Ultrashort pulse characterization

Much of my research has concerned the characterization of ultrashort pulses of light. I pioneered multiple spectral shearing interferometry, performed the first spatio-temporal reconstructions of few-cycle long wavelength pulses, and have been part of a range of other developments.

Introduction to ultrashort pulse characterization

The phase of electromagnetic radiation defines both its direction and temporal structure, and is therefore essential information in characterizing ultrashort pulses. As a general rule, electronic detectors for infrared and shorter wavelengths are not sensitive to the phase of incident radiation - only to the energy.

Shearing interferometry

In general, interferometry provides the phase difference between two fields. When the two fields are duplicates of one another but with one being shifted along a certain dimension such as space, time or frequency, the phase difference is just the approximation to the derivative as seen in elementary calculus. In this context, the amount of shift is called the shear. Shearing in space is known as lateral shearing interferometry and is a common means of measuring optical wavefronts and testing optical surfaces. Shearing in frequency is the basis of the well-known SPIDER technique.

Multiple-spectral-shearing interferometry

If the unknown field has a region of zero intensity along the shearing axis which is larger than the shear, then shearing interferometry cannot determine the phase difference between the adjacent (nonzero) regions. Because the shear determines the resolution, it cannot in general be increased without a loss of fine details. This leads to a relative phase ambiguity in shearing interferometry, as illustrated in the following figure.

With Dr Tobias Witting and Professor Ian Walmsley, I developed the first algorithm for reconstructing the phase of a field from a set of interferometry measurements with different shears. We demonstrated the algorithm with data acquired experimentally using the SEA-SPIDER technique, and later with the SEA-CAR-SPIDER arrangement for acquiring the different shears simultaneously. We also used the technique to study a classical analogues of sub-Planck structure in a bichromatic double pulse.