Panda type polarization maintaining optical fiber and preparation method thereof

Abstract

The invention belongs to the technical field of optical fibers, and discloses a panda type polarization maintaining optical fiber and a preparation method thereof. According to the panda type polarization maintaining optical fiber, the stress areas are symmetrically distributed on the two sides of the fiber core at intervals, the whole stress areas are wrapped by the inner wrapping layer, and the grinding performance and the mechanical performance of the optical fiber can be improved by combining fiber core fluorine-doped viscosity matching, stress area diameter and stress area gradual change design; therefore, the optical fiber has better birefringence performance, full-temperature performance, reliability and deformation resistance. Meanwhile, the polarization-maintaining mother rod is prepared through the PCVD large rod technology, the fiber core, the inner cladding and the outer cladding are integrally formed, the stress area can be completely wrapped by the inner cladding through the technology, the preparation procedure of the panda type polarization-maintaining optical fiber can be greatly simplified, and the performance of the optical fiber can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fibers, and more specifically, relates to a panda-type polarization-maintaining optical fiber and a preparation method thereof. Background Art

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

[0003] However, the existing Panda-type polarization-maintaining optical fiber has the following main problems: (1) the optical fiber has poor grinding performance; (2) the optical fiber has poor full-temperature performance; (3) the stability of the optical fiber or its ability to resist deformation needs to be improved; (4) the stress distribution inside the optical fiber is uneven. The existing preparation methods of Panda-type polarization-maintaining optical fiber mainly have the problems of complicated preparation procedures or poor practical operability.

[0004] For example, the technical solution disclosed in patent CN104445912A includes: making an optical fiber core rod and a polarization-maintaining stress rod, symmetrically fixing two polarization-maintaining stress rods on both sides of the core rod according to the stress distribution requirements, using an external deposition method to deposit an outer cladding on the core rod and the polarization-maintaining stress rod, obtaining a preform loose body, sintering the preform loose body, and obtaining a polarization-maintaining optical fiber preform with a circular cross-section. The above method has poor practical operability and obvious defects. The two stress rods are symmetrically fixed on both sides of the core rod by a fixture for deposition, and then the rods will shrink during the sintering process, which will cause the position of the stress rods to change. Therefore, it is difficult to ensure symmetry during actual operation, resulting in uneven stress distribution of the optical fiber.

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

[0006] For example, the technical solutions disclosed in patent CN102910812B and patent CN112327405A are similar, and mainly include: using PCVD process to prepare polarization-maintaining core rod and two boron-doped stress rods, melting the polarization-maintaining core rod with a quartz sleeve at high temperature and drawing it into a polarization-maintaining mother rod, drilling holes in the polarization-maintaining mother rod, embedding the boron-doped stress rods into the holes to form a polarization-maintaining optical fiber preform, and drawing the polarization-maintaining optical fiber preform in a molten state into a polarization-maintaining optical fiber. The above method is a common domestic method, but it has many defects, including: 1. After the core rod is prepared by PCVD, it needs to be stretched through the sleeve to obtain the polarization-maintaining mother rod, and the preparation process is complicated; 2. During the high-temperature melt stretching process, additional thermal stress will be introduced, and the magnitude of the axial thermal stress will be different, resulting in uneven stress on the mother rod, and the stress will affect the change of the material refractive index, resulting in poor consistency of the optical parameters of the drawn optical fiber; 3. The core rod is sleeved in the sleeve, and there is actually a gap between the core rod and the inner hole of the sleeve. The absolute middle position of the core rod cannot be guaranteed during the stretching process, which will lead to a large geometric deviation of the concentricity of the optical fiber core package; 3. Due to the large cross-sectional area of ​​the sleeve and the small area of ​​the core rod, the width of the deposited inner cladding during the preparation of the core rod is limited; 4. The parameters of the polarization-maintaining mother rod obtained by the sleeve method are affected by the parameters of the sleeve. The roundness and wall deviation of the sleeve itself will affect the geometric parameters of the polarization-maintaining mother rod. At present, the domestic sleeve production process is not mature. In most cases, the sleeve relies on imports, resulting in rising costs and poor controllability of raw materials; 5. The secondary contamination of the core rod is easy to cause problems in attenuation and strength during the stretching preparation process. Summary of the invention

[0007] The present invention provides a panda-type polarization-maintaining optical fiber and a preparation method thereof, thereby solving the problems in the prior art that the preparation process of the panda-type polarization-maintaining optical fiber is complicated and the optical fiber performance needs to be improved.

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

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

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

[0011] Preferably, the single-side thickness of the gradient layer is 0.1D3, where D3 is the diameter of the stress zone.

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

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

[0014] In a second aspect, the present invention provides a method for preparing the above-mentioned Panda-type polarization-maintaining optical fiber, comprising the following steps:

[0015] Through the PCVD rod process, the inner cladding and the core layer are sequentially deposited in the liner, and the deposited liner is melted and sintered to obtain a polarization-maintaining mother rod, wherein the inner cladding of the polarization-maintaining mother rod wraps the entire stress zone; wherein the inner hole size of the lathe resonant cavity of the PCVD rod process is greater than or equal to 45 mm, and the power of the high-frequency system power of the PCVD is greater than or equal to 10 KW;

[0016] A pair of symmetrical circular holes are longitudinally processed on both sides of the center line of the polarization-maintaining master rod, wherein the center line of the circular holes is parallel to the center line of the polarization-maintaining master rod and is located in the same plane;

[0017] preparing two stress rods, and performing grinding and corrosion treatment on the stress rods;

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

[0019] The panda-type polarization-maintaining optical fiber preform is drawn in a molten state to obtain a panda-type polarization-maintaining optical fiber.

[0020] Preferably, when depositing the inner cladding, a mixed gas of silicon tetrachloride, oxygen and hexafluoroethane is introduced into the liner tube to react and generate fluorine-doped silicon dioxide which is deposited on the liner tube;

[0021] When depositing the inner cladding, the flow rate of silicon tetrachloride is 100-1000sccm, the flow rate of oxygen is 1000-3000sccm, the flow rate of hexafluoroethane is 10-100sccm, the temperature is 1000-1500°C, and the number of deposited layers is 19000-20000 layers.

[0022] Preferably, when depositing the core layer, a mixed gas of silicon tetrachloride, germanium tetrachloride, oxygen and hexafluoroethane is introduced into the liner tube to react and generate fluorine-doped silicon dioxide and germanium dioxide which are deposited on the liner tube;

[0023] When depositing the core layer, the flow rate of silicon tetrachloride is 100-1000sccm, the flow rate of germanium tetrachloride is 2-100sccm, the flow rate of oxygen is 300-3000sccm, the flow rate of hexafluoroethane is 2-100sccm, the temperature is 1000-1500°C, and the number of deposited layers is 500-4800 layers.

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

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

[0026] In terms of optical fiber design, the width of the inner cladding of the conventional panda-type polarization-maintaining optical fiber is relatively small, and it can only wrap the inner edge of the stress zone, and it is impossible to make the entire stress zone inside the inner cladding. However, the inner cladding of the present invention is extremely wide, and the stress zone can be completely wrapped inside the inner cladding, and the stress zone includes a flat layer and a gradient layer from the inside to the outside (that is, the edge of the stress zone adopts a gradient design), so it can reduce the viscosity difference between the outer edge of the stress zone and the interface position of the inner cladding, improve the situation that the edge of the stress zone of the optical fiber is easy to crack during polishing, and improve the polishing performance of the optical fiber.

[0027] In addition, considering that when polarization-maintaining optical fiber is used in the field of gyroscopes, the full-temperature performance of the optical fiber is an important factor restricting the accuracy of the gyroscope, and the poor full-temperature performance of the polarization-maintaining optical fiber is mainly due to the expansion of the material causing compressive stress or tensile stress on the fiber core. The polarization-maintaining optical fiber provided by the present invention includes an extremely wide fluorine-doped inner cladding. Due to the SiO 2 The expansion coefficient is 0.5×10 -6 / ℃, the expansion coefficient of fluorine-doped glass is about 0.3×10 -6 / ℃, therefore, the present invention can reduce the expansion coefficient of the optical fiber cladding by increasing the effective area of ​​the fluorine-doped inner cladding by increasing the diameter of the fluorine-doped inner cladding, thereby reducing the tensile stress or compressive stress of the fiber core caused by the expansion of the material due to temperature changes, and improving the full-temperature performance of the optical fiber. At the same time, the fluorine-doped quartz glass shows higher performance in strength and elastic modulus than pure quartz glass. The improvement of strength and elastic modulus means that the fluorine-doped quartz glass can maintain better shape stability when subjected to external forces and has a higher ability to resist deformation.

[0028] The stress zone of conventional polarization-maintaining optical fiber is borosilicate glass with very low viscosity, while the cladding is pure silica glass with very high viscosity, which makes the viscosity difference at the interface of the stress zone large, resulting in large stress at the interface of the stress zone. The optical fiber is subjected to external stress, such as optical fiber grinding, which can easily cause cracking at the interface of the stress zone, affecting the reliability of the optical fiber. The present invention optimizes the optical fiber viscosity matching, and wraps the entire stress zone through a fluorine-doped extremely wide inner cladding to reduce the viscosity of the outer cladding at the boundary of the stress zone, thereby reducing the viscosity difference between the inner and outer sides of the boundary of the stress zone, thereby optimizing the interface stress of the optical fiber stress zone and improving the reliability of the optical fiber.

[0029] At the same time, the use of an extremely wide fluorine-doped inner cladding can effectively increase the area ratio of the fluorine-doped inner cladding to the entire cladding part, which can reduce the viscosity of the entire optical fiber. The reduction in the overall viscosity of the optical fiber means that a lower furnace temperature can be used for drawing, which can reduce the diffusion of boron elements in the stress zone at high temperatures and cause changes in optical fiber attenuation, which can effectively improve the attenuation uniformity.

[0030] In terms of optical fiber preparation, the present invention first prepares a polarization-maintaining mother rod through a PCVD large rod process (for example, a PCVD-R47 process) in which the inner hole size of the lathe resonant cavity is greater than or equal to 45 mm and the high-frequency system power is greater than or equal to 10 KW. The obtained polarization-maintaining mother rod has an extremely wide fluorine-doped inner cladding, which can wrap the entire stress area; prepare two stress rods; then process a pair of circular holes in the polarization-maintaining mother rod, and then embed the two polished and corroded stress rods into the circular holes on both sides of the polarization-maintaining mother rod respectively, and combine them to obtain a panda-type polarization-maintaining optical fiber preform rod; finally, the panda-type polarization-maintaining optical fiber preform rod is drawn in a molten state to obtain a panda-type polarization-maintaining optical fiber. Taking PCVD-R47 as an example, the present invention can give full play to the advantages of the PCVD-R47 process. Based on the size characteristics of the inner hole of the resonant cavity used by PCVD-R47, the core (corresponding to the core layer in the panda-type polarization-maintaining optical fiber preform), inner cladding and outer cladding (corresponding to the sleeve in the panda-type polarization-maintaining optical fiber preform) in the panda-type polarization-maintaining optical fiber prepared by the present invention are integrally formed. Compared with the existing conventional panda-type polarization-maintaining optical fiber preparation method which requires the preparation of polarization-maintaining mother rods by sleeve tube and stretching, the present invention can greatly simplify the preparation process of polarization-maintaining mother rods, and thus can greatly simplify the preparation process of panda-type polarization-maintaining optical fibers. Moreover, since the present invention does not require the sleeve tube and stretching processes, it can avoid the problem of core rod centering during the core rod sleeve tube process, avoid the problem of large concentricity of the optical fiber core package from the source, and avoid the introduction of impurities during the stretching process. The problem of large optical fiber attenuation can be avoided, and the problem of secondary contamination of the core rod caused by the stretching preparation process can be avoided; since the present invention does not involve a high-temperature melting and stretching process, no additional thermal stress will be introduced, and the uniform stress of the mother rod can be ensured, and the consistency of the optical fiber optical parameters can be improved; the present invention utilizes the PCVD-R47 process, and the deposition efficiency of the inner cladding and the core layer can reach 100%, and the core rod profile can be accurately controlled to ensure that the core rod profile is consistent with the calculated theoretical value, thereby improving the optical fiber optical performance; compared with the conventional preparation method, the present invention utilizes the PCVD-R47 process, and the area ratio before and after melting and shrinking is larger, which is more conducive to the control of the non-circularity of the core layer and the inner cladding. The non-circularity of the drawn optical fiber core and the inner cladding can be less than 0.5%. Compared with the sleeve stretching method, the present invention can avoid the geometric parameters of the polarization-maintaining mother rod being affected by the sleeve parameters (including the sleeve's own roundness and wall deflection).

[0031] In addition, the present invention can obtain an extremely wide inner cladding by preparing a polarization-maintaining mother rod through the PCVD-R47 process, can ensure that the stress area is completely wrapped inside the inner cladding, and can improve the relevant performance of the panda-type polarization-maintaining optical fiber by combining the stress area diameter design, the gradual change design of the edge position of the stress area, the inner core fluorine doping, and the optical fiber viscosity matching design, so as to improve the grinding performance and mechanical properties of the optical fiber, so that the optical fiber has better birefringence performance, full-temperature performance, reliability and deformation resistance, can avoid or improve the situation that the internal stress distribution of the optical fiber is uneven due to the large viscosity difference, and can effectively improve the axial consistency. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

[0036] Figure 5 This is a process flow chart of a method for preparing a panda-type polarization-maintaining optical fiber provided in Example 2 of the present invention.

[0037] Among them, 1-core, 2-inner cladding, 3-stressed area, 4-outer cladding, 5-inner coating layer, 6-outer coating layer. DETAILED DESCRIPTION

[0038] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0039] Embodiment 1:

[0040] Example 1 provides a Panda-type polarization-maintaining optical fiber, see Figure 1 , Figure 2 , mainly including a core 1, an inner cladding 2, a stress zone 3, and an outer cladding 4, and in addition, the outer cladding 4 may be coated with an inner coating layer 5 and an outer coating layer 6. The core 1 is located at the center of the inner cladding 2, and the stress zones 3 are symmetrically spaced on both sides of the core 1, and the stress zones 3 are completely wrapped in the inner cladding 2; the stress zone 3 includes a flat layer and a gradient layer from the inside to the outside.

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

[0042] The diameter of the stress region 3 needs to be controlled within a certain range compared to the diameter of the outer cladding 4. If the diameter is too small, the optical fiber birefringence performance will be insufficient, and if the diameter of the stress region 3 is too large, the optical fiber mechanical strength will be poor. Figure 3 , the birefringence coefficient of the optical fiber increases with the increase of the stress zone diameter. When the stress zone diameter is less than 0.25D4, the birefringence coefficient increases rapidly with the increase of the stress zone diameter; when the stress zone diameter is greater than 0.25D4, the birefringence coefficient tends to be flat as the stress zone diameter increases. The breaking force of the optical fiber decreases with the increase of the stress zone diameter. When the stress zone diameter is less than 0.3D4, the breaking force decreases slowly with the increase of the stress zone diameter; when the stress zone diameter is greater than 0.3D4, the breaking force decreases rapidly with the increase of the stress zone diameter. Therefore, considering the birefringence performance and mechanical strength of the light, when the diameter D3 of the stress zone 3 is in the range of [0.25D4, 0.3D4], the birefringence performance and mechanical strength of the optical fiber can reach ideal values, and the birefringence can reach 7.72×10 -4 Above, the optical fiber breaking force can reach above 4.12Gpa.

[0043] See also Figure 1 and Figure 2 The inner cladding 2 is a fluorine-doped inner cladding, and the relative refractive index of the inner cladding 2 is Δ2∈[-0.2%, -0.1%]. The corresponding viscosity of the inner cladding at 1950°C is 5.8 to 6.1 Pas.

[0044] The core 1 in the present invention adopts a viscosity matching design, and the viscosity needs to be less than the viscosity of the inner cladding. According to the optical waveguide design, the relative refractive index of the core 1 is Δ1∈[0.55%, 0.75%]. Conventional optical fibers adopt a pure germanium-doped design, and the viscosity is 6.1-6.25 Pas at 1950°C, so the core viscosity is larger than the inner cladding. The core 1 in the present invention adopts a fluorine-germanium co-doped design, and the core is doped with germanium and the viscosity of the core is adjusted by adding a certain amount of fluorine. When the core is doped with fluorine Δ∈[-0.15%, -0.05%] and the corresponding germanium-doped Δ∈[0.6%, 0.9%], the viscosity of the core 1 at 1950°C is 5.6-5.75 Pas, which can make the core viscosity less than the inner cladding viscosity, where the core Δ1=germanium-doped Δ+fluorine-doped Δ.

[0045] The stress region 3 is a boron-doped stress region. Boron doping in the stress region directly affects the birefringence performance of the optical fiber. If the mol content is less than 15%, the birefringence performance of the optical fiber will be insufficient and the polarization ability of the optical fiber will be weak. If the mol content is greater than 25%, the internal stress of the optical fiber will be large and the mechanical properties of the optical fiber will be poor. 2 O 3 The mol content ∈[15%, 25%], the corresponding viscosity at 1950°C is 4.7-5.2Pas. Among them, the cracking of the optical fiber polishing stress zone is mainly caused by the large stress at the boundary of the stress zone. Therefore, the stress zone 3 adopts a special design, which includes a flat layer and a gradient layer from the inside to the outside. The gradient layer design at the edge of the stress zone can effectively reduce the stress at the edge of the stress zone.

[0046] See also Figure 4 , where the unilateral thickness of the gradient layer in the stress zone will affect the probability of optical fiber cracking due to grinding and will also affect the birefringence performance of the optical fiber. When the unilateral thickness diameter of the gradient layer is less than 0.1D3, the birefringence coefficient of the optical fiber decreases slowly as the gradient layer increases; when the unilateral thickness of the gradient layer is greater than 0.1D3, the birefringence coefficient of the optical fiber decreases rapidly as the unilateral thickness of the gradient layer increases; and the probability of optical fiber cracking due to grinding decreases rapidly as the gradient layer increases. When the unilateral thickness of the gradient layer is greater than 0.1D3, the probability of cracking due to grinding is 0%. Therefore, the unilateral thickness of the gradient layer is controlled at 0.1D3 as an ideal value, which can ensure that the birefringence coefficient of the optical fiber reaches 7.72×10 -4 , while the probability of grinding cracking is 0%.

[0047] The relative refractive index Δ4 of the outer cladding 4 is 0%, and the corresponding viscosity at 1950° C. is 6.35 Pas.

[0048] By matching the core viscosity, controlling the inner cladding diameter, the stress zone diameter, and the thickness of the gradient layer on one side of the stress zone, the birefringence performance of the optical fiber can be guaranteed to reach 7.72×10 -4 , the optical fiber breaking force can reach 4.12Gpa, and the probability of optical fiber cracking due to grinding is 0%.

[0049] Embodiment 2:

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

[0051] Example 2 provides a method for preparing the Panda-type polarization-maintaining optical fiber as described in Example 1, see Figure 5 , mainly including the following steps:

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

[0053] Through the PCVD large rod process, the inner cladding and the core layer are sequentially deposited in the liner, and the deposited liner is melted and sintered to obtain a polarization-maintaining mother rod, wherein the inner cladding of the polarization-maintaining mother rod wraps the entire stress zone.

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

[0055] For example, a polarization-maintaining mother rod (ie, a core rod) can be prepared by a PCVD-R47 process, and two stress rods can be prepared by a PCVD-R47 process or a PCVD-R31 process.

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

[0057] When depositing the inner cladding, a mixed gas of silicon tetrachloride, oxygen and hexafluoroethane is introduced into the liner tube to react and generate fluorine-doped silicon dioxide which is deposited on the liner tube. The flow rate of silicon tetrachloride is 100-1000sccm, the flow rate of oxygen is 1000-3000sccm, the flow rate of hexafluoroethane is 10-100sccm, the temperature is 1000-1500°C, and the number of deposited layers is 19000-20000 layers.

[0058] When depositing the core layer, a mixed gas of silicon tetrachloride, germanium tetrachloride, oxygen and hexafluoroethane is introduced into the liner tube to react and generate fluorine-doped silicon dioxide and germanium dioxide, which are deposited on the liner tube. The flow rate of silicon tetrachloride is 100-1000sccm, the flow rate of germanium tetrachloride is 2-100sccm, the flow rate of oxygen is 300-3000sccm, the flow rate of hexafluoroethane is 2-100sccm, the temperature is 1000-1500°C, and the number of deposited layers is 500-4800 layers.

[0059] When the deposited liner is melted and shrunk, the pressure inside the tube is adjusted to a slight positive pressure or a slight negative pressure, the pressure range is -50mBar to +20mBar, the temperature is adjusted to 1600-2000℃, the moving speed of the heating furnace is adjusted to 20-100mm / min, and the indentation of each layer of the liner (i.e. the change in the thickness of the core rod) is controlled to 1-5mm.

[0060] When the melted core rod is sintered, the pressure in the tube is adjusted to a slight negative pressure in the range of -950mba to 0mbar, the temperature is adjusted to 1800-2200°C, and the moving speed of the heating furnace is adjusted to 10-40mm / min.

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

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

[0063] In addition, after obtaining the polarization-maintaining mother rod and before processing the round hole, the method may further include: annealing the polarization-maintaining mother rod to eliminate residual stress.

[0064] (2) Grinding and corrosion of stress rods.

[0065] Specifically, after the stress rod is prepared, it is necessary to perform grinding and corrosion treatment on the stress rod.

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

[0067] Among them, after the stress rod is polished, hydrofluoric acid is used to remove impurities introduced during the stress rod polishing machining process through corrosion, and the corrosion time is controlled at 20 to 40 minutes.

[0068] (3) Core rod drilling.

[0069] Specifically, a pair of symmetrical circular holes are longitudinally processed on both sides of the center line of the polarization-maintaining mother rod, and the center line of the circular holes is parallel to the center line of the polarization-maintaining mother rod and is located in the same plane.

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

[0071] Among them, the process from rod to fiber is a proportional reduction process. The center position of the hole determines the geometric position of the optical fiber stress zone. The distance between the center of the hole and the center of the polarization-maintaining mother rod is 11.0-12.1mm. In addition, the aperture size needs to be controlled within a certain range compared to the outer diameter of the stress rod after polishing, which can ensure that the geometric deviation of the optical fiber stress zone is controlled within 1um, ensure the geometric symmetry of the stress zone, and the single-side gap needs to be controlled within 0.2mm, that is, the aperture size needs to be controlled within 0-0.4mm compared to the outer diameter of the stress rod after polishing.

[0072] It should be noted that the processing sequence of (2) grinding and etching of the stress rod and (3) drilling of the core rod in the present invention is not limited and can be performed simultaneously or adjusted according to the preparation conditions. The above sequence is only used as an example. The processing of the two can be performed after manufacturing the core rod and the stress rod and before assembling them.

[0073] (4)Combination.

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

[0075] (5) Wire drawing.

[0076] The panda-type polarization-maintaining optical fiber preform is drawn in a molten state to obtain a panda-type polarization-maintaining optical fiber.

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

[0078] (6) Coating.

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

[0080] In the present invention, the fiber core, inner cladding and outer cladding are integrally formed to directly obtain a polarization-maintaining mother rod, and an extremely wide fluorine-doped inner cladding can also be obtained to completely wrap the stress area. Specifically, this can be achieved by using the R47 large rod process (i.e., PCVD-R47 process) of plasma chemical vapor deposition (PCVD).

[0081] Among them, the inner hole of the resonant cavity used by PCVD-R47 is 47mm. After deposition and sintering, the outer diameter of the rod can reach 45mm, and the maximum diameter of the inner cladding can reach 40mm. The inner cladding can completely cover the stress zone; while the inner hole of the conventional PCVD-R31 resonant cavity is 31mm. After deposition and sintering, the maximum outer diameter of the rod can only reach 25mm, and the maximum diameter of the inner cladding is only 21mm. The inner cladding and the stress zone only partially overlap, and the inner cladding cannot completely cover the stress zone.

[0082] Among them, the high-frequency system power used by PCVD-R47 is 10KW. The silicon tetrachloride, germanium tetrachloride, oxygen, hexafluoroethane and other gases introduced during the inner cladding and core deposition process can react fully, and the reaction efficiency can reach 100%. The core rod profile can be controlled more accurately, so that the core rod profile is consistent with the designed theoretical value; while the conventional PCVD-R31 high-frequency system power is 6KW, the gas reaction efficiency during the deposition process is difficult to reach 100%, and there is a certain difference between the core rod profile and the designed theoretical value, which will affect the optical performance.

[0083] The present invention can give full play to the advantages of the PCVD-R47 process, including:

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

[0085] (2) No need to use a rod for the stretching process, which can avoid the problem of core rod centering during the core rod sleeve process, avoid the problem of large concentricity of the optical fiber core package from the source, and at the same time avoid the problem of large optical fiber attenuation caused by the introduction of impurities during the stretching process;

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

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

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

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

[0090] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A Panda-type polarization-maintaining optical fiber, characterized in that: include: The core, inner cladding, stress region and outer cladding; The stress regions are symmetrically spaced and distributed on both sides of the core, and the inner cladding wraps the entire stress region.

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

3. The Panda-type polarization-maintaining optical fiber according to claim 1, characterized in that: The stress zone includes a flat layer and a gradient layer from the inside to the outside.

4. The PANDA-type polarization-maintaining optical fiber according to claim 3, characterized in that: The single-side thickness of the gradient layer is 0.1D3, where D3 is the diameter of the stress zone.

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

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

7. A method for preparing a panda-type polarization-maintaining optical fiber as claimed in any one of claims 1 to 6, characterized in that: The following steps are involved: Through the PCVD rod process, the inner cladding and the core layer are sequentially deposited in the liner, and the deposited liner is melted and sintered to obtain a polarization-maintaining mother rod, wherein the inner cladding of the polarization-maintaining mother rod wraps the entire stress zone; wherein the inner hole size of the lathe resonant cavity of the PCVD rod process is greater than or equal to 45 mm, and the power of the high-frequency system power of the PCVD is greater than or equal to 10 KW; A pair of symmetrical circular holes are longitudinally processed on both sides of the center line of the polarization-maintaining master rod, wherein the center line of the circular holes is parallel to the center line of the polarization-maintaining master rod and is located in the same plane; preparing two stress rods, and performing grinding and corrosion treatment on the stress rods; The two treated stress rods are respectively embedded into the circular holes on both sides of the polarization-maintaining mother rod to obtain a panda-shaped polarization-maintaining optical fiber preform; The panda-type polarization-maintaining optical fiber preform is drawn in a molten state to obtain a panda-type polarization-maintaining optical fiber.

8. The method for preparing the panda-type polarization-maintaining optical fiber according to claim 7, characterized in that: When depositing the inner cladding, a mixed gas of silicon tetrachloride, oxygen and hexafluoroethane is introduced into the liner tube to react and generate fluorine-doped silicon dioxide which is deposited on the liner tube; When depositing the inner cladding, the flow rate of silicon tetrachloride is 100-1000sccm, the flow rate of oxygen is 1000-3000sccm, the flow rate of hexafluoroethane is 10-100sccm, the temperature is 1000-1500°C, and the number of deposited layers is 19000-20000 layers.

9. The method for preparing the panda-type polarization-maintaining optical fiber according to claim 7, characterized in that: When depositing the core layer, a mixed gas of silicon tetrachloride, germanium tetrachloride, oxygen and hexafluoroethane is introduced into the liner tube to react and generate fluorine-doped silicon dioxide and germanium dioxide which are deposited on the liner tube; When depositing the core layer, the flow rate of silicon tetrachloride is 100-1000sccm, the flow rate of germanium tetrachloride is 2-100sccm, the flow rate of oxygen is 300-3000sccm, the flow rate of hexafluoroethane is 2-100sccm, the temperature is 1000-1500°C, and the number of deposited layers is 500-4800 layers.

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

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