High Performance Fiber Lasers with Spectral, Thermal and Lifetime Control

This thesis contains the results of research in the fields of spectral control, efficiency and lifetime of high-power, rare-earth doped fiber lasers, properties which are of great importance for scientific and industrial applications. Volume Bragg gratings (VBGs) has for the first time been used together with fiber lasers and the laser performance in terms of spectral purity, thermal stability, and tunability was evaluated. It was found that VBGs are an excellent high-contrast spectral filter for many fiber laser designs where bulk optics are necessary, or for speciality fibers such as photonic crystal fibers or large-mode area fibers. It is also shown that they work equally well in low power and very high-power configurations, i.e. for fiber lasers ranging three orders of magnitude in output power, from ~100 mW to > 100 W. Furthermore, VBGs are shown to work very well as tunable spectral filters, producing a narrow emission linewidth in a compact setup. Concerning efficiency, it was shown how cryogenic cooling of the fiber gain-media substantially increased the efficiency. The reasons are an increased pump absorption, an increased gain cross-section, and a decreased threshold. The broad spectral output resulting from the low temperatures is shown to be easily mitigated by implementing a VBG as one of the cavity mirrors. The low operating temperature is also shown to efficiently suppress self-pulsing in the fiber laser, which, if left unchecked, can lead to catastrophic break-down of the fiber end-faces. The increased absorption and suppressed self-pulsing allowed a fiber length long enough to almost completely absorb the pump, which meant that, for the same pump power, more than 60 % higher output-power was attained. Finally, the lifetime issue of Yb-doped fiber lasers was addressed. It was found that Cecodoping substantially reduced the photodarkening-rate while leaving other fiber parameters essentially unchanged. This is especially important for Yb-doped fiber lasers emitting at 980 nm, as the high inversion required make them very susceptible to photodarkening. It was shown that the output power in Yb-doped fiber lasers degraded quickly when no Cecodoping was present and, conversely, with the right Yb/Ce-codoping ratio, degradationfree lasing could be achieved for many hours. The research results obtained in this work could be of great interest to scientists and engineers working with spectroscopy, display systems, non-linear optics to just name a few examples.