Fig. 1. Schematic of probes. (a) Conventional probe; (b) multi-mode interference probe without focusing elements ; (c) multi-mode interference probe with focusing elements

Fig. 2. Schematic of probes. (a) Intravascular OCT probe^[23]; (b) imaging needle for OCT^[24]

Fig. 3. Miniature probe based on ball lens. (a)Glass hemisphere for reflecting and focusing beam^[25]; (b) sapphire ball lens with 500 μm^[26]; (c) 70 μm diameter probe for common-path OCT in air and liquids^[27]; (d) curve of beam size of the probe changing with the distance from the end of the probe^[27]

Fig. 4. Fabrication of lensed fibers and their output beam spots. (a)-(c) Radii of the fiber lens increase with the diameters of the coreless fibers^[28]; (d) radii of the fiber lens decrease with arc power of the splicer^[29]; (e) side viewing probe design with angle polishing fiber lens^[30]; (f) transverse intensity distribution of the spot without the pro

Fig. 5. Subminiature lateral probe^[32-33]. (a) Schematic of probe; (b) three dimensional OCT image of sheep lung obtained by needle probe; (c) OCT image of skeletal muscle obtained by the needle probe; (d) micrograph of skeletal muscle

Fig. 6. Probe with free form surface lens and its imaging effect^[34]. (a) 3D printing of off-axis parabolic total-reflection surface and optical fiber assembly. (b) OCT image of tape phantom; (c) OCT image of cucumber; (d) OCT image of human hand

Fig. 7. Lens-free probes and their imaging performances. (a) Human finger nail obtained from single SMF probe; (b) OCT image of human finger tip^[35]; (c) schematics of ultra-thin probe based on stepwise transitional core fiber; (d) OCT image of human finger tip^[7]

Fig. 8. Lens-free probe with tunable output beam^[9]. (a) Layout of the probe; (b) two-dimensional light intensity distribution of the outgoing beam under four typical parameters; (c) photograph and micrograph of the fabricated probe; (d) OCT system based on the probe; (e) OCT image of the human finger based on galvanometer desktop system

Fig. 9. Probes based on micro conical lens. (a) Electric field intensity distribution diagram of probe based on micro conical lens made by chemical corrosion^[17]; (b) microscope image of probe based on micro conical lens made by polishing and grinding; (c) light intensity distribution diagram on focal plane; (d) normalized light intensity distributions curve in x axis^[18]; (e) schematic of micro co

Fig. 10. Miniature probes with extended focus depth. (a) Using GIF phase plate to expand the focus depth of the probe; (b) its light intensity distribution of the outgoing beam in water^[20]; (c) self-imaging wavefront division optical system; (d) its field intensity distribution in tissue; (e) edge beam trace of the 0th-order mode; (f) edge beam trace of the 1st-order mode; (g) edge beam trace of the 2nd-order mode^[21<

Fig. 11. All fiber OCT probe based on GIF-LCF pupil filter to extend depth of focus^[22]. (a) Schematic layout of probe; (b)-(d) normalized light intensity distributions of the probes with three designed filters; (e) normalized light intensity distribution of the conventional probe without filter

Fig. 12. All-fiber probe. (a) Layout of the probe; (b) simulated field intensity of the output beams in air for three typical cases of the designed probe; (c) microscope images of the proposed probe

Fig. 13. Cross-sectional OCT images of the microbeads-agarose phantom taken by probes. (a) Image taken by the conventional probe; (b) image taken by the proposed probe