定义：具有内置微结构的特种光纤，一般玻璃中都包含小的空气孔。

自从1990s，Philip St. J. Russell的小组做了关于光子晶体光纤的先驱性的工作之后，它的发展受到了广泛的关注并且发展了很多应用。这一领域不仅包含了一部分光子带隙结构还结合了其它的方法，是目前光学研究最活跃的领域之一。光子晶体光纤在设计上具有非常高的自由度能够实现很多特殊的性质，因此可以广泛应用到很多领域。 

图1：常用的实心光子晶体光纤设计示意图。空气孔为三角形的，中间的心缺失。灰色区域代表玻璃，白色的圆形空气孔典型的尺寸为几微米。这里只显示了纤芯附近的区域。

图2：空心光纤底端的显微镜图片。
光子晶体光纤（也称为多孔光纤，空气孔辅助型光纤，微结构光纤）的波导特性不是来自于随空间变化的玻璃组分而实现的，而是通过贯穿整个光纤长度的紧密排列的小的空气孔来实现的一种光纤。空气孔可以通过包含孔状结构的预制棒来得到，该预制棒可通过首先堆叠毛细管和固体管（堆叠管技术），然后再将这些管子插入更大的管子来得到。通常，需要将预制棒纤拉制成直径为1 mm的棒，再拉制成需要的直径为125微米的光纤。如果采用非常软的玻璃和聚合物，那么可以通过挤压方法制备光子晶体光纤的预制棒。空气孔可以有很多种排列方式，因此得到PCFs具有很多不同的性质。所有的这些PCFs都可看做是特种光纤。 

最简单的光子晶体光纤（也是最常用的）的空气孔为三角形，中间的孔缺失（如图1）。即实心的芯周围是排列好的空气孔。这种类型的PCF可以通过有效折射率模型来理解：缺失的孔处具有更高的有效折射率，与普通光纤中的纤芯类似。 

还有光子带隙光纤（PGB光纤）具有完全不同的导波机制，利用包层区域的光子晶体带隙结构。后者甚至可以是吸纳在空心中导波（即，在低折射率区域）（如图2），这时大部分的光在中心的孔中传播（参阅空心光纤）。 这种空气导引空心光子晶体光纤（或者空心带隙光纤）可以得到很低的非线性效应和很高的损伤阈值。它通常可以传播很窄波长范围的光，宽度为100-200nm，由于大部分光在空心区域传播，因此这种光纤可以用于高光强的脉冲压缩。 

大多数的PCFs是由纯石英制造的（参阅石英光纤），是可以采用之前所述的制 造技术制造的。但是，也出现了很多采用其它材料制造的PCFs，尤其值得提出的是重金属软玻璃和聚合物（塑料光纤），有时甚至用于太赫兹辐射。 

目录

• 用于放大器和激光器的活性光纤
• 通过设计可以实现的性质
• 技术问题
• 应用

用于放大器和激光器的活性光纤 

图3：具有空气包层的光子晶体光纤结构。
可以将稀土掺杂棒作为预制棒的中心部分来制造激光活性PCFs，可用于光纤激光器和放大器中。稀土掺杂（镱或铒）可以提高折射率，但是采用附加的氟掺杂可以完全补偿掉折射率改变值，于是导波特性只由光子微结构来决定而不是由通常的折射率差来决定。采用稀土掺杂PCFs，可以得到工作在1000nm波长区域的孤子锁模光纤激光器，其中光纤的色散一般处于正常色散区域，但是如果采用合适的设计可以使其处于反常色散区域。

 如果用在高功率光纤激光器和放大器中，可以采用双包层PCFs（图3），其中泵浦包层周围为空气包层区域（空气包层光纤）。由于折射率差很大，泵浦包层具有很高的数值孔径（NA），因此极大的减小了对泵浦光源的光束质量和亮度的要求。这种PCF设计的光纤纤芯通常具有非常大模式面积，然而只传播具有衍射极限输出的单模，因此非常适合得到非常高光束质量的高输出功率的光。这种光纤另一个优势就是泵浦光远离聚合物涂层，因此避免了涂层的过热问题。 
掺杂光子晶体光纤也具有很好的特性可以用到光纤耦合的啁啾脉冲放大器系统中，得到很高的输出峰值功率。 

通过设计可以实现的性质 
通过设计空气孔类型（基本的几何排列，孔的相对尺寸，以及一些小的偏移量）可以得到具有非常卓越的性能，与设计细节密切相关： 

• 可以得到具有非常高数值孔径的多模光纤，例如0.6或者0.7（也可以得到双包层光纤的泵浦包层）。 
• 孔尺寸与孔间隙比值较小时可以得到很大波长范围内的单模导波（无截止单模光纤）。 
• 也可以得到非常小或者非常大模式面积的光纤（可能得到比普通光纤更小的NA）。这样可以得到非常强或非常弱的光学非线性效应。这种情况下，PCFs即使具有大模式面积也对弯曲损耗的灵敏度很低。 
• 一定的空气孔排列能够得到光子带隙（参阅光子带隙光纤），其中导波甚至可以在空心中实现，因为这种情况下不需要中心部分具有更高的折射率。这种空气导引空心光纤在高脉冲能量的色散脉冲压缩中很重要。 
• 尤其是空气孔很大时，可以将气体或液体充进孔中。充气PCFs可以用于光纤传感器中，用于很高功率情况下的非线性光谱展宽，或者得到可变的功率衰减器。 
• 不对称的孔排列可以得到非常强的双折射得到保偏光纤。也可以同时得到大模式面积。 
• 可以采用不同的方法得到与偏振相关的衰减器（偏振光纤）。例如，基模截止可能与偏振有关，因此在特定波长范围内光纤只传播单个偏振的光。 
• 类似的，也可以通过抑制拉曼散射来强烈衰减长波长的光。 
• 尤其是小模式面积的PCFs可以实现非常不寻常的色散特性，例如在可见光区域是反常色散。具有很大的设计自由读，可以结合需要的不同的参数。 
• 将光纤端口的孔进行热处理熔化后就得到了无芯端盖。这种密封的端面可以在大模式面积光纤端面得到，因此具有更高的损伤阈值，可以用于放大很强的纳秒脉冲。 
• 也可以设计多芯光纤，例如单根光纤的纤芯结构规则排列，纤芯之间可能发生耦合也可能不发生耦合。 

技术问题 
总体来讲，光子晶体光纤在很多方面与标准光纤类似。但是，在有些方面需要进行特殊处理： 

• PCFs端面不能采用液体溶液（例如乙醇）进行清洁，因为毛细力会将液体吸入孔中。空中有一点液体都会对光纤的导波特性造成很大的影响。还有专门的研究课题是基于这种效应，通过控制液体进入空气孔中的程度得到可调谐的光学损耗。 
• 可以切割和熔接PCFs，但是非常困难，尤其是光纤中空气孔占的比例很大时。在熔接过程中，空气会发生膨胀然后使光纤结构发生畸变。光纤也可以通过机械接点、光纤连接器、插接电缆、光束扩展单元等连接在一起。 
• 即使熔接过程非常顺利，但是也可能由于模式面积不匹配引起很大的耦合损耗，例如，小纤芯PCF耦合到标准单模光纤中。存在特制的锥形单模光纤和锥形PCFs提高耦合效率，但是这些锥形光纤也不是很容易制备。 

应用 
光子晶体光纤特殊的性质使其可以应用到很多领域。例如： 

• 光纤激光器和放大器，包括高功率装置，锁模光纤激光器等 
• 非线性装置中，例如超连续光产生，拉曼转换，参量放大，或者脉冲压缩 
• 通信器件，例如，用于色散控制，滤波或者开关 
• 各种光纤传感器 
• 量子光学，例如，产生相干光子对，电磁诱导透明或者传播冷原子

Acronym: PCF (also used for polymer cladded fibers!)

Definition: specialty optical fibers with a built-in microstructure, in most cases consisting of small air holes in glass

Alternative terms: microstructure fibers, holey fibers

More general term: optical fibers

More specific terms: photonic bandgap fibers, hollow-core fibers

Pioneered by the research group of Philip St. J. Russell in the 1990s, the development of photonic crystal fibers and the exploration of the great variety of possible applications have attracted huge interest. The field, which constitutes a part of the wider field of photonic bandgap structures while utilizing other concepts as well, can be considered as one of the most active fields of current optics research. This is partly because these fibers offer many degrees of freedom in their design to achieve a variety of peculiar properties, which make them interesting for a wide range of applications (see below).

Fabrication of Photonic Crystal Fibers

Figure 1: A frequently used solid-core photonic crystal fiber design. There is a triangular pattern of air holes, where the central hole is missing. The gray area indicates glass, and the white circles air holes with typical dimensions of a few micrometers.

A photonic crystal fiber (also called holey fiber, hole-assisted fiber, microstructure fiber, or microstructured fiber) is an optical fiber which obtains its waveguide properties not from a spatially varying glass composition but from an arrangement of very tiny and closely spaced air holes which go through the whole length of fiber. Such air holes can be obtained by using a preform with (larger) holes, made e.g. by stacking capillary and/or solid tubes (stacked tube technique) and inserting them into a larger tube. Usually, that preform is then first drawn to a cane with a diameter of e.g. 1 mm, and thereafter into a fiber with the final diameter of e.g. 125 μm.

Particularly soft glasses and polymers (plastics) also allow the fabrication of preforms for photonic crystal fibers by extrusion [13, 30]. There is a great variety of hole arrangements, leading to PCFs with very different properties. All these PCFs can be considered as specialty fibers.

Figure 2: Inspection of a preform for a photonic crystal fiber. The image has been kindly provided by the Optoelectronics Research Centre, University of Southampton.

Most PCFs are made of pure fused silica (→ silica fibers), which is compatible with the above-mentioned fabrication techniques. However, various PCFs made of other materials have been demonstrated, most notably of heavy metal soft glasses and of polymers (plastic optical fibers), sometimes used even for terahertz radiation [12]. An interesting method is also to produce soft glass PCF preforms with 3D printing [38].

Photonic Crystal Fiber Designs

Depending on the design, substantially different physical mechanisms providing the guidance of light may be obtained. The most important designs are explained in the following.

Triangular Hole Patterns

The simplest (and most often used) type of photonic crystal fiber has a triangular pattern of air holes, with one hole missing (see Figure 1), i.e. with a solid core surrounded by an array of air holes. The guiding properties of this type of PCF can be roughly understood with an effective index model: the region with the missing hole has a higher effective refractive index, similar to the fiber core in a conventional fiber. That guiding principle can work in a very wide range of wavelengths; even single-mode guidance may be obtained in a very wide spectral region (endlessly single-mode fibers) [3].

Photonic Bandgap Fibers

Figure 3: Microscope picture of the end of a hollow-core fiber. The photograph has been kindly provided by NKT Photonics.

There are also so-called photonic bandgap fibers (PBG fibers) [6] with a totally different guiding mechanism, based on a photonic bandgap of the cladding region, which is considered as a two-dimensional photonic crystal. Based on that mechanism, one can even obtain guidance in a hollow core (i.e. in a low-index region) (see Figure 2), such that most of the power propagates in the central hole (→ hollow-core fibers). Such air-guiding hollow-core photonic crystal fibers (or air core bandgap fibers) can have a very low nonlinearity and a high damage threshold.

Photonic bandgap fibers typically guide light only in a relatively narrow wavelength region with a width of e.g. 100–200 nm and can be used e.g. for pulse compression with high optical intensities, as most of the power propagates in the hollow core.

Active Fibers for Amplifiers and Lasers

Laser-active PCFs for fiber lasers and amplifiers can be fabricated, e.g., by using a rare-earth-doped rod as the central element of the preform assembly. Rare earth dopants (e.g. ytterbium or erbium) tend to increase the refractive index, but this can be accurately compensated, e.g. with additional fluorine doping of the core or with germanium doping of the cladding, so that the guiding properties are determined by the photonic microstructure only and not by a conventional-type refractive index difference. With rare-earth-doped PCFs, it is possible to realize, for example, soliton mode-locked fiber lasers operating in the 1-μm region, where a fiber's chromatic dispersion would usually be in the normal dispersion regime, but can be anomalous for suitable designs [7, 15, 16]. However, one may also implement dispersion management by incorporating dispersion-modified passive fibers in the laser resonator.

Figure 4: Structure of a photonic crystal fiber with an air cladding.

For high-power fiber lasers and amplifiers, double-clad PCFs (Figure 4) can be used, where the pump cladding is surrounded by an air cladding region (air-clad fiber). Due to the very large contrast of refractive index, the pump cladding can have a very high numerical aperture (NA), which significantly lowers the requirements on the pump source with respect to beam quality and brightness. Such PCF designs can also have very large mode areas of the fiber core [4, 26] while guiding only a single mode for diffraction-limited output, and are thus suitable for very high output powers with excellent beam quality. Another advantage is that the pump light is kept away from any polymer coating, thus avoiding possible problems with overheating of a coating.

Doped photonic crystal fibers have favorable properties also for use in fiber-based chirped-pulse amplification systems with very high output peak power.

Mode Profiles

In most cases, photonic crystal fibers either have single-mode propagation only or support a small number of guided modes (few-mode fibers). The intensity profiles of the fundamental guided modes are largely concentrated in the core, while higher-order modes tend to extend more into the cladding area. Normally, the mode fields extend into the air holes only to a very small amount; the intensity pattern are “squeezed” between the holes.

The calculation of mode properties of such fibers is relatively difficult, (a) because no radial symmetry is given (in contrast to most all-glass fibers), and (b) because of the large refractive index contrast between air and glass. Advanced mode solver software is required, which generally requires substantially longer computation times than simple mode solvers which are restricted to LP modes.

Properties Achievable by Design

Photonic crystal fibers with different designs of the hole pattern (concerning the basic geometry of the lattice, the relative size of the holes, and possibly small displacements) can have very remarkable properties, strongly depending on the design details:

• It is possible to obtain a very high numerical aperture of e.g. 0.6 or 0.7 of multimode fibers (also for the pump cladding of a double-clad fiber) [20].
• Single-mode guidance over very wide wavelength regions (endlessly single-mode fiber) is obtained for small ratios of hole size and hole spacing [3].
• Extremely small effective mode areas are possible, so that obtains highly nonlinear fibers.
• Also, extremely large mode areas are possible; the numerical aperture may be smaller than possible with a conventional all-glass fiber design. That leads to very weak optical nonlinearities, as are beneficial e.g. when ultrashort pulses with high peak power need to be guided. PCFs can be made with a low sensitivity to bend losses even for large mode areas [4].
• Certain hole arrangements result in a photonic bandgap (→ photonic bandgap fibers). Based on that principle, guidance is possible even in a hollow core, as a higher refractive index in the inner part is no longer required. Such air-guiding hollow-core fibers are interesting e.g. for dispersive pulse compression at high pulse energy levels.
• Particularly for larger holes, there is the possibility to fill gases or liquids into the holes. Gas-filled PCFs can be exploited for fiber-optic sensors, for nonlinear spectral broadening at very high power levels, or for variable power attenuators.
• Asymmetric hole patterns can lead to extremely strong birefringence for polarization-maintaining fibers [8]. This can also be combined with large mode areas.
• Strongly polarization-dependent attenuation (polarizing fibers) [18, 24] can be obtained in different ways. For example, there can be a polarization-dependent fundamental mode cut-off, so that the fiber guides only light with one polarization in a certain wavelength range.
• Similarly, it is possible to suppress Raman scattering [25] by strongly attenuating longer-wavelength light.
• Very unusual chromatic dispersion properties, e.g. anomalous dispersion in the visible wavelength region [7], result particularly for PCFs with small mode areas. There is substantial design freedom, allowing for different combinations of desirable parameters.
• Multicore designs are possible, e.g. with a regular pattern of core structures in a single fiber, where there may or may not be some coupling between the cores.

Technical Issues with Fiber Ends

Overall, photonic crystal fibers are handled in similar ways as standard optical fibers. However, special care is required in various respects:

• Ends of PCFs may not be cleaned with liquid solvents, such as ethanol, as capillary forces may pull liquid substances into the hole. Of course, the guiding properties can be strongly modified with any liquid in the holes. There is even research on exploiting such effects, e.g. for variable optical attenuators: a tunable amount of optical loss is obtained by controlling the degree to which a liquid penetrates the holes.
• Cleaving and fusion splicing PCFs is possible, but may be more difficult, particularly for fibers with large air content. During fusion splicing, the air may expand and distort the fiber structure. Connections between fibers are also possible with a variety of mechanical splices, fiber connectors, protected patch cables, beam expansion units, etc.
• Even when the splicing process works well, there may be a substantial coupling loss due to a mismatch of mode areas, e.g. when a small-core PCF is coupled to a standard single-mode fiber. There are special tapered single-mode fibers and tapered PCFs for enhancing the coupling efficiency, but these may not be easily available.

Core-less end caps can be fabricated simply by fusing the holes near the fiber end with a heat treatment. The sealed end facets allow for larger mode areas at the fiber surface and thus a higher damage threshold, e.g. for amplifying intense nanosecond pulses.

Applications

Their special properties make photonic crystal fibers very attractive for a very wide range of applications. Some examples are:

• fiber lasers and amplifiers, including high-power devices, mode-locked fiber lasers, etc.
• nonlinear devices e.g. for supercontinuum generation [9, 28] (→ frequency combs), Raman conversion [11], parametric amplification, or pulse compression
• telecom components, e.g. for dispersion control, filtering or switching
• fiber-optic sensors of various kinds
• quantum optics, e.g. generation of correlated photon pairs [23], electromagnetically induced transparency, or guidance of cold atoms.

Even though PCFs have been around for a number of years, the huge range of possible applications is far from being fully explored. It is to be expected that this field will stay very lively for many years and many opportunities for further creative work, concerning both fiber designs and applications.

Bibliography