The emerging mid-infrared photoacoustic microscopy (MIR-PAM) is a potential imaging modality in revealing special biomolecules compositions in thick samples by utilizing the light-excited ultrasound signals. The development of a nanosecond and high-energy MIR fiber laser source 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 is developed and combined with PAM for the first time targeting at the lipid-rich mouse brain sample due to main absorption band of myelin sheaths. This laser source is based on the cascading of two ARHCFs, where a high-energy (~26.5 μJ) Raman Stokes line at 1409 nm is generated through the 1st-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 2nd-stage hydrogen-filled ARHCF, to generate the Raman laser at 3.4 μm with ~2.7 μJ pulse energy. Our label-free ex-vivo imaging depicted the lipid-rich myelin region in the mouse brain, showing the feasibility of extending the novel gas-filled laser platform into PAM imaging modalities.
This report presents a state-of-the-art multimodality imaging device that combines multi-spectral optoacoustic microscopy (OAM) and optical coherence tomography (OCT) to chart absorbers in live tadpoles (Xenopus laevis) accurately. The OAM channel captures maps of five internal contrast agents: melanin, hemoglobin, collagen, glucose, and lipids. A novel method was developed to achieve this by assuming that each voxel in the 3D-OAM image exhibits a single chromophore contributing to the optoacoustic signal. The device is powered by a single optical source (SuperK Compact, NKT Photonics) that operates across an ultra-wide spectral range of 450 to 2400 nm. The set-up was optimized by minimizing optical aberrations and attenuation on optical components to stimulate the sample effectively. Using optical pulses of 2 ns duration and a repetition rate of 20 kHz, the device imaged tadpoles in their embryonic stage at multiple wavelengths, using narrow spectral windows of 25 nm bandwidth within the broad spectrum of the supercontinuum source at a time. In addition, an ultra-high-resolution OCT imaging channel operating at 1300 nm (spectral bandwidth 180 nm) was created and incorporated into the device. The OCT channel, also powered by a commercial supercontinuum source (SuperK EXTREME EXR9, NKT Photonics), was used for guidance purposes and to help determine the location of the chromophores.
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