A mid-IR laser source for muonic hydrogen spectroscopy: The FAMU laser system

A pulsed, tunable, narrow linewidth mid-infrared (mid-IR) laser radiation source was developed to meet the needs of the FAMU project (Fisica Atomi MUonici). The main goal of this experiment is to measure the hyperfine splitting of the ground state of muonic hydrogen using pulsed laser spectroscopy. The experiment requires a high energy, mid-IR source around the resonance energy ΔE1Shfs∼ 0.1828 eV (6.79μm) of the muonic hydrogen atom to excite it from the para (F = 0) to the ortho (F = 1) spin state. The laser system designed to fulfill these requirements is based on Difference Frequency Generation (DFG) using a non-linear crystal pumped by two laser beams at 1064 nm and 1262 nm. For the first time, the details of the entire laser system are described in this paper. The laser light at 6.79μm, generated with a BaGa4Se7 non-linear crystal, is being directed into the FAMU target multi-pass optical cavity. To date, an energy above 1 mJ, a linewidth below 30 pm, and a tunability step of 9 pm have all been attained. Here, we present the unique features of the laser system and the characterization results.