Ultra-broadband wavelength swept Tm-doped fibre laser

Wavelength-swept laser sources with broad wavelength tunability and narrow instantaneous linewidth in the two-micron band have potential applications in a number of areas, including spectroscopic characterisation and optical coherence tomography (OCT). The use of OCT for non-invasive investigation of paintings to provide the information necessary for effective restoration and to aid conservation is one example of an emerging application where operation in two-micron band brings the advantage of increased penetration depth due to lower loss in commonly used pigments compared to the situation at near-infrared wavelengths. Tm-doped silica fibre lasers provide an efficient way to access the relevant wavelength region due to the wide emission line that extends from ~1700 nm to ~2100 nm. Unfortunately, wavelength tuning across the entire emission band with a single fibre gain stage is extremely difficult due to the combination of varying quasi-three-level character as a function of wavelength and gain saturation due to short wavelength amplified spontaneous emission. In order to access the full emission bandwidth potential of Tm-doped fibres, a different approach must be employed with two or more fibre gain stages and with each gain stage tailored to provide emission in complementary bands.