We show that the problem of a \ensuremath{\Lambda}-type (and also a V-type) three-level single-mode laser (maser) is isomorphic to that of a two-level single-mode laser (maser) under the conditions of energy-level degeneracy and equal decay rates. This correspondence reveals that lasing without population inversion can occur in the \ensuremath{\Lambda}-type system with or without an initial atomic coherence, and also gives immediately the master equation of the laser (maser) field. We analyze in detail the operational and noise properties of a \ensuremath{\Lambda}-type laser using a Fokker-Planck approach. We also present a vector model to visualize the population trapping and the absorption and emission processes in such a laser (maser) system.
The well-known sub-Doppler polarization gradient cooling in a type-I transition (
Fe=Fg+1) is caused by red-detuned lasers. On the other hand, in a type-II transition (${F}_{\mathrm{e}}\ensuremath{\le}{F}_{\mathrm{g}}$), sub-Doppler cooling takes place through blue-detuned lasers. This opposite behavior for the two types of transitions is due to spontaneously transferred coherence (STC). In the absence of STC, both types of transitions show blue-detuned cooling. In this paper, we experimentally and theoretically demonstrate blue-detuned cooling for both types of transitions in
87Rb. For completeness, we compare the temperatures in various configurations.