Demonstration of a Cold Start Procedure for a Laser Source Frequency-Locked to Molecular Absorption Lines

We have recently proposed a referencing scale for multifrequency (WDM) optical communication systems with exact multiples of 100 GHz1. In such a system, the absolute frequency control of the sources is imperative since semiconductor lasers possess a nominal operating frequency which is difficult to control precisely upon fabrication and which fluctuates with injection current, junction temperature and aging. Locking the frequency of a laser source to an absorption line of an atomic or molecular gas improves the frequency stability by several orders of magnitude and sets its value precisely. Acetylene (C2H2), for example, provides lines whose absolute frequencies can be known to better than 30 MHz2, with drifts of less than 100 kHz/°C and 200 kHz/Torr3. Typical full width at half maximum (FWHM) of the lines are around 600 MHz at ambient temperatures. Once the laser is stabilized, a long term frequency stability of better than 20 kHz can be obtained. These figures meet the dense optical frequency multiplexing requirements.