Phase modulation conversion and conservation in optical parametric amplifier of femtosecond laser pulses

Abstract

The influence of the pump pulse, group-velocity mismatch, dispersion and selfphase modulation & cross phase modulation on the phase transfer in the OPA process were analyzed. The results show that very sharp phase modulation can be maintained in mid infrared range by optical parametric amplifier. The period and depth of the periodic modulation can also be maintained due to small group-velocity mismatching in the calculated wavelength range.Keywords Femtosecond pulse · OPA · Shaped pulse Recently, generation of tunable shaped femtosecond pulses becomes increasingly interesting for coherently controlling physical and chemical process (Pfeifer et al. 2005;Vogt et al. 2005;Aeschlimann et al. 2007). Generally, there are two typical methods for pulse shaping: the directly pulse shaping by modulators (Roth et al. 2005;Hacker et al. 2003) or transfering the modulated pulses by nonlinear frequency conversion process. For direct pulse shaping, SLM has many advantages, such as fast, programmable for modulation of phase or amplitude (Weiner 2000). However, it is impossible to achieve high power shaped pulses by use of SLM because of the limitation of the damage threshold of liquid crystal. In addition, it is difficult to shape pulses in arbitrary wavelength ranges because of the limitation of the transmission spectra of SLM.It is attractive to obtain ultrashort shaped pulses in more than 2 µm wavelength range or below 400 nm range for coherently controlling chemical molecules with several tens nanometers absorption spectra width in these ranges. Some groups had proved the possibility of shaped pulses by frequency conversion in both mid infrared and ultraviolet range (Tan et al.