1. A broadband single polarization residual dispersion compensating photonic crystal fiber, comprising: the optical fiber structure comprises an optical fiber structure taking quartz as a substrate material, wherein the optical fiber structure is sequentially provided with a fiber core area, an inner cladding and an outer cladding from inside to outside, the fiber core area is composed of a fiber core central hole and a surrounding quartz substrate, the interior of the fiber core central hole is filled with a liquid crystal material, the outer cladding comprises second hexagonal array arrangement air holes and first hexagonal array arrangement air holes which are sequentially arranged from inside to outside, and the first hexagonal array arrangement air holes and the second hexagonal array arrangement air holes and the inner cladding form a microstructure end surface structure which is completely symmetrical in the horizontal and vertical directions;

and applying an external electric field to the liquid crystal material filled in the central hole of the fiber core, and adjusting the rotation angle of the director direction of the liquid crystal molecules by adjusting the size of the external electric field to compensate the residual dispersion in the orthogonal polarization direction.

2. The broadband single-polarization residual dispersion compensating photonic crystal fiber of claim 1, wherein the first and second hexagonal arrays of air holes are each formed by regular hexagonal arrangement of air holes, the first hexagonal array of air holes has a number of 18 air holes, the second hexagonal array of air holes has a number of 12 air holes, the air holes have a pitch of 3.5-3.52 μm, and the air holes have a diameter of 1.39-1.41 μm.

3. The broadband single polarization residual dispersion compensating photonic crystal fiber of claim 2, wherein the central core hole is an elliptical core hole, the elliptical core hole and the surrounding quartz substrate form a C2v symmetric structure, and the long axis direction of the elliptical core hole is parallel to the x-axis direction.

4. The broadband single polarization residual dispersion compensation photonic crystal fiber of claim 3, wherein the inner cladding is composed of four first elliptical air holes and two second elliptical air holes arranged in a regular hexagon, the major axis direction of the first elliptical air holes is parallel to the y-axis direction, and the major axis direction of the second elliptical air holes is parallel to the x-axis direction.

5. The broadband single polarization residual dispersion compensating photonic crystal fiber of claim 4, wherein the major axis of the core elliptical hole is 1.8-1.82 μm, the minor axis of the core elliptical hole is 0.9-0.92 μm, the major axis of the first elliptical air hole is 4 μm, the minor axis of the first elliptical air hole is 2.5 μm, the major axis of the second elliptical air hole is 4.8 μm, and the minor axis of the second elliptical air hole is 1.6 μm.

6. The broadband single-polarization residual dispersion compensation photonic crystal fiber of claim 2, wherein the central core hole is a circular core hole, two first air holes are disposed on two sides of the circular core hole, the inner cladding is composed of four enlarged air holes arranged in a regular hexagon and two second air holes, the two enlarged air holes are grouped in pairs and disposed on the upper and lower sides of the circular core hole, respectively, and the second air holes are disposed on the left and right sides of the first air holes, respectively.

7. The broadband single-polarization residual dispersion compensating photonic crystal fiber of claim 6, wherein the first and second air holes have the same diameter as the air holes of the first and second hexagonal arrays of air holes, each diameter being 1.39-1.41 μm, the diameter of the enlarged air hole is 0.94-0.95 times the hole pitch of the air holes, the diameter of the core round hole is 0.79-0.82 μm, and the hole pitch of the first air hole and the core round hole is 1.2 μm.

Background

In long-haul optical fiber transmission systems, conventional optical fibers are used as transmission media, wherein dispersion accumulation generated in the line often generates large positive dispersion, leading to pulse broadening, causing signal distortion, thereby increasing the error rate, and affecting the transmission rate and transmission bandwidth of transmission signals, so that it is necessary to compensate for the dispersion in the line. In order to compensate for the large dispersion generated during the optical fiber transmission process, the compensation can be performed by the positive dispersion of the large negative dispersion fiber, however, after the dispersion compensation, a part of residual positive dispersion always exists, which is called residual dispersion, and at this time, the residual dispersion compensation fiber becomes the necessary dispersion compensation. Such residual dispersion compensating fiber should provide large negative flat dispersion so as to compensate for the positive dispersion accumulated in a wide wavelength band by a standard single mode fiber. For a high-speed transmission system, another dispersion existing in a single-mode fiber is polarization mode dispersion, which is based on the birefringence characteristic of the fiber, during the drawing process of the fiber, the internal stress is not uniform, and external factors such as bending, twisting and extrusion can generate unavoidable birefringence, so that different transmission constants can be generated when two orthogonal electric fields in a fiber core are transmitted along the axial direction due to the birefringence, thereby different group velocities are generated, different time delays are generated at the output end of the fiber by optical signals, and the optical pulse broadening is caused. Polarization mode dispersion and polarization crosstalk are main factors influencing long-distance transmission of optical fibers, and residual dispersion compensation optical fibers with single polarization transmission have wide application prospects in high-speed long-distance optical fiber communication transmission systems.

The photonic crystal fiber is widely applied to a dispersion compensation fiber due to a flexible and adjustable structure, and most of the commonly used dispersion compensation photonic crystal fibers are designed to have large negative dispersion only at the communication wavelength of 1.55 mu m and are used for dispersion compensation in a narrow-band wavelength range. The common technical scheme for realizing the dispersion compensation optical fiber is that an outer fiber core is formed by reducing the diameter of an air hole on the left and right of a 3 rd layer of an outer cladding, an inner fiber core is a solid core formed by a central air hole defect, and the refractive index matching is generated at the communication wavelength of 1.55 mu m by utilizing an outer core mode field and an inner core fundamental mode, so that the large negative dispersion characteristic at the communication wavelength of 1.55 mu m is realized. The two cladding layers are very close and can respectively support independent modes for transmission, when the transmission wavelength is near a certain phase matching wavelength, the two modes can be coupled, and therefore the effective refractive index of the modes which are respectively propagated has an obvious change, so that a large dispersion with negative and positive is generated, and the negative dispersion generated by the large dispersion is the main reason for carrying out dispersion compensation.

The technical scheme is utilized to design a broadband dispersion compensation photonic crystal fiber, an equivalent double-core structure is formed by utilizing an inner fiber core grapefruit type, the limit of the inner fiber core to transmission light is weakened by utilizing extrusion among the inner fiber cores, the light which is transmitted in an inner core is diffused into an outer fiber core, the mode coupling of the inner fiber core and the outer fiber core is realized in a certain wavelength range, and the broadband dispersion compensation PCF with the dispersion value increased from-380 ps/(nn.km) to-420 ps/(nn.km) and the dispersion change range of 40ps/(nn.km) is designed in the wavelength range of 1.52 to 1.58 mu m and is used for residual dispersion compensation.

However, the broadband residual dispersion compensation implemented as described above has a disadvantage that the introduction of the shaddock-type structure into the fiber core region causes the fiber itself to generate two polarization direction mode transmission, while the fiber with the designed structure can only implement dispersion compensation in one polarization direction, and does not consider the polarization dispersion brought by the residual dispersion compensation fiber itself while performing dispersion compensation, which undoubtedly brings obstruction to dispersion compensation of a high-speed and large-capacity fiber communication transmission line, and in addition, the absolute value of the dispersion difference of the dispersion thereof in the range of 1.52 to 1.58 μm is too large, and thus residual dispersion compensation in the broadband range cannot be better implemented. Therefore, it is necessary to design a broadband single polarization residual dispersion compensation photonic crystal fiber.

Disclosure of Invention

The invention aims to provide a broadband single-polarization residual dispersion compensation photonic crystal fiber which is simple in structure, easy to realize, free of changing the structure of the fiber and capable of adjusting the rotation angle of a liquid crystal molecule director direction by changing an external electric field to compensate residual dispersion in a cross-polarization direction.

In order to achieve the purpose, the invention provides the following scheme:

a broadband single polarization residual dispersion compensating photonic crystal fiber, comprising: the optical fiber structure comprises an optical fiber structure taking quartz as a substrate material, wherein the optical fiber structure is sequentially provided with a fiber core area, an inner cladding and an outer cladding from inside to outside, the fiber core area is composed of a fiber core central hole and a surrounding quartz substrate, the interior of the fiber core central hole is filled with a liquid crystal material, the outer cladding comprises second hexagonal array arrangement air holes and first hexagonal array arrangement air holes which are sequentially arranged from inside to outside, and the first hexagonal array arrangement air holes and the second hexagonal array arrangement air holes and the inner cladding form a microstructure end surface structure which is completely symmetrical in the horizontal and vertical directions;

and applying an external electric field to the liquid crystal material filled in the central hole of the fiber core, and adjusting the rotation angle of the director direction of the liquid crystal molecules by adjusting the size of the external electric field to compensate the residual dispersion in the orthogonal polarization direction.

Optionally, the first hexagonal array arrangement air holes and the second hexagonal array arrangement air holes are formed by arranging air holes in regular hexagons, the number of the air holes in the first hexagonal array arrangement air holes is 18, the number of the air holes in the second hexagonal array arrangement air holes is 12, the hole pitch of the air holes is 3.5-3.52 μm, and the diameter of the air holes is 1.39-1.41 μm.

Optionally, the central core hole is an elliptical core hole, the elliptical core hole and a surrounding quartz substrate form a C2v symmetric structure, and a long axis direction of the elliptical core hole is parallel to an x axis direction.

Optionally, the inner cladding is formed by four first elliptical air holes and two second elliptical air holes which are arranged in a regular hexagon shape, the major axis direction of the first elliptical air holes is parallel to the y-axis direction, and the major axis direction of the second elliptical air holes is parallel to the x-axis direction.

Optionally, the major axis of the core elliptical hole is 1.8 to 1.82 μm, the minor axis of the core elliptical hole is 0.9 to 0.92 μm, the major axis of the first elliptical air hole is 4 μm, the minor axis of the first elliptical air hole is 2.5 μm, the major axis of the second elliptical air hole is 4.8 μm, and the minor axis of the second elliptical air hole is 1.6 μm.

Optionally, the fiber core central hole is a fiber core circular hole, two first air holes are arranged on two sides of the fiber core circular hole, the inner cladding is composed of four enlarged air holes and two second air holes which are arranged in a regular hexagon, the two enlarged air holes are grouped in pairs and respectively arranged on the upper side and the lower side of the fiber core circular hole, and the second air holes are respectively arranged on the left side and the right side of the first air holes.

Optionally, the diameters of the first air hole and the second air hole are the same as those of the first air hole arranged in the hexagonal array and the second air hole arranged in the hexagonal array, and are both 1.39 to 1.41 μm, the diameter of the enlarged air hole is 0.94 to 0.95 times of the hole pitch of the air holes, the diameter of the core round hole is 0.79 to 0.82 μm, and the hole pitch of the first air hole and the core round hole is 1.2 μm.

According to the specific embodiment provided by the invention, the invention discloses the following technical effects: the broadband single-polarization residual dispersion compensation photonic crystal fiber provided by the invention does not need to change the structure of the fiber, and can realize the residual dispersion compensation of the fiber cores in two orthogonal polarization directions only by changing the rotation angle of the director direction of liquid crystal molecules in the photonic crystal fiber through an external electric field; the optical fiber comprises two optical fiber structures, wherein each of the two optical fiber structures comprises a fiber core area, an inner cladding layer, an outer cladding layer and a quartz substrate material, one of the fiber core areas is a fiber core elliptical hole, the inner cladding layer is composed of four first elliptical air holes and two second elliptical air holes which are arranged in a regular hexagon shape, the long axis direction of the first elliptical air holes is parallel to the y-axis direction, the long axis direction of the second elliptical air holes is parallel to the x-axis direction, the fiber core mode refractive index can be adjusted in a higher mode by the arrangement mode, the negative dispersion flatness can be improved by the arrangement mode which is consistent with the fiber core elliptical holes, an external electric field is applied to a liquid crystal material filled in the fiber core elliptical holes, and the rotation angle of the liquid crystal molecule director direction is adjusted by adjusting the size of the external electric field to compensate for the residual dispersion in the orthogonal polarization direction; the fiber core structure comprises a fiber core circular hole, wherein first air holes are formed in two sides of the fiber core circular hole, an inner cladding layer is composed of four increase air holes and two second air holes which are arranged in a hexagonal mode, the increase air holes and the first air holes reduce the fiber core mode refractive index in a single polarization direction, broadband single polarization transmission can be achieved, an external electric field can be applied to liquid crystal materials filled in the fiber core circular hole, the rotation angle of the liquid crystal molecule director direction is adjusted by adjusting the size of the external electric field, and the residual dispersion in the orthogonal polarization direction is compensated.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.

FIG. 1 is a schematic diagram of a fiber configuration in which the central core hole is an elliptical core hole;

FIG. 2a is a diagram of refractive index of a core elliptical hole fiber structure in a 90 ° mode at a rotation angle in the director direction of liquid crystal molecules;

FIG. 2b is a diagram of the mode refractive index of the fiber core elliptical hole fiber structure at a rotation angle of 0 ° in the director direction of the liquid crystal molecules;

FIG. 3a is a graph showing the variation of core mode loss with wavelength for two polarization directions when the rotation angle of the fiber core elliptical hole fiber structure in the liquid crystal molecular director direction is 90 °;

FIG. 3b is a graph showing the variation of the core mode loss with wavelength for two polarization directions when the rotation angle of the fiber core elliptical hole fiber structure in the liquid crystal molecular director direction is 0 °;

FIG. 4 is a graph of the effect of different arrangements of second elliptical air holes in the inner cladding of a core elliptical hole fiber structure on the dispersion of the fiber structure;

FIG. 5 is a graph showing the variation of modal dispersion with wavelength for a fiber core elliptical hole fiber structure with transmission in only the x-polarization and only the y-polarization directions when the rotation angles in the director direction of the liquid crystal molecules are 0 and 90, respectively;

FIG. 6 is a schematic diagram of an optical fiber structure in which the central core hole is a circular core hole;

FIG. 7a is a diagram of refractive index of a core round hole fiber structure in a 90 ° mode at a rotation angle in the director direction of the liquid crystal molecules;

FIG. 7b is a diagram of the refractive index of the core round hole fiber structure in the mode of 0 ° rotation angle in the director direction of the liquid crystal molecules;

fig. 8 is a graph showing the variation of modal dispersion with wavelength for transmission in the x-polarization only and the y-polarization only directions when the rotation angles of the core round hole fiber structure in the director direction of the liquid crystal molecules are 0 ° and 90 °, respectively.

Reference numerals: 1. a fiber core elliptical hole; 2. a first elliptical air hole; 3. a second elliptical air hole; 4. an air hole; 5. a quartz substrate; 6. a fiber core circular hole; 7. a first air hole; 8. enlarging the air hole; 9. a second air hole.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The invention aims to provide a broadband single-polarization residual dispersion compensation photonic crystal fiber which is simple in structure, easy to realize, free of changing the structure of the fiber and capable of adjusting the rotation angle of a liquid crystal molecule director direction by changing an external electric field to compensate residual dispersion in a cross-polarization direction.

In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.

As shown in fig. 1 and fig. 6, the broadband single-polarization residual dispersion compensation photonic crystal fiber provided by the embodiment of the present invention includes: the optical fiber structure comprises an optical fiber structure taking quartz as a substrate material, wherein the optical fiber structure is sequentially provided with a fiber core area, an inner cladding and an outer cladding from inside to outside, the fiber core area is composed of a fiber core central hole and a surrounding quartz substrate 5, the interior of the fiber core central hole is filled with a liquid crystal material, the outer cladding comprises second hexagonal array arrangement air holes and first hexagonal array arrangement air holes which are sequentially arranged from inside to outside, and the first hexagonal array arrangement air holes and the second hexagonal array arrangement air holes and the inner cladding form a microstructure end face structure which is completely symmetrical in the horizontal and vertical directions;

and applying an external electric field to the liquid crystal material filled in the central hole of the fiber core, and adjusting the rotation angle of the director direction of the liquid crystal molecules by adjusting the size of the external electric field to compensate the residual dispersion in the orthogonal polarization direction.

The first hexagonal array arrangement air holes and the second hexagonal array arrangement air holes are formed by arranging air holes 4 in a regular hexagon, the number of the air holes 4 of the first hexagonal array arrangement air holes is 18, the number of the air holes 4 of the second hexagonal array arrangement air holes is 12, the hole pitch Lambda of the air holes 4 is 3.5-3.52 mu m, and the diameter d of the air holes 4 is 1.39-1.41 mu m.

As shown in fig. 1, the core central hole is a core elliptical hole 1, the core elliptical hole 1 and the surrounding quartz substrate 5 form a C2v symmetric structure, and the long axis direction of the core elliptical hole 1 is parallel to the x axis direction.

The inner cladding is composed of four first elliptical air holes 2 and two second elliptical air holes 3 which are arranged in a regular hexagon shape, the major axis direction of the first elliptical air holes 2 is parallel to the y-axis direction, and the major axis direction of the second elliptical air holes 3 is parallel to the x-axis direction. High birefringence can be generated due to different arrangement modes of the first elliptical air holes 2 and the second elliptical air holes 3 of the inner cladding in the horizontal and vertical directions and filling of the liquid crystal material of the elliptical core holes 1, wherein the different arrangement modes of the first elliptical air holes 2 and the second elliptical air holes 3 in the horizontal and vertical directions can better adjust the refractive index distribution of the core mold, and the arrangement mode consistent with that of the elliptical core holes 1 can improve the flatness of negative dispersion.

Major axis a of the core elliptical hole 1₀1.8-1.82 μm, the minor axis b of the elliptical core hole 1₀0.9-0.92 μm, major axis b of the first elliptical air holes 2₂Is 4 μm, the minor axis a of the first elliptical air hole 2₂2.5 μm, major axis a of the second elliptical air hole 3₁Is 4.8 μm, and the minor axis b of the second elliptical air hole 3₁It was 1.6 μm.

As shown in fig. 2a, when the rotation angle of the director direction n of the liquid crystal molecules is 90 °, i.e., Φ is 90 °, the long axis direction of the liquid crystal molecules coincides with the y axis of the cross section of the optical fiber structure. The effective refractive index of the E7 material when Φ is 90 ° is represented by the diagonal matrix as:

n_e＝A_e+B_e/λ²+C_e/λ⁴

n_o＝A_o+B_o/λ²+C_o/λ⁴

wherein λ is optical transmission wavelength, and coefficient of Cauchy equation A_e、B_e、C_e、A_o、B_oAnd C_oAt room temperature are respectively: a. the_e＝1.6933、B_e＝0.0078μm²、C_e＝0.0028μm⁴、A_o＝1.4994、B_o＝0.007μm²And C_o＝0.0004μm⁴. Refractive index of liquid crystal material divided by ∈_xxLess than epsilon_yyThat is, the refractive index of the fiber core in the y polarization direction is far higher than that of the fiber core in the x polarization direction, and the refractive index of the fiber core in the x polarization direction is close to that of the cladding, so that the leakage of incident light in the x polarization direction to the cladding is enhanced; because the refractive index of the fiber core in the y polarization direction is far higher than that of the cladding, the mode field energy in the y polarization direction can be better limited in the fiber core according to the total internal reflection principle of optical fiber transmission, so that single polarization transmission only with light transmission in the y polarization direction is realized, and polarization mode dispersion, polarization mode loss and polarization mode crosstalk are effectively avoided.

As shown in fig. 2b, the long axis direction of the liquid crystal molecules is changed by adjusting the magnitude of the applied electric field, when the rotation angle of the director direction n of the liquid crystal molecules is 0 °, that is, when Φ is 0 °, the long axis direction of the liquid crystal molecules is consistent with the x axis of the cross section of the optical fiber structure, and the effective refractive index of the E7 material when Φ is 0 ° is represented by a diagonal matrix:

wherein the refractive index distribution ε of the liquid crystal material_xxGreater than epsilon_yyThe refractive index of the fiber core in the x polarization direction is far higher than that of the fiber core in the y polarization direction, the refractive index of the fiber core in the y polarization direction is close to that of the cladding mode, and the leakage of incident light in the y polarization direction to the cladding is enhanced.

In addition, since the elliptical hole of the core and the inner cladding are both elliptical hole structures, when the rotation angle of the liquid crystal molecular director direction n is adjusted, the corresponding polarization direction of the high refractive index does not completely coincide with the core mode.

FIG. 3 shows the variation of the core mode loss with wavelength of two polarization directions when the rotation angle of the director direction n of the liquid crystal molecule is 0 ° and 90 °, respectively, as shown in FIG. 3, in order to more clearly show the loss difference of the two polarization directions, the loss value is logarithmically processed, as shown in FIG. 3a, when the rotation angle of the director direction n of the liquid crystal molecule is 90 °, the core mode loss in the x polarization direction is greater than 80dB/km and the core mode loss in the y polarization direction is less than 0.0002dB/km within the wavelength range of 1.35 to 1.7 μm (E + S + C + L + U communication band), the core mode loss in the two polarization directions is far greater than 100, so as to satisfy the single polarization transmission condition of only the core mode in the y polarization direction in the E + S + C + L + U communication band, and match the refractive index distribution of the two polarization directions when the rotation angle of the director direction n of the liquid crystal molecule is 90 °, the refractive index of the fiber core in the y polarization direction is far greater than that of the cladding mode, so that polarized light of the fiber core is better limited in the fiber core for transmission, and the refractive index of the fiber core in the x polarization direction is close to that of the cladding mode, so that the polarized light of the fiber core is leaked into the cladding, and higher loss is generated.

As shown in FIG. 3b, whenWhen the rotation angle of the director direction n of the liquid crystal molecules is 0 DEG, the fiber core mold loss in the y polarization direction is more than 100dB/km and the fiber core mold loss in the x polarization direction is less than 1.8 multiplied by 10 in the E + S + C + L + U communication waveband^-5dB/km, the fiber core module loss of the two polarization directions is far more than 100, single polarization transmission only with x polarization transmission is realized, and the refractive index distribution result of the two polarization directions is consistent with the refractive index distribution result of the liquid crystal molecule when the rotation angle of the director direction n is 0 degrees.

Fig. 4 is a dispersion influence diagram of the inner cladding second elliptical air holes of the core elliptical hole optical fiber structure in different arrangement modes on the optical fiber structure, as shown in fig. 4, when the elliptical major axis directions of the first elliptical air holes and the second elliptical air holes of the inner cladding are perpendicular to each other, flatter negative dispersion can be obtained, and the broadband residual dispersion compensation is more suitable; when the arrangement mode of the first elliptical air holes of the inner cladding is opposite to the arrangement mode of the second elliptical air holes, the long axes of the second elliptical air holes are parallel to the x-axis direction, the direction of the long axes of the first elliptical air holes is parallel to the y-axis direction, the area of the air holes at the positions of the long axes of the elliptical holes is larger, the average refractive index distribution of the area range of the air holes is effectively reduced, the orthogonal arrangement of the long axes of the elliptical holes at the positions orthogonal to each other is realized, the change of the refractive index along with the wavelength is effectively reduced, the flatter dispersion can be realized according to a dispersion equation, and the change of the fiber core mode refractive index along with the wavelength is flatter, wherein the dispersion equation is as follows:

fig. 5 is a diagram showing the change of the modal dispersion with wavelength of the fiber core elliptical hole fiber structure having only x polarization when the rotation angle of the liquid crystal molecular director direction is 0 ° and having only y polarization transmission when the rotation angle of the liquid crystal molecular director direction is 90 °, as shown in fig. 5, when the rotation angle of the liquid crystal molecular director direction n is 90 °, the dispersion value of the fiber core in the y polarization direction in the communication band of wavelengths 1.3 to 1.65 μm is-217.2 to-214.5 ps/(nn.km), and the change value of the fiber dispersion core in the y polarization direction in the communication band range is only 2.7ps/(nn.km) under the condition of satisfying the single polarization transmission of the fiber core in the y polarization direction;

by adjusting the external electric field, when the rotation angle of the liquid crystal molecular director direction n is changed to be 0 degree, the single polarization direction transmission of only x polarization direction fiber core-mold transmission in a communication waveband of 1.3 to 1.65 mu m can be realized, the corresponding x polarization fiber core-mold dispersion value is-123.2 to-119.5 ps/(nn.km), the residual dispersion in the optical fiber communication transmission process can be compensated under the condition that the single polarization transmission is ensured in both polarization directions only by changing the rotation angle of the liquid crystal molecular director through the external electric field, and the polarization mode dispersion can not be brought to a transmission system.

As shown in fig. 6, the core center hole is a core round hole 6, two first air holes 7 are disposed on two sides of the core round hole 6, the inner cladding is composed of four enlarged air holes 8 and two second air holes 9 arranged in a regular hexagon, the enlarged air holes 8 are grouped in pairs and respectively disposed on the upper side and the lower side of the core round hole 6, and the second air holes 9 are respectively disposed on the left side and the right side of the first air holes 7.

The first air holes 7 and the second air holes 9 have the same diameter as the air holes 4 of the first hexagonal array arrangement air holes and the second hexagonal array arrangement air holes, and are each 1.39 to 1.41 μm, and the diameter d of the enlarged air holes 8₁The diameter of the fiber core round hole is 0.79-0.82 mu m, and the hole spacing between the first air hole and the fiber core round hole is 1.2 mu m.

As shown in fig. 7a, when the rotation angle of the director direction n of the liquid crystal molecules is 90 °, the y-polarization direction core mode refractive index is higher than the x-polarization direction core mode refractive index, and the increasing air holes and the first air holes largely reduce the x-polarization direction core mode refractive index, so that the x-polarization direction core mode refractive index is cut off in the wavelength range of more than 0.85 μm due to being smaller than the cladding mode refractive index and failing to satisfy the total internal reflection condition of the optical fiber transmission, and the y-polarization direction core mode is cut off in the wavelength range of more than 1.8 μm due to being lower than the cladding mode refractive index, so that the optical fiber structure satisfies the single polarization transmission in the wavelength range of 0.85 to 1.8 μm;

as shown in fig. 7b, when the rotation angle of the director direction n of the liquid crystal molecules is 0 °, the x-polarization-direction core mode refractive index is higher than the y-polarization-direction core mode refractive index, and increasing the air holes and the first air holes largely reduces the x-polarization-direction core mode refractive index, so that the x-polarization direction is cut off in a range of wavelengths greater than 1.67 μm due to being lower than the cladding mode refractive index, while the first air holes have little influence on the y-polarization-direction core mode refractive index, and the y-polarization-direction core mode refractive index is cut off in a range of wavelengths less than 0.86 μm due to being smaller than the cladding mode, so that single polarization transmission in a range of wavelengths from 0.86 to 1.67 μm is satisfied, and the single polarization transmission wavelength is shifted to a short wavelength range compared to 90 ° of the rotation angle of the director direction n of the liquid crystal molecules, and therefore, no matter whether the rotation angle of the director direction n of the liquid crystal molecules is 0 ° or 90 °, single polarization transmission can be ensured in the O + E + S + C + L + U communication wave band.

Fig. 8 is a diagram showing the change of the modal dispersion with wavelength for transmission only in the x-polarization direction when the rotation angle of the core circular-hole fiber structure in the director direction of the liquid crystal molecule is 0 ° and in the y-polarization direction when the rotation angle of the director direction of the liquid crystal molecule is 90 °, as shown in fig. 8, when the rotation angle of the director direction n of the liquid crystal molecule is 90 °, the dispersion value of the fiber core module in the y-polarization direction in the communication band of wavelengths 1.4 to 1.6 μm is-547.2 to-550.1 ps/(nn.km), and the change value of the fiber core module in the y-polarization direction in the communication band range is only 3.9 ps/(nn.km);

when the rotation angle of the director direction n of the liquid crystal molecules is changed to be 0 degrees by adjusting an external electric field, the transmission of a single polarization direction of only x-polarization direction fiber core mode transmission in a communication waveband of 1.45 to 1.65 mu m can be realized, and the corresponding x-polarization fiber core mode dispersion value is large negative flat dispersion of-892.9 to-900.8 ps/(nn.km).

According to the broadband single-polarization residual dispersion compensation photonic crystal fiber provided by the invention, the residual dispersion compensation of fiber cores in two orthogonal polarization directions can be realized only by changing the rotation angle of the liquid crystal molecule director direction n in the photonic crystal fiber through an external electric field without changing the structure of the fiber; the optical fiber comprises two optical fiber structures, wherein each of the two optical fiber structures comprises a fiber core area, an inner cladding layer, an outer cladding layer and a quartz substrate material, one of the fiber core areas is a fiber core elliptical hole, the inner cladding layer is composed of four first elliptical air holes and two second elliptical air holes which are arranged in a regular hexagon shape, the long axis direction of the first elliptical air holes is parallel to the y-axis direction, the long axis direction of the second elliptical air holes is parallel to the x-axis direction, the fiber core mode refractive index can be adjusted in a higher mode by the arrangement mode, the negative dispersion flatness can be improved by the arrangement mode which is consistent with the fiber core elliptical holes, an external electric field is applied to a liquid crystal material filled in the fiber core elliptical holes, and the rotation angle of the liquid crystal molecule director direction is adjusted by adjusting the size of the external electric field to compensate for the residual dispersion in the orthogonal polarization direction; the fiber core structure comprises a fiber core circular hole, wherein first air holes are formed in two sides of the fiber core circular hole, an inner cladding layer is composed of four increase air holes and two second air holes which are arranged in a hexagonal mode, the increase air holes and the first air holes reduce the fiber core mode refractive index in the polarization direction, broadband single-polarization transmission can be achieved, an external electric field can be applied to liquid crystal materials filled in the fiber core circular hole, the rotation angle of the liquid crystal molecule director direction is adjusted by adjusting the size of the external electric field, and the residual dispersion in the orthogonal polarization direction is compensated.

The principles and embodiments of the present invention have been described herein using specific examples, which are provided only to help understand the method and the core concept of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the above, the present disclosure should not be construed as limiting the invention.