Ultra-Low Phase Noise Atomic Clock using Coherent Population Trapping (CPT) in Rubidium

This thesis describes the design and implementation of a complete atomic clock system from first principles. The system is based on Coherent Population Trapping (CPT) in 85Rb and incorporates an ultra-low phase noise multi-element local oscillator, consisting of a 10MHz crystal oscillator and 1.5GHz Dielectric Resonator Oscillator (DRO) in a phase locked configuration. It is shown that the crystal oscillator can achieve excellent close to carrier phase noise performance of -122dBc/Hz at 1Hz offset and -147.7dBc/Hz at 10Hz offset, as well as Allan deviation of 3×10^(-13) at 100ms averaging time. The DRO exhibits excellent medium offset and far from carrier performance with a noise floor of about -180dBc/Hz. When phase locked to the crystal oscillator, the phase noise at 1Hz is shown to approach that of the crystal oscillator (scaled up to 1.5GHz).
A frequency synthesis chain, incorporating a Direct Digital Synthesizer (DDS), low noise digital divider, single sideband mixer and notch filter is used to upconvert the 1.5GHz local oscillator frequency to the ground state hyperfine splitting frequency of 85Rb (1.5178GHz). The DDS is controlled by a microcontroller and used to produce an offset frequency of 17.8MHz, which can be tuned, modulated or swept. The single sideband mixer is used to mix the 1.5GHz and 17.8MHz signals and suppress the lower sideband of the output. The notch filter is used to further suppress the LO feedthrough of the mixer by another >30dB.
A Rb CPT physics package incorporates a vertical cavity surface-emitting laser, which is modulated by the output of the synthesizer and used to interrogate the atomic resonance in a 85Rb vapour cell. The package was built with custom made optical mounts, low noise laser driving electronics and temperature control loops. The parameters affecting the shape, amplitude and stability of the atomic resonance were experimentally investigated and the construction of the physics package was adjusted for the optimal conditions.
A digital frequency locked loop is used to lock the local oscillator to the atomic resonance and the performance of the full system is tested.