CN103091769B - A kind of annular microstructured optical fibers - Google Patents

Abstract

The invention discloses an annular microstructure optical fiber, which is composed of a matrix material (1), a first dielectric column (2) and a second refractive index medium column (3). Wherein the fiber core (4) is located at the center of the optical fiber and is composed of a matrix material. The centers of the first dielectric columns (2) are equally spaced on a circle centered on the center of the optical fiber; the centers of the second dielectric columns (3) are equally spaced on the other circle centered on the center of the optical fiber ; The distance between the center of the first dielectric column (2) and the center of the optical fiber is L ₁ , the distance between the center of the second dielectric column (3) and the center of the optical fiber is L ₂ , and L ₁ >15 μm and L ₂ >L ₁ . The optical fiber uses the first dielectric column (2) with a smaller diameter to bind the fundamental mode of the fiber, and uses the second dielectric column (3) with a larger diameter to reduce the bending loss of the fundamental mode of the fiber, realizing single-mode, large-mode field, The purpose of low bending loss transmission.

Description

A ring microstructure optical fiber

technical field

The invention relates to a symmetrical optical fiber structure, in particular to a microstructured optical fiber with large mode field, low bending loss characteristics and effective single-mode operation.

Background technique

Photonic crystal fiber and silica micro-air were first proposed in 1995. Due to the special application of this fiber in ultra-broadband transmission, supercontinuum generation, high-power optical transmission, optical amplification and other functional devices, Therefore, it has aroused the interest of many researchers. Bending loss is generally considered to be a disadvantage in light transmission.

Since the photonic crystal fiber was proposed and successfully manufactured, people began to try to use the photonic crystal fiber structure to make large mode field fiber. Since the photonic crystal fiber can realize endless single-mode transmission in theory, the single-mode transmission with super large mode field area can be realized by using the photonic crystal fiber structure. At present, the mode field diameter of single-mode photonic crystal fibers that have been successfully fabricated in experiments can reach 100 μm. But its bending performance is poor. Although single-mode transmission can be realized in theory, in fact, most of the high-order modes are removed through the loss attenuation difference, that is, the high-order mode loss is large (generally above 1 dB/m), while the fundamental mode loss is low (generally below 0.1 dB/m). An effective way to achieve single-mode transmission of LMA is to use channel leaky fiber, such as LCF with a layer of air holes surrounding the fiber core proposed by Dong, but it is difficult to manufacture the air holes. K. Iizawa et al obtained single-mode transmission by doping ytterbium in the fiber core and coupling, but the high-order mode loss of this fiber is less than 1dB/m, which cannot completely eliminate single-mode transmission.

The patent application number is 200610119574.6 "Large mode field double-clad single-mode fiber" discloses a single-mode large mode field fiber, the core radius can reach 72 μm, but the bending loss problem of the fiber is not solved, and the structure is relatively complicated. Not good for production.

Contents of the invention

Aiming at the deficiencies of the prior art, the object of the present invention is to provide a large mode field microstructured optical fiber which effectively filters out high-order modes and has relatively low bending loss.

The technical solution of the present invention is: a ring-shaped microstructure optical fiber, including a core and a cladding, the cladding is composed of a matrix material, a first dielectric column and a second dielectric column; the core is composed of a matrix material, located at The center of the optical fiber; the centers of the first dielectric columns are arranged at equal intervals on a circle centered on the center of the optical fiber, and the centers of the second dielectric columns are arranged at equal intervals on another circle centered on the center of the optical fiber above; the distance between the center of the first dielectric column and the center of the optical fiber is L ₁ , the distance between the center of the second dielectric column and the center of the optical fiber is L ₂ , then L ₁ >15 μm and L ₂ >L ₁ ; the first The diameter d ₁ of the dielectric column and the diameter d ₂ of the second dielectric column satisfy d ₁ < d ₂ ; the refractive index of the first dielectric column and the second dielectric column are the same and lower than the refractive index of the matrix material .

The number N ₁ of the first medium columns is ≥12, and the number N ₂ of the second medium columns is 4-12.

The diameter d ₁ of the first dielectric column satisfies 5≥d ₁ ≥2 μm; the diameter d ₂ of the second dielectric column (3) satisfies 1.5πL ₂ /N ₂ ≥d ₂ ≥0.4πL ₂ /N ₂ .

The distance L ₁ between the center of the first dielectric column and the center of the optical fiber and the distance L ₂ between the center of the second dielectric column and the center of the optical fiber satisfy: 4L ₁ >L ₂ >1.5L ₁ .

The difference between the refractive index n _rod of the first dielectric column and the second dielectric column and the refractive index n _clad of the matrix material satisfies: n _clad −n _rod =0.001˜0.01.

The technical effect of the invention is that a layer of low-refractive-index dielectric columns with smaller diameters is arranged around the fiber core to form a lower-refractive-index layer, thereby effectively confining light and making the optical fiber base mold have lower bending loss. Arranging the low-refractive-index dielectric cylinders around the center of the fiber core makes the fundamental mode field of the fiber more circularly symmetrical. At the same time, a layer of low-refractive-index dielectric pillars with larger diameters and larger gaps between the dielectric pillars is arranged outside the low-refractive-index dielectric pillars to reduce the binding loss and bending loss of the fundamental mode. The gap between them is large, so as to ensure that the high-order mode light of the fiber can be effectively leaked to achieve the purpose of single-mode transmission. Thus, the purpose of single-mode, large-mode-field, and low-bending-loss transmission is achieved. Since the cladding only adopts two layers of holes, the structure is simple and symmetrical, which ensures that the cladding has a small size.

Description of drawings

Fig. 1 is a schematic cross-sectional view of an embodiment of the present invention;

FIG. 2 is the confinement loss curves of the fundamental mode and higher-order modes of the optical fiber when the diameter of the first dielectric column 2 in FIG. 1 changes.

FIG. 3 is the confinement loss curves of the fundamental mode and higher-order modes of the optical fiber when the first dielectric column 2 in FIG. 1 takes different central angles.

FIG. 4 is the loss curves of the fundamental mode and high-order mode of the second dielectric column 3 with different diameters in the structure shown in FIG. 1 .

Fig. 5 is a graph showing the variation curve of the bending loss of the fundamental mode of the optical fiber with the structure shown in Fig. (1) as a function of the bending radius of the optical fiber.

Among them: 1-matrix material, 2-first dielectric column 2, 3-second dielectric column 3, 4-fiber core.

Detailed ways

Fig. 1 shows a schematic cross-sectional view of an embodiment of the present invention, the fiber core 4 is surrounded by two layers of low-refractive-index medium pillars with different diameters. The diameter of the first dielectric columns 2 is relatively small, the arrangement is relatively dense, and the ratio between the diameter of the first dielectric columns 2 and the distance between the adjacent first dielectric columns 2 is relatively large. According to the photonic crystal fiber theory, the cladding structure composed of dielectric columns with small spacing has a low equivalent refractive index, so the fundamental mode of the fiber can be bound to a certain extent during straight waveguide and bending. However, since there is only one layer of dielectric column, its ability to confine light is limited, and the high-order mode energy of the fiber is easier to expand to the cladding region, so the high-order mode of the fiber can still maintain high loss.

If the optical fiber is only composed of the matrix material 1 and the first dielectric column 2, the loss difference between the fundamental mode and the high-order mode is very small. Fig. 2 shows the confinement loss curves of the optical fiber fundamental mode and higher order modes when the diameter of the first dielectric column 2 changes. It can be seen from the figure that the difference between the two is small, and the fundamental mode loss of the fiber is relatively large, which is not suitable for practical applications.

According to the photonic crystal fiber theory, the introduction of multi-layer low refractive index dielectric pillars can effectively confine light. However, in the large mode field fiber, due to the large core size, the introduction of two layers of low refractive index columns can make the high-order mode loss of the fiber very small, and the purpose of single-mode transmission cannot be achieved. For example, as shown in the example in Figure 2, when the diameter of the first dielectric column 2 is 5 μm, if there is only one layer of dielectric column, the high-order mode confinement loss is 2.98 dB/m, and if an additional layer with the same diameter as For the low-refractive-index dielectric pillars with the same distance between the first dielectric pillars 2, the confinement losses of the fundamental mode and the high-order mode are reduced to 0.51 dB/m and 2.13 dB/m, respectively. Therefore, the fiber is still a non-single-mode fiber.

To this end, we add a layer of second dielectric column 3 on the outside of the first dielectric column 2, which is farther away from the fiber core, has a larger diameter, and has a smaller ratio of its diameter to the distance between the dielectric column to reduce the loss of the fundamental mode while maintaining the high-order mode. high-loss purposes. The function of the second dielectric column 3 can be seen in this way: if there is no first dielectric column 2, the fiber core is surrounded by the second dielectric column 3 to confine light. Because the second dielectric column 3 is far away from the fiber core, and the ratio of its diameter to the distance between the second dielectric column 3 is large, it is similar to the photonic crystal fiber that only has a single layer of holes, and is single-mode transmission, that is, the high-order mode can be transmitted from the second The gap of medium column 3 leaks out. Therefore, the introduction of the second dielectric column 3 will not have a great impact on the high-order mode loss of the optical fiber. However, the second dielectric column 3 can effectively reduce the binding loss and bending loss of the fundamental mode of the fiber. For example, after the second dielectric column 3 is introduced, its fundamental mode confinement loss can be reduced by 1200 times.

Since the effective refractive index of the cladding area composed of the first dielectric column 2 and the matrix material 1 should be low, the diameter of the first dielectric column ₂ should be relatively small, and its number N should be relatively large, that is, the first dielectric column 2 The arrangement is denser. The second dielectric column 3 is to bind the fundamental mode and leak high-order modes, therefore, its diameter should be relatively large, and its number N ₂ should be relatively small. In particular, the diameter d ₂ of the second medium column 3 should satisfy 1.5πL ₂ /N ₂ ≥d ₂ ≥0.4πL ₂ /N ₂ . That is, the ratio of the diameter of the second dielectric pillar 3 to the distance between adjacent second dielectric pillars 3 is not greater than 0.75, so as to meet the requirement of keeping high-order mode loss at a high level.

If the second dielectric column 3 is very close to the first dielectric column 2 , the space for high-order mode leakage becomes smaller. Therefore, the second dielectric column 3 should keep a certain distance from the first dielectric column 2 . But if it is too far away, it will weaken the effect of its binding schema. Therefore, it is required that: the distance L ₁ between the center of the first dielectric column 2 and the center of the optical fiber, and the distance L ₂ between the center of the second dielectric column 3 and the center of the optical fiber satisfy: 4L ₁ >L ₂ >1.5L ₁ .

Figure 3 shows the confinement of higher-order modes and fundamental modes when the parameters of the second dielectric column 3 are constant, the central angle θ of the adjacent first dielectric column 2 in the inner layer is from 6° to 15°, and the diameter d1 is from ₂ μm to 5 μm loss and loss ratio. It can be seen from the figure that increasing the diameter of the second dielectric column 3 can increase the loss ratio between the high-order mode and the fundamental mode, and achieve good single-mode transmission.

Figure 4 shows the relationship between the confinement loss of the high-order mode and the fundamental mode and the diameter _d2 when the L2 of the _second dielectric column 3 does not change to 60 μm, the central angle of the inner layer hole is 15°, and the diameter d1 is ₅ μm. It can be seen from the figure that the loss ratio reaches the maximum when the normalized diameter of the outer hole is about 0.7.

Fig. 5 shows the relationship between the bending loss of the fundamental mode and the bending radius of the optical fiber when the diameter of the second dielectric column 3 is d ₂ =42 μm, L ₂ =59 μm, the central angle θ of the inner layer hole is 18°, and the diameter d ₁ is 4 μm. It can be found from the figure that when the bending radius is 0.5m, the bending loss is less than 0.1 dB/m.

The following takes quartz as the matrix material as an example to illustrate the transmission characteristics of the optical fiber. In practical application, the corresponding optical fiber structure can be selected according to different matrix materials and applicable wavelength ranges of the matrix materials.

Example:

The cross-sectional structure of the optical fiber is shown in Figure 1. The matrix material adopts pure quartz, and the hole material adopts doped quartz material. The diameter of the first dielectric column 2 is d ₁ =5 μm, the distance from the center of the optical fiber is L ₁ =30 μm, and the number N ₁ =20. The diameter of the second dielectric column 3 is d ₂ =42 μm, the distance from the center of the optical fiber is L ₂ =60 μm, and the number N ₂ =6. The low refractive index dielectric pillars have a refractive index 0.004 lower than the matrix material. When the transmission wavelength is 1064 nm, the mode field area of the fundamental mode can reach 1498 μm ² in a straight fiber, and the mode field area of the fundamental mode is 1384 μm ² when the bending radius is 50 cm. The bending loss of the optical fiber is 0.095dB/m when the bending radius is 50cm. When the fiber is straight, its fundamental mode leakage loss is less than 0.002dB/m, and its high-order mode loss is greater than 1.11dB/m.

The above drawings are only illustrative diagrams, and do not limit the protection scope of the present invention. It should be understood that these examples are only for illustration of the present invention, but not to limit the scope of the present invention in any way.

Claims (3)

1. A large mode field single-mode annular microstructured optical fiber, comprising a core (4) and a cladding, is characterized in that: the cladding consists of a matrix material (1), a first dielectric post (2) and a second medium Composed of pillars (3); the core (4) is composed of a matrix material and is located at the center of the optical fiber; the centers of the first dielectric pillars (2) are arranged at equal intervals on a circle centered on the center of the optical fiber, so The centers of the second dielectric columns (3) are arranged at equal intervals on another circle centered on the center of the optical fiber; the distance between the center of the first dielectric column (2) and the center of the optical fiber is L ₁ , and the second medium The distance between the center of the column (3) and the center of the fiber is L ₂ , then L ₁ >15 μm and L ₂ >L ₁ ; the diameter d 1 of the first dielectric column (2) and the diameter d ₁ of the second dielectric column (3) The diameter d ₂ satisfies d ₁ <d ₂ ; the refractive index of the first dielectric column (2) and the second dielectric column (3) are the same and lower than the refractive index of the matrix material (1); the first The diameter d ₁ of the medium column (2) satisfies 5≥d ₁ ≥2 μm; the diameter d ₂ of the second medium column (3) satisfies 1.5πL ₂ /N ₂ ≥d ₂ ≥0.4πL ₂ /N ₂ ; the The distance L ₁ between the center of the first dielectric column (2) and the center of the optical fiber and the distance L ₂ between the center of the second dielectric column (3) and the center of the optical fiber satisfy: 4L ₁ >L ₂ >1.5L ₁ . 2. A large mode field single-mode annular microstructured optical fiber according to claim 1, characterized in that: the number N ₁ of the first dielectric column (2) is ≥ 12, and the number of the second dielectric column (3) The number N ₂ is 4-12. 3. A kind of large mode field single-mode annular microstructure fiber according to claim 1, is characterized in that: the refractive index n _rod of described first dielectric post (2) and described second dielectric post (3) and The difference of the refractive index n _clad of the matrix material (1) satisfies: n _clad −n _rod =0.001˜0.01.