Flat dispersion photonic crystal fiber with low nonlinearity and low loss

Abstract

The invention discloses a flat dispersion photonic crystal fiber with low nonlinearity and low loss. The flat dispersion photonic crystal fiber comprises a cladding and a fiber core ring, a first circular air hole is formed in the fiber core ring; a layer of elliptical air hole ring and three or more layers of circular air hole rings are arranged in the cladding; the elliptical air hole ring is tightly attached to the outer ring of the fiber core ring and is formed by transversely arranging 16 elliptical air holes with the same size; the circular air hole rings are arranged on the outer circle of the oval air hole ring layer by layer at equal intervals, and each layer is of a regular octagonal structure and is an air hole array formed by second circular air holes consistent in size. Three second circular air holes are evenly distributed in each edge of the circular air hole ring on the innermost layer at intervals, and one more second circular air hole is distributed in each edge of the circular air hole ring on the outer side of the innermost layer. According to the invention, more transmission modes are supported, the transmission loss is lower, the nonlinear effect is lower, and the overall dispersion change is flat.

Description

Technical Field

[0001] The invention relates to the field of optical fiber technology, and in particular to a low-nonlinear and low-loss flat dispersion photonic crystal optical fiber. Background Art

[0002] With the rapid development of intelligent applications, the demand for data transmission has exploded, and many fields have put forward higher requirements for the transmission capacity and efficiency of communication systems. For example, intelligent transportation systems, sensor technology, intelligent security systems, etc. These application scenarios not only require high-speed data transmission, but also ensure that the communication system has strong anti-interference capabilities and reduces data distortion. However, the existing communication system faces problems of bandwidth bottleneck and signal interference. In recent years, vortex beams carrying orbital angular momentum (OAM) have attracted people's research interest due to their unique orthogonality. Using orbital angular momentum as an information carrier to improve communication capabilities provides a new idea for solving the problem of tight communication capacity.

[0003] Since information transmission in optical fiber is more stable, more resistant to interference, more secure, and has a longer transmission distance than in space, OAM optical fiber communication has become a research hotspot in the field of communications. However, due to the complex spatial structure of OAM light beams, their modes are usually not easy to maintain alone in traditional optical fibers, and are easily coupled with other modes, resulting in interference and distortion between modes. In recent years, photonic crystal fiber (PCF) has become a popular research object for researchers due to its design flexibility and unique optical properties (such as low loss, flat dispersion, controllable nonlinearity, etc.). In 2018, Zhang et al. proposed a circular photonic crystal fiber that can support 110 OAM modes in the C+L band, and the limiting loss of the supported vector modes is less than 10 -7 In 2021, Fu et al. designed a circular photonic crystal fiber composed of doped amethyst and large and small circular air holes, which can support the transmission of 118 OAM modes with a nonlinear coefficient of less than 1.25W. -1 / km, and the dispersion variation of low-order modes is relatively flat. In 2023, Wang et al. proposed a dual-ring photonic crystal fiber that supports 116 OAM modes, and the maximum dispersion of the supported modes reached 313ps / nm / km.

[0004] In long-distance fiber-optic communication systems and optical sensor systems, it is usually required that the optical fiber has relatively flat dispersion and low nonlinearity within the communication band, as well as low transmission loss, in order to reduce the influence of self-phase modulation (SPM) and other nonlinear effects, thereby allowing the signal to be transmitted over longer distances without relays or to improve sensing accuracy in sensor systems. However, the existing photonic crystal fibers still have shortcomings in these specific application scenarios, such as high nonlinear coefficients and uneven fiber dispersion. Summary of the invention

[0005] Purpose of the invention: In order to overcome the shortcomings of the background technology, the present invention discloses a low nonlinear and low-loss flat dispersion photonic crystal fiber.

[0006] Technical solution: The low nonlinear and low-loss flat dispersion photonic crystal fiber disclosed in the present invention comprises a cladding and a high-refractive-index core ring located in the middle of the cladding;

[0007] The interior of the core ring is a first circular air hole;

[0008] The envelope is provided with a layer of elliptical air hole rings and three or more layers of circular air hole rings;

[0009] The elliptical air hole ring is arranged closely to the outer ring of the fiber core ring, and is formed by 16 elliptical air holes of the same size arranged transversely;

[0010] The circular air hole rings are arranged in layers at equal intervals on the outer ring of the elliptical air hole ring, and each layer is a regular octagonal structure, which is an air hole array formed by second circular air holes with the same size;

[0011] Three second circular air holes are evenly distributed on each side of the innermost circular air hole ring, and one more second circular air hole is distributed on each side of the circular air hole ring on the outer side, and so on.

[0012] Furthermore, the core ring is made of silicon-doped germanium dioxide, and the cladding is made of silicon dioxide.

[0013] Furthermore, the radius of the first circular air hole is 18.5um-18.9um.

[0014] Furthermore, the radius of the core ring is 21.3um-21.8um; the thickness is 2.4um-3.3um.

[0015] Furthermore, the distance from the center of the fiber core ring to the center of the elliptical air hole is 24.8um-25.2um, and the distance to the center of the innermost second circular air hole is 31.8um-32.3um.

[0016] Furthermore, the major semi-axis size range of the elliptical air hole is 4.2um-4.6um; the minor semi-axis size range is 2.6um-3.1um; and the radius range of the second circular air hole is 2.9um-3.3um.

[0017] Furthermore, the spacing between the circular air hole ring layers ranges from 7um to 7.4um; and the spacing between adjacent second circular air holes on each side ranges from 7um to 7.4um.

[0018] Furthermore, the core ring adopts germanium dioxide doped with 60%±0.5% silicon dioxide; the radius of the first circular air hole is 18.7um; the radius of the core ring is 21.5um; the thickness is 2.8um; the distance from the center of the core ring to the center of the elliptical air hole is 25um, the distance to the center of the innermost second circular air hole is 32um, and the optical fiber radius is 62.5um; the major semi-axis size range of the elliptical air hole is 4.6um; the minor semi-axis size range is 2.6um; the radius range of the second circular air hole is 3um; the spacing range of the circular air hole ring layer is 7um; the spacing range of adjacent second circular air holes on each side is 7um.

[0019] Furthermore, the normal operating wavelength range of the optical fiber is 1.5um-1.6um.

[0020] Beneficial effects: Compared with the prior art, the advantages of the present invention are: the number of OAM modes supported by the optical fiber is basically the same as that of the existing photonic crystal fiber, and 114 OAM modes can be stably transmitted. The dispersion variation of the supported modes is flatter. In the wavelength range of 1.5-1.6um, the maximum dispersion variation is 17.22ps / nm / km. The limiting loss is one order of magnitude lower than that of the current traditional photonic crystal fiber. The limiting loss of the OAM mode supported by the optical fiber is mainly concentrated in 10 -14 ~10 -11 The order of dB / m, nonlinear coefficient is less than 0.6W -1 / km. In addition, the mode quality of the OAM mode of stable optical fiber transmission is higher than 95%. These excellent performances, on the one hand, reduce the nonlinear effect of the communication system and improve the accuracy of the sensor system. On the other hand, they also provide an effective solution to the current problem of tight communication capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a cross-sectional structural diagram of the present invention;

[0022] Figure 2 The photonic crystal fiber of the present invention supports vector mode HE 1,1 , EH 7,1 and HE 30,1Schematic diagram of electric field intensity distribution and phase distribution;

[0023] Figure 3 A refractive index distribution diagram of the photonic crystal fiber supporting the vector mode of the present invention;

[0024] Figure 4 The nonlinear coefficient of the photonic crystal fiber of the present invention is distributed as a function of wavelength;

[0025] Figure 5 The distribution diagram of the limiting loss of the photonic crystal optical fiber of the present invention as a function of wavelength;

[0026] Figure 6 The dispersion distribution diagram of the photonic crystal fiber of the present invention as a function of wavelength;

[0027] Figure 7 This is a distribution diagram of the mode quality of the photonic crystal fiber of the present invention as a function of wavelength. DETAILED DESCRIPTION

[0028] like Figure 1 The low nonlinear and low-loss flat dispersion photonic crystal fiber shown comprises a cladding 1 and a core ring 2 with a high refractive index located in the middle of the cladding 1;

[0029] The interior of the core ring 2 is a first circular air hole 3;

[0030] The cladding 1 is provided with a layer of elliptical air hole rings and three layers of circular air hole rings;

[0031] The elliptical air hole ring is arranged closely to the outer ring of the fiber core ring 2, and is formed by 16 elliptical air holes 4 of the same size arranged transversely;

[0032] The circular air hole rings are arranged in layers at equal intervals on the outer ring of the elliptical air hole ring, and each layer is a regular octagonal structure, which is an air hole array formed by second circular air holes 5 of the same size;

[0033] Three second circular air holes 5 are evenly distributed on each side of the innermost circular air hole ring, and one more second circular air hole 5 is distributed on each side of the circular air hole ring on the outer side, and so on.

[0034] The core ring 2 is made of silicon-doped germanium dioxide, and the cladding 1 is made of silicon dioxide. The radius of the first circular air hole 3 is 18.5um-18.9um. The radius of the core ring 2 is 21.3um-21.8um; the thickness is 2.4um-3.3um. The distance from the center of the core ring 2 to the center of the elliptical air hole 4 is 24.8um-25.2um, and the distance to the center of the innermost second circular air hole 5 is 31.8um-32.3um. The major semi-axis size range of the elliptical air hole 4 is 4.2um-4.6um; the minor semi-axis size range is 2.6um-3.1um; the radius range of the second circular air hole 5 is 2.9um-3.3um. The distance between the circular air hole ring layers ranges from 7um to 7.4um; the distance between adjacent second circular air holes 5 on each side ranges from 7um to 7.4um.

[0035] In the preferred embodiment of the present invention, the core ring 2 is doped with 60%±0.5% silicon dioxide with germanium dioxide; the radius of the first circular air hole 3 is r0=18.7um; the radius of the core ring 2 is r1=21.5um; the thickness is 2.8um; the distance from the center of the core ring 2 to the center of the elliptical air hole 4 is r2=25um, the distance to the center of the innermost second circular air hole 5 is r3=32um, and the fiber radius r4=62.5um; the major semi-axis size range of the elliptical air hole 4 is b=4.6um; the minor semi-axis size range is a=2.6um; the radius range of the second circular air hole 5 is D / 2=3um; the distance range of the circular air hole ring layer is d1=7um; the distance range of the adjacent second circular air holes 5 on each side is d2=7um. The normal working wavelength range of the optical fiber is 1.5um-1.6um.

[0036] This embodiment is simulated, and the finite element method is used for analysis and modeling. The dispersion, nonlinear coefficient and limiting loss are obtained through model optimization and calculation. Figure 2 The partial vector modes (in terms of HE) supported by the photonic crystal fiber of the present invention are shown. 1,1 , E.H. 7,1 , H.E. 30,1 The electric field intensity distribution diagram and phase distribution diagram are shown in Figure 1, where the small arrow in the electric field intensity diagram indicates the direction of the electric field. The HE and EH modes that are stably transmitted in the photonic crystal fiber are linearly superimposed to obtain the OAM mode. The superposition formula is as follows:

[0037]

[0038] In the above formula, l represents the topological charge number, m represents the radial order, even represents the even mode, odd represents the odd mode, and j represents the π / 2 phase difference between the odd mode and the even mode of the HE / EH mode.

[0039] Figure 3 The relationship between the effective refractive index of all vector modes supported by the photonic crystal fiber of the present invention and the wavelength is described. It can be seen that within the wavelength range of 1.5um to 1.6um, as the wavelength increases, the effective refractive index of the vector mode decreases.

[0040] The following is an explanation of the theoretical formula used in the calculation process of the implementation scheme of the present invention and the conclusions obtained: The nonlinear coefficient is a key parameter that reflects the degree of influence of nonlinear effects on optical signals during transmission in optical fibers. The lower the nonlinear coefficient value, the smaller the nonlinear effect on the optical signal, which is more conducive to the transmission of OAM modes in optical fibers. In sensor systems, the sensing accuracy will be improved. The calculation formula of the nonlinear coefficient is as follows:

[0041]

[0042] Where λ is the wavelength, A eff is the effective mode field area, is the nonlinear refractive index of the doping material (GeO2-SiO2), E(x,y) is the electric field distribution on the cross section of the optical fiber, and S represents the entire cross section of the optical fiber. Through experimental determination, the nonlinear refractive index of the 60% doping material GeO2-SiO2 used in the photonic crystal fiber of the present invention is 2.81×10 -20 m 2 / W. Figure 4 The nonlinear coefficients of all vector modes stably transmitted in the photonic crystal fiber of the present invention vary with wavelength. It can be seen that the nonlinear coefficients of all vector modes are less than 0.6w -1 / km, low nonlinearity helps reduce signal distortion and improves sensing accuracy in sensor systems. As the wavelength increases, the nonlinear coefficient tends to decrease overall, and the nonlinear coefficient of the high-order mode is lower than that of the low-order mode.

[0043] The limiting loss can directly reflect the ability of the optical fiber to constrain the OAM mode. The smaller the limiting loss value, the less energy in the core ring leaks into the cladding, thus ensuring that the signal can achieve long-distance and high-quality transmission. The limiting loss calculation formula is as follows:

[0044]

[0045] Where λ represents the wavelength, Im(n eff ) represents the imaginary part of the effective refractive index of the vector mode. Figure 5 As shown, the limiting loss of the vector mode supported by the photonic crystal fiber of the present invention is mainly concentrated in 10 -14 ~10 -11The order of dB / m is very helpful for high-quality signal transmission in long-distance communication systems.

[0046] The flattening of optical fiber dispersion will reduce the signal broadening caused by dispersion, thus helping the signal to be transmitted over a longer distance without the need for additional dispersion compensation. The dispersion in optical fiber is mainly caused by material dispersion (CD m ) and waveguide dispersion (CD w ), the specific formula is as follows:

[0047]

[0048] CD total =CD m +CD w .

[0049] Where λ is the wavelength, c is the speed of light in a vacuum, and Re(n eff ) represents the real part of the effective refractive index of the vector mode. Figure 6 As shown, in the wavelength range of 1.5 to 1.6 μm, the dispersion variation of all modes is relatively flat, with the maximum variation of 17.22 ps / nm / km (HE 8,1 ).

[0050] In addition, the relationship between the mode quality of the vector mode stably transmitted by the photonic crystal fiber of the present invention and the wavelength is as follows: Figure 7 It can be seen that the mode qualities of all vector modes are greater than 95.8%, which is beneficial for long-distance communication of OAM modes in optical fibers and for improving sensing accuracy.

Claims

1. A low nonlinear and low loss flat dispersion photonic crystal fiber, characterized in that: It comprises a cladding (1) and a core ring (2) with a high refractive index located in the middle of the cladding (1); The interior of the fiber core ring (2) is a first circular air hole (3); The envelope (1) is provided with a layer of elliptical air hole rings and three or more layers of circular air hole rings; The elliptical air hole ring is arranged closely to the outer ring of the fiber core ring (2), and is formed by 16 elliptical air holes (4) of the same size arranged transversely; The circular air hole rings are arranged in layers at equal intervals on the outer ring of the elliptical air hole ring, and each layer is a regular octagonal structure, forming an air hole array of second circular air holes (5) of uniform size; Three second circular air holes (5) are evenly distributed on each side of the innermost circular air hole ring, and one more second circular air hole (5) is distributed on each side of the circular air hole ring on the outer side, and so on.

2. The low nonlinear and low loss flat dispersion photonic crystal fiber according to claim 1, characterized in that: The fiber core ring (2) is made of silicon-doped germanium dioxide, and the cladding (1) is made of silicon dioxide.

3. The low nonlinear and low loss flat dispersion photonic crystal fiber according to claim 1, characterized in that: The radius of the first circular air hole (3) is 18.5 um-18.9 um.

4. The low nonlinear and low loss flat dispersion photonic crystal fiber according to claim 1, characterized in that: The radius of the fiber core ring (2) is 21.3um-21.8um; the thickness is 2.4um-3.3um.

5. The low nonlinear and low loss flat dispersion photonic crystal fiber according to claim 1, characterized in that: The distance from the center of the fiber core ring (2) to the center of the elliptical air hole (4) is 24.8um-25.2um, and the distance to the center of the innermost second circular air hole (5) is 31.8um-32.3um.

6. The low nonlinear and low loss flat dispersion photonic crystal fiber according to claim 1, characterized in that: The size range of the major semi-axis of the elliptical air hole (4) is 4.2um-4.6um; the size range of the minor semi-axis is 2.6um-3.1um; and the radius range of the second circular air hole (5) is 2.9um-3.3um.

7. The low nonlinear and low loss flat dispersion photonic crystal fiber according to claim 1, characterized in that: The spacing between the circular air hole ring layers ranges from 7um to 7.4um; and the spacing between adjacent second circular air holes (5) on each side ranges from 7um to 7.4um.

8. The low nonlinear and low loss flat dispersion photonic crystal fiber according to claim 1, characterized in that: The fiber core ring (2) is made of germanium dioxide doped with 60%±0.5% silicon dioxide; the radius of the first circular air hole (3) is 18.7um; the radius of the fiber core ring (2) is 21.5um; the thickness is 2.8um; the distance from the center of the fiber core ring (2) to the center of the elliptical air hole (4) is 25um, the distance to the center of the innermost second circular air hole (5) is 32um, and the fiber radius is 62.5um; the major semi-axis size range of the elliptical air hole (4) is 4.6um; the minor semi-axis size range is 2.6um; the radius range of the second circular air hole (5) is 3um; the circular air hole ring layer spacing range is 7um; the spacing range of adjacent second circular air holes (5) on each side is 7um.

9. The low nonlinear and low loss flat dispersion photonic crystal fiber according to claim 1, characterized in that: The normal working wavelength range of optical fiber is 1.5um-1.6um.

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