Development And Imaging Applications Of All-fiber Probe For Optical Coherence Tomography

Optical coherence tomography(OCT)is a non-invasive imaging method that can obtain three-dimensional structure and/or function information of tissues and organs in vivo.Combined with endoscopy,OCT can obtain high-resolution tomographic images of human internal tissues and organs,which is of great significance to biomedical research.The OCT probe is the key to the endoscopic OCT system,but the main optical properties of the probe such as lateral resolution,depth of focus,and working distance contradict each other,which is especially prominent among miniature probes.This research studies and exploits the mode interference of light in step-index fibers to develop all-fiber probes with an optimized output beam for OCT.In this dissertation,I develop the miniaturized flexible all-fiber probes with high transmission efficiency and enhanced imaging quality that,and explore their imaging applications.In addition,I also manufacture an ultrasound-OCT dual-modality probe and demonstrate the advantages of the dual-modality imaging in both high resolution and deep penetration depth.The main research contents and innovation results are as follows:1.A lens-free all-fiber probe with high transmission efficiency was developed.Based on the previous work "all-fiber OCT probe based on the tapered structure",I propose to use the fundamental mode and high-order mode of the large-core fiber to control the output beam of the lens-free probe.The critical taper angle of the taper section is derived to obtain a shorter rigid length while maintaining high transmission efficiency.When the taper angle is fixed,the manipulation of the mode power in the large-core fiber by adjusting the length of the tapered section is studied.The manipulation of the output beam by adjusting the lengths of the tapered section and the large core fiber is studied.To verify the feasibility of multimode all-fiber probes in OCT imaging applications,the following researches are conducted:(1)study the influence of multimode transmission on OCT imaging when there are obvious high-order modes;(2)study the wavelength-dependent collection efficiency of multimode fiber probes under broadband light illumination.The imaging applications are studied in biological samples in vivo and tubular samples.A side-view probe with diameter of 340?m was fabricated,which provides an effective imaging range of?0.6 mm with a beam diameter less than 30?m.2.An all-fiber probe with extended depth range by a fiber-based pupil filter was developed.I exploit dual-mode interference of the large-core fiber to generate annular illumination on the pupil of the fiber lens and extend the depth of focus of the all-fiber probe.I investigate the focal depth expansion ratios of fiber-based pupil filters under typical parameters.I investigate the collection efficiency and sidelobe level of the probe under different filters.I study the reduction of the reflection from the internal facets of the probe by adjusting the filter parameters.For the parameter combination with the largest focal depth expansion,the influence of the cutting error of each fiber in the probe on the optical performance is studied,the manufacturing tolerance of the probe is determined,and the advantages of the focal depth expansion technology in terms of easy fabrication are verified.I develop a precise optical fiber cleaver,manufacture the optical fiber probe,and experimentally verify the probe's lateral resolution and the focal depth expansion.A DOF extension ratio of 2.6 over conventional Gaussian beam is obtainable in one proposed probe under a focused beam diameter of 4.6?m.The probe has a working distance of 130?m,a depth of focus of 195?m,light eficiency of?90%and sidelobe intensity of 3%.Length tolerence of the proposed probe is determined to be-28/+20?m.The fabricated probe has an outer diameter of 0.125mm,and achieves a lateral reslution of 4.9?m and a depth of focus of 186?m.3.A probe with optimized focal depth,working distance,and on-axis intensity uniformity was developed.Based on our previous research "Focus Depth Expansion Technology of Optical Fiber Probe",I further study and compare two beam expansion methods for multimode interference field:diffraction amplification and imaging amplification,so as to obtain a longer working distance.In regard to the non-uniformity of the on-axis intensity in the output beam of the multi-mode fiber probes,it is proposed that controlling the mode phase difference is the key.A fast simulation method based on eigenmode expansion(EME)is proposed to optimize the multimode fiber probes.The probe with optimized parameters is fabricated,and the probe's application in 3D imaging of living biological tissues is studied.The advantages of the proposed probe compared to the traditional fiber probe in imaging quality are verified.The probe is fabricated with a diameter of 0.125 mm and a total length of 2.6 mm for its distal fiber optics,achieves depth of focus of 211?m and working distance of 174?m.Compared to the conventional probe with similar minimal lateral resolution of better than 4.4?m,the proposed probe achieves two times that of the DOF gain,1.7 times that of the WD and 1.4 dB sensitivity penalty.The computation time of the EME-based simulation method is 0.6 second.The results using the EME method are in good agreement with that by the beam propagation method(BPM),while the latter takes 1-2 orders more computation time.4.The OCT part of the ultrasound-OCT imaging system and the dual-modality probe were developed.Based on the previous research in the laboratory,a compact fiber-type swept-source OCT system is established,and a proximal scanning device suitable for the dual-modality probe is established.The dual-modality probe was fabricated with diameter of 3.5 mm and rigid length of 19 mm.Imaging experiments were conducted for verification.The optical component of the probe has an outer diameter of?1mm,lateral resolution of 10?m at the focus and lateral resolutions better than 30?m over depth range of 1 mm.The maximum rotation speed of the designed proximal scanning device is 25 rps.