Ultrafast axial-scanning multifocus photoacoustic microscope with extended depth-of-field based on a tunable acoustic gradient lens and fiber delay network

Abstract

Photoacoustic imaging is a high-resolution and high-contrast technique, which combines optical contrast with ultrasonic detection to map the distribution of the absorbing pigments in biological tissues. As an important branch of photoacoustic imaging, optical-resolution photoacoustic microscopy (OR-PAM) suffers from narrow depth-of-field (DoF), since the lateral resolution is determined by tight optical focusing. The small DoF will prevent OR-PAM to achieve large volumetric imaging. Here, we developed an ultrafast axial-scanning multifocus photoacoustic microscope with extended depth-of-field based on a tunable acoustic gradient lens (TAG) and fiber delay network. The TAG lens is used to high -speed focus-shift. And a fiber delay network consists of three optical fibers with different lengths is used to split a single laser pulse into three sub-pulses with different delay time. A function generator generates a sinusoidal signal to drive the TAG lens at an eigenmode. The focusing power of the TAG lens will exhibit a sinusoidal oscillation at the frequency of the driving signal. Then, the three sub-pulses synchronizes with three vibration states of the TAG lens, respectively. Finally, we can obtain three focuses with different depth in one A-line data acquisition to improve the DoF. The DoF we measured by a vertically tilted carbon fiber is estimated to larger than 775 μm, which is ~ three times of that of single-focus PAM. The large DoF of large volumetric PAM was also verified by imaging a tungsten wire network. This system can achieve rapid and large-scale monitoring of physiological activities, which could expand the application of OR-PAM in biomedical researches.