Optimization of plasma-blue-shift spectral shear interferometry for characterization of few-cycle pulses

Measurement of ultrashort ultraweak laser pulses with over-an-octave wide spectra poses a well-known challenge because of the spectral overlap of harmonic orders. The problem of the interference between the nonlinear signal and the fundamental spectrum can be mitigated in both frequency-resolved optical gating (FROG) and spectral shearing interferometry (SPIDER) [1] techniques by using non-collinear sum-frequency generation in very thin crystals. However, the suitable phase-matching bandwidth for frequency conversion is ensured as a rule by using low conversion efficiency several-µm-thick nonlinear crystals. Additional issues that become increasingly important as the pulse duration shrinks are the minimum added dispersion of the pulse characterization apparatus [2] and the time delay accuracy/jitter between the interfering pulse replicas in SPIDER [3]. These issues were addressed in newly developed technique based on ionization induced blueshifting called iSPIDER [4]. Here we investigate optimal parameters for the use of this technique.