A microcontroller based self-locking laser system

We present a self-locking laser system that does not require operator interventions. The system automatically finds a desired atomic transition and subsequently locks to it. Moreover, it has the ability to automatically detect if the laser is out of lock and activate the re-locking process. The design was implemented on two different diode lasers, a distributed Bragg reflector (DBR) diode laser and a Fabry Perot (FP) diode laser, used as a repump laser for a magneto-optical trap in a laser cooling experiment and a Raman laser for a four-level Raman transition experiment, respectively. The design relies on frequency modulation transfer spectroscopy to obtain a sub-Doppler atomic spectrum of rubidium-85. This spectrum is then demodulated to obtain zero-crossing linear slopes at the exact points of each atomic and crossover transition. The frequency modulation, the signal analysis, and the automatic locking and re-locking of the lasers are all implemented using an Arduino Due microcontroller. The lock loop has a bandwidth of 7 kHz. The lasers used for the design are characterized, and the robustness of the lock is analyzed. The achieved linewidths of DBR and FP lasers are 1.4 and 5.5 MHz, respectively. The frequency drifts of both lasers are a few 100 kHz over a course of days. The capture range of the locking system is up to 4.9 GHz for the DBR laser and 725 MHz for the FP laser. Both lasers performed well under actual experimental conditions.