Pierre Didier

Mid-infrared frequency combs provide highly sensitive and selective access to molecular vibrational transitions, making them attractive for integrated spectroscopic sensing. However, compact sources offering broad wavelength coverage together with independent electronic control of comb parameters remain challenging to realize. Here, we demonstrate mid-infrared electro-optic frequency-comb generation through difference-frequency generation in periodically poled thin-film lithium niobate waveguides implemented on two substrates, namely silicon dioxide and sapphire. Our architecture combines a continuous-wave near-infrared pump with a telecom-band electro-optic comb operated in either single- or dual-comb configurations, generated on-chip. Difference-frequency generation allows downconversion of the telecom comb to the mid-infrared. The comb spacing is defined by the drive radio frequency, while the comb center frequency is independently controlled by the telecom seed wavelength. The quasi-phase-matching condition can be further tuned through temperature control. We generate mid-infrared combs with an instantaneous bandwidth of approximately 6 nm and a center-wavelength tuning range exceeding 200 nm on a single device. A direct comparison of the device on two substrates reveals the advantage of lithium niobate on sapphire for operation beyond 3.5 µm, providing higher output power, improved conversion efficiency, and extended spectral coverage up to 4.5 µm. Finally, we demonstrate on-chip dual-comb generation in the mid-infrared with a tunable relative frequency offset between the two combs. Low-frequency beat notes between corresponding comb lines are detected over a 140 nm MIR wavelength range. This unique integrated mid-infrared electro-optic comb platform combines independent electronic control of the comb center frequency, line spacing, and frequency offset with on-chip dual-comb generation.

Nouveaux dispositifs à cascade quantique dans l'infrarouge moyen pour des applications dans l'optique espace libre, la sécurité des données et la photonique des micro-ondes: Cette thèse de doctorat porte sur la transmission optique en espace libre (FSO) dans la région de l’infrarouge moyen couvrant trois aspects principaux : la transmission FSO à grande vitesse, la communication privée par synchronisation chaotique, et l’intégration de systèmes de communication FSO-RF. Dans le domaine de la transmission FSO à grande vitesse, cette recherche s’intéresse à différents schémas de modulation, de filtrage et d’équalization. Des dispositifs en cascade tels que les lasers en cascade interbande (ICL) et les lasers en cascade quantique (QCL) sont utilisés. Des débits de données allant jusqu’ ´ a 14 Gbps sont obtenus en utilisant des ICL et des photodétecteurs interbandes en cascade (ICIP) dans la fenêtre 3-5 µm. Des QCL et des modulateurs externes bases sur l’effet stark sont également mis en œuvre, permettant d’atteindre des débits de données allant jusqu’ à 68 Gbps. Ces dispositifs offrent une puissance modulée élevée, ce qui les rend adaptes aux transmissions sur de longues distances. La thèse explore également l’application des techniques de synchronisation chaotique pour la communication privée sur les liaisons FSO. Les propriétés chaotiques des sources lumineuses dans l’infrarouge moyen sont exploitées, ce qui permet de dissimuler des messages dans des signaux chaotiques. La complexité du chaos généré permet une transmission privée; les utilisateurs légitimes obtenant de faibles taux d’erreur alors que ceux des utilisateurs non légitimes sont important. En outre, la thèse étudie l’intégration des systèmes de communication FSO et RF. L’objectif est de créer une conversion entre les liaisons FSO à grande vitesse et les liaisons RF. Des techniques de battement hétérodyne sont utilisées, combinant des lasers à cascade quantique pour générer des signaux de battement. Cette approche permet la transmission de signaux FSO sur la bande Ka par l’intermédiaire d’un détecteur QWIP. La recherche démontre la faisabilité de l’intégration FSO-RF, ouvrant des possibilités de combiner les avantages des deux systèmes de communication. Dans l’ensemble, cette thèse présente des avancées en matière de transmission FSO à grande vitesse, de communication sécurisée utilisant la synchronisation chaotique, et d’intégration des systèmes FSO et RF.

The mid-infrared spectrum, spanning from 3 to 14 µm, holds great promise for molecular spectroscopy and free space optical communication, benefiting from strong molecular absorption and reduced atmospheric attenuation. While progress in MIR photonics has accelerated due to improved sources and detectors, integrated low-loss, high -performance modulators remain limited. In order to address this gap, we demonstrate a broadband, high speed lithium niobate on sapphire Mach Zehnder electro optic modulator operating from 3.95 to 4.5 µm. The device shows a 3 dB bandwidth above 20 GHz, 17 dB extinction ratio, and V _π L = 22 V ⋅ cm, with optical output power at the half milliwatt level. We demonstrate 10 Gbit s ⁻¹ data transmission and a 70 GHz broad frequency comb, uniquely combining integration, low propagation loss, extinction ratio and high-speed operation.

Free-space optical communications in the mid-infrared transparency windows (4–5 and 8–14 μm wavelength regions) is emerging as a viable solution for high bitrate data transmission. Unipolar quantum optoelectronics is the technology of choice for data communication in this wavelength region, thanks to the high frequency response of detectors and modulators. In this work, it is demonstrated that the performances of these devices can be substantially enhanced by embedding them into metamaterials. It is also shown that metamaterials have to be engineered differently in detectors than in modulators, as the role of light–matter interaction must be tuned adequately in the two devices. Metamaterial-enhanced performances allow the realization of data transmission with a record rate of 68 Gbit/s, while ensuring robustness and consistency, as it should be for real-world applications. These findings underscore the promising role of metamaterial-enhanced unipolar devices in advancing free-space optical communication systems.

Interband cascade lasers typically have significantly lower threshold current and power consumption than quantum cascade lasers. They can also have advantages regarding costs and compactness with the photonic integration onto silicon substrates by epitaxial growth. This research introduces a novel examination of the relative intensity noise and the modulation dynamics of a silicon-based Fabry–Perot interband cascade laser emitting at 3.5 μm. The investigation delves into crucial parameters, such as relaxation oscillation frequency, differential gain, gain compression, and K-factor. The resonance patterns identified in relative intensity noise curves can provide essential insights for the thorough characterization of high-defect mid-infrared semiconductor structures intended for high-speed applications. Moreover, this study demonstrates the feasibility of reaching 10 Gbit/s free-space transmission using a silicon-based interband cascade laser in conjunction with an interband cascade infrared photodetector.

Nonlinear dynamics at mid-infrared wavelength is of interest for various applications but has been mainly limited to quantum cascade lasers so far. In this article, we show that interband cascade lasers can generate complex optical chaos with bandwidth in the GHz range and this outperforms the performances of quantum cascade laser chaos. The chaos nature of our signal is confirmed by a thorough time-series analysis. Modeling of the semiconductor laser under short-cavity external optical feedback allows confirming the high bias chaos operation that we observe experimentally. These results pave the way for applications at mid-infrared wavelength, such as private free-space communication.

In environments where traditional fiber optic cables are impractical, free-space optical communications offer a promising solution for transmitting large amounts of data, especially in the mid-infrared wavelength range. Despite the advantages of minimal atmospheric interference and stable signals, the vulnerability of wireless optical communications to eavesdropping poses a significant challenge. This study addresses this challenge by demonstrating a method for privately transmitting optical data using photonic chaos from distributed feedback quantum cascade lasers operating at 9.3 µm. Signal processing techniques are applied to enhance the quality of the transmission over distances exceeding 30 m, accompanied by a comprehensive analysis of the photonic chaos complexity to ensure data confidentiality. These findings mark a significant advancement in developing private communications systems within the thermal atmospheric window, with a substantially reduced risk of interception by adversaries. The research not only contributes to secure communications but also has potential implications for enhancing security of data transmission in challenging environments, impacting various industries and applications.

Lack of critical communication components (external modulators, high-sensitivity detectors, amplifiers) has long hindered the development of high-speed free-space transmission in the 8-12 μm thermal atmospheric window. Unipolar quantum technology has emerged as a game-changer by developing key elements that outperform conventional direct-modulation schemes in terms of performance. In particular, we demonstrated a free-space communication at 30 Gbits/s. High-speed modulation of the 9 μm-wavelength beam from a quantum cascade laser is implemented with a Stark-effect external modulator while fast detection relies on a quantum well infrared photodetector. In between, a multi-pass cell allows increasing the propagation distance to 31 meters.

Space-to-ground high-speed transmission is of utmost importance for the development of a worldwide broadband network. Mid-infrared wavelengths offer numerous advantages for building such a system, spanning from low atmospheric attenuation to eye-safe operation and resistance to inclement weather conditions. We demonstrate a full interband cascade system for high-speed transmission around a wavelength of 4.18 µm. The low-power consumption of both the laser and the detector in combination with a large modulation bandwidth and sufficient output power makes this technology ideal for a free-space optical communication application. Our proof-of-concept experiment employs a radio-frequency optimized Fabry–Perot interband cascade laser and an interband cascade infrared photodetector based on a type-II InAs/GaSb superlattice. The bandwidth of the system is evaluated to be around 1.5 GHz. It allows us to achieve data rates of 12 Gbit/s with an on–off keying scheme and 14 Gbit/s with a 4-level pulse amplitude modulation scheme. The quality of the transmission is enhanced by conventional pre- and post-processing in order to be compatible with standard error-code correction.