Comparison of two methods for frequency stabilization of a laser to an optical transition

Abstract

The most common method applied to lock the frequency of a laser source to an optical transition is to use a lock-in amplifier. Using a lock-in amplifier has two distinct disadvantages: (1) A lock-in amplifier is rather expensive, and (2) tuning of a lock-in amplifier to frequency stabilize a laser to an optical transition is a delicate experimental procedure that can be rather time consuming and demanding ofthe experimenter.
At the University of San Carlos research is ongoing on the cooling and trapping of rubidium atoms in a magneto-optical trap. For the cooling and trapping of rubidium two optical transitions between different hyperfine levels of the ground- and second excited state have to be considered. One of the diode laser systems used in the cooling and trapping experiment if frequency stabilized to the optical transition using a lock-in amplifier. The diode laser used for the second optical transition is frequency stabilized using an alternative digital locking scheme which shows two very attractive characteristics: (1) It is much cheaper as compared to using a lock-in amplifier, and (2) it is extremely user-friendly.
In this paper first the technique of frequency stabilization using a lock-in amplifier is presented. Then the digital locking scheme is presented. Results of the digital locking scheme applied to our cooling and trapping experiment are briefly discussed as well.