LU102720B1

LU102720B1 - A 2-5THz broadband hexagonal porous core ultra-high birefringence terahertz fiber

Info

Publication number: LU102720B1
Application number: LU102720A
Authority: LU
Inventors: Zhanqiang Hui
Original assignee: Xian Univ Of Posts And Telecommunications; Nanjing Blue Wave Bay Photoelectric Tech Co Ltd
Priority date: 2021-03-31
Filing date: 2021-03-31
Publication date: 2021-10-11

Abstract

The invention discloses a 2-5 THz broadband hexagonal porous core ultra-high birefringence terahertz optical fiber. The terahertz optical fiber includes the following materials: a cladding and a core. The cladding and the core are arranged in a same base material. The core includes: a central inner layer and an outer layer. The central inner layer is a regular hexagonal area surrounded by 6 circular air holes; the outer layer is composed of 36 circular air holes, with each 6 circular air holes as the basic unit, enclosing 6 regular hexagonal areas. The core is composed of a plurality of micro air holes, including a core microstructure composed of 1 regular hexagon unit air hole core in the central inner layer and 6 regular hexagon units in the outer layer.

Description

DESCRIPTION A 2-5THz broadband hexagonal porous core ultra-high birefringence terahertz fiber

TECHNICAL FIELD The invention relates to the technical field of optical fiber communication, in particular to a 2-5THz broadband hexagonal porous core ultrahigh birefringence terahertz optical fiber.

BACKGROUND Terahertz fiber with high birefringence is a kind of fiber with high birefringence by changing the size, shape, or arrangement of air holes in two polarization directions and adjusting the refractive index distribution of the fiber. Terahertz fiber with high birefringence can provide important support for polarization-maintaining transmission of terahertz waves, and plays an important role in imaging technology, broadband communication, security detection, biomedicine and so on, which has very high research value and application prospect. The earliest high birefringence fiber is polarization- maintaining fiber reported by Blanch et al. of Bath University. By using two kinds of air holes with different diameters, the fiber has double rotational symmetry, and the birefringence value is 3 .7x107 Subsequently, a great deal of research has been done on high birefringence fiber, which has been applied to various wavebands. Because of its unique position in the electromagnetic spectrum, terahertz wave has many superior characteristics. Terahertz fiber can be widely used in the fields of terahertz wave generation, transmission, and detection, so high birefringence terahertz fiber has a very promising future.

Current reports can be roughly divided into the following two categories: (1) The cladding of optical fiber 1s anisotropic. This design method mainly improves birefringence by adjusting anisotropic parameters of cladding lattice. (2) Asymmetry is introduced into the local cladding near the core. This design method mainly realizes high birefringence by adjusting the lattice parameters of the cladding near the core. For terahertz wave transmission, reducing its absorption loss and improving birefringence have important engineering practical value. However, the birefringence and absorption loss of terahertz fiber in the prior art are not ideal.

SUMMARY In order to solve the problems in the prior art, the embodiment of the invention provides a broadband hexagonal porous core ultrahigh birefringence terahertz fiber within 2-5THz. The terahertz optical fiber comprises a cladding and a fiber core; The fiber core comprises a central inner layer and an outer layer. The cladding and the fiber core are arranged in the same base material. The central inner layer is a regular hexagonal area surrounded by 6 circular air holes. The outer layer is composed of 36 circular air holes, and every 6 circular air holes are taken as basic units to enclose 6 regular hexagonal areas. Taking the central inner layer as the center, the six regular hexagonal regions of the outer layer surround into a double-layer hexagonal cascade region. The fiber core is embedded in the cladding.

Further, the cladding (1) is a circular air hole arranged by six layers of regular hexagonal lattices. The innermost layer is composed of six circular air holes, and the second, third, fourth, fifth, and sixth layers have 12, 18, 24, 30, and 36 circular air holes respectively.

Preferably, the radius R of the circular air holes constituting the cladding (1) is 45um— 4%9um.

The distance between two adjacent circular air holes composing the cladding (1) is 100um—104um.

Furthermore, the radius R of the circular air hole forming the core (2) 1s 1-3um.

The distance between two adjacent circular air holes constituting the fiber core (2) is A, A=L/V3 and L is 13pm ~ 15um.

The distance d between two adjacent regular hexagonal regions constituting the core (2) 1s 27um-31pm.

The base material (3) is cycloolefin copolymer COC.

Beneficial effects: In the present invention, the core consists of a plurality of micro air holes, including a core microstructure consisting of one regular hexagonal unit air hole core in the central inner layer and six regular hexagonal units in the outer layer, with the central inner layer as the center and the six regular hexagonal areas in the outer layer surrounding a double- layer hexagonal cascade area. In this way, due to the asymmetric arrangement of the core structure, the symmetry of the core structure is broken, which not only improves the birefringence of the terahertz fiber, but also reduces the absorption loss. Secondly, in the invention, the cladding of the terahertz optical fiber adopts air holes arranged in a typical regular hexagonal lattice, and the cladding structure also breaks the symmetry of the cross section of the optical fiber, which is combined with the asymmetrically arranged core structure to further improve the birefringence of the terahertz optical fiber and reduce the absorption loss. In addition, the terahertz optical fiber provided by the invention realizes the birefringence change range from 0.077 to 0.0965 in the frequency range from 2THz to STHz, and obtains the ultra-high birefringence of 0.0965, the ultra-low confining loss of 10" dB/cm and the effective material loss of less than 1ecm™ when the working frequency is 3.5THz. The proposed structure exhibits a very low near-zero flat waveguide dispersion of £0.2 ps/THz/cm in the frequency range of 2. 25 ~ 5 THz.

BRIEF DESCRIPTION OF THE FIGURES In order to explain the technical scheme in the embodiments of the present invention more clearly, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention, and other drawings can be obtained according to these drawings on the premise of not paying creative labour.

Fig. 1 is the schematic cross-sectional structure diagram of a 2-5THz broadband hexagonal porous core ultrahigh birefringence terahertz fiber provided by the present invention; Fig. 2 1s the graph of birefringence changing with frequency under different optical fiber structure parameters provided by the present invention; Fig. 3 is the graph showing the variation of confining loss with frequency of a 2-5THz broadband hexagonal porous core ultrahigh birefringence terahertz fiber provided by the present invention; Fig. 4 is the waveguide dispersion curve of a 2-5THz broadband hexagonal porous core ultrahigh birefringence terahertz fiber at 1.3-5THz; Fig. 5 is the mode field distribution diagram of TE and TM polarization states provided by the present invention at 1.3THz, 3THz and STHz.

Reference number: 1- Cladding; 2- Fiber core; 3- Base material; 4- Central inner layer; 5- Outer layer.

DESCRIPTION OF THE INVENTION In order to make the object, technical scheme, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail with reference to the accompanying drawings.

It should be noted that when a component is “connected” to another component, it may be directly connected to another component or there may be an intermediate component at the same time.

Unless otherwise defined, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those skilled in the technical field of the present invention. Terms used in the specification of the present invention are only for the purpose of describing specific embodiments, and are not intended to limit the present invention.

Fig. 1 is the schematic cross-sectional structure diagram of a 2-5THz broadband hexagonal porous core ultrahigh birefringence terahertz fiber provided by the present invention. Referring to Fig. 1, the terahertz optical fiber includes a cladding 1 and a core

2. The cladding 1 and the core 2 are arranged in the base material 3. The core 2 comprises a central inner layer 4 and an outer layer 5. The central inner layer 4 is a regular hexagonal area surrounded by six circular air holes. The outer layer 5 is composed of 36 circular air holes, with every 6 circular air holes as the basic unit, enclosing 6 regular hexagonal areas. Taking the central inner layer 4 as the center, the six regular hexagonal regions of the outer layer 5 surround into a double-layer hexagonal cascade region. The core 2 1s embedded in the cladding 1.

It should be noted that since the core lattice has greater influence on the mode characteristics of optical waveguide than the cladding lattice, it is easier to obtain high birefringence characteristics by introducing anisotropic microstructure lattice into the core of optical fiber, and the mode field area of optical fiber can be increased. Therefore, in the present invention, the core consists of a plurality of micro air holes, including a core microstructure consisting of one regular hexagonal unit air hole core in the central inner layer and six regular hexagonal units in the outer layer, with the central inner layer as the center and the six regular hexagonal areas in the outer layer surrounding a double-layer hexagonal cascade area. In this way, due to the asymmetric arrangement of the core structure, the symmetry of the core structure is broken, and the absorption loss of terahertz wave is reduced by the microstructure core, thus improving the birefringence of terahertz fiber and reducing the absorption loss.

Further, the cladding 1 is composed of six layers of circular air holes arranged in regular hexagonal lattice. The innermost layer is composed of six circular air holes, and the second, third, fourth, fifth and sixth layers are composed of 12, 18, 24, 30 and 36 circular air holes respectively.

It should be also noted that the cladding layer 1 is made of air holes arranged in a typical regular hexagonal lattice, and the number of cladding layers is determined to be 6 considering the birefringence, confining loss characteristics and manufacturing difficulty of the optical fiber. In this way, the cladding structure also breaks the symmetry of the fiber cross section, and the cladding structure is combined with the core microstructure, thus further improving the birefringence of the terahertz fiber and reducing the absorption loss.

Further, the radius R of the circular air holes constituting the cladding 1 1s 45um~4%m Fig. 2 is the graph of the variation of birefringence with frequency under different optical fiber structure parameters provided by the present invention. Referring to Figure 2(d), when the radius of the circular air hole constituting the cladding 1 is R=45um, R=46um, R=47um, R=48um, R=49um, the birefringence and frequency are calculated respectively. The relationship can be concluded from the figure: when R=49um, the birefringence value is the largest.

Further, the distance A between two adjacent circular air holes constituting the cladding 1 is 100um ~ 104um.

It should be noted that, referring to Figure 2(e), the relationship between birefringence and frequency is calculated when the distances between two adjacent circular air holes constituting the cladding 1 are A = 100um, A = 101um, A = 102um, A = 103um and A = 104um, and it can be concluded from the figure that when A = 100 is selected. A is the distance from center to center of two adjacent circular air holes in cladding 1.

Further, the radius r of the circular air holes constituting the core 2 is 1um”3um With reference to Fig. 2(a), the relationship between birefringence and frequency is calculated when the radius of the circular air hole constituting the core 2isr=1 pum, r= 1 Sum,r=2yum,r=2 5yum,r=3 jm, and it can be concluded from the figure that when r =2 u m is selected, the birefringence is the largest.

The distance between two adjacent circular air holes constituting the core 2 is A, A=L/+3,andLis 13 um 15pm.

It should be noted that, referring to Figure 2(b), the relationship between birefringence and frequency is calculated when L = 13um, L = 13. Sum, L = 14um, L = 14. Sum and L = ] Sum respectively. It can be concluded from the figure that when L = 14. Sum is selected, the birefringence is the largest. Therefore, the distance between two adjacent circular air holes constituting the core 2 is the maximum birefringence value. A is the distance from the center of two adjacent circular air holes forming the core 2 to the center of the circle.

Further, the distance d between two adjacent regular hexagonal regions constituting the core 2 is 27um~31 um.

With reference to fig. 2(c), the relationship between birefringence and frequency is calculated when the distance between two adjacent regular hexagonal regions constituting the core 2 is d = 27um, d = 28um, d = 29um, d = 30um and d = 31um, and it can be concluded from the figure that when d = 29um is selected, the birefringence is the largest. d is the center-to-center distance between two adjacent regular hexagonal regions constituting the core 2.

Further, the base material 3 is a cycloolefin copolymer COC. It should be noted that cycloolefin copolymer COC is an amorphous polymer. It has a relatively stable refractive index of 1.52 in the range of 2-4.5thz, and a low bulk absorption coefficient anat = 1cm”.

Fig. 3 is the graph showing the variation of confining loss with frequency of a 2-5THz broadband hexagonal porous core ultrahigh birefringence terahertz fiber provided by the present invention. The relationship between confining loss and frequency is calculated when the structural parameters of optical fiber are set as | = 14.5um, d = 29um, r = 49um, A = 100um, r = Zum. It can be concluded from the figure that when the optimized parameters are selected, the maximum confining loss of TM polarization mode is

0.0551dB/cm at the low frequency of 1.3THz. Wide-band and low-loss transmission is realized in the whole working frequency band.

Fig. 4 is the waveguide dispersion graph of a 2-5THz broadband hexagonal porous core ultrahigh birefringence terahertz fiber provided by the present invention at 1. 3-STHz. In Fig. 4, the waveguide dispersion curve of the proposed THz fiber after optimization is calculated when the fiber structure parameters are set as L= 14. 5 u m, d =29 um, R = 49 um, A=100 um, R=2 yum. It can be concluded from the figure that the near-zero flat waveguide dispersion is kept in a wide frequency range of 2. 25 ~ STHz and always in the range of 0.2ps/THz/cm, which is beneficial to the efficient transmission of broadband THz waves.

Fig. 5 is the mode field distribution diagram of TE and TM polarization states provided by the present invention at 1.3THz, 3THz and STHz. The mode field distributions of the two polarization states at 1.3THz, 3THz and STHz respectively are obtained when the fiber structure parameters are set as L = 14. Sum, d = 29um, R = 49um, A = 100um and r = 2um. Fig. 5(a) shows the TE mode field distribution at 1.3THz. Fig. 5(b) shows the TM mode field distribution at 1.3THz. Fig. 5(c) shows the TE mode field distribution at 3THz. Fig. 5(d) shows the TM mode field distribution at 3THz. Fig. 5(e) shows TE mode field distribution at STHz. Fig. 5(f) shows the TM mode field distribution at STHz. The results show that the designed fiber can work at these three frequencies.

In the present invention, the core consists of a plurality of micro air holes, including a core microstructure consisting of one regular hexagonal unit air hole core in the central inner layer and six regular hexagonal units in the outer layer, with the central inner layer as the center and the six regular hexagonal areas in the outer layer surrounding a double-

layer hexagonal cascade area. In this way, due to the asymmetric arrangement of the core structure, the symmetry of the core structure is broken, which not only improves the birefringence of the terahertz fiber, but also reduces the absorption loss. Secondly, in the invention, the cladding of the terahertz optical fiber adopts air holes arranged in a typical regular hexagonal lattice, and the cladding structure also breaks the symmetry of the cross section of the optical fiber, and is combined with the asymmetrically arranged core structure to further improve the birefringence of the terahertz optical fiber and reduce the absorption loss. In addition, the terahertz optical fiber provided by the invention realizes the birefringence change range from 0.077 to 0.0965 in the frequency range from 2THz to STHz, and obtains ultra-high birefringence of 0.0965, ultra-low confining loss of 10° BdB/cm and effective material loss of less than 1cm”! when the working frequency is

3.5THz; The proposed structure exhibits a very low near-zero flat waveguide dispersion of +0.2 ps/THz/cm in the frequency range of 2.25 ~ 5 kHz.

The above-mentioned serial numbers of the embodiments of the present invention are only for description, and do not represent the advantages and disadvantages of the embodiments.

The above 1s only a preferred embodiment of the present invention, and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A 2-5THz broadband hexagonal porous core ultra-high birefringent terahertz fiber, which is characterized in that the terahertz fiber includes the following materials: a cladding (1) and a core (2); the cladding (1) and the core (2) is arranged in the base material 3); the core (2) includes: a central inner layer (4) and an outer layer (5); the central inner layer (4) is a regular hexagonal area surrounded by 6 circular air holes; the outer layer (5) is composed of 36 circular air holes, and each 6 circular air holes is used as a basic unit to enclose 6 regular hexagonal regions; with the central inner layer (4) as the center, the 6 regular hexagonal regions of the outer layer (5) surround a double- layer hexagonal cascaded region; the core (2) is embedded in the cladding (1).

2. The 2-5THz broadband hexagonal porous core ultra-high birefringent terahertz fiber according to claim 1, wherein the cladding (1) is composed of six layers of circular air holes arranged in regular hexagonal lattice; the innermost layer is composed of 6 circular air holes, and the second, third, fourth, fifth, and sixth layers have 12, 18, 24, 30, and 36 circular air holes respectively.

3. The 2-5THz broadband hexagonal porous core ultra-high birefringent terahertz fiber according to claim 2 is characterized in that the radius R of the circular air holes constituting the cladding (1) 1s 45pum~ 49um.

4. The 2-5THz broadband hexagonal porous core ultra-high birefringence terahertz fiber according to claim 3 is characterized in that, the distance À between two adjacent circular air holes composing the cladding (1) is 100um~104um.

5. The 2-5THz broadband hexagonal porous core ultrahigh birefringence terahertz optical fiber according to claim 4, which is characterized in that the radius R of the circular air hole forming the core (2) is 1-3um.

6. The 2-5THz broadband hexagonal porous core ultrahigh birefringence terahertz optical fiber according to claim 5, characterized in that the distance between two adjacent circular air holes constituting the core (2) is A, A=L/ NE) ‚and L is 13pum-15um.

7. A 2-5THz broadband hexagonal porous core ultrahigh birefringence terahertz optical fiber according to claim 6, which is characterized in that the distance d between two adjacent regular hexagonal regions constituting the core (2) is 27um-31um.

8. The 2-5THz broadband hexagonal porous core ultrahigh birefringence terahertz optical fiber according to claim 1, which is characterized in that the base material (3) 1s cycloolefin copolymer COC.