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In this chapter, starting from the description of the characteristics of photonic crystals, materials with a refractive index periodic distribution, the passage from conventional optical fibers to photonic crystal ones, introduced for the first time in 1995, is explained.

This Chapter summarizes the results obtained by analyzing the PCF guiding properties. These can be evaluated starting from a parameter which characterizes the PCF modes, that is the value of the complex propagation constant = +, being the attenuation constant, the effective index and the wave number in the vacuum.

In this chapter results regarding the PCF dispersion properties are reported. The analyses performed have shown that, by properly changing the geometric characteristics of the air-holes in the PCF cross-section, the waveguide contribution to the dispersion parameter can be significantly changed, thus obtaining unusual positions of the zero-dispersion wavelength, as well as particular values of the dispersion curve slope. In particular, by manipulating the air-hole radius or the lattice period of the microstructured cladding, it is possible to control the zero-dispersion wavelength, which can be tuned over a very wide range [3.1–3.3], or the dispersion curves, which can be engineered to be ultraflattened [3.4–3.7].

In this chapter the nonlinear properties of PCFs are deeply analyzed. Firstly, supercontinuum generation, one of the most important applications of the fibers with enhanced nonlinear properties, is described, starting from the numerous results, both experimental and theoretical, which have been presented in literature so far.

The importance of Raman amplification in optical communication systems has become more and more relevant in the last years. The gain mechanism in Raman amplification is the SRS, that is, a nonlinear scattering process by which energy is transferred from a pump wavelength to a signal one, which can be longer, in the Stokes process, or shorter, in the anti-Stokes one [5.1, 5.2]. The gain flexibility, that is, the possibility to obtain amplification at any wavelength in any fiber, is one of the key advantage of Raman fiber amplifiers. Broadband and low noise-figure Raman amplifiers can be obtained with multipumping schemes [5.3–5.5]. Moreover, distributed Raman amplification provides a significant improvement of the noise performances and an increase of the signal power budget in transmission fibers [5.6, 5.7].

In recent years, PCFs have emerged as an attractive alternative and also as active fibers. PCFs used as active fibers have been first reported in [6.1]. In particular, the possibility of obtaining very small– or very large–mode area with this new kind of optical fibers has been exploited to realize new fiber lasers [6.1, 6.2] or fiber amplifiers with single transverse mode operation and efficiency higher than in conventional doped fibers.