An adaptive intensity stabilizer for optical lattices

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2014
Thesis identifier
  • T13874
Qualification Level
Qualification Name
Department, School or Faculty
  • The depth of a dipole trap for ultracold atoms is directly related to the intensity of the corresponding trapping laser beam. Therefore, accurate preparation and control of a trapped atomic cloud requires fine control over the intensity of the involved laser beams. This can be achieved with a controller that monitors the intensity on a photodiode and regulates it via an acousto-optical modulator, stabilizing it to a given value via a feedback loop. Here, two important benchmarks are the regulation bandwidth and the accuracy of the controller. Our experimental context requires fast and accurate intensity control of laser beams that form an optical lattice. An experimental sequence requires both intensities on a mW and on a W scale, where the controller performance at low intensities is particularly important as the physical systems of interest are studied in relatively shallow optical lattices. This thesis presents an intensity controller that satisfies these requirements. To ensure good resolution in the low intensity regime, the controller employs a logarithmically amplified photodiode that is very sensitive in this regime. Since the gain of this photodiode varies drastically across different intensity regimes, it is impossible to optimize the gain of a static feedback loop for all intensities. Here, our solution is to dynamically adjust the gain of the controller using an Arduino microcontroller. We implement a gain schedule with five different gain regimes that optimizes the controller bandwidth across the entire intensity spectrum. For intensities below 10% of the total power we observe a substantial improvement, with a feedback bandwidth up to five times higher than in the case of a static PI-controller. The control accuracy at low intensities, i.e. how accurate a setpoint level can be specified, is at least 0.01%. The noise of the stabilization and thus the precision is better than 0.1%. The implementation of the Arduino allows for further controller configurations, enabling a wide range of applications such as completely digital PID control, storage of output values and prewarming of the AOM.
Resource Type
Date Created
  • 2014
Former identifier
  • 1041612