Martin Romme Henriksen

A thesis for the degree of Doctor of Philosophy defended October 2019.

The PhD School of Science, Faculty of Science, Quantum Optics, Niels Bohr Institute, University of Copenhagen

Supervisor:
Prof. Jan W. Thomsen

Optical Frequency References

Ultra-stable and accurate frequency references have a large number of applications within the fields of metrology, communication, and spectroscopy. This work presents three projects with focus on compactness and cost: An acetylene frequency reference, micro-resonator Kerr frequency combs, and a strontium atomic clock. The acetylene frequency reference is a compact, frequency stabilized laser system with a frequency noise at the Hz-level. Here, a fiber laser is stabilized to the P(16)1 + 3 ro-vibrational line in carbon-13 ethyne (acetylene) at 1542 nm. The setup is based on the Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) technique. This technique generates a spectroscopy signal of the acetylene line with a signal-to-noise ratio of 104 and a signal bandwidth of 2 MHz, allowing for stabilization using the molecular line alone. A frequency stability of 25 Hz at 0.2 s is achieved.

The combination of the acetylene frequency reference with a compact frequency comb will provide not only a broad bandwidth reference at optical frequencies but also at microwave frequencies. In this work chip-based micro-resonator Kerr frequency combs are investigated. A waveguide resonator design in aluminium gallium arsenide (AlGaAs) with tapered regions is presented. This design shows high flexibility of dispersion engineering while maintaining single-mode operation. The fabricated micro-resonators’ dispersion profiles are measured on a system using a low-FSR Fabry-Pérot reference cavity. With this simple and lowcost system, low-noise measurements of the micro-resonators’ dispersion are obtained. Alternative material platforms are also discussed as well as different stabilization techniques suitable for optical Kerr frequency combs.

The NICE-OHMS technique is also applied to a cold ensemble of strontium atoms. In this proof-of-principle experiment the 1S0 $ 3P1 transition in 88Sr is used as a reference. Interrogation of a cold ensemble of strontium atoms, with a cycle time as low as 10 ms, is achieved producing a spectroscopic signal with a signal-to-noise ratio of 115.