Ultra-wideband double-hollow-core optical fiber polarization beam splitter

Abstract

The invention relates to an ultra-wideband double-hollow-core optical fiber polarization beam splitter. An optical fiber structure comprises a pear-shaped tube unit, an air fiber core, an outer cladding sleeve and a circular hollow-core nesting tube, the pear-shaped tube unit on the x axis is nested and connected to the inner wall of the outer cladding sleeve around the fiber core; and the pear-shaped tube unit, the fiber core on the y axis and the outer cladding sleeve form the ultra-wideband double-hollow-core fiber polarization beam splitter. According to the invention, by introducing the pear-shaped tube units, asymmetry in x and y directions is increased, so that the effect of increasing birefringence is achieved. According to the optical fiber polarization beam splitter, a mixed structure of the nested tube and the pear-shaped tube unit is designed on the optical fiber cladding, so that the loss of a fundamental mode is reduced, the high-order mode rejection ratio is improved, and the optical fiber polarization beam splitter with more excellent single-mode performance and wider application bandwidth is obtained. The method has great application value in the field of optical communication.

Description

(I) Technical field

[0001] The present application relates to an ultra-wideband dual hollow-core optical fiber polarization beam splitter, belonging to the technical field of optical communication and polarization beam splitters. (II) Background technology

[0002] With the continuous advancement of social informatization, polarization effects play an important role in many fields such as optical communication networks, fiber lasers, fiber gyroscopes, and interferometers. In optical fibers, manufacturing defects and environmental disturbances (such as bending and twisting) can introduce random birefringence and lead to unpredictable output. In optical fiber devices, unwanted birefringence can lead to two different polarization eigenmodes, and this mode competition may cause polarization instability. Hollow-core optical fibers have attracted widespread attention in recent years due to their unique properties, such as low material absorption, high damage threshold, and low transmission delay, and have been applied to optical communications, gyroscopes, high-power pulse transmission, supercontinuum generation, and infrared transmission. Compared with hollow-core photonic bandgap fibers, hollow-core antiresonant fibers have wider bandwidths and lower field overlap with glass. Since the invention of the first hollow-core antiresonant fiber in 2002, hollow-core antiresonant fibers with different structures have been reported to reduce transmission losses. By 2021, the transmission loss of nested nodeless hollow-core antiresonant fibers has been reduced to 0.22dB / km, which is similar to the transmission loss of traditional single-mode optical fibers. This major breakthrough will further promote the application of hollow-core antiresonant optical fibers.

[0003] Polarization effects have also been used to create fast saturable absorbers in passively mode-locked fiber lasers. When an intense light pulse propagates in an optical fiber, the polarization state in the high-intensity peak is different from that in the low-intensity wing, thus allowing a polarization filter to attenuate the low-intensity wing.

[0004] The cladding structure of hollow-core antiresonant fiber is relatively simple, in which the proportion of silica glass material is small, so that most of the inside of the fiber is air. Therefore, in the near-infrared band, the fiber has lower material absorption loss and higher light-induced damage threshold. Compared with traditional solid-core optical fiber, its structure is simpler and can be prepared relatively easily through the stacking drawing process.

[0005] The dual-core hollow microstructure fiber polarization beam splitter is a major application form based on bandgap microstructure fiber. The challenge is that the energy coupling between the two cores is relatively difficult, and the harsh conditions required to achieve the bandgap formation increase the complexity of fiber preparation. There is a problem with the existing dual-core hollow antiresonant fiber polarization beam splitter, that is, the distance between the two symmetrical cores is too large, resulting in an increase in the overall length, which limits the use scenarios of the beam splitter and may increase production costs.

[0006] Patent application number "CN202310563591.2" provides a dual hollow core anti-resonant fiber polarization beam splitter, which designs a semicircular and semi-elliptical tube on the x-axis, and increases the asymmetry in the x and y directions to achieve the effect of increasing the birefringence; at the same time, the fiber core is divided into two symmetrical fiber cores A and B, and the energy coupling mechanism between the modes of the two cores is introduced to adjust the refractive index of the four supermode modes of the dual cores and the coupling length of the two polarization directions; in addition, the anti-resonance wall is nested in the semicircular and semi-elliptical tube, and adding a quartz layer will bring two quartz-air interfaces. By adjusting the position of the anti-resonance wall, the effective refractive index of the tube mode is changed, so as to reduce the core fundamental mode loss and increase the high-order mode loss. Although it is close to the limiting loss magnitude of the present invention, the high-order mode suppression ratio of the present invention is the highest at 1530nm, and the high-order mode suppression ratio is 2193.54.

[0007] Patent application number "CN202310513074.4" provides a hollow-core antiresonant fiber polarization beam splitter, which has a simpler design and a low manufacturing process, but its coupling length ratio is lower than 0.5, about 0.3. At 1300nm, the present invention is 0.508, which is close to the optimal coupling length ratio.

[0008] Patent application number "CN202310290686.1" provides a terahertz double hollow-core anti-resonant fiber polarization splitter. By specially designing the fiber cladding wall and adopting a double-thickness structure, the double-thickness structure is used to achieve polarization and high birefringence of light, but its application bandwidth is relatively short. The application bandwidth of the present invention reaches 520nm.

[0009] Patent application number "CN202210549137.7" provides an octagonal double-core photonic crystal fiber polarization beam splitter based on sulfur glass. The air hole arrangement of the octagonal double-core photonic crystal fiber polarization beam splitter based on sulfur glass is simple, so the preparation method is easy. However, traditional polarization-maintaining optical fibers have technical problems such as low extinction ratio, long length, and short wavelength. The wavelength length of the device used in the present invention is 520nm.

[0010] Scheme (YTNi, JHYuan, S.Qiu, GYZhou, CMXia, X.Zhou, BBYan, Q.Wu, KRWang, XZSang, and CXYu. Dual hollow-core negative curvature fiber polarization beam splitter covering the O+E+S+C+Lcommunication band[J].Journal of the Optical Society of America B.2022,39(9):2493-2501.): The core of the optical fiber is surrounded by eight densely arranged circular quartz glass tubes, six of which have small tubes nested inside, while the remaining two tubes are not nested. By introducing two small tubes in the x-axis direction, the air core is divided into two symmetrical cores A and B. Simulation results show that the length of the polarization beam splitter is 4.1cm, achieving a broadband of 400nm, covering the O+E+S+C+L communication band, and has excellent single-mode characteristics. However, the parameters that can be changed in the large quartz tube with nested small quartz tubes described in the document are limited, and it is impossible to optimize more structural parameters to achieve that the coupling length in the y polarization direction is always greater than the coupling length in the x polarization direction, and the coupling length does not change much when the wavelength changes. The application bandwidth of the present invention is 520nm, which is much larger than 400nm.

[0011] Another solution (CJWang, HJDu, SGLi, JSLi, XJMeng, ZYYin, XXMa, C.Wang“,Wide bandwidth and short-length polarization beam splitter based ontellurite glass dual hollow-core anti-resonant fiber,”Optik-InternationalJournal for Light and Electron Optics.280(2023)): Ten closely arranged circular tellurite glass tubes surround the fiber core, six of which have small tubes nested inside, and the glass tubes located on the outermost sides of the x-axis and y-axis have no nested tubes. By nesting two small tubes in the x-axis direction, the circular tellurite glass tube divides the air core into two parts. From the simulation results, it can be seen that the length of the polarization beam splitter is 2.122cm, achieving a broadband of 400nm, and has single-mode characteristics in the frequency band of 1.26~1.66μm, and its polarization extinction ratio is as high as -292.83dB. However, the tellurite glass used in the document has high nonlinear characteristics and a wide infrared transmission range, but is not sufficiently symmetric in the x and y directions, resulting in a low high-order mode suppression ratio, which is lower than that of the present invention.

[0012] Another solution (YTNi, JHYuan, S.Qiu, GYZhou, CMXia, X.Zhou, BBBYan, Q.Wu, KRWang, XZSang, and CXYu. “Dual hollow-core anti-resonant fiber polarization beam splitter with excellent single-mode characteristics for ultra-bro adband” Applied Physics B 129:153.(2023)): Introduce a U-shaped nested tube and six circular quartz nested tubes in the optical fiber. The core is divided into two parts by two U-shaped tubes and the air core width is shorter to achieve a wide band. However, this structure also leads to the inability to effectively change the refractive index of the tube mode, resulting in the period length being too long at 8.1 cm. At the same time, the coupling length ratio CLR of the structure is less than 2.0, which is not the optimal coupling length ratio. The coupling length ratio CLR of the present invention is around 0.5 in the operating band, which is the optimal coupling ratio.

[0013] In summary, the performance of the double hollow-core antiresonant fiber can be improved by adjusting the ratio of the nested tube diameters, the thickness of the nested tubes, the air gap, and the diameter of the hollow-core front sleeve. Adjusting the air gap can optimize the value of the coupling length ratio CLR to an optimal value of 0.5 or 2.0; at the same time, the high-order mode suppression ratio can be optimized by adjusting the thickness of the nested tubes, the diameter of the hollow-core nested tubes, and the diameter ratio. The former affects the coupling length ratio by adjusting the coupling channel between the two cores, while the latter increases the wavelength independence by using the inner tube in the outer tube, which is beneficial to improving the working bandwidth. In order to enhance the coupling between the high-order mode and the cladding mode, a nested tube with an inner core is designed, and the circular polygonal tube unit distance is reasonably designed. Finally, a double hollow-core antiresonant fiber polarization beam splitter with good single-mode performance is realized. (III) Summary of the invention

[0014] In order to solve the deficiencies in the prior art, a series of measures can be taken to improve the optical fiber polarization beam splitter. First, a pear-shaped tube unit is introduced to increase the asymmetry in the x and y directions, thereby increasing the birefringence. At the same time, the core is divided into two symmetrical parts, and an energy coupling mechanism between the modes of the two cores is introduced to adjust the refractive index of the four high-order mode modes of the dual cores and the coupling lengths in the two polarization directions. By nesting multiple layers of pear-shaped tubes in the pear-shaped tube unit and adding multiple quartz layers, two interfaces between quartz and air will be generated. By adjusting the position of the antiresonance wall, the effective refractive index of the tube mode is changed, thereby reducing the loss of the core fundamental mode and increasing the loss of the high-order mode. Finally, a fiber polarization beam splitter with good single-mode characteristics and excellent polarization splitting performance is realized.

[0015] The object of the present invention is achieved in that:

[0016] An ultra-wideband double hollow-core optical fiber polarization beam splitter comprises a pear-shaped tube unit, an anti-resonance layer band, an air fiber core and an outer cladding sleeve.

[0017] The invention discloses a hollow-core antiresonant optical fiber polarization beam splitter, wherein the core region is an air core region, and includes two symmetrical cores A and B; the cladding region includes cladding circular tubes at two ends in the y direction, pear-shaped tube units at two ends in the x direction, and large circular tubes closely arranged between adjacent cladding circular tubes and pear-shaped tube units, wherein the radii of the cladding circular tubes and the large circular tubes are d3 and d2 respectively, wherein the ranges of d2 and d3 are 15.3-17.3 μm and 12.5-13.5 μm respectively, and a second circular tube with a diameter of k*d3 and a first circular tube with a diameter of k*d2 are nested inside, respectively, d3<d2, the outer first-layer semicircular diameters of the pear-shaped tube units are d0 and d1 respectively, wherein the ranges of d0 and d1 are 11.5-13.5 μm and 14.5-16.5 μm respectively, the arc diameter close to the core is d0, and the arc diameter close to the optical fiber cladding wall is d1, and the pear-shaped tubes are connected in a nested manner, and the nested polygonal tubes are inside. The dielectric materials for thin layer walls and nested tube walls are SiO 2 ; The thickness at the connection node is maintained at t±0.1t; the two arc nested tubes rotate at a specific angle tr, and the specific angle tr ranges from 39.4° to 40.4°; the spacing g between the two pear-shaped tube units ranges from 3.0 to 5.0 μm; the essence is to limit the transmission of light to the core area through the anti-resonance principle, thereby reducing leakage loss, and the difference in dielectric thickness between large and small pear-shaped tubes can reduce fundamental mode loss and further improve the birefringence level of the optical fiber.

[0018] A further improvement of the technical solution of the present invention is that the thickness of the air core, the circular hollow core nested tube, the pear-shaped tube unit and the outer cladding sleeve are the same, which is 0.63-0.66 μm.

[0019] A further improvement of the technical solution of the present invention is that the material of the air core, the circular hollow core nested tube, the pear-shaped tube unit and the outer cladding sleeve is SiO with a refractive index of 1.45. 2 , the rest of the space is air with a refractive index of 1.

[0020] A further improvement of the technical solution of the present invention is that the value range of k is 0.56 to 0.68

[0021] A further improvement of the technical solution of the present invention is that in the circular polygonal tube unit, the value range of d2 is 15.3 to 17.3 μm.

[0022] Due to the adoption of the above technical solution, the technical progress achieved by the present invention mainly includes the following three points:

[0023] 1. There is enough difference between the coupling lengths in the two polarization directions and the coupling length in the y-polarization direction is always smaller than the coupling length in the x-polarization direction. The coupling length does not change much with the wavelength and is relatively short.

[0024] 2. The present invention provides a hollow-core anti-resonant optical fiber polarization beam splitter, in which two symmetrical cores A and B are both air, so the two cores A and B use the refractive index of air as the upper limit of the refractive index of their respective core modes. The structure of this hollow-core anti-resonant optical fiber polarization beam splitter introduces a pear-shaped tube unit, which ensures the asymmetry of the hollow-core anti-resonant optical fiber polarization beam splitter in two orthogonal directions and introduces high birefringence. At the same time, the introduction of dual cores destroys the symmetry of the traditional optical fiber circle center, resulting in the existence of four non-degenerate modes in such a waveguide, namely: x-polarization even mode, x-polarization odd mode, y-polarization even mode, and y-polarization odd mode. The odd mode refers to the opposite direction of the mode electric field in the two cores, and the even mode refers to the same direction of the mode electric field in the two cores. After the core region passes light, four non-degenerate modes will be excited, which are called high-order modes. The asymmetry of the structure in the two polarization directions makes the transmission constants of the odd mode and the even mode different. The x-polarized light and the y-polarized light are periodically transferred in the core A and the core B. The shortest distance required for any polarized light in the x and y directions to be fully coupled from one fiber core to another is the coupling length, and the calculation formula is: This structure must ensure that the coupling lengths in the two polarization directions have sufficient difference in the operating band.

[0025] The cladding region includes 14 quartz tubes, which are symmetrical about the x and y axes, which makes the areas of the cores A and B the same size. A circular tube is nested in the cladding circular tube in the y direction, and multiple layers of pear-shaped tubes are nested in the pear-shaped tube unit in the x direction, so that the coupling length in the y polarization direction is always greater than the coupling length in the x polarization direction. At the same time, the nested second circular tube and the antiresonance wall will form two quartz-air interfaces, enhancing the suppression coupling effect between the core mode and the tube mode, thereby limiting the energy to the core. The wall thickness of all quartz tubes, including the large circular tube, cladding circular tube, pear-shaped tube unit and circular nested tube, as well as the nested antiresonance wall, is consistent. Due to the antiresonance effect, the thickness of the tube determines the resonant wavelength λ res In order to obtain lower loss, the selected wavelength should be the anti-resonance wavelength. In the anti-resonance wavelength region, the loss is low, the effective refractive index of the four supermodes coupled in the two cores is not much different, the shortest distance required for light of any polarization direction to be fully coupled from one core to another is relatively stable, and the coupling length does not change much with the change of wavelength, thus overcoming the dependence on wavelength, making the working wavelength of the beam splitter wider, and thus increasing the working bandwidth of the beam splitter.

[0026] Increasing the thickness of all quartz tubes and antiresonance walls will increase the area of ​​all quartz tubes and antiresonance walls in the cladding region, while reducing the area of ​​the core region, which will affect the refractive index of the four high-order modes. It will also increase the lower limit of the mode refractive index of the four high-order modes in the two polarization directions of the core, and the coupling length in the two polarization directions will become shorter. In summary, this technical solution is not conducive to achieving a sufficient difference in the coupling lengths of the two polarization directions and the coupling length in the y polarization direction is always greater than the coupling length in the x polarization direction, and the coupling length does not change much with the wavelength.

[0027] 3. At a wavelength of 1.55μm, since the dual-core coupling mechanism and coupling length do not change much with wavelength, the energy of the two cores is converted: one core has only x-polarized light, and the other core has only y-polarized light.

[0028] The beam splitter length Z coupling length ratio needs to meet certain conditions, and the calculation formula is: When the CLR is 1:2 or 2:1, the performance of the polarization beam splitter is most ideal, the x-polarized light and the y-polarized light in the fiber core can be fully separated to achieve the function of polarization beam splitting, and the length of the beam splitter is the shortest. In the anti-resonance wavelength region, two fiber cores of the same size are tightly wrapped by 8 quartz tubes, and the 4 supermode refractive indices of the core fundamental mode do not change much with the wavelength, and the coupling lengths of the two polarization directions do not change much with the wavelength, overcoming the wavelength dependence. Therefore, in the operating band, the CLR ratio is very stable and does not change much with the wavelength.

[0029] At the same time, the polarization extinction ratio is also an indicator for judging the function of the polarization beam splitter. It is used to describe the ability to separate different polarization states after the light wave propagates a certain distance in the fiber core. The calculation formula is: in and They represent the power of the x- and y-polarized light output in a certain fiber core. Generally, when |ER|>20dB, it means that the two polarization states of light are effectively separated.

[0030] The technical solution of the present invention utilizes the dual-core coupling mechanism and the coupling length that does not vary much with wavelength at a wavelength of 1550nm, so that the energy of the two fiber cores is converted: the distance when one core has only x-polarized light and the other core has only y-polarized light is the shortest.

[0031] In summary, the current fiber polarization beam splitter has the disadvantage of strong wavelength dependence, which leads to the disadvantages of too long fiber length and short application wavelength. We designed a pear-shaped tube dual-core hollow-core antiresonant fiber polarization beam splitter to overcome the limitations of traditional structures in wavelength dependence and bandwidth, thereby achieving wider bandwidth and better single-mode characteristics. The design takes advantage of structural asymmetry and introduces birefringence in the x and y directions, so that the slope of the four supermode mode refractive index of the dual core is consistent with the wavelength curve, which helps to reduce wavelength dependence. At the same time, we adjust the coupling lengths of the two polarization directions to ensure that there is enough difference between them to achieve a wider bandwidth. Specifically, in the x polarization direction, the pear-shaped tube unit with nested antiresonant walls is used to match the effective refractive index of the tube mode with the high-order mode refractive index of the core at the working wavelength, thereby obtaining excellent single-mode characteristics. This design combines antiresonant walls and hollow core structures, which can effectively suppress the generation of multimode modes and ensure the single-mode characteristics of the output signal, while providing a wider bandwidth, making it suitable for more optical application scenarios. (IV) Description of the drawings

[0032] Figure 1 A schematic cross-sectional view of an ultra-wideband dual hollow core fiber polarization beam splitter provided in the present application;

[0033] Figure 2 A graph showing the relationship between the coupling length in the x-polarization direction and the y-polarization direction and the coupling length ratio of an ultra-wideband dual hollow-core optical fiber polarization beam splitter provided in an embodiment of the present application and the wavelength;

[0034] Figure 3 A graph showing the relationship between the normalized power of an ultra-wideband dual hollow core fiber polarization beam splitter and the transmission length provided in an embodiment of the present application;

[0035] Figure 4 A graph showing the relationship between the extinction ratio and the high-order mode suppression ratio of an ultra-wideband double hollow-core fiber polarization beam splitter provided in an embodiment of the present application when the length is 5.499 cm and the wavelength; (V) Specific implementation methods

[0036] In order to make the present invention more clear and understandable, the specific implementation methods of the present invention will be described in detail with reference to the accompanying drawings.

[0037] This invention introduces an ultra-wideband dual hollow-core optical fiber polarization beam splitter. Compared with the method of introducing a pear-shaped tube body to split a larger core into two cores A and B in the prior art, the polarization beam splitter of the present invention adopts a pear-shaped tube unit with an embedded anti-resonance wall. The radii of the two ends of the pear-shaped tube unit tube body are different, and the radius of the semicircle can be adjusted independently, so as to more effectively adjust the effective refractive index of the tube mode, so that the coupling length is insensitive to the change of wavelength, thereby shortening the distance of energy conversion between the two cores and reducing the total length of the beam splitter. Compared with the solution of using eight compactly arranged arc quartz glass tubes in the prior art, the pear-shaped tube unit structure adopted in the present invention has higher structural flexibility and adjustable parameters, and overcomes the wavelength dependence by relying on the mechanism of coupling length, so as to achieve a shorter beam splitter length. Compared with the solution of using tellurite as the base material in the prior art, the present invention selects quartz as the material, and realizes the low sensitivity of the coupling length to the wavelength change through the non-central symmetric design in the structure, so as to effectively separate the polarized light in two directions within the operating band.

[0038] Example

[0039] Please refer to Figure 1 , a detailed structural diagram of the hollow core optical fiber;

[0040] In this embodiment, the detailed structure of the hollow core optical fiber includes:

[0041] (1) Outer sheath casing;

[0042] The radius of the outer cladding tube is 45.5 μm, the thickness is 15 μm, and the material is SiO 2 .

[0043] (2) pear-shaped tube unit;

[0044] The pear-shaped tube unit is composed of an outer large pear-shaped tube and a plurality of internally nested small pear-shaped tubes; the diameters d0 and d1 of the outermost large pear-shaped tube are 12 μm and 16 μm respectively; the diameter ratios of the large arc and the small arc of the internally nested small pear-shaped tube to the outer pear-shaped tube are 0.3, 0.49, and 0.78 respectively; the dielectric thickness t1 of the outer large pear-shaped tube and the internally nested small pear-shaped tube is 510 nm; the connection method of the outer large pear-shaped tube and the internally nested small pear-shaped tube is a nested connection, and the thickness at the connection node is kept at 510±58 nm; the dielectric material is SiO 2 .

[0045] (3) Circular hollow nested tube unit;

[0046] The circular hollow nested tube unit is composed of 6 pairs of 2-layer concentric circular hollow tubes, and 3 pairs form a group symmetrical about the x-axis; the diameter of the circular hollow tube of the outer layer of the circular hollow tube on the y-axis is: 13.5μm, and the diameter ratio of the inner circular hollow tube to the outer circular hollow tube is 0.56; the diameter of the circular hollow tube located on both sides of the y-axis is: 17.3μm, and the diameter ratio of the inner circular hollow tube to the outer circular hollow tube is 0.59; the dielectric thickness of the circular hollow front casing unit is consistent, and the dielectric thickness t is: 510nm; the dielectric material is SiO 2 .

[0047] (4) Optical fiber core;

[0048] The optical fiber core is composed of a pear-shaped tube unit and a circular hollow core nested tube unit, which is divided into A core and B core; the material is air; please refer to the optical fiber core light guide effect diagram Figure 2 .

[0049] The present invention can achieve a fundamental mode loss of 0.267916 dB / m in the y direction and a fundamental mode loss of 0.102865 dB / m in the x direction at 1550 nm. Please refer to Figure 3 , 4 The present invention can provide great application value for the development of the field of fiber optic gyroscope.

[0050] The specific technical solution of this patent can be described as:

[0051] The dual hollow-core antiresonant fiber polarization splitting structure proposed in this study consists of eight quartz tube antiresonant cladding regions of different sizes and shapes and two air core regions of equal size. Due to the existence of the antiresonance effect, the thickness of the tube wall determines the resonant wavelength λ res , the calculation formula is (m is an arbitrary integer, n is the refractive index of quartz), when m = 1 and the thickness of the tube t is 0.51 μm, λ res is 1.1μm, and in order to reduce transmission loss, the working band of 1370~1890nm is selected. After light passes through the core area, four supermodes will be excited, namely x-polarization odd mode, x-polarization even mode, y-polarization odd mode and y-polarization even mode. Due to the asymmetry of the two polarization directions, the transmission constants of the odd mode and the even mode are different, resulting in periodic transfer of x-polarized light and y-polarized light between the first core and the second core. When a certain polarization of light is completely coupled from one core to another, the shortest distance required is called the coupling length, and its calculation formula is (n represents the effective refractive index of the even mode and odd mode in the x-polarization direction and the y-polarization direction respectively; λ is the wavelength of the incident light). When the coupling length ratio meets certain conditions, its calculation formula is: In the formula, both m and n need to be integers and their parities need to be opposite. When the coupling length ratio is 1:2 or 2:1, the x-polarized light and y-polarized light in the optical fiber can be fully separated, realizing the polarization beam splitting function, and the length of the beam splitter is the shortest. After introducing the pear-shaped tube unit in this study, the ratio of the lowest high-order mode (HOM) loss to the highest fundamental mode (FM) loss of the fiber core increases. This ratio is called the high-order mode extinction ratio (HOMER), which is used to describe the single-mode performance of the optical fiber. Its calculation formula is. When the HOMER value is larger, the single-mode performance is better. When the HOMER value is greater than 100, it is generally considered that the optical fiber can achieve single-mode transmission. The HOMER values in this study are all higher than 100 in the 1370 - 1890 nm band.

[0052] The polarization extinction ratio (ER, also known as the polarization separation ratio PER) is a parameter that describes the ability of light waves to separate different polarization states at the output port after propagating a certain distance in the fiber core. It is defined as the normalized power ratio between one polarization and another polarization in the same fiber core, and the calculation formula is ( respectively represent the powers of the x-polarized light and y-polarized light output from a certain fiber core). When When, PER > 0; when When, PER < 0. The larger the absolute value of ER, the better the separation effect of the polarized light in the two directions. Generally speaking, when PER is higher than 20 dB or lower than -20 dB, it means that the power of one polarization state is at least 100 times that of the other polarization state, which enables the two different polarization states in the optical fiber to be completely and effectively separated. Therefore, the wavelength range covered by |PER|≥20 dB is defined as the bandwidth of the polarization beam splitter. In this embodiment, the ratio of CLR is stable at about 0.5 and changes little with the wavelength. Therefore, in the 1370 - 1890 nm band, the polarized light in the two directions can be excellently separated.

[0053] According to Figure 2 shown, the double-hole-core fiber polarization beam splitter based on the anti-resonance mechanism proposed in this study shows the trends of the coupling lengths of the x-polarization direction and y-polarization direction and their coupling length ratios changing with the wavelength. It can be observed that as the wavelength increases, the coupling lengths of the x- and y-polarized lights first increase and then decrease, and the coupling length of the x-polarization is less than that of the y-polarization. The coupling length ratio also gradually decreases with the increase of the wavelength and fluctuates around 0.5. Therefore, generally, the change of the coupling length with the wavelength is relatively stable.

[0054] Refer to Figure 3 , which shows the relationship between the normalized power and the transmission length of the double-hole-core fiber polarization beam splitter based on the anti-resonance mechanism proposed in this study at a wavelength of 1.55 μm. According to Figure 3It can be obtained that when the length is 5.499 cm, the difference between the x-polarization power and the y-polarization power is the largest.

[0055] like Figure 4 As shown in the figure, the relationship between the extinction ratio and HOMER value of the double hollow-core fiber polarization beam splitter based on the antiresonance mechanism provided in this study varies with wavelength when the length is 5.499 cm. Figure 4 It can be observed that when the beam splitter length is 5.499cm, at a wavelength of 1550nm, the absolute value of the extinction ratio reaches -20.58dB. In the wavelength range of 1370-1890nm, the absolute value of the polarization extinction ratio is greater than 20dB, which can effectively separate polarized light and realize the splitting of two polarization states of light. Therefore, the bandwidth of the double hollow-core fiber polarization beam splitter in this study is 520nm. The HOMER values ​​are all above 100, indicating that it has excellent single-mode characteristics.

[0056] The specific parameters in the embodiments of the present invention may be modified according to actual needs, but the modifications and changes are within the scope of the present invention described in the claims.

Claims

1. An ultra-wideband dual hollow-core fiber polarization beam splitter, the dual hollow-core anti-resonant fiber structure comprising: Air core, circular hollow core nested tube, pear-shaped tube unit and outer cladding sleeve. The air fiber core is an air channel surrounded by a circular hollow core nested tube and a pear-shaped tube unit, and is the main light-guiding area of ​​the hollow core optical fiber. The circular hollow-core nested tube is a light-beaming unit inside the hollow-core optical fiber, and is composed of an outer large circular tube and an inner nested small circular tube. The pear-shaped tube unit is a light beam unit inside the hollow-core optical fiber, and is composed of an external large pear-shaped tube and a plurality of internally nested small pear-shaped tubes, and is a main unit for improving single-mode characteristics. The outer cladding tube is a protective layer outside the hollow core optical fiber. The ultra-wideband double hollow-core optical fiber polarization beam splitter; the double hollow-core optical fiber introduces high birefringence and low limiting loss characteristics through two ways: pear-shaped tube and optical path difference in x and y directions; according to the pear-shaped tube-tube unit, the asymmetry of the geometric structure of the core is formed, the energy coupling mechanism between the two core modes is introduced, and the refractive index of the four supermode modes of the double core and the coupling length of the two polarization directions are adjusted. In addition, the pear-shaped tube-tube unit will increase the quartz-air interface, and the refractive index can be increased by adjusting the internal nested unit ratio, so as to achieve the effect of reducing the core fundamental mode loss and increasing the high-order mode loss.

2. The air core according to claim 1, characterized in that: The air fiber core is essentially an air hole surrounded by circular hollow core nested tubes and pear-shaped tube units; the air fiber core is divided into an A core and a B core, and the width ranges from 12.1 to 16.5 μm.

3. The circular hollow nested tube according to claim 1, characterized in that: The pear-shaped tube units are concentrically distributed and have the same dielectric thickness. The circular hollow front sleeve has two sizes, wherein the outer diameters d2 and d3 range from 15.3 to 17.3 μm and 12.5 to 13.5 μm, respectively. The outer diameter of the inner circular tube is tangent to the inner diameter of the outer circular tube, and the diameter of the inner circular hollow tube is 0.56 of the diameter of the outer tube. Since the size of the air gap between the double cores affects the coupling length of the two core modes, if it is below this range, the coupling length ratio will gradually increase; if it is above this range, the coupling length ratio will decrease.

4. The circular hollow nested tube unit according to claim 1, characterized in that: The circular hollow front sleeve unit rotates itself at a specific angle tr, the specific angle tr ranges from 39.4° to 40.4°, and two circular nested tube units are added distributed on the y-axis; the intervals between two adjacent circular front sleeve units remain consistent.

5. The pear-shaped tube unit according to claim 1, characterized in that: The pear-shaped tube units are located on the x-axis and are symmetrical about the center of the circle; the interval g between the two pear-shaped tube units is consistent, and the interval g ranges from 3.0 to 5.0 μm; and the number of the pear-shaped tube units is fixed at 2.

6. The pear-shaped tube unit according to claim 1, characterized in that: The sizes of the pear-shaped tube units are consistent; the diameters d0 and d1 of the two ends of the outer large pear-shaped tube range from 11.5 to 13.5 μm and 14.5 to 16.5 μm, and the gap g between the two pear-shaped tubes is 3.0 to 5.0 μm; the length and width of the outer nested rectangular tubes range from 7.93 μm to 1.18 μm, respectively; the dielectric thickness t of the outer large pear-shaped tube ranges from 500 to 560 nm. The diameter ratios of the large arc and the small arc of the inner nested small pear-shaped tube to the outer pear-shaped tube range from 0.25 to 0.45, 0.46 to 0.66, and 0.67 to 0.87, respectively.

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