LIPhy acquires a frequency comb referenceable to the REFIMEVE network

The LAME (Lasers, Molecules, and Environment) team at LIPhy has acquired a frequency comb (Menlo Systems FC1500-ULNnova), a light source whose name refers to its spectrum composed of regularly spaced lines. This state-of-the-art equipment represents an investment of 350 k€, funded by the REFIMEVE network, UGA, CNRS, LIPhy, and the LAME team. This type of source is equivalent to millions of optical tuning forks whose frequencies are precisely known and cover a range extending from the infrared (2.1 µm) to the visible (500 nm), and allows for the extremely precise calibration of the frequency of any laser within this range.

At the same time, LIPhy becomes a key node in the REFIMEVE network, a research infrastructure that distributes ultra-precise time and frequency references across Europe via the telecom fiber network. The fact that the frequency comb and the network node are located in the same place offers a dual mutual benefit.

• On the one hand, this improves the quality of the REFIMEVE signal coverage, which is initially generated at the Laboratoire Temps Espace (LTE, Paris) using atomic clocks but degrades as it propagates over hundreds of kilometers and must therefore be reprocessed at various points in the network.
• On the other hand, this stabilizes the spectrum emitted by the frequency comb, thereby providing LIPhy with laser sources whose frequency is extremely stable and known with high precision in absolute terms. For example, an innovative source, funded by the FIRST-TF Labex and locked to the comb, produces a laser beam with a frequency of 473612337576230 Hz (corresponding to a wavelength in vacuum of 632.991234 nm) equivalent to that of a He-Ne laser, but with a spectral width of less than 5 kHz—making it 200 times better—and long-term stability guaranteed to be better than 10 Hz!

Having laser sources with high spectral purity that are properly calibrated is absolutely crucial for metrology experiments conducted in the laboratory. Within the LAME team, this enables, for example, measurements of isotopic ratios essential to geosciences, of spectral line profiles underlying satellite measurements, of THz spectroscopy for astrophysics, and of quantities of fundamental interest. These state-of-the-art laser beams are also intended to be distributed to neighboring laboratories, such as the Institut des Géosciences de l'Environnement (IGE) via the ERC DocPAST project, as well as to all LIPhy experiments that can benefit from them, such as the interferometers of the dynamic Surface Force Apparatus developed by the MODI team, which enables the study of the behavior of confined fluids at the nanoscale.