Spectral phase shifts in femtosecond parametric down-conversion with media dispersion

Abstract

We theoretically study the spectral phase shift of a midinfrared (MIR) femtosecond pulse generated through parametric frequency down-conversion in dispersive media, with an emphasis on the effect of group-velocity dispersion (GVD), with the aim of obtaining guidelines for dispersion compensation and pulse compression in such systems. It is found that the spectral phase of the generated MIR pulse differs significantly from that of due to its linear propagation. We consider two common ways of MIR femtosecond pulse generation: difference frequency generation (DFG) and optical parametric amplification (OPA). The resultant spectral phase is determined by the initial injection condition, and the transfer of the spectral phase accumulated by the higher-frequency fields. Our results show that in DFG, pump depletion has little effect on the spectral phase, while in OPA, the combined effect of linear GVD and high parametric gain leads to a notable dependence of spectral phase on gain---the higher the gain, the less the spectral phase. Quantitatively, under conditions where the MIR pulse is generated through injection of two higher-frequency fields, its spectral phase is close to only half of that accumulated through linear propagation in DFG, while it is even smaller in OPA. To obtain the shortest MIR femtosecond pulse, these results determine the amount of dispersion that needs compensation.