  Photoacoustic microscopy (PAM) is a promising imaging modality because it is
able to reveal optical absorption contrast in high resolution on the order of a
micrometer. It can be applied in an endoscopic approach by implementing PAM
into a miniature probe, termed as photoacoustic endoscopy (PAE). Here we
develop a miniature focus-adjustable PAE (FA-PAE) probe characterized by both
high resolution (in micrometers) and large depth of focus (DOF) via a novel
optomechanical design for focus adjustment. To realize high resolution and
large DOF in a miniature probe, a 2-mm plano-convex lens is specially adopted,
and the mechanical translation of a single-mode fiber is meticulously designed
to allow the use of multi-focus image fusion (MIF) for extended DOF. Compared
with existing PAE probes, our FA-PAE probe achieves high resolution of 3-5
{\mu}m within unprecedentedly large DOF of >3.2 mm, more than 27 times the DOF
of the probe without performing focus adjustment for MIF. The superior
performance is demonstrated by imaging both phantoms and animals including mice
and zebrafishes in vivo. Our work opens new perspectives for PAE biomedical
applications.
