All-fiber modulators for laser applications
Abstract: The objective of this thesis was to explore the usefulness of all-fiber modulators for laser applications. The modulators were all based on refractive index change achieved in the core of the studied fiber- components, exploiting either the elasto-optic effect or the electro-optic effect. This was realized with the aid of electrodes inside the fiber cladding close to the core that provided either thermal stress in the core, or an electric field across the core. The electrodes consisted of low melting-point alloys, such as BiSn and AuSn, which were pushed into the hole-fiber, in the liquid state, which then solidified to form solid electrodes filling the entire hole.Together with an analyzer such as a polarizer or an interferometer the achieved refractive index modulation in the core could then be translated into an amplitude modulation of the guided light, which was subsequently utilized for switching fiber-lasers to generate cavity dumped, Q-switched, or mode-locked pulses. The fast rise/fall-time of a few nanoseconds for the elasto-optic devices was due to the fast thermal expansion of the electrodes. The maximum repetition rate, however, was limited to a few tens of kHz, due to the slow thermal processes for dissipation of the applied energy. The electro-optic fiber components, which displayed similar rise/fall-times on the other hand, showed a much higher cut-off frequency of 16 MHz.The electro-optic, all-fiber switch was also employed to select single pulses at 1 MHz repetition rate out of a 7 MHz train of pulses. Additionally, simulations using the finite element method were performed in order to gain insight and to explain the underlying processes of the observed response of a long-period grating written in a 2-hole fiber with electrodes, when applying HV-pulses to one of these.The thesis shows that the studied fiber-components show great potential of becoming complementary devices with high damage threshold for all-fiber laser applications in the future.
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