High-coherence dual-comb interferometry with free-running lasers
|Authors:||Bourbeau Hébert, Nicolas|
|Abstract:||Dual-comb spectroscopy is a technique where two slightly detuned laser frequency combs are interfered together after probing a sample under study in order to retrieve its spectral signature with a high resolution and at high speed. However, it requires two lasers that are mutually coherent, a constraint that is most often satisfied by active stabilization at the cost of an increased hardware complexity. This thesis tackles this issue by presenting solutions that allow the use of free-running combs, thus simplifying the dual-comb technique. First, we demonstrate a compact laser platform able to generate a pair of frequency combs that are similarly affected by environmental perturbations. It is based on an erbium-doped glass chip containing a number of ultrafast-laser-inscribed waveguides separated by a few hundred microns. Two adjacent waveguides are pumped simultaneously and passively mode-locked at ∼1 GHz in the 1.5 μm band to deliver a pair of correlated frequency combs. The free-running frequency noise of this source is characterized thoroughly and its mutual coherence time is found to exceed the measurement time required to retrieve a high-resolution spectrum. This is made possible by the use of intrinsically low-noise waveguide lasers, by the dual-comb source’s mechanical integration, and by the use of a large repetition rate difference between the combs. We also present two correction algorithms that, when combined with our dual-comb source, allow to artificially extend its coherence time in order to increase the useful averaging time without sacrificing the spectral resolution. These algorithms work by estimating and compensating the phase and timing of the measured interferograms without relying on any external measurement of the combs’ fluctuations. They are first described in detail and their limitations are determined quantitatively in terms of the combs’ parameters and relative frequency noise, where a large repetition rate difference appears to be the key to a successful correction. Finally, the performance of the dual-comb spectrometer assisted by a software correction is demonstrated by measuring the transmission spectrum of acetylene and hydrogen cyanide with a spectral sampling of ∼1 GHz. The quality of the measurements is validated by comparison to spectra simulated from known data.|
|Document Type:||Thèse de doctorat|
|Open Access Date:||15 October 2019|
|Collection:||Thèses et mémoires|
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