Supercontinuum generation is demonstrated in a 5-cm-long water-core photonic crystal fiber pumped near water's zero-dispersion wavelength. Up to 500-nm spectral width (evaluated at -20 dB from the peak) is achieved, while spectral widths were over 4 times narrower with a bulk setup at the same wavelength and peak power, and over 3 times narrower if the PCF was pumped away from the zero-dispersion wavelength. The supercontinuum generation mechanisms for bulk and waveguide setups are compared and tuning of the zero-dispersion wavelength via waveguide dispersion is theoretically investigated.

Photonic crystal fibres (PCFs) have important applications in sensing the optical properties of fluids. To this end, the material should be inserted into the fibre holes in order to interact with the propagating field. When dealing with liquids, it is particularly interesting to exclusively insert the sample into the core of a hollow-core PCF, which then guides light through the liquid via total internal reflection. Nevertheless, there is still a series of issues to be addressed before fluid sensing with PCFs becomes practical. The work described here proposes and demonstrates possible solutions for two of these issues: (a) how to insert the sample through a lateral access to the fibre longitudinal holes so that the fibre tips are free for optical handling and accessing and (b) in the case of a liquid-core PCF, how to reduce the number of propagating modes.

We experimentally demonstrate a simple and novel technique to simultaneously insert a liquid into the core of a hollow-core photonic crystal fiber (PCF) and a different liquid into its cladding. The result is a liquid-core, liquid-cladding waveguide in which the two liquids can be selected to yield specific guidance characteristics. As an example, we tuned the core-cladding index difference by proper choice of the inserted liquids to obtain control over the number of guided modes. Single-mode guidance was achieved for a particular choice of liquids. We also experimentally and theoretically investigated the nature of light confinement and observed the transition from photonic bandgap to total internal reflection guidance both with the core-cladding index contrast and with the PCF length.

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