High-resolution Interferometric Diagnostics for Ultrashort Pulses

2.3 Introduction to ultrashort pulse metrologyĨ (ω)EE (t )+E (t − τ)χ (2)Ĩ (ω)Ωχ (2)τFigure 2.11: Schematic of a temporally encodedSPIDER device; E is an etalon.Figure 2.12: Schematic of a ZAP-SPIDER —see text for an explanation.or correlation-based methods because of the quasi-monochromaticity of the ancilla; a phasematchingresponse that encompasses the test pulse is only required for one of the two fields involvedin the nonlinear interaction. Furthermore, since the spectral phase information is encodedin the fringe spacing, it is invariant to the spectral response of the apparatus. This simplifies implementationsof SPIDER for broadband and few-cycle pulses [50, 190, 191]. Reduced to the barestof essentials, SPIDER is only a spectral phase measurement; external measurement of the spectralintensity is necessary for a complete characterisation. However in several common cases it ispossible to extract the spectral intensity from the same data trace as the phase [192, 193].The cost of these advantages is, in general, additional complexity. Equation (2.42) makes evidentthree parameters ω 0 , τ and Ω, which must be calibrated. A pulse stretcher for the ancillawith an appropriate chirp must be designed and constructed, unless a chirped pulse happens tobe available. Because the test pulse is mixed with a stretched pulse, the signal intensity is generallylower than in spectrographic methods. This does not in itself imply that SPIDER is less sensitive,because its single-shot nature means that for a given total measurement time one may integratethe signal for longer than in a multi-shot method. However, it does introduce the practical difficultyof aligning a weak signal.There is a menagerie of variations on SPIDER, with several playing an important role in thisdissertation. It should be noted that the various features discussed below are not necessarily mutuallyexclusive.Zero-additional phase SPIDER (ZAP-SPIDER) has the useful property that the test pulse does notpass through any transmissive optics before the nonlinear mixing step; only reflections are used45