High-resolution Interferometric Diagnostics for Ultrashort Pulses

2.3 Introduction to ultrashort pulse metrologybeen used to characterise nanojoule pulses from an oscillator [49] and in telecommunication systems[153–155]. Enhanced sensitivity can be achieved by conducting the interaction in waveguides[156]. By taking into account various spectral factors arising from the response of the opticalelements and the phase-matching, and temporal factors arising from the crossing angle of thebeams, few-cycle optical pulses can be measured using SHG-FROG [61, 157, 158]. Characterisationof mid-IR free electron laser pulses [159] also suggest the method’s versatility.The aforementioned SHG-FROG geometries were noncollinear — essentially a spectrally resolvedintensity autocorrelator. A collinear geometry is also possible, avoiding temporal blurringartefacts arising from the beam crossing angle. Using type II sum-frequency generation, in whichthe polarizations of the replicas are mutually orthogonal [160], the acquired signal is the same asthe noncollinear case [161–163]. Using a type I interaction, in which the polarizations are parallel,one obtains a spectrally resolved interferometric autocorrelation. Although the interference termscan be numerically removed to retrieve a normal SHG-FROG trace [161], they can be exploited foran independent reconstruction [164, 165].Whilst SHG-FROG is sensitive and versatile, it has the major drawback of possessing a time reversalambiguity, stemming from the fact that the temporal gate is multiplication by the unknownpulse itself. In the frequency domain, this corresponds to a phase sign ambiguity; positive andnegative dispersion cannot be distinguished. This is resolveable by applying recognisable featuresto the pulse — introducing glass of known dispersion or using multiple reflections to producetrailing satellite pulses [166].Third-order nonlinearities. Self-diffraction FROG (SD-FROG) is one of a number of FROG geometriesbased on the third order nonlinearity. Whilst generally possessing a lower sensitivitythan SHG-FROG, third-order spectrograms are often more intuitive [167] and lack the reversal oftime ambiguity. In SD-FROG, the test pulse and its delayed replica cross in a third-order medium,forming a refractive index grating via the Kerr effect. The test pulse and replica scatter from this39