Multifocus optical-resolution photoacoustic microscope for extended depth of field

Abstract

Photoacoustic microscopy with large depth of focus is significant to the biomedical research. Here, we developed a multifocus photoacoustic microscopy by using a tunable acoustic gradient (TAG) lens and optical delay pathways. We split a single laser pulse into three sub-pulses and introduce them into three multimode fibers with a length of 1 m, 26 m and 51 m, respectively. The sub-pulses out of the fibers were combined by a single-mode fiber thereafter. We then obtained a pulse train with a time interval of 120 ns. The output of the single-mode fiber is collimated by a fiber port, and then guided into homemade TAG lens vertically. 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. By controlling the fire time of the pulse train and the driving signal of the TAG lens, the laser pulses out of three multimode fibers synchronize with three vibration states of the TAG lens. And we finally achieved three focal spots in one A line data acquisition using a single input laser pulse. The depth of focus (DoF) of the system was measured to be 360 μm, which is three times of that of single-focus system without the sacrifice of time resolution. A mouse cerebral vasculature were imaged in-vivo to demonstrate the feasibility of the extended DoF of our system.