Femtosecond laser micromachining is a laser processing technology with proven potential for the fabrication of a wide range of photonic devices. In the context of developing integrated components for microfluidics, a key issue is the machining of high aspect ratio micro and nano-channels, and the use of diffraction-free Bessel beams for this purpose has attracted much attention. However, although Bessel beams possess several attractive characteristics for this purpose, we show here that their practical use for high aspect ratio micromachining requires a careful selection of focussing and pump laser parameters. We report results of a systematic study of Bessel beam micromachining of structures of diameter < 5 mum in fused silica, and we describe conditions under which high quality and high aspect ratio structures can be reproducibly obtained. Within the parameter regimes identified here, Bessel beams present quantitative advantages when compared to diffractive Gaussian beams, especially for the drilling of channels of sub-10 mum diameter. Outside this parameter regime, there is little benefit in the Bessel beam approach.
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