Mid-infrared photoacoustic microscopy by gas-filled hollow-core fiber laser

Mid-infrared photoacoustic microscopy (MIR-PAM) is an emerging imaging modality with high potential in revealing spatially-resolved biomolecules compositions in thick samples by utilizing light-excited ultrasound signals. The development of a nanosecond and high-energy MIR fiber laser source, which is crucial for MIR-PAM, is still at an early age, facing the challenges of either low peak power or large footprint. This work aims to develop a new Raman laser source for MIR-PAM based on the gas-filled anti-resonant hollow core fiber (ARHCF) technology. As a proof of concept, a MIR laser source at 3.4 µm wavelength, which overlaps with the main absorption band of myelin sheaths, is developed and combined with PAM for the first time to image the lipid-rich mouse brain slices. This laser source is based on the cascading of two ARHCFs, starting with a high-energy (~26.5 µJ) Raman Stokes line at 1409 nm generated through the 1^st stage nitrogen-filled ARHCF with a pump fiber laser at 1060 nm. The output Raman laser from the 1st stage ARHCF is used as a pump for the 2^nd stage hydrogen-filled ARHCF, to generate a Raman laser at 3.4 µm with ~2.7 µJ pulse energy. Our label-free ex-vivo imaging was able to map myelin in the mouse brain, showing the feasibility of extending the novel gas-filled laser platform into PAM imaging modalities.