High-resolution Interferometric Diagnostics for Ultrashort Pulses

7.3 Experiment: longitudinal gas jet scanGas inletLaserDia. = 0.5 mmHarmonics2 mmFigure 7.4: Pulsed gas jet target used in the longitudinal gas jet scan experiment.tuations about the mean value. For example,inferred φ i .(V) ii is a meaningful size for an error bar on theThe definition of the covariance matrix in the previous paragraph describes fluctuations due tothe shot-to-shot fluctuations. For χ 2 red> 1, the dominant errors are due to inconsistency betweenthe shears. In the present experiment, I did not attempt to estimate empirically the magnitudeof these errors. Instead, I used the standard technique of assuming that these errors were suchthat the fit was “good but not too good” i.e. χ 2 red= 1 [420]. Effectively, this means replacing thedefinition of the covariance matrix with V = χ 2 red (BT B) −1 .7.3 Experiment: longitudinal gas jet scanWe constructed an experimental implementation at the attosecond beamline at Imperial CollegeLondon [421, 422]. The laser source was a hollow core fibre compressor [60] pumped by aTi:Sapphire chirped-pulse amplifier (CPA). For a convincing agreement with theory, we soughtto scan a parameter of the high-harmonic generation process, taking LSI measurements at eachpoint. An attractive choice would have been the peak laser intensity. However, we needed the fullouput of our laser system just to get any measurable signal at all. Therefore, we choose to scan thegas jet z -position. The gas jet target is shown in Fig. 7.4.The procedure was the following: we scanned the gas jet z -position over a distance of 8 mm in0.5 mm steps, centred around the position of maximum harmonic yield. At each gas jet z -position,we used 7 different shear stage positions. At each shear stage position, we took a several far-field165