Slow Light Cavities for Thermal Noise Mitigation in Laser Frequency Stabilization
Author
Summary, in English
Precision laser frequency stabilization is an important part of modern optical metrology, with applications ranging from atomic clocks to fundamental tests of physics. However, the frequency stability of the ultrastable cavities used in these experiments is fundamentally limited by thermal noise in the mirrors of the cavity. The thermal noise causes the atoms that form the mirrors to move randomly, which results in small variations in the length of the cavity used as a reference.
This thesis explores a design based on slow-light cavities, which are optical resonators characterized by strong dispersion. The dispersion effectively increases the optical path length of the cavity, resulting in reduced sensitivity to cavity length changes due to thermal fluctuations. The cavity was constructed from a yttrium orthosilicate crystal doped with the rare-earth element europium, coated with plane-parallel mirrors. These ions have narrow optical transitions and long hyperfine lifetimes, so that highly dispersive transmission windows can be created in the inhomogeneous profile via optical pumping. The frequency stability was assessed in the presence of such strong dispersion where the group velocity of light is drastically reduced. A dual beam interrogation scheme is then introduced to measure the differential short-term frequency stability of the locked laser system. The results show that careful alignment of the cavity modes within the transmission window can significantly mitigate drift, which is important to improve frequency stability by enabling longer averaging times.
A method to reset the population in longlived hyperfine states of rare-earth ions to thermal equilibrium is also demonstrated. This method was used to restore the locking conditions of the transmission window after it had been degraded by the locking beam, enabling subsequent locking at the same frequency.
This thesis explores a design based on slow-light cavities, which are optical resonators characterized by strong dispersion. The dispersion effectively increases the optical path length of the cavity, resulting in reduced sensitivity to cavity length changes due to thermal fluctuations. The cavity was constructed from a yttrium orthosilicate crystal doped with the rare-earth element europium, coated with plane-parallel mirrors. These ions have narrow optical transitions and long hyperfine lifetimes, so that highly dispersive transmission windows can be created in the inhomogeneous profile via optical pumping. The frequency stability was assessed in the presence of such strong dispersion where the group velocity of light is drastically reduced. A dual beam interrogation scheme is then introduced to measure the differential short-term frequency stability of the locked laser system. The results show that careful alignment of the cavity modes within the transmission window can significantly mitigate drift, which is important to improve frequency stability by enabling longer averaging times.
A method to reset the population in longlived hyperfine states of rare-earth ions to thermal equilibrium is also demonstrated. This method was used to restore the locking conditions of the transmission window after it had been degraded by the locking beam, enabling subsequent locking at the same frequency.
Department/s
Publishing year
2025
Language
English
Full text
Document type
Dissertation
Publisher
Department of Physics, Lund University
Topic
- Physical Sciences
Keywords
- laser frequency stabilization
- slow light effect
- optical clocks
- rare earths
- RF erasure
- Fysicumarkivet A:2025:Lindén
Status
Published
Supervisor
- Lars Rippe
- Stefan Kröll
- Martin Zelan
ISBN/ISSN/Other
- ISSN: 0281-2762
- ISBN: 978-91-8104-533-8
- ISBN: 978-91-8104-534-5
Defence date
13 June 2025
Defence time
09:15
Defence place
Lecture Hall Rydbergsalen, Department of Physics, Professorsgatan 1, Faculty of Engineering LTH, Lund University, Lund.
Opponent
- Yann Le Coq (Dr.)