  We discuss the radiation generated by positrons channeling in a crystalline
undulator. The undulator is produced by periodically bending a single crystal
with an amplitude much larger than the interplanar spacing. Different
approaches for bending the crystal are described and the restrictions on the
parameters of the bending are discussed. We also present numeric calculations
of the spontaneous emitted radiation and estimate the conditions for stimulated
emission. Our investigations show that the proposed mechanism could be an
interesting source for high energy photons and is worth to be studied
experimentally.
