Fig. 1. Schematic diagram of the spacer-removed probe.n_f, n_g, n_c, and n_air are the refractive index of SMF core, GRIN fiber at the center, NCF, and air, respectively; l_g and l_c are the length of GRIN fiber and NCF; P and θ are the polishing surface and tilt angle.

Fig. 2. (a) Refractive index of the GRIN fiber. Inset: cross section of GRIN fiber captured by the refractive index profiler. (b) FWHM beam size at different position. Inset: beam profile at 2-mm position (left top); beam measurement setup (right bottom).

Fig. 3. (a) Working distance and Rayleigh length versus different GRIN fiber length. (b) Beam size and divergence angle versus different GRIN fiber length.

Fig. 4. (a) Angle-polished probe. (b) Distal optics with coil. (c) Distal image of assembled catheter with a guide wire and radiopaque marker. (d) Full view of the whole catheter. (e) Connection of the catheter and PIU. Inset: cap used to house the SC/APC connector. U: UV adhesive; C: double-wrapped torque coil; P: plastic sheath; M: radiopaque marker; G: 0.014-inch guide wire; T: catheter; L: Luer connector.

Fig. 5. Catheter FWHM beam size at different imaging depth.X: horizontal direction, Y: vertical direction. Inset: beam profile at 2-mm position (right bottom); beam size measurement setup (left top).

Fig. 6. (a) in vivo imaging arrangement. (b) X-ray image of porcine heart in this experiment. (c) Cross-section image without stent attachment. (d) Cross-section image with the stent. (e) Cutaway view of the 3D image of a segment of coronary artery with stent and guide wire. ECG: electrocardiogram; G: guidewire; H: healthy blood vessel wall; T: catheter; L: small blood vessel branch; S: stent.