Recent progress on high efficiency Tm-doped silica fibers pumped at 790nm has enabled the
demonstration of a 2μm CW fiber laser operating at the 1kW power level and with single mode
beam quality . In addition to this state-of-the-art high power research, Tm-doped fibers are now
starting to find applications in lasers with nsec  and psec  pulsed operation, as well as lower
power CW lasers (50-100W) in the 1940nm wavelength region for medical use .
As with Yb-doped fibers, the question of photodarkening needs to be addressed to ensure the long
term fiber reliability is appropriate for the application. In a recent paper , we proposed that the
up-conversion process in highly doped Tm-fibers was significantly quenched when compared with
lower concentration fibers, under 790nm pumping. This trend was also observed in the
photodarkening rate as measured in a CW 2μm fiber laser cavity operating around 20W output
power. In this controlled experiment, the rate of photodarkening dropped from 15% per 1000 hours
in low concentration fiber to less than 1% in a fiber doped with 4.6% Tm.
In this paper we review the 20W results of our earlier work and then confirm the long term
reliability of 1940nm CW fiber lasers operating at higher (40W) output power, presenting results for
a laser operating for 1200 hours without significant loss of output power (around 12% total power
variation). Over any given 24-hour period during this experiment, the laser operates open loop with
around 8% total power variation, in an air cooled configuration. We believe these results confirm
the appropriate level of long term reliability of Tm-doped fibers for CW fiber lasers at the ~50W
power level, suitable for medical laser applications.
The capability of Tm-doped silica fibers pumped at 790nm to efficiently produce high power emission in the 1.9~2.1μm
region has been well documented to date but little has been presented on the reliability of this technology. Early
experiments highlighted that photodarkening can be a significant concern when Tm-doped silica fibers are exposed to
high intensity blue light. We present a discussion of the processes responsible for the production of blue light in Tmdoped
fibers pumped at 790nm and how fiber composition influences these processes. Through optimization of fiber
composition we have demonstrated highly efficient lasers exhibiting less than 1% output power degradation per thousand
790nm-pumped Tm-doped fibre lasers provide a number of distinct benefits for integration into next generation DIRCM
systems. Incorporation of Tm-doped fibre technology into mid-IR laser systems has been demonstrated in two main
architectures to date; in early works the fibre laser was used as a low quantum defect pump source for Q-switched solidstate
holmium laser which was subsequently shifted to the mid-IR using a ZnGeP2 OPO  and more recently, a pulsed
fibre laser systems was used for directly pumping the OPO . He we present two fibre laser systems for integration into
DIRCM systems. Firstly we present a 70W MOPA system (pump power limited) operating at 1908nm with 53% slope
efficiency from the amplifier stage for pumping Ho:YAG. Secondly we present a pulsed fibre laser system producing
over 4kW peak power at 1910nm using all single-mode fibres.
This report presents a discussion of the engineering issues and results of high power 2μm Tm3+-doped fibre lasers pumped at 790nm. To date we have achieved up to 85W from such devices with 54% slope efficiency relative to launched pump. More recently, through using Tm3+ concentrations of approximately 4(wt.)% to enhance the cross-relaxation process (3H4,3H6->3F4,3F4) we have demonstrated slope efficiencies of up to 67% relative to launched power. This represented ~170% quantum slope efficiency for the 790nm pumped 2μm laser.
Using Tm3+-doped double-clad silica fibre we have produced high power, high efficiency 2μm lasers. To date we have achieved a 59% slope efficiency relative to launched pump power using single end pumping and double passing the pump light. By pumping the fibre laser from both ends, we achieved up to 118W peak output power with 54% slope efficiency relative to launched power at 25% duty cycle. The quantum efficiency of this laser was 120% relative to launched pump power, which we attribute to a cross-relaxation process in Tm3+ (3H4,3H6→3F4,3F4). We have also demonstrated fixed wavelength operation of the laser near 1.9μm by using fibre Bragg gratings.