Quantum mode-locked Faraday laser

We report an external-cavity mode-locked semiconductor laser that uses a Faraday atomic filter as a saturable absorber (SA), termed as the quantum mode-locked Faraday laser. The unique SA exhibits nonlinear transmission characteristics exclusively in the vicinity of the atomic quantum transition frequency, which narrows down the spectral bandwidth of the mode-locked pulses to the gigahertz level and results in a central wavelength of the mode-locked pulses corresponding to the 87Rb (F=2) component of the D2 quantum transition line. Simultaneously, influenced by the slow-light effect of rubidium-dispersive vapor in the SA configuration, the fundamental repetition rate of the mode locking can vary between 261 and 228 MHz. Pulse delay tests conducted outside the resonator provide conclusive evidence of a gigahertz-bandwidth slow light within the Faraday laser. The mode-locking technique presented here can be applied to pulsed light sources of other quantum transition lines by setting appropriate atomic filter parameters. In addition, this narrow-spectrum mode-locked laser, with a tunable repetition rate and a central wavelength corresponding to a quantum transition line, has potential applications in the fields of atomic precision spectroscopy and quantum precision metrology.