Ultrafast Laser Laboratory

Ultrashort-laser-pulse measurement: Frequency-Resolved Optical Gating (FROG)

Frequency-resolved optical-gating (FROG) is a novel nonlinear technique to measure the time-dependent intensity and phase, the full complex electric field, of arbitrary-shaped ultrashort laser pulses. A FROG trace is obtained by measuring the spectrum of each time slice of the laser pulse. Using second-harmonic generation (SHG) as the nonlinear element, numerous excellent FROG measurements have been made. However, SHG-FROG has an unavoidable ambiguity in the direction-of-time due to its nature of second-order process where the time axis is symmetric.

The next higher-order process, third-harmonic generation (THG), is dipole-allowed and do not have direction-of-time ambiguity. But third-order process in the bulk of nonlinear media do not have sufficient strength to yield usable FROG traces from unamplified ultrashort laser oscillators. However, the strength of THG on the surface of dielectric media was found to be, surprisingly, many orders of magnitude stronger than the bulk of the same material. By selectively focusing the laser beam on the surface of a transparent dielectric medium, one can easily obtain THG FROG traces of unamplified ultrashort laser oscillators. For instance, focusing 300 mW, 100-fs mode-locked Ti:sapphire laser pulses on a piece of cover glass slide, a total of several nanowatts of average THG signal power can be obtained. The optical arrangement is shown here.

A standard background free autocorrelation is used in these measurements. A pulse from a self-mode-locked Ti:sapphire laser is divided by a beam splitter. After one replica of the pulse is delayed with respected to the other, a 20x microscope objective is used to focus the two collinearly propagating beams on to the back surface of a 160-micron thick piece of cover glass. Note that by focusing the fundamental beam at the back surface one can use dielectric materials that are opaque to the third-harmonic radiation. Two autocorrelated THG beams, each carries a fraction of a nanowatt signal power sufficient for the required spectral measurements. One of the THG beam is recollimated and sent to a spectrometer equipped with a linear diode array for spectral recording. Spectrograms at varies time delays, with a 10-fs interval, are collected and converted into a 256x256 pixel FROG trace.

Because the interaction length of surface THG is extremely short, on can in principle measure the shortest pulses without potential distortion caused by geometric and dispersive effects. Furthermore, this surface THG does not rely on the efficient phase-matching condition nor does it strongly depend on wavelengths. Thus the simplicity of THG-FROG is particularly useful in the far infrared and wavelengths below the ultraviolet where suitable SHG crystals are lacked. A set of measurements are shown here.

For non-transform-limited pulse THG FROG are asymmetric with respect to the time delay axis, therefore, it has no direction-of-time ambiguity that is otherwise contained in SHG-FROG. The results here show a set of measured and reconstructed FROG trace of a (spectral cubic) phase distorted beam by reflecting the Ti:sapphire laser beam off a multilayer coated dielectric mirror at an angle of 50 degree. Also displayed are the retrieved fields for this pulse. The characteristic feature of a small satellite pulse and the pi-phase jump between the main pulse and the satellite pulse are clearly reconstructed. The slight variation in the position of the intensity maximum of the main pulse is caused by a drift of the laser during measurements, which also demonstrated the powerful nature on the self consistency check of the FROG retrieval algorithm.

For more information contact Thomas Y. F. Tsang

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