A compact electro-optic modulator integrated within a SiGe waveguide operating in the $5-9 \mu \mathrm{m}$ wavelength range is demonstrated. Utilizing a Schottky diode and freecarrier absorption effect, the device achieves at least $\mathbf{3 GHz}$ bandwidth and an ER of $\mathbf{1.75 \sim dB}$.

ABSTRACT Free‐space optical (FSO) communication is rapidly advancing as a high‐capacity wireless technology, with growing interest in extending its operation from the traditional near‐infrared to the mid‐infrared (mid‐IR) spectral region. Particularly, the mid‐wave infrared (MWIR) and long‐wave infrared (LWIR) atmospheric transmission windows in the mid‐IR region provide advantages regarding reduced turbulence effects, scattering and absorption, enabling more robust links under adverse conditions. This review presents recent progress in MWIR and LWIR FSO communication systems, highlighting advances in transmitter and receiver technologies, such as quantum cascade lasers (QCLs), lithium niobate modulators and detectors, including quantum cascade detectors (QCDs), mercury cadmium telluride (HgCdTe and MCT) photodiodes and quantum‐well infrared photodetectors (QWIPs). Experimental demonstrations achieving multigigabit to tens‐of‐gigabit data rates with advanced modulation formats, digital equalisation and coherent detection are summarised. The paper discusses integration trends in MWIR and LWIR photonics, coherent communication strategies and hybrid FSO–radio frequency (RF) architectures. Together, these developments outline the pathway for MWIR and LWIR FSO systems to evolve from laboratory demonstrations towards scalable, practical platforms for next‐generation terrestrial and space communication networks.

We report on the fabrication and investigation of compact microresonators (100 µm radius) operating in the long-wave infrared range, exploiting silicon-germanium graded waveguides. Two designs are considered and demonstrate intrinsic quality factors above 4 × 104 at 7.5 µm wavelength and 2 × 104 at 8 µm wavelength. The resonances measured in these systems attain a 0 dB extinction ratio. These properties could be leveraged for Kerr-comb generation, on-chip optical filtering or multiplexing systems in the long-wave infrared range.

Recent works on the development of mid-infrared frequency comb sources based on graded-SiGe photonic integrated circuits will be presented. More specifically, we will focus the presentation on (i) current development of high performance micro resonators for nonlinear optics based microcombs sources, (ii) recent demonstrations of electro-optical frequency comb based on Schottky modulators operating around 8 mu m wavelength

Free space optical (FSO) communication is considered a critical part of future ICT infrastructure, particularly in non-terrestrial communication segments. In this context, the ability to achieve fast and reliable FSO propagation through long-distance atmospheric channels is the most important factor in choosing technological solutions. One property of optics directly related to this factor is the choice of wavelength. It has been identified that the mid-infrared (mid-IR) regime, which includes two atmospheric transmission windows-the mid-wave IR (MWIR, 3-5 mu m) and the long-wave IR (LWIR, 8-12 mu m)-can potentially offer a promising solution for achieving such performance. Additionally, viable semiconductor sources and detectors that support high-speed and efficient signal transmission are also considered critical to generating sufficient critical mass to advance the application of mid-IR FSO. Unipolar quantum optoelectronics, including quantum cascade lasers (QCL), Stark modulators, quantum cascade detectors (QCD), and quantum-well IR photodetectors (QWIP), among other components, emerge as potential candidates to build such FSO subsystems and systems. We present our recent efforts in conducting subsystem and system-level studies with different variants of these unipolar quantum optoelectronics and demonstrate the potential for feasible transmitter and receiver performance in a laboratory environment. We also discuss the key challenges and considerations of such technologies towards practical development. Finally, we summarize recent research and development efforts worldwide in advancing this highly promising direction.

A photonics platform based on graded SiGe waveguides has been developed for operating in the mid-IR range, with experimental demonstrations up to 11 mu m wavelength. Applications are related with absorption spectroscopy and sensing in the fingerprint region, and with free space communications. After an overview of the platform, the presentation will focus on : (i) integrated high speed modulators operating in a wide spectral range, based on free carrier plasma dispersion effect, (ii) integrated resonators operating around 8 mu m wavelength with quality factors beyond 10(5). We will also show that mid-IR graded SiGe photonics circuits can be fabricated on industrial-scale 200 mm wafers, and that low propagation losses can be obtained in a wide spectral range of the mid-IR. These results thus pave the way for a scalable silicon compatible mid-infrared platform.

The development of on-chip spectroscopic systems has attracted a lot of attention in recent years. Such systems require broadband mid-infrared sources, resulting in the exploration of mid-infrared supercontinuum generation. This study focuses on reducing the pump peak power to obtain on-chip octave-spanning supercontinua at wavelengths from 4 to 13 mu m, leveraging the high nonlinear refractive index and low propagation losses of Ge-rich graded SiGe photonic circuits. Two distinct platforms are explored, one grown in an industrial-scale epitaxy tool and the other using low-energy plasma-enhanced chemical vapor deposition. These silicon-compatible platforms are used to design nonlinear waveguides exhibiting anomalous group velocity dispersion, enabling soliton fission based octave-spanning supercontinuum generation at a record-low peak power of 311 W in this wavelength range. This work paves the way toward chip-based commercially available supercontinuum sources in the mid-infrared. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/).

Mid-infrared spectroscopy is a powerful technique for identifying chemical and biological substances, increasing the demand for compact and cost-effective integrated systems. In this context, integrated electro-optic modulators are essential to implement advanced techniques such as synchronous detection or dual-comb spectroscopy. In this paper, we present a waveguide-integrated electro-optic modulator operating in a broad spectral range from 5 to 9 µm wavelength, based on a Schottky diode embedded in a graded silicon germanium waveguide. By using the free-carrier plasma absorption effect, the device achieves an extinction ratio of up to 1.9 dB at a wavelength of 9 µm. Two different methods have been used to evaluate the device speed. In both cases, high-speed operation up to 7 GHz is obtained, while the optical bandwidth is estimated to be around 3 GHz. These results represent a significant step forward in both the development of characterization techniques and in the performance of mid-IR integrated modulators, paving the way for high-performance on-chip spectroscopic systems.

We report on the development of free-form silicon nitride waveguides for pulse compression of ps pJ optical pulses. A genetic algorithm is used to determine the best waveguide profiles, e.g. a 15cm length pathway ensuring a compression factor of 20 giving rise to 44fs wide output pulses. Experiments are being conducted to confirm these trends.

Mid-infrared (mid-IR) spectroscopy can be used to identify chemical and biological substances. Indeed, most molecules absorb light at specific wavelengths in the mid-infrared (mid-IR). Mid-IR photonic circuits on silicon chips have recently gained a lot of attention. Indeed, they can provide high-performance with a low power consumption, while being cheap, compact, and light. Germanium (Ge) and silicon-germanium (SiGe) alloys with a high Ge concentration are particularly interesting because of the wide transparency window of Ge extending up to 15 µm. It has been demonstrated for a few years now that a Ge-rich graded SiGe platform relying on a graded SiGe layer epitaxially grown on a Si substrate can be used as a photonics platform for mid-IR operation in a wide spectral range. In this invited talk, recent developments concerning graded SiGe photonic integrated circuits will be presented. First, passive devices will be reviewed. It will be shown that graded-SiGe waveguides can be used in an unprecedented spectral range, e.g. up to 11 µm. Mach Zehnder interferometers, resonators and integrated Fourier transform spectrometers will be discussed. Then, a large bandwidth light source based on non-linear optical effects in SiGe waveguides [1] will be presented, together with results on integrated mid-IR optoelectronic modulators [2, 3]. Tunable electro-optical frequency-comb generation around 8 µm wavelength based on such integrated modulators will also be discussed [4]. Finally, the recent scale-up of this platform will be shown, with the first demonstration of low-loss mid-infrared waveguides fabricated on a Ge-rich SiGe strain-relaxed buffer grown on an industrial-scale 200 mm wafer [5]. Acknowledgment: This work was co-funded by ANR Light-up Project (ANR-19-CE24-0002-01) and by European Union (ERC, Electrophot,101097569). This work was partly supported by the French RENATECH network. References: [1] M. Montesinos-Ballester, et al, ACS Photonics, 7, 3423-3429, (2020). [2] T.H.N. Nguyen, et al, Optics Express, 30, (26), 47093 (2022). [3] T.H.N. Nguyen, et al, Advanced Photonics Research, 2200237 (2022). [4] V. Turpaud, et al, Laser & Photonics Reviews, 2300961 (2024). [5] V. Turpaud, et al, to be published, 2024.

This study investigates the potential of long-wave infrared (LWIR) free-space optical (FSO) transmission using multilevel signals to achieve high spectral efficiency. The FSO transmission system includes a directly modulated-quantum cascade laser (DM-QCL) operating at 9.1 μm and a mercury cadmium telluride (MCT) detector. We conduct small-signal characterization of the system, including the DM-QCL chip and MCT detector, evaluating the end-to-end response of both components and all associated electrical elements. For large-signal characterization, we employ a range of modulation formats, including on-off keying (OOK), 4-level pulse amplitude modulation (PAM4), and 6-level PAM (PAM6), with the objective of optimizing both the bit rate and spectral efficiency of the FSO transmission by applying pre- and post-processing equalization. At 15˚C, the studied LWIR FSO system achieves gross rates of 16 Gbps with an OOK signal and 18 Gbps with PAM4, both below the 6.25% overhead hard decision-forward error correction (6.25%-OH HD-FEC) limit, and 10 Gbps OOK below the KR-FEC limit. At 20˚C, we obtain 15 Gbps with OOK, 18 Gbps with PAM4, and 17.4 Gbps with PAM6. Furthermore, we evaluate the BER performance as a function of the decision feedback equalization (DFE) tap number to explore the role of equalization in enhancing signal fidelity and reducing errors in FSO transmission. Our findings accentuate the competitive potential of DM-QCL and MCT detector-based FSO transceivers with digital equalization for the next generation of FSO communication systems.

Integrated ultrashort pulsed lasers are widely used in the telecom wavelength range. However, the mid-infrared spectral range still lacks compact integrated ultrashort pulsed lasers. In this work, we present integrated chirped Bragg gratings for pulse compression working in the 7.8–8.2 µm wavelength range. It will be shown that group delay dispersion of a few tens of ps2 can be obtained in such spectral range. Considering an input pulse of 11.8 ps, a compression factor of 9 can thus be obtained, with an increase of the peak power of a factor 9.

We experimentally demonstrate a room-temperature LWIR FSO link with a 9.1-μm directly modulated QCL and an MCT detector. Net bitrate of up to 16.9 Gb/s is achieved at both 15°C and 20°C over a 1-meter distance.

We summarize our recent experimental studies of free-space communications enabled by directly modulated quantum cascade lasers at both MWIR and LWIR regions. Different detector types with different characteristics are compared.

This article presents low-loss mid-infrared waveguides fabricated on a Ge-rich SiGe strain-relaxed buffer grown on an industrial-scale 200 mm wafer, with propagation losses below 0.5 dB/cm for 5-7 µm wavelengths and below 5 dB/cm up to 11 µm. Investigation reveals free-carrier absorption as the primary loss factor for 5-6.5 µm and silicon multiphonon absorption beyond 7 µm wavelength. This result establishes a foundation for a scalable, silicon-compatible mid-infrared platform, enabling the realisation of photonic integrated circuits for various applications in the mid-infrared spectral region, from hazard detection to spectroscopy and military imaging.

Abstract The large mid-infrared (MIR) spectral region, ranging from 2.5 µm to 25 µm, has remained under-exploited in the electromagnetic spectrum, primarily due to the absence of viable transceiver technologies. Notably, the 8–14 µm long-wave infrared (LWIR) atmospheric transmission window is particularly suitable for free-space optical (FSO) communication, owing to its combination of low atmospheric propagation loss and relatively high resilience to turbulence and other atmospheric disturbances. Here, we demonstrate a direct modulation and direct detection LWIR FSO communication system at 9.1 µm wavelength based on unipolar quantum optoelectronic devices with a unprecedented net bitrate exceeding 55 Gbit s−1. A directly modulated distributed feedback quantum cascade laser (DFB-QCL) with high modulation efficiency and improved RF-design was used as a transmitter while two high speed detectors utilizing meta-materials to enhance their responsivity are employed as receivers; a quantum cascade detector (QCD) and a quantum-well infrared photodetector (QWIP). We investigate system tradeoffs and constraints, and indicate pathways forward for this technology beyond 100 Gbit s−1 communication.

Mid-Wave Infrared (MIR) free-space optical communication offers multiple advantages, such as improved transmission capacity through the atmosphere and immunity to electromagnetic interference. In addition, MIR transmission between 8-12 microns provides stealth for the communication signal thanks to the random thermal blackbody radiation having a strong background at these wavelengths, hence greatly reducing the probability of adversaries intercepting a MIR laser signal. Quantum Cascade Lasers (QCL) are optical sources of choice to target this wavelength domain. They are unipolar semiconductor lasers from which stimulated emission is obtained via electronic transitions between discrete energy states inside the conduction band. This work reports on a full unipolar quantum optoelectronics communication system based on a 9-micron QCL and on a Stark-effect modulator. Two different receivers are considered for high-speed detection, namely an uncooled Quantum Cascade Detector (QCD) and a nitrogen-cooled Quantum Well Infrared Photodetector (QWIP). We evaluate the maximum data rate of our link in a back-to-back (B2B) configuration before adding a multi-pass Herriott cell so as to increase the transmission length of the light path up to 31 meters. By using pulse shaping, pre- and post-processing, we reach a record bitrate both two-level (OOK) and four-level (PAM-4) modulation scheme for a 31-meter propagation link and a Bit Error Rate (BER) compatible with standard error-correction codes. Overall, we believe that our unipolar quantum system is of paramount importance for the development of cost-effective, reliable and versatile free-space optics data links.

High bitrate mid-infrared links using simple (NRZ) and multi-level (PAM-4) data coding schemes have been realized in the 8 µm to 14 µm atmospheric transparency window. The free space optics system is composed of unipolar quantum optoelectronic devices, namely a continuous wave quantum cascade laser, an external Stark-effect modulator and a quantum cascade detector, all operating at room-temperature. Pre- and post-processing are implemented to get enhanced bitrates, especially for PAM-4 where inter-symbol interference and noise are particularly detrimental to symbol demodulation. By exploiting these equalization procedures, our system, with a full frequency cutoff of 2 GHz, has reached transmission bitrates of 12 Gbit/s NRZ and 11 Gbit/s PAM-4 fulfilling the 6.25 % overhead hard-decision forward error correction threshold, limited only by the low signal-to-noise ratio of our detector.