Polarization-maintaining hollow-core photonic crystal fiber

Abstract

The invention relates to a polarization-maintaining hollow-core photonic crystal fiber which comprises a hexagonal cladding area (1) and an air hole fiber core (2), the air hole fiber core (2) is located in the center of the photonic crystal fiber, the cladding area is formed by periodically arranging SiO2 and air holes, and the air holes are arranged in a triangular mode. Two of the innermost layer cladding air holes (4) in the cladding area comprise nested hole structures (3), the nested hole structures (3) are internally tangent to the innermost layer cladding air holes (4), and the internally tangent points are in contact and coincide with the cladding walls of the innermost layer cladding air holes (4). According to the optical fiber disclosed by the invention, two symmetrical nested hole structures are added in a cladding region, so that low loss and wide bandwidth transmission are kept while high birefringence is introduced.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fibers, and in particular relates to a polarization-maintaining hollow-core photonic crystal optical fiber. Background Art

[0002] The bandgap hollow core photonic crystal fiber is formed by periodically arranging SiO 2 The photonic bandgap effect is generated by the air hole, which limits the light wave to the air core in the optical fiber for propagation. Because it uses air to guide light, it greatly reduces the nonlinear coefficient of the optical fiber, which is conducive to suppressing harmful nonlinear processes. At the same time, it has excellent characteristics such as bending insensitivity, high temperature resistance, radiation resistance, low dispersion, low delay and high laser damage threshold. It is a subversive optical fiber technology.

[0003] Hollow-core photonic crystal fiber can change its refractive index distribution by adjusting the size, shape, position and arrangement of the air holes in the fiber cladding, thereby obtaining a higher birefringence. It has unique propagation characteristics that traditional optical fibers cannot achieve, such as flexible structural design, non-cutoff single-mode transmission, high mode birefringence, and high polarization-maintaining performance.

[0004] In the existing bandgap polarization-maintaining hollow-core optical fiber structure, SiO 2 The fiber structure composed of an air hole can achieve a maximum birefringence of 10 -4 However, it has the disadvantages of high loss or narrow bandwidth. Therefore, it is of great significance to design a bandgap photonic crystal polarization-maintaining hollow-core fiber with high birefringence, low loss and wide bandwidth. Summary of the invention

[0005] To solve the above problems, the present invention proposes a polarization-maintaining hollow-core photonic crystal fiber with a nested hole structure added in the cladding region, nested holes are added in the cladding hole region of the basic bandgap structure, and specific fiber structure parameters are adjusted to meet the photonic bandgap effect to confine light in the air hole core, so that high birefringence, low loss and wide bandwidth can be achieved simultaneously.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A polarization-maintaining hollow-core photonic crystal fiber, comprising a cladding region 1 and an air hole core 2 which are hexagonal in shape as a whole, characterized in that: the air hole core 2 is located at the center of the photonic crystal fiber, and the cladding region is made of SiO 2 The air holes are arranged in a periodic manner, and the air holes are arranged in a triangular arrangement. Two of the innermost cladding air holes 4 in the cladding area include a nested hole structure 3. The nested hole structure 3 is inscribed in the innermost cladding air hole 4, and the inscription point is in contact and coincides with the cladding wall of the innermost cladding air hole 4.

[0008] Preferably, the air hole core 2 is quasi-circular, and is composed of 7 removed air holes to form a large air hole, and the 7 removed air holes are the central air hole of the photonic crystal fiber and an innermost air hole layer centered thereon.

[0009] Preferably, the nested hole structure 3 is symmetrically distributed in the horizontal direction in the innermost cladding air holes 4 on the left and right sides of the air hole core 2, with the air hole cladding wall close to the core 2 as the inner side, and the tangent point between the nested hole structure 3 and the innermost cladding air hole 4 is located on the outside of the air hole cladding wall, that is, on the side away from the core.

[0010] Preferably, the wall thickness of the air hole core 2 is t, and the normalized wall thickness is Where Λ is the center distance of the air holes in the cladding region, and d is the diameter of the air holes in the cladding region.

[0011] Preferably, the inner diameter d of the nested hole structure 3 is q is n×d, the wall thickness of the nested hole structure 3 is t q It is m×(Λ-d), where Λ is the center distance of the air holes in the cladding region, d is the diameter of the air holes in the cladding region, m is 0.83-1, and n is 0.35-0.7.

[0012] Preferably, the center spacing Λ of the air holes in the cladding region of the photonic crystal optical fiber is 3.45 μm, and the diameter of the air holes in the cladding region is 3.312 μm.

[0013] Preferably, the wall thickness t of the air hole core 2 of the photonic crystal optical fiber is 0.276 μm.

[0014] Preferably, the operating wavelength of the photonic crystal fiber is 1.55 μm.

[0015] Preferably, the photonic crystal fiber matrix material is SiO 2 .

[0016] Preferably, the inner diameter of the nested pore structure 3 is 2.15 μm, and the wall thickness of the nested pore structure 3 is 0.12 μm.

[0017] The advantages and positive effects of the present invention are as follows:

[0018] The photonic crystal fiber proposed in the present invention introduces high birefringence while maintaining low loss and wide bandwidth transmission by adding two symmetrical nested hole structures in the cladding region. In addition, the fiber proposed in the present invention has a simple structure and high process feasibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the cross section of a polarization-maintaining hollow-core photonic crystal fiber.

[0020] Figure 2 This is a partially enlarged schematic diagram of the cross section of a polarization-maintaining hollow-core photonic crystal fiber.

[0021] Figure 3 for Figure 1 Spectral curve of polarization-maintaining hollow-core photonic crystal fiber with the structure shown.

[0022] Figure 4 for Figure 1 The loss spectrum curves of the polarization-maintaining hollow-core photonic crystal fiber with no nested hole structure added to the cladding region (Structure A) and with the nested hole structure added (Structure B) are compared.

[0023] Figure 5 for Figure 1 Dispersion curves of the core fundamental mode and the nested hole mode in the y direction of the polarization-maintaining hollow-core photonic crystal fiber of the structure shown.

[0024] Among them, 1 is the cladding region, 2 is the air hole core, 3 is the nested hole structure, and 4 is the innermost cladding air hole in the cladding region. DETAILED DESCRIPTION

[0025] In order to better understand the technical solution of the present invention, the content of the present invention includes but is not limited to the specific implementation methods described below, and similar technologies and methods should be considered to be within the scope of protection of the present invention. In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the following will be described in detail with reference to the accompanying drawings and specific embodiments.

[0026] The cross-sectional structure of a polarization-maintaining hollow-core photonic crystal fiber in this example is as follows: Figure 1 As shown, it includes a cladding region 1 which is hexagonal as a whole and an air hole core 2, wherein the air hole core 2 is located at the center of the photonic crystal fiber, and the cladding region is made of SiO 2 The air holes are periodically arranged to satisfy the photon bandgap effect to confine light in the air hole core 2. The air holes are arranged in a triangular arrangement. Two of the innermost cladding air holes 4 in the cladding region include a nested hole structure 3. The nested hole structure 3 is inscribed in the innermost cladding air hole 4 and the inscription point is in contact and coincides with the cladding wall of the innermost cladding air hole 4.

[0027] The overall shape of the cladding region 1 is hexagonal, and the air hole core 2 is quasi-circular, consisting of 7 removed air holes to form a large air hole. The 7 removed air holes are the central air hole of the photonic crystal fiber and an innermost air hole centered thereon.

[0028] The nested hole structure 3 is located in the innermost cladding air hole corresponding to the cladding area, where the shape of the air hole is not easily deformed during the optical fiber drawing process, and the nested hole structure can be well maintained. The nested hole structure 3 is symmetrically distributed in the innermost cladding air holes 4 on the left and right sides of the air hole core 2 in the horizontal direction to break the six-fold symmetry, form a two-fold symmetric structure, and introduce high birefringence. In order to reduce the drawing difficulty and achieve better optical fiber performance, the air hole cladding wall close to the core 2 is the inner side, and the intangent point between the nested hole structure 3 and the innermost cladding air hole 4 is located on the outer side of the air hole cladding wall, that is, on the side away from the core, so that the core wall thickness will not be increased.

[0029] The air hole in the cladding region is approximately hexagonal, which is different from the sharp corners of the hexagon. The approximately hexagonal shape in the present application is rounded. This is because the air hole in the cladding region is prepared using a circular capillary, but during drawing, due to the expansion of air pressure and the existence of a triangular area between the circular capillaries, the air hole expands into a shape approximately hexagonal. The matrix material of the photonic crystal fiber is SiO 2 , the refractive index is 1.44 at 1.55μm.

[0030] By introducing a nested hole structure, an additional nested hole mode is introduced into the optical fiber. By adjusting the inner diameter and wall thickness of the nested hole structure, the propagation constant of the nested hole mode is close to or equal to the propagation constant of a certain polarization direction of the core fundamental mode, and it is made to occur in the optical fiber guide band, the core mode and the nested hole mode are coupled, and an anti-crossing phenomenon occurs. At this time, the effective refractive index of the y-polarization fundamental mode changes dramatically, and the difference with the effective refractive index of the x-polarization fundamental mode becomes larger, thereby generating high birefringence.

[0031] Since this optical fiber design is used in optical fiber communication, the parameter optimization aims to achieve the best effect at 1.55μm. The position of the optical fiber guide band is mainly determined by the center spacing Λ of the air holes in the cladding area, the diameter d of the air holes in the cladding area, the inner diameter of the nested hole structure, and the normalized wall thickness T of the air hole core, where the normalized wall thickness Where t is the wall thickness of the air hole core, and the normalized wall thickness will affect the width of the conduction band. In a preferred embodiment of the present application, the air hole diameter d is 3.312 μm, the air hole spacing Λ is 3.45 μm, and the normalized wall thickness T is 0.5. T of 0.5 can ensure that the surface mode scattering is at a low level while achieving a wider bandwidth. The above parameters are only the implementation values ​​of the best effect of the present invention, and the parameter range is not limited to the above values.

[0032] Assume the inner diameter d of the nested hole structure q is n×d, the wall thickness of the nested hole structure is t qis m×(Λ-d); where Λ is the center spacing Λ of the air holes in the cladding region, and d is the diameter of the air holes in the cladding region. The values ​​of m and n will affect the birefringence effect in the conduction band. In order to make the optical fiber have a better birefringence effect, the present application uses the finite element method to simulate the wall thickness and inner diameter of the nested hole structure 3, as shown in the following Table 1. It is finally determined that when n is 0.35-0.7, a higher birefringence can be achieved. Then the inner diameter size of the nested hole structure is divided into units of 0.05, and n is obtained as 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, and 0.7 respectively. After determining the inner diameter size of the nested hole structure, the wall thickness parameters of the nested hole structure with different inner diameter sizes are optimized to minimize the x-direction limiting loss loss(x) and the y-direction limiting loss loss(y) to obtain the maximum birefringence. The inner diameters of different nested hole structures and the wall thickness parameters of the corresponding nested hole structures, the corresponding loss (x) and loss (y) and the bandwidth are shown in Table 1.

[0033] Parameter settings Birefringence loss(x)(dB / km) loss(y)(dB / km) Bandwidth (below 10dB / km) (nm) n=0.35,m=1 3.05E-04 14 25.4 110 n=0.4,m=0.95 5.95E-04 3.13 9.07 110 n=0.45,m=0.89 6.97E-04 0.234 9.50 110 n=0.5,m=0.85 9.55E-04 0.245 7.81 120 n=0.55,m=0.83 8.71E-04 0.312 1.94 120 n=0.6,m=0.84 1.14E-03 0.378 1.45 120 n=0.65,m=0.87 1.32E-03 0.431 1.36 120 n=0.7,m=0.94 1.60E-03 0.553 10.6 120

[0034] Table 1

[0035] A partial enlarged cross-section of a polarization-maintaining hollow-core photonic crystal fiber in this embodiment is shown in FIG. Figure 2 As shown, the parameters are set as follows: the center spacing of the air holes in the cladding region Λ=3.45μm, the diameter of the air holes in the cladding region d=0.96Λ=3.312μm, the normalized wall thickness T of the air hole core 2 is 0.5, the wall thickness t of the air hole core 2 is 0.276μm, and the inner diameter d of the nested hole structure 3 is q =0.65×d=2.15μm, the wall thickness t of the nested pore structure 3 q =0.87×(Λ-d)=0.12μm.

[0036] according to Figure 2 The photonic crystal fiber of the structure shown in the figure is simulated and calculated using the finite element method to obtain the birefringence and loss of the photonic crystal fiber in the 1.48μm-1.66μm band, as shown in FIG. Figure 3 As shown, at 1.55μm working wavelength, the fiber birefringence can reach 1.32x10 -3 , the loss in the x direction is 0.43dB / km, and the loss in the y direction is 1.36dB / km. Figure 2 The loss spectrum curves of the polarization-maintaining hollow-core photonic crystal fiber without nested hole structure (structure A) and with nested hole structure (structure B) are compared, as shown in Figure 2. Figure 4As shown, it can be seen that the optical fiber fundamental mode can be transmitted with low loss in the 1.52μm-1.64μm band, and the minimum loss in the x-direction can reach 0.43dB / km, which is 0.029dB / km lower than when the nested hole structure is not added. The minimum loss in the y-direction can reach 1.36dB / km. Although it is higher than the loss when the nested hole structure is not added, it is lower than the loss value of other existing structures, and the best birefringence effect can be achieved at this time.

[0037] Figure 5 for Figure 1 , Figure 2 The dispersion curves of the core fundamental mode and the nested hole mode of the polarization-maintaining hollow-core photonic crystal fiber in the y direction are shown. Figure 5 It can be seen that with the increase of wavelength, the effective refractive index of the nested hole mode and the core fundamental mode gradually approach each other, and mode coupling occurs between the two near 1.55μm, resulting in high birefringence.

[0038] The polarization-maintaining hollow-core photonic crystal fiber of the present invention symmetrically adds two nested hole structures in the innermost cladding air hole 4 in the cladding region, thereby introducing high birefringence while maintaining low loss and wide bandwidth transmission. It can transmit at low loss in the 1.52μm-1.64μm band, and at a working wavelength of 1.55μm, the birefringence reaches 1.32x10 -3 , at this time, the loss in the x direction is 0.43dB / km, and the loss in the y direction is 1.36dB / km. It can be adapted to existing quartz core optical fibers and related devices, and can be applied to low-loss optical signal transmission.

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. A person skilled in the art may modify the technical solutions described in the present invention, or replace some of the technical features therein with equivalents, and these modifications or replacements do not make the corresponding technical solutions deviate from the scope described in the present invention.

Claims

1. A polarization-maintaining hollow-core photonic crystal fiber, comprising a cladding region (1) which is generally hexagonal and an air-hole core (2), characterized in that: The air hole core (2) is located at the center of the photonic crystal optical fiber, the cladding region is composed of SiO2 and air holes arranged periodically, the air holes are arranged in a triangular arrangement, two of the innermost cladding air holes (4) in the cladding region include a nested hole structure (3), the nested hole structure (3) is inscribed with the innermost cladding air hole (4), and the inscription point is in contact with and coincides with the cladding wall of the innermost cladding air hole (4).

2. A polarization-maintaining hollow-core photonic crystal fiber according to claim 1, characterized in that: The air hole core (2) is quasi-circular and is composed of 7 removed air holes to form a large air hole. The 7 removed air holes are the central air hole of the photonic crystal optical fiber and an innermost air hole layer centered thereon.

3. A polarization-maintaining hollow-core photonic crystal fiber according to claim 1, characterized in that: The nested hole structures (3) are symmetrically distributed in the horizontal direction in the innermost cladding air holes (4) on the left and right sides of the air hole core (2), with the air hole cladding wall close to the core (2) as the inner side, and the intangent point between the nested hole structures (3) and the innermost cladding air holes (4) is located on the outer side of the air hole cladding wall, that is, on the side away from the core.

4. A polarization-maintaining hollow-core photonic crystal fiber according to claim 1, characterized in that: The wall thickness of the air hole core (2) is t, and the normalized wall thickness is Where Λ is the center distance of the air holes in the cladding region, and d is the diameter of the air holes in the cladding region.

5. A polarization-maintaining hollow-core photonic crystal fiber according to claim 1, characterized in that: The inner diameter d of the nested hole structure (3) q is n×d, the wall thickness t of the nested hole structure (3) q It is m×(Λ-d), where Λ is the center distance of the air holes in the cladding region, d is the diameter of the air holes in the cladding region, m is 0.83-1, and n is 0.35-0.

7.

6. A polarization-maintaining hollow-core photonic crystal fiber according to claim 1, characterized in that: The center spacing Λ of the air holes in the cladding region of the photonic crystal optical fiber is 3.45 μm, and the diameter of the air holes in the cladding region is 3.312 μm.

7. A polarization-maintaining hollow-core photonic crystal fiber according to claim 1, characterized in that: The wall thickness t of the air hole core (2) of the photonic crystal optical fiber is 0.276 μm.

8. A polarization-maintaining hollow-core photonic crystal fiber according to claim 1, characterized in that: The working wavelength of the photonic crystal optical fiber is 1.55 μm.

9. A polarization-maintaining hollow-core photonic crystal fiber according to claim 1, characterized in that: The photonic crystal optical fiber matrix material is SiO2.

10. A polarization-maintaining hollow-core photonic crystal fiber according to claim 1, characterized in that: The inner diameter of the nested pore structure (3) is 2.15 μm, and the wall thickness of the nested pore structure (3) is 0.12 μm.

Images