Spectral broadening of visible ultrashort pulses in tapered photonic crystal fiber

Here we study propagation of ultrashort laser pulses in tapered PCFs. A PCF with a core diameter of 1.23 mum and high air-filling fraction was tapered to a core diameter of 590 nm using the flame-brush technique. The group-velocity dispersion (GVD) was measured for both untapered and tapered fibers using white-light interferometry. The tapered fiber has two closely-spaced zero dispersion wavelengths (ZDWs) at 509 nm and 640 nm. The laser source was a commercial non-collinear optical parametric amplifier with tuning range 490-700 nm, repetition rate 250 kHz, pulse energy up to 250 nJ and pulse duration 35 fs. The spectral evolution of the laser pulses strongly depends on the phase-matching conditions, and is driven by the position of the ZDWs relative to the pump wavelength, the peak power as well as the magnitude and the slope of the GVD. The conversion efficiency to the blue (450plusmn20 nm) was measured to be more than 10%. As can be seen, up-conversion to the UV is very efficient.