Research

Master of Engineering:

Saturated absorption spectroscopy of rubidium and feedback control of LASER frequency for Doppler cooling

This research investigates the absorption spectra of rubidium and the feedback control of an external cavity diode laser. This research is a necessary prerequisite for laser (Doppler) cooling and trapping of rubidium atoms.

Cooling rubidium atoms down to such low temperatures can be achieved using the Doppler cooling technique. Here a laser is tuned to remain resonant with a specific atomic transition. To do this, the absorption spectra of rubidium must therefore be observed. All of the above require a reasonable knowledge about topics such as atomic physics, laser cooling and trapping, feedback control systems, and absorption spectroscopy. A discussion of these topics is provided.

We have utilised an experimental setup which allowed for measurements of the Doppler broadened and Doppler free absorption spectra of rubidium, as well the analysis of the Zeeman effect on the Doppler free spectra. The setup consisted of a saturated absorption spectrometer for high resolution spectroscopy and a Michelson interferometer for calibrating our measurements. In analysing the Zeeman effect we added a set of Helmholtz coils to the saturated absorption spectroscopy arrangement to measure the splitting of the hyperfine energy levels. We also performed a theoretical analysis of the magnetic energy level splitting of rubidium.

Our estimations of the separations and lifetimes of the 5S_1/2 and 5P_3/2 hyperfine energy levels of rubidium 85 and 87 showed agreement with current literature. The analysis of the Zeeman splitting at the 5S_1/2 and 5P_3/2 hyperfine energy levels showed large variations between theory and experiment. However, this was improved by comparing measurements with theoretical predictions of transitions that were different to our initial assumptions.

In addition to observing the absorption spectra we used a pre-built laser frequency feedback control system to maintain the laser frequency at a specific hyperfine absorption line. While the laser was locked to this transition we subjected the system to various disturbances; the laser remained locked. We managed to lock the laser to the same transition for roughly an hour.

The results of our absorption spectroscopy experiment as well as the laser locking experiment show that a good foundation has been laid for future work involving the study of cold atoms.

Publications:

Wyngaard, A., Govender, K., and Steenkamp, C. (2016), “Measurements of
the Hyperfine and Weak-Field Zeeman Spectra of Rb 85 and Rb 87″, Poster
presentation at the 61st Annual Conference of the South African Institute of
Physics (SAIP 2016).

Patel, M., De Jager, G., Nkosi, Z., Wyngaard, A., and Govender, K. (2017),
“On the non-linear spectroscopy including saturated absorption and four-wave
mixing in two and multi-level atoms: a computational study”, IOP
Conf. Series: Journal of Physics: Conf. Series 905.

Wyngaard, A., De Jager, G., Steenkamp, C., and Govender, K. (2017), “Experimental
study of the weak field Zeeman spectra of 85Rb and 87Rb”, The
Proceedings of the 62nd Annual Conference of the South African Institute of
Physics (SAIP 2017).

Opeolu, V., Govender, K., Wyngaard, A., Ouassini, N., De Jager, G., and
Scarrot, J. (2017), “Analysis and Performance of a closed loop external cavity
diode laser control system”, The Proceedings of the 62nd Annual Conference
of the South African Institute of Physics (SAIP 2017).

Opeolu, V., Govender, K., Wyngaard, A., Ouassini, N., De Jager, G., and
Scarrot, J. (2017), “Analysis and Performance of a closed loop external cavity
diode laser control system”, Poster presentation at the 62nd Annual Confer-
ence of the South African Institute of Physics (SAIP 2017).