Recently, much effort has been devoted to reducing the linewidth of lasers used in spectroscopy. For example, a He-Ne laser spectrometer with a linewidth of only 7 Hz has been reported. A specially constructed continuous wave dye laser has been stabilized to 1 kHz. With the use of a feedback loop and a Fabry-Perot reference cavity to improve short term stability, the linewidth of a commercially available continuous wave argon ion laser was reduced from 20 MHz to 10 kHz.
M. J. Lavan, "Simple Technique for Precise Changes in Laser Beam Frequency," Appl. Spectrosc. 32, 400-401 (1978)