Innovative single-shot 2D pulse front tilt diagnostic

The generation of ultrashort pulses implies a large bandwidth, increasing complexity of the beam propagation, and diagnostics to fully characterize the final pulse in the spatial-temporal domain. The angular dispersion (AD) of the beam is one of the spatial-temporal couplings (STC) playing a key role in a wide range of scientific areas. The presence of this chromatic aberration will distort the laser pulse, causing pulse front tilt. In turn, when the beam is focused, the actual pulse length in the focal spot is different from the pulse length in the near field, where usually is measured.


The presence of PFT in the near field has been proved to have an effect on the electron acceleration experiments, so its control and measurement are important for the development of compact laser-driven plasma accelerators and, more in general, to guarantee the best performances of high power laser facilities. On Chirped pulse amplification laser system, slightly misalignment of the compressor and/or the stretcher could generate angular dispersion.


Different diagnostics have been proposed to characterize these aberrations. Unfortunately, these diagnostics adopt complex setup and/or complex retrieval routines, sometimes leading to ambiguities in the STCs detection or just providing qualitative measurements.


The simple innovative single-shot method, presented by M Galimberti, F Bisesto and M Galletti, addresses most of the main issues of the existing STC diagnostics: a simple retrieval routine able to retrieve different STCs without ambiguities, simple experimental setup presenting simple alignment procedures and it is not a self-referenced method so it is not required a good quality beam in the spatial-temporal domain. The related research results are published on High Power Laser Science and Engineering, Vol.9, Issue 2 (M. Galimberti, F. G. Bisesto, M. Galletti. Innovative single-shot 2D pulse front tilt diagnostic[J]. High Power Laser Science and Engineering, 2021, 9(2): 02000e16).


The basic idea of the proposed instrument is to use a diffraction element to disperse the spectral components angularly. If a transmission grating is used as a diffraction element in the horizontal direction, by focusing the diffraction order and analyzing the diffracted spot, the spectral components will be diffracted at different horizontal positions while the vertical position will encode information about the angular dispersion. By using a 2D diffraction element and analyzing the diffraction orders in both directions, the angular dispersion in both directions can be retrieved.


Among all, the main advantage of this new diagnostics is the simplicity of the setup and the capability to show straight from the acquired image the presence of pulse front tilt prior to the analysis in both directions.


The proposed single-shot diagnostic, requiring a minimal number of optics and a compact layout, could be used as a real-time diagnostic both on a high repetition rate tens of Hz laser system and on a low repetition rate, high power large size laser system. Moreover, the ability to work also with stretched pulses will allow monitoring the AD across the entire amplification chain. The performances of the instrument were tested numerically and experimentally proving to be robust and accurate over a wide range of parameters.


On this work Dr Ian Musgrave, head of the Vulcan Laser at the Central Laser Facility in UK, said: "With the increasing interest in laser driven particle acceleration there is ever more focus being placed on the characteristics of the laser pulse and its impact on the generated particle beam. It has been shown that pulse-front tilt has an impact on electron beams and therefore a simple device capable of measuring the pulse front-tilt is of great benefit to the community. The added advantage of the device being presented in this report is the single-shot nature of the device therefore broadening its potential application."


On the left, the experimental setup of the instrument used to characterize the angular dispersion after a glass-block stretcher in the VOPPEL front end at the Vulcan Laser Facility, CLF, STFC, UK. On the right the simulated performance of the instrument for different amplitude of angular dispersion