Browsing by Author "Ludvigsen, Hanne; Doc."

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Novel sensor and telecommunication applications of photonic crystal fibers
(2006-08-04) Ritari, Tuomo
Doctoral dissertation (article-based)
Photonic crystal fibers are novel optical waveguides containing a periodic array of air holes running along the fiber around a solid or hollow core. These fibers have recently attracted great interest in many research areas such as in nonlinear optics and measurement science as their manufacturing process allows for a high flexibility in the fiber design. Index-guiding photonic crystal fibers are commonly referred to as microstructured fibers whereas hollow-core photonic crystal fibers which guide light through a photonic bandgap effect are called photonic bandgap fibers. The thesis provides results on the polarization properties of microstructured fibers and presents novel applications which utilize both the unique physical structure and optical characteristics of microstructured and photonic bandgap fibers. Polarization effects can have a great impact on the operation of fiber-based devices and communication systems. In particular, polarization-mode dispersion can limit long distance high bit-rate data transmission. In this thesis, the polarization characteristics of microstructured fibers and the sensitivity of these properties to the temperature and wavelength are studied. Also, polarization-mode dispersion of large mode-area microstructured fibers is investigated. One of the earliest applications of microstructured fibers is supercontinuum generation. In the thesis, supercontinuum generation is studied in large mode-area microstructured fibers by employing nanosecond laser pulses. The special properties of these fibers allow for the realization of single-mode supercontinuum sources with a high spectral density, low output beam divergence and low polarization dependence. Moreover, a novel nanosecond supercontinuum source based on an acetylene-filled nonlinear microstructured fiber is presented. The approach provides a broadband source that is self-referenced to the absorption lines of the gas. Air-guiding photonic bandgap fibers can guide more than 98% of the light in air thus reducing the influence of the material parameters of silica on the optical properties of the fiber. By filling the fiber holes with gas, such a fiber can provide a very long optical path length in compact fashion. In this thesis, the use of photonic bandgap fibers in gas sensing is studied. The high sensitivity and long interaction length provided by PBFs allows for the detection of molecules with weak absorption lines. In addition, a compact optical wavelength reference based on a gas-filled photonic bandgap fiber is presented. By utilizing lock-in technique, the output frequency of the laser was stabilized to the center point of weak acetylene absorption lines coinciding with wavelength division multiplexing channels.