Low phase noise diode laser oscillator for 1S-2S spectroscopy in atomic   hydrogen
  We report on a low-noise diode laser oscillator at 972 nm actively stabilized to an ultra-stable vibrationally- and thermally compensated reference cavity. To increase the fraction of laser power in the carrier we designed a 20 cm long external cavity diode laser with an intra-cavity electro-optical modulator. The fractional power in the carrier reaches 99.9% which corresponds to a rms phase noise of $\phi^2_\textrm{rms}=1\,\textrm{mrad}^2$ in 10\,MHz bandwidth. Using this oscillator we recorded 1S-2S spectra in atomic hydrogen and have not observed any significant loss of the excitation efficiency due to phase noise multiplication in the three consecutive 2-photon processes. 
N.Kolachevsky, J.Alnis, C.G.Parthey, A.Matveev, R.Landig, T.W.Haensch
quant-ph physics.ins-det physics.optics