Microstructured optical fibers

Monro, T.M. (2005) Microstructured optical fibers. In, Pal, Bishnu P. (ed.) Guided Wave Optical Components and Devices. Oxford, UK. Academic Press.

Record type: Book Section

Abstract

The development of core-clad silica glass optical fibers has revolutionized communications systems over the past 30 years. These 'conventional' optical fibers have also made a significant impact in areas as diverse as sensing, medical imaging, laser welding and machining, and the realization of new classes of lasers and amplifiers. All of these advances have been enabled by one key factor: the reduction of the fiber loss. Reducing loss was a topic of intensive research and development for two decades, and dramatic improvements in the transmission of silica-based fibers in the 1.5 micron telecommunications window were achieved as a result. The widely used Coming SMF-28 fiber has a loss of less than 0.2 dB/km at 1550nm.

In the early 1970s, when the fabrication processes for the manufacture of core-clad preforms had not yet reached maturity, Kaiser et al. proposed an alternative route to achieving low fibre losses. Kaiser’s concept was to confine light within a pure (undoped) silica core by surrounding it with air [1], [2]. The core was supported by a sub-wavelength strand of silica glass and then jacketed in a silica cladding for strength. Although this new class of fibers showed promise, the fabrication methods used to produce these early single-material fibers were limited, and this new technology was quickly overtaken by improvements in the MCVD (modified chemical vapour deposition process), which allowed the definition of high quality preforms for the production of low loss core-clad silica fibres.

In the late 1980s, work by Yablonovitch [3] on the development of three dimensional photonic crystals identified micron-scale structuring to be a powerful means of modifying the optical characteristics of a material. The earliest photonic crystal samples were formed by drilling cm-scale holes to produce photonic bandgaps within which light propagation was forbidden. These samples were confirmed to have photonic bandgaps located at microwave wavelengths. In the 1990s, a number of groups worked to extend this concept to infrared and visible wavelengths by scaling down the dimensions of the photonic crystal structure to micron-scale feature sizes. The technique that has been used most extensively for defining two dimensional photonic crystals is electron beam lithography [4]. However, this technique is not well suited for defining structures that are truly extended in the third dimension to avoid non-uniformities in this direction modifying the properties of the photonic bandgaps. Fabricating two dimensional photonic crystals is an engineering challenge, and although a number of approaches exist, there is a continued drive to develop cheap and flexible techniques for the large scale production of hgh quality photonic crystals.

In 1995, Birks et al. proposed a novel technique for producing two dimensionally structured silica/air photonic crystal structures by taking advantage of optical fiber manufacturing techniques [5] . The fabrication concept was to stack macroscopic silica capillary tubes together into a hexagonal lattice to form a preform with mm-scale features, and then to pull this preform to a fiber with micron-scale features on a drawing tower. Thus the scale reduction and longitudinal uniformity inherent to the fiber drawing process could be utilized to produce tlie first photonic crystals that could truly be considered infinite in the third dimension.

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