Powerful fast triggerable 6 μm laser for the muonic hydrogen 2S-Lamb shift experiment

Laser spectroscopy of the 2S-Lamb shift in muonic hydrogen (μ−p) is being performed at the Paul Scherrer Institute, Switzerland, to determine the root-mean-square (rms) proton charge radius with 10−3 precision. A multistage laser system has been developed which provides 0.2 mJ pulse energy tunable at 6 μm wavelength. An excimer pumped dye laser is used to drive a titanium sapphire (Ti:Sa) system whose wavelength is then shifted to 6 μm using a multipass Raman cell filled with hydrogen. The short cavity length of the Ti:Sa oscillator (7 cm) guarantees a pulse width of 7 ns and a pulse energy of 1.2 mJ at 708 nm, a wavelength controlled by a mono-mode cw-Ti:Sa laser. The laser is triggered at a maximum 60 s−1 repetition rate by muons entering the apparatus at random times. A new type of multipass cavity has been developed to provide a homogeneously illuminated volume (25 × 7 × 170 mm³).