17  μm wavelength tunable gain-switched fiber laser and its application to spectroscopic photoacoustic imaging

Recently demonstrated bond-selective photoacoustic (PA) imaging has revealed the importance of 1.7 μm laser sources. In this Letter, we demonstrate a gain-switched thulium-doped fiber laser with continuous tuning from 1690 to 1765 nm by using an electrically driven acousto-optical tunable filter. Micro-joule laser pulses with a shot-to-shot intensity variation of 1.6% and a pulse duration of 150 ns are obtained. The laser source is then harnessed to implement a PA microscopy system, of which the lateral resolution is estimated to be 15.6 μm by scanning the edge of a black tape. The PA spectra of butter, rapeseed oil, and adipose tissue are measured, and they show a consistent absorption peak of around 1720 nm. Photoacoustic microscopy imaging of the adipose tissue demonstrates a high optical absorption contrast of lipids and the superiority of the laser for spectroscopic PA detection.