Design and fabrication of advanced lasers and spectrometers: towards molecular metrology
Alexis Libert (1,2), Baptiste Fabre (3), Séverine Robert (1), Clément Lauzin (2)
1) Royal Belgian Institute for Space Aeronomy, B-1180 Uccle, Belgium
2) Institute of Condensed Matter and Nanosciences, Université catholique de Louvain,
B-1348 Louvain-la-Neuve, Belgium
3) Université de Bordeaux, CNRS, CEA, CELIA, UMR5107, F33405 Talence, France
Molecular spectroscopy is a fundamental tool in studying the structure, dynamics and interactions of molecules. It allows to derive important reference parameters later used for fundamental physics, planetology and chemical kinetics applications. It relies on the precise measurement of the quantity of absorbed photons and of the frequency of light. However, achieving accurate measurements over a broadband range has always been a challenge. Frequency combs, which provide a precisely spaced grid of frequencies spanning a broad spectrum, have emerged as invaluable tools for addressing this challenge in molecular spectroscopy. Any comb line can be expressed as follows:
fm = fCEO + m*frep'
where 'fm' is the offset frequency, 'frep' the frequency difference between adjacent comb modes and 'm' an integer. These two characteristic frequencies, which lie in the radiofrequency domain can be measured very precisely and phase locked with microwave frequency standards. When measuring the comb spectrum, eventually after its interaction with a gas sample, with enough resolution to resolve the different comb lines, the precision on the determination of the frequency axis is ultimately limited by the precision on the offset and repetition frequencies which can reach (dn/n~10e-14).
In this talk, the design and fabrication of a frequency comb will be described. Details about the conditions these oscillators should fulfill to provide on the order of 80 million pulses per second, each of them having a duration of about 50 x 10e-15 seconds. The free space Ti:Sapphire femtosecond oscillator built in house at UCLouvain will support these points. Then we will present the next steps we are working on to transform this femtosecond laser into a stabilized frequency comb. We will also describe how we intend to use it to perform broadband molecular spectroscopy with metrological level of precision on the intensity and frequency axes. Finally, we will emphasize the gain of such light sources when coupled with Fourier transform spectrometers like the one we are currently building at BIRA-IASB.