Photonic crystal fiber for reducing the occurrence of high-order radial modes and improvement method thereof

Abstract

The present invention discloses a photonic crystal fiber for reducing the occurrence of high-order radial modes and an improvement method thereof. The photonic crystal fiber reduces the material refractive index of the air hole region of the photonic crystal fiber. When the wavelength of light is 1.55 micrometers, the reduction range of the material refractive index is between 0 and 0.444. When processing the air hole region of the photonic crystal fiber, the region where the material refractive index is reduced increases by corresponding distances at the inner diameter and the outer diameter of the air hole region. The material of the photonic crystal fiber is silica, and the material of the region where the material refractive index is reduced is fluorine-doped silica. The present invention enables more OAM modes to be transmitted in the optical fiber without interference. At the same time, the proposed method not only does not damage other properties of the optical fiber, but even can improve some properties.

Description

Technical Field

[0001] This invention patent relates to a photonic crystal fiber for reducing the occurrence of higher-order radial modes and an improvement method thereof, specifically a method for reducing the refractive index of the material in the region where air holes are located. Background Art

[0002] In recent years, with the development of big data, cloud computing, artificial intelligence, and fifth-generation communication technologies, the existing communication channel capacity can no longer meet the explosive growth demand of new technologies for channel capacity. The demand for higher data transmission capacity has prompted researchers to explore new technologies beyond traditional multiplexing methods, such as wavelength-division multiplexing (WDM) and time-division multiplexing (TDM). Mode-division multiplexing (MDM) emerged as a solution, further increasing the transmission capacity by leveraging the spatial dimension of light propagation. MDM utilizes different orbital angular momentum (OAM) modes within the optical fiber to support multiple data channels. By taking OAM as an additional degree of freedom, the MDM-OAM system can achieve higher capacity. This combined method can achieve transmission of an unprecedented number of channels, thus greatly enhancing the data transmission ability.

[0003] In the field of optical communication, photonic crystal fiber (PCF) offers unique advantages for the generation, manipulation, and transmission of OAM in optical communication systems. PCF can achieve multiplexing of multiple OAM modes, allowing multiple independent OAM-carrying channels to be transmitted through a single optical fiber. This OAM multiplexing ability provides a larger channel capacity and higher data transmission rate in optical communication systems. PCF provides a platform for the combination of MDM and OAM, enabling simultaneous transmission of multiple spatial and OAM channels within the same optical fiber. And PCF can maintain high OAM mode quality during long-distance propagation. Different from traditional optical fibers, PCF has a unique waveguide structure, which helps to maintain the spatial and phase characteristics of OAM modes. This characteristic is crucial for maintaining the quality and integrity of OAM-carrying light beams, allowing reliable transmission of OAM-encoded information. PCF provides a highly flexible platform for customizing the waveguide structure and refractive index profile, allowing precise control of the characteristics of OAM modes. This flexibility enables the design and optimization of PCF to meet specific OAM generation and transmission requirements. Although PCF can support the transmission of a very high number of OAM modes, it also generates unwanted higher-order radial modes, and the existence of these modes will interfere with the OAM modes whose effective refractive index (n eff ) is close to theirs. Summary of the Invention

[0004] Objective of the Invention: For the first time, the present invention proposes to reduce the material refractive index of the air hole region of a photonic crystal fiber to reduce the high-order radial modes that appear in the fiber, enabling more OAM modes to be transmitted in the fiber without interference. At the same time, the proposed method not only does not damage other properties of the fiber, but can even improve some properties.

[0005] Technical Solution: A photonic crystal fiber for reducing the appearance of high-order radial modes, wherein the improved photonic crystal fiber reduces the material refractive index of the air hole region of the photonic crystal fiber. When the wavelength of light is 1.55 microns, the reduction range of the material refractive index is between 0 and 0.444.

[0006] Furthermore, when processing the air hole region of the photonic crystal fiber, the region where the material refractive index is reduced increases by corresponding distances at the inner diameter and outer diameter of the air hole region.

[0007] Furthermore, the material of the photonic crystal fiber is silica, and the material of the region where the material refractive index is reduced is fluorine-doped silica.

[0008] An improved method for a photonic crystal fiber to reduce the appearance of high-order radial modes, comprising:

[0009] Step (1): Reduce the material refractive index of the air hole region of the photonic crystal fiber;

[0010] Step (2): Analyze the effective refractive index n when high-order radial modes appear after being processed in Step (1) eff and the change in the performance of the fiber.

[0011] Furthermore, in Step (1), when processing the air hole region of the photonic crystal fiber, the region where the material refractive index is reduced increases by corresponding distances at the inner diameter and outer diameter of the air hole region.

[0012] Furthermore, in Step (1), the material of the photonic crystal fiber is silica, and the material of the region where the material refractive index is reduced is fluorine-doped silica.

[0013] Furthermore, in Step (1), when the wavelength of light is 1.55 microns, the reduction range of the material refractive index is between 0 and 0.444.

[0014] Furthermore, after being processed in Step (1), the fiber can support the transmission of the 21st-order OAM mode without interference.

[0015] Furthermore, after being processed in Step (1), the confinement loss of the 21st-order mode in the improved photonic crystal fiber is lower than 10 -10 dB / m.

[0016] Furthermore, after being processed in Step (1), the mode quality of the improved photonic crystal fiber increases.

[0017] Beneficial effects: The present invention can reduce the high-order radial modes that appear in the optical fiber, enabling more OAM modes to be transmitted in the optical fiber without interference. At the same time, it improves the mode quality of the optical fiber and reduces the confinement loss. Description of the Drawings

[0018] Figure 1 Schematic diagram of an improved method for a photonic crystal fiber to reduce the occurrence of high-order radial modes;

[0019] Figure 2 n when high-order radial modes appear in a conventional PCF and the PCF improved by the present invention eff Comparison diagram;

[0020] Figure 3 Comparison diagram of the mode quality between a conventional PCF and the PCF improved by the present invention;

[0021] Figure 4 Comparison diagram of the confinement loss between a conventional PCF and the PCF improved by the present invention. Detailed Embodiments

[0022] The technical solution of the present invention will be further described below in conjunction with the drawings.

[0023] The OAM modes in the optical fiber are linearly superimposed by vector eigenmodes (HE or EH). The specific superposition formula is as follows:

[0024]

[0025] Among them, l is the topological charge, j refers to the π / 2 phase difference between the even and odd modes of the same vector mode, and m describes the radial index. The superscript ± represents the left- or right-handed circular polarization direction, and the subscript ± represents the right- or left-handed direction of the phase wavefront.

[0026] As Figure 1 shown, a conceptual diagram of an improved method for a photonic crystal fiber to reduce the occurrence of high-order radial modes is provided. Among them Figure 1 (a) is a conventional photonic crystal fiber, Figure 1 (b) is a photonic crystal fiber improved by applying the proposed method.

[0027] As Figure 1As shown in (a), first, a model of a traditional photonic crystal fiber is established. Its center is a air hole with a radius r1 = 14 μm. The outside of the central air hole is a ring core with a thickness of 3 μm. The outer layer of the ring core is four layers of circular air holes with a diameter d = 2 μm arranged around the ring core. The distance from the center of the innermost air hole to the center of the fiber is r2 = 18.5 μm, and the distance between the air hole layers is D = 2.5 μm. The diameter of the entire fiber is 125 μm, and the material used is pure silica, whose refractive index is 1.444 at a wavelength of 1.55 μm. Figure 1 (b) shows the proposed method. The refractive index of the material in the area where the air holes are located in the cladding is reduced. Considering the manufacturing difficulty, to prevent an air hole from straddling two different materials, the areas where the material refractive index is reduced are increased by 0.5 μm each in the inner diameter and outer diameter (r3 = 17 μm, r4 = 27.5 μm). The material used in this part is fluorine-doped silica with a doping concentration of 10%, and its refractive index is 1.4 at 1.55 μm. The remaining parameters of the fiber are Figure 1 exactly the same as those in Figure 1 (a). The model in

[0028] is an example of the method. The reduction of the material refractive index is about 3.14%. This value is not fixed and can be adjusted as needed. At a wavelength of 1.55 μm, the refractive index of the silica material is 1.444, and the refractive index of air is 1. The reduction of the material refractive index generally does not go below the value of air, so the reduction range is 0 - 0.444.

[0028] As Figure 2 shown, analyze the n eff when the high-order radial mode appears. Analyze the number of vector modes with n eff greater than the high-order radial mode before and after using the proposed method. Figure 2 (a) shows the n eff of some modes in the traditional PCF, (b) shows the n eff of some modes in the improved PCF, and (c) shows the comparison of the n eff of the same vector modes in the two PCFs.

[0029] Figure 2 (a) shows the effective refractive indices of some modes in the traditional PCF. Since the photonic crystal fiber supports a large number of modes, for a concise display, we selected low-order modes (HE1,1 - HE4,1), high-order modes (HE17,1 and EH15,1), and the highest-order mode before the appearance of the high-order radial mode for display. From Figure 2 (b), it is observed that before the appearance of the high-order radial fundamental mode (HE1,2), the improved PCF supports two additional modes, EH20,1 and HE22,1, which means that the fiber can support the transmission of the 21st-order OAM mode without interference. Although the proposed method reduces the neff while also reducing the neff of other vector modes, but through Figure 2 (c) It is observed that the decrease of the high-order radial mode is significantly greater than that of other modes. The decrease of other modes is about 0.00191, while the decrease of HE1,2 reaches 0.00701, which is 3.67 times that of other modes.

[0030] As Figures 3-4 shown, analyze the changes in other performance of the optical fiber. Analyze the other performance of the optical fiber before and after applying the method to observe whether it causes deterioration of other performance. Figure 3 is the comparison of the mode quality between the traditional PCF and the improved PCF. Figure 4 (a) is the confinement loss of the mode in the traditional PCF, and (b) is the confinement loss of the mode in the improved PCF.

[0031] The mode quality is an important parameter to describe the ability of the optical fiber to confine vector modes. A higher mode quality indicates that the energy of the electric field can be better confined within the ring core. The calculation of the mode quality is shown in formula (2):

[0032]

[0033] where E(x,y)ring represents the electric field energy in the ring core, and E(x,y)whole-section represents the electric field energy of the entire optical fiber cross-section. Figure 3 shows the mode quality of the same vector mode in the traditional PCF and the improved PCF. The mode in the improved PCF has a higher mode quality. Compared with the traditional PCF, the average mode quality increases by about 3.3%

[0034] The confinement loss can evaluate the transmission distance of the mode in the optical fiber. A low confinement loss is very beneficial to the long-distance transmission of the optical fiber. Its calculation formula is as follows:

[0035]

[0036] where Im(n eff ) represents the imaginary part of n eff . The confinement loss of the mode in the traditional PCF is shown in Figure 4 (a). The confinement loss of the low-order mode is about 10 - 12 dB / m, while that of the high-order mode is greater than 10 - 10 dB / m. In the improved PCF, even the confinement loss of the 21st-order mode is lower than 10 - 10 dB / m. Compared with the traditional PCF, the improved PCF has significantly better performance in terms of confinement loss.

[0037] The proposed method can not only reduce n when the high-order radial mode appears in the PCF eff, other properties of the optical fiber can also be enhanced, and this improvement method can be used in any photonic crystal fiber.

[0038] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A photonic crystal fiber for reducing the occurrence of high-order radial modes, the photonic crystal fiber being composed of a ring core, a cladding, and a central air hole, characterized in that, The cladding consists of an annular air-hole region and a silica ring outside the annular air-hole region, the refractive index of the material in the annular air-hole region is reduced, and the refractive index of the material after reduction is not lower than that of air; when processing the annular air-hole region of the photonic crystal fiber, the region where the refractive index of the material is reduced increases by corresponding distances at both the inner diameter and the outer diameter of the annular air-hole region.

2. A photonic crystal fiber for reducing the occurrence of high-order radial modes according to claim 1, wherein When the wavelength of light is 1.55 microns, the reduction range of the refractive index of the material is between 0 and 0.

444.

3. A photonic crystal fiber for reducing the occurrence of high-order radial modes according to claim 1, characterized in that, The material of the photonic crystal fiber is silica, and the material in the region where the refractive index of the material is reduced is fluorine-doped silica.

4. An improved method for a photonic crystal fiber to reduce the occurrence of higher-order radial modes, characterized in that, Including: Step (1): The photonic crystal fiber consists of a ring core, a cladding and a central air hole. The cladding consists of an annular air-hole region and a silica ring outside the annular air-hole region. The refractive index of the material in the annular air-hole region is reduced, and the refractive index of the material after reduction is not lower than that of air; when processing the annular air-hole region of the photonic crystal fiber, the region where the refractive index of the material is reduced increases by corresponding distances at both the inner diameter and the outer diameter of the annular air-hole region. Step (2): Analyze the effective refractive index n when the high-order radial mode appears after being processed by Step (1). eff And the changes in the optical fiber performance.

5. An improved method for a photonic crystal fiber to reduce the occurrence of high-order radial modes, characterized in that, In the step (1), the material of the photonic crystal fiber is silica, and the material in the region where the refractive index of the material is reduced is fluorine-doped silica.

6. An improved method for a photonic crystal fiber to reduce the occurrence of high-order radial modes, characterized in that, In the step (1), when the wavelength of light is 1.55 microns, the reduction range of the refractive index of the material is between 0 and 0.

444.

7. An improved method for a photonic crystal fiber to reduce the occurrence of high-order radial modes, characterized in that After being processed by the step (1), the optical fiber can support the transmission of the 21st-order OAM mode without interference.

8. An improved method for a photonic crystal fiber to reduce the occurrence of high-order radial modes, characterized in that, After being processed by step (1), the confinement loss of the 21st order mode in the improved photonic crystal fiber is lower than 10 - 10 dB / m.

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