Supercontinuum
In optics, a supercontinuum is formed when a collection of nonlinear processes act together upon a pump beam in order to cause severe spectral broadening of the original pump beam, for example using a microstructured optical fiber. The result is a smooth spectral continuum (see figure 1 for a typical example). There is no definitive explanation of how much broadening constitutes a supercontinuum; however researchers have published work claiming as little as 60 nm of broadening as a supercontinuum. There is also no agreement on the spectral flatness required to define the bandwidth of the source, with authors using anything from 5 dB to 40 dB or more. In addition the term supercontinuum itself did not gain widespread acceptance until this century, with many authors using alternative phrases to describe their continua during the 1970s, 1980s and 1990s.
During the last decade, the development of supercontinua sources has emerged as an interesting and active research field. This is largely due to new technological developments, which have allowed more controlled and accessible generation of supercontinua. This renewed research has created a variety of new light sources which are finding applications in a diverse range of fields, including optical coherence tomography, frequency metrology, fluorescence lifetime imaging, optical communications, gas sensing and many others. The application of these sources has created a feedback loop whereby the scientists utilising the supercontinua are demanding better customisable continua to suit their particular applications. This has driven researchers to develop novel methods to produce these continua and to develop theories to understand their formation and aid future development. As a result, rapid progress has been made in developing these sources since 2000. While supercontinuum generation has for long been the preserve of fibers, in recent years, integrated waveguides have come of age to produce extremely broad spectra, opening the door to more economical, compact, robust, scalable and mass-producible supercontinuum sources.
In 1964 Jones and Stoicheff reported using a continua generated by a maser to study induced Raman absorption in liquids at optical frequencies. It had been noted by Stoicheff in an early publication that "when the maser emission was in a single sharp spectral line, all the Raman emission lines were sharp; whenever the maser emission contained additional components, all of the Raman emission lines, with the exception of the first Stokes line, were considerably broadened, sometimes up to several hundred cm−1." These weak continua, as they were described, allowed the first Raman absorption spectroscopy measurements to be made.
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