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.