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.

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