A frequency-resolved optical gating (FROG) technique that combines autocorrelation (second-harmonic-generation FROG, SHG-FROG) and cross correlation FROG (XFROG) is reported for simultaneous characterization of two unknown optical pulses. Two SHG-FROG signals and a XFROG signal are acquired in a single measurement using a single diagnostic system. Unlike the conventional combination of SHG-FROG for reference-pulse characterization and XFROG for test-pulse characterization, the ambiguity in the direction of time in SHG-FROG is removed during phase retrieval by simultaneously analyzing the three FROG signals. Furthermore, overall characterization is faster, more robust, and highly convergent than the conventional combination of SHG-FROG and XFROG techniques.

Abstract:We investigate four-wave mixing in hydrogen gas using a gas cell and a hollow fiber for the generation of high-energy, multicolor femtosecond (fs) optical pulses. Both a hydrogen-filled gas cell and hollow fiber lead to the generation of multicolor fs pulses in a broad spectral range from the deep ultraviolet to the near infrared. However, there is a difference in the energy distribution of the multicolor emission between the gas cell and the hollow fiber. The hydrogen-filled gas cell generates visible pulses with higher energies than the pulses created by the hollow fiber. We have generated visible pulses with energies of several tens of microjoules. The hydrogen-filled hollow fiber, on the other hand, generates ultraviolet pulses with energies of a few microjoules, which are higher than the energies of the ultraviolet pulses generated in the gas cell. In both schemes, the spectral width of each emission line supports a transform-limited pulse duration shorter than 15 fs. Four-wave mixing in hydrogen gas therefore can be used for the development of a light source that emits sub-20 fs multicolor pulses in a wavelength region from the deep ultraviolet to the near infrared with microjoule pulse energies.

A diagnostic system using three frequency-resolved optical gating (FROG) techniques-crosscorrelation, second harmonic generation, and self-diffraction-is reported for the reliable characterization of femtosecond laser pulses. The latter two FROG techniques are employed to evaluate suitability in measurements of the reference pulse. A train of optical pulses generated by the superposition of two femtosecond pulses emitting at 800 nm and 1180 nm has been characterized by the cross-correlation FROG to evaluate the reliability of the present diagnostic system.

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