Unidirectional isolator-free 2 micron fiber lasers

This thesis presents results of a research on unidirectional isolator-free cavity lasers, exploiting thulium-doped fiber (TDF) as amplification medium and operating in \unit[2]{\mu m} wavelength range. For fibre ring resonator, an optical isolator should be used to ensure unidirectional lasing. The isolator suppresses backward propagating light within a given bandwidth, which generally does not exceed few hundreds of nm. Therefore, isolator-free unidirectional ring fibre cavity (referred to "theta" resonators) represents an attractive alternative solution. In theta cavities, non-reciprocal losses are introduced via an S-shape feedback, which is set by incorporating two directional couplers into the main loop. The first part of the thesis is devoted to different aspects of thulium-doped fiber characterization. We used the software package VPITransmissionMaker to model the active fiber, incorporated in different resonators, and ensured that experimentally measured TDF characteristics matched the simulation results. In the second part, the theoretical model of isolator-free resonators is stated. It directly links performance indicators of unidirectional lasers to the cavity parameters. For the theta lasers, the split ratios of intracavity couplers constitute main design degrees of freedom. The theta laser with BPF was found to maintain the highest extinction ratio (ER) between directional modes for any split ratios. The theta laser with fiber Bragg grating (FBG) is also analysed. As it represents a superposition of Sagnac loop and theta with bandpass filter BPF configurations, the ER is always limited, but the steady-state gain coefficient is significantly lower, comparing with the one of the ring- or theta with BPF cavities. The design rules suggest that considerably asymmetrical couplers should be used to achieve a high directivity in this case. In the last part, the results of experimental investigation of non-reciprocal cavities TDF lasers (TDFL) are presented. Initially, the first unidirectional theta TDFL with BPF is demonstrated, providing sub-Watt output power with a slope efficiency of 25%, 2dB flat tuning range of 1900--2050 nm, and linewidth of 0.2 nm, and achieving the ER of 18--25 dB. It is proven that these characteristics are competitive with the ones of similar ring cavities. Next, truly all-fiber unidirectional laser, based on the theta resonator with FBG, is investigated. Based on the theoretical model, we identify and experimentally confirm the set of cavity design parameters that enables the theta laser to generate up to 1W output power with a slope efficiency of about 30%, preserving ER better than 22dB, and the linewidth smaller than 0.20 nm (for FBG FWHM of 0.8--0.9nm). Furthermore, we enhance a functionality of the theta cavity toward a dual-band operation in two different gain media, by attaching a second FBG to another coupler and inserting additional holmium-doped fiber section. In our current laser layout, 100--200~