Ultra-stable lasers and high performance cavities
- Silicon cavity
Atomic optical clocks are currently beginning to outperform the best caesium fountain clocks with respect to their accuracy and stability. However, the short-term stability of optical clocks is limited by the frequency stability of the ultra-stable lasers that are used to interrogate the atomic or ionic quantum transition used as the pendulum of the atomic clock. Therefore our research project aims for improving the stability of the laser system. Besides atomic clocks, narrow-linewidth lasers are indispensable in high-resolution spectroscopy, for atom interferometers, fundamental physics tests as well as for interferometric measurements like future space missions including e.g. the Laser Interferometer Space Antenna.
To realize such a stable oscillator a laser is locked to a narrow resonance of an optical cavity which consists of two mirrors with the highest reflectivity. The mirrors are optically contacted to a spacer that keeps them at a well defined distance. Once the cavity is setup the stabilized laser light can then be linked to any other laser system using a frequency comb. As any length change of the cavity transfers to a change in laser frequency the cavity needs to be insensitive of thermal length fluctuations as well as vibrations. Nowadays the frequency fluctuations of the best lasers are ultimately limited by thermal fluctuations of the length of the optical cavities in a way that is analogous to the Brownian motion of the molecules in the cavity material. This poses a serious problem for the development of ultra-stable lasers which hinder a number of promising fields to live up to their far reaching expectations.
- Assembly of the cryostat
Our research project concentrates on solving this problem with high priority. By reducing the temperature and by choosing an appropriate material of high mechanical quality we want to reduce the thermal fluctuations of the resonator. For this purpose we are currently working on a novel single-crystal silicon cavity operated at a temperature of 120 K. The optical resonator should reach a fractional frequency instability below 10-16 .
This program is carried out in close collaboration with the research group of Prof. Jun Ye (JILA, USA). To specify the performance of our silicon resonator we aim for a direct comparison with a long ULE type optical cavity which is being built by our colleagues.
Additionally we want to investigate the possible application of new high-reflective materials which might replace standard multilayer coatings. Those coating-free mirrors might open up ways to monolithic cavities of unprecedented stability. In the QUEST framework we are going to investigate silicon based coatings in close collaboration with Laser Zentrum Hannover (LZH) and Albert-Einstein Institut (AEI).