Alessandro Pesatori Michele Norgia V. Calabrese Gianluca Galzerano Elio Bava Cesare Svelto Optical Frequency Standard by HI Doppler-Broadened Absorption and External-Cavity Laser Diode at 1.541 muhboxm. 1708-1712 2008 57 IEEE Trans. Instrum. Meas. 8 https://doi.org/10.1109/TIM.2008.925716 db/journals/tim/tim57.html#PesatoriNCGBS08