Fig. 1. Comparison of resolution among different imaging technologies^[3]

Fig. 2. Configurations of fiber OCT systems. (a) TD-OCT; (b) SD-OCT; (c) SS-OCT

Fig. 3. Fiber OCT imaging catheters. (a) Forward-viewing catheter; (b) side-viewing catheter; (c) proximal scanning catheter; (d) distal scanning catheter

Fig. 4. Fiber-prism-based fiber OCT endoscopes. (a) Catheter type^[21]; (b) needle type^[24]; (c) balloon type^[29]; (d) capsule type^[30]

Fig. 5. Design of fiber OCT endoscope with high resolution and long depth of focus. (a) High resolution and long depth of focus achieved with diffractive lens^[32]; (b) long depth of focus imaging achieved by self-imaging wavefront division^[34]

Fig. 6. All-fiber OCT endoscopes. (a) Forward-viewing fiber OCT endoscope based on GRIN fiber^[38]; (b) side-viewing fiber OCT endoscope based on angle polished ball lens^[40]

Fig. 7. Design of all-fiber OCT endoscope with long depth of focus. (a) Endoscope with phase mask^[43]; (b) axicon lens^[44]; (c) no-focus collimated light beam^[45]

Fig. 8. Composite fiber OCT endoscopes. (a) BPSF endoscope^[46]; (b) metalens endoscope^[48]

Fig. 9. Composite fiber OCT endoscopes based on two-photon polymerization 3D printing. (a) Separate probe^[56]; (b) monolithic probe^[57]

Fig. 10. Endoscopic imaging of human lung resections using OCT probe based on metalens^[48], the fine features of lung tissue are clearly visible in the magnified OCT images, including moderately scattering epithelium (epi), highly scattering basement membrane (bm), cartilage (car), alveoli (alv), and the small irregular glands (g). (a) Normal airway; (b) distal bronchiole; (c) abnormal airway

Fig. 11. OCT images of esophagus of different animals and their histological images of the specimen. (a) Swine esophagus^[34]. Images obtained with the conventional Gauss beam (1) are significantly blurred when it is out of focus, while the cells are visualized with high contrast and resolution in images obtained with the CAFM beam (2) due to longer depth of focus; (b) rat esophagus^[40]. The cut-away view of a reconstructed 3D image (1) and cross-sectional image (2) obtained by OCT scanning, where the keratinized stratified squamous epithelium (EP), lamina propria (LP), muscularis mucosae (MM), muscularis propria (MP), and submucosa (SM) are observed in enlarged view

Fig. 12. White light endoscopic images and OCT images of esophagus of the colon model^[75]. (a) Healthy tissue with layered structure of mucosa (M), submucosa (S), and muscular layer (ML); (b) non neoplastic mucosal growth phantom. Yellow arrows represent a benign lesion with visible thickening of the mucosa; (c) pedunculated polyp phantom; (d) flat cancerous tissue (CT) and healthy tissue (HT) sessile

Fig. 13. Endoscopic OCT images of porcine ureter^[81], urothelium (U), lamina propria (LP), smooth muscle (SM), and adipose tissue (AD) are observed with high resolution. (a) Cross-sectional image; (b) histologic image; (c) 3D renderings of ureter segment consisting of multiple cross-sectional images; (d) longitudinal lumen view

Fig. 14. OCT scanning images of different arteries. (a) Human cadaver coronary artery^[35]. The smooth muscle cells (red arrow) and macrophages undergoing diapedesis (green arrow) are observed in image (1), image (2) shows probable thrombus (blue arrow); (b) mouse aorta^[57]. The adventitial and perivascular adipose tissues (AT) in image (1) and cholesterol crystals (yellow arrow) in image (2) are observed

Fig. 15. 3D reconstructed OCT images of arteries with implanted stents. (a) Rabbit artery^[35]. The purple and red arrows indicate the implanted stents; (b) porcine artery^[45]. The blood vessel wall (H), guidewire (G), and stents (S) are observed

Fig. 16. Micro-OCT system with long depth of focus and fiber probe based on few mode interferometry^[35]

Fig. 17. Fiber OCT probes for multimodal imaging. (a) Ultrasound/OCT^[98]; (b) photoacoustic/ultrasound/OCT^[99]

Fig. 18. Fiber OCT probes for multimodal imaging. (a) Fluorescence/OCT^[100]; (b) polarization-sensitive OCT^[101]