An ultrabroadband frequency comb spanning the ultraviolet to the mid-infrared, 350 nm to 4.4 μm, has been generated in a waveguide of periodically poled lithium niobate (PPLN). The scientists from Japan behind the achievement claim that the 3.6 octave optical frequency comb is the broadest ever reported. The long- and short-wavelength measurements of the comb were limited by the spectral coverage of the team's spectral analysers. Significantly, the comb covers the absorption lines of CO2 at 2.7 and 4.3 μm and water's absorption band at 2.6–3.1 μm, making it a potentially valuable tool for sensing or spectroscopy. The team pumped the PPLN waveguide with a train of amplified light pulses from a mode-locked erbium-fibre laser (wavelength 1.56 μm, 48 MHz repetition rate) that was passed through a 15-cm-long highly nonlinear fibre (HNLF) for spectral broadening before entering the waveguide. The apparatus has been running for more than six months without the need to replace either the PPLN waveguide or HNLF and the team says that its repeatability and reproducibility are good.
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Graydon, O. Ultrawide span. Nature Photon 10, 691 (2016). https://doi.org/10.1038/nphoton.2016.219