  The FAMU (Fisica degli Atomi Muonici) experiment at the RIKEN RAL pulsed muon
facility will measure the proton Zemach radius with high precision, thus
contributing to the solution of the so-called proton radius puzzle. The
situation is now confused and new measurements will help reduce the discrepancy
as measured with electrons or muons. To this aim, FAMU will make use of a
high-intensity pulsed muon beam at RIKEN-RAL impinging on a cryogenic hydrogen
target and a tunable Mid-infrared (MIR) laser emitting at around 6.78 um, to
measure the hyperfine (HFS) splitting of the 1S state of muonic hydrogen. The
injection of light into the cryogenic target and its monitoring via dedicated
MIR sensors is an asset for the experiment. A possible solution is via
specialty MIR fibres based on hollow core waveguides or polycrystalline fibres.
Two dedicated setups based on pulsed QCL lasers, from Alpes Laser, emitting
around 6.78 um have been used to study the fibre attenuation while the high
energy laser developed at Elettra Sincrotrone in Trieste has been used to study
the damage threshold at high energies. The alignment of MIR laser radiation
into an optical cavity presents criticalities due to geometrical constraints.
Its control requires the use of small and sensitive detectors. The results
obtained with a quantum IR sensor with an integrated circuit for signal
processing are reported.
