Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11474170) and the Natural Science Foundation of Tianjin, China (Grant No. 16JCYBJC16900).
Received Date:17 March 2019
Accepted Date:09 April 2019
Available Online:01 June 2019
Published Online:05 June 2019
Abstract:In an optical fiber communication system, vortex beams have aroused great interest in the last several decades. Vortex beams possess many intriguing properties. For example, they have the ability to carry orbital angular momentum (OAM) which is mutually orthogonal. The OAM is a fundamental physical quantity of light which can be used as information carriers for transmission channel of optical fiber. Combined with the existing multiplexing techniques such as wavelength division multiplexing technique, advanced multilevel amplitude modulation formats, etc., the vortex beams provide an alternative to the increase of the transmission capacity and spectral efficiency of the optical fiber transmission system. Recently, long-length transmission of vortex-beam in optical fiber has been realized and there have also occurred some new designs of optical fiber on vortex beams, such as air-core ring shaped fiber, graded index vortex fiber, multi-ring fiber, and supermode fiber. Photonic crystal fiber (PCF) is flexible in design. Therefore, it is easy to regulate the transmission performance of PCF by adjusting the radius and the pitch of the air holes and so on. In this paper, we propose a newly designed sixfold photonic quasi-crystal fiber (SPQCF) to transmit vortex beams stably. Transmission characteristics of this newly designed fiber are simulated and calculated by using COMSOL multiphysics software. When the wavelength of the incident light is 1550 nm, the effective index difference between the vortex modes in a group is more than 10–4 which is large enough to preclude the LP modes from being formed, and to transmit 7 vector modes (10 OAM modes). Changing the radius and pitch of the air holes, we can regulate the dispersion characteristic and confinement loss of the SPQCF flexibly. At 1550 nm, the confinement loss of the SPQCF maintains 10–8?10–7 which is low enough to confine the vortex beams in the fiber core. When the incident light wavelength of HE21 ranges from 1500 nm to 1800 nm (r0 = 1.9 μm), the dispersion coefficient of the SPQCF is between 63.51?65.42 ps·nm–1·km–1 which tends to be flat. By changing r0, the flat trend is adjusted to different wavelength range. This dispersion characteristic possesses great potential for the transmission of optical solitons. The effective mode area (HE21) is about 40 μm2 and the nonlinear coefficient (HE21) is maintained on the order of 10–3 between 1500?1600 nm. These features suppress the generation of nonlinear effect in the fiber and benefit the transmission of vortex beams. The stable transmission distance is longer than 1 km. In summary, we design a new type of PCF featuring quasi-crystal structure which has a ring shaped fiber core and supports the transmission of vortex beams stably. Keywords:photonic quasi-crystal fiber/ vortex beams/ dispersion/ confinement loss
其中$\lambda $为自由空间光波长, ${\rm{Im}} ({n_{{\rm{eff}}}})$为模式有效折射率虚部, 代表光能量衰减参量. 以${\rm{H}}{{\rm{E}}_{21}}$模为例, 对其各项性能进行研究分析. 通过模拟计算, 分析了1000—2000 nm波段内, 光子晶体光纤限制性损耗随中心空气孔半径${r_0}$的变化曲线, 如图4所示. 从图4中可以看到, 随着波长的增大, ${\rm{H}}{{\rm{E}}_{21}}$的限制性损耗增大; 当中心空气孔的半径变大时, ${\rm{H}}{{\rm{E}}_{21}}$模的限制性损耗也相应增大. 在波段1500—1600 nm内, 此光纤具有相当低的限制性损耗(${10^{ - 8}}$—${10^{ - 7}} $量级), 因此在传输的过程中, 光能很好地局域在纤芯, 有效地抑制了限制性损耗对涡旋光传输的影响. 图 4 HE21模的限制性损耗随波长的变化(不同中心空气孔半径), 插图为1500—1600 nm波段内HE21模的限制性损耗 Figure4. Confinement loss as a function of wavelength for HE21 mode with different r0, the inset shows the loss between 1500?1600 nm.
D为光纤模式的总色散系数, 包括材料色散系数和波导色散系数; c为真空中的光速; ${\rm{Re}} ({n_{\rm eff}})$为模式有效折射率的实部. 图5所示为不同的中心空气孔半径下, ${\rm{H}}{{\rm{E}}_{21}}$模色散系数随波长$\lambda $的变化曲线. 从图5中可以看出, 随着中心空气孔半径${r_0}$的改变, 能够实现特定波段的色散平坦趋势. 当中心空气孔为1.9 μm时, 光纤在1500—1800 nm波段保持色散平坦, 色散系数维持在63.51—5.42 ps/(nm·km)之间 插图部分表示在不同中心空气孔半径下, 色散平坦趋势所对应的波段, 我们可以通过调节中心空气孔的大小, 对其色散平坦区域进行调节, 使光纤在特定波段实现色散平坦. 并且随着中心空气孔的增大, 色散平坦区域向短波段移动, 在光孤子传输方面具有潜在的应用. 图 5${\rm{H}}{{\rm{E}}_{21}}$模色散系数随波长的变化(不同中心空气孔半径), 插图为${r_0}$对涡旋光平坦趋势的影响 Figure5. Dispersion as a function of wavelength for ${\rm{H}}{{\rm{E}}_{21}}$ with different ${r_0}$, the inset shows the flat trend with di-fferent ${r_0}$
33.2.3.光纤的模场面积和非线性特性 -->
3.2.3.光纤的模场面积和非线性特性
图6(a)表示不同中心空气孔半径下, ${\rm{H}}{{\rm{E}}_{21}}$模模场面积随波长的变化, 模场面积表征光纤模式传输过程中实际的模场分布的大小, 图 6 (a) HE21模的模场面积; (b) HE21模的非线性系数随波长的变化 (不同中心空气孔半径); (a), (b) 的插图分别为 波段内的模场面积和非线性系数 Figure6. (a) Effective mode area of HE21, (b) nonlinear coefficient as a function of wavelength for HE21 mode with different r0, the inset shows the (a) effective modes area and (b) nonlinear coefficient between 1500?1600 nm.
其中$\lambda $, c, $\Delta t$分别为波长、真空中光速、走离时间. 图7表示光纤中不同本征模10 ps走离长度${L_{10\;{\rm{ps}}}}$随波长的变化. 对于光纤中涡旋光的稳定传输, 模式的10 ps走离长度越大, 其传输特性越好, 信号失真越小. 因此, 相比较而言, 由${\rm{H}}{{\rm{E}}_{21}}$模组成的${\rm{OA}}{{\rm{M}}_{1,1}}$模的传输性能最好, 其保持信道稳定传输的距离大于4.9 km. 但就全部的涡旋模式而言, 稳定的传输距离大于1 km, 所以其整体的传输性能保持在很好的水平. 图 7 光纤中不同本征模的${L_{10\;{\rm{ps}}}}$随波长的变化 Figure7.${L_{10\;{\rm{ps}}}}$ as a function of wavelength for vortex modes in SPQCF.