STAGE Offre de stage: Optical phase locked loop for time and frequency metrology

The future of time and frequency metrology lies in the optical domain : the second, the unit of time
will soon be defined via an optical transition in atoms ; optical clocks around the world have reported
unprecedented performance in both stability and accuracy ; and optical fiber networks provide
means of comparing distant clocks and disseminating frequency and time. In the heart of such
developments, an essential tool is the ultra-stable lasers.
Standard ultra-stable lasers nowadays derive their frequency stability from a Fabry-Perot cavity,
which has hit its fundamental stability limit since the early 2000s. Due to thermodynamic noise in
the cavity, the fractional frequency stability of these lasers are at a few 10-16 at 1 s average time.
Among the various paths taken in the search for improvement, a promising candidate is laser
spectroscopy with rare earth doped ions at cryogenic temperatures. With an optimal choice of the
crystalline matrix and dopant ion, very narrow spectroscopic features can be realized using the socalled ‘Spectral Hole Burning’ technique. These holes are then used as a reference to stabilize the
frequency of lasers. Fundamental limit of such a system is currently unknown but expected to be
below 10-18 at 1 s. This internship will contribute to the Spectral Hole Burning experiment of SYRTE.
Project
Central to the SHB experiment is the ultra-low noise heterodyne detection using two phase-locked
lasers. The phase relation between these two continuous wave lasers is locked on a stable microwave signal using an optical phase locked loop (OPLL). The residual relative phase noise between the
lasers enters directly into the detection noise. For the ultimate performance of our detection
scheme, it is therefore crucial to optimize the OPLL.
Conventional solutions based on analog electronics exist since more than 30 years. They constitute
the standard phase lock schemes already present on our experiment. However, the advent of digital
electronics provides the attractive possibilities of agile frequency control and atomization. We have
therefore collaborated with the startup company Koheron for a digital OPLL based on commercial
FPGA (Red Pitaya). The intern will first test the functionalities of the digital OPLL in order to evaluate
its performance in terms of locking bandwidth and residual noise. The former could be potentially
limited by digitization. Optimization should be carried out for our specific system.