High-resolution Interferometric Diagnostics for Ultrashort Pulses

2.4 Extending ultrashort metrology to the space-time domainuniform reference that were discussed in the previous section. To complete the spatio-temporalcharacterisation, one or more spectral phase measurements at specific spatial locations are required.In fact, there are several advantages to measuring the spectral phase at all spatial pointsusing one of the methods described in section 2.4.2, and this is one motivation for the work presentedin chapter 5 of this dissertation.A Shack-Hartmann wavefront sensor may be placed in the detector plane of an imaging spectrometer[262], a highly practical solution because such sensors are a mature and robust technology.Care must be taken to calibrate out the quadratic spatial phase caused by the imaging system.In ref. [262] this approach was combined with FROG to perform complete characterisation in twodimensions.Frequency-resolved tomographic imaging is performed by making intensity measurements inthe near and far field. For each frequency, the spatial phase profile is then retrieved using theGerchberg-Saxton algorithm [215, 216]. This has been combined with FROG to form CROAK [263],a complete characterisation method.Spectrally resolved lateral shearing interferometry is the method of choice in this dissertation.The principle is identical to the spatial case discussed in section 2.4.1.3, except that an imagingspectrometer is used so that one of the spatial dimensions is replaced by frequency. The beamsare combined with a time delay and a tilt, and Fourier-transform filtering is used to extract thefringe phase [57, 264]. As with all forms of one-dimensional shearing interferometry, there is arelative phase ambiguity across null regions.The combination of lateral and spectral shearing interferometry is space-time SPIDER (ST-SPIDER) [58, 59, 265].2.4.5 SummaryThe most common approach to extending ultrashort phase measurement to the spatial domainis using interferometry, although the use of tomography and wavefront sensors has also beendemonstrated. There are only a handful of spatio-temporal characterisation methods which canbe considered self-referenced and complete. Many of these involve using spatial filtering or selec-61