Stabilization of a lead-salt tunable diode laser : measuring and controlling chaotic frequency emission

Detailed observations of the emission from a lead-salt tunable diode laser (TDL) reveal that the observed instrumental linewidth is actually a temporal average of many narrow (less than 0.5 megahertz) emission "modes." Though the instantaneous laser output width is less than 0.5 MHz, the work-ing width of the laser is limited to several MHz by frequency jitter. Since the jitter frequency is a dynamic system variable which occurs within a bounded range but varies in what appears to be a random fashion, chaotic behavior is suggested. With this in mind we designed a procedure to monitor and measure these frequency fluctuations on our own lead-salt TDL system. We then analyzed the data and discovered that the fluctuations are indeed in-dicative of a chaotic process. Utilizing a variation of the chaos control tech-nique of occasional proportional feedback (OFF) developed in 1991 by E. R. Hunt, we constructed an electronic OFF controller. With this controller we succeeded in decreasing the frequency variations by at least a factor of twenty over the same laser emission without the controller, and in some experi-ments, by a factor greater than 70.