High-resolution Interferometric Diagnostics for Ultrashort Pulses

2.3 Introduction to ultrashort pulse metrologySGCτFDFFigure 2.1: Czerny-Turner spectrometer; thesymbols are defined in the text.Figure 2.2: Apparatus for measuring thefield autocorrelation using a balanced interferometer.self-referenced methods.2.3.3 Linear time-stationary measurementsMeasurements performed using linear time-stationary elements and slow integrating detectorsare a fundamental building block in most forms of ultrashort pulse characterisation.2.3.3.1 Single-pulse spectral intensity measurementsAll measurements of a single pulse using linear time-stationary elements and slow integratingdetectors are of the form (2.19) — they are some approximation to the spectral intensity.A typical Czerny-Turner [82] spectrometer design is shown in Fig. 2.1. Light spreads out fromthe input slit S and is collimated by the curved mirror C. It is angularly dispersed by diffractiongrating G, and during propagation from the grating to the curved focusing mirror F the wavelengthsbecome spread. The focused beam falls upon the spatially resolving detector D with aspace-to-frequency mapping which enables Ĩ (ω) to be inferred.An alternative to the spectrometer is measurement of the field autocorrelation, defined as ∞B AC1 (τ)= E (t )E ∗ (t − τ) dt . (2.21)−∞This can be obtained using a balanced interferometer as shown in Fig. 2.2. The signal obtained onthe photodetector is ∞−∞|E (t )+E (t − τ)| 2 dt (2.22)25