An extreme short duration and high intensities are the two unique properties of ultrashort laser pulses making them an unparalleled tool for studying as well as transforming the matter. For many applications, the information about ultrashort pulse shape – both its intensity and its phase - is of great importance. However, even the fastest optoelectronic components are orders of magnitude too slow to trace out the fast changing envelope of an ultrashort laser pulse.
One of the first, and today probably the most wide-spread characterization method for ultrashort light pulses is Frequency-Resolved Optical Gating (FROG). It can determine the pulse shape from the measured approximation of a pulse’s spectrogram followed by reconstruction of the exact field by an iterative algorithm. In this work, we have studied and developed an improved version of FROG in terms of sensitivity, capable of characterizing relatively long and weak, few-picoseconds pulses with low energy.
The spectrogram is recorded by spectrally resolving the signal from the nonlinear interaction of a pulse and its delayed replica for the varying delay times. Second harmonic generation (SHG) is the nonlinear process of choice for the most sensitive measurements due to its lowest order of nonlinearity. Phase-matching the broad spectrum of an ultrashort pulse typically requires very thin nonlinear crystals, which leads to low conversion efficiencies. We have tested here the three non-standard SHG-FROG geometries - a thick nonlinear crystal in combination with the tight focusing, a walk-off compensating stack of crystals and a quasi phase-matching in a periodically-poled crystal. High sensitivity was achieved by optimization of SHG process, use of collinear geometry and a slitless spectrometer. The same improvements were applied to another variant of FROG, called GRENOUILLE, which extracts the spectral information through narrow and angular-dependent phase-matching bandwidth of a thick birefringent crystal. The sensitivity and the measurement range (the shortest and the longest pulse) of both experimental setups were tested on specific pulses and also related to the critical design parameters.
Both here presented concepts are extending the operation range of FROG method towards longer and weaker pulses, which were so far impossible or hardly possible to measure. We strongly believe, that an in-depth description of the main concepts of these novel, yet technologically simple enough FROG adaptations, will serve researchers in characterization and development of new sources of ultrashort light pulses and its applications in the future.
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