  Transmission of high power laser beams through partially absorbing materials
modifies the light propagation via a thermally-induced effect known as thermal
lensing. This may cause changes in the beam waist position and degrade the beam
quality. Here we characterize the effect of thermal lensing associated with the
different elements typically employed in an optical trapping setup for cold
atoms experiments. We find that the only relevant thermal lens is represented
by the $TeO_2$ crystal of the acousto-optic modulator exploited to adjust the
laser power on the atomic sample. We then devise a simple and totally passive
scheme that enables to realize an inexpensive optical trapping apparatus
essentially free from thermal lensing effects.
