CN115712168B - High-refractive-index double-ring microstructure optical fiber - Google Patents

Abstract

The invention discloses a high-refractive-index double-ring microstructure optical fiber which comprises a central air hole, an inner layer high-refractive-index ring, an inner cladding layer provided with the air hole, an outer layer high-refractive-index ring and an outer cladding layer which are sequentially arranged from inside to outside, wherein the refractive indexes of materials of the inner layer high-refractive-index ring and the outer layer high-refractive-index ring are equal, and the difference between the refractive indexes of the materials of the inner layer high-refractive-index ring and the outer layer high-refractive-index ring and the refractive index of the material of the inner cladding layer is larger than 0.14. The invention can avoid resonance interference, has higher signal transmission quality and wider application range.

Description

A high refractive index double ring microstructure optical fiber

Technical Field

The invention relates to optical fiber technology, in particular to a high refractive index double-ring microstructure optical fiber.

Background Art

With the rapid development of new technologies, communication needs continue to challenge the capacity of communication systems. To solve this problem, many technologies have been proposed, such as space division multiplexing (SDM), time division multiplexing (TDM), polarization division multiplexing (PDM), and wavelength division multiplexing (WDM). But these solutions still cannot keep up with the growth of communication needs. Recently, orbital angular momentum (OAM) mode and its multiplexing, mode division multiplexing (MDM) have been proposed and are considered to have great potential to improve the transmission capacity of optical fiber communication systems.

The study found that some light beams have angular momentum, which can be divided into orbital angular momentum (OAM) and spin angular momentum (SAM). Among them, the OAM beam is a spiral phased beam with a unique spiral phase wavefront. where l and Refers to topological charge and azimuth, respectively. In recent years, OAM beams have received widespread attention in the field of optical communications due to their infinite topological charge values and inherent orthogonality. By multiplexing multi-order OAM beams, the transmission capacity of the communication system can be greatly improved. Therefore, OAM-based mode division multiplexing (MDM) technology is considered to be a good solution to the insufficient capacity of single-mode systems. OAM modes can propagate in optical fibers and free space, but they are easily affected by atmospheric turbulence and cannot be transmitted over long distances. In contrast, optical fiber is a good transmission medium that has the ability to transmit over long distances and avoid interference from external factors. As a carrier for optical signal transmission, optical fiber has become the most important part of optical communication systems. Traditional single-mode optical fibers cannot support more OAM modes, so microstructures need to be introduced into optical fibers to support more OAM mode transmission.

In 2020, Huang Wei and others from Tianjin University of Technology proposed an OAM mode dispersion compensation microstructure fiber based on dual-ring resonance. To solve some situations where dispersion compensation is required during transmission, the structure has a high-refractive index dual-ring structure. The material used for the high-refractive index inner and outer rings is doped with silica. The refractive index of the high-refractive index inner ring and the high-refractive index outer ring are unequal, with a difference of 0.03. The maximum difference in the refractive index of the high-refractive index inner and outer rings compared to the cladding material is no more than 0.08, so that the mode refractive index of the same mode is close in the inner and outer rings and equal at a certain wavelength point. At this time, the dual rings resonate, and the energy of the mode is evenly distributed on the inner and outer rings. The dispersion of the mode will reach an extremely high negative dispersion value, which can achieve dispersion compensation. However, the high dispersion compensation point of the structure in this document is different for different modes, resulting in the use of only the band near the specific wavelength point where the dual-ring resonance can occur and a specific single mode. If multiple modes are transmitted at the same time, the signal interference is serious due to the dual-ring resonance, and high-quality signal transmission cannot be achieved.

Summary of the invention

Purpose of the invention: In view of the problems existing in the prior art, the present invention provides a high-refractive-index double-ring microstructure optical fiber with a wider range of applications and higher transmission signal quality.

Technical solution: The high-refractive-index double-ring microstructure optical fiber described in the present invention comprises a central air hole, an inner high-refractive-index ring, an inner cladding provided with an air hole, an outer high-refractive-index ring and an outer cladding, which are arranged in sequence from the inside to the outside. The material refractive index of the inner high-refractive-index ring and the outer high-refractive-index ring is equal, and the difference between the material refractive index of the inner high-refractive-index ring and the outer high-refractive-index ring is greater than 0.14.

Furthermore, the material of the inner high refractive index ring and the outer high refractive index ring is germanium dioxide.

Furthermore, the material of the inner cladding is silicon dioxide.

Furthermore, the material of the outer cladding layer is silicon dioxide.

Furthermore, the thickness of the inner high refractive index ring is 1.3-1.5 microns.

Furthermore, the thickness of the outer high refractive index ring is 1-1.5 microns.

Beneficial effect: Compared with the prior art, the present invention has the following significant advantages: the dual-ring structures designed for the optical fiber of the present invention each perform signal transmission as an independent channel, the material refractive index of the inner high-refractive index ring and the outer high-refractive index ring are set to be equal, and the difference with the material refractive index of the inner cladding is greater than 0.14, which increases the difference in mode refractive index between modes under the condition of simultaneous multi-mode transmission, and can effectively avoid resonance interference between different modes, thereby supporting the transmission of more OAM modes, and does not require a specific wavelength, so the scope of application is wider, and because resonance interference is avoided, the transmission signal quality is higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a structural diagram of a high refractive index double-ring microstructure optical fiber provided by the present invention;

Fig. 2 is a schematic diagram of the electric field intensity, optical field intensity and phase of the inner ring HE _3,1 , HE _5,1 and EH _4,1 modes;

Fig. 3 is a schematic diagram of the electric field intensity, optical field intensity and phase of the inner ring HE _4,1 , HE _8,1 and EH _14,1 modes;

FIG4 is a refractive index distribution diagram of the inner ring 1st to 6th order OAM modes;

FIG5 is a refractive index distribution diagram of the outer ring 1st to 17th order OAM modes;

FIG6 is a distribution diagram of the inner ring 6th order OAM mode constraint loss;

FIG7 is a distribution diagram of the outer ring 13th to 17th order OAM mode constraint loss;

FIG8 is a graph showing the variation of the dispersion of optical fiber materials with wavelength;

Figure 9 is a diagram of the dispersion distribution of the inner ring 1st to 6th order OAM modes;

FIG10 is a diagram showing the dispersion distribution of the outer ring 1st to 17th order OAM modes.

DETAILED DESCRIPTION

This embodiment provides a high refractive index double ring microstructure optical fiber, as shown in FIG1, comprising a central air hole 1, an inner high refractive index ring 2, an inner cladding 3 provided with air holes 31, an outer high refractive index ring 4 and an outer cladding 5, which are arranged in sequence from the inside to the outside. The center of the central air hole 1 is located at the center of the optical fiber, and its radius is 1.5 microns. The thickness of the inner high refractive index ring 2 is 1.5 microns. In other embodiments, it can also be any value between 1.3-1.5 microns, with the same effect. The inner boundary is 1.5 microns away from the center of the central air hole 1. Six circular air holes 31 with a diameter of 1 micron are arranged in the inner cladding 3. The circular air holes 31 are evenly arranged on a circle with a radius of 4.5 microns from the central air hole 1. The thickness of the outer high refractive index ring 4 is 1 micron. In other embodiments, it can also be any value between 1-1.5 microns, with the same effect. The inner boundary is 6 microns away from the center of the central air hole 1, and the outer boundary of the outer cladding 5 is 62.5 microns away from the center of the central air hole 1. The materials of the inner high refractive index ring 2 and the outer high refractive index ring 4 are the same and have the same refractive index, both of which are germanium dioxide. The materials of the inner and outer claddings are silicon dioxide. The difference in refractive index between the materials of the inner high refractive index ring 2 and the outer high refractive index ring 4 and the material of the inner cladding is greater than 0.14.

The refractive index of the outer high-refractive index ring and the inner high-refractive index ring is 1.5871 at a wavelength of 1.55 microns, and the refractive index of the inner and outer cladding is 1.444 at a wavelength of 1.55 microns. The difference between the two is greater than 0.14. Each high-refractive index ring transmits signals independently. The larger material refractive index difference can support more modes, increase the difference between modes, and reduce resonant interference, thereby eliminating the need for a specific wavelength. As long as the wavelength is within the communication band, the performance will not be significantly affected. In addition, if the radius of the high refractive index ring is relatively large, the number of supported modes will increase, but the effective mode refractive index of the fundamental mode (HE _1,1 ) of the ring will decrease, which will cause the effective mode refractive index of the subsequent high-order mode to decrease. When the material refractive index remains unchanged, the number of modes increases, which will cause the difference between the modes to be close and the interference to increase; and the increase in the thickness of the ring core will also produce high-order radial modes (such as HE _2,2 , HE _2,3 , HE _3,2, etc.), which cannot be used to transmit signals, and the mode refractive index of these modes may be close to or even equal to a certain radial high-order fundamental mode (such as HE _8,1 ), which will cause mode interference within the ring. If it is close to or equal to a certain radial fundamental mode of another ring, interference between rings will occur. Therefore, in order to avoid this situation, the present invention sets the thickness of the high refractive index ring to be designed to be thinner, suppressing the interference of all high-order radial modes, and only having radial fundamental modes.

The finite element method is an effective numerical method for analyzing microstructured optical fibers and is widely used in the industry. The finite element method is used to model, calculate and analyze the above-mentioned example structure, and the characteristics of this optical fiber structure can be analyzed. This embodiment calculates the optical fiber described in the example at a wavelength of 1500nm to 1600nm, and obtains the field intensity and phase of the mode supported by the optical fiber. As shown in Figures 2 and 3, in Figure 2, the contours of the field intensity and phase of the 2nd, 4th, and 5th order OAM modes of the inner ring of the optical fiber are clearly visible. In Figure 3, the field intensity and phase of the 3rd, 7th, and 15th order OAM modes of the outer ring of the optical fiber are clearly visible. The OAM vector mode in the optical fiber is formed by the linear superposition of intrinsic odd and even modes. The small arrows in the figure represent the direction of the electric field lines, and different modes can be judged. The specific superposition method is as shown in the following formula:

In the above formula, even is the odd mode, odd is the even mode, and j is the phase difference.

Figure 4 shows the variation of the refractive index of the OAM mode that can be transmitted in the inner ring of the optical fiber with wavelength, and Figure 5 shows the variation of the refractive index of the OAM mode that can be transmitted in the outer ring of the optical fiber with wavelength. The effective refractive index difference between adjacent modes is greater than ^10-4 . Figure 6 shows the variation of the confinement loss of the 6th order OAM mode in the inner ring with wavelength. The confinement loss is the largest at a wavelength of 1.6 microns, which is lower than 1.62× ^10-8 db/m. Figure 7 shows the variation of the confinement loss of the 13th to 17th order OAM modes in the outer ring with wavelength. The 17th order with the largest confinement loss is also lower than 4× ^10-9 db/m at a wavelength of 1.6 microns, both of which meet the transmission requirements of orbital angular momentum optical fiber. Figure 8 shows the variation of the dispersion of the materials used in the optical fiber cladding and high refractive index ring with wavelength. The dispersion value of silica material is about 0 in the 1500nm-1600nm band, and the dispersion value of germanium dioxide material in the 1500nm-1600nm band is negative. Figure 9 shows the dispersion value of the orbital angular momentum mode supported by the inner ring. The dispersion value shown in the figure is the result of the addition of material dispersion and mode dispersion. The maximum dispersion value of the inner ring is less than 300ps/nm/km. Figure 10 shows the dispersion value of the orbital angular momentum mode supported by the outer ring. The maximum value is less than 800ps/nm/km. Except for the 10th order mode, the others are less than 500ps/nm/km, and the dispersion of some modes is negative.

Performance parameter simulation is performed on this embodiment. Compared with ordinary ring optical fiber, one ring is added, the cladding is shared, and more modes are supported. The inner ring supports 1st to 5th order; the outer ring supports 1st to 16th order stable transmission, with a total of 80 OAM modes (18 in the inner ring and 62 in the outer ring), and the OAM mode quality is guaranteed to be greater than 70%, and the difference between each mode is greater than ^10-4 . Within the communication band, no resonance phenomenon is found between different modes of the inner ring and the outer ring, avoiding inter-ring interference. Secondly, neither the inner ring nor the outer ring generates any radial high-order mode, avoiding not only intra-ring interference but also inter-ring interference.

The above disclosure is only a preferred embodiment of the present invention, which cannot be used to limit the scope of the present invention. Therefore, equivalent changes made according to the claims of the present invention are still within the scope of the present invention.

Claims (6)

1. A high refractive index double-ring microstructure optical fiber, comprising a central air hole, an inner high refractive index ring, an inner cladding with an air hole, an outer high refractive index ring and an outer cladding, which are arranged in sequence from the inside to the outside, characterized in that the material refractive index of the inner high refractive index ring and the outer high refractive index ring is equal, and the difference between the material refractive index of the inner high refractive index ring and the outer high refractive index ring is greater than 0.14. 2. The high refractive index double-ring microstructure optical fiber according to claim 1, characterized in that the material of the inner high refractive index ring and the outer high refractive index ring is germanium dioxide. 3. The high refractive index double-ring microstructure optical fiber according to claim 1, characterized in that the material of the inner cladding is silicon dioxide. 4. The high refractive index double-ring microstructure optical fiber according to claim 1, characterized in that the material of the outer cladding is silicon dioxide. 5. The high refractive index double-ring microstructure optical fiber according to claim 1, characterized in that the thickness of the inner high refractive index ring is 1.3-1.5 microns. 6. The high refractive index double-ring microstructure optical fiber according to claim 1, characterized in that the thickness of the outer high refractive index ring is 1-1.5 microns.