We present an ultrathin fiber-optic endoscopy probe for optical coherence tomography (OCT), which is made of a series of fused optical fibers instead of the conventional scheme based on an objective lens. The large-core fiber with a core diameter of 20 μm was utilized for the probe, while a single-mode fiber of core diameter 8.2 μm mainly delivered the OCT light. Those fibers were spliced with a bridge fiber of an intermediate core size. The guided light was stepwise converted to a beam of a large mode-field diameter to be radiated with a larger depth of focus. We obtained a 125 μm thick all-fiber endoscopy probe with a side-viewing capability implemented by an angled fiber end. Successful OCT imaging was demonstrated with a swept-source OCT system and showed the practical applicability of our lens-free endoscopy probe.
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