An absolute frequency reference at 1556 nm for WDM communication systems

Standardization of channel frequency allocation and calibration of the frequency scale at 1.55 /spl mu/m will require some frequency standards in this range. A promising avenue consists of making use of the two-photon transitions of rubidium at 778 nm to frequency-lock a 1556 nm laser. These lines have been recently measured by direct comparison to the Cs frequency standard with an uncertainty of 2 kHz. The method which we develop consists in using the second harmonic of the 1556 nm laser to injection-lock a low-cost laser diode operating at 778 nm which is then used to observe the transitions in rubidium.