  The temperature distribution inside a double-cladding optical fiber laser or
amplifier is examined in detail. Traditionally, the quantum defect in the core
is taken to be the main source of heating in an active optical fiber. However,
contributions from the parasitic absorption of the signal and the pump may also
play an important role, especially for low quantum defect or radiation-balanced
lasers and amplifiers. The contributions to the heating in both the core and
the inner-cladding are considered and analyzed in general terms in this paper.
In particular, it is shown that if the maximum tolerable surface temperature of
the fiber relative to the ambient is taken to be 300 degrees Celsius to avoid
damaging the fiber's outer polymer cladding, the core temperature rises only in
the range of 0-5 degrees Celsius relative to the inner-cladding for an
air-cooled fiber. However, for a water-cooled fiber, the core temperature can
be higher than the inner-cladding by as much as 50 degrees Celsius, potentially
changing a single-mode core to multimode due to the thermo-optic effect.
