Low-loss double-hollow-core optical fiber polarization beam splitter

Abstract

The invention relates to a low-loss double-hollow-core optical fiber polarization beam splitter. An optical fiber structure comprises a C-shaped nested tube, an air fiber core, an outer cladding sleeve and a circular hollow tube, a pair of C-shaped nesting pipes and circular hollow pipes which are symmetrical about the y axis is distributed on the x axis; a pair of C-shaped nested tubes connected with the cladding wall is distributed on the two sides of the y axis; a pair of C-shaped nesting pipes symmetrical about the y axis is distributed on the two sides of the y axis; the eight C-shaped nesting tubes, 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 C-shaped nested tube unit, the effect of reducing loss is achieved. By designing a mixed structure of the circular hollow tube and the C-shaped tube unit on the optical fiber cladding, the loss of a fundamental mode is reduced, the high-order mode rejection ratio is improved, and the optical fiber polarization beam splitter with lower loss 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 a low-loss 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 development of communication technology, polarization beam splitters are widely used in optical communication networks, wavelength division multiplexing, high-power transmission and other fields. The main function of polarization beam splitters is to split a beam of light into two mutually perpendicular polarized beams. In optical fibers, manufacturing defects and environmental disturbances can introduce random birefringence and lead to unpredictable output. In optical fiber devices, unnecessary birefringence can lead to two different polarization eigenmodes. Polarization beam splitters made of traditional optical fibers face problems such as narrow bandwidth and long device length. Polarization beam splitters designed with double solid core fibers have the disadvantages of high absorption material loss and nonlinearity. Hollow core antiresonant fiber, also known as hollow core negative curvature fiber, follows the suppressed coupling mechanism. The optical fiber light guiding mechanism is based on the antiresonant planar waveguide theory. Therefore, compared with traditional optical fibers, this fiber has the advantages of high damage threshold, low delay, and radiation resistance. The transmission of incident light in optical fiber is supported by the suppressed coupling between the core mode and the cladding mode. Therefore, the dual hollow core fiber polarization beam splitter overcomes the limitations of traditional optical fiber polarization beam splitters.

[0003] Polarization effect is used as a fast saturation absorber in the design of passively mode-locked fiber lasers. During the transmission of a strong light pulse along an optical fiber, the high-intensity region of the light pulse and the low-intensity shoulder have different polarization states. This property enables the polarization filter to selectively attenuate the low-intensity shoulder.

[0004] The cladding design of hollow-core antiresonant fiber is relatively simple, and the amount of quartz glass material used is significantly reduced, resulting in the interior of the fiber being mainly composed of air. This structural feature enables the fiber to exhibit lower material absorption loss and higher light-induced damage threshold in the near-infrared band. Compared with traditional solid optical fibers, the preparation process of hollow-core antiresonant fibers is simpler, and an efficient manufacturing process can be achieved through stacking and drawing technology.

[0005] The dual-core hollow microstructured fiber polarization beam splitter is an advanced application based on bandgap microstructured fiber. The main challenges faced by this technology include the difficulty of energy coupling between the two cores and the strict conditions required to achieve the bandgap, which increase the complexity of the fiber preparation process. The existing dual-core hollow anti-resonant fiber polarization beam splitter has certain limitations in design, especially because the distance between the two symmetrical cores is long, which increases the length of the overall structure, which not only limits the applicability of the beam splitter in specific application scenarios, but also may have an adverse impact on production costs.

[0006] Patent application number "CN 202111350765.4" provides a rhombus double-core photonic crystal fiber polarization beam splitter with air holes filled with liquid crystal. By designing a multi-layer structure core and a rhombus structure core, the beam splitting effect is increased by filling the central air hole with anisotropic liquid crystal material. The bandwidth reaches 157nm, while the bandwidth of the present invention is 320nm.

[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 0.44, which is close to the optimal coupling length ratio.

[0008] Patent application number "CN202210471342.6" provides a double hollow-core anti-resonant fiber polarization beam splitter based on the anti-resonance mechanism, which generates x-polarized light and y-polarized light and periodically transfers them between the first fiber core and the second fiber core. The bandwidth is 240nm, which is lower than the bandwidth of the present invention of 320nm.

[0009] Patent application number "CN202310513074.4" provides a dual-core hollow anti-resonant fiber polarization beam splitter. This design shortens the distance between the two pairs of symmetrical fiber cores of the beam splitter. Although this design shortens the device length of the polarization beam splitter, it increases the limiting loss of the device. The minimum loss is higher than 2dB / m. The maximum limiting loss of the present invention is 1.6dB / m, and the minimum limiting loss is 0.03dB / m.

[0010] Solution (Zhao T, Jia H, Lian Z, et al. Cltra-broadband dCal hollow-core anti-resonant fiber polarization splitter [J]. Academic Press, 2019. DOI: 10.1016 / j.yofte.2019.102005.): The core of the optical fiber is surrounded by eight densely arranged circular quartz glass tubes, four of which have small tubes nested inside, while the remaining four tubes are not nested. By introducing two elliptical hollow tubes in the x-axis direction, the air core is divided into two symmetrical cores A and B. The simulation results show that the length of the polarization beam splitter is 6.75 cm, a broadband of 310 nm is achieved, and it has excellent single-mode characteristics. However, the large quartz tube with nested small quartz circular tubes described in the document can change only a limited number of parameters, 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 320nm which is greater than 310nm.

[0011] Another solution (CJWang, HJDC, SGLi, JSLi, XJMeng, ZYYin, XXMa, C.Wang, “Wide bandwidth and short-length polarization beam splitter based on tellCrite glass dCal hollow-core anti-resonant fiber,” Optik-International Journal 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. It can be seen from the simulation results that the tellurite glass used in the polarization beam splitter itself has high nonlinear characteristics and a wide infrared transmittance range, but the symmetry in the x and y directions is insufficient, resulting in a low high-order mode suppression ratio, which is lower than that of the present invention.

[0012] Another solution (YTNi, JHYCan, S.QiC, GYZhoC, CMXia, X.ZhoC, BBYan, Q.WC, KRWang, XZSang, and CXYC. "D-Cal hollow-core anti-resonant fiber polarization beam splitter with excellent single-mode characteristics for C-band" Applied Physics B 129: 153. (2023)): Introduce C-type nested tubes and six circular quartz nested tubes in the optical fiber. The core is divided into two parts by two C-type tubes and the air core width is shorter to achieve a wide band. However, this structure also makes it impossible to effectively change the refractive index of the tube mode, resulting in the coupling length ratio CLR of the structure being 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 within the working band, which is the optimal coupling ratio.

[0013] Comprehensive analysis shows that the performance of dual hollow-core antiresonant fibers can be optimized by adjusting their structural parameters. Specifically, adjusting the diameter ratio of the nested tubes, the wall thickness, the air gap, and the diameter of the hollow outer tube are key control measures. The adjustment of the air gap is crucial for optimizing the coupling length ratio (CLR), and the goal is to achieve the optimal value of the coupling length ratio, i.e., 0.5 or 2.0. In addition, the high-order mode suppression ratio can be improved by adjusting the wall thickness of the nested tubes, the diameter of the hollow nested tubes, and the diameter ratio. Optimizing the coupling length ratio is mainly achieved by changing the geometric characteristics of the coupling channel between the two cores, while improving the high-order mode suppression ratio involves adjusting the size of the nested tubes, and using the inner tube structure in the outer tube to enhance the wavelength independence, which helps to broaden the working bandwidth. In order to enhance the coupling efficiency between the high-order mode and the cladding mode, a nested tube structure with an inner core is designed, and the spacing of the circular polygonal tube units is precisely designed. Through these comprehensive design strategies, a dual hollow-core antiresonant fiber polarization beam splitter with excellent single-mode characteristics is successfully developed. (III) Summary of the invention

[0014] In order to overcome the limitations of the existing technology, several strategies can be implemented to improve the performance of fiber polarization beam splitters. First, by introducing an arc-shaped polygonal tube unit, the geometric asymmetry of the device in the x and y axes is enhanced, thereby effectively improving the birefringence. In addition, the core is divided into two symmetrical parts, and the refractive index of the four high-order modes of the dual core is adjusted by using the inter-mode energy coupling mechanism of the two parts, and the coupling length of the two polarization directions is optimized. Multiple layers of similar structures are nested inside the arc-shaped polygonal tube unit, and multiple interfaces between quartz and air are created by adding multiple quartz layers. By adjusting the position of the antiresonant wall, the effective refractive index of the tube mode can be changed, which not only reduces the loss of the core fundamental mode, but also increases the loss of the higher-order mode. Through the comprehensive application of these strategies, a fiber polarization beam splitter that exhibits both excellent single-mode characteristics and efficient polarization splitting capabilities is finally developed.

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

[0016] A low-loss double hollow-core optical fiber polarization beam splitter comprises a C-shaped hollow nested tube, a circular hollow tube, an air fiber core and an outer cladding sleeve.

[0017] A hollow core anti-resonant optical fiber polarization beam splitter, wherein the core region is an air core region, including two symmetrical cores A and B; the distance g between the two C-shaped hollow nested tubes located on the x-axis is in the range of 2.5 to 3 μm; the cladding region includes C-shaped hollow nested tubes located at both ends of the y-direction, a pair of symmetrical C-shaped hollow nested tubes located on the x-axis, and a circular hollow tube. The diameter of the pair of hollow circular tubes located on the x-axis is, and the diameter of the C-shaped hollow nested tubes located on both sides of the y-axis is fixed to the radius of the optical fiber cladding tube minus half of the interval g; the radius R1 of the pair of C-shaped hollow nested tubes located on the x-axis is 7.5 to 8.5 μm, the ratio of the radius of the inner C-shaped tube to the radius of the outer circular tube is k1, which is in the range of 0.6 to 0.7, and the rectangular length of the C-shaped tube is 3.5 to 4.2 μm. The radius R2 of the C-shaped hollow nested tubes located on both sides of the y-axis is 7.5-8.5 μm, the ratio of the radius of the inner C-shaped tube to the radius of the outer circular tube is k2, ranging from 0.6 to 0.7, and the rectangular length of the C-shaped tube is 6-6.5 μm; the radius R3 of the C-shaped hollow front sleeve located in the y direction is 7-8 μm, the ratio of the radius of the inner C-shaped tube to the radius of the outer circular tube is k3, ranging from 0.6 to 0.7, and the rectangular length of the C-shaped tube is 4-4.5 μm; the thickness of the wall of the hollow circular tube and the C-shaped hollow nested tube is 0.45-0.65 μm, and the dielectric material of the wall is SiO 2 The C-shaped hollow nested tubes on both sides of the y-axis rotate at a specific angle tr, and the range of the specific angle tr is 40° to 45°; the essence is to limit the transmission of light in the core area through the anti-resonance principle, thereby reducing leakage loss, and the difference in the semicircle radius of the C-shaped tubes inside the large and small C-shaped hollow nested tubes and the C-shaped hollow nested tubes can reduce the 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 C-shaped hollow nested tube, the circular hollow tube and the outer cladding sleeve are the same, which is 0.45 to 0.65 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 arc polygonal 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 ranges of k1, k2, and k3 are all 0.6 to 0.7.

[0021] A further improvement of the technical solution of the present invention is that in the C-shaped hollow nested tubes, the value range of R1 and R2 is 7.5-8.5 μm.

[0021] A further improvement of the technical solution of the present invention is that in the C-shaped hollow nested tubes, the value range of R3 is 7 to 8 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. In order to overcome the limitations of existing technologies, this study proposed a series of strategies to improve optical fiber polarization beam splitters. First, by introducing arc polygonal tube units, the geometric asymmetry of the device in the x and y axes is enhanced, thereby effectively improving the birefringence. In addition, 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 in the dual-core structure, as well as the coupling length in the two polarization directions. Furthermore, a C-shaped tube is nested inside the C-shaped hollow nested tube, and multiple quartz layers are added to create multiple interfaces between quartz and air. By adjusting the position of the antiresonance wall, the effective refractive index of the tube mode can be changed, which not only helps to reduce the loss of the core fundamental mode, but also promotes the increase of high-order mode loss. Through the comprehensive application of these strategies, a fiber polarization beam splitter that exhibits both excellent single-mode characteristics and efficient polarization splitting capabilities has been successfully developed. The shortest distance required for any polarized light in the x and y directions to be fully coupled from one 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] In order to improve the performance of the fiber polarization beam splitter, this study made a special design for the cladding region, which consists of 10 quartz tubes and is symmetrical about the x and y axes. This design ensures that the cross-sectional areas of core A and core B are equal. In the y direction, a C-shaped tube is embedded inside the cladding circular tube; in the x direction, a C-shaped hollow nested tube is designed to connect with the circular hollow tube. This structural configuration makes the coupling length in the y polarization direction constantly greater than the coupling length in the x polarization direction. In addition, the nested circular hollow tubes and C-shaped hollow nested tubes together form two quartz-air interfaces. This design significantly enhances the suppression coupling effect between the core mode and the tube mode, effectively confining the light energy in the core region. All quartz tubes, including circular hollow tubes and C-shaped hollow nested tubes, adopt a consistent wall thickness design. According to the antiresonance effect, the thickness of the tube wall determines the resonant wavelength λres. In order to achieve lower losses, the selected working band should be within the antiresonance band. In the anti-resonance wavelength region, the loss is low, and the coupling of the four high-order modes in the two cores has a small effective refractive index difference. For light of any polarization direction, the shortest distance required for complete coupling from one core to another is also relatively stable, and the coupling length is insensitive to changes in wavelength. This design overcomes the dependence on a specific wavelength, thereby broadening the operating wavelength range of the beam splitter and correspondingly increasing the operable bandwidth of the beam splitter.

[0026] Increasing the thickness of all quartz tubes and their antiresonant walls will increase the cross-sectional area of ​​the quartz tubes and antiresonant walls in the cladding region, and correspondingly, the cross-sectional area of ​​the core region will decrease. This change will affect the refractive index of the four higher-order modes in the core. Specifically, increasing the thickness will increase the lower limit of the mode refractive index of the four higher-order modes in the two polarization directions in the core, thereby shortening the coupling length in the two polarization directions. Taking all factors into consideration, this technical solution may have an adverse effect on achieving a significant difference in the coupling length in the two polarization directions, especially when it is expected that the coupling length in the y polarization direction is always greater than the coupling length in the x polarization direction. In addition, the sensitivity of the coupling length to wavelength changes may increase, which is contrary to the design goal of achieving a coupling length that is insensitive to wavelength changes. Therefore, this solution may not be conducive to maintaining the stability of the coupling length under wavelength changes, affecting the performance of the beam splitter over a wide wavelength range.

[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 ratio of the beam splitter length to the coupling length needs to meet certain conditions, and the calculation formula is: When the coupling length ratio (CLR) is set to 1:2 or 2:1, the performance of the fiber polarization beam splitter reaches the optimal state. Under this condition, the x-polarized light and the y-polarized light in the core can be efficiently separated, thereby completing the task of polarization beam splitting, and the required length of the beam splitter is the shortest at this time. In the anti-resonance wavelength region, two cores of equal size are tightly surrounded by 8 quartz tubes, and the refractive index of the four higher-order modes of the core fundamental mode remains relatively stable when the wavelength changes. The coupling length in the two polarization directions also shows insensitivity to wavelength, effectively reducing the dependence on a specific wavelength. Therefore, within the operating band, the ratio of the CLR remains highly stable, and its fluctuation varies very little with wavelength. This design strategy is conducive to achieving broadband operation within the wavelength range and maintaining consistency in the performance of the beam splitter.

[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] Comprehensive analysis shows that the traditional optical fiber structure has the disadvantages of wavelength dependence and bandwidth limitation and high loss. We propose to introduce structural asymmetry to produce birefringence effects in the x and y directions, ensuring that the slopes of the refractive index of the four high-order modes in the dual-core remain consistent with the wavelength, so as to solve the device's dependence on wavelength and reduce the transmission loss of the optical fiber. In addition, the bandwidth is further broadened by precisely controlling the coupling lengths in the two polarization directions and ensuring that there is a significant difference between them. Specifically, in the x-polarization direction, a C-shaped hollow nested tube unit with nested antiresonant walls is designed to achieve precise matching of the effective refractive index of the tube mode with the refractive index of the core high-order mode at the operating wavelength, thereby optimizing the single-mode characteristics. This design combines the advantages of antiresonant walls and hollow core structures, effectively suppresses the excitation of multimode modes, and ensures the single-mode characteristics of the output signal. At the same time, the proposed structural design also significantly expands the bandwidth, enabling it to meet the needs of more diverse optical applications. (IV) Description of the drawings

[0032] Figure 1A cross-sectional schematic diagram of a low-loss double hollow-core optical fiber polarization beam splitter provided in the present application;

[0033] Figure 2 A relationship diagram of the coupling length in the x-polarization direction and the y-polarization direction and the coupling length ratio thereof as a function of wavelength of a low-loss double hollow-core optical fiber polarization beam splitter provided in an embodiment of the present application;

[0034] Figure 3 A relationship diagram of the normalized power of a low-loss double hollow-core optical fiber polarization beam splitter versus transmission length provided in an embodiment of the present application;

[0035] Figure 4 A graph showing the relationship between the limiting loss and the high-order mode suppression ratio in the x-polarization direction of a low-loss double hollow-core fiber polarization beam splitter provided in an embodiment of the present application when the length is 35.32 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 a low-loss dual hollow-core optical fiber polarization beam splitter. Compared with the method of introducing a C-shaped hollow nested tube 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 C-shaped hollow nested tube with an embedded anti-resonance wall. The radii of the two ends of the C-shaped hollow nested 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 C-shaped hollow nested tube 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 achieves the purpose of low loss through the non-central symmetric design of the structure, thereby effectively separating 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) C-shaped hollow nested tube;

[0044] The hollow core anti-resonant optical fiber polarization beam splitter has an air core region, including two symmetrical cores A and B; the distance g between the two C-shaped hollow nested tubes located on the x-axis is in the range of 2.5 to 3 μm; the cladding region includes C-shaped hollow nested tubes located at both ends of the y-direction, a pair of symmetrical C-shaped hollow nested tubes located on the x-axis, and a circular hollow tube. The radius R1 of the pair of C-shaped hollow nested tubes located on the x-axis is 7.5 to 8.5 μm, the ratio of the radius of the inner C-shaped tube to the radius of the outer circular tube is k1, which is in the range of 0.6 to 0.7, and the rectangular length of the C-shaped tube is 3.5 to 4.2 μm. The radius R2 of the C-shaped hollow nested tubes on both sides of the y-axis is 7.5-8.5 μm, the ratio of the radius of the inner C-shaped tube to the radius of the outer circular tube is k2, ranging from 0.6-0.7, and the rectangular length of the C-shaped tube is 6-6.5 μm; the radius R3 of the C-shaped hollow front sleeve located in the y direction is 7-8 μm, the ratio of the radius of the inner C-shaped tube to the radius of the outer circular tube is k3, ranging from 0.6-0.7, and the rectangular length of the C-shaped tube is 4-4.5 μm. The C-shaped hollow nested tubes on both sides of the y-axis rotate at a specific angle tr, and the specific angle tr ranges from 40° to 45°; the essence is to limit the transmission of light in the core area through the anti-resonance principle, thereby reducing leakage loss, and the difference between the large and small C-shaped hollow nested tubes and the semicircular radius of the C-shaped tube inside the C-shaped hollow nested tube can reduce the fundamental mode loss and further improve the birefringence level of the optical fiber.

[0045] (3) Circular hollow tube;

[0046] The diameter of the pair of hollow circular tubes on the x-axis is , and the diameter of the C-shaped hollow nested tubes on both sides of the y-axis is fixed to the radius of the optical fiber cladding tube minus half of the interval g. The thickness of the walls of the hollow circular tubes and the C-shaped hollow nested tubes is 0.45-0.65 μm, and the dielectric material of the wall is SiO 2 , located on the x-axis and connected to the cladding wall.

[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 0.121203dB / m in the y direction and a fundamental mode loss of 0.127281dB / m in the x direction at 1550nm. 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 10 quartz tube antiresonant cladding regions of different 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. In order to reduce transmission loss, the working band of 1420~1740nm 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 odd mode and 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: ( 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 this study, a specific coupling length ratio (CLR) value, i.e., 1:2 or 2:1, is considered to be the condition for achieving the most ideal polarization splitting performance. Under this condition, the x-polarized light and the y-polarized light in the core can be effectively separated to complete the function of polarization splitting, and this effect is achieved with the shortest beam splitter length. By introducing the pear-shaped tube unit, this study successfully improved the ratio of the lowest higher-order mode (HOM) and the highest fundamental mode (FM) loss in the core, which is called the higher-order mode extinction ratio (HOMER). HOMER is a key indicator for measuring the single-mode performance of optical fiber, and its calculation formula is described in the article. This study shows that when the HOMER value exceeds 100, the optical fiber can usually achieve single-mode transmission. In the 1420-1740nm band of this study, all measured HOMER values ​​are higher than 100, indicating that the optical fiber studied has excellent single-mode transmission performance in this band.

[0052] Polarization extinction ratio (ER, also known as polarization separation ratio PER) is a parameter that describes the ability of light waves to separate different polarization states at the output port after transmitting 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 output power of x-polarized light and y-polarized light in a certain fiber core). When PER>0; when When PER is greater than 20 dB or less than -20 dB, PER is less than 0. The greater the absolute value of ER, the better the separation of polarized light in 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 another 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 the definition of |PER| ≥ 20 dB is the bandwidth of the polarization beam splitter. In this embodiment, the ratio of CLR is stable at about 0.5 and does not change much with wavelength. Therefore, in the 1370-1890 nm band, polarized light in two directions can be separated very well.

[0053] according to Figure 2 The results show that the dual hollow-core fiber polarization beam splitter based on the antiresonance mechanism proposed in this study reveals how the coupling length in the x-polarization direction and the y-polarization direction and their coupling length ratio change with the increase or decrease of wavelength. The observation results show that with the increase of wavelength, the coupling length of the x-polarization and y-polarization light shows a trend of first increasing and then decreasing. In this process, the coupling length of the x-polarization is always smaller than the coupling length of the y-polarization. In addition, the coupling length ratio gradually decreases with the increase of wavelength and fluctuates around the value of 0.5. Combining the above observations, the change of coupling length relative to wavelength has maintained a relatively stable state as a whole. This finding is of great significance for the design of polarization beam splitters with a wide wavelength operating window.

[0054] refer to Figure 3 , showing the relationship between the normalized power and the transmission length of the double hollow-core fiber polarization beam splitter based on the anti-resonance mechanism proposed in this study at a wavelength of 1.55 μm. Figure 3 It can be obtained that when the length is 35.32 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 35.32 cm. Figure 4It can be observed that when the beam splitter length is 35.32cm, at a wavelength of 1550nm, the limiting loss of the optical fiber polarization beam splitter is less than 0.1dB / m. In the wavelength range of 1420-1740nm, the limiting loss of the optical fiber polarization beam splitter is less than 1dB / m. At the same time, the absolute value of the polarization extinction ratio is greater than 20dB, which can achieve effective separation of polarized light and splitting of two polarization states of light. Therefore, the bandwidth of the double hollow-core fiber polarization beam splitter in this study is 320nm. 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. A low-loss double hollow-core fiber polarization beam splitter, the double hollow-core anti-resonant fiber structure comprising: Air core, C-shaped hollow nested tube, circular hollow tube and outer cladding sleeve. The air fiber core is an air channel surrounded by five C-shaped hollow nested tubes and is the main light-guiding area of ​​the hollow core optical fiber. The C-shaped hollow nested tube is a light beam unit inside the hollow core optical fiber, and is composed of an outer large circular tube and an inner nested small C-shaped tube. The hollow circular tube is a light beam unit inside the hollow core optical fiber, and is a hollow silica tube. The outer cladding tube is a protective layer outside the hollow core optical fiber. A low-loss dual hollow-core optical fiber polarization beam splitter; the dual hollow-core optical fiber introduces high birefringence and low limiting loss characteristics through two methods: C-shaped hollow nested tubes and optical path difference in the x and y directions; according to the C-shaped hollow nested tubes, the asymmetry of the geometric structure of the core is formed, the energy coupling mechanism between the modes of the two cores is introduced, and the refractive index of the four supermode modes of the dual core and the coupling length of the two polarization directions are adjusted. In addition, the C-shaped hollow nested tube increases 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 C-shaped hollow nested tubes; the air fiber core is divided into an A core and a B core, and the width ranges from 12 to 16 μm.

3. The C-shaped hollow nested tube according to claim 1, characterized in that: The C-shaped hollow nested tube is composed of a hollow circle and a C-shaped hollow tube, and the C-shaped hollow tube is composed of a semicircle and two rectangles, wherein the center of the semicircle is the same as the center of the circular hollow tube, wherein the radius of the semicircle is 0.4 to 0.7 of the radius of the outer circle, and the width of the rectangles constituting several C-shaped tubes is the same, which is 0.4 to 0.7 μm, and the length is different. The radius R1 of a pair of C-shaped hollow nested tubes located on the x-axis is 7.5 to 8.5 μm, the ratio of the radius of the inner C-shaped tube to the radius of the outer circular tube is k1, which ranges from 0.6 to 0.7, and the length of the C-shaped tube rectangle is 3.5 to 4.2 μm.

4. The C-shaped hollow nested tube according to claim 1, characterized in that: The C-shaped hollow nested tubes on both sides of the y-axis rotate themselves at a specific angle tr, and the specific angle tr ranges from 41.4° to 43.4°. At the same time, two circular nested tube units distributed on the y-axis are added; the sizes of the C-shaped hollow nested tubes on both sides of the y-axis remain consistent, and the radius R2 of the C-shaped hollow nested tubes on both sides of the y-axis is 7.5 to 8.5 μm, the ratio of the radius of the inner C-shaped tube to the radius of the outer circular tube is k2, which ranges from 0.6 to 0.7, and the rectangular length of the C-shaped tube is 6 to 6.5 μm.

5. The C-shaped nested tube according to claim 1, characterized in that: The two C-shaped nested tubes are located on the x-axis and are symmetrical about the center of the circle; the two C-shaped hollow nested tubes are spaced apart by a distance g, and the range of the distance g is 2.5 to 4.5 μm; the number of the C-shaped hollow nested tubes is fixed at 2.

6. The hollow circular tube according to claim 1, characterized in that: The hollow circular tube is located symmetrically about the center of the circle on the x-axis, and its diameter is fixed to the sum of the radius of the optical fiber cladding tube minus half of the interval g and the C-shaped hollow nested tube located on the x-axis, and the hollow circular tube is connected to the cladding wall.

7. The optical fiber cladding tube according to claim 1 has a thickness of 5 μm and serves as a protective layer on the outside of the light.

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