Chirp And Self-phase Modulation In Induced-grating Autocorrelation Measurements Of Ultrashort Pulses

While simple second-harmonic-generation autocorrelation techniques have been popular for measuring ultrashort pulse lengths, they do not provide the phase information that is crucial for a full understanding of the nonlinear-optical effects that can occur to pulses in ultrashort-pulse lasers and in optical fibers. Interferometric second-harmonic generation autocorrelation (ISHGA) does provide phase information, but it is difficult to align and use. Here, we demonstrate theoretically and experimentally that induced-grating autocorrelation (IGA) techniques (or, equivalently, four -wave-mixing autocorrelation techniques) using slowly responding media yield the same phase information as ISHGA. IGA traces clearly display linear-chirp and self-phase-modulation (SPM) effects; linear chirp causes a narrowing of the trace, and SPM in the absence of group-velocity dispersion (GVD) causes easily recognized oscillations in the wings. These variations in the trace are the same as those that occur to the envelope of the high-frequency fringes in ISHGA traces, which are generally used to indicate the presence of the above two phase distortions. Conveniently, however, no high-frequency fringes occur in IGA methods. Other advantages of IGA methods include automatic phase-matching, single-wavelength operation, and applicability to most wavelength ranges (using a thermal grating simply requires an absorbing medium).