  Multimode fibres are becoming increasingly attractive in optical endoscopy as
they promise to enable unparalleled miniaturisation, spatial resolution and
cost as compared to conventional fibre bundle-based counterpart. However,
achieving high-speed imaging through a multimode fibre (MMF) based on wavefront
shaping has been challenging due to the use of liquid crystal spatial light
modulators with low frame rates. In this work, we report the development of a
video-rate dual-modal forward-viewing photoacoustic (PA) and fluorescence
endo-microscopy probe based on a MMF and a high-speed digital micromirror
device (DMD). Light transmission characteristics through the fibre were
characterised with a real-valued intensity transmission matrix algorithm, and
subsequently, optimal binary patterns were calculated to focus light through
the fibre with wavefront shaping. Raster-scanning of a tightly focused beam
(1.5 {\mu}m diameter) at the distal end of the fibre was performed for imaging.
With the DMD running at 10 kHz, the PA imaging speed and spatial resolution of
were controlled by varying the scanning step size, ranging from 1 to 25 frames
per second (fps) and from 1.7 to 3 {\mu}m, respectively, over a field-of-view
of 50 {\mu}m x 50 {\mu}m. High-resolution PA images of carbon fibres, and mouse
red blood cells were acquired through a MMF with high image fidelity at
unprecedented speed with MMF-based PA endoscope. The capability of dual-modal
PA and fluorescence imaging was demonstrated by imaging phantoms comparing
carbon fibres and fluorescent microspheres. We anticipate that with further
miniaturisation of the ultrasound detector, this probe could be integrated into
a medical needle to guide minimally invasive procedures in several clinical
contexts including tumour biopsy and nerve blocks.
