  We propose an experimental set up allowing to convert an input light of
wavelengths about $1-2 \mu m$ into an output light of a lower frequency. The
basic principle of operating relies on the nonlinear optical properties
exhibited by a microcavity filled with glass. The light inside this material
behaves like a 2D interacting Bose gas susceptible to thermalise and create a
quasi-condensate. Extension of this setup to a photonic bandgap material (fiber
grating) allows the light to behave like a 3D Bose gas leading, after
thermalisation, to the formation of a Bose condensate. Theoretical estimations
show that a conversion of $1 \mu m$ into $1.5 \mu m$ is achieved with an input
pulse of about $1 ns$ with a peak power of $10^3 W$, using a fiber grating
containing an integrated cavity of size about $500 \mu m \times 100 \mu m^2$.
