Quasiperiodic photonic crystal fiber

Microstructure Fiber

Microstructure optical fiber (MOF) offers a wide range of applications in short-distance communication systems, fiber lasers, supercontinuum generation, sensors, modulators and wavelength converters, as well as in various scientific fields such as optics, electronics, medicine, biology and environmental science. Conventional step-index fibers suffer from the constraints of the monotonic structure construction and material properties, which limits the further optimization of their optical performance in terms of work bandwidth, dispersion and loss. For practical purposes such as high-capacity transmission and high-power lasers, they face the challenges such as maintaining single-mode operation with a small core size, ensuring a low cutoff wavelength and matching the thermal characteristics of the core and cladding materials. In contrast, MOF, such as the well-known photonic crystal fiber (PCF), exhibit a strong dependence of the effective refractive index of the cladding on the structural parameters and wavelength, which enables a broad tunability of the effective refractive index contrast (0 ~ 90%) between the core and cladding modes. Therefore, by adjusting the structural parameters of the fiber, such as the size, shape and lattice constant of the air holes, the fundamental mode propagation constant will be effectively controlled, easily achieving single-mode transmission with zero cutoff wavelength, high numerical aperture, ultra-flat dispersion, low loss or high nonlinearity coefficient, among other desirable fiber features. These features endow PCF with an unparalleled advantage over traditional fiber.

Photonic quasicrystal fiber: concept, structure classification, development milestones and prospective applications

Photonic quasicrystal fiber (PQF) (also named quasiperiodic PCF) is a special type of PCF that features rotational symmetry and long-range order, but not translational symmetry, in its air-hole arrangement structure. Unlike conventional PCF, PQF offers more structural flexibility, versatile mode manipulation techniques, diverse defect modes, and benefits for optimizing dispersion, confinement loss and nonlinearity coefficients, as well as designing novel fibers and fiber devices. PQF has emerged as a new frontier of innovation for fiber sensing, communication and other applications. The PQF structure can be divided into Stampfli-type, Penrose-type and Sunflower-type, which employs either total reflection-type or bandgap-type guiding mechanisms for mode transmission. The current researches on PQF applications covers dispersion compensation, polarization maintaining, sensor, filter, orbital angular momentum mode transmission and supercontinuum generation. The concept proposal, cross-section structure, development history and potential applications of PQF are shown in Fig. 1.

Review of photonic quasicrystal fiber

Jianjun Liu, associate professor at the School of Physics and Electronics, Hunan University, and Exian Liu, lecturer at the Central South University of Forestry and Technology, have comprehensively reviewed the PQF. They first introduce the basic concept, working mechanism and development history of PQF, then summarize their optical performance optimization (such as dispersion control, polarization maintaining and high nonlinearity) and applications in optics (such as supercontinuum generation, orbital angular momentum mode transmission, plasmonic resonance sensing, filtering and topological mode transmission). The challenges faced by PQF in revealing the mode guidance mechanism and developing practical fabrication techniques are also addressed in detail. Finally, they outlook the future trends and prospects of PQFs. Their work is published in the 6^th issue of the 21^st volume of Chinese Optics Letters (Exian Liu and Jianjun Liu, Quasiperiodic photonic crystal fiber [Invited]), and is selected as the cover paper of that issue, as shown in Fig.2.

封面|光子准晶光纤的“前世今生

湖南大学物理与微电子科学学院刘建军副教授与中南林业科技大学刘娥贤讲师合作，全面综述了光子准晶光纤。概述了光子准晶光纤的基本概念、工作机制及发展历程，并总结了其光学特性优化（色散控制、保偏能力及高非线性系数等）及在光学领域的潜在应用（超连续谱产生、轨道角动量模式传输、等离子体共振传感器、滤波器及拓扑模式传输等）。同时，讨论了其在导模传输机制的揭示及实际制备技术等方面面临的挑战，展望了其发展趋势及应用前景。相关成果投稿后仅用5天就被录用，并发表于Chinese Optics Letters第21卷第6期上（Exian Liu and Jianjun Liu, Quasiperiodic photonic crystal fiber [Invited]），并入选为当期封面论文。

背景介绍

以光子晶体光纤（Photonic Crystal Fiber，PCF）为代表的微结构光纤凭借包层灵活的结构可调性及基模有效折射率强烈的波长相关性，使得包层与芯层的有效折射率差具有较宽的可调范围（0 ~ 90%）。因此，通过合理设置结构参数（如：晶格常数、空气孔尺寸及形状等）可改变基模传输常数，进而可实现无截止波长单模传输、较高数值孔径、超平坦色散、低损耗及高非线性系数等光纤特性。通过结构定制实现光学特性的灵活调控，突破了传统阶跃光纤的局限性，如：有限的纤芯尺寸或截止波长以保证单模传输，芯层与包层材料须拥有相同热学特性等。自世界上第一根PCF成功制备，PCF得到了极其快速的发展，已在传感器、激光器、调制器、波长变换器及超连续谱产生等方面，以及在光学、电学、医学、生物学及环境学等领域呈现巨大应用价值。因此，开发新型PCF对于微结构光纤及其器件的发展具有重要意义。

光子准晶光纤的概念提出、结构类型及发展历程

1998年，二维光子准晶（也称为准周期光子晶体）被提出，并呈现诸多优异、独特的光学特性。该晶格结构具有旋转对称性与长程有序性，但不具备平移对称性，这为光子准晶与光纤的结合奠定了基础。2007年，光子准晶光纤（Photonic Quasicrystal Fiber，PQF）被提出。至今，已发展出多种类型PQF，包括Stampfli型、Penrose型及Sunflower型，其模式传输机制包括全反射型及带隙型导光机制，相关应用研究主要包括色散补偿、偏振保持、传感器、滤波器、轨道角动量模式传输及超连续谱产生等。其概念提出、截面结构及发展历程，如图1所示。

光子准晶光纤的传感器应用

PQF因高结构自由度、灵活可控的空气孔结构或多组分材料，为无标记、抗电磁干扰、实时监测、结构紧凑的光纤传感器的设计提供了新思路，有望应用于生物医疗、环境食物及航天军事等领域。目前，PQF传感器的研究主要集中在干涉型PQF传感器、等离子体共振PQF传感器及轨道角动量PQF传感器，设计不同结构和沉积不同金属材料可提升传感器的灵敏度、传感参数范围、分辨率等性能参数。干涉型PQF传感器以纯二氧化硅为背景材料，基于波动光学中光干涉原理产生与外界环境变化相关的干涉条纹，主要传感压力、温度、应力、电场等物理参量。

传统的干涉型PCF传感器有马赫•曾德、萨格•纳克、法布里•珀罗等，干涉型PQF传感器的研究才刚刚起步，其中双传导芯PQF可产生对外界温度和压力的干涉条纹，灵敏度高，如图2(a)，而等离子共振PQF传感器主要以增强PQF导模与金属表面等离激元极化子模式的耦合作用为主（图2(b)-2(f)），通常通过结构调控或者金属薄膜选择，此类传感器对环境折射率变化极其灵敏，灵敏度最高可达到62 000 nm/RIU，传感折射率范围为1.3至1.5。此外，PQF轨道角动量模式也可激发金属表面等离激元，灵敏度可达4466.5 nm/RIU，分辨率2.3 × 10⁻⁵ , 如图2(g)和2(h)。

光子准晶光纤的轨道角动量模式传输

轨道角动量（Orbital Angular Momentum，OAM）模式的螺旋相位及空间正交特性使其传输更为稳定，由于OAM模式在理论上的拓扑荷数无限大，OAM模式被认为可以实现无数个信号通道，是建立高质量模分复用系统以及大幅度提升信号传输容量的最重要的通信方案之一，对解决现代网络通信日益增加的传输数据的问题具有指导意义。自由空间OAM通信已有大量研究，传输距离可跨越几百米到几千米，但自由空间的OAM模式在传输过程中易受到空气中灰尘散射、气体密度不均及大气湍流等效应的影响，导致模式传输不够稳定。采用光纤传输OAM模式能够有效解决此问题，是目前研究OAM传输的主要方法之一。传统阶跃型光纤通常通过构建较高折射率环形芯层以实现OAM模式传输，如：反抛物线折射率环形光纤、空气中心环形纤芯光纤、双纤芯光纤及螺旋环形光纤等。然而，这些设计仍存在亟待解决的问题：（1）产生OAM模式的LP模组内有效折射率差Δn_eff过小，可能造成OAM模式再次耦合成LP模；（2）通过掺入高折射率材料构建环形芯层需要精确的掺杂工艺，制备过程复杂。

微结构光纤因灵活的结构设计在构建高折射率芯层具有很大优势。PQF包层气孔旋转对称性结构更易构建环形芯传输层，与所传导的OAM螺旋相位结构匹配度高，多数OAM-PQF以调控结构为主（图3(a)-3(d)）。然而，环形微结构光纤仍存在需要改善之处：（1）高数量级Δn_eff对于低阶OAM有效，而对于高阶OAM模式Δn_eff数值仍为较低数量级，约为10^-4；（2）虽采用高折射率光纤背景材料，但由于较高非线性效应，此种光纤并不适合光通信系统，且支持OAM模式的数量极少；（3）多数光子晶体光纤呈现较大的光纤色散或色散变化，例如，一种大尺寸环形纤芯支持26个OAM模式，但在1.55 µm通信波长，多数模式的色散大于200ps/nm/km。过高色散将限制光纤长距离传输，并会引起信号传输时的误码率。

光子准晶光纤的超连续谱产生

相对于早期的固体、气体及液体等非线性介质，光纤能够将泵浦光约束在纤芯中以增强光与物质相互作用，可诱发丰富的非线性效应，是超连续谱产生的理想介质。特别是微结构光纤，可定制色散与非线性系数的优良特性，极大地促进了超连续谱的发展。目前，产生超连续谱的PQF结构集中于Sunflower型和Stampfli型，以非线性折射率系数较低的二氧化硅为背景材料的结构，因导模非线性系数低，在近红外波段产生的光谱带宽约0.6 μm，如图4(a)。若以非线性折射率较高的背景材料（图4(b)-4(d)），如硫族化物、氟化物等，得到的非线性系数可达2079 W^-1 km^-1，诱发的非线性效应更为丰富，如：自相位调制效应、拉曼效应、孤子频移及色散波等，且光谱范围覆盖1 μm至12.5 μm，带宽宽度大于11 μm。