Circularly polarized light beam splitter based on spiral double-core anti-resonance hollow-core optical fiber

Abstract

The invention discloses a circularly polarized light beam splitter based on a spiral double-core anti-resonance hollow-core optical fiber, a cladding of the circularly polarized light beam splitter comprises six circular nested tubes and two elliptical nested tubes dividing the interior of the optical fiber into two symmetrical fiber cores, and when light is propagated in one fiber core, the light is coupled to the other fiber core mainly through gaps of the elliptical tubes; meanwhile, the optical fiber is designed into a double-core spiral chiral structure along the light transmission direction, so that four super-mode fundamental modes of the optical fiber are changed into circular polarization modes, namely a left circular polarization odd mode and a coupling mode and a right circular polarization odd mode and an even mode; afterwards, the effective refractive indexes of two odd-even modes of different polarization are adjusted by optimizing optical fiber structure parameters, so that a proper mode coupling length is obtained, and a good beam splitting effect on circular polarization is achieved. And finally, by adjusting the size of the nested inner tube, the coupling of a fiber core high-order mode and a cladding mode is increased, so that the high-order mode limiting loss is increased, and the single-mode performance of the optical fiber is ensured.

Description

Technical Field

[0001] The invention belongs to the technical field of polarization beam splitters, and in particular relates to a double-core hollow-core anti-resonance optical fiber polarization beam splitter with a spiral structure. Background Art

[0002] Solid core optical fibers based on quartz glass cannot achieve better optical transmission performance due to their inherent Rayleigh scattering, dispersion, material absorption and nonlinear effects, while the emergence of hollow core optical fibers has broken through these limitations. Hollow core optical fibers include photonic crystal hollow core optical fibers and antiresonant hollow core optical fibers. Among them, photonic crystal hollow core optical fibers confine light in the hollow core through the principle of photonic band gap, but their development is also limited due to their inability to reduce surface scattering losses. Compared with photonic crystal hollow core optical fibers, antiresonant hollow core optical fibers can overcome the defects of surface scattering losses of photonic crystal hollow core optical fibers while ensuring the transmission of light in the air medium, and have a simpler cladding structure. Therefore, antiresonant hollow core optical fibers have great potential. At present, the minimum loss of antiresonant hollow core optical fibers has been as low as 0.8dB / km, and they have become a strong competitor to solid core optical fibers.

[0003] Optical passive devices are an indispensable part of building an optical fiber system. If a solid-core optical fiber device is directly connected to an antiresonant hollow-core optical fiber, the stability and efficiency of the optical system will be seriously affected due to the large Fresnel reflection or mode mismatch at the silica-air interface where the solid-core device and the hollow-core optical fiber are connected. Therefore, it is of great significance to study optical passive devices based on hollow-core antiresonant optical fibers.

[0004] Polarization beam splitters play an important role in optical passive devices. They can split a beam of light into two mutually orthogonal polarized lights in space and are widely used in information processing, communication, sensing and other aspects. At present, linear polarization beam splitters based on dual-core antiresonant hollow-core fibers have been fully studied. The beam splitting principle is: dual-core antiresonant hollow-core fibers produce four linear polarization supermode fundamental modes, which are generally divided into odd and even modes polarized in the x direction, and odd and even modes polarized in the y direction. The odd and even modes with the same polarization direction will couple with each other when propagating in the optical fiber. Due to the different supermode coupling lengths in different polarization directions, when the light propagates a certain distance in one core, one polarization light is completely coupled to the other core, and the other polarization light remains in the original core, thereby achieving the purpose of linear polarization beam splitting. At present, there are some problems in the application of linear polarization beam splitters based on antiresonant hollow-core fibers. For example, there may be misalignment of the polarization axis during connection, and there is squeezing in actual application, resulting in polarization errors and noise. At this time, the use of circular polarization beam splitters can suppress such problems. Summary of the invention

[0005] The object of the present invention is to provide a circularly polarized light beam splitter based on a spiral double-core anti-resonant hollow-core optical fiber with low loss, good single-mode performance and good beam splitting effect.

[0006] In order to solve the above technical problems, the present invention is achieved through the following technical solutions:

[0007] The circular polarization beam splitter based on helical double-core anti-resonant hollow-core fiber has the following characteristics:

[0008] The optical fiber includes a circular outer tube and 8 nested tubes inscribed with the circular outer tube; the 8 nested tubes are arranged at intervals on the inner wall of the circular outer tube and fused with the circular outer tube at the inscribed point; the 8 nested tubes are arranged in a spiral along the axial direction of the optical fiber. The 8 nested tubes are arranged symmetrically along the cross section of the optical fiber, one pair of nested tubes are elliptical nested tubes, and the remaining nested tubes are circular nested tubes; the two elliptical nested tubes divide the optical fiber into two symmetrical cores, which are respectively recorded as A core and B core. When in use, light is injected from the A core. Due to the different coupling lengths of different circularly polarized lights, after propagating for a certain distance, the left circularly polarized light and the right circularly polarized light of a specific wavelength are completely separated in the A core and the B core, realizing the role of a circularly polarized light beam splitter.

[0009] Furthermore, the eight nested tubes form a left-handed helical structure along the axial direction of the optical fiber, that is, along the light propagation direction, and the twist rate α is 0.9 rad / mm.

[0010] Furthermore, the included angle between the central axes of the cross sections of two adjacent nested tubes is 45°.

[0011] Furthermore, the nested tube material is quartz glass, and the refractive index is determined by the Sellmeier formula; the other areas in the optical fiber are air, and the refractive index is 1.

[0012] In order to get rid of the defects of the linear polarization beam splitter, the present invention adopts a dual-core anti-resonant hollow-core optical fiber with a spiral structure. Different from the traditional non-spiral linear polarization beam splitter, the spiral structure makes the supermode in the dual-core anti-resonant hollow-core optical fiber a circular polarization mode, and the four supermodes generated are left circular polarization odd mode, left circular polarization even mode, and right circular polarization odd mode and right circular polarization even mode. When the phase difference between the modes in the two cores is π, it is an odd mode, and when the phases are the same, it is an even mode. When light propagates in the core, modes with the same polarization direction will couple with each other. According to the symmetric waveguide mode coupling theory, the coupling length of modes with different polarization directions can be calculated by the following formula:

[0013]

[0014] in, represents the coupling length corresponding to the circularly deflected mode, Δβ LCP / RCPIt represents the effective refractive index difference between the odd mode and the even mode corresponding to the circularly deflected mode. When the coupling length ratio between the modes is 0.5 or 2, the best beam splitting effect exists.

[0015] In order to achieve the above effect, the radius of the spiral dual-core anti-resonant hollow-core optical fiber designed by the present invention (that is, the inner radius R of the circular outer sleeve) is 36μm, and the cladding structure is composed of eight nested tubes evenly arranged, and the wall thickness t of the nested tubes is 0.55μm. Each nested tube includes an outer tube and an inner tube, and the inner tube is inscribed with the inner wall of the outer tube, and the tangent point position is consistent with the tangent point position of the outer tube and the circular outer sleeve. The cross-sections of the outer tube and the inner tube of the elliptical nested tube are both elliptical, and the cross-sections of the outer tube and the inner tube of the circular nested tube are both circular.

[0016] The outer tube of the elliptical nested tube has a major semi-axis length of 17.875 μm and a minor semi-axis length of 8.3 μm; the inner tube of the elliptical nested tube has a major semi-axis length of 10 μm and a minor semi-axis length of 5 μm. It divides the interior of the optical fiber into two symmetrical fiber cores, so as to spatially split the left circular polarization and the right circular polarization.

[0017] The outer tube diameter (D 1 ) is 18 μm, and the inner diameter of the two circular nested tubes that are not adjacent to the two elliptical nested tubes (D 2 ) is 10 μm, and the inner diameter of the four circular nested tubes adjacent to any elliptical nested tube (D 3 ) is 6μm. Specifically, the central axis direction of the cross section of the two elliptical nested tubes can be defined as the Y-axis direction, that is, the two elliptical nested tubes are distributed in the Y direction; then the circular nested tubes that are not adjacent to the two elliptical nested tubes are circular nested tubes distributed in the X-axis direction perpendicular to the Y-axis, and the circular nested tubes adjacent to any elliptical nested tubes are circular nested tubes distributed in the direction of 45° with both the Y-axis and the X-axis. Different inner tube diameters allow the high-order mode of the core to couple more with the cladding mode, thereby increasing the loss of the high-order mode. At the same time, the nested structure reduces the core fundamental mode loss, thereby improving the high-order mode suppression ratio, where the high-order mode suppression ratio is the ratio of the lowest loss of the high-order mode to the highest loss of the fundamental mode.

[0018] The beneficial effects of the present invention are embodied in:

[0019] The circular polarization beam splitter based on the dual-core anti-resonant hollow-core fiber disclosed in the present invention comprises 6 circular nested tubes and 2 elliptical nested tubes, the angle between each nested tube is 45°, and the overall distribution is circular. Among them, the two elliptical tubes separate the inside of the optical fiber into two symmetrical cores, and when the light propagates in one of the cores, it is mainly coupled to the other core through the gap of the elliptical tubes. At the same time, the optical fiber is designed into a dual-core spiral chiral structure along the light transmission direction, so that the four supermode fundamental modes of the optical fiber are changed into circular polarization modes, namely, left circular polarization odd mode and even mode, right circular polarization odd mode and even mode. Afterwards, the effective refractive index of the two odd and even modes of different polarization is adjusted by optimizing the optical fiber structure parameters to obtain a suitable mode coupling length, so as to achieve a good splitting effect for circular polarization. Finally, by adjusting the size of the nested inner tube to increase the coupling of the core high-order mode and the cladding mode, the high-order mode loss is increased, thereby ensuring the single-mode performance of the optical fiber. Finally, a spiral hollow-core optical fiber circular polarization beam splitter capable of spatially splitting the circularly biased mode and having good single-mode characteristics is realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for describing the embodiments are briefly introduced below.

[0021] Figure 1 A schematic diagram of the cross-sectional structure of a circular polarization beam splitter provided in an embodiment of the present invention;

[0022] Figure 2 A schematic diagram of a three-dimensional spiral structure of a circularly polarized beam splitter provided in an embodiment of the present invention;

[0023] Figure 3 A graph showing the relationship between the polarization mode coupling length ratio of the circular polarization beam splitter provided by an embodiment of the present invention and the wavelength;

[0024] Figure 4 A graph showing the relationship between the normalized power of the circularly polarized beam splitter provided in an embodiment of the present invention and the transmission length;

[0025] Figure 5 A graph showing the relationship between the polarization extinction ratio of the circularly polarized beam splitter provided in an embodiment of the present invention and the wavelength;

[0026] Figure 6 A graph showing the relationship between the high-order mode suppression ratio of the circularly polarized beam splitter provided in an embodiment of the present invention and the wavelength. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0028] like Figure 1 As shown, a schematic cross-sectional view of a circular polarization beam splitter designed based on a spiral double-core anti-resonant hollow-core optical fiber proposed in the present invention is given. The optical fiber includes a circular outer tube and 8 nested tubes inscribed with the circular outer tube; the 8 nested tubes are arranged at intervals on the inner wall of the circular outer tube and fused with the circular outer tube at the inscribed point. The 8 nested tubes are arranged symmetrically in pairs along the cross section of the optical fiber, wherein a pair of nested tubes are elliptical nested tubes, and the remaining nested tubes are circular nested tubes.

[0029] like Figure 2 As shown in the figure, the eight nested tubes form a left-handed spiral structure along the axial direction of the optical fiber, that is, along the direction of light propagation, with a twist rate of 0.9 rad / mm. The spiral structure generates a left circular polarization mode and a right circular polarization mode in the optical fiber. The two elliptical nested tubes divide the optical fiber into two symmetrical cores, which are respectively recorded as A core and B core, so as to split the two circular polarization modes in space. Among them, the wall thickness t of the nested tubes is 0.55μm. The long semi-axis (R 1 ) and the minor axis (R 2 ) are 17.875 μm and 8.3 μm in length, respectively. The semi-major axis of the inner tube of the elliptical nested tube (R 3 ) and the minor axis (R 4 ) lengths are 10μm and 5μm respectively. Under this parameter, the elliptical nested tube can reduce the supermode fundamental mode loss generated by the optical fiber while maintaining an appropriate coupling length ratio. The outer tube diameter D of the circular nested tube 1 The inner diameter of the circular nested tubes located in the x direction (i.e., the two circular nested tubes that are not adjacent to the two elliptical nested tubes) is larger, at 10μm. The inner diameter of the circular nested tubes located in the 45° direction (i.e., the circular nested tubes adjacent to any elliptical nested tubes) is smaller, at 6μm. Under this structural design, the high-order modes generated in the optical fiber are more coupled with the cladding tube modes in the 45° direction, which increases the high-order mode loss while ensuring the low loss of the fundamental mode, thereby improving the high-order mode suppression ratio.

[0030] like Figure 3 As shown, the coupling length and coupling length ratio of the two circularly polarized modes in the helical double-core hollow-core antiresonant fiber are calculated based on the coupled mode theory and transformation optics. The mode coupling length can be calculated by formula (1), and the mode coupling length ratio can be calculated by the following formula:

[0031]

[0032] The length of the designed circular polarization beam splitter When the CLR value is 0.5 or 2, the length of the circular polarization beam splitter is the shortest and can ensure a good beam splitting effect. In this embodiment, when the operating wavelength increases from 1.35 μm to 1.65 μm, From 17.6cm to 9.1cm, From 8.5cm to 5.9cm, CLR increases from 0.4829 to 0.6483. Set the length of the circular polarization beam splitter to 15.6cm. At this time, its operating wavelength is at 1.4μm. 15.6cm, It is 8.1cm and the CLR is 0.5192.

[0033] According to the mode coupling theory, the relationship between the polarization mode output power and the propagation distance in the optical fiber can be calculated by the following formula:

[0034]

[0035] in, Indicates the output power of left-handed polarization mode or right-handed polarization mode, P in represents input power, and L represents propagation distance. Based on the mode coupling length of the designed circular polarization beam splitter at 1.4 μm, the output power of different polarization modes is calculated. The results are as follows: Figure 4 As shown in the figure, the best beam splitting effect is achieved at 15.6 cm.

[0036] The polarization extinction ratio is an important indicator for judging the beam splitting performance of a polarization beam splitter and can be calculated by the following formula:

[0037]

[0038] The polarization extinction ratio of the circular polarization beam splitter in the embodiment is calculated by formula (4) to obtain the relationship between the polarization extinction ratio and the wavelength. The result is as follows: Figure 5 When |ER|>20dB, the polarization beam splitter is considered to have effective beam splitting performance. In the embodiment, when the working wavelength is between 1.374 and 1.428 μm, the polarization extinction ratio is greater than 20 dB, that is, the designed circular polarization beam splitter has an operating bandwidth of 54 nm.

[0039] The designed circular polarization beam splitter should have good single-mode performance, and its single-mode performance can be calculated based on the high-order mode suppression ratio of the optical fiber, which is obtained by dividing the minimum loss of the high-order mode by the maximum loss of the fundamental mode. In this embodiment, the relationship between the high-order mode suppression ratio (homer), the minimum loss of the high-order mode (CLhm), and the maximum loss of the fundamental mode (CLfm) and the wavelength is as follows: Figure 6 Structurally, by reducing the size of the inner tube of the nested tube at 45°, the coupling between the high-order mode and the inner tube mode is increased to increase the high-order mode loss, thereby improving the high-order mode suppression ratio, and finally achieving a high-order mode suppression ratio greater than 100 in the range of 1.30 to 1.55 μm.

[0040] This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can better understand and utilize the present invention.

Claims

1. A circularly polarized beam splitter based on a helical double-core anti-resonant hollow-core fiber, characterized by: The optical fiber comprises a circular outer sleeve and 8 nested tubes inscribed in the circular outer sleeve; the 8 nested tubes are arranged at intervals on the inner wall of the circular outer sleeve; The eight nested tubes are arranged in a spiral pattern along the axial direction of the optical fiber; The eight nested tubes are arranged symmetrically along the cross section of the optical fiber, one pair of the nested tubes are elliptical nested tubes, and the remaining nested tubes are circular nested tubes; the two elliptical nested tubes divide the optical fiber into two symmetrical cores, which are respectively recorded as A core and B core; When in use, light is injected from core A. Due to the different coupling lengths of different circularly polarized lights, after a certain distance of transmission, the left circularly polarized light and the right circularly polarized light of a specific wavelength are completely separated in core A and core B, thus realizing the role of a circularly polarized light beam splitter.

2. The circularly polarized light beam splitter based on helical double-core anti-resonant hollow-core fiber according to claim 1, characterized in that: The eight nested tubes form a left-handed spiral structure along the axial direction of the optical fiber, that is, along the direction of light propagation, with a twist rate α of 0.9 rad / mm.

3. The circularly polarized light beam splitter based on helical double-core anti-resonant hollow-core fiber according to claim 1, characterized in that: The inner radius R of the circular outer sleeve is 36 μm.

4. The circularly polarized light beam splitter based on helical double-core anti-resonant hollow-core fiber according to claim 1, characterized in that: The included angles of the central axes of the cross sections of two adjacent nested tubes are all 45°.

5. The circularly polarized light beam splitter based on helical double-core anti-resonant hollow-core fiber according to claim 1, characterized in that: The nested tube material is quartz glass, and the refractive index is determined by the Sellmeier formula; the other areas in the optical fiber are air, and the refractive index is 1.

6. The circularly polarized light beam splitter based on helical double-core anti-resonant hollow-core fiber according to claim 1, characterized in that: The wall thickness t of the nested tubes is 0.55 μm.

7. The circularly polarized light beam splitter based on helical double-core anti-resonant hollow-core fiber according to claim 1, characterized in that: The outer tube major semi-axis R1 of the elliptical nested tube is 17.875 μm, the minor semi-axis R2 is 8.3 μm, the inner tube major semi-axis R3 is 10 μm, and the minor semi-axis R4 is 5 μm.

8. The circularly polarized light beam splitter based on helical double-core anti-resonant hollow-core fiber according to claim 1, characterized in that: The outer tube diameter D1 of the circular nested tubes is 18 μm, the inner tube diameter D2 of the two circular nested tubes not adjacent to the two elliptical nested tubes is 10 μm, and the inner tube diameter D3 of the four circular nested tubes adjacent to any elliptical nested tube is 6 μm.

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