SRS of Chirped Pulses as a Method of Wavelength Conversion of Femtosecond Pulses

Abstract

The stimulated Raman scattering (SRS) of femtosecond pulses is strongly transient with high threshold intensity which is comparable to threshold of optical element damage. As a rule the SRS of femtosecond pulses accompanies many other nonlinear effects like a self-phase modulation [1] and a white light continuum generation [2]. To prevent these undesirable effects we propose to use femtosecond pulses stretched by diffraction gratings up to duration of 0.5–1 ns for SRS conversion. In this case SRS process is quasi stationary in the field of broadband pump and has some peculiarities [3]. Thus for original pulse duration of 100 fs spectrum of stretched emission is f = 150 cm1 and because of that SRS will be generated effectively only in forward direction as the coherent length of backward interaction is order 1/f = 6 μm. The SRS threshold is also increased in the comparison with that of monochromatic pump. For gain (M = I g z) calculation of forward SRS in the field of broadband pump we can use the expression [3]: $$[M = Mp - \ln \left\{ {\left[ {\left( {fp + fc} \right)df} \right]\left( {Mp} \right)1/2} \right\}$$ (1) where Mp — is a gain in the field of monochromatic pump, fp, fc — are pump and Stokes emission bandwidths respectively, df — is a linewidth of spontaneous Raman scattering. From equation (1) we see that the gain of stretched pump decreases with the increase of fp and fc. It is important that with SRS of broadband pump, pump spectrum is reproduced in Stokes spectrum [3] i.e. fc = fp and the chirped structure of stretched pulse is preserved and it is possible to compress the SRS pulse up to the original (100 fs) duration.