High-resolution Interferometric Diagnostics for Ultrashort Pulses

2. BACKGROUNDWavefronts Lenslets DetectorffFigure 2.16: Shack-Hartmann wavefront sensor. Measurement of a flat wavefront (blue) and adistorted wavefront (red) is shown.sition or wavenumber are trivially accomplished. Every single pixel on a detector array performsa space-nonstationary selection, whilst a spatial filter selects one particular wavenumber. Thischanges the game somewhat, allowing the use of techniques whose temporal analogue would becompletely impractical.2.4.1.1 Shack-Hartmann wavefront sensorThe Shack-Hartmann wavefront sensor [227–229], depicted in Fig. 2.16, is widely used. An array oflenslets each sample a small portion of the beam, producing an array of foci on a detector placedin the common focal plane. The foci are displaced according to the local wavefront tilt fk x /k .The combination of a Shack-Hartmann sensor with FROG to measure a space-time factorablepulse is called Shackled-FROG [230].2.4.1.2 InterferometryInterferometry is also widely used to obtain the spatial phase difference between two fields. Thethree main applications of this information mentioned in the spectral phase context (section 2.3.3.2)also apply. Comparison of the phase of a beam before and after the application of an optical instrument,such as a lens, provides the phase of the transfer function of the instrument, and is thebasis of optical testing. Where the phase difference of the beams is related to the phase of an unknownbeam by some invertible operation, then self-referenced phase measurement is possible.Finally, if the phase of one of the beams is known then the other may be inferred.Many of the spatial interferometric methods used in this thesis can be traced back to optical54