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08
2025
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ELight | Hollow Core Fiber - The Perfect Optical Waveguide
Author:
编者按（郭宸孜）
菲利普·罗素（Philip Russell）是世界公认的光子晶体光纤之父，也是空芯光纤的创始人，历任英国皇家学会会士、德国马普光学所创所所长以及美国光学学会主席，现为中国科学院外籍院士、罗素先进光波科学中心（RCALS）主任。Philip Russell基于空芯光纤在光通信及高功率紫外光刻光源的应用曾创建过两家高科技初创公司(BlazePhotonics和Ultralumina,后分别被NKT Photonics和全球光刻机巨头ASML收购)。近年来，空芯光纤在下一代光纤通信与超快激光等领域展现出巨大优势与应用潜力，引起了世界范围内研究热潮，相关应用技术开发呈现爆发式增长（例如去年底微软公布了将在两年内部署1.5万公里空芯光纤的重大部署）。据此，eLight邀请空芯光纤创始人Philip Russell院士撰写空芯光纤评述，回顾空芯光纤的发明历程，并揭示其关键原理与技术难点。
使用空芯波导替代自由空间来传输电磁辐射的理念由来已久。自20世纪40年代起，横截面尺寸仅支持基模的空芯金属波导就已广泛应用于包括雷达装置在内的各种微波电路中。许多早期研究激光的科学家与工程师都具备微波技术背景，这对光通信的早期发展产生了深远影响。不过，由于金属波导在光频段的损耗极高，在20世纪60年代激光出现后，研究人员开始尝试在地下管道中安装透镜组以构建自由空间光链路。虽然这种方法能够克服光束衍射问题，但人们很快发现，由路面交通振动等因素引发的光束指向误差会随着距离迅速累积，致使此类系统无法满足远程通信的需求[1]。与此同时，玻璃毛细管也曾被短暂研究过，然而由于其泄漏损耗过高（与λ2d-3m-1成正比，其中λ为真空中的波长，d为孔径），因而被认为不适用于通信[2]。不过，在20世纪90年代初，孔径大（d/λ~300）、长度短（数米）的充气毛细管曾被用于拓宽超短钛宝石脉冲（<0.1ps，~10mJ）的频谱，脉冲经压缩后宽度可接近单周期[3]。此后，基于毛细管的脉冲压缩技术成为超快与阿秒科学领域的核心技术[4,5]。
1978年的一项理论研究提出了一种布拉格光纤结构，如图1a所示，它由环绕中心空芯的多层同心环构成，通过布拉格反射来实现对光的束缚[6]。虽然在可见光与近红外波段实现这一想法后来被证明难以做到 （后文将讨论单环空芯光子晶体光纤的类似问题），但麻省理工学院的团队通过在多层同心环中交替使用聚合物和硫系玻璃材料并拉制成光纤，成功制备出适用于10μm波段的空芯布拉格光纤[7]。这种光纤损耗约为1dB/m，足以灵活传输10μm的CO2激光，并且目前已被应用于激光手术。
1991年5月，在美国巴尔的摩的激光与电光会议（CLEO）会议上，我受到了一个观点的启发——即除了所有周期性材料中都具有的众所周知的电磁波阻带外，在一些具有恰当结构的材料中也可能形成完全的三维光子带隙（PBG）[8]——提出了开发一种空芯光纤的构想：其包层采用周期性结构设计以支持二维光子带隙，从而实现对光场的约束[9,10]。在光纤轴周围特定范围的锥角内，无论以何种偏振态或方位角入射的光线都将被完全反射回纤芯，并在一系列满足纤芯谐振条件的反射角度上形成本征模式。这种空芯波导的低损耗导光特性首次打破了已有百年历史的瑞利判据（Rayleigh criterion）[11]。瑞利准则限制了光束在真空环境中保持聚焦的传输距离，一旦超过这个距离，衍射效应就会导致光束发散。
这一新型导光机制的独特之处在于其低损耗传输并不依赖全内反射（TIR）—— 毕竟在空芯结构中，全内反射本就不可能存在（若要满足TIR条件，模场折射率需小于1）。理想的空芯光纤结构应包含周期性排列的空芯微通道阵列，结构中心为一个空芯，且整个结构沿光纤传输方向贯穿。回溯到20世纪90年代初，为了避免被冠以 “又一个疯狂的点子”，我特意将其命名为 “多孔光纤（holey fibre）”,并指出第一个单词中有一个“e”。要实现一个有效的“多孔光纤”原型，不仅要开发新型光纤拉制技术，还需要设计一种新的求解麦克斯韦方程组的数值算法[9]。在1991-1995年之间的四年时间里，我的团队（当时在南安普顿大学，1996年后转至巴斯大学）致力于研发适用的制备工艺，最终确立了如今全球通用的 “堆栈-拉丝（stack and draw）” 技术。在研究初期，团队因在空芯环绕的实芯结构中发现改进的全内反射现象而有所分心；这一发现直接催生了首个覆盖倍频程的飞秒频率梳，也为特奥多尔·亨施（Theodore Hänsch）和约翰·霍尔（Jan Hall）赢得了2005年的诺贝尔奖[12]。在这个 “意外收获” 之后，首根空芯光子晶体光纤（HC - PCF）于1998年在巴斯大学的拉丝塔中诞生。其透射光谱呈现窄带高透与宽带高损交替出现的特征，这与光子带隙导光理论的预期完全吻合（见图1b、c）[13]。
Figure 1: Collection of structural diagrams of hollow-core photonic crystal fibers. (a) Schematic diagram of the hollow-core Bragg fiber structure. For a pure silica cladding structure, the grating period is ~0.47λ. (b) Scanning electron microscope image of the first hollow-core photonic bandgap fiber, with a core diameter of 14 μm [13]. (c) Guided mode spot of the photonic bandgap fiber in (b) when excited by white light, showing its spectral filtering characteristics. (d) Scanning electron microscope image of an optimized photonic bandgap hollow-core photonic crystal fiber [14]. (e) Scanning electron microscope image of a hollow-core photonic crystal fiber with a Kagomé structured cladding [15]. (f) Kagomé structured hollow-core photonic crystal fiber realizing white light waveguide. (g) Schematic diagrams of three single-ring hollow-core photonic crystal fiber structures with six-fold rotational symmetry: The left is a non-contact capillary ring array [16], the middle is a stacked capillary structure [17], and the right is a nested capillary configuration [18]. Introducing a complex radial structure can enhance the radial stopband effect, significantly reducing leakage loss and bending loss. (h) A conical beam formed by the radial stopband reflection of light in the core of a single-ring photonic crystal fiber. When the grazing incidence condition is satisfied, its reflectivity approaches 100%. When the transverse resonance condition is satisfied, a stable mode will be formed in the core.
From the current perspective, HC-PCF does have many advantages worth exploring in depth, because it occupies an area in the parameter space considered a “sweet spot” by engineers. Firstly, since most of the light in the core propagates in gas or vacuum, this significantly reduces glass-related absorption and Rayleigh scattering (Rayleigh scattering is proportional to 1/λ 4 and by adjusting the thickness and shape of the glass microstructure to form an anti-resonance with the core mode, the proportion of light in the glass can be minimized. Secondly, for a core diameter d c much larger than the wavelength (typical value d c /λ∼40) HC-PCF, the light constituting the guided mode is incident on the core boundary at a grazing angle, producing near 100% Fresnel reflection, which greatly enhances the binding strength of any mechanism that can bind the light field.
To understand the mechanism by which the hollow-core fiber core confines light, we need to start with the physical concept of electromagnetic wave stopbands—a concept that has been studied for over a hundred years and explains many phenomena, such as Bragg scattering of X-rays in crystals, the iridescence of opals, and the iridescent colors of butterfly wings and beetle shells. In fact, stopbands are a common feature of periodic structures with periods on the scale of wavelengths. In a periodic structure composed of two materials with different refractive indices, when the refractive index difference Δn is much smaller than the average refractive index n, the Bragg condition has a clear definition. However, as Δn approaches n, this condition becomes increasingly ambiguous—in this case, at a fixed light frequency, the range of incident angles satisfying the Bragg reflection condition will gradually increase; at a fixed incident angle, the range of light frequencies satisfying the Bragg reflection condition will gradually broaden. When the Bragg reflection condition is satisfied, light cannot propagate in the multilayer periodic structure (although tunneling can occur when the number of layers is small), and will therefore be strongly reflected.
To form a complete two-dimensional photonic bandgap structure, light of all directions and all polarization states should not be able to propagate in this structure (Figure 1d shows an optimized photonic bandgap hollow-core photonic crystal fiber developed by BlazePhotonics in the early 2000s [14]). However, for the fundamental mode in a hollow-core photonic crystal fiber, it is only necessary to suppress the propagation of light in a specific cone angle in the cladding. This can be achieved by forming a radial stopband, which is like placing a cylindrical "mirror" around the core. When the two eigenvalues (e ±q ，q is a positive number) of the transfer matrix of a single-period structure are real numbers, a stopband can be formed. In a structure with N periods, the light field amplitude will decay by a factor of e ± N q . When the incident angle is close to grazing incidence, the value of q will be much greater than 1, so a single-period structure can also achieve a very high reflection coefficient, while the reflection spectrum bandwidth (which narrows with increasing N) will become very wide.
After the publication of the first paper on photonic bandgap hollow-core fibers [13], researchers unexpectedly found that hollow-core fibers with a Kagomé lattice structure (Figure 1e–f) cladding can also be used to transmit white light, with a transmission loss of 0.1 dB/m [15]. Since numerical simulations did not show the existence of a photonic crystal bandgap, the light transmission mechanism of this type of hollow-core fiber was initially unclear. However, it is speculated that the design of the first layer of hollow tube rings may be crucial. In 2008, a research group in Moscow reported a hollow-core structure fiber with a ring of capillaries bonded to the inner wall of its hollow core tube. The research group experimentally measured multiple guiding spectral bands of this fiber structure (some guiding spectral bands have losses below 1 dB/m), confirming that this greatly simplified structure can also be used for light transmission [16]. This work ushered in a wave of research on single-ring HC-PCF (Figure 1g). In a recent report, the loss of this type of hollow-core fiber has been optimized to be lower than that of currently widely used silica communication fibers [17-21]. It should be pointed out that the optimization of hollow-core fiber loss requires a large amount of numerical simulation work, trial-and-error process, and new preparation techniques.
In single-tube ring-core photonic crystal fibers, for modes with a mode factor n m close to 1, except for some relatively narrow "photonic band windows" at certain wavelengths, the fiber exhibits strong reflection in all directions for a conical beam centered on the fiber axis (Figure 1h). Within these photonic band windows, the eigenvalues of the transmission matrix are e ± i q , light in the band can propagate in the cladding, resulting in spectral narrowband windows with high transmission loss. These high-loss spectral narrowbands are usually attributed in the literature to "resonance" or "anti-crossings" of the core and ring wall. In spectral regions far from these high-loss spectral narrowbands, the light guiding mechanism of the fiber is "anti-resonant reflection," which is essentially that the single-period structure forms a photonic bandgap. Single-ring core photonic crystal fiber has some similarities to Bragg fiber proposed in 1978 [6], but the single-tube ring core photonic crystal fiber has gaps between the rings, which leads to possible light leakage. However, at grazing incidence, the wall reflectivity of the ring is very high, equivalent to specular reflection, and the gap between the rings is much smaller than the transverse effective wavelength, which allows light to be strongly reflected back into the core.
When applied to the communication field, the loss of the hollow-core fiber needs to be very low. However, in other applications, the low-loss characteristic is not the most important. In a hollow-core photonic crystal fiber with a loss of 0.1 dB/m, the beam can maintain a high light intensity in the core while achieving a transmission distance of 30 meters with only half the light intensity attenuation, which greatly enhances the interaction strength and distance between light and gas [15, 20]. Because the dispersion properties of hollow-core photonic crystal fibers can be controlled by the pressure of the filled gas, gas-filled hollow-core photonic crystal fibers have played a revolutionary role in the development of gas nonlinear optics. Using this type of fiber, pulses with a pulse width of 30 fs and pulse energy of several μJ can achieve extreme compression and generate dispersion waves in the vacuum ultraviolet band [23]. Because the required pulse energy is three orders of magnitude lower than that of ordinary wide-bore capillaries, the pulse repetition frequency can be increased to the MHz level without increasing the average power. High repetition frequency is a parameter pursued in many experiments. Laser pulses with a pulse repetition frequency of 10 MHz, wavelength of 1030 nm, pulse energy of 7 μJ, and pulse width of 320 fs generated by a ytterbium-doped fiber laser can be compressed to a single cycle with 60% efficiency in a two-stage hollow-core photonic crystal fiber compression device [24]. Moreover, the entire system can be integrated into a relatively compact device. In a recent report, using a hydrogen-filled hollow-core photonic crystal fiber, by controlling the pressure of the filling gas to adjust the dispersion, R. Tyumenev et al. realized high-fidelity quantum state single-photon frequency up-conversion [25]. Spiral hollow-core photonic crystal fibers have polarization-maintaining characteristics for circular polarization, and this type of fiber can also exhibit circular polarization dichroism under specific structures [26]. Liquid-filled hollow-core photonic crystal fibers can be used for high-sensitivity monitoring of photochemical reactions [27, 28]. Hollow-core photonic crystal fibers can also be used in the field of optical tweezers to achieve long-distance trapping and propulsion of particles on curved paths [29]. In addition, hollow-core photonic crystal fibers can achieve high-quality and flexible transmission of mid-infrared and ultraviolet light, which allows hollow-core photonic crystal fibers to be applied in medical, spectral, and remote sensing fields [30, 31].
These are just a few examples of the many disruptive applications of hollow-core photonic crystal fibers. The most noteworthy research on hollow-core photonic crystal fibers is the reduction of transmission loss, If the transmission loss can be significantly lower than that of traditional communication fibers, this may completely change future optical communication systems. In the early 2000s, BlazePhotonics, a startup company based in Bath, UK, first attempted to develop low-loss hollow-core photonic crystal fibers for communication [14]. A loss of less than 1 dB/km was achieved at 1550 nm, but the company went bankrupt in 2004 due to lack of funding [32]. Twenty years later, the telecommunications industry finally followed up on the technology and established Lumenisity, which was acquired by Microsoft in 2022 [33]. The latest research on single-tube ring-core photonic crystal fibers has significantly reduced the loss caused by Rayleigh scattering. This allows near-infrared light to be transmitted over long distances in the fiber, avoiding the use of expensive erbium-doped amplifiers as repeaters. In the future, the loss of hollow-core photonic crystal fibers is likely to be much lower than that of traditional fibers, thus greatly reducing the demand for repeater amplifiers and lowering the cost of long-distance communication.
Paper Information
Russell, P. Invited commentary: hollow core glass fibre—the perfect pipe for light. eLight 5, 4 (2025).
https://doi.org/10.1186/s43593-025-00086-w
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