A fiber-coupled laser diode design for reflection mode optical resolution photoacoustic microscopy

Pulsed Laser Diodes (PLD) are compact and high pulse repetition rate laser sources that show a great potential for low-cost Optical Resolution Photoacoustic Microscopes (OR-PAM). Nevertheless, their non-uniform multimode laser beams are of low quality so that high lateral resolutions with tightly focused beams are difficult to realize at long focusing distances, as required for reflection mode OR-PAM devices of clinical application. A new strategy based on homogenizing and shaping the laser diode beam with a square-core multimode optical fiber allowed to attain competitive lateral resolutions while keeping one centimeter working distance. The theoretical expressions for the laser spot size, determining optical lateral resolution, and the depth of focus are also written for general multimode beams. An OR-PAM system was built in confocal reflection mode with a linear phased-array as the ultrasound receiver in order to test its performance, first, on a resolution test target and, afterwards, on ex vivo rabbit ears to show the system potential for subcutaneous imaging of blood vessels and hair follicles.