The Optical Communications Reference

modes can lead to narrowband regions of strong anoma-

lous dispersion (via avoided crossings) that may find use

in novel device applications [136,137].

Now that high-quality, low-loss, air-guiding fibers can

be made, many new applications can be envisaged. These

include low-loss/high-power transmission at wavelengths

where the absorption in silica is high, in gas-sensing ap-

plications, and for spectral filtering exploiting the narrow-

band nature of the guided-mode transmission bands.

Applications for laser-assisted atom transport or even as

particle accelerators have also been proposed [138].

One recently demonstrated example of an application

of air-guiding PBGF is all-fiber chirped pulse amplifica-

tion [137]. Pulses from a wavelength- and duration-

tunable femtosecond/picosecond source at 10 GHz were

dispersed in 100 m of dispersi on compensating fiber

before being amplified in an erbium-doped fiber amplifier

and subsequently recompressed in 10 m of the anoma-

lously dispersive PBGF. Pulses as short as 1.1 ps were

obtained. The advantage of air-core fibers for this appli-

cation is that they present negligible nonlinearity and

thus can potentially be used to obtain ultra-high pulse

peak powers. The novel dispersion properties of air-core

PBGFs have also been used for dispersion compensation

within a fiber laser [139].

Hollow-core PBGFs are also ideally suited to deliver

high power laser beams. Recent work in this area has

concluded that 7-unit cell cores are currently most suit-

able for transmission of femtosecond and sub-picosecond

pulses, whereas larger cores (e.g., 19-cell cores) are better

for delivering nanosecond pulses and continuous-wave

beams [140].

3.3.8 Conclusion and the future

Microstructured optical fibers have now developed to

the point where they are not only of interest from

a research perspective, but are also becoming available

commercially. As this chapter demonstrated, good

quality index-guiding HFs and PBGFs based on a clad-

ding structure with a triangular lattice of air holes em-

bedded in pure silica glass can now routinely be

fabricated over a wide parameter range. More compli-

cated structures, such as double-clad fibers and active

fibers, have also been demonstrated.

The unique properties and design flexibility of these

fibers opens up a wide range of possible applications as

functional components in fiber commu nication net-

works (including devices for amplification or dispersion

compensation), in novel broadband sources, or for

high-power transmission, to name a few. One area in

which significant progress has been made recently is

the field of highly nonlinear index-guiding HFs. The

generation of broad super-continuum spectra and all-

optical data regeneration are examples of the signifi-

cant advances that have resulted from this silica-based

HF technology. Moreover, nonsilica index-guiding HFs

with extremely high nonlinearity can now be routinely

fabricated [21], and such fibers promise to offer

nonlinear fiber devices at unprecedentedly low oper-

ating powers (1–10 mW) and short device lengths

(0.1–1 m).

By filling the holes of these fibers with a range of

materials, it should be possible to significantly extend

their functiona lity. One example is the filling of index-

guiding HFs with a high index liquid to form tunable

PBGFs [141].

The synergy of novel nonsilica glass materials and

microstructured fiber technology promises a broad

range of new and potentially useful optical fibers. The

large index contrast possible in nonsilica micro-struc-

tured fibers is promising for the development of fibers

based on photonic bandgap effects. However, it has

been demonstrated that increasing the refractive index

contrast beyond that available in air/silica does not

necessarily broaden the photonic bandgaps that are

available [142]. One particularly promising application

of this new fiber type will be the development of new

air-guiding fibers for broadband high power IR

transmission.

It was demonstrated that solid microstructured

fibers can be produced using a relatively small number

of cladding features. It is expected that the use of

a solid fiber structure may lead to a number of practical

advantages relative to air/glass fibers. For example,

edge polishin g, angle polishing, a nd splicing should all

be more straightforward in solid microstructured

fibers. By combin ing a solid cladding with an air core, it

is possible to demonstrate that low fiber attenuation

can be achieved through structural design rather than

high-transparency material selection [124].Asthe

fabrication techniques used to produce structured

preforms and to draw high quality fibers continue to

improve, it is anticipated that more novel optical

properties and promising applications will continue to

emerge.

Acknowledgments

I thank a number of colleagues at the ORC, University of

Southampton who have made many important contri-

butions to the research described within this chapter. In

particular, warm thanks to Joanne Baggett, Heike

Ebendorff-Heidepriem, Xian Feng, Vittoria Finazzi,

Kentaro Furusawa, John Hayes, Ju Han Lee, Periklis

Petropoulous, Jonathan Price, and David Richardson.

I also acknowledge the support of a Royal Society

University Research Fellowship.

146

SECTION THREE Optical Fibers