High-sensitivity ultrashort mid-infrared pulse characterization by modified interferometric field autocorrelation.

We report on spectral phase retrieval of 43 MHz, ∼100  fs, 3.3 μm pulses at energies down to 8.9 pJ by a modified interferometric field autocorrelation method. The simple setup consists of a Michelson interferometer, a 266 μm thick AgGaSe2 crystal, and a homemade spectrometer with an InGaAs point detector, which is readily applicable to measuring a 20 fs (1.8 cycles) pulse at 3.3 μm. The feasibility is verified by comparing with the results obtained by simulation and frequency-resolved optical gating for the spectral phase modulation because of a 4 mm thick germanium plate.