光子晶体光纤：特性与应用探索

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" Photonic Crystal Fibers---Properties and Applications" 光子晶体光纤（Photonic Crystal Fibers，PCFs）是一种新型的光纤结构，也被称为微结构光纤或孔洞光纤。自20世纪90年代初首次被提出以来，光子晶体光纤在全球范围内引发了大量激动人心的研究活动，其在电信、计量学、光谱学、显微镜、天文学、微机械加工、生物学以及传感等领域都展现出了独特的优势和应用潜力。 PCFs的独特之处在于它们的内部结构，不同于传统光纤的均匀芯层和包层，而是通过一系列微小孔洞的排列来形成光的传播路径。这些孔洞的排列方式形成了光子带隙，即在特定波长范围内，光不能在光纤内部传播，从而实现了对光传播的精确控制。这种结构使得PCFs在光的传输、耦合、模式控制等方面具有显著的优越性。在通信领域，光子晶体光纤可以提供超宽的无色散区，这对于长距离的光信号传输至关重要，因为这可以减少色散导致的信号质量下降。此外，它们的非线性效应也比传统光纤更加强烈，这在光学信号处理中非常有用，例如用于频率转换、光学参量振荡等。在光谱学中，PCFs由于其独特的光传播特性，可以作为微型光谱仪的核心组件，实现紧凑、便携的光谱测量设备。在显微镜应用中，PCFs可以作为微小样本的光路，提供高分辨率的成像，特别适用于生物组织的非侵入性检测。在生物医学传感方面，PCFs的微结构使其能够直接与生物介质相互作用，用于检测温度、压力、化学物质浓度等参数，成为生物传感器的理想选择。同时，它们的小尺寸和灵活性使得在复杂环境下的布设变得可能。在天文观测中，PCFs可以用于构建紧凑的光收集系统，提高望远镜的光采集效率，并减少光路中的失真。在微机械加工中，PCFs由于其高度可控的光传输特性，可以用于精确的激光切割和打标操作。光子晶体光纤通过其独特的物理性质和设计灵活性，极大地扩展了光纤技术的应用范围，推动了光电子学的发展。随着研究的深入，我们可以期待更多的创新应用和技术突破将由光子晶体光纤引领。

8 Chapter 1. Basics of photonic crystal ﬁbers

In general, a loss reduction for PCFs can be obtained by improving the

fabrication process, reported in the last part of the chapter. The stack-and-

draw process is described with other fabrication techniques, like extrusion,

usually employed to realize ﬁbers with materials diﬀerent from silica, such as

soft-glasses or polymers. Once reached the technological maturity, the advan-

tages oﬀered by PCFs with respect to conventional ﬁbers will be completely

exploited for diﬀerent applications, as described in the ﬁnal part of the chapter,

and the new ﬁbers will enter concretely in the market.

1.1 From conventional optical ﬁbers to PCFs

Optical ﬁbers, which transmit information in the form of short optical

pulses over long distances at exceptionally high speeds, are one of the major

technological successes of the 20th century. This technology has developed

at an incredible rate, from the ﬁrst low-loss single-mode waveguides in 1970

to being key components of the sophisticated global telecommunication net-

work. Optical ﬁbers have also non-telecom applications, for example, in beam

delivery for medicine, machining and diagnostics, sensing, and a lot of other

ﬁelds. Modern optical ﬁbers represent a careful trade-oﬀ between optical losses,

optical nonlinearity, group velocity dispersion, and polarization eﬀects. After

30 years of intensive research, incremental steps have reﬁned the capabilities

of the system and the fabrication technology nearly as far as they can go.

The interest of researchers and engineers in several laboratories, since the

1980s, has been attracted by the ability to structure materials on the scale of

the optical wavelength, a fraction of micrometers or less, in order to develop

new optical medium, known as photonic crystals. Photonic crystals rely on a

regular morphological microstructure, incorporated into the material, which

radically alters its optical properties [1.1]. They represent the extension of the

results obtained for semiconductors into optics. In fact, the band structure

of semiconductors is the outcome of the interactions between electrons and

the periodic variations in potential created by the crystal lattice. By solving

the Schr¨odinger’s wave equation for a periodic potential, electron energy states

separated by forbidden bands are obtained. PBGs can be obtained in photonic

crystals, where periodic variations in dielectric constant, that is in refractive

index, substitute variations in electric potential, as well as the classical wave

equation for the magnetic ﬁeld replaces the Schr¨odinger’s equation [1.2].

1.1. From conventional optical ﬁbers to PCFs 9

PBG, originally predicted in 1987 by Sajeev John, from University of

Toronto, and Eli Yablonovitch, from Bell Communications Research, has

become the really hot topic in optics in the early 1990s. The idea was to

build the right structures, in order to selectively block the transmission of

photons with energy levels, that is wavelengths, corresponding to the PBGs,

while allowing other wavelengths to pass freely. Moreover, slight variations in

the refractive index periodicity would introduce new energy levels within the

PBG, as it happens with the creation of energy levels within the bandgap of

conventional semiconductors.

Unfortunately, building the right structures has proved extremely diﬃcult.

The ﬁrst PBG material was created in 1991 by Yablonovitch and his colleagues by

drilling holes with a diameter of 1 mm in a block of material with a refractive index

of 3.6. Since the bandgap wavelength is of the order of the spacing between the

air-holes in the photonic crystal, this structure had a bandgap in the microwave

region.

In 1991, Philip Russell, who was interested in Yablonovitch’s research,

got his big “crazy” idea for “something diﬀerent,” during CLEO/QELS

conference [1.2]. Russell’s idea was that light could be trapped inside a ﬁber

hollow core by creating a two-dimensional photonic crystal in the cladding,

that is a periodic wavelength-scale lattice of microscopic air-holes in the glass.

The basic principle is the same which is the origin of the color in butterﬂy

wings and peacock feathers, that is all wavelength-scale periodic structures

exhibit ranges of angle and color, stop bands, where incident light is strongly

reﬂected. When properly designed, the photonic crystal cladding running

along the entire ﬁber length can prevent the escape of light from the hol-

low core. These new ﬁbers are called PCFs, since they rely on the unusual

properties of photonic crystals.

The ﬁrst ﬁber with a photonic crystal structure was reported by Russell

and his colleagues in 1995 [1.3]. Even if it was a very interesting research

development, the ﬁrst PCF did not have a hollow core, as shown in Fig. 1.1,

and, consequently, it did not rely on a photonic bandgap for optical conﬁne-

ment. In fact, in 1995 Russell’s group could produce ﬁber with the necessary

air-hole triangular lattice, but the air-holes were too small to achieve a large

air-ﬁlling fraction, which is fundamental to realize a PBG. Measurements have

shown that this solid-core ﬁber formed a single-mode waveguide, that is only

the fundamental mode was transmitted, over a wide wavelength range. More-

over, the ﬁrst PCF had very low intrinsic losses, due to the absence of doping

elements in the core, and a silica core with an area about ten times larger

1.2. Guiding mechanism 11

They realized that the photonic bandgap guiding mechanism is very robust,

since light remains well conﬁned in the hollow core, even if tight bends are

formed in the ﬁber. However, it is highly sensitive to small ﬂuctuations in the

ﬁber geometry, for example, to variations in the air-hole size.

Initial production techniques were directed simply at the task of mak-

ing relatively short lengths of ﬁber in order to do the basic science, but

many research teams are now working hard to optimize their PCF production

techniques, in order to increase the lengths and to reduce the losses.

1.2 Guiding mechanism

In order to form a guided mode in an optical ﬁber, it is necessary to introduce

light into the core with a value of β, that is the component of the propagation

constant along the ﬁber axis, which cannot propagate in the cladding. The

highest β value that can exist in an inﬁnite homogeneous medium with

refractive index n is β = nk

, being k

the free-space propagation constant.

All the smaller values of β are allowed. A two-dimensional photonic crystal,

like any other material, is characterized by a maximum value of β which can

propagate. At a particular wavelength, this corresponds to the fundamental

mode of an inﬁnite slab of the material, and this β value deﬁnes the eﬀective

refractive index of the material.

1.2.1 Modiﬁed total internal reﬂection

It is possible to use a two-dimensional photonic crystal as a ﬁber cladding,

by choosing a core material with a higher refractive index than the cladding

eﬀective index. An example of this kind of structures is the PCF with a silica

solid core surrounded by a photonic crystal cladding with a triangular lat-

tice of air-holes, shown in Fig. 1.3. These ﬁbers, also known as index-guiding

PCFs, guide light through a form of total internal reﬂection (TIR), called

modiﬁed TIR. However, they have many diﬀerent properties with respect to

conventional optical ﬁbers.

Endlessly single-mode property

As already stated, the ﬁrst solid-core PCF, shown in Fig. 1.1, which consisted

of a triangular lattice of air-holes with a diameter d of about 300 nm and a

hole-to-hole spacing Λ of 2.3 µm, did not ever seem to become multi-mode in

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