Fig. 3: Liquid-shaped microendoscope.

a(i) Fabrication of liquid-shaped OCT fiber probe. Optical liquid of pre-calculated volume on a glass substrate with a modified wetting property forms a microlens of custom contact angle, shape, and size, which is then polymerized and used as distal optics of fiber probe made of single-mode fiber (SMF) and non-core fiber (NCF). Here an incident angle θ is formed in between the OCT light axis with respect to the normal axis of the reflective surface of microlens. a(ii) Schematic of OCT microendoscope. The fiber probe is guarded in a hypodermic tube (rigid version) or a torque coil (flexible version) and protected with a glass capillary tube (rigid version) or a plastic sheath (flexible version). a(iii) Zoomed-in view of the distal part of the microendoscope, consisting of an NCF of length L and a semi-spherical microlens of radius r and utilizing an incident angle θ. Here the working distance (WD) is measured as the normal distance between the protective sheath surface and the center of the focal plane. b Calculated back-reflection in microendoscope versus NCF length L for four representative combinations of lens radius r and incident angle θ. The back-reflection is found to decrease by about 0.55 dB (black line), 0.33 dB (red line), 0.89 dB (blue line), and 0.41 dB (green line), respectively, as NCF length L increases from 350 μm to 650 μm for each combination. c Calculated back-reflection versus lens radius r and incident angle θ in the microendoscope using an NCF length of 500 μm. The desired incident angle of 52.5° was indicated by a red dot. Chromatic focal shift (d), focused spot size (e), astigmatism ratio (f), effective depth of focus (DOF, g), and working distance (h) calculated at different NCF lengths L and lens radii r when the incident angle is 52.5°. i Optimal design region compiled from the calculated results in (d–h). The design adopted to fabricate the microendoscope is indicated with a red dot.