UV Extension of Supercontinuum via Tapered Single-ring PCF

Short wavelength light sources in the blue and ultraviolet (UV) are extremely useful for applications such as semiconductor metrology, photolithography, molecular spectroscopy and high performance liquid chromatography [1] . Gas-filled single-ring hollow-core photonic crystal fibre (SR-PCF), guiding by anti-resonant-reflection, is highly suitable for efficient generation of broadband supercontinua and tunable narrowband UV light because its nonlinearity and dispersion can be tuned by varying the species and pressure of gas [2] . We report the use of tapered SR-PCF filled with 15 bar krypton, pumped with ultrashort (220 fs) 1030 nm pulses carrying 7.8 μJ of energy from a compact laser source, to extend the UV bandwidth and achieve significant spectral power in the 200-350 nm region. The pulses are launched into the untapered end of a SR-PCF with core diameter of 36 μm and an average core-wall thickness of 386 nm (standard deviation 23 nm). Anti-crossings between the core modes and the resonances in the cladding walls result in loss peaks that interrupt the broad transmission window of the SRPCF. Upon propagation, the pulses break up due to modulational instability, resulting in a broad supercontinuum, the blue side of which forms by dispersive wave (DW) emission. By tapering the SR-PCF to half its original dimensions, the phase-matching wavelength for DW emission shifts to higher frequencies [3] , following the shift in zero dispersion wavelength indicated in Fig. 1(a) by dashed lines. As the thickness of the capillary walls falls, the loss peaks (marked by the shaded grey areas in Fig. 1(b) ) shift further into the blue, as previously reported [4] . As a result, the generated UV band extends down to 220 nm in the tapered fibre, a considerable improvement over untapered fibre, where the high frequency edge ends at 310 nm ( Fig. 1(a) ). The tapers were fabricated by brushing an oxy-butane flame across the fibre while pulling it apart axially, keeping the core at near vacuum pressure (60 mbar) and the cladding capillaries at atmospheric pressure [4] . The insets in Fig. 1(b) show optical micrographs of the untapered (above) and tapered (below) ends of the SR-PCF.