Electromagnetically induced transparency in Raman gain for realizing a superluminal ring laser.

We describe an approach for realizing a superluminal ring laser using a single isotope of Rb vapor by producing electromagnetically induced transparency (EIT) in Raman gain. We show that by modifying the detuning and the intensity of the optical pump field used for generating the two-photon population inversion needed for generating Raman gain, it is possible to generate a dip in the center of the gain profile that can be tuned to produce a vanishingly small group index, as needed for making the Raman laser superluminal. We show that two such lasers, employing two different vapor cells, can be realized simultaneously, operating in counter-propagating directions in the same cavity, as needed for realizing a superluminal ring laser gyroscope. This technique, employing only one isotope, is much simpler than the earlier, alternative approach for realizing a superluminal Raman laser based on Raman gain and Raman dip in two isotopes [Zhou et. al, Opt. Express27, 29738 (2019)10.1364/OE.27.029738]. We present both an approximate theoretical model based on four levels as well as the results of a model that takes into account all relevant hyperfine states corresponding to the D1 and D2 transitions in 85Rb atom. We also present experimental results, in good agreement with the theoretical model, to validate the approach.