Charge Radius Experiment with Muonic Atoms
Lamb shift measurement in muonic hydrogen : experimental set-upThe experimental set-up is made of two parts : the production of muonic hydrogen and the 6 µm laser source. Our group is involved in the laser development part : it is in charge of the seeded oscillator-amplifier titanium-sapphire laser system and of the frequency measurement of the full laser system.
Production of muonic hydrogen
The laser source
The 6 µm radiation needed to excite the 2S-2P transition is obtained from the conversion of the 708 nm pulse by a third-Stockes Raman shifter operated with 14 bar H gas. The 6 µm pulses must be triggered in less than 1 µs, since the muons lifetime is about 2 µs. Since 2007, we use a frequency-doubled disk laser which pumps an injection-seeded oscillator-amplifier Ti:Sa laser system operating at 708 nm. Our group has developed the oscillator-amplifier system, the oscillator being seeded by a tunable cw-Ti:Sa laser.
708 nm-Ti:Sa oscillator
This set-up has been developed in Paris and then brought to PSI. Using a very short oscillator cavity (the cavity is a few cm long with no Pockels cell) associated with an 8 pass-amplifier, we obtain pulses of 12 mJ, with a length shorter than 7 ns for a pump energy between 90 and 100 mJ.
The frequency of the 6 µm radiation is given by the cw Ti:Sa laser. This one is frequency locked on a stable Fabry Perot cavity whose length is fixed. A lambdameter and an iodine cell are also used to measure the laser frequency. The mesurement of the frequency difference between the cw laser and the oscillator is in very good agreement with the simulations. The calibration of the absolute frequency of the 6 µm source is performed by measuring a water absorption line. The Raman shift due to the dihydrogen cell and deduced from the 708 nm and 6 µm calibrations is in good agreement with the previous measurements, demonstrating the consistency of the frequency measurements in the whole system.