Supercontinuum - Historical Overview - Progress Since 2000

Progress Since 2000

Advances made during the 1980s meant that it had become clear that to get the broadest continua in fibre, it was most efficient to pump in the anomalous dispersion regime. However it was difficult to capitalise upon this with high power 1 μm lasers as it had proven extremely difficult to achieve a zero dispersion wavelength of much less than 1.3 μm in conventional silica fibre. A solution appeared with the invention of Photonic-crystal fibers (PCF) in 1996 by Knight et al. The properties of PCFs are discussed in detail elsewhere, but they have two properties which make PCF an excellent medium for supercontinuum generation, namely: high nonlinearity and a customisable zero dispersion wavelength. Amongst the first was Ranka et al. in 2000, who used a 75 cm PCF with a zero dispersion at 767 nm and a 1.7 μm core diameter. They pumped the fibre with 100 fs, 800 pJ pulses at 790 nm to produce a flat continuum from between 400 and 1450 nm.

This work was followed by others pumping short lengths of PCF with zero dispersions around 800 nm with high power femtosecond Ti:sapphire lasers. Lehtonen et al. studied the effect of polarization on the formation of the continua in a birefringent PCF, as well as varying the pump wavelength (728-810 nm) and pulse duration (70-300 fs). They found that the best continua were formed just inside the anomalous region with 300 fs pulses. Shorter pulses resulted in clear separation of the solitons which were visible in the spectral output. Herrmann et al. provided a convincing explanation of the development of femtosecond supercontinua, specifically the reduction of solitons from high orders down to the fundamental and the production of dispersive waves during this process. Fully fibre integrated femtosecond sources have since been developed and demonstrated.

Other areas of development in since 2000 have included: supercontinua sources that operate in the picosecond, nanosecond and CW regimes; the development of fibres to include new materials, production techniques and tapers; novel methods for generating broader continua; novel propagation equations for describing supercontinuum in photonic nanowires, and the development of numerical models to explain and aid understanding of supercontinuum generation. Unfortunately, an in depth discussion of these achievements is beyond this article but the reader is referred to an excellent review article by Dudley et al.

Read more about this topic:  Supercontinuum, Historical Overview

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