TY - JOUR AU - L.-S. Ma AU - John Hall BT - IEEE Journal of Quantum Electronics DA - Dec-31-1969 DO - 10.1109/3.62120 PY - 1990 SP - 2006 EP - 2012 T2 - IEEE Journal of Quantum Electronics TI - Optical heterodyne spectroscopy enhanced by an external optical cavity: toward improved working standards VL - 26 SN - 00189197 ER -