In this letter, the feasibility of label-free in vivo GRIN-lens optical resolution photoacoustic micro-endoscopy is demonstrated. An image guide with 100 000 single-mode fibers in a 1.4 mm diameter bundle in conjunction with a 0.29 pitch GRIN lens is used in order to transfer a focused scanning spot through the image guide and refocus it into tissue. A high-repetition-rate (up to 600 kHz) ytterbium fiber laser is used in order to enable near real-time imaging capability. Phantom studies indicate 6 μm resolution. The system, with ～2 mm working distance, overcomes the penetration depth limitation and hence improves the surface laser fluence of previously reported fiber based optical resolution photoacoustic microscopy (OR-PAM). The proposed system with a sub-mm probe footprint is very flexible and now has a significant penetration depth which is another step towards clinical applications.
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