An all-optical photoacoustic microscopy system for remote, noncontact characterization of biological tissues

Conventional photoacoustic microscopy (PAM) employs light pulses to produce a photoacoustic (PA) effect and detects the resulting acoustic waves using an ultrasound transducer acoustically coupled to the target tissue. The resolution of conventional PAM is limited by the sensitivity and bandwidth of the ultrasound transducer. We have investigated an all-optical, pump-probe method employing interferometric detection of the acoustic signals that overcomes limitations of conventional PAM. This method does not require contact with the specimen and provides superior resolution. A 532-nm pump laser with a pulse duration of 5 ns excited the PA effect in tissue. Resulting acoustic waves produced surface displacements that were sensed interferometrically with a GHz bandwidth using a 532-nm continuous-wave (CW) probe laser and a Michelson interferometer. The pump and probe beams were coaxially focused using a 50X objective giving a diffraction-limited spot size of 0.5 µm. The phase-encoded probe beam was demodulated using a homodyne interferometer. The detected time-domain signal was time reversed using k-space wave-propagation methods to produce a spatial distribution of PA sources in the target tissue. Performance was assessed using 3D images of fixed, ex vivo, retina specimens.