Methods of ECDL Frequency Stabilization*

Resonant Faraday rotation on alkali metals can be used to monitor the polarization of dense spin-polarized helium-3 targets. If spin exchange optical pumping is used to polarize the helium-3 nuclei, sparse amounts of Rb and K will be present in the target. Tuning external-cavity diode lasers (ECDL) to the D2 transitions of Rb or K maximizes the rotation of linearly polarized light due to the Faraday effect; which allowed measurement of small magnetic fields produced by the spin-polarized nuclei. To accomplish this goal, the laser frequencies must remain stable over long periods of time due to various environmental changes. We performed diagnostic interferometry to determine the rate of frequency drift and to locate the D2 transition frequencies via custom methods of automation on data acquisition and laser parameter control. Then, using the doppler free absorption spectrum of Rb or K as a feedback mechanism, a lock-in technique was used to generate an error signal and a PID feedback system allowed us to minimize the frequency drift of our ECDL to provide sufficient laser frequency stability for the Faraday rotation experiment.