4.1 Heating the rear face of a mirror
Technical issues make it difficult to install any device in front of the cavity input mirrors. Thus, it has
generally been proposed that one heat the rear face of these mirrors, which is more accessible
from the central part of the vacuum tank. The heating is due to infrared radiation produced
either by a hot material or by a CO
auxiliary laser, but in both cases, the wavelength of the
heating radiator is such that absorption by the silica substrate occurs in a thin layer. If we
assume another heat source located on the rear face of a mirror, we must modify the model
developed above. The extension is straightforward. For instance, let us consider the case in
which the thermal lens to be compensated for is caused by thermalization on the coating of the
intracavity-stored power
. As usual, we denote by
the FB coefficients related to the
main readout beam and by
and
the FB coefficients of the power distribution of the
compensation system, radiating an integrated power
. We get the following temperature field
with
(For brevity, we have set
and
, with
defined as before). By integrating
Equation (4.2) on the thickness, we get the thermal lens
which yields
with
with
. As expected, this shows that the global thermal lens is simply the sum
of the primary lens (caused by the stored light) plus the compensation lens. Thus, it is possible to imagine
power profiles compensating for the primary lens. This is the starting point for thermal compensation
systems.