An integrated polarization maintaining optical fiber, a preparation method thereof and an integrated gyroscope comprising the same

Abstract

The application discloses an integrated polarization maintaining optical fiber and a preparation method thereof and an integrated gyro containing the same, and belongs to the technical field of special optical fiber preparation. The integrated polarization maintaining optical fiber is characterized by the following steps: a stress area is symmetrically arranged on both sides of the long axis direction of an elliptical core of the polarization maintaining optical fiber; and the core ellipticity, the ratio of the inner cladding short axis diameter to the core short axis diameter, and the relative refractive index difference between the inner cladding and the stress area are optimally designed, so that the integrated polarization maintaining optical fiber can effectively meet the characteristic requirements of the integrated gyro optical fiber. The integrated polarization maintaining optical fiber has the advantages of simple structure, convenient preparation, four characteristics of the integrated optical fiber gyro preparation required optical fiber, sufficient application requirements of the integrated polarization maintaining optical fiber in the integrated gyro, improved preparation efficiency and preparation precision of the integrated optical fiber gyro, reduced preparation and application cost of the integrated optical fiber gyro, and good economic value and practical value.

Description

Technical Field

[0001] This invention belongs to the field of special optical fiber preparation technology, specifically relating to an integrated polarization-maintaining optical fiber and its preparation method, as well as an integrated gyroscope containing the same. Background Technology

[0002] Fiber optic gyroscopes are widely used in inertial navigation systems. With industry development, fiber optic gyroscopes are gradually evolving towards higher precision, lower cost, and miniaturization. A conventional fiber optic gyroscope comprises an ASE light source, coupler, detector, Y-waveguide, and fiber optic loop. However, with the development of integrated circuits and optical device chips, these four independent components—ASE light source, coupler, detector, and Y-waveguide—are being integrated, forming a new integrated design scheme for fiber optic gyroscopes that combines optical device chip integration, ASIC-based circuitry, and miniaturized fiber optic loops.

[0003] Based on the structural characteristics of integrated gyroscopes, the polarization-maintaining fiber used typically needs to meet the following characteristics: Characteristic 1: The fiber has high birefringence performance, with a beat length of less than or equal to 2.5 mm in the 1310 nm and 1550 nm operating bands, and excellent loop winding performance; Characteristic 2: The fiber end face can be polished, and the probability of cracking on the fiber end face after polishing is no greater than 2%; Characteristic 3: The polished fiber end face is coupled to the optical device chip, requiring mode field matching, and the coupling loss is no greater than 3 dB; Characteristic 4: The fiber has a small bending diameter to meet the miniaturization requirements of the gyroscope, and under the condition of a 5 mm bending diameter and 10 turns, the macro-bending loss in the 1310 nm and 1550 nm bands is no greater than 0.1 dB.

[0004] Currently, polarization-maintaining fibers used in domestic gyroscopes are generally divided into two main categories: loop-wound and waveguide-type. Polishable waveguide-type fibers have poor birefringence and cannot be used for winding fiber loops. Loop-wound fibers, which can be used for winding fiber loops, have strong birefringence but lack polishing properties and suffer from high macro-bending loss at a bending diameter of 5mm. For this reason, conventional polarization-maintaining fibers are difficult to effectively meet the application requirements of integrated gyroscopes. Although existing technologies have made targeted improvements to the structure of polarization-maintaining fibers, these improvements still have some problems and are difficult to perfectly adapt to the aforementioned four characteristics, thus affecting the development and application of integrated fiber optic gyroscopes. Summary of the Invention

[0005] To address one or more of the above-mentioned defects or improvement needs of existing technologies, this invention provides an integrated polarization-maintaining fiber, its fabrication method, and an integrated gyroscope containing the same. This effectively meets the four characteristics of optical fibers required for integrated gyroscopes, fully satisfies the application requirements of polarization-maintaining fibers in integrated gyroscopes, significantly simplifies the fabrication process of the integrated polarization-maintaining fiber, and reduces the application cost of polarization-maintaining fibers for integrated gyroscopes.

[0006] To achieve the above objectives, one aspect of the present invention provides an integrated polarization-maintaining optical fiber, comprising a fiber core and an inner cladding and an outer cladding arranged sequentially from the inside to the outside of the fiber core, wherein two stress regions are symmetrically arranged in the outer cladding with respect to the fiber core. The fiber core is elliptical, and the two stress zones are symmetrically located on both sides of the long axis of the fiber core; and The ellipticity of the fiber core is between [20%, 35%]; the ratio of the minor axis diameter D2 of the inner cladding to the minor axis diameter D1 of the fiber core is between [2, 3]; and The relative refractive index difference Δ2 of the inner cladding is between [-0.35%, -0.2%]; the relative refractive index difference Δ3 of the stress zone is between [-0.85%, -0.7%].

[0007] As a further improvement of the present invention, after the bare fiber of the integrated polarization-maintaining optical fiber is drawn, it is annealed at a temperature of 500~700°C before coating.

[0008] As a further improvement of the present invention, after the bare fiber is annealed, an inner coating and an outer coating are sequentially coated on its outer periphery from the inside to the outside. As a further improvement of the present invention, the distance from the edge of the stress zone to the edge of the fiber core is greater than 1.5 μm.

[0009] As a further improvement of the present invention, the ratio of the sum of the cross-sectional areas of the two stress zones to the cross-sectional area of ​​the bare fiber is between 13.5% and 20.5%.

[0010] As a further improvement of the present invention, the refractive index profile at the edge of the stress zone is designed with a gradient.

[0011] Another aspect of the present invention provides a method for fabricating an integrated polarization-maintaining optical fiber for an integrated gyroscope, comprising the following steps: (1) Prepare a core rod; deposit an inner cladding layer doped with F and a core layer co-doped with Ge and F sequentially from the outside to the inside, and control the ratio of the inner cladding diameter to the core diameter to be between [2,3], and control the relative refractive index difference Δ2 of the inner cladding layer to be between [-0.35%, -0.2%]; (2) Determine the grinding size of the core rod according to the core ellipticity range of the integrated polarization-maintaining fiber, and grind the two sides of the outer surface of the core rod symmetrically into a crescent shape; the core ellipticity range is [20%, 35%]; (3) The polished mandrel is embedded into the corresponding quartz sleeve for stretching to obtain an elliptical core polarization-maintaining solid optical rod. (4) Prepare stress bars, control the relative refractive index difference Δ3 of the stress bars to be between [-0.85%, -0.7%], and grind the prepared stress bars to the required size; (5) Determine the positions for drilling holes in the stress zone on both sides of the long axis of the core layer of the elliptical core solid optical rod and drill holes symmetrically. (6) The two stress rods after polishing are combined and embedded into the opening on the solid optical rod, and the combined optical rod is stretched to obtain an elliptical core panda-shaped polarization-maintaining fiber preform. (7) The integrated polarization-maintaining fiber for integrated gyroscopes is prepared by drawing the elliptical core panda-type polarization-maintaining fiber preform.

[0012] As a further improvement of the present invention, in process (1), the preparation process of the mandrel includes a mandrel deposition process and a mandrel melting and shrinking process performed sequentially; and During the core rod deposition process, the deposition rate of the inner cladding and the core layer is controlled within the range of 0.5~1.5 g / min, and the temperature inside the tube is <1200℃, while the pressure inside the tube is controlled within 10~20 mbar. and / or During the mandrel melting process, a process with micro-positive pressure, low speed, and low power is adopted for mandrel melting. The pressure inside the tube is controlled at 5~20 mbar higher than atmospheric pressure, the melting speed is controlled at 10~20 mm / min, and the melting power is controlled at 5~10 kW.

[0013] As a further improvement of the present invention, in process (3), the power of the stretching furnace in the stretching process is controlled at 40~50KW, the stretching speed is controlled at 10~20mm / min, and an elliptical core solid light rod with a diameter of 50±1mm and a rod roundness of ≤1% is obtained.

[0014] As a further improvement of the present invention, in process (4), the ratio of the sum of the cross-sectional areas of the two polished stress bars to the cross-sectional area of ​​the elliptical core solid optical bar is between 13.5% and 20.5%.

[0015] As a further improvement of the present invention, in process (4), the control of the relative refractive index difference Δ3 of the stress bar is achieved by constraining the molar doping content of B2O3 in the stress bar, which ranges from 18% to 22%.

[0016] As a further improvement of the present invention, in process (6), the stretching process of the combined light rod adopts a low temperature, low speed and low pressure stretching process, the power of the stretching furnace is controlled at 5~10kw, the stretching speed is controlled at 2~5mm / min, and the pressure inside the tube is controlled at 0~50mbar.

[0017] As a further improvement of the present invention, in process (7), after the bare fiber is drawn and before coating, an annealing operation with bare fiber is performed, and the annealing temperature is set to 500~700℃.

[0018] Another aspect of the present invention also provides an integrated gyroscope comprising a spool and an integral polarization-maintaining fiber wound on the spool; The integrated polarization-maintaining fiber is prepared using the same method described for preparing integrated polarization-maintaining fibers for integrated gyroscopes.

[0019] As a further improvement of the present invention, the diameter of the spool is between 5 and 30 mm, and the length of the integrated polarization-maintaining fiber on the spool is no more than 300 m. or The diameter of the spool is greater than 30mm, and the length of the integrated polarization-maintaining optical fiber wound on the spool is no greater than 3000m.

[0020] The aforementioned improved technical features can be combined with each other as long as they do not conflict with each other.

[0021] In summary, the beneficial effects of the above-described technical solutions conceived by this invention compared with the prior art include: (1) The integrated polarization-maintaining fiber of the present invention optimizes the structure of the polarization-maintaining fiber, symmetrically sets stress regions on both sides of the major axis of the elliptical core, and optimizes the core ellipticity, the ratio of the inner cladding minor axis diameter to the core minor axis diameter, and the relative refractive index difference between the inner cladding and the stress region. This enables the integrated polarization-maintaining fiber to effectively meet the characteristic requirements of the fiber for integrated gyroscopes, thereby meeting the setting requirements of integrated fiber optic gyroscopes, realizing the miniaturization design of fiber optic gyroscopes, and reducing the cost of using fiber optic gyroscopes.

[0022] (2) The integrated polarization-maintaining fiber of the present invention can further improve the performance of the integrated polarization-maintaining fiber by annealing the bare fiber before coating the outer periphery of the bare fiber and further optimizing the distance from the edge of the stress zone to the edge of the fiber core, thereby reducing the intrinsic attenuation and beat length of the fiber, making it better suited to meet the usage requirements of integrated fiber optic gyroscopes, and further improving the performance of the fiber.

[0023] (3) The preparation method of the integrated polarization-maintaining fiber for integrated gyroscope of the present invention is simple in steps and easy to control. By optimizing the ellipticity of the core layer, the ratio of the minor axis diameter of the core layer to the inner cladding, and the dopants and their doping amount in the inner cladding and stress zone during the preparation of the integrated polarization-maintaining fiber preform, the prepared fiber preform can accurately produce integrated polarization-maintaining fiber, effectively ensuring that the integrated polarization-maintaining fiber meets the four characteristics of the fiber required by the integrated gyroscope, fully meeting the fiber requirements of the integrated fiber optic gyroscope, improving the efficiency of fiber preparation, and effectively reducing the application cost of the integrated fiber optic gyroscope.

[0024] (4) The method for preparing the integrated polarization-maintaining fiber for integrated gyroscopes of the present invention can fully meet the requirements for setting the structural features of each part of the integrated polarization-maintaining fiber by optimizing the forming process of the core rod and the doping elements and doping ratios of the core layer, inner cladding layer and stress layer, and quickly realize the forming of the integrated polarization-maintaining fiber preform; at the same time, by setting an annealing and heat preservation furnace between the tail end of the drawing tower and the coating mechanism, the preparation efficiency and preparation quality of the integrated polarization-maintaining fiber can be fully guaranteed by optimizing the bare fiber annealing temperature.

[0025] (5) The integrated gyroscope of the present invention includes a spool and an integrated polarization-maintaining fiber wound on the spool. By designing the performance parameters of the integrated polarization-maintaining fiber and optimizing the manufacturing process, it is possible to ensure that the integrated polarization-maintaining fiber can fully meet the miniaturization and intelligent setting requirements of the integrated gyroscope, fully guarantee the product performance of the integrated gyroscope, reduce the cost of using the integrated gyroscope, and promote the development of the integrated gyroscope and related industries.

[0026] (6) The integrated polarization-maintaining fiber of the present invention has a simple structure and is easy to prepare. It can effectively meet the four characteristics of the fiber required for the preparation of integrated fiber optic gyroscopes, fully meet the application requirements of integrated polarization-maintaining fiber in integrated gyroscopes, improve the preparation efficiency and preparation accuracy of integrated fiber optic gyroscopes, reduce the preparation and application costs of integrated fiber optic gyroscopes, and has good economic and practical value. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a schematic cross-sectional view of the integrated polarization-maintaining fiber in an embodiment of the present invention; in the figure, the technical features corresponding to the reference numerals are: 1. Core; 2. Inner cladding; 3. Stress zone; 4. Outer cladding; 5. Inner coating; 6. Outer coating.

[0029] Figure 2 This is a schematic cross-sectional view of the relative refractive index difference of each layer of the integrated polarization-maintaining fiber in an embodiment of the present invention. Figure 3 This is a macrobending loss diagram of an integrated polarization-maintaining fiber when the ratio of the inner cladding short axis diameter to the core short axis diameter is different under the condition of 5mm diameter and 10 turns. Figure 4 This is a macrobending loss diagram of an integrated polarization-maintaining fiber under the condition of 5mm diameter and 10 turns, with a relative refractive index difference Δ2 between different inner cladding layers. Figure 5 This is a graph showing the relationship between the probability of end-face grinding cracking of the integrated polarization-maintaining fiber and the change in boron trioxide doping concentration in the stress region in an embodiment of the present invention. Figure 6 This is a graph showing the relationship between the crosstalk value of the integrated polarization-maintaining fiber and the change in boron trioxide doping concentration in the stress region in an embodiment of the present invention. Figure 7 This is a schematic diagram showing the correspondence between different core ellipticities and fiber crosstalk values ​​of the integrated polarization-maintaining fiber in this embodiment of the invention. Figure 8 This is a schematic diagram showing the correspondence between different core ellipticities of the integrated polarization-maintaining fiber and the coupling loss of the optical device chip in an embodiment of the present invention. Figure 9 This is a precision test diagram of an integrated gyroscope assembled after the integrated polarization-maintaining fiber loop with an inner diameter of 98mm and a loop length of 1280m is wound in an embodiment of the present invention. Figure 10 This is a comparison test diagram of the accuracy of a conventional polarization-maintaining fiber with an inner diameter of 98mm and a ring length of 1280m, after which the fiber loop pigtail is fused with polished fiber and then used in an integrated gyroscope. Figure 11 This is the beat length variation curve of the optical rod used in the polarization-maintaining fiber in this embodiment of the invention under different annealing temperatures during fiber drawing. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0031] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0032] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0033] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0034] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0035] Example: Please see Figure 1 In a preferred embodiment of the present invention, the integrated polarization-maintaining optical fiber includes a fiber core 1 located in the middle of the cross section and an inner cladding 2, an outer cladding 4, an inner coating 5, and an outer coating 6 arranged sequentially from the inside to the outside of the fiber core 1, as well as two stress zones 3 arranged in the outer cladding 4 and symmetrically arranged relative to the fiber core 1.

[0036] For the integrated polarization-maintaining fiber in the preferred embodiment, it is mainly used to meet the application requirements of integrated fiber optic gyroscopes. As described above, the polarization-maintaining fiber in an integrated gyroscope needs to meet four characteristics, namely: Feature 1: The optical fiber has high birefringence performance, operates in the 1310nm and 1550nm wavelength bands, has a beat length of no more than 2.5mm, and has excellent loop winding performance. Feature 2: The fiber end face can be polished, and the probability of cracking on the fiber end face after polishing is no more than 2%; Feature 3: The end face of the optical fiber after polishing is coupled to the optical device chip, which requires mode field matching, and the coupling loss is no more than 3dB.

[0037] Feature 4: Small fiber bending diameter, meeting the miniaturization requirements of gyroscopes. Under the condition of a 5mm bending diameter and 10 turns, the macro bending loss in the 1310nm and 1550nm bands is no more than 0.1dB.

[0038] The birefringence coefficient of panda-type polarization-maintaining fiber can be calculated using the following formula:

[0039] In the formula, L B λ is the birefringence coefficient; B is the operating wavelength; obviously, the smaller the beat length, the stronger the birefringence performance of the optical fiber.

[0040] The beam length of the optical fiber is calculated using the following formula:

[0041] In the formula, C The photoelastic coefficient; E Δ is the elastic modulus; T ω is the difference between the glass softening temperature and room temperature; ω is Poisson's ratio; Δα is the difference in thermal expansion coefficients between the stress region and the cladding material, which depends on the doping concentration of the stress region. R The radius of the stress zone; r R is the distance from the edge of the stress zone to the center of the fiber core. The larger R is, the smaller r is, and the smaller the beat length is.

[0042] Regarding the four characteristics mentioned above, researchers, through engineering practice, discovered that for polarization-maintaining fibers, the deeper the doping concentration in the stress region during end-face grinding, the greater the probability of grinding cracks. To satisfy characteristic two, the doping concentration in the stress region needs to be controlled within a small range. Simultaneously, to satisfy characteristic one (the fiber's birefringence performance), this can be achieved by adjusting the size of the stress region and the distance from its inner edge to the fiber core center. Secondly, for characteristic four, adding a fluorine-doped inner cladding depression around the fiber core can reduce the fiber's bending insensitivity, meeting the bending requirements of polarization-maintaining fibers used in fiber optic gyroscopes. As for characteristic three, it can be satisfied by controlling the fiber core mode field diameter to match the optical device chip.

[0043] However, researchers have also found that simply increasing the birefringence performance of optical fibers by adjusting the size of the stress region and the distance from the edge of the stress region to the center of the fiber core is insufficient to meet characteristic one. This is because the boron trioxide doped in the stress region diffuses during the high-temperature drawing process. r If the amount of boron oxide is reduced to a certain level, it will cause boron trioxide to diffuse into the fiber core, resulting in high fiber attenuation.

[0044] Specifically, for conventional panda-type polarization-maintaining fiber, its slow axis is usually along the axis of the stress region, and its fast axis is perpendicular to the axis of the stress region; while for conventional elliptical polarization-maintaining fiber, its slow axis is often along the long axis of the fiber core, and its fast axis is often along the short axis.

[0045] Therefore, in the preferred embodiment, by combining the physical characteristics of panda-type polarization-maintaining fiber and elliptical core polarization-maintaining fiber, the core 1 of the integrated polarization-maintaining fiber is designed to be elliptical, and the long axis of the core 1 is made to coincide with the direction of the line connecting the centers of the two stress regions 3, so as to enhance the birefringence performance of the fiber and meet the setting requirement of "high birefringence performance" in feature one.

[0046] Furthermore, under normal circumstances, the stronger the birefringence of an optical fiber, the better its crosstalk stability. Therefore, while ensuring the fiber can be polished, the depth of the relative refractive index difference Δ3 in stress region 3 should be increased as much as possible. Whether the fiber meets the polishing requirements is mainly determined by the doping concentration of stress region 3. Therefore, whether the integrated polarization-maintaining fiber meets the requirements of feature two should first be determined by the doping concentration of stress region 3.

[0047] In practical calculations, the preferred formula for calculating the relative refractive index difference of a material is:

[0048] In the formula, n 1 represents the absolute refractive index of the material. n 2 represents the absolute refractive index of pure quartz (silicon dioxide, SiO2).

[0049] Studies have found that the smaller the relative refractive index difference Δ3 in the stress zone, the stronger the birefringence performance of the polarization-maintaining fiber. However, when Δ3 exceeds a certain range, the fiber polishing performance deteriorates sharply. Engineering practice has verified that when Δ3 is less than -0.85%, the probability of polishing cracking increases dramatically. Figure 5 As shown in the figure; however, when Δ3 is greater than -0.7%, it will lead to insufficient birefringence performance of the optical fiber, which in turn will result in high crosstalk in the optical fiber, such as... Figure 6 As shown in the image.

[0050] Therefore, in the preferred embodiment, in order to ensure the birefringence and crosstalk performance of the optical fiber, the relative refractive index difference Δ3 of the stress region 3 of the optical fiber is selected within the range of [-0.85%, -0.7%].

[0051] Corresponding to the selection of the relative refractive index difference Δ3 in the fiber stress region 3 in the preferred embodiment, the preferred embodiment controls the molar doping content of B2O3 during the stress rod manufacturing process. In actual operation, corresponding to the selection range of the relative refractive index difference Δ3 in the stress region, the molar doping content of B2O3 ranges from 18% to 22%. Based on this range of molar doping content, the flow rate opening range of B2O3 during stress rod deposition manufacturing can be calculated.

[0052] Furthermore, the content described in feature three above is mainly related to the ellipticity of the fiber core 1. In the preferred embodiment, it is preferably achieved by determining the ellipticity of the fiber core.

[0053] In fact, the smaller the ellipticity, the weaker the geometric birefringence of the optical fiber; while the larger the ellipticity, the worse the mode field matching of the optical fiber and the optical device chip, resulting in greater coupling loss.

[0054] In a preferred embodiment, the fiber's birefringence and coupling matching properties are considered, along with the actual variations in fiber crosstalk and coupling loss under different core ellipticities, such as... Figure 7 , Figure 8 As shown, the ellipticity of the fiber core 1 is set between [20%, 35%]. Within this ellipticity range, both the birefringence performance of the fiber and the coupling loss between the fiber and the optical device are guaranteed to be less than 3.0dB.

[0055] In the actual preform preparation process, by controlling the thickness of the core rod grinding, the ellipticity of the core layer in the core rod can be controlled, thereby ensuring that the ellipticity of the core 1 of the integrated polarization-maintaining fiber obtained by subsequent drawing meets the design requirement of 20%~35%.

[0056] In specific measurements, the ellipticity of the core layer is calculated by (major axis - minor axis) / major axis * 100%. Based on the obtained ellipticity result and the known minor axis length, the major axis length can be calculated, thereby determining the dimensions of the mandrel grinding and stretching.

[0057] Furthermore, corresponding to the content described in feature four above, it is necessary to ensure that the optical fiber has minimal bending to meet the requirements of miniaturized gyroscope configuration. To this end, the inner cladding structure of the optical fiber needs to be designed, and the matching inner cladding doping depth and width need to be determined.

[0058] Research has shown that adding a wide, fluorine-doped matching depression channel around the fiber core, with wider and deeper channels, results in better macrobending performance. Engineering experiments have verified that when the ratio of the inner cladding minor axis diameter to the core minor axis diameter is between [1, 2], the macrobending loss is relatively high, and decreases significantly with increasing ratio. When the ratio is between [2, 3], the macrobending loss is significantly reduced to below 0.1 dB, and decreases slowly with increasing ratio. When the ratio is greater than 3, the macrobending loss remains essentially unchanged with increasing ratio. Figure 3 As shown in the image.

[0059] Considering the size and functional design requirements of optical fiber, in the preferred embodiment, the ratio of the minor axis diameter of the inner cladding 2 to the fiber core 1 is set to [2,3]. Within this range, it can ensure that the macro bending loss of the optical fiber is less than 0.1dB, and also avoid the inner cladding 2 of the optical fiber being too large.

[0060] Furthermore, practical verification revealed that the macrobending loss of optical fibers varies significantly depending on the relative refractive index difference of the inner cladding. Figure 4 It can be seen that the larger the relative refractive index difference Δ2 of the inner cladding 2, the greater the macrobending loss and the worse the macrobending performance. When Δ2 is greater than -0.2%, the fiber macrobending loss is greater than 0.1dB, and the performance at this time cannot meet the requirement of a bending diameter of 5mm. At the same time, when Δ2 is less than -0.35%, the inner cladding 2 cannot meet the viscosity matching requirement, that is, the melting point of the inner cladding 2 will be lower than the melting point of the fiber core 1. At this time, when the fiber is drawn, the inner cladding 2 will melt before the fiber core 1, resulting in a higher intrinsic attenuation of the fiber.

[0061] Therefore, in the preferred embodiment, the relative refractive index difference Δ2 of the inner cladding 2 should be controlled within the range of [-0.35%, -0.2%]. According to... Figure 4 As shown in the diagram, when Δ2 is -0.2%, under the condition of 5mm diameter and 10 turns, the macro bending loss of 1310nm band optical fiber is 0.09dB and that of 1550nm band optical fiber is 0.1dB, both of which meet the 0.1dB standard.

[0062] In the actual preparation process, the relative refractive index difference Δ2 of the inner cladding layer 2 is controlled by constraining the molar doping content of fluorine during the deposition of the inner cladding layer 2. Corresponding to the range of the above-mentioned relative refractive index difference, in the preferred embodiment, the molar doping content of fluorine should be controlled within the range of 0.55% to 0.95%, and can be further achieved by controlling the opening degree of fluorine doping flow rate during the deposition process.

[0063] Furthermore, based on the determination of the doping amount in the stress zone and the ellipticity of the fiber core, the birefringence performance of the optical fiber can be further adjusted by regulating the distance r from the edge of the stress zone to the center of the fiber core, according to the cross-sectional structure design of the optical fiber.

[0064] Specifically, the smaller the value of r, the stronger the birefringence performance of the fiber. However, engineering research has shown that when r ≤ 1 / 2D + 1.5μm (where D is the major axis diameter of the fiber core), meaning the distance between the edge of the stress region and the edge of the fiber core is no greater than 1.5μm, the boron trioxide doped in the stress region during fiber drawing will diffuse into the fiber core, resulting in significant intrinsic attenuation of the fiber. Therefore, for integrated polarization-maintaining fibers, the distance from the edge of the stress region to the edge of the fiber core needs to be greater than 1.5μm.

[0065] In addition to limiting the distance from the edge of the fiber stress zone to the edge of the fiber core, in the preferred embodiment, the ratio of the sum of the cross-sectional areas of the two stress zones to the cross-sectional area of ​​the bare fiber (excluding the inner and outer coating areas) is also limited. This is because the stress zone is a mixture of boron trioxide and silicon dioxide, which has relatively high stress. If the area occupied by the stress zone is too large, it will lead to poor mechanical properties of the fiber; while according to the calculation formula for birefringence of polarization-maintaining fiber, if the area of ​​the stress zone is too small, it will lead to insufficient birefringence performance of the fiber.

[0066] Therefore, in a preferred embodiment, the ratio of the sum of the cross-sectional areas of the two stress regions to the cross-sectional area of ​​the bare fiber is between 13.5% and 20.5%. Practical experience has shown that within this range, the integrated polarization-maintaining fiber can meet both the requirements for birefringence performance and ensure the mechanical properties of the fiber.

[0067] Furthermore, since the stress in the fiber stress region is greater near the inner edge of the fiber core, in a preferred embodiment, the refractive index profile at the edge of the stress region is designed with a gradient, which can effectively reduce the stress at the inner edge of the stress region and optimize the polishing performance of the fiber.

[0068] Specifically, within the stress zone, in a 0.1D3 region starting from the edge of the stress zone and moving inwards, the refractive index gradient decreases linearly, with the relative refractive index decreasing from 0 to Δ3. The linearity is Δ3 / 0.1D3, where D3 is the diameter of the stress zone and Δ3 is the relative refractive index difference within the stress zone. This configuration can eliminate stress concentration points at the edge of the stress zone by reducing the doping concentration, thereby increasing the stress uniformity of the fiber stress zone and effectively optimizing the fiber polishing performance.

[0069] In addition, there is an overlapping area between the inner side of the stress zone and the sunken inner cladding, such as... Figure 1As shown, from the stress region to the fiber core, there is only a fluorine-doped inner cladding region and no pure silicon region. The fiber stress region is doped with boron trioxide, the inner cladding is fluorine-doped, the fiber core is co-doped with fluorine and germanium, and the outer cladding is pure silicon dioxide. The viscosity of each part is ordered as follows: stress region > inner cladding > fiber core > outer cladding.

[0070] In short, there is an overlapping area between the inner side of the stress zone (near the fiber core) and the sunken inner cladding, and there is no pure silicon outer cladding area from the stress zone to the fiber core. This results in a gradient increase in fiber viscosity from the fiber core to the stress zone, which can reduce the difficulty of fiber manufacturing, reduce fiber attenuation, and increase the internal stress stability of the fiber.

[0071] Meanwhile, in order to better improve the looping performance of the optical fiber and reduce the fiber beat length, the preferred embodiment further improves the drawing process of the integrated polarization-maintaining optical fiber. In particular, during the optical preform drawing process, after the bare fiber comes out of the drawing furnace, an annealing process is added before the optical fiber coating. The birefringence performance of the optical fiber is effectively adjusted by the annealing temperature.

[0072] Actual research has found that when the annealing temperature rises from 0℃ to between 500 and 700℃, the fiber stave length gradually decreases. When the annealing temperature is in the range of 500 to 700℃, the fiber stave length can be reduced by 0.5 mm compared to 0℃. As the annealing temperature increases further, the fiber stave length gradually increases, which may have an adverse effect on the drawn fiber.

[0073] Therefore, during the optical fiber drawing process, the annealing operation after bare fiber drawing and before coating is preferably carried out at a temperature of 500~700℃ to further ensure the parameter requirements in feature one mentioned above. After the bare fiber annealing operation is completed, it is even more preferable to coat the outer periphery of the bare fiber with an inner coating 5 and an outer coating 6 sequentially from the inside to the outside.

[0074] In summary, the integrated polarization-maintaining fiber in the preferred embodiment is an elliptical-core panda polarization-maintaining fiber. Two stress regions 3 are symmetrically located on both sides of the major axis of the elliptical core 1. The ellipticity of the core 1 is between [20%, 35%], the ratio of the minor axis diameter D2 of the inner cladding to the minor axis diameter D1 of the core is between [2, 3], the relative refractive index difference Δ2 of the inner cladding 2 is between [-0.35%, -0.2%], and the relative refractive index difference Δ3 of the stress regions 3 is between [-0.85%, -0.7%]. Furthermore, during the fiber drawing process, the bare fiber undergoes annealing at a temperature of 500~700℃ before coating.

[0075] As another aspect of the present invention, a method for preparing the aforementioned integrated polarization-maintaining optical fiber is also provided, the specific preparation process of which is as follows: (1) Preparation of the mandrel; The preparation process of the mandrel mainly includes the mandrel deposition process and the mandrel melting and shrinking process.

[0076] In a preferred embodiment, the core rod deposition process is carried out by chemical vapor deposition, specifically using PCVD or MCVD processes to complete the deposition preparation. Before deposition preparation, the corresponding quartz liner is selected according to the design parameters of the integrated polarization-maintaining fiber, and the deposition parameters of each layer in the quartz liner, such as the deposition thickness and the corresponding element doping amount, are determined.

[0077] In the specific deposition process, a pure quartz outer cladding layer is deposited first, followed by an F-doped inner cladding layer of corresponding thickness, such that the molar doping content of fluorine in the inner cladding layer is between 0.55% and 0.95%, and then the relative refractive index difference of the inner cladding layer is controlled within the range of -0.35% to -0.2%. Finally, a Ge and F co-doped core layer is deposited.

[0078] In the actual process of co-doping Ge and F core layers, the molar doping amount of Ge is controlled within the range of 12% to 15%, and the molar doping amount of F is controlled within the range of 0.1% to 0.4%. This results in the relative refractive index contribution of Ge being 0.43% to 0.7%, and the relative refractive index difference contribution of F being -0.03% to -0.1%, thereby controlling the relative refractive index difference of the final core layer within the range of 0.4% to 0.6%.

[0079] Meanwhile, during the deposition process from the outside in, the ratio of the deposition diameter of the inner cladding to the deposition diameter of the core is controlled to be between [2,3], so as to ensure that the ratio of the short axis diameter of the inner cladding to the short axis diameter of the core is within the design range during the subsequent polarization-maintaining fiber drawing process.

[0080] To ensure the consistency of axial doping in the mandrel, the deposition rate of the inner cladding and core layer is preferably controlled within the range of 0.5~1.5 g / min, and the temperature inside the tube is <1200℃, while the pressure inside the tube is controlled within 10~20 mbar.

[0081] After the core plate deposition is completed, the mandrel melting and shrinking process is carried out. In order to ensure the consistency of the axial grinding area of ​​the mandrel during subsequent grinding, the roundness of the mandrel must be controlled below 0.5%, the roundness of the core must be controlled below 2%, and the curvature must be controlled below 0.1mm / 250mm.

[0082] To meet the above requirements, in the preferred embodiment, a process of micro-positive pressure, low speed, and low power is used for mandrel melting and shrinking. During melting and shrinking, the pressure inside the tube is controlled to be slightly higher than atmospheric pressure by 5~20 mbar, the melting and shrinking speed is controlled at 10~20 mm / min, and the melting and shrinking power is controlled at 5~10 kW.

[0083] (2) Determine the grinding dimensions of the core rod according to the core ellipticity range of the integrated polarization-maintaining fiber, and grind the outer surface of the core rod symmetrically into a crescent shape on both sides. (3) After the polished mandrel is embedded into the corresponding quartz sleeve, it is stretched to produce an elliptical core polarization-maintaining solid optical rod. Specifically, during the actual stretching process, the power of the stretching furnace is controlled at 40~50KW, the stretching speed is controlled at 10~20mm / min, and the final product is an elliptical core solid bright rod with a diameter of 50±1mm and a roundness of ≤1%. (4) Prepare stress rods; In a preferred embodiment, stress bars are manufactured by chemical vapor deposition, and the deposition of stress bars is completed using PCVD or MCVD processes.

[0084] In actual operation, the molar doping content of B2O3 in the stress bar is controlled to be within the range of 18%~22% to ensure that the relative refractive index difference Δ3 of the stress region meets the design requirements. At the same time, the final size of the stress bar to be polished after preparation is determined according to the size of the prepared elliptical core polarization-maintaining solid optical rod and the proportion of the stress region area on the cross section of the optical fiber.

[0085] In a preferred embodiment, the size of the stress bar after grinding is 13~15mm.

[0086] In addition, in actual setup, the ratio of the sum of the cross-sectional areas of the two polished stress bars to the cross-sectional area of ​​the elliptical core solid optical bar is further ensured to be between 13.5% and 20.5%.

[0087] (5) Based on the distance between the edge of the stress zone and the edge of the core layer on the cross section of the integrated polarization-maintaining fiber and the fabrication dimensions of the stress rod, determine the specific locations for symmetrical drilling on both sides of the long axis of the core layer.

[0088] In practice, the edge of the hole partially overlaps with the inner cladding, and the distance from the edge of the drill hole to the edge of the core layer is between 1.3mm and 3.0mm, more preferably between 1.8mm and 2.5mm.

[0089] (6) The stress bar is embedded in the opening on the solid optical rod, and the assembled optical rod is stretched to prepare an elliptical core panda-shaped polarization-maintaining fiber preform. To avoid air bubbles and air lines being trapped during the stretching process, a low-temperature, low-speed, and low-pressure stretching process is adopted in the preferred embodiment. Specifically, the power of the stretching furnace is controlled at 5~10 kW, the stretching speed at 2~5 mm / min, and the pressure inside the tube at 0~50 mbar.

[0090] (7) The integrated polarization-maintaining fiber is drawn. During the fiber drawing process, after the bare fiber is drawn and before coating, it is annealed in a heat preservation furnace and the annealing temperature is set to 500~700℃. Then, the inner and outer coatings are coated on the surface of the bare fiber to obtain the panda-shaped elliptical core polarization-maintaining fiber.

[0091] To better demonstrate the performance advantages of the integrated polarization-maintaining fiber in the preferred embodiment, specific embodiments 1 to 8 are used to compare the performance characteristics of the integrated polarization-maintaining fiber prepared. The fiber preparation process in each embodiment is carried out in accordance with the aforementioned preparation method, with the only difference being the selection of the corresponding parameter values.

[0092] In Examples 1-5, the design parameters of each optical fiber conform to the parameter range determined for features 1-4 in the aforementioned preferred embodiments; while in Examples 6-8, the design parameters of each optical fiber differ from some of the design parameter ranges in the aforementioned designs. In this way, integrated polarization-maintaining optical fibers that meet the design of different parameter ranges are prepared, and the performance indicators of each optical fiber are directly compared.

[0093] The specific settings and fiber performance parameters are shown in Table 1 below.

[0094] Table 1. Comparison of experimental data for specific embodiments 1-8

[0095] In Table 1 above, the test method for fiber optic crosstalk is defined as follows: a 1000m long fiber is wound with a tension of 25g onto a spool with a diameter of 30mm, and the polarization crosstalk of the fiber is measured accordingly. For the fiber used in the integrated gyroscope in the preferred embodiment, the polarization crosstalk under the above test conditions should not be greater than -25dB.

[0096] Based on the test results in Table 1 above, it is easy to see that for the integrated polarization-maintaining fiber in Examples 1 to 5, under the condition of 5mm diameter and 10 turns, the macrobending loss in the 1310nm and 1550nm bands is less than 0.1dB; the coupling loss between the fiber and the optical device chip is less than 3dB; the beat length of the fiber in the 1310nm and 1550nm bands is less than 2.5mm, and the crosstalk is less than -25dB; the wound 98 fiber loop used for integrated gyroscope has a full temperature range of -40℃ to 70℃ of less than 0.12° / h, and a nonlinear error of less than 1.5%.

[0097] In short, the integrated polarization-maintaining optical fibers in the above embodiments 1 to 5 can all meet the characteristics 1 to 4 required for polarization-maintaining optical fibers used in integrated gyroscopes, and can effectively meet the requirements for the preparation and use of integrated gyroscopes.

[0098] As for the optical fiber sample in Example 6, its core ellipticity is 15%, which is lower than the design range in the preferred embodiment; and the ratio of the inner cladding diameter to the fiber core diameter is 1.5, which is also lower than the design range in the preferred embodiment. For the optical fiber sample in this embodiment, although its coupling loss is lower, it is not difficult to see that under the above conditions, the crosstalk value and macrobending loss of the optical fiber are relatively large, and the beat length of the optical fiber in the 1550nm band is also greater than 2.5mm, which cannot meet the requirements for the use of optical fiber for integrated gyroscopes.

[0099] For the optical fiber sample in Example 7, the relative refractive index difference Δ2 of its inner cladding is -0.15%, which is higher than the design range in the preferred embodiment; the relative refractive index difference Δ3 of the boron-doped stress region is -0.9%, which is also higher than the design range in the embodiment. Under these design conditions, although the fiber has a small beat length, strong birefringence, and good crosstalk performance, it is easy to see that the macrobending loss of the fiber is relatively large, and the grinding cracking reaches 11.3%, which cannot meet the requirements for the use of optical fibers for integrated gyroscopes.

[0100] For the optical fiber sample in Example 8, its core ellipticity is 40%, which is higher than the design range in the preferred embodiment. Under this design condition, although its fiber beat length is small, its birefringence performance is strong, and its crosstalk performance is good, it is not difficult to see that the coupling loss of the optical fiber reaches 3.87dB, which cannot meet the requirements for use in integrated gyroscopes.

[0101] In short, for the optical fiber samples in Examples 6 to 8 above, some of their design parameters did not strictly follow the parameter conditions determined in the aforementioned preferred embodiments. Although some performance of the prepared optical fiber was improved, some performance indicators of the optical fiber could not meet the performance requirements of optical fiber for integrated gyroscopes, and could not simultaneously and accurately meet the requirements of features 1 to 4 above, thus making it unsuitable for the preparation of integrated fiber optic gyroscopes.

[0102] As another aspect of the present invention, a preferred embodiment further provides an integrated gyroscope based on the aforementioned integrated polarization-maintaining fiber. The integrated gyroscope includes a spool and an integrated polarization-maintaining fiber wound on the spool. The diameter of the spool is not less than 5 mm, and the length of the integrated polarization-maintaining fiber wound on it may vary depending on the diameter of the spool.

[0103] Specifically, when the winding diameter of the spool is between 5 and 30 mm, the length of the integrated polarization-maintaining fiber on the spool is no more than 300 m.

[0104] When the winding diameter of the spool is greater than 30mm, the length of the integrated polarization-maintaining fiber on the spool shall not exceed 3000m.

[0105] The integrated polarization-maintaining fiber of this invention has a simple structure and is easy to manufacture. It can effectively meet the four characteristics of optical fibers required for the manufacture of integrated fiber optic gyroscopes, fully meet the application requirements of integrated polarization-maintaining fiber in integrated gyroscopes, improve the manufacturing efficiency and accuracy of integrated fiber optic gyroscopes, and reduce the manufacturing and application costs of integrated fiber optic gyroscopes. It has good economic and practical value.

[0106] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An integrated polarization-maintaining optical fiber, comprising a fiber core and an inner cladding and an outer cladding arranged sequentially from the inside to the outside of the fiber core, wherein two stress regions are symmetrically arranged in the outer cladding with respect to the fiber core; characterized in that, The fiber core is elliptical, and the two stress zones are symmetrically located on both sides of the long axis of the fiber core; and The ellipticity of the fiber core is between [20%, 35%]; the ratio of the minor axis diameter D2 of the inner cladding to the minor axis diameter D1 of the fiber core is between [2, 3]; and The relative refractive index difference Δ2 of the inner cladding is between [-0.35%, -0.2%]; the relative refractive index difference Δ3 of the stress zone is between [-0.85%, -0.7%].

2. The integrated polarization-maintaining optical fiber according to claim 1, characterized in that, After the bare fiber of this integrated polarization-maintaining optical fiber is drawn, it undergoes an annealing process at a temperature of 500~700℃ before coating.

3. The integrated polarization-maintaining optical fiber according to claim 2, characterized in that, After annealing, the bare fiber is coated with an inner coating and an outer coating sequentially from the inside to the outside.

4. The integrated polarization-maintaining optical fiber according to any one of claims 1 to 3, characterized in that, The distance from the edge of the stress zone to the edge of the fiber core is greater than 1.5 μm.

5. The integrated polarization-maintaining optical fiber according to any one of claims 1 to 3, characterized in that, The ratio of the sum of the cross-sectional areas of the two stress zones to the cross-sectional area of ​​the bare fiber is between 13.5% and 20.5%.

6. The integrated polarization-maintaining optical fiber according to any one of claims 1 to 3, characterized in that, The refractive index profile at the edge of the stress zone is designed with a gradient.

7. A method for fabricating an integrated polarization-maintaining optical fiber for an integrated gyroscope, characterized in that, The process includes the following: (1) Prepare a core rod; deposit an inner cladding layer doped with F and a core layer co-doped with Ge and F sequentially from the outside to the inside, and control the ratio of the inner cladding diameter to the core diameter to be between [2,3], and control the relative refractive index difference Δ2 of the inner cladding layer to be between [-0.35%, -0.2%]; (2) Determine the grinding size of the core rod according to the core ellipticity range of the integrated polarization-maintaining fiber, and grind the two sides of the outer surface of the core rod symmetrically into a crescent shape; the core ellipticity range is [20%, 35%]; (3) The polished mandrel is embedded into the corresponding quartz sleeve for stretching to obtain an elliptical core polarization-maintaining solid optical rod. (4) Prepare stress bars, control the relative refractive index difference Δ3 of the stress bars to be between [-0.85%, -0.7%], and grind the prepared stress bars to the required size; (5) Determine the positions for drilling holes in the stress zone on both sides of the long axis of the core layer of the elliptical core solid optical rod and drill holes symmetrically. (6) The two stress rods after polishing are combined and embedded into the opening on the solid optical rod, and the combined optical rod is stretched to obtain an elliptical core panda-shaped polarization-maintaining fiber preform. (7) The integrated polarization-maintaining fiber for integrated gyroscopes is prepared by drawing the elliptical core panda-type polarization-maintaining fiber preform.

8. The method for fabricating an integrated polarization-maintaining optical fiber for an integrated gyroscope according to claim 7, characterized in that, In process (1), the preparation of the mandrel includes a mandrel deposition process and a mandrel melting and shrinking process performed sequentially; and During the core rod deposition process, the deposition rate of the inner cladding and the core layer is controlled within the range of 0.5~1.5 g / min, and the temperature inside the tube is <1200℃, while the pressure inside the tube is controlled within 10~20 mbar. and / or During the mandrel melting process, a process with micro-positive pressure, low speed, and low power is adopted for mandrel melting. The pressure inside the tube is controlled at 5~20 mbar higher than atmospheric pressure, the melting speed is controlled at 10~20 mm / min, and the melting power is controlled at 5~10 kW.

9. The method for fabricating an integrated polarization-maintaining optical fiber for an integrated gyroscope according to claim 7, characterized in that, In process (3), the power of the stretching furnace is controlled at 40~50KW and the stretching speed is controlled at 10~20mm / min, so as to obtain an elliptical core solid light rod with a diameter of 50±1mm and a roundness of ≤1%.

10. The method for fabricating an integrated polarization-maintaining optical fiber for an integrated gyroscope according to claim 7, characterized in that, In process (4), the ratio of the sum of the cross-sectional areas of the two polished stress bars to the cross-sectional area of ​​the elliptical core solid optical bar is between 13.5% and 20.5%.

11. The method for fabricating an integrated polarization-maintaining optical fiber for an integrated gyroscope according to any one of claims 7 to 10, characterized in that, In process (4), the relative refractive index difference Δ3 of the stress bar is controlled by constraining the molar doping content of B2O3 in the stress bar, which ranges from 18% to 22%.

12. The method for fabricating an integrated polarization-maintaining optical fiber for an integrated gyroscope according to any one of claims 7 to 10, characterized in that, In process (6), the stretching process of the combined light rod adopts a low temperature, low speed and low pressure stretching process. The power of the stretching furnace is controlled at 5~10kw, the stretching speed is controlled at 2~5mm / min, and the pressure inside the tube is controlled at 0~50mbar.

13. The method for fabricating an integrated polarization-maintaining optical fiber for an integrated gyroscope according to any one of claims 7 to 10, characterized in that, In process (7), after the bare fiber is drawn and before coating, an annealing operation is performed on the bare fiber, and the annealing temperature is set to 500~700℃.

14. An integrated gyroscope, comprising a spool and an integrated polarization-maintaining optical fiber wound on the spool; characterized in that, The integrated polarization-maintaining fiber is prepared by the method for preparing an integrated polarization-maintaining fiber for an integrated gyroscope as described in any one of claims 7 to 13.

15. The integrated gyroscope according to claim 14, characterized in that, The diameter of the spool is between 5 and 30 mm, and the length of the integrated polarization-maintaining fiber on the spool is no more than 300 m. or The diameter of the spool is greater than 30mm, and the length of the integrated polarization-maintaining optical fiber wound on the spool is no greater than 3000m.

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