Vertical cavity surface emitting lasers for miniature atomic clocks

Start date 1 October 2017
Duration 3.5 years
Funding body EPSRC

Miniature coherent population trapping (CPT) based clocks use the following fundamental physics package components:

-a single mode laser diode (typically a VCSEL)
-beam conditioning optics such as an ND filter and a quarter-wave plate
-a cell containing a vapour of alkali atoms (generally atoms that have a three-state lambda energy structure such as caesium or rubidium)
-a photodetector.

The laser diode operates at the D1 resonance and is modulated at half of the hyperfine ground states separation frequency (4.6 GHz for caesium, 3.4 GHz for rubidium), such that a superposition of the two ground states being resonant to a third state is achieved, enabling coherent population trapping of the atoms in the third state and causing optical transparency, i.e. the atoms no longer absorb the light.

As the modulation is swept through this feature, a spike in the optical transmission through the atoms is detected by the photodetector positioned at the other end of the cell, providing a means to identify the precise resonant frequency of the atoms and thereby serving as a stable frequency discriminant.

The semiconductor laser properties affect the performance of the clock and the project will investigate these relationships in detail.