While ultra-thin optical coherence tomography (OCT) probes using rotating fibers have been demonstrated in side-viewing configurations, achieving fully miniaturized forward-viewing OCT endoscopy remains challenging because of the need to incorporate a lateral scanning mechanism. Here, we report a forward-viewing OCT fiber endoscope that can fit inside a protective tube with an inner diameter of 0.16 mm, enabling an endoscope with an outer diameter of 0.31 mm. The rotating fiber assembly incorporates a gradient-index fiber lens and an angle-cleaved length of no-core fiber at the distal tip. This causes the beam to exit at an angle to the endoscope axis, such that rotating the fiber using an external rotary coupler generates circular scanning of the beam over the tissue. Lateral and axial resolutions of 25 µm and 11 µm, respectively, are achieved at focus, with a usable imaging depth range of over 2 mm and an image circle circumference of 950 µm at 1 mm from the fiber tip. An exemplar application in vitreoretinal surgery is demonstrated by ex vivo porcine retinal imaging and real-time monitoring of a surgical tool interacting with the tissue.
