Dual-comb spectroscopy with high coherence and high repetition rate

Abstract

With the technological advent of optical frequency combs providing low-noise, well-defined and equidistantly spaced spectral references over a broad bandwidth, unprecedented opportunities for optical precision metrology arose. One elegant spectroscopic technique combines two optical frequency combs to achieve scan-less spectroscopy via massively multiplexed heterodyning. This technology, aptly named dual-frequency comb spectroscopy, relies on the mixing of a probing comb with a second comb of slightly different repetition rate for the down-conversion of an optical signature to the radio frequency domain. Attractive characteristics of dual-comb spectroscopy are comb-based precision and accuracy, high signal-to-noise ratio, broadband operation, fast acquisition times and absence of moving parts. One of the key challenges in dual-comb spectroscopy is to establish coherence between the two optical frequency combs over time scales exceeding the measurement times. Systems with high mutual coherence in combination with high repetition rate lasers create new opportunities for high bandwidth, fast spectroscopy at high optical resolution. This thesis focuses on the demonstration of the potency of high repetition rate, high mutual coherence dual-comb spectrometers through multiple experiments. The high coherence of a 1 GHz dual-electro-optic comb spectrometer is used to demonstrate mapping and compression of optical spectra by 8 orders of magnitude from the hundreds of THz to the kHz range and below. Leveraging this compression from optical to acoustic frequencies, we show the first demonstration of photo-acoustic dual-comb spectroscopy, which combines the assets of broadband dual-comb spectroscopy with the high sensitivity of a photoacoustic detection scheme. The technique can operate background-free in the entire electromagnetic spectrum, and weak absorption features of acetylene are rapidly and precisely sampled as an example. Secondly, pushing the use of the system’s high mutual coherence even further, real-time dual-comb hyperspectral imaging is realized based on a fast infrared detector array, providing high spectral and spatial resolution hyperspectral movies of a sample. A neural network is trained to achieve real-time treatment of the 16’384 pixels’ data recorded at kHz rate based on 30 spectral channels, providing gas concentration imaging with a 10 Hz refresh rate. To overcome the bandwidth limitations inherent to electro-optic combs, a dual-comb spectrometer based on 1 GHz Erbium oscillators is built. High mutual coherence between the combs on the seconds scale is established via a fully digital locking scheme implemented in field-programmable gate arrays. The potential for the system to provide simultaneously fast and broadband measurements is demonstrated by performing water vapor spectroscopy around 1375 nm, as well as, by acquiring narrow gas absorption features across 0.6 THz in only 5 μs. Using this spectrometer, a new calibration technique for astronomical spectrographs is demonstrated. By performing dual-comb spectroscopy of a Fabry Pérot calibrator, the cavity’s spectrum can be measured and linked to an atomic time standard with comb precision. The calibration lines resulting from the filtering of a white-light source by the cavity can then be linked back to the atomic time standard, providing properly conditioned, absolutely known calibration markers for astronomical spectrographs. This novel technique importantly provides astronomical spectrograph calibration based on lasers with repetition rates in the tens of MHz to a few GHz, circumventing the need for so-called astro-combs with repetition rates in the tens of GHz and their associated challenging operation. Finally, to fully leverage the potential of high repetition rate, high mutual coherence dual-comb spectrometers, a software for simulating and optimizing supercontinuum generation from mixed and cascaded nonlinearities is presented. Via this software, extension paths of dual-comb spectroscopy to e.g. the mid-infrared for molecular absorption fingerprint spectroscopy or to the astronomy-critical visible and ultraviolet regions can be investigated and designed. The software is an open-source python package called pychi and simulates the propagation of short pulses in media exhibiting quadratic, cubic, mixed and cascaded nonlinearities. A dedicated solver is developed for fast computational times, as well as, an anti-aliasing technique preventing spurious spectral contamination from the finite bandwidth of the simulation. The simulations are shown to quantitatively agree with experimental results, enabling the use of the software not only for crystal and fiber-based simulations, but importantly in highly nonlinear chip-integrated waveguides.