Thibault Voumard

Astronomical precision spectroscopy supports searches for exoplanets and may enable the direct observation of the Universe’s expansion. A key challenge in these efforts is the precise and long-term calibration of astronomical spectrographs. White light-illuminated Fabry–Pérot cavities, cross-calibrated with hollow-cathode lamps, are reliable calibrators. However, they exhibit temporal variability and nontrivial dispersion of the resonance frequencies, and the calibration lamps provide only sparse spectra with complex line shapes, which are also prone to aging. In contrast, laser frequency combs (“astrocombs”) can offer SI-referenced accuracy and precision, but the requirement of resolvable comb lines implies pulse repetition rates >10 GHz, which is challenging to implement. Here, we explore an alternative technique where lower-repetition rate dual-frequency comb lasers (comb lines unresolvable by the spectrograph) measure the modes of a 12 GHz Fabry–Pérot cavity across a spectral span of 30 THz (175–205 THz). This effectively transfers the comb’s accuracy to the cavity and enables precise near real-time tracking of the cavity modes. These measurements are of immediate relevance to currently used calibration techniques, and with further advances, this approach could potentially support spectrograph calibration at the level needed to search for Earth-like exoplanets.

Optical frequency combs and their spectra of evenly spaced discrete laser lines are essential to modern time and frequency metrology. Recent advances in integrated photonic waveguides enable efficient nonlinear broadening of an initially narrowband frequency comb to multi-octave bandwidth. Here, we study the nonlinear dynamics in the generation of such ultrabroadband spectra where different harmonics of the comb can overlap. We show that a set of interleaved combs with different offset frequencies extending across the entire spectrum can emerge, which can be arranged into a practically evenly spaced ultrabroadband frequency comb when the initial comb is offset-free.

Frequency combs from continuous-wave-driven Kerr-nonlinear microresonators have evolved into a key photonic technology with applications from optical communication to precision spectroscopy. Essential to many of these applications is the control of the comb’s defining parameters, i.e., carrier-envelope offset frequency and repetition rate. An elegant and all-optical approach to controlling both degrees of freedom is the suitable injection of a secondary continuous-wave laser into the resonator onto which one of the comb lines locks. Here, we experimentally study such sideband injection locking in microresonator soliton combs across a wide optical bandwidth and derive analytic scaling laws for the locking range and repetition rate control. As an application example, we demonstrate optical frequency division and repetition rate phase-noise reduction to three orders of magnitude below the noise of a free-running system. The presented results can guide the design of sideband injection-locked, parametrically generated frequency combs with opportunities for low-noise microwave generation, compact optical clocks with simplified locking schemes, and, more generally, all-optically stabilized frequency combs from Kerr-nonlinear resonators.

Nonlinear optical frequency conversion is of fundamental importance in photonics and underpins countless of its applications: Sum- and difference-frequency generation in media with quadratic nonlinearity permits reaching otherwise inaccessible wavelength regimes, and the dramatic effect of supercontinuum generation through cubic nonlinearities has resulted in the synthesis of broadband multi-octave spanning spectra, much beyond what can be directly achieved with laser gain media. Chip-integrated waveguides permit to leverage both quadratic and cubic effects at the same time, creating unprecedented opportunities for multi-octave spanning spectra across the entire transparency window of a nonlinear material. Designing such waveguides often relies on numeric modeling of the underlying nonlinear processes, which, however, becomes exceedingly challenging when multiple and cascading nonlinear processes are involved. Here, to address this challenge, we report on a novel numeric simulation tool for mixed and cascaded nonlinearities that uses anti-aliasing strategies to avoid spurious light resulting from a finite simulation bandwidth. A dedicated fifth-order interaction picture Runge–Kutta solver with adaptive step-size permits efficient numeric simulation, as required for design parameter studies. The simulation results are shown to quantitatively agree with experimental data, and the simulation tool is available as an open-source Python package (pychi).

Low-noise Kerr-combs with detectable microwave repetition rate are generated for the first time in chip-integrated high-Q Fabry–Pérot m croresonators. Steps-features characteristic of dissipative Kerr solitons formation are observed in the transmitted power.

Precise and accurate calibration of astronomical spectrograph is key in exo-planet searches and cosmological studies. Dual-comb spectroscopy of a Fabry-Pérot cavity represents a new approach to absolute calibration avoiding the need for high-repetition rate lasers.

Hyperspectral imaging provides spatially resolved spectral information. Utilizing dual-frequency combs as active illumination sources, hyperspectral imaging with ultra-high spectral resolution can be implemented in a scan-free manner when a detector array is used for heterodyne detection. Here, we show that dual-comb hyperspectral imaging can be performed with an uncooled near-to-mid-infrared detector by exploiting the detector array’s high frame rate, achieving 10 Hz acquisition in 30 spectral channels across 16,384 pixels. Artificial intelligence (AI) enables real-time data reduction and imaging of gas concentration based on characteristic molecular absorption signatures. Owing to the detector array’s sensitivity from 1 to 5 µm wavelength, this demonstration lays the foundation for real-time versatile imaging of molecular fingerprint signatures across the infrared wavelength regime with high temporal resolution.