Pulsed Laser Beam Quality Assessment via Phase Retrieval

Abstract: In the search for a better understanding of the fundamental nature of the atom, the development of femtosecond pulsed laser sources has opened up for many applications, in particular in time-resolved physics. For many of those, like the generation of attosecond pulses through high-order harmonic generation, high intensity is needed and the quality of the beam is a critical factor. Most commonly, beam quality is quantified by the M^2-values, which relates the actual beam size to an ideal Gaussian beam. This procedure however is often very time consuming, the quality of the beam is only defined by one parameter and no information of the spatial phase or the focused intensity is obtained. This is what has prompted the development of a novel beam diagnostic tool, inspired by diffractive imaging. The diagnostic tool relies on the Fourier transform relation between the beam in two different planes. Through an iterative algorithm the spatial phase of the beam can be retrieved. With both the intensity profile and phase of the beam known, the beam profile through the focus as well as in the far-field, can be reconstructed. From which parameters like the M^2-value and the Strehl ratio, which relates the intensity to the theoretical best, can be derived. As the need for experimental set-ups with better resolution and with higher fidelity is evergrowing, we apply the developed tool to characterize and assess the attosecond pump-probe interferometric set-up that is being upgraded at the Atomic Physics Division at Lund University. The general set-up, together with improvements to its stability and flexibility are presented. Where the upgraded set-up shows promise for producing better resolved attosecond pump-probe experiments with a high fidelity. The beam diagnostic tool is very well-suited to evaluate the beam quality in different places in the set-up. Future prospects of improvements to the diagnostic tool, involves a full decomposition of the spatial phase into e.g. Zernike polynomials, allowing for a characterization of the aberrations present in the beam.