero angle of 5° to 90° with the optical axis of the single-axis crystal material. The single-axis crystal material is selected from any one of sapphire, lithium niobate, and yttrium vanadate. The optical fiber body is a single crystal core. The emissivity of the cladding layer is lower than that of the single crystal core. The material of the cladding layer is a crystal homogeneous with the core, a heterogeneous crystal, or optical glass, and is suitable for extreme high-temperature environments above 500°C.

2. A method for fabricating a single-crystal polarization-maintaining fiber based on intrinsic birefringence, based on the single-crystal polarization-maintaining fiber based on intrinsic birefringence as described in claim 1, characterized in that, include: Seed crystal and source rod preparation: An optical-grade non-cubic uniaxial crystal is selected to prepare an oriented seed crystal. The axial direction of the seed crystal is at a non-zero angle of 5° to 90° with the optical axis of the crystal. The crystal orientation of the seed crystal is calibrated using an X-ray diffractometer, and the deviation of the seed crystal axial direction from the preset crystal orientation is controlled within ±0.5°. A high-purity polycrystalline material of the corresponding material is selected and cut into a raw material rod of a preset size. The raw material rod and the seed crystal are respectively fixed to a ceramic rod with high-temperature ceramic adhesive and dried and cured for later use. Laser melting clamping: The raw material rod and the seed crystal are clamped in the lower feed device and the upper lifting device of the laser heating base growth equipment, respectively, and the centers of the two are adjusted to be coaxial; the furnace cavity and the laser reflector are cleaned, a protective atmosphere is introduced, the laser is started and the power is gradually increased to melt the top of the raw material rod, the center of the raw material rod is adjusted to be located at the laser reflection focus, the power is further adjusted to form a stable hemispherical melting zone, and the melt state is stabilized at a constant temperature. Directional crystal growth: Slowly lower the seed crystal to ensure its bottom end is fully wetted and in contact with the molten zone. Adjust the seed crystal position to the center of the molten zone, adjust the laser power until the seed crystal diameter is stable, and start the lifting device to complete the necking, shouldering, and equal-diameter growth processes in sequence. During the growth process, the crystal growth direction is guided by the directional seed crystal to ensure that the angle deviation between the fiber axis and the crystal optical axis is controlled within ±0.5°. Final removal: After the fiber has grown to the preset length, adjust the laser power and slowly remove the crystal from the molten zone. Reduce the laser power stepwise at a fixed rate until it is turned off to avoid thermal shock causing the crystal to crack. After the furnace cavity cools to room temperature, remove the fiber.

3. The method for fabricating a single-crystal polarization-maintaining optical fiber based on intrinsic birefringence according to claim 2, wherein in the directional crystal growth step, the pulling speed of the constant diameter growth process is 0.3-1.0 mm / min, and the feed speed of the raw material rod is 0.05-0.2 mm / min.

4. The method for fabricating a single-crystal polarization-maintaining optical fiber based on intrinsic birefringence according to claim 2, wherein after the tailing and stripping step is completed, a high-temperature annealing step is further included: the grown single-crystal optical fiber is placed in a high-temperature sintering furnace and annealed in air or an inert atmosphere to eliminate internal stress during crystal growth; the specific annealing procedure is as follows: the temperature is raised to 800°C at a heating rate of 5°C / min and held for 2 hours; then the temperature is raised to the target annealing temperature at a heating rate of 3°C / min and held at a constant temperature; then the temperature is slowly lowered to 800°C at a cooling rate of 2°C / min, and finally the temperature is naturally cooled to room temperature with the furnace.

5. The method for fabricating a single-crystal polarization-maintaining fiber based on intrinsic birefringence according to claim 2, further comprising a polarization-maintaining axis detection and calibration step after the growth process is completed: constructing an extinction ratio test optical path consisting of a linearly polarized laser, an optical fiber coupler, a precision rotating fixture, a rotatable analyzer, and an optical power meter; clamping the fiber under test onto the precision rotating fixture, fixing the polarization direction of the input linearly polarized light, unidirectionally rotating the fiber to change the relative angle between its polarization-maintaining axis and the input polarization direction, while applying dynamic mechanical perturbation in the middle section of the fiber to suppress multimode interference noise; when rotated to a certain angle, if the output light power remains stable and reaches a maximum value under the perturbation, it is determined that the input polarization direction has been aligned with one polarization-maintaining principal axis of the fiber; continuing to rotate by about 90° to obtain another orthogonal polarization-maintaining principal axis; determining the crystal c-axis orientation using an X-ray diffractometer, distinguishing the fast axis and slow axis based on the difference in refractive index between the ordinary and extraordinary rays of the material, and testing and calculating the extinction.

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