Microfiber-based Passive Optical Micro-devices

As an emerging important area of micro/nanophotonics, optical micro/nanofibers (MFs) have being attracting vast interest in various fields including optical fiber sensing, microwave photonics, optical fiber communications and signal processing, atom and quantum optics, due to unique characteristics such as wavelength/sub-wavelength diameter, high optical field confinement, ultra-low surface roughness, low loss, ease of fabrication and loss-negligible integration with standard optical fibers. It holds a bright prospect as a building block of constructing optical micro-devices in future. Moreover, it offers plenties of advantages in investigating the interaction between photons and materials. With the subject of optical micro-devices, this dissertation proposes and demonstrates several novel MF devices and their applications in microwave photonics. The content of this dissertation includes the following aspects.The first part in this dissertation is mainly about state-of-art of optical MF devices. Firstly, the investigation background of optical MFs is introduced. Then the world-wide institutions where the research on optical MF was carried out are summarized. As last, optical MF divices are categorized into three types:passive, active and nonlinear devices. The state-of-art of each type is summarized.The second part introduce the fundamental knowledge and properties of optical MF and forms a basis for the whole work of this dissertation. Light propagation along MFs is governed by "Maxwell Equations. The mode, energy distribution, dispersion and nonlinear properties of optical MFs are quantitatively analized. Then the fabrication and characterization of optical MFs utilized in our laboratory is introduced in detail.The third part is on the study of microfiber Bragg gratings (MFBGs). Firstly, longitudinal coupling effect (LCE) in air-cladding MFBGs is theoretically investigated. Distinct from conventional weakly-guiding optical fibers, large longitudinal electric field (Ez) exists in wavelength-scale microfibers. Due to the LCE, MFBG reflectivity is reduced by more than30%within the band-gap and the full width at half maximum (FWHM) is obviously narrowed. This theoretical analytical work is instructive in designing MFBGs. Secondly, high-index-contrast nanohole-induced MFBGs are fabricated using phase-mask technique under femtosecond laser ablation. These submicrometer-diameter holes, benefited from the resolution of femtosecond laser micromachining beyond-diffraction limit, results in an effective negative refractive index change Δn～-10-3. Transmission dips over-23dB are achieved for the gratings with excellent Gaussian apodization and-3dB reflection bandwidths up to1.14nm. Moreover, the grating reflectivity increased by3dB, the resonant wavelength blue-shifted1.35nm after two weeks’ placement at room temperature and these gratings exhibit excellent stability in the following time. This makes them attractive elements in sensing, nanophotonics and nonlinear optics.In the fourth part, a novel in-line polarization-dependent microfiber interferometer (PD-MFI) is proposed and experimentally demonstrated, which is tapered from a commercial polarization-maintaining fiber. Different from conventional MFIs, the transmission spectra of the MFI are highly polarization-dependent, due to the mode-sensitive birefringence. The experimental results agree well with the theoretical analysis. Moreover, exploiting the polarization-dependent property of the PD-MFI, a simple and flexible scheme of generating polarity-switchable ultra-wideband pulses in the optical domain is experimentally demonstrated. Doublet pulses with a central frequency of6.28GHz and a10-dB bandwidth of7.86GHz are obtained, of which the electrical spectrum meets the FCC mask.The fifth part proposes and demonstrates a novel compact in-line optical filter with a narrow rejection bandwidth based on an asymmetric MF coupler. Composed of silica and bismuth-oxide-glass MFs, it has a length less than500μm. Single transmission dip in a wavelength range of up to300nm is obtained, and the transmission extinction ratio is more than30dB. Additionally, a-20dB bandwidth of0.88nm is achieved. This narrow bandwidth of the transmission notch benefits from the huge refractive index difference between the two kinds of MFs. These experimental results agree well with the theoretical predictions. With the advantages of having a simple structure, being fiberized, and having a small footprint, this device can be an attractive element for micro/nanophotonics, optical sensing, optical signal processing, and optical fiber communications.In the sixth part, a novel silica microsphere resonator embedded with iron-oxide nanoparticles is demonstrated, which possesses broad all-optical wavelength tunability up to terahertz. This microsphere is generated by using in-line laser ablation of a MF with the assistance of magnetic fluid. To the best of our knowledge, this simple method of fabricating such microsphere resonators is reported for the first time. Strongly absorbing lightwaves running through the sphere core, the resulted sphere exhibits low loss of whispering gallery modes. Prominent photothermal effect is realized by the nanoparticles, which leads to a wavelength shift of the resonator over13nm (1.6THz). This tuning range is the largest with respect to silica microsphere resonators reported so far. Moreover, a linear tuning efficiency up to0.2nm/mW is realized. With distinct features of excellent robustness and high optical pumping efficiency, the spheres can be attractive elements in building up novel micro-illuminators, point heaters, optical sensors and fiber communication modules.