Optical gains as small as 2 × 10-5 are detectable by placing the gain medium inside the cavity of a cw dye laser and operating the laser slightly above threshold to take advantage of moderate spatial hole burning. This technique was used to measure gain on the 40′ → 6″, 7″, and 8″ transitions in optically pumped Br2.
© 1983 Optical Society of America
Keith A. Truesdell and Richard A. Keller, "Intracavity gain detection: example I2, Br2," Appl. Opt. 22, 339-346 (1983)