All-Fiberised Picosecond 1.75 μm Thulium Fibre Laser

Pulsed fibre lasers in the 1.7 μm spectral band are attractive for many important applications including polymer processing and lipid-targeted photoacoustic imaging, due to the overlap of this spectral band with the C-H bond resonance wavelength [1]. Methods of frequency conversion, such as optical parametric oscillators and Raman shifting, applied to ytterbium or erbium doped fibre lasers can generate wavelengths in the 1.7 μm band, however such systems are complex with low efficiency [2], [3]. Thulium (Tm) doped fibre provides a wide emission spectral range that covers the wavelength of interest, however, generation and amplification of the 1.7 μm wavelength using Tm-fibres encounters a challenge due to the short-wavelength signal reabsorption and the long-wavelength gain competition. A 1738 nm Tm-fibre laser amplification system has been reported to generate 34 ps pulses at 85 MHz with an average power of 5.2W [4]. However, this system contains complicated chirped-pulse amplifiers, a mode-locked laser, and free-space optics, which can create problems in real-life applications.