Spatio-spectral metrology at focus of ultrashort lasers: a phase-retrieval approach

Abstract

The complete characterization of an ultrashort laser beam ultimately requires the determination of its spatio-temporal electric field E(x, y, t), or its spatio-spectral counterpart E(x, y, ω). We describe a new measurement technique called INSIGHT, which determines E(x, y, ω), up to an unknown spatially-homogeneous spectral phase. Combining this information with a temporal measurement at a single point of the beam then enables the determination of the spatio-temporal field E(x, y, t). This technique is based on the combination of spatially-resolved Fourier-transform spectroscopy with an alternate-projection phase-retrieval algorithm. It can be applied to any reproducible laser source with a repetition rate higher than about 0.1 Hz, relies on a very simple device, does not require any reference beam, and circumvents the difficulty associated with the manipulation of large beam diameters by working in the vicinity of the beam focus. We demonstrate INSIGHT on a 100 TW-25 fs laser, and use the measurement results to introduce new representations for the analysis of spatio-temporal/spectral couplings of ultrashort lasers.

Multipass cells: 1D numerical model and investigation of spatio-spectral couplings at high nonlinearity

“…In order to further asses the spatio-spectral properties of the broadened laser pulses, we use the INSIGHT technique [13], which involves spatially-resolved Fourier-transform interferometry at three positions around the focus. Using a phase retrieval algorithm this allows for reconstruction of the spatio-spectral phase in addition to the spectral amplitude already afforded by the Fourier-transform spectroscopy.…”

“…In a second part, we experimentally study the spatiospectral profile at the output of MPCs as the peak power approaches the critical power. This is done using both an imaging spectrometer and a complete 3D characterization technique known as INSIGHT [13]. The results show that the output pulses are essentially free of spatio-temporal couplings even at peak power levels close to the critical power.…”

Spatio-temporal structure of a petawatt femtosecond laser beam

“…This is used to measure the spatially-resolved first order autocorrelation function of the beam at each point of the detector (standard Fourier-transform spectroscopy), from which one can deduce the frequency-resolved spatial intensity profile of the beam. As detailed in [19], any pointing fluctuations must be compensated for in order to calculate the correct spectrum at the positions around focus. This was implemented in these measurements where the fluctuations on the BELLA focus were approximately 16 and 24 μm RMS for the horizontal and vertical axis respectively.…”

Controlling the velocity of a femtosecond laser pulse using refractive lenses

“…The effect of the doublet was characterized with an INSIGHT device [23], which provides the complete spatio-spectral properties of the output laser beam in amplitude and phase. We used the ∼24 fs pulses delivered by the UHI100 laser source at CEA-Saclay, with a beam apertured to roughly 4 cm diameter in order to avoid clipping in the specific version of the INSIGHT device used.…”