Inline structure with longitudinal dispersion variation based on a silica micro-structured optical fiber for highly coherent supercontinuum generation
Date of Issue2017-03-11
School of Electrical and Electronic Engineering
The supercontinuum generation in the pure silica micro-structured optical fibers (MOF), or photonic crystal fiber (PCF) when the cladding air holes are arranged periodically, has been investigated for over more than a decade since its first debut in the year 2000. The mechanism of the supercontinuum generation is mainly affected by the dispersion of the pumping wavelength when short pulse laser is used as a pumping source. The supercontinuum generated by pumping at anomalous dispersion wavelengths usually has a large bandwidth. The drawbacks of this generated spectrum is that the coherence can degrade fast due to influence of modulation instability in the anomalous dispersion region. Thus, it is sensitive to the input shot to shot noises and there can be spectral intensity fluctuation in the generated supercontinuum spectrum. This hinders its uptake in a wide range of applications. On the other hand, pumping at the normal dispersion regime can inhibit the problematic modulation instability. Therefore, highly coherent supercontinuum generation is achievable when pumped at normal dispersion regime. However, usually the bandwidth is limited due to fast peak power dropping by the rapid temporal. In this work, I develop an inline optical fiber structure with a longitudinal variation based on the silica PCF for the highly coherent supercontinuum generation. In the inline fiber structure, the input pulse is first compressed for a peak power enhancement to overcome the problem of insufficient nonlinear interaction due to the power dropping in normal dispersion region. Then, the spectrum is rapidly broadened in a short length of normal dispersion taper. The normal dispersion taper can inhibit the modulation instability and yield high coherent supercontinuum spectrum. The short length of the small structure taper could reduce the loss from the low confinement in the fiber, as well as the absorption in the silica when the spectrum extends to above 2 μm region. Besides, the pulse compression and the spectrum broadening occur in one fiber structure, which avoid the additional loss in coupling. The whole inline fiber structure has a short length and could be pumped with commercial picosecond laser for a better performance, which make it compatible with other systems and promising in the real application. In this thesis, I introduce the background physics including the linear and nonlinear properties of the PCF, theoretical investigation of the supercontinuum generation in the inline fiber structure, fabrication and characterization of the fiber and fiber taper, and finally the experimentally investigation of the pulse compression and supercontinuum generation in the inline fiber structure. The obtained supercontinuum generation is over one octave in bandwidth and extends to above 2158 nm in this silica fiber. The simulation agrees with the experiment result and confirms that the generated spectrum is highly coherent.