Master Thesis Defense by Stefan Alaric Schäffer – University of Copenhagen

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Master Thesis Defense by Stefan Alaric Schäffer

Lasers with exceedingly narrow spectral linewidths are important within the field of high
precision measurements and are used in atomic clocks for e.g. GPS systems, gravitational
interferometry, and in high tech industries. Optical atomic clocks rely on ultra stable lasers, prestabilized to empty cavities. However, this method is now limited by the
thermal fluctuation in the mirror substrates, and further increase in the performance
of such lasers is becoming exceedingly challenging.
As such, new methods have been proposed, where a low-finesse cavity acts as an
amplifier for the atom-light interaction in atomic or molecular spectroscopy. By
implementing atoms with narrow-linewidth transitions in such a cavity the shot noise limited linewidth of a laser stabilized to the system should be able to reach the millihertz, or even microhertz regime.

Here we present work on four separate spectroscopic systems. A proof-of-principle
realization of a stabilization system based on cold strontium atoms trapped in a MOT is
presented, and the results leading to a laser feedback signal, as well as predictions for an
obtainable shot noise limited linewidth of 500 mHz is given. We use the NICE-OHMS
technique to obtain a phase-response with a high signal-to-noise ratio. While this setup
operates in a cyclic manner, a continuous interrogation is necessary if the laser lock is
to be realized, and the design and construction of a strontium beamline experiment is
thus presented. Finally, preliminary considerations for two molecular clocks on diiodine
and ethyne are presented, and a cavity constructed to significantly reduce transit time
broadening effects in these systems is designed and tested. This work thus shows the way for several approaches to creating ultra stable lasers using cavity enhanced saturated spectroscopy. Continued work on these systems should be able to reach laser stabilities comparable to the current state of the art in stabilization to empty cavities.