Bachelor defense by Anne J. Mortensen

Intensity Stabilisation of a 309 nm Laser

Laser spectroscopy can be used to examine transitions between hyperfine levels of aluminium. To get precise measurements, it is important that the laser intensity is constant. This project aims to construct a closed-loop control system, which can be used to stabilise the intensity of a 309 nm laser. Theoretical concepts relating to acousto-optic modulation, control systems and PID controllers, and conversion between photodetector voltage and laser beam intensity, are explained. An experimental setup including an acousto-optic modulator, a PID controller, and a voltage-variable attenuator is presented. Tests of the individual feedback loop components, as well as of the total system, are performed. It is concluded that the system bandwidth is limited by the bandwidth of the photodetectors used, which is found to be 40 kHz. Furthermore, the tuned feedback loop is observed to suppress noise in the range 10-50 kHz by up to 5 dB, increasing the stability of the laser intensity. The feedback loop may be more effective if used on a laser with slower intensity variation, or if electronics with a higher bandwidth are used. This would improve the signal-to-noise ratio in the spectroscopy experiments.