Mid-IR narrow bandwidth tuneable laser source for the FAMU experiment

¹ Sezione INFN di Trieste, via A. Valerio 2, Trieste, Italy
² Dipartimento di Scienze Matematiche, Informatiche e Fisiche, Università di Udine, via delle Scienze 206, Udine, Italy
³ The Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, Trieste, Italy
⁴ Elettra-Sincrotrone Trieste, SS14, km 163.5, Basovizza, Italy
⁵ Consiglio Nazionale delle Ricerche, Istituto Nazionale di Ottica, CNR-INO, Via Madonna del Piano 10, Sesto Fiorentino, Italy
⁶ Sezione INFN di Napoli, Complesso Universitario di Monte S. Angelo ed. 6, via Cintia, Napoli, Italy
⁷ Dipartimento di Matematica e Fisica, Università della Campania “Luigi Vanvitelli”, Viale Lincoln 5, Caserta, Italy
⁸ Institute of Solid State Physics, Bulgarian Academy of Sciences, Tzarigradsko Chaussee Blvd. 72, 1784 Sofia, Bulgaria

^* Corresponding author: marco.baruzzo@ts.infn.it

Published online: 31 October 2024

Abstract

The FAMU (Fisica degli Atomi MUonici) collaboration aims to measure the proton Zemach radius through muonic hydrogen (μp) spectroscopy. The experimental setup relies on a custom-developed pulsed mid-IR laser source that can be tuned over a specific 6780-6790 nm wavelength range needed to ignite the hyperfine transition of the μp ground state 1S (also known as spin-flip transition). The excitation is observed as a distinctive muonic X-rays emission resulting from the oxygen impurity present in the hydrogen target. The mid-IR emission is produced by Difference Frequency Generation (DFG) in a non-linear crystal, pumped with a fixed wavelength 1064 nm Nd-YAG laser and a tuneable Cr:forsterite laser centred on 1262 nm. This setup produces more than 1.2 mJ at 6780 nm with a linewidth smaller than 30 pm. The experiment requires the laser to run continuously 24/7 in a restricted/radiation-controlled area and for this reason a specifically developed control software permits to remotely act on the laser. The characterization of a series of different non-linear crystals was performed during the development of the laser, resulting in the choice of BaGa₄Se₇.