Microstructured optical fibres

Monro, Tanya M., Bjarklev, Anders and Lægsgaard, Jesper (2002) Microstructured optical fibres. In, Handbook of Optoelectronics. Institute of Physics.

Abstract

A new class of optical fibre has emerged in recent years: the microstructured fibre [1]. These fibres guide light by means of an arrangement of air holes that run along the fibre, and some examples are shown in figure 1-1. Note that the hole-to-hole spacing is typically labelled A, and d is the hole diameter. In these fibres, light can be guided using either one of two quite different mechanisms.
The first class of microstructured fibres is the index-guiding microstructured fibre. Such fibres are widely known as holey fibres (HFs). Holey fibres guide light due to the principle of modified total internal reflection. The holes act to lower the effective refractive index in the cladding region, and so light is confined to the solid core, which has a relatively higher index. Some examples are shown in figure 1-1 (left) and (centre). HFs can be made entirely from a single material, typically pure undoped silica, although HFs have also been fabricated in chalcogenide glass [2] and in polymers [3]. The effective refractive index of the cladding can vary strongly as a function of the wavelength of light guided by the fibre. For this reason, it is possible to design fibres with spectrally unique properties that are not possible in conventional solid optical fibres. The basic operation of index guiding fibres does not depend on having a periodic array of holes; in fact the holes can even be arranged randomly [4].

Fig.1-1: Some typical microstructured silica optical fibres. Left: a small-core index-guiding holey fibre (picture supplied by the ORC, Southampton). Centre: a polarization-maintaining index-guiding holey fibre (picture provided by Crystal Fibre NS). Right: a bandgap guiding fibre (picture provided by Crystal Fibre)

The optical properties of holey fibres are determined by the configuration of air holes that forms the cladding region, and HFs can have mode areas ranging over three orders of magnitude by scaling the dimensions of the structure [5]. Small mode area fibres can be used for devices based on nonlinear effects [6], whereas the large mode fibres allow high power delivery [7]. In addition, these fibres can exhibit optical properties not readily attainable in conventional fibres, including endlessly single-mode guidance [8] and anomalous dispersion well below 1.3 microns [9]. Dispersion and birefringence are two properties that can depend strongly on the cladding configuration, particularly when the hole-to-hole separation is small. By exploiting the innate flexibility provided by the choice of hole arrangement, it is thus possible to design fibres with a wide range of characteristics. Note that the modes of all single-material HFs are leaky modes because the core index is the same as that beyond the finite holey cladding, and for some designs this can lead to significant confinement loss [10].

The second guidance mechanism in microstructured fibres can occur, if the air holes that define the cladding region are arranged on a strictly periodic lattice. For such structures, photonic bandgaps may appear [1,11]. These are effective index regions, below the effective cladding index, in which no periodic cladding modes are allowed. By breaking the periodicity of the cladding (e.g., by adding an extra air hole to form a low-index core-region), it is possible to introduce a mode that is only allowed in the low-index core-region, while being forbidden in the cladding region because of the photonic bandgap. This core-mode will, therefore, be guided along the fibre, because of the photonic bandgap of the cladding region. If the core mode has an effective index that is either below or above the effective index range covered by the photonic bandgap at the particular wavelength, the core mode will not be guided.

It has been found that silica-air photonic crystals with air holes arranged in a so-called honeycomb lattice are capable of exhibiting PBG effect for much smaller air holes than triangular (close-packed hexagonally arranged) photonic crystals [12]. Based on this finding, the first bandgap guiding microstructured fibres (inset to Fig. 1-2) were designed. The inset shows how an extra air hole is introduced into the center of the fibre to act as a low-index defect region.

To understand the waveguiding mechanism, it is valuable to consider the fibre using a modal-index illustration, and Fig.1-2 shows such an illustration. It can be seen that two forbidden regions open up by photonic bandgap effects below the effective cladding index. No modes appear above the effective cladding index, in agreement with the fact that the low index core-region should not allow guidance of light by modified total internal reflection. However, the extra air hole causes a single mode to be confined to the core defect and correspondingly be guided through the fibre in the frequency range for which the defect mode falls within the bandgap. Hence, the PBG fibre may only support guided modes within certain transmission windows. Depending on the extent of the PBGs, these transmission windows may be several microns wide and centered at near-infrared wavelengths [13].

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