15 August 2019


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

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.

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