An Yb3+-doped optical fiber is laser cooled in vacuo from 298 to 282 K
. Cooling results from anti-Stokes fluorescence of the ytterbium ions
after optical pumping at a wavelength of 1015 nm. The sample temperatu
re is deduced from the emission spectrum, which is sensitive to the po
pulations in the excited-state multiplet of the ions. The temperature
change is limited by the coupling between the fiber and ambient blackb
ody radiation, as confirmed when samples suddenly exposed to the pump
laser are found to exponentially relax towards thermal steady state wi
th the expected time constants.