High-resolution Interferometric Diagnostics for Ultrashort Pulses

3.1 Motivationration of information from a larger shear, which links disparate frequencies directly without anyconcatenation.The noise sensitivity of SSI also increases with the degree of spectral intensity modulation ofthe spectrum. In the extreme case of a spectral null, SSI develops an ambiguity in the relativephase across the null, as discussed in section 2.3.6.1. However, ultrashort pulses with highly modulatedspectra arise frequently in ultrafast optics. Examples include the output of hollow-corefiber compression systems [60], soliton molecules [272], shaped two-color pulses for differencefrequency generation [273], supercontinua generated in photonic crystal fibers [268] and shapedpulse sequences for spectroscopy and coherent control [274]. Furthermore, the relative phase betweenspectrally disjoint subpulses is usually physically significant. For example, the output ofpulse compression systems based on self-phase modulation often consist of the superposition oftemporally overlapped but spectrally disjoint subpulses, and the resulting temporal intensity iscritically dependent on the relative phase of the subpulses. Also, the relative phase of spectrallydisjoint subpulses can be important in pump-probe and coherent control experiments [275–277].A multiple shear scheme could potentially alleviate the problems associated with spectral intensitymodulation in the single shear case. A small shear would resolve spectral details, whilst alarge shear could cross spectral nulls, precisely linking the spectral phase between frequencies ofsignificant spectral intensity. The idea is depicted in Fig. 3.1.Another motivation is the redundancy inherent in a multiple shear dataset, which should alsooffer an automatic self-consistency check, useful for validation of the measurement and identificationof an incorrect setup. This is an advance upon the comparison of several single-shearreconstructions taken at different shears, an approach that is already available to a conscientiousexperimenter with a variable-shear apparatus.Of course, in motivating multiple SSI one must justify the use of SSI in the first place, particularlyover less experimentally complex spectrographic methods. Besides the features of SSIdescribed in section 2.3.6, there are a number of specific reasons one may prefer SSI over spectographyfor complex and/or highly modulated pulses. The most complex ultrashort pulse measurementshave been done with crosscorrelation FROG (XFROG) [175], which has elucidated the65