The availability of efficient high power lasers and the development of dispersion and nonlinearity tailored optical fibers have allowed significant advances to be achieved in the field of nonlinear fiber optics over the last decade or so. The generation of new frequencies lies at the heart of nonlinear optics, and usually the extreme spectral broadening that leads to the generation of “white light” is coined supercontinuum generation. Broadband supercontinuum spectra spanning more than one octave have already found application in such fields as optical frequency metrology, optical coherence tomography and spectroscopy. Their generation has often benefitted from the design freedom of specialty optical fibers, which has allowed the nonlinear systems to be adapted to the wavelengths and laser sources of interest according to the application. Advances in experimentation have been followed by important theoretical works, which have provided a detailed insight into the spectral broadening mechanisms. It is nowadays possible to model supercontinuum generation in optical fibers with remarkable accuracy.

It is probably straight-forward to understand how short, intense pulses can give rise to the generation of new optical frequencies when launched into a nonlinear optical medium (such as a length of an optical fiber). Somewhat more subtle perhaps, is supercontinuum generation originating from a continuous wave source. Yet supercontinuum spectra spanning over several hundreds of nanometers have been generated from continuous wave laser sources emitting (just) a few tens of watts of optical power. Supercontinuum generation in such systems is initiated by modulation instability, a nonlinear mechanism present in anomalously dispersive media, responsible for the formation of soliton pulses out of small perturbations in a continuous wave beam. Modulation instability is normally followed by Raman effects that red-shift the energy towards longer wavelengths and soliton collisions that eventually form the desired broadened spectrum. Apart from the simplicity in the implementation of supercontinuum generation systems based just on simple, suitably scaled in power continuous wave pump sources (rather than on amplified short pulse laser systems), continuous wave supercontinua also benefit from exhibiting remarkably smooth spectra and high average spectral densities (typically higher than 10mW/nm). On the other hand, the initiation of the modulation instability process is intimately linked with the noise properties of the pump source.

This last observation forms the basis of the study presented in the paper by Kelleher et al. In a detailed study encompassing both simulations and experiments, the authors have studied the influence of the pump coherence in the formation of a continuous wave supercontinuum. Through a clearly and logically structured presentation, a hypothesis that some degree of wave incoherence is beneficial for supercontinuum generation is made, and convincingly proven. This is an important paper, not only because of the significance of the results that it communicates, but also because the methodology of the work and its presentation are exemplary.

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