Conversion of broadband incoherent pump to narrowband signal in an optical parametric amplifier

The parametric amplification of the signal beam with a narrow angular spectrum in the field of an incoherent broadband pump is investigated theoretically as well as experimentally. It is demonstrated that in the absence of beam walk-off a significant enhancement of the parametric gain takes place, and the power of the incoherent pump can be effectively transferred into the signal beam. The background of the signal spectrum appearing in the process of amplification due to pump amplitude fluctuations can be removed by filtering, and the coherence of the input signal can be recovered.