Design of optimized thulium-doped fiber amplifiers through experiment and performance simulation

This thesis presents detailed and precise experimental and theoretical studies of Thulium-doped based fiber amplifier architectures emitting around 2 mm. This contribution describes the optimization of Thulium-doped fiber laser topologies through the development of a simulation tool for Thulium-doped silica fiber amplifiers combined with measurements of fundamental spectroscopic properties on Thulium-doped silica fibers, and a comparison of simulation and experimental demonstration. The outcome is a significant maturing of the scientific and engineering understanding of Thulium (Tm) doped fiber amplifier and laser technology. All the amplifiers studied have Master Oscillator - Power Amplifier (MOPA) architectures: we use a single-frequency semiconductor laser as the Master Oscillator and the Power Amplifier consists of a single or multi-stage Tm-doped fiber amplifier based on single or double clad fibers. We have developed a comprehensive model for predicting the performance of Tm-doped fiber amplifiers (TDFA) based on a review in the literature of the physical behavior of the Thulium system in silica fibers. Procedures were developed to evaluate the fundamental fiber properties, like the absorption coefficient, the gain coefficient, and the lifetime of the 2 mm transition. Different multiple single- or double-clad fibers have been characterized. These obtained fibers parameters have been used as input parameters into the developed simulation tool to predict the TDFA performances. The combination of a simulation model with the measured parameters is then demonstrated to be an accurate method to assist in the design of single clad and double clad TDFAs. Several single clad amplifier architectures based on core-pumping around 1560 nm were investigated with the intent of providing high signal gain, low noise figure, large input signal dynamic range over a wide operating bandwidth in the 2 mm region. Gains greater than 40 dB, and noise figures close to the quantum limit, with operation estimated over more than 100 nm, are demonstrated. Using the single-clad pumping scheme we demonstrate output power scaling up to 8 W in a 4 mm core diameter fiber without non-linear effects. Scaling to higher output powers in the 2 mm region is achieved through double clad amplification with multimode laser diode pumps at 793 nm. Two topologies are demonstrated with output power greater than 20 W with signal wavelengths centered around 1950 nm and 2050 nm and efficiencies greater than 50 % relying on the ""two-for-one"" effect. Operation is limited only by the available pump power and will allow us to scale to even higher output powers. Finally, using the developed single and double clad amplifiers, we investigate the pulsed amplification of a directly modulated single-frequency semiconductor laser at 1950 nm in the nano-second regime. Three configurations are compared and peak powers greater than 1 kW are demonstrated over pulse widths from 6 to 21 ns and repetition rates from 10 kHz to 500 kHz. We also observed a supercontinuum generation and report on our first results.