Pulsed cascaded Raman fiber laser widely tunable in the second near-infrared and visible window for hyperspectral photoacoustic imaging

Pulsed laser sources with nanosecond pulse duration are widely used in numerous noninvasive biomedical imaging applications, notably photoacoustic imaging (PAI), an <i>in vivo</i> imaging modality. Multiple laser sources are required to target the absorption features of multiple endogenous chromophores (optical absorbers inside tissues). We demonstrate a widely tunable pulsed cascaded Raman fiber laser continuously tunable in the second near-infrared window (NIR-II, 1060-1600 nm) and through harmonic conversion in the visible window (530-600 nm, limited by crystal availability in-house) for PAI in multiple wavelength bands. The laser generates pulse energy ∼10 μJ in the NIR-II window and ∼0.1 μJ in the visible window with a high repetition rate tunable from 20 to 80 kHz and a pulse duration tunable from 40 to 200 ns. The source is then used for PAI, demonstrating photoacoustic spectroscopy of lipids in the second resonance of C-H bonds (1145-1257 nm).