17 April 2023
Ultra-stable lasers are an important component of optical clocks. As a matter of fact, they are used to probe ultra-narrow atomic "clock" transitions and therefore constitute the local oscillator in these setups. Currently, ultra-stable lasers are realized by locking to very high finesse (typically 106) Fabry-Pérot cavities. These cavities need, of course, to be set in very well controlled vacuum environments, well shielded from environmental vibrations and temperature fluctuations. Such ultra-stable lasers reach nowadays their fundamental limits, which is due to thermal agitation (at room temperature), at fractional stabilities of the order of 3-4×10-16 at 1 second for typical state-of-the-art devices. At such an order of magnitude, the local oscillator is a limiting factor of the performance of the lattice optical clocks developed at SYRTE and other laboratories in the world. In particular, these new atomic clocks cannot reach there own fundamental limits (induced by the number of atom which are probed at each cycled, the so-called "quantum projection noise"). One could expect dramatic performance improvements should better quality local laser oscillators be available.
Several different routes can be explored for improving Fabry-Perot cavity based ultra-stable lasers. For example, one can realize large size cavities, where thermal agitation effects are averaged more and induce a lower fundamental limit stability. One can also realize cavities in cryogenic environments, where thermal noise is lower. At SYRTE, we have recently started a new research activity to realize, use and study ultra-stable lasers locked on "spectral hole burning" patterns in rare-earth doped crystals at cryogenic temperatures ( 4K, near absolute zero). Indeed, one can expect that such device may be an order of magnitude or more better than Fabry-Perot based systems, even though many studies and technological developments will need to be realized to reach such a high level of performance.
The idea behind our system is that rare-earth elements in crystalline matrices at cryogenic temperatures exhibit absorption lines of a few GHz linewidth (inhomogeneous broadening) in which one can imprint narrow spectral patterns ("spectral hole burning", homogeneous linewidth 1kHz, lifetime as long as several hours at low temperature) with a laser whose frequency is pre-stabilized by standard techniques via a suitable experimental protocol. Once the pattern is photo-inscribed, the same laser can be "post-stabilized" on them, and have its spectral purity and stability improved (as long as the spectral pattern itself is sufficiently stable over time). The fundamental limits of such a cryogenic system are very low (due to the very low level of thermally activated excitations). However, the experimental challenges that will need to be overcome to approach them are many and difficult: Highly temperature stable and low vibration continuously operated cryogenic environments, matrix/dopant complex with low sensitivity to residual environmental perturbation, photo-inscription/post-stabilization protocol that are both performance optimized and compatible with continuous operation for weeks,...
This research program is building on a strong collaboration with other French laboratories which bring in there respective and complementary know-how. The Laboratoire de Chimie de la Matière Condensée de Paris (LCMCP, Chimie ParisTech/Collège de France/CNRs, Paris) for the realization and caracterization of rare-earth doped cristals, the Laboratoire Aimé Cotton (LAC, CNRS, Orsay) for spectroscopy and utilization of rare-earth doped cristals and the Franche-Comté Electronique, Mécanique, Thermique et Optique - Sciences et Technologies institute (FEMTO-ST, CNRS/ENS de Mécanique et des Microtechniques de Besancon) for highly stable cryogenic environments for metrological applications.
Eventually, the goal of our project, once we reach a satisfactory level of performance, is to transfer the spectral purity of our laser to other wavelengths of metrological interests ("clock" atomic transitions used at SYRTE), so as to benefit from it on the lattice optical clocks developed in the laboratory.
Contact
Bess Fang
- Email: Bess.Fang (at) obspm.fr
- Tel.: +33 (0) 1 40 51 22 91
Yann Le Coq
- Email: Yann.LeCoq (at) obspm.fr
- Tel.: +33 (0) 1 40 51 21 01