All-fiber modulators for laser applications

The objective of this work is to explore the usefulness of all-fiber modulators for laser applications. The modulators are 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 is realized with the aid of electrodes inside the fiber cladding close to the core that provides either thermal stress in the core, or an electric field across the core. The electrodes consists 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 can then be translated into an amplitude modulation of the guided light, which is 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 is due to the fast thermal expansion of the electrodes. The maximum repetition rate, however, is so far 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 display similar rise/fall-times on the other hand, show a much higher cut-off frequency of 16 MHz.

The electro-optic, all-fiber switch is 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 have  been 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 studied fiber-components show great potential of becoming complementary devices with high damage threshold for all-fiber laser applications in the future.

PhD student: Micke Malmström. 

The thesis has been carried out at Dept of Applied Physics at KTH, group of Laser Physics in collaboration with Acreo Fiber Optics. The supervisors of the work were Fredrik Laurell (KTH) and Walter Margulis (Acreo).