Spectral narrowing and temporal expanding of femtosecond pulses by use of quadratic nonlinear processes

A nonlinear system capable of expanding pulse with spectral narrowing, an analogy to a spatial beam expander in linear optics, is studied theoretically and experimentally. The system, with features of high efficiency and maintaining near Fourier-transform limit (FTL), is constructed by using quadratic nonlinear processes with chirped pulses. The spectral and temporal characteristics of such a pulse expander are investigated analytically and computationally. It shows that group-velocity mismatch of nonlinear crystal plays a detrimental role, which leads to a deviate operation of pulse expander from its ideal case, e.g., temporal shortening, spectral broadening, and a deviation from the FTL of the expanded pulses. The criteria for designing a near aberration-free pulse expander are given based on these analyses. As as demonstration, we experimentally expand broadband 70-fs pulses from a Ti:sapphire regenerative amplifier to narrowband 60-ps longer pulses. The conversion efficiency from pump to idler of the nonlinear pulse expander is currently limited to a few percent and can be practically improved to 10% to 20%.