Hole-size increasing PCFs for blue-extended supercontinuum generation

Supercontinuum (SC) sources with spectra extending into the deep-blue region below 400 nm are highly desirable in areas such as fluorescent microscopy [1]. Tapering of photonic crystal fibers (PCFs) with high air-fill fractions has proven an effective way of extending the spectra into the deep-blue [1-4]. This facilitates the ideal combination of (1) an initial fiber section to allow a pulse break-up in the vicinity of the zero-dispersion wavelength (ZDW) and an efficient energy transfer into the visible, and (2) a subsequent fiber section with group-velocity match (GVM) from the long wavelength spectral edge to wavelengths in the deep-blue or even UV. Previous reports on blue-extended SC generation were typically achieved in tapered PCFs where the airhole structure was preserved [1-4], i.e. the relative hole-size constant. However, such PCFs with high air-fill fractions are inevitably (highly) multi moded at the pump, which complicates coupling and interfacing. In [5] this was overcome by increasing the air-hole size in a short section of a single mode PCF using a post processing technique, but only to enhance the visible power. Here we present the first high-power SC generation into the deep-blue in a single mode PCF with varying hole-size and pitch fabricated directly at the draw-tower.