Ultra-broadband dual-core negative curvature fiber polarization beam splitter with paperclip cladding structure

Abstract

The present invention discloses an ultra-wideband dual-core negative curvature optical fiber polarization beam splitter with a paperclip cladding structure, comprising an outer supporting tube layer with a radius R, an inner fiber core region, and a cladding region therebetween. The cladding region is composed of eight quartz tubes in a paperclip structure, consisting of an outer cladding tube and an inner nested nested tube. The nested tubes and the cladding tubes are both semi-oblong, formed by two straight sides and a semicircular side on one side, and are coaxially arranged. The present invention controls the beam splitting capability of the beam splitter by adjusting the effective refractive index of the four supermode modes in the two fiber cores. By adjusting the diameter of the semicircular side of the cladding tube, the mode field area is changed and the coupling length is increased, achieving a technical effect of an extinction ratio higher than 20 dB over a bandwidth of 1.34 to 1.88 μm and up to 540 nm, and a high-order mode extinction ratio greater than 100 in the 1.48 to 1.55 μm band.

Description

Technical Field

[0001] The invention relates to an ultra-wideband dual-core negative curvature optical fiber polarization beam splitter with a paperclip cladding structure, belonging to the technical field of polarization beam splitters. Background Art

[0002] Since Dr. Charles Kao proposed using optical fiber as a long-distance transmission medium, the information age has rapidly developed. Over the past decade, internet traffic has skyrocketed at a rate of tenfold annually. The transmission capacity of traditional single-mode optical fiber has reached the physical limit set by Shannon's theorem. To address this capacity crisis, wavelength division multiplexing (WDM) technology has matured. Fiber optic beam splitters, passive devices that split, combine, and distribute optical signals, are essential optical components in WDM, fiber-optic local area networks, and certain measuring instruments. Polarization beam splitters primarily separate or combine input polarized optical signals according to their polarization directions. However, the limited operating band and device complexity of conventional optical fiber-based polarization beam splitters limit their application. Hollow-core antiresonant fiber, leveraging the antiresonance effect and suppressed coupling between cladding and core modes, confines significant optical power to the air core for transmission. This eliminates concerns about quartz material nonlinearity, dispersion, and absorption losses. Compared to traditional optical fibers, hollow-core antiresonant fiber offers advantages such as low dispersion, low nonlinearity, a high power damage threshold, low temperature sensitivity, low latency, and a wide transmission window. Hollow-core antiresonant fiber has developed rapidly in recent years. Double-nested hollow-core antiresonant fibers in the telecommunications band have consistently broken previous loss records, approaching the loss threshold of commercial single-mode silica glass fibers. Hollow-core antiresonant fiber technology has also reached the early stages of commercialization and is finding applications in optical communications, high-power pulse transmission, infrared transmission, and many other fields. Therefore, designing a new hollow-core antiresonant fiber polarization beam splitter to address the technical challenges of traditional optical fiber beam splitters, such as narrow bandwidth, long device length, and low extinction ratio, is of great significance for promoting the application of hollow-core antiresonant fibers.

[0003] Hollow-core antiresonant optical fibers have a variety of design structures. Taking advantage of this feature, several antiresonant optical fiber beam splitter designs have emerged in recent years. One of them uses special materials as a substrate to design a polarization beam splitter, such as the optical fiber beam splitter designed by Ma Rui et al. using cycloolefin polymer as a substrate (Ma Rui, Liu Shuo, Chen Haodong et al. Design of antiresonant optical fiber polarization beam splitter in the terahertz band [J]. Optoelectronic Technology Application, 2021, 36(01): 28-33.). 18 hollow-core circular cladding tubes are arranged in three layers in the optical fiber core to form two cores, of which 12 hollow-core circular cladding tubes are placed close to the outer support tube layer of the optical fiber, and the remaining 6 hollow-core circular cladding tubes are arranged symmetrically in groups of three, leaving coupling channels in the two cores. According to simulation results, the length of this polarization beam splitter is 119 cm, and the bandwidth with an extinction ratio less than -15 dB is about 2.2 μm. Due to the special internal structure of this optical fiber polarization beam splitter, it can only be used in the terahertz band. In another fiber polarization beam splitter designed by Jia et al. in 2021 (Jia H, Wang X, Zhao T, et al. Ultrawide bandwidth single-mode polarization beam splitter based on dual-hollow-core anti-resonant fiber [J]. Applied optics), eight pure quartz glass cladding tubes are arranged adjacent to each other to form two symmetrical cores. Each core is surrounded by five pure quartz glass cladding tubes similar to the five petals of cherry blossoms, leaving an air gap between the two cores as a coupling channel. A nested tube is added to each quartz cladding tube to form a double anti-resonance to reduce the leakage of mode energy. According to simulation results, the operating bandwidth of this fiber polarization beam splitter is as long as 460nm. However, when forming the double cherry core, the cladding tubes contact each other due to the lack of stable mechanical support to form a stable structure, so a resonance point is formed on the surface of the cladding tube, and the beam splitter is relatively long.

[0004] In addition, researchers have also proposed another method for designing optical fiber polarization beam splitters, which is to introduce an asymmetric structure into the core of the optical fiber to destroy the rotational symmetry of the optical fiber, such as (Lu, Y., et al., A THz fiber polarization splitter based on anti-resonant hollow-core fiber with the asymmetric dual-suspended cores. Frontiers in Physics, 2023.11.) Introducing 8 circular cladding tubes arranged along the outer support tube layer, two hollow-core glass tubes are supported by a thin bracket in the fiber core, and a hollow glass tube is supported by a thin bracket at the other end of the x-direction symmetrically, and the radius of this glass tube is different from that of the previous two glass tubes. The coupling length of this optical fiber polarization beam splitter at 1THz reaches 0.37cm, but because the asymmetric structure of this optical fiber forms a single core in the fiber core, the single-mode transmission effect is poor, and the complex structure is not conducive to production.

[0005] Antiresonant beam splitters made based on existing technologies are limited by their structural characteristics. While ensuring a simple structure, they often cannot guarantee stable transmission characteristics within a longer wavelength range. The present invention adopts a special cladding structure to enable the antiresonant beam splitter to overcome the wavelength dependence to a certain extent and achieve good single-mode transmission characteristics at the operating wavelength. Summary of the Invention

[0006] To solve the above technical problems, the present invention provides an ultra-wideband dual-core negative curvature fiber polarization beam splitter with a paperclip cladding structure. By using eight paperclip-shaped quartz glass tubes with nested tubes inside to form a special cladding structure, each cladding tube has stable mechanical support, avoiding contact between the cladding tubes to produce resonance points, and overcoming wavelength dependence to a certain extent. Two fiber cores are formed in the structure that are symmetrical in the y direction, which can effectively confine the four supermode modes in the fiber cores. By introducing the mode coupling theory between the two fiber cores, the limiting loss and output power of the four supermode modes in the two fiber cores are effectively controlled. At the same time, by controlling the fiber radius and the radius of the cladding tube in the fiber core, the core coupling channel width is controlled, achieving the technical effects of an extinction ratio higher than 20dB in a bandwidth of 1.34-1.88μm and up to 540nm, and an extinction ratio of higher-order modes greater than 100 in the 1.48-1.55μm band.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0008] An ultra-wideband dual-core negative-curvature fiber polarization beam splitter with a paperclip cladding structure comprises an outer support tube layer with a radius R, an inner core region, and a cladding region between the outer and inner core regions. The cladding region comprises eight quartz tubes in a paperclip-shaped structure, consisting of an outer cladding tube and an inner nested tube. The nested tubes and cladding tubes are both semi-oblong, formed by two straight sides and a semicircular side, and are coaxially arranged.

[0009] The specific arrangement of the eight quartz tubes is as follows: the two mutually perpendicular diameters of the cross-section circle of the outer support tube layer are the y-axis and the x-axis respectively, including the third cladding tubes arranged at both ends of the x-axis and arranged along the x-axis direction and the third nested tubes nested inside; the second cladding tubes and the second nested tubes nested inside the third cladding tubes located at the upper and lower sides of the x-axis and parallel to the third cladding tubes; and the first cladding tubes and the first nested tubes nested inside the third cladding tubes located at both ends of the y-axis and arranged along the y-axis direction.

[0010] The lengths of the straight sides of the first, second, and third cladding tubes are l0, l1, and l2, respectively, and l2>l1>l0; the diameters of the semicircular sides are d0, d1, and d2, respectively, and d2>d1>d0; the lengths of the straight sides of the first, second, and third nested tubes are l3, l4, and l5, respectively, and l5>l4>l3; the diameters of the semicircular sides are k×d0, k×d1, and k×d2, respectively, and k<1; and the spacing between two third cladding tubes arranged on the x-axis is g.

[0011] A further improvement of the technical solution of the present invention is that the thickness of the first cladding tube, the second cladding tube, the third cladding tube, and the first nested tube, the second nested tube, and the third nested tube inside are all the same, which is 0.4 to 0.62 μm.

[0012] A further improvement of the technical solution of the present invention is that the first cladding tube, the second cladding tube, the third cladding tube, and the first nested tube, the second nested tube, and the third nested tube inside are made of pure quartz glass with a refractive index obtained by the Sellmeier equation, and the remaining space is filled with air with a refractive index of 1.

[0013] A further improvement of the technical solution of the present invention is that the length l0 of the straight side of the first cladding tube is The straight side length l1 of the second cladding tube is The straight side length l2 of the third cladding tube is

[0014] A further improvement of the technical solution of the present invention is that the straight side length l3 of the first nested tube is The straight side length l4 of the second nested tube is The straight side length l5 of the third nested tube is

[0015] A further improvement of the technical solution of the present invention is that the semicircular side diameter d0 of the first cladding tube is 9-11 μm, the semicircular side diameter d1 of the second cladding tube is 14-15 μm, and the semicircular side diameter d2 of the third cladding tube is 16-18 μm.

[0016] A further improvement of the technical solution of the present invention is that the ratio k is 0.4 to 0.6.

[0017] A further improvement of the technical solution of the present invention is that the radius R of the outer support tube layer is 30 μm.

[0018] A further improvement of the technical solution of the present invention is that the spacing g between two third cladding tubes arranged on the x-axis is 2.0-3.0 μm.

[0019] Due to the adoption of the above technical solution, the technical advancements achieved by the present invention are:

[0020] The present invention adopts a special cladding structure, which greatly expands the applicable wavelength of the beam splitter while maintaining good single-mode characteristics.

[0021] (1) The present invention uses a non-contact paper clip structured quartz tube, so that the quartz tubes do not contact each other, eliminating the resonance point, and enabling the beam splitter to overcome the wavelength dependence to a certain extent, achieving a good beam splitting effect over an ultra-long bandwidth of 1.34 to 1.88 μm.

[0022] The present invention utilizes a paperclip-shaped quartz tube to provide stable mechanical support between the cladding tube and the outer support tube, eliminating contact between the cladding tubes to maintain stable support while also forming a dual-core structure. Appropriate gaps are left between each paperclip-shaped quartz tube to control the coupling between the core mode and the cladding mode. While keeping the limiting loss of the core mode at a low level, the refractive index of the x-polarized light and the refractive index of the y-polarized light gradually decrease, achieving numerical matching near the operating wavelength. The transmission distance required for x-polarized light to be fully coupled from one core to another is approximately 1:2 compared to the transmission distance required for y-polarized light. The polarization extinction ratio of the beam splitter is well above 20 dB, resulting in an excellent beam splitting effect.

[0023] In addition, increasing the thickness of the cladding tube changes the antiresonance reflection phase of the core optical signal at the cladding tube, and the antiresonance window of the optical fiber moves from the original wavelength to a longer wavelength. The overlapping area between the antiresonance wavelength and the working wavelength becomes longer, and the loss of the optical fiber is controlled, so that the loss of the optical fiber at a longer bandwidth is kept at a low level, overcoming the wavelength dependence to a certain extent.

[0024] On this basis, increasing the coupling channel width not only increases the spacing between the two cladding tubes at both ends in the x-direction, but also enhances the coupling strength between the two fiber cores. This raises the lower limit of the core mode's refractive index, thereby re-enabling the refractive index in the two polarization directions to numerically match as the wavelength varies, consistently achieving excellent beam splitting effects over ultra-long bandwidths. Furthermore, the present invention adds nested tubes within the fiber cladding tubes, leveraging the dual antiresonance effect to shift the fiber's antiresonance window to the right. This increases the fiber's refractive index for the core mode, achieving numerical matching of the refractive indices of x- and y-polarized light. The transmission distance required for complete coupling of x-polarized light from one core to the other stabilizes at approximately a 1:2 ratio, overcoming wavelength dependence.

[0025] (2) The present invention uses a paperclip-shaped cladding tube, and the energy leakage of the core fundamental mode is suppressed near the operating wavelength, achieving low-loss transmission while achieving good single-mode transmission performance.

[0026] In order to stabilize the single-mode performance of the optical fiber polarization beam splitter, the present invention uses 8 paperclip-shaped cladding tubes to form a dual-core structure, and at the same time adds nested tubes in the cladding tubes to achieve double anti-resonance to limit the leakage of core energy. The core's ability to limit optical signals is proportional to the number of layers of the cladding tube. When the optical signal is transmitted in the core, part of the light returns to the core through anti-resonance for transmission, while the other part of the light leaks into the cladding tube. When energy leaks into the outer cladding tube, it encounters the inner anti-resonance cladding tube, achieving multiple anti-resonances, and further reflecting from the cladding back to the core, further reducing the loss. The realization of multiple anti-resonances depends on the reflection and leakage process of light in the cladding tube. This beam splitter adopts a double-layer anti-resonance tube design, which makes the design of the optical fiber simple and easy to manufacture while maintaining the low loss of the optical fiber.

[0027] On this basis, the wall thickness of the cladding tube and the nested tube is increased to enhance the antiresonance reflection, but energy still leaks into the cladding tube to form a tube mode. As the wall thickness increases, an intersection appears between the refractive index of the tube mode and the refractive index of the core mode. At this time, the tube mode's inhibitory effect on the core fundamental mode is enhanced, and the energy of the core fundamental mode leaking into the cladding tube returns to the core for transmission. The core fundamental mode is separated from the tube mode, and due to the high refractive index of the high-order mode, the core high-order mode is mixed with the tube mode, and the loss is aggravated. At this time, the limitation loss of the high-order mode in the core is much higher than that of the fundamental mode, and the single-mode performance of the beam splitter is stable.

[0028] In addition, the technical solution of increasing the ratio of the nested tube diameter to the cladding tube diameter enhances the inhibitory coupling effect of the tube mode on the core fundamental mode, enhances the anti-resonance effect of the part of the core energy leaking into the cladding tube, and increases the restriction of the core fundamental mode. In addition, the coupling efficiency of the high-order mode and the tube mode is increased, and its restriction loss is higher than that of the fundamental mode. The HOMER value reaches a peak, and the single-mode performance is stable.

[0029] In summary, the present invention adopts a contactless cladding tube design to provide an ultra-wideband dual-core negative curvature fiber polarization beam splitter with a paperclip cladding structure. Since the resonance point between the quartz cladding tubes is eliminated, the present invention overcomes the wavelength dependence to a certain extent and obtains good single-mode transmission characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a schematic cross-sectional structural diagram of an embodiment of the present invention;

[0031] Figure 2 is the variation of the coupling length and the coupling length ratio with wavelength in the embodiment of the present invention;

[0032] Figure 3 is the variation of the limiting loss and HOMER with wavelength in the embodiment of the present invention;

[0033] Figure 4 This is a normalized power diagram of the fiber core A in the transmission distance range of 0 to 30 cm according to an embodiment of the present invention;

[0034] Among them, 1. outer supporting tube layer, 2. fiber core region, 3. first cladding tube, 4. first nested tube, 5. second cladding tube, 6. second nested tube, 7. third nested tube, 8. third cladding tube. DETAILED DESCRIPTION

[0035] The present invention is described in further detail below in conjunction with the embodiments:

[0036] like Figure 1 The figure shows the specific structure of an embodiment of the present invention. From the outside to the inside, it includes an outer support tube layer 1, a cladding region, and an internal core region 2. The radius of the outer support tube layer 1 is R, which ranges from 25 to 35 μm, and in this embodiment is 30 μm. The cladding region is composed of 8 quartz tubes in a paper clip structure, which are composed of an outer cladding tube and an inner nested nested tube. The nested tube and the cladding tube are both semi-circular with two straight sides and a semicircular side on one side, and the two are arranged coaxially. The 8 quartz tubes in a paper clip structure divide the core into two parts, A and B, forming a dual-core structure.

[0037] The two mutually perpendicular diameters of the cross-section circle of the outer support tube layer are the y-axis and the x-axis respectively. The specific arrangement of the 8 quartz tubes is as follows:

[0038] The third cladding tube 8 is located at both ends of the x-axis and arranged along the x-axis, and the third nested tube 7 is nested inside. The third cladding tube 8 and the third nested tube 7 are symmetrical about the x-axis. The length of the straight side of the third cladding tube 8 is l2, and the value of l2 is calculated using the following formula: The diameter of the semicircular side of the third cladding tube 8 is d2, and the value of d2 is 16-18 μm. In this embodiment, the value is 17.3 μm. The length of the straight side of the third nested tube 7 is l5, and the value of l5 is calculated by the following formula: The diameter of the semicircular side of the third nested tube 7 is k×d2, and the ratio k is 0.4 to 0.6, and is 0.4 in this embodiment.

[0039] The first cladding tube 3 and the first nested tube 4 are located at both ends of the y-axis and arranged along the y-axis. The first cladding tube 3 and the first nested tube 4 are symmetrical about the y-axis. The length of the straight side of the first cladding tube 3 is l0. The value of l0 in this embodiment is calculated using the formula: The diameter of the semicircular side of the first cladding tube 3 is d0, and the value of d0 is 9 to 11 μm. In this embodiment, the value is 11 μm. The length of the straight side of the first nested tube 4 is calculated by the following formula: The diameter of the semicircular side of the first nested tube 4 is k×d0.

[0040] There are four second cladding tubes 5 located on the upper and lower sides of the x-axis, which are parallel to the third cladding tube 8. The second nested tube 6 is nested inside the second cladding tube 5. The length of the straight side of the second cladding tube 5 is l1, which is taken as The diameter of the semicircular side of the second cladding tube 5 is d1, and the value of d1 is 14-15 μm. In this embodiment, the value is 15 μm. The length of the straight side of the second nested tube 6 is The diameter of the semicircular side of the first nested tube 4 is k×d1.

[0041] The quartz tubes in the three aforementioned positions use pure quartz glass as their base material. The size of their diameters determines the size of the core area. When the core area becomes smaller, the core's binding of light energy will increase, and the lower limit of the core mode's refractive index will rise. In addition, due to the special structure of the optical fiber, the light propagating in the optical fiber will be decomposed into four non-degenerate states and couple with each other. According to mode coupling theory, when any polarized light in the x and y polarization directions is fully coupled from one core to another, the shortest distance required is the coupling length. The calculation formula is: This formula shows that to maintain a stable coupling length, the propagation constant and effective refractive index difference between the even and odd modes in the fiber core must be stable. Changing the diameter of the cladding tube in the fiber core changes the effective mode field area of ​​the core, and the refractive index difference between the even and odd modes in the same polarization direction will also change. Furthermore, in the present invention, the even mode refractive index is always higher than the odd mode refractive index. In this embodiment, d0 is set to 11μm, d1 is set to 15μm, and d2 is set to 17.3μm. This ensures that the effective refractive indices of the four fundamental modes in the fiber core vary in the same direction with wavelength, and the coupling length ratio approaches 1:2 and remains essentially unchanged.

[0042] The radius of the outer support tube layer of the optical fiber is R, and the width of the coupling channel between the two fiber cores is g. The cladding tube surrounding the two fiber cores is adjacent to the outer support tube, and the width of the coupling channel between the two fiber cores also depends on the diameter d2 of the cladding tube placed in the fiber core. When d2 remains unchanged, the width of the coupling channel between the two fiber cores will change by changing the radius R of the outer support tube layer of the optical fiber. Change the radius R of the outer support tube layer of the optical fiber: First, as the radius R of the optical fiber increases, the proportion of the cladding tube area and the core area of ​​the optical fiber in the fiber core increases. This means that the mode field area of ​​the optical fiber will also change. This change leads to a decrease in the coupling efficiency between the core mode and the tube mode in the fiber core. As a result, the energy of the core mode is not easily leaked into the tube mode, and therefore the limiting loss of the core mode will be reduced. In the present invention, this reduction is manifested as the fundamental mode LP 01 and higher-order modes LP 11 The limiting loss decreases with the increase of R.

[0043] Adding nested tubes to all cladding tubes achieves double anti-resonance. The diameter of the nested tube is d ni The diameter d of the nested cladding tube i The ratio is k, where i = 0, 1, or 2. When light propagates within the fiber core, a portion of the light returns to the core through the first layer of antiresonance, while another portion leaks into the cladding. When this energy leaks into the first layer of cladding, it encounters the second layer of antiresonance, achieving second-layer antiresonance. This second layer of antiresonance allows the light to be further reflected back into the core, further reducing losses. Specifically, when light leaks into the first layer of cladding, it encounters the second layer of antiresonance. This antiresonance allows the light to reflect within the cladding and re-enter the core along its path. This process of reflection and re-entry further reduces light energy loss. Therefore, by utilizing the multi-layer antiresonance cladding mechanism, optical fibers can effectively reduce losses. This mechanism allows light to reflect and propagate multiple times between the core and cladding, minimizing energy leakage and loss. According to the dual-core fiber coupled mode theory, when light enters the fiber core, the light wave propagates in the form of sine and cosine trigonometric functions. We use P ni The incident power of the fundamental mode in the fiber core is expressed as follows: When the transmission length is 2.55 cm, the normalized power of the x-polarized optical signal in core A is 1, so all optical signals deflected in the x-direction are completely concentrated in core A. In core B, the normalized power of the x-polarized optical signal is 0, indicating that there is no longer any x-polarized optical signal in core B. Conversely, all optical signals deflected in the y-direction are concentrated in core B. Therefore, this design enables the shortest beam splitting effect at a transmission length of 14.1 cm. While maintaining the aforementioned short beam splitter length, the coupling between the core mode and the tube mode can be weakened by varying the width of the coupling channel in the core. This results in an increase in the ratio of the optical power output in the x-polarization direction to the optical power output in the y-polarization direction. The polarization extinction ratio of the optical beam splitter in the 1.34-1.88 μm band exceeds 20 dB, with a bandwidth of 540 nm.

[0044] According to the influence of the above structural parameters on the present invention, it can be known that:

[0045] (1) The diameter of the two cladding tubes at both ends of the y direction is d0, and the diameter of the two cladding tubes at the x direction is d2. A cladding tube with a diameter of d1 is arranged between the two cladding tubes. The diameter of the cladding tube is adjusted to d i The following technical effects will be produced: ① The calculation formula for the core refractive index of hollow-core antiresonant optical fiber is: Where A refers to the effective mode field area of ​​the fiber core. When the cladding tube diameter is adjusted to increase the effective mode field area of ​​the fiber core, the lower limit of the refractive index of the core mode increases. However, the geometric structure of the fiber in the x and y directions does not change significantly. The refractive index of the even mode in the core is always higher than that of the odd mode, so that the coupling length values ​​in the x and y polarization directions at the operating wavelength match, and the coupling length ratio is stable at around 1:2. ② Change the cladding tube diameter d i The overall rotational symmetry of the optical fiber is further destroyed, and the difference between the refractive index in the x-polarization direction and the refractive index in the y-polarization direction changes, but the refractive index in the x-polarization direction is always higher than the refractive index in the y-direction. The slope of the coupling length of the optical fiber in the x- and y-polarization directions changes with the wavelength. The coupling length ratio is always stable at around 1:2 within the range of 540nm, which overcomes the wavelength dependence to a certain extent.

[0046] (2) R is the radius of the outer support tube layer of the optical fiber. Adjusting the size of R will produce the following technical effects: ① Increasing the radius of the outer support tube layer of the optical fiber will increase the spacing between the cladding tubes, increase the core area, and increase the lower limit of the refractive index of the core mode, but will not change the geometric structure of the antiresonant optical fiber. The birefringence of the optical fiber will not change. The coupling length in the y-polarization direction of the core will always be higher than that in the x-polarization direction, and will achieve numerical matching near the operating wavelength. ② As the optical fiber radius increases, the lower limit of the core refractive index increases and the difference in refractive index between the cladding tube and the core increases. The suppression of the fundamental mode by the tube mode increases, and the coupling strength decreases. The limitation loss of the core fundamental mode decreases in an approximately linear relationship with the optical fiber radius, and the limitation loss of the higher-order mode also decreases accordingly. At the operating wavelength, the loss of the higher-order mode is much higher than the loss of the fundamental mode, and the HOMER value reaches a peak value as the optical fiber radius increases.

[0047] (3) k is the ratio of the diameter of all nested tubes to the diameter of the nested cladding tube. Adjusting the size of k will produce the following technical effects: when the ratio of the nested tube diameter to the cladding tube diameter increases, the spacing between the cladding tube and the nested tube becomes smaller, the coupling between the tube mode and the higher-order mode is enhanced, the limiting loss of the higher-order mode increases, and the coupling of the fundamental mode is suppressed, resulting in a decrease in the limiting loss of the fundamental mode. However, because the suppression of higher-order modes is not obvious at the beginning, the HOMER value at the operating wavelength shows a trend of first decreasing and then increasing as the k value increases.

[0048] (4) g is the width of the coupling channel between the two fiber cores. Adjusting the size of g will produce the following technical effects: ① Increasing the coupling channel g will narrow the tube mode transmission channel in the x direction. In addition, since the radius of the outer supporting tube layer of the optical fiber has not changed, the core area increases, which increases the lower limit of the refractive index of the core mode. However, the coupling distance of the optical signal in each polarization direction coupled to the other fiber core remains basically unchanged, and the coupling length ratio always remains near 1:2. ② As the coupling channel increases, the mode field energy between the two fiber cores affects the mode field distribution of the other fiber core, and the loss increases. However, due to the increase in the distance between the cladding tubes, the resonance point between the cladding tubes disappears, and the limiting losses of the high-order mode and the fundamental mode are suppressed. At g of 2.8μm, numerical matching is achieved to achieve good single-mode transmission characteristics.

[0049] (5) t is the thickness of all nested tubes and cladding tubes. Adjusting the size of t will produce the following technical effects: ① When the cladding tube wall t increases, the refractive index difference between the cladding and the hollow core area decreases. This will lead to a decrease in the number of reflections of light between the cladding and the hollow core, thereby limiting the core mode, and the antiresonance window shifts to longer wavelengths as the thickness increases. ② When t increases, the confinement loss of the core high-order mode will first decrease and then increase. At the same time, the confinement loss of the core fundamental mode changes more complexly, resulting in a large range of changes in the HOMER value. Ultimately, the high-order mode suppression ratio is optimized when t is 0.58μm.

[0050] In summary, the present invention discloses an ultra-wideband dual-core negative-curvature fiber polarization beam splitter with a paperclip cladding structure. This structure utilizes a unique cladding structure consisting of an outer support tube and eight paperclip-shaped quartz glass tubes. This structure forms two symmetrical cores connected by an air gap in the y-direction of the core. This design effectively confines four supermode modes within the core. Furthermore, by incorporating the theory of mode coupling between the two cores, the confinement loss and output power of the modes in the core can be effectively regulated. To achieve control over the width of the core coupling channel, the present invention achieves this by controlling the fiber radius and the radius of the cladding tubes within the core. This control method enables the present invention to achieve a bandwidth of 540 nm in the 1.34-1.88 μm range and an extinction ratio exceeding 20 dB. In the 1.48-1.55 μm band, the extinction ratios of higher-order modes are all greater than 100. This demonstrates that the present invention exhibits excellent single-mode characteristics and can achieve high performance across a wide wavelength range.

[0051] As attached Figure 2 The figure shows the relationship between the coupling length in the x-polarization direction, the coupling length in the y-direction, and their coupling length ratio as a function of wavelength for an ultra-wideband dual-core negative curvature fiber polarization beam splitter with a paperclip cladding structure disclosed in the present invention. The figure shows that as the wavelength of polarized light in the x- and y-directions changes toward the long-wave direction, the coupling length increases as the difference between the even-mode refractive index and the odd-mode refractive index decreases. However, due to the better symmetry of polarized light in the y-direction, the refractive index difference changes more slowly than that in the x-direction, and the slope of its coupling length as a function of wavelength is greater than that of polarized light in the x-direction. This results in the coupling length ratio of the anti-resonant fiber polarization beam splitter with a special cladding structure proposed in the present invention gradually increasing within a small range and then decreasing continuously. However, the overall change is not significant and has little impact on performance.

[0052] As attached Figure 3 As shown, it is the fundamental mode LP in the core mode of the present invention 01 and higher-order mode LP 11 The limiting loss and the HOMER value of the higher-order mode suppression ratio vary with wavelength. As can be seen from the figure, due to the double anti-resonance effect caused by the nested tubes in the cladding tube, most of the energy leaking into the cladding tube near the operating wavelength of 1.55μm returns to the fiber core. The loss of the fundamental mode and higher-order modes does not vary significantly with wavelength. However, the HOMER value increases twice with wavelength, then decreases, reaching a peak at the operating wavelength of 1.55μm.

[0053] As attached Figure 4 As shown, Figure 4This is a diagram of the normalized power of core A for the present invention, with a transmission distance range of 0 to 30 cm. After entering the fiber core, the optical signal propagates in a nearly sinusoidal pattern, with the signal's normalized frequency periodically varying between the two cores. The figure shows that when the wavelength is fixed at 1.55 μm, the coupling length ratio (CLR) is fixed at 2, and the transmission length is 14.1 cm, the normalized power of the x-polarization optical signal in core A is 1. At this point, the entire x-polarization optical signal is in core A, the normalized power of the x-polarization optical signal in core B is 0, and there is no x-polarization optical signal in core B. The y-polarization optical signal is also exclusively in core B.

[0054] The above-described embodiments are merely descriptions of preferred embodiments of the present invention and do not limit the scope of the present invention. Persons skilled in the art may make various modifications and improvements to the technical solution of the present invention, provided that they are consistent with the design concept of the present invention. Such modifications and improvements shall be included within the scope of protection defined by the claims of the present invention.

Claims

1. An ultra-wideband dual-core negative curvature optical fiber polarization beam splitter with a paperclip cladding structure, comprising an outer support tube layer (1) with a radius R, an inner core region (2), and a cladding region therebetween; characterized in that: The cladding area is composed of eight quartz tubes in a paper clip structure, consisting of an outer cladding tube and an inner nested tube. The nested tube and the cladding tube are both semi-oblong, formed by two straight sides and a semi-circular side, and are coaxially arranged. The specific arrangement of the eight quartz tubes is as follows: the two mutually perpendicular diameters of the cross-section circle of the outer supporting tube layer are the y-axis and the x-axis respectively, including a third cladding tube (8) located at both ends of the x-axis and arranged along the x-axis direction and a third nested tube (7) nested inside, a second cladding tube (5) located at the upper and lower sides of the x-axis and parallel to the third cladding tube (8) and a second nested tube (6) nested inside, and a first cladding tube (3) located at both ends of the y-axis and arranged along the y-axis direction and a first nested tube (4) nested inside; The lengths of the straight sides of the first cladding tube (3), the second cladding tube (5), and the third cladding tube (8) are l0, l1, and l2, respectively, and l2>l1>l0, and the diameters of the semicircular sides are d0, d1, and d2, respectively, and d2>d1>d0; the lengths of the straight sides of the first nested tube (4), the second nested tube (6), and the third nested tube (7) are l3, l4, and l5, respectively, and l5>l4>l3, and the diameters of the semicircular sides are k×d0, k×d1, and k×d2, respectively, and k<1, and the spacing between the two third cladding tubes (8) arranged on the x-axis is g.

2. The ultra-wideband dual-core negative curvature fiber polarization beam splitter with a paperclip cladding structure according to claim 1, characterized in that: The thickness of the first cladding tube (3), the second cladding tube (5), the third cladding tube (8), and the first nested tube (4), the second nested tube (6), and the third nested tube (7) inside are all the same, ranging from 0.4 to 0.62 μm.

3. The ultra-wideband dual-core negative curvature fiber polarization beam splitter with a paperclip cladding structure according to claim 1, characterized in that: The materials of the first cladding tube (3), the second cladding tube (5), the third cladding tube (8), and the first nested tube (4), the second nested tube (6), and the third nested tube (7) inside are pure quartz glass, the refractive index of which is obtained by the Sellmeier equation, and the other spaces are filled with air with a refractive index of 1.

4. The ultra-wideband dual-core negative curvature fiber polarization beam splitter with a paperclip cladding structure according to claim 1, characterized in that: The straight side length l0 of the first cladding tube (3) is The straight side length l1 of the second cladding tube (5) is The straight side length l2 of the third cladding tube (8) is 5. The ultra-wideband dual-core negative curvature optical fiber polarization beam splitter with a paperclip cladding structure according to claim 1, characterized in that: The straight side length l3 of the first nested tube (4) is The straight side length l4 of the second nested tube (6) is The straight side length l5 of the third nested tube (7) is 6. The ultra-wideband dual-core negative curvature optical fiber polarization beam splitter with a paperclip cladding structure according to claim 4 or 5, characterized in that: The semicircular side diameter d0 of the first cladding tube (3) is 9 to 11 μm, the semicircular side diameter d1 of the second cladding tube (5) is 14 to 15 μm, and the semicircular side diameter d2 of the third cladding tube (8) is 16 to 18 μm.

7. The ultra-wideband dual-core negative curvature fiber polarization beam splitter with a paperclip cladding structure according to claim 4 or 5, characterized in that: The ratio k is 0.4 to 0.

6.

8. The ultra-wideband dual-core negative curvature fiber polarization beam splitter with a paperclip cladding structure according to claim 4 or 5, characterized in that: The radius R of the outer supporting tube layer (1) is 30 μm.

9. The ultra-wideband dual-core negative curvature fiber polarization beam splitter with a paperclip cladding structure according to claim 4 or 5, characterized in that: The distance g between two third cladding tubes (8) arranged on the x-axis is 2.0-3.0 μm.

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