Straight plasma channels are widely used to guide relativistic intense laser pulses over several Rayleigh lengths for laser wakefield acceleration. Recently, a curved plasma channel with gradually varied curvature was suggested to guide a fresh intense laser pulse and merge it into a straight channel for staged wakefield acceleration [Phys. Rev. Lett. 120, 154801 (2018)]. In this work, we report the generation of such a curved plasma channel from a discharged capillary. Both longitudinal and transverse density distributions of the plasma inside the channel were diagnosed by analyzing the discharging spectroscopy. Effects of the gas-filling mode, back pressure and discharging voltage on the plasma density distribution inside the specially designed capillary are studied. Experiments show that a longitudinally uniform and transversely parabolic plasma channel with a maximum channel depth of 47.5 μm and length of 3 cm can be produced, which is temporally stable enough for laser guiding. Using such a plasma channel, a laser pulse with duration of 30 fs has been successfully guided along the channel with the propagation direction bent by 10.4°.

中红外光纤作为中红外领域的重要器件，在中红外激光产生与传输、生物医学检测、环境检测等领域有着重要应用。然而中红外光纤长期存在制备困难、制备材料化学稳定性差等问题，限制了其发展。与实芯光纤相比，空芯光纤通过构建包层微结构将光波限制在空气中传输，可以大幅降低光纤光学性能对制备材料的依赖，从而为光波传输提供一个低损耗、低色散、低延迟、低非线性、高损伤阈值的理想传输通道，这为中红外光纤的发展拓宽了道路。文中从光纤结构、拉制方式、材料吸收、传输性能等方面分析了石英基和软玻璃基中红外空芯光纤的发展历程、研究现状和应用前景。并通过理论仿真分析了石英基单圈结构和嵌套管结构反谐振空芯光纤吸收损耗、限制损耗与纤芯、壁厚、波长之间的关系，为低损耗中红外反谐振空芯光纤的制备和应用提供了理论指引。

In the fields of light manipulation and localization, quasiperiodic photonic crystals, or photonic quasicrystals (PQs), are causing an upsurge in research because of their rotational symmetry and long-range orientation of transverse lattice arrays, as they lack translational symmetry. It allows for the optimization of well-established light propagation properties and has introduced new guiding features. Therefore, as a class, quasiperiodic photonic crystal fibers, or photonic quasicrystal fibers (PQFs), are considered to add flexibility and richness to the optical properties of fibers and are expected to offer significant potential applications to optical fiber fields. In this review, the fundamental concept, working mechanisms, and invention history of PQFs are explained. Recent progress in optical property improvement and its novel applications in fields such as dispersion control, polarization-maintenance, supercontinuum generation, orbital angular momentum transmission, plasmon-based sensors and filters, and high nonlinearity and topological mode transmission, are then reviewed in detail. Bandgap-type air-guiding PQFs supporting low attenuation propagation and regulation of photonic density states of quasiperiodic cladding and in which light guidance is achieved by coherent Bragg scattering are also summarized. Finally, current challenges encountered in the guiding mechanisms and practical preparation techniques, as well as the prospects and research trends of PQFs, are also presented.

空芯反谐振光纤（HCARF）的科学研究取得了突破性进展，其有望突破现有传统光纤的一些固有本征限制。HCARF将光束缚在空气芯中，在传输及其应用上具有传统光纤不可比拟的优势，因此，HCARF成为当前光通信领域的研究热点。文章介绍了HCARF的导光机理，并分析了其在光纤通信系统中的容量优势，阐述了HCARF的发展机遇与面临的挑战，希望能够为我国下一代通信大容量超宽带长距离传输光纤提供一定的参考价值与借鉴。

We measured the parameter reproducibility and radial electron density profile of capillary discharge waveguides with diameters of 650 $\mathrm{\mu} \mathrm{m}$ to 2 mm and lengths of 9 to 40 cm. To the best of the authors’ knowledge, 40 cm is the longest discharge capillary plasma waveguide to date. This length is important for $\ge$10 GeV electron energy gain in a single laser-driven plasma wakefield acceleration stage. Evaluation of waveguide parameter variations showed that their focusing strength was stable and reproducible to $<0.2$% and their average on-axis plasma electron density to $<1$%. These variations explain only a small fraction of laser-driven plasma wakefield acceleration electron bunch variations observed in experiments to date. Measurements of laser pulse centroid oscillations revealed that the radial channel profile rises faster than parabolic and is in excellent agreement with magnetohydrodynamic simulation results. We show that the effects of non-parabolic contributions on Gaussian pulse propagation were negligible when the pulse was approximately matched to the channel. However, they affected pulse propagation for a non-matched configuration in which the waveguide was used as a plasma telescope to change the focused laser pulse spot size.

模式是光纤光学中的基本概念，也是光纤激光器研究中最关注的问题之一。在一般的教材、文献中对模式的简并问题涉及较少，没有给出清晰、直观的物理图像。文中采用经典的电磁场理论，阐明了矢量模的简并度、标量模的简并度以及矢量模和标量模之间的简并关系。在弱导阶跃折射率光纤中，矢量模和标量模都是描述光场的正交完备基，矢量模[HE_1n^(o)、HE_1n^(e)]与标量模[LP_0n^(ye)、LP_0n^(xe)]描述的光场空间是2维的，矢量模[TE_0n、HE_2n^(o)、HE_2n^(e)、TM_0n]与标量模[LP_1n^(yo)、LP_1n^(ye)、LP_1n^(xo)、LP_1n^(xe)]描述的光场空间是4维的，矢量模[EH_{m-1, n}^(o)、EH_{m-1, n}^(e)、HE_{m+1, n}^(o)、HE_{m+1, n}^(e)]与标量模[LP_mn^(yo)、LP_mn^(ye)、LP_mn^(xo)、LP_mn^(xe)]描述的光场空间也是4维的。