Low phase noise diode laser oscillator for 1S–2S spectroscopy in atomic hydrogen

We report on a low-noise diode laser oscillator at $972\mathrm{nm}$ actively stabilized to an ultrastable vibrationally and thermally compensated reference cavity. To increase the fraction of laser power in the carrier we designed a $20\mathrm{cm}$ long external cavity diode laser with an intracavity electro-optical modulator. The fractional power in the carrier reaches 99.9%, which corresponds to an rms phase noise of ${\phi }_{\mathrm{rms}}^2=1{\mathrm{mrad}}^2$ in $10\mathrm{MHz}$ 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 two-photon processes.