The advancement of laser technology

producing increasingly shorter and more intricate optical pulses

has elevated the significance of precise characterization of a transient electric field

including the carrier-envelope phase. This characterization must cover progressively larger spectral bands and be performed as close as possible to the experimental site to enable a detailed understanding of the coherent light–matter interaction. Furthermore

in many experiments

two (or more) different ultrashort pulses are used

calling for a technique capable of characterizing multiple electric fields simultaneously. Here

we introduce the TREX (third-order reconstruction of electric fields via cross(X)-correlation) method

which allows the all-optical

in situ characterization of the complete electric fields of 2 broadband pulses with different central wavelengths. The method relies on the measurement of the perturbative third-order nonlinear response generated in a noble gas target while varying the delay between 2 pulses. The resulting spectrograms can be reconstructed using a custom evolutionary algorithm. The technique is demonstrated by retrieving the complete electric field

including the carrier-envelope phase

generated by the coherent synthesis of 2 ultrashort pulses. These synthesized waveforms reach time durations below a single optical cycle

demonstrating the ability of TREX to characterize complex multioctave-spanning electric fields.