CN103645535B - A kind of high birefringence Hz optical fiber - Google Patents

Abstract

The invention discloses a terahertz optical fiber, which comprises a layered medium (1) and a medium tube (3), characterized in that the layered medium (1) is arranged on the medium tube at equal intervals in two directions. (3), and the layered medium (1) in two directions forms a diamond-shaped cross structure in the center of the medium tube (3), and the range of the acute angle θ formed by the intersection of the two directions is: 40 ^o ≤ θ ≤ 70 ^o ; the diamond-shaped cross structure and the medium tube (3) form four air holes (4); the layered medium (1) is fixed on the medium tube (3); the rhombic cross structure part The layered medium (1) and the air layer (2) are the fiber core, and the layered medium (1), the air layer (2), the medium tube (3) and the air hole (4) other than the orthogonal structure are the package layer. The whole is a total internal reflection structure. The fiber is suitable for terahertz wave transmission and has the advantages of low loss and high birefringence.

Description

A high birefringence terahertz fiber

technical field

The invention relates to the field of optical fiber communication, in particular to an optical fiber for transmitting terahertz waves.

Background technique

Terahertz (Terahertz, THz) usually refers to electromagnetic waves with a frequency in the range of 0.1 to 10 THz, and its band is between microwave and infrared in the electromagnetic spectrum. THz radiation has potential applications in many fields, such as communication, sensing, imaging, spectroscopy, and medicine. In recent years, more and more research groups at home and abroad have carried out research on low-loss THz waveguides. Because materials have strong absorption for THz waves, reducing the absorption loss of materials is the focus of people's research on THz waveguides. The existing optical fiber designs have Sub-wavelength fiber, holey fiber and hollow-core fiber, etc. The main idea is to distribute more core mode energy in the air, thereby effectively reducing material absorption loss.

Due to the important applications of high birefringence fibers in the fields of optical signal detection and processing, more and more research groups at home and abroad have recently carried out research on low-loss high birefringence THz waveguides. Haibin Chen et al. proposed a high birefringence THz holey fiber, the shape of the core and the air holes in the core are elliptical, so the fiber has high birefringence characteristics, but the core and the air holes in the core are not The circular shape brings difficulties to the preparation of optical fibers [Haibin Chen, et al., “Squeezed lattice elliptical-hole terahertz fiber with high birefringence,” Applied Optics, 2009, 48 (20): 3943]. In view of this, the literature [Daru Chen, et al., “Highly birefringent terahertz fibers based on super-vell structure,” Journal of lightwave technology, 2010, 28 (12): 1858] proposed a new supercell structure High birefringence THz holey fiber, the dumbbell-shaped or diamond-shaped structural unit composed of circular air holes introduces high birefringence characteristics to the fiber, and each air hole is circular, which is conducive to fiber drawing. The literature [S. Atakaramians, et al., "THz porous fibers: design, fabrication and experimental characterization," Opt. Express, 2009, 17 (16): 14053] discloses a highly birefringent porous fiber based on a rectangular hole structure , and its birefringence can reach 0.012. The literature [Fu Xiaoxia et al., "Research on low-loss, high-birefringence optical fiber for terahertz wave transmission," Acta Physica Sinica, 2011, 60 (7): 074222] modified and optimized its structure to improve its birefringence Refracted properties. However, the high birefringence THz fibers proposed above can be attributed to holey fibers, all of which use the air outside the porous core as the cladding, so their THz wave transmission characteristics are easily disturbed by the environment outside the core, and are inconvenient to touch and difficult to control. The literature [M. Cho, et al., “Highly birefringent terahertz polarization maintaining plastic photonic crystal fibers,” Opt. Express, 2008, 16 (1): 7] proposes a high birefringent fiber core composed of two solid rods Terahertz optical fiber, THz wave transmission characteristics are not easily disturbed by the outside world, and its birefringence can reach 0.021. However, the core conduction mode is mainly transmitted in the matrix material, and its absorption loss cannot be reduced.

Contents of the invention

Aiming at the above deficiencies, the present invention provides a high birefringence THz wave microstructure optical fiber capable of realizing low loss, broadband and immunity from external interference.

The technical solution of the present invention is: a high birefringence terahertz optical fiber, including a layered medium and a medium tube, the layered medium is arranged in the medium tube at equal intervals in two directions, and the two directions The layered medium forms a diamond-shaped cross structure in the center of the medium tube, and the range of the acute angle θ formed by the intersection of the two directions is: 40 ^o ≤ θ ≤ 70 ^o ; the rhombus cross structure and the medium tube form four Air holes; the layered medium is fixed on the medium tube; the layered medium and the air layer of the diamond-shaped cross structure are the fiber core, and the layered medium (1), air layer and medium other than the orthogonal structure Round tubes and air holes are cladding.

The arrangement period of the layered medium is Λ, the width of the medium layer is d, the inner diameter of the medium tube is D, and the thickness is D ₁ .

If the arrangement period of the dielectric layer is Λ constant, the reduction of its width d can reduce the core air filling factor f ₁ , thereby achieving the purpose of reducing the material absorption loss. However, if the width d is too narrow, it will increase the difficulty of fabrication and reduce the A small difference between f ₁ and f ₂ increases confinement loss, so the dielectric layer width d≥10 μm is required here.

In order to ensure that the fundamental mode of the fiber core is effectively confined in the fiber core, the ratio of f ₁ to f ₂ is required to be lower than 0.9.

In order to obtain a fiber with low loss and high birefringence value, 40 ^o ≤ θ ≤ 70 ^o is required.

The technical effect of the present invention is: the air filling factor of the fiber core can be expressed as f ₁ =(Λ-d) ² /Λ ² , and the air filling factor of the layered medium region in the cladding is f ₂ =(Λ-d)/Λ , f ₁ < f ₂ , the core conduction mode can be effectively bound in the fiber core, and the change of the external environment of the fiber will not affect the transmission of THz waves, which is convenient for practical application. Both the fiber core and the air holes in the fiber core are parallelograms, and the fiber core matrix has high birefringence characteristics. In addition, because the fiber core is porous, more core mode energy can be distributed in the air holes, which effectively reduces the absorption loss and can realize the transmission of THz wavelength distance. The outer thicker dielectric tube can stabilize the fiber structure and facilitate fiber fabrication.

Description of drawings

Fig. 1 is the structural representation of high birefringent optical fiber of the present invention;

Fig. 2 is the variation graph of the birefringence of Fig. 1 embodiment with angle θ;

Fig. 3 is the variation curve diagram of the absorption loss of Fig. 1 embodiment with included angle θ;

Fig. 4 is the curve diagram of the variation of the confinement loss with the included angle θ of the embodiment of Fig. 1;

Fig. 5 is the variation graph of the birefringence with frequency of Fig. 1 embodiment;

Fig. 6 is the variation graph of the absorption loss with the frequency of Fig. 1 embodiment;

Fig. 7 is a curve diagram showing the variation of limiting loss with frequency in the embodiment of Fig. 1;

Fig. 8 is the variation curve diagram of the birefringence of Fig. 1 embodiment with period Λ;

Fig. 9 is the variation curve diagram of the absorption loss with the period Λ of the embodiment of Fig. 1;

Fig. 10 is a curve diagram showing the variation of the limiting loss with the period Λ in the embodiment of Fig. 1;

Fig. 11 is the electric field intensity distribution figure of the x polarization mode of Fig. 1 embodiment;

FIG. 12 is a diagram showing the electric field intensity distribution of the y-polarization mode of the embodiment in FIG. 1 .

Detailed ways

Fig. 1 has provided the schematic diagram of the cross section of the holey optical fiber of the present invention, and the optical fiber comprises core and cladding, and the layered medium 1 of periodic distribution and air layer 2 are alternately arranged, and the arrangement direction forms angle θ/2 with x-axis , the same structures are arranged at symmetrical positions about the x-axis, and a stable structure is formed by the outer dielectric tube 3 . The intersecting part is used as the core of the optical fiber, and the layered medium 1, air layer 2, outer dielectric tube 3 and four large air holes 4 outside the intersecting part are the fiber cladding, and the whole is a total internal reflection structure. The core air filling factor can be expressed as f ₁ =(Λ-d) ² /Λ ² , the air filling factor of the layered medium region in the cladding is f ₂ =(Λ-d)/Λ, f ₁ <f ₂ , The core conduction mode can be effectively confined in the core.

The air hole is filled with air, the refractive index is n _air =1.0, the matrix material of the optical fiber is polytetrafluoroethylene, the refractive index is n=1.5, and the material absorption loss is selected as 130dB/m.

The absorption loss of the fundamental mode is expressed as:

Among them, Sz is the Poynting vector in the z direction, and the subscripts x and total represent the material area and the total area, respectively.

Embodiment one:

The structure of the porous core fiber is shown in Figure 1. When the width of the layered PTFE is 20 μm, the period Λ=60 μm, the inner diameter of the tetrafluoroethylene tube is D=1400 μm, and the thickness of the tetrafluoroethylene tube is D ₁ = 200 μm.

Figure 2 shows the change curve of birefringence with the included angle θ. It can be seen from the figure that when θ is 90 ^o , the birefringence is 0, and as the θ angle becomes smaller, the birefringence begins to increase. When θ =30 ^o , the birefringence can reach 0.072. The reason for the high birefringence is that the ratio of x-polarized mode energy in the material increases and the ratio of y-polarized mode energy in the material decreases as the θ angle decreases. This characteristic also causes The increase in x-polarization mode absorption loss and the decrease in y-polarization mode absorption loss are shown in Figure 3. Figure 4 shows the variation of the confinement loss of the two polarization modes with the angle θ. With the decrease of θ, the confinement loss of the x-polarization mode does not change much, close to zero loss, and the confinement loss of the y-polarization mode increases, but even When θ is 30 ^o , the limiting loss of y polarization mode is only 0.0000027dB/cm, which is much smaller than the magnitude of absorption loss and has little effect on the total loss. Both polarization modes can be effectively confined in the core.

Since the absorption loss of the x-polarized mode increases with the decrease of θ, in order to obtain a low-loss birefringent fiber, θ≥40 ^o is required here. The birefringence decreases with the increase of θ. In order to obtain a fiber with high birefringence, θ≤70 ^o is required here.

For a fixed angle θ=40 ^o , the variation of birefringence with frequency is shown in Figure 5. In the range of 0.5-1.6THz, the birefringence remains at the order of 10 ^-2 . Figure 6 shows the variation of absorption loss with frequency. As the frequency increases, the absorption loss of both polarization modes increases, which is due to the gradual increase of the core fundamental mode energy distribution in the material. At 1.6THz, x The polarization mode absorption loss is 0.74dB/cm, and the y polarization mode absorption loss is 0.57dB/cm. Figure 7 shows the variation of the confinement loss with frequency. For the x-polarization mode, the confinement loss is almost zero loss, while for the y-polarization mode, the confinement loss will gradually increase only when the frequency is less than 0.7THz. Therefore, in In the range of 0.7-1.6THz, THz waves can be transmitted in this fiber with low loss and have high birefringence characteristics.

With a fixed angle θ=40 ^o and a frequency of 1THz, Figure 8 shows the change curve of the birefringence with the arrangement period Λ. With the increase of Λ, the birefringence value first increases, and then gradually decreases after reaching the maximum value. Even at Λ=110μm, the birefringence value can still reach 0.62. The width d of layered PTFE remains unchanged, and Λ increases, and now the air filling factor f ₁ increases, thereby reducing the absorption loss. Figure 9 shows the variation curve of absorption loss with arrangement period Λ, as Λ increases, the absorption loss of both polarization modes decreases. Figure 10 shows the variation curve of the confinement loss with the arrangement period Λ. For the x-polarized mode, the proportion of the mode energy in the material is relatively large, so the confinement loss hardly changes, and the confinement loss is much lower than the absorption loss. have little effect on the total loss. For the y-polarization mode, as the arrangement period Λ increases, the confinement loss increases gradually. At Λ=110μm, the confinement loss of the y-polarization mode is 0.00132dB/cm, which is still two orders of magnitude smaller than the confinement loss.

Here, in order to keep the core matrix with low limiting loss, it is required that Λ≤110 μm, and in order to ensure that the core matrix has low absorption loss, it is required that Λ≥60 μm.

When the layered polytetrafluoroethylene width=20μm, period Λ=60μm, angle θ=40 ^o , and frequency 1THz, the electric field intensity distribution diagrams of x-polarization mode and y-polarization mode are shown in Figure 11 and Figure 12 respectively, from It can be seen from the figure that the energy distribution ratio of the x-polarized mode in the material is higher than that of the y-polarized mode, so a high birefringence can be obtained. At the same time, the x-polarized mode and the y-polarized mode are effectively confined to the fiber core for transmission, THz The wave transmission in the fiber is not disturbed by the external environment, which is better than the holey fiber.

The above drawings are only illustrative diagrams, and do not limit the protection scope of the present invention. It should be understood that this example is only to illustrate the present invention, but not to limit the scope of the present invention in any way.

Claims (5)

1. A high birefringence terahertz optical fiber, comprising a layered medium (1) and a medium tube (3), characterized in that the layered medium (1) is arranged in the medium at equal intervals in two directions respectively In the circular tube (3), the layered medium (1) in two directions forms a rhombus cross structure in the center of the medium circular tube (3), and the range of the acute angle θ formed by the intersection of the two directions is: 40°≤θ ≤70°; the diamond-shaped cross structure and the medium tube (3) form four air holes (4); the layered medium (1) is fixed on the medium tube (3); the rhombic cross structure Part of the layered medium (1) and air layer (2) is the fiber core, and the layered medium (1), air layer (2), medium tube (3) and air hole (4) other than the diamond cross structure for cladding. 2. A high birefringence terahertz optical fiber according to claim 1, characterized in that: the width d of the layered medium (1) is greater than or equal to 10 μm. 3. A high birefringence terahertz optical fiber according to claim 1, characterized in that: the arrangement period Λ of the layered medium (1) should be: 60 μm≤Λ≤110 μm. 4. A high birefringence terahertz optical fiber according to claim 1, characterized in that: f ₁ /f ₂ ≤ 0.9 is required, wherein the core air filling factor is f ₁ =(Λ-d) ² /Λ ^2. The air filling factor of the layered medium area in the cladding is f ₂ =(Λ-d)/Λ, f ₁ <f ₂ , the arrangement period of the layered medium is Λ, and the width of the layered medium is d. 5. A high birefringence terahertz optical fiber according to claim 1, characterized in that: the number of said layered media is the same in both directions.