High-resolution Interferometric Diagnostics for Ultrashort Pulses

7. LATERAL SHEARING INTERFEROMETRY FOR HIGH-HARMONIC GENERATIONTable 7.1: Gas jet z-scan parameters.ParameterValueLaserLaser centre wavelength785 nmLaser pulse duration13.6 fsLaser waist 50 µmLaser peak intensity 1.4 × 10 14 W/cm 2TargetGas speciesKryptonGas jet effective thickness 1.25 mmGas peak pressure0.1 atmGas jet z -scan range 8 mm in 16 stepsLateral shearing interferometerMach-Zhender lateral shear range 0.6 mmFocal spot separation 130 µmFocal length40 cmFocii-to-detector distance118 cmCamera exposure time5 msspectra: background (5 images), A arm (10 images), B arm (10 images), and AB interferogram (50images). A servo-driven beam blocking system automated the acquisition of the different arms,whilst a linear actuator adjusted the shear stage position. We moved the gas jet z position manually.The entire run took 51 minutes.The parameters are listed in Table 7.1. The broadened laser spectrum was measured usingan Ocean Optics HR2000 spectrometer, and the centre wavelength was taken as the centroid ofthe spectrum. The pulse duration was taken by assuming a transform-limited pulse. The waistwas taken from the focal spot measurements of the beams. The peak intensity was inferred frompower measurements before the vacuum chamber. The gas jet thickness was taken as the tubelength in the target. The gas target pressure was rather uncertain, with original estimations basedon experience with different designs [421]. The middle of the gas jet (along the z -axis) was locatedto a precision of ±2 mm with respect to the focus. The values of the target thickness, pressure andlocation with respect to the focus were refined using the the simulations described below.Figure 7.5 shows the position and shape of the two laser focii for the different shears and gasjet positions. We took care to adjust the collimation of the incoming laser beam such that itswaist coincided with the geometric focus of the focusing optic used for HHG. Our method was to166