Flexibly adjustable depth-of-focus photoacoustic microscopy with spatial light modulation

Through focusing the excitation laser, optical-resolution photoacoustic microscopy (OR-PAM) is capable of measuring optical absorption properties down to micrometer-scale lateral resolution within biological tissues. The focused Gaussian beam routinely employed in the OR-PAM setups is inadequate for acquiring the volumetric images of biological specimens with thickness from tens micrometers to millimeter without scanning in depth because of the inconsistent lateral resolution along the depth direction due to its short depth of focus (DoF). Here, we integrate a spatial light modulator (SLM) into the optical path of an OR-PAM for realizing the flexibly adjustable DoF. By simply switching the phase patterns assigned onto the SLM interface, three representative illumination beams are produced, including conventional short-DoF Gaussian beam (GB), needle-like Bessel beam (BB), and extended depth-of-focus beam (EDFB). These modulations can be well realized based on the extended Nijboer-Zernike theory. The photoacoustic excitations show variable DoFs ranging from hundreds of micrometers (GB and BB) up to 1.38 mm (EDFB) but a consistent lateral resolution of ∼3.5 μm. The proposed method is confirmed by volumetric imaging of multiple tungsten fibers positioned at different depths.