Throughout the development of ultrafast optics, the ability to accurately measure the temporal properties of ultrashort light pulses has been crucial. Several well-established techniques are now available to measure their electric field E(t) from the femtosecond down to the attosecond range. However, within any ultrashort beam, the temporal properties of the light pulse can vary spatially and vice versa.
Such correlations between the spatial and temporal properties are called spatiotemporal couplings (STCs) and prevent the decomposition of the laser field E(t,r) as E(t,r) = f(t)g(r). A simple Fourier transform with respect to time shows that STCs are the temporal counterpart of conventional chromatism, that is, the frequency dependence of the spatial properties of a light beam.
STCs are ubiquitous in ultrafast optics. In the femtosecond range, chirped-pulse amplification (CPA), the key technology of amplified ultrashort pulses, relies on the use of massive STCs induced at different locations in laser systems (for instance by gratings or prisms), which should all eventually perfectly cancel out at the laser output. Residual STCs, for example resulting from imperfect compensation, decrease the peak intensity at focus by increasing both the focal spot size and the pulse duration. This is particularly detrimental for UHI lasers, which aim for the highest possible peak intensities.
Accurately measuring STCs is thus essential in ultrafast optics. In the past 6 years, we have developed two different and complementary techniques to fully characterize the spatio-temporal (or spatio-spectral) structure of femtosecond lasers, which are particularly suited to high-power systems. These are the TERMITES and INSIGHT techniques. We have now applied these techniques to several high-power femtosecond lasers, such as UHI100@CEA Saclay, Salle Jaune @ LOA, and the PW BELLA laser @LBLN.
Movie of the E-field of the BELLA beam in (x,y,t) as the beam travels across the best focus.
3D view of the E-field of the UHI100 laser beam in a given propagation plane (collimated beam)
“Space-time characterization of ultra-intense femtosecond laser beams”, G. Pariente, V. Gallet, A.Borot, O. Gobert and F. Quéré, Nat. Photonics (2016) 10 p547
“Spatio-spectral metrology at focus of ultrashort lasers: a phase retrieval approach”, A. Borot and F. Quéré, Optics Express (2018) 26 p.26444