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

Abstract

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.