Increasing the blue-shift of a picosecond pumped supercontinuum

The first experiments with supercontinuum generation in a photonic crystal fibre (PCF) demonstrated impressive spectra spanning from 400 nm to 1500 nm using 100 fs pulses (Ranka et al., 2000). Often, one does not require the use of the entire supercontinuum bandwidth, or the spectrum needs to be concentrated in a specific spectral region where other lasers are not readily available. One method is to use the soliton self-frequency shift to simply red-shift a laser pulse over a desired wavelength range, which can be done over 900 nm (Chan et al., 2008). This provides a basis for tunable lasers with applications including broadband spectroscopy (Walewski et al., 2004), and coherent anti-Stokes Raman scattering (CARS) microspectroscopy (Andresen et al., 2007). ZBLAN fluoride (a mixture of zirconium, barium, lanthanum, aluminium, and sodium fluorides) fibres have been used to extend a supercontinuum spectrum beyond 4.5 μm with potential applications in spectroscopy (Xia et al., 2006). Besides these examples of generating light in the near- or mid-infrared, one also finds examples of generating light in the ultraviolet–blue region of the spectrum. This wavelength region is highly interesting for several reasons. Primarily, many fluorescent molecules are excited in a wavelength range from ∼600 nm down to ∼350 nm (Prasad, 2003). Supercontinuum light sources covering this wavelength range are highly useful for fluorescence microscopy. In particular, a high spectral density over a broad wavelength range removes the need for using several lasers, each corresponding to the excitation wavelength of a specific fluorescent molecule.