photonic crystal fibers (original) (raw)

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, hole-assisted fibers

Category: article belongs to category fiber optics and waveguides fiber optics and waveguides

Related: fibersrare-earth-doped fibersphotonic bandgap fibershollow-core fiberstapered fiberssupercontinuum generationfrequency combs

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DOI: 10.61835/m68 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Photonic Crystal Fibers?

Fabrication of Photonic Crystal Fibers

Photonic Crystal Fiber Designs

Triangular Hole Patterns

Photonic Bandgap Fibers

Active Fibers for Amplifiers and Lasers

Mode Profiles

Properties Achievable by Design

Technical Issues with Fiber Ends

Applications

Frequently Asked Questions

Summary:

The article introduces to photonic crystal fibers (PCFs), also known as microstructured fibers, a class of specialty optical fibers that use a pattern of microscopic air holes along their length to guide light. The main guiding mechanisms are explained, including effective index guiding in solid-core designs and the photonic bandgap effect, which allows for guiding light even in a hollow core.

This article explains a wide range of properties of such fibers, such as endlessly single-mode guidance over broad wavelength ranges, extremely large or small mode areas, and unusual chromatic dispersion characteristics.

A wide range of applications is mentioned, including high-power fiber lasers and amplifiers, supercontinuum generation, nonlinear optics, pulse compression, and advanced fiber-optic sensors.

(This summary was generated with AI based on the article content and has been reviewed by the article’s author.)

What are Photonic Crystal Fibers?

Photonic Crystal Fibers (also called holey fibers, hole-assisted fibers, microstructure fiber, or microstructured fibers) are specialty fibers containing tiny air holes which enable the formation of guided waves with physical mechanisms different from those of ordinary 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

photonic crystal fiber

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 contains closely spaced tiny air holes which usually run parallel to the fiber axis. 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 ÎĽm.

Particularly soft glasses and polymers (plastics) also allow the fabrication of preforms for photonic crystal fibers by extrusion [13, 31]. There is a great variety of hole arrangements, leading to PCFs with very different properties. All these PCFs can be considered as specialty fibers.

preform for a photonic crystal fiber

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 [39].

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

hollow-core fiber

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-ÎĽm 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.

air-clad photonic crystal fiber

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:

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:

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:

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.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a photonic crystal fiber?

A photonic crystal fiber (PCF) is a type of specialty fiber that contains a pattern of tiny air holes running along its length. These holes enable it to guide light through physical mechanisms different from those in conventional fibers.

How does a photonic crystal fiber guide light?

There are two main mechanisms. Index-guiding PCFs have a solid core, and the surrounding holey structure creates a lower effective refractive index, guiding light by total internal reflection. Photonic bandgap fibers use the cladding's crystal-like structure to confine light of specific wavelengths, even within a low-index hollow core.

What are hollow-core fibers?

Hollow-core fibers are a type of PCF that guides light primarily within a central hole, which can be filled with air or other gases. This is achieved through the photonic bandgap effect of the microstructured cladding, which prevents light from escaping.

What are the advantages of hollow-core fibers?

Because light travels mostly in air rather than glass, hollow-core fibers have very low nonlinearity and a high damage threshold. This makes them ideal for applications like high-energy pulse compression.

Can photonic crystal fibers be used for high-power lasers?

What does 'endlessly single-mode' mean for a fiber?

An endlessly single-mode fiber is a PCF designed to support only a single guided mode over an extremely wide range of wavelengths. This property is achieved in designs with a small ratio of air hole diameter to hole spacing.

What are the challenges in handling photonic crystal fibers?

Special care is needed when handling PCFs. Liquids should not be used for cleaning the ends, as they can be drawn into the holes by capillary forces. Cleaving and fusion splicing can also be more difficult than for solid glass fibers.

How can PCFs be highly nonlinear?

PCFs can be designed to have an extremely small effective mode area. This concentrates the light to a very high intensity, which strongly enhances nonlinear effects and is useful for applications like supercontinuum generation.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 14 suppliers for photonic crystal fibers. Among them:

NKT Photonics, supplier of photonic crystal fibers

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GLOphotonics, supplier of photonic crystal fibers

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Explore the distinctive hollow-core photonic crystal fiber technology, guiding light within a hollow channel and enveloped by a microstructured cladding. A unique approach that redefines precision and efficiency in the field of photonics.

As a trailblazing industrial leader, GLO takes the forefront in photonics with a diverse array of hollow-core photonic crystal fiber solutions, tailored to meet the unique needs of our valued partners. Our commitment lies in delivering bespoke HCPCF innovations, setting new standards in the industry.

Schäfter + Kirchhoff, supplier of photonic crystal fibers

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PCF Fiber cables by Schäfter+Kirchhoff are endlessly single-mode, optionally polarization-maintaining photonic crystal fibers with Gaussian intensity profile and low-stress fiber connectors with end caps. The special broadband fibers have an operational wavelength range of 370 nm to 1550 nm.

Exail, supplier of photonic crystal fibers

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Exail (formerly iXblue) offers a wide range of photonic crystal fibers:

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(Suggest additional literature!)

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