  Optical waveguides, covered with thin films, which transmittance can be
controlled by external action, are widely used in various applications from
optical modulators to saturable absorbers. It is natural to suggest that the
waveguide losses will be proportional to the covering material absorption. We
demonstrate that under certain conditions this simple assumption fails.
Instead, we observe the reduction of the film material absorption can lead to
an increase in the waveguide propagation losses. For this, we use a side
polished fiber covered with a single-walled carbon nanotube thin film whose
absorption is attenuated either due to saturable absorption or electrochemical
gating. For the films thicker than 50 nm, we observe saturable absorption to
turn into light induced absorption with nonmonotonic dependence on the incident
power. With a numerical simulation and analytical approach, we identify that
this nontrivial behavior comes from mode reshaping and predict required
parameters for its observation.

Optics;Materials Science