Devices and pumping architectures for 2μm high power fiber lasers

Thulium-doped fiber lasers are gaining in popularity since they emit at about 2 μm, a wavelength particularly interesting for many industrial, sensing and medical applications, and, moreover, in the so-called "eye-safe" spectral region. Despite the many advantages, however, thulium-doped fiber lasers with power high enough to allow practical applications have still limited deployment mainly due the high cost per emitted watt. The paper investigates alternative paths to high power CW emission at about 2 μm by exploring two complementary approaches: the development of specific pump combiners and the study of new pumping schemes that take advantage of co-doped fibers. The developed pump combiners are based on fused fiber technology and are characterized either by the use of "non-standard" fiber dimensions to allow pumping through an ytterbium-doped fiber laser or by a large number of input ports (up to 39) to provide adequate levels of pump power through the efficient coupling of several fiber pigtailed diodes with emission wavelength suitable for pumping thulium. On the other hand, a co-doped ytterbium-thulium fiber is also studied to analyze the possibility of using ytterbium ions as pump source for thulium ions. The use of ytterbium, either as co-dopant or as laser source, is particularly interesting because it allows taking advantage of the remarkable advancements made in the pump diodes for such a laser system, and specifically of the favorable cost per emitted watt. Preliminary experimental results have demonstrated the feasibility of the proposed approaches and have shown that the joint use of the "ad-hoc" pump combiners and of the ytterbium-thulium co-doping can lead to the development of lasers with power suitable for industrial applications, although the efficiency needs further improvement