An absolute frequency reference at 192.6 THz (1556 nm) based on a two-photon absorption line of rubidium at 778 nm for WDM communication systems

A 192.6-THz (1556 nm) distributed-feedback (DFB) laser is frequency-locked on a two-photon transition of rubidium at 385.3 THz (778 nm) using second-harmonic (SH) generation. With 43 mW at 1556 nm, we obtain a SH power of 15 μW using a KNbO
₃
crystal placed in a ring cavity. Optical feedback from this cavity is used to reduce the DFB laser linewidth to the 10-kHz level and control its frequency. The SH signal is used to injection-lock a 778-nm Fabry-Perot laser in order to increase the interrogation power. With this scheme, we observe two-photon transitions in rubidium and lock the 1556-nm laser frequency.