AN ABSOLUTE FREQUENCY REFERENCE AT 192.6 THZ (1556 NM) BASED ON A 2-PHOTON ABSORPTION-LINE OF RUBIDIUM AT 778 NM FOR WDM COMMUNICATION-SYSTEMS

A 192.6-THz (1556 nm) distributed-feedback (DFB) laser is frequency-lo cked on a two-photon transition of rubidium at 385.3 THz (778 nm) usin g second-harmonic (SH) generation, With 43 mW at 1556 nm, we obtain a SH power of 15 mu W using a KNbO3 crystal placed in a ring cavity. Opt ical feedback from this cavity is used to reduce the DFB laser linewid th to the 10-kHz level and control its frequency, The SH signal is use d to injection-lock a 778-nm Fabry-Perot laser in order to increase th e interrogation power, With this scheme, we observe two-photon transit ions in rubidium and lock the 1556-nm laser frequency.