Novel All-fiber Endoscopic Optical Coherence Tomography And Study On The Formation Mechanism Of Long Period Fiber Gratings

Due to its small volume, capable of being bent, extra low loss, high bandwidth, insensitivity to electro-magnetic and capable of measuring lots of physical quantities, Optical fiber-based devices are wildly used in optical communications and sensing applications; Optical coherence tomography (OCT) is a non-invasive, high-resolution and non-destructive optical technique based on Michelson interferometry which has great potentials in biomedical applications such as clinical medicine and in vivo detection of living tissues. To circumvent the problems mentioned above, an all-fiber common-path beam-splitting OCT endoscopic probe is proposed in this paper. In a common-path interferometer, the reference and sample arm overlap for a significant portion. As a result, it has the advantages of adjustable probe length, reduced sensitivity to vibration and temperature variation, and better stability due to automatically maintained beam overlap. In addition, this miniaturized configuration reduces the complexity and cost of the system. In accordance with the photosensitivity of optical fiber, another research is focused on CO2 laser fabricated long period fiber gratings (LPFG), including the fabrication technique and principles.The research mainly includes:1. A novel common-path OCT probe is demonstrated, which contains a virtual reference plane by using common-path technique. This plane could be located anywhere in front of, in between or behind the sample, free from overlapping of reference plane and investigated sample.2. In the common-path OCT endoscopic system, rotors, stators and clutches are employed to separate A-Scan and B-Scan, and in the meanwhile, to adjust the working distance of OCT probe.3. In order to achieve beam expansion and collimation, and satisfy the need for miniaturized device used in common-path OCT endoscopic system, an all-fiber device using graded-index multimode fiber collimator is investigated.4. A new method to measure the CO2 laser irradiation-induced refractive index modulation in the core of a single-mode optical fiber is demonstrated for the purpose of design and fabrication of LPFGs. The total refractive index change was measured using an optical fiber Fabry–Perot interferometer. The CO2-laser irradiation-induced refractive index change in the fiber core has a negative value and its magnitude is a sensitive function of the laser exposure time following a linear relation. This study provides valuable guidance for LPFG fabrication.5. Owing to the fact that deep scan information from Fourier Domain OCT system is obtained by inverse Fourier transform of back scattered signal spectrum, the acquired images contains parasitic information such as DC signal and self-interference signal along with useful information. Given this drawback, this paper puts forward a new algorithm of fixed-step two frames phase shift. This algorithm significantly lowers the demand for the accuracy of virtual reference plane movement, and only one time movement is needed for eliminating DC signals and self-interference signals inside the sample.