Ultra-short pulse reconstruction software in high power laser system

Abstract

An ultra-short pulse reconstruction software, validated through a set of experimental measurements on the front-end of the Vulcan laser at the RAL Central Laser Facility is presented. The measurements were acquired in Target Area Petawatt of the Vulcan laser, both using a conventional autocorrelation technique and the GRENOUILLE technique in order to compare the results. The FWHM of the laser pulses considered came out to be comparable for the two techniques. In this experimental campaign for the first time a technique different from the autocorrelation one has been used for a PW class laser as Vulcan.

Introduction

The characterization of the ultra-short high power laser pulses has always been a challenge [1].

It requires using the same pulse to measure itself, because it does not exist a shorter event (do not exist an event shorter) to compare to. Moreover, there are many other difficulties to take into account, concerning the pulse itself (no-linear effects, distortion, etc.) and the complexity of the set-up required to perform the measurements. One of the most promising techniques is GRENOUILLE [2], whose set-up is much simpler than popular techniques like FROG [3], SPIDER [4] and others. Moreover, GRENOUILLE is considerably more sensitive, extremely easy to set-up and to align. GRENOUILLE is based on a previous technique, FROG, with respect to which the experimental configuration provides simple changes: the simplification of the splitting and the recombination of the pulse to be analyzed, obtained by replacing the beam-splitter, the delay line and the optical recombination of the two copies of the initial impulse with a Fresnel bi-prism; the measurement, obtained by replacing the thin crystal for the generation of the laser second harmonic (SHG) and the related spectrometer with a thick crystal. This choice also offers the advantage of having a second harmonic signal more intense because it depends on the thickness of the crystal frequency doubler.

The laser beam, impinging into the Fresnel bi-prism, is split and recombined at the same time to a certain angle θ relative to the other copy pulse. Thus exploiting the transverse spatial extension of the crystal, one gets a time delay between the two pulses at different points in the crystal. The thick crystal acts also as a spectrometer, as due to the small bandwidth of the phase-matching it generates a radiation whose wavelength varies with the angle of incidence along the vertical axis. Then a cylindrical lens makes an image of the crystal on the horizontal axis (time) and focuses the signal in the vertical axis (spectral). The output image is acquired by a CCD camera. It provides a trace that yields the pulse intensity FWHM and phase, a spectrogram, that involves temporal and frequency resolution simultaneously. In the paper we present the development of the analysis software for the data acquired by a GRENOUILLE set-up. This innovative laser-pulse diagnostics is based on the acquisition of an experimental image, which is cleaned up (subtraction of the background) and calibrated before being used in the program, so it can be compared to the calculated analytic image. Starting from an arbitrary pulse, we create a simulated image, immediately after compared to the experimental one. By using a suitable algorithm that changes at each step the arbitrary pulse, we minimize the distance between the two images, finally obtaining the laser pulse shape of the investigated laser pulse. The software has been tested on experimental images acquired in the laser Vulcan front-end (low-power) at the Central Laser Facility (RAL). Afterward the same measurements have been performed in the Target Area Petawatt (TAP), with the laser Vulcan at full power.