The group Metrology, Molecules and Fundamental Tests is performing ultra-high resolution molecular spectroscopy in the near and middle infrared and optical frequency metrology. The main experimental projects are the following:
Absolute frequency control using optical frequency comb
Anne Amy-Klein, Benoît Darquié, Olivier Lopez, Rosa Santagata (postdoc), Dang Bao An Tran (PhD)
We have developed an experiment to measure the absolute frequencies of molecular transitions in the mid-infrared (MIR) or visible spectral range. The resolution (for 1-s averaging time) and the accuracy is better than 10-14. This set-up uses a femtosecond pulsed laser as a frequency comb. The measurement uncertainty is directly linked to the frequency reference used to control the comb repetition rate.
Recently we built a new set-up which is more reliable and efficient thanks to a fiber optical comb emitting around 1.55 µm. This comb is controlled with an optical reference signal transmitted with an optical fiber link from the lab SYRTE, where its absolute frequency is controlled with primary standards. The optical comb is then used to control the frequency of a MIR laser emitting around 10 µm. By using sum-frequency generation, one generates a beatnote between a high-harmonic of the repetition rate and the MIR laser which is sent to a phase-lock loop. This set-up allows us to control the MIR laser frequency with an uncertainty below 10-14 for a 100-s averaging time and also to correct its frequency noise up to a few hundreds of kHz bandwidth. After some first developments with CO2 lasers, it is now applied to Quantum Cascade Lasers. Although these lasers are quite noisy, we demonstrated a stability of a few 10-14 at 1-s averaging time and a laser linewidth below 1 Hz (3x10-14 in relative value). This performance exceeds those demonstrated so far with quantum cascade lasers by almost two orders of magnitude. We are currently developing a scanning for the comb repetition rate which will enable us to scan the QCL frequency continuously on more than 1GHz.
With this project we have shown that the optical link signal can be used to control the frequency of remote lasers up to the mid-infrared. We were able to benefit from a much better optical reference that the ones available in this MIR spectral range. This set-up is now currently used for high-precision molecular spectroscopy and open the way to high sensitivity experiments with molecules at the same level than with atoms.
- B. Argence, B. Chanteau, O. Lopez, D. Nicolodi, M. Abgrall, C. Chardonnet, C. Daussy, B. Darquié, Y. Le Coq and A. Amy-Klein,
Quantum cascade laser frequency stabilization at the sub-Hz level,
Nature Photonics (2015).
- B. Chanteau, O. Lopez, W. Zhang, D. Nicolodi, B. Argence, F. Auguste, M. Abgrall, C. Chardonnet, G. Santarelli, B. Darquié, Y. Le Coq and A. Amy-Klein,
Mid-infrared laser phase-locking to a remote near-infrared frequency reference for high precision molecular spectroscopy,
New J. Phys. 15, 073003 (2013).
- A. Shelkovnikov, R.J. Butcher, C. Chardonnet and A. Amy-Klein,
Stability of the proton-to-electron mass ratio,
Phys. Rev. Lett. 100, 150801, (2008).