Novel Optical Fiber Modal Interference Devices Based On Micro/Nanofabrication Technologies

Miniaturizaion and integration are the current trends in photonictechnology. Micro/nanofabrication technology is the basis to manufacure theminiaturized or integrated optical devices. With the help ofmicro/nanofabrication technology, people can design and prepare variousoptical fiber micro/nano structures and devices. The optical devices exhibitssignificantly difference in characteristics, compared with the bulky countparts,as the functional structure size decreases to micro and/or nanometers. Thesespecial physical properties have wide theoretical research prospect andpractical application. In this dissertation, two kinds of novel fiber modalinterference devices with micro/nano structures are theoretically analized andexperimentally studied, including the mode coupling properties and relevantmicro/nanofabrication technologies. The operation principle, preparationprocess and functional properties of the proposed optical fiber devices arestudied in detail. By comparison with that of the traditional sized optical fiberdevices, the advantages in performance of the micro/nano structure basedones are verified. The main contents of this dissertation include:Two kinds of typical optical fiber based modal interference devices andthe relevant micro/nanofabrication technology are proposed in the firstchapter. One typical optical fiber modal interference device is the opticalfiber grating. Adopting optic fiber surface micro/nano structure technology, a wide range tunable optical fiber grating based optical device is achieved. Theother typical device is the tapered optical fiber. Through the micro/nanooptical fiber fabrication technology, optical micro/nano fiber taper basedoptical devices is realized, which can be used for high resolution sensing.In the second chapter, the properties of the cladding modes in longperiod fiber gratings coated with a high refractive index micro/nanometeroverlay are theoretically studied. The resonant wavelength and spectralcharacteristics of the four layer model long period grating are also analizedbased on the coupled-mode theory. Besides, the transmission spectra of longperiod grating with different overlay thickness and refractive indices arenumerically calculated. After investigating the relation of the resonantwavelength with the cladding thickness, we analyze the methods to improvethe tuning range of the long period fiber grating.In the third chapter, the mode coupling and interference characteristicsin a locally-bent microfiber taper are theoretically analized. Themathematical model for locally curved micro/nano fiber taper is established.The working principle of its modal interference is described and the influenceof the fiber taper geometry on the interference fringes is discussed. Theoptimization principle for geometry parameters is presented.In the forth chapter, a new structure LPFG coated with nanosizedhigh-refractive-index liquid crystal (LC) layer is experimentally realized. Therefractive indices of LC at different temperatures are measured. Using thesample brush coating technology, we have prepared different thickness LClayers on the surface of LPFGs. The cladding mode reorganization in highrefractive index coated LPFGs is theoretically analyzed and experimentallyobserved with the aim of exploring the sensitivity of the resonance wavelength to the change of the refractive index in a nanoscale overlay.Experimental results show that the transition between cladding modes andoverlay modes occurs when the refractive index (HRI) of the LC overlay ischanged from1.477to1.515by increasing its temperature from20°C to65°C. The spectral tuning ability of LPFGs coated with a HRI LC layer byelectro-optic modulation on a LC layer is also demonstrated, and themaximum tuning range can reach approximately10nm by choosing a highlysensitive operating point in the transition region, which is vertified with thetheoretical results in chapter2.In the fifth chapter, the fabrication process and optic sensing propertiesof the locally bent microfiber taper based modal interferometer are presented.The microfiber taper fabricated by adiabatically stretching a heatedsingle-mode fiber is made in a C-shape bent to form a modal interferometer.The microfiber taper waist diameter can be optimized to minimize thespectral shift of the interferometer owing to the environmental temperaturechange. We show that the transmission spectrum of a microfiber taper withdiameter of about1.92μm has substantially small temperature dependence,which agrees well with the theoretical estimation. The proposed modalinterferometer has a high refractive index sensitivity of~658nm/RIU for RI=1.333-1.353and a high microdisplacement sensitivity of~102pm/μm. Theproposed device can be used for precision sensing applications.Conclusions and expectation are made for the dissertation in the lastchapter.