Ultra Frequency-Stable Laser via Spectral Hole Burning in Rare-earth Ion Doped Crystals

The UltraStabLaserViaSHB project seeks to catch the currently elusive grand prize of time and frequency metrology: a frequency source with a relative stability on the order of 10^-18 at 1 s. The desirability of this goal is borne of the near-future redefinition of the SI unit of time, the second.

As optical atomic clocks surpass microwave-frequency atomic clocks in accuracy, the switch to an optical definition of the second drives the metrology field to strive to the fundamental performance limit of optical clocks, the quantum projection noise limit.

Currently, optical clock performance is limited by frequency fluctuations of the optical-cavity-stabilized laser field which probes the atoms' optical transition.

The optical lattice clocks located at SYRTE could reach their quantum projection limit if a probe laser with a sufficient frequency stability could be realized.The project proposed here seeks develop an ultra frequency stable laser at SYRTE to reach this performance via a paradigm shift in laser stabilization, away from optical cavity frequency references (which themselves approach their fundamental limit, Brownian noise) and toward a novel method: laser stabilization via spectroscopy of rare-earth ion doped crystals.

This is achieved through a technique called Spectral Hole Burning (SHB) where a spectral pattern is imprinted on the crystal at cryogenic temperatures by a pre-stabilized laser (a spectral ""hole"" is ""burnt"").

A probe beam then interacts with this spectral hole and the resulting de-phasing of the probe beam provides the source for a control signal which allows us to actuate the probe laser, stabilizing it to the narrow line of the rare earth ion. Early results in this young technique are extremely promising and its limits are yet undiscovered.

The result will impact not only time metrology, but all fields which rely on ultra-stabilized lasers such as gravitational-wave detection, fundamental constant measurements, and tests of general relativity.