Ph.D. Thesis: All-Fiber Modulators for Laser Applications
by
Mikael Malmström(KTH, Applied Physics, Laser Physics)
→
Europe/Stockholm
FD5
FD5
Description
The objective of this thesis was to explore the usefulness of all-fiber modulators for laser applications. In particular refractive index change was achieved in the core of the studied fiber-components, with either the elasto-optic effect or the electro-optic effect. This was realized with the
aid of electrodes placed inside the fiber cladding and ran along the core and provided either stress in the core due to thermal expansion, or an electric field over the core. The electrodes consisted of low melting-point alloys, such as BiSn and AuSn, which were pushed into the hole-fibers
used, in the liquid state which then solidified to form continues electrodes filling the entire space in the 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 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 the elasto-optic devices was only a few nanoseconds owing to fast thermal expansion of the electrodes, the maximum repetition rate, however, was limited to a few tens of kHz, due to slow thermal processes. The electrooptic fiber components displayed similar rise/fall-time, although, with a
much higher cut-off frequency of ~ 16 MHz.
The electro-optic all-fiber switch was also employed to select singe 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 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 the electrodes.
The thesis show that the all-fiber components used show great potential of becoming price-worthy high damage threshold components for laser applications in the future.
