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 μm) and the long-wave IR (LWIR, 8-12 μm)—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.

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.

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. The laser operated at the temperature settings of 15°C and 20°C. The experiment was conducted over a distance of 1 m and in a lab as a controlled environment. 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 non-return-to-zero on-off keying (NRZ-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 net bitrates of 15 Gbps with an NRZ-OOK signal and 16.9 Gbps with PAM4, both below the 6.25% overhead hard decision-forward error correction (6.25%-OH HD-FEC) limit, and 10 Gbps NRZ-OOK below the 2.7% overhead Reed-Solomon RS(528,514) pre-FEC (KR-FEC limit). At 20°C, we obtained net bitrates of 14.1 Gbps with NRZ-OOK, 16.9 Gbps with PAM4, and 16.4 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.

This article highlights the potential of photonic-assisted analog fronthaul solutions, particularly analog radio-over-fiber (ARoF) and analog radio-over-free-space-optics (ARoFSO), as prospective alternatives for the development of 6G applications. First, we present (New-Radio) NR/5G conformance testing of ARoF and ARoFSO fronthaul links, including the assessment of the error vector magnitude (EVM) and adjacent channel leakage power ratio (ACLR) to demonstrate compliance with the minimum transmitter requirements outlined by the 3rd Generation Partnership Project (3GPP) standards. Then, with focus on future 6G Distributed-MIMO (D-MIMO) networks, we conduct experimental validations of coherent joint transmissions (CJT) using ARoF and ARoFSO fronthaul links in a 2-transmitter D-MIMO network, demonstrating MIMO gains of up to 5.35 dB and that these links meet the stringent synchronization demands for CJT. These tests represent the first realizations of CJT utilizing ARoF and ARoFSO links. Finally, for consistency, we validate CJT in a 4-transmitter D-MIMO network with ARoF fronthaul links, with MIMO gains up to 9.4 dB and confirming our previous results. This evidence indicates that these technologies hold significant potential for applications in future 6G systems.

We experimentally validate coherent joint transmission (CJT) in a D-MIMO system with four transmitters using analog fronthaul and RoF links, fulfilling CJT stringent synchronization requirements. MIMO gains close to theoretical values are demonstrated.

We experimentally validate coherent joint transmission (CJT) in a D-MIMO system with four transmitters using analog fronthaul and RoF links, fulfilling CJT stringent synchronization requirements. MIMO gains close to theoretical values are demonstrated.

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.

Free space optical (FSO) communication is increasingly recognized as a critical component of future Information and Communication Technology(ICT) infrastructure, especially in non-terrestrial networks. This thesis explores the application of mid-infrared (MIR) wavelengths—specifically within themed-wave IR (MWIR, 3-5 μm) and long-wave IR (LWIR, 8-14 μm) atmospheric transmission windows—to enhance FSO system performance. Thesewavelengths are pivotal for achieving fast, reliable data transmission overlong atmospheric distances due to their reduced atmospheric absorption andscattering.Advancements in semiconductor sources and detectors that enable high speed and efficient signal transmission are essential for realizing the potential of mid-IR FSO. Unipolar quantum optoelectronics, including components such as quantum cascade lasers (QCLs), Stark modulators, quantum cascade detectors (QCDs), and quantum-well IR photodetectors (QWIPs), offer significant promise for developing advanced FSO systems. Additionally, the use of advanced modulation formats, such as pulse amplitude modulation (PAM)and discrete multi-tone (DMT), combined with intensity modulation direct detection (IM/DD) techniques, further enhances system performance. The integration of digital signal processing (DSP) is also explored to mitigate channel impairments and optimize the overall transmission quality. This work provides a comprehensive analysis of these technologies through subsystem and system-level experiments, demonstrating the feasibility of such optoelectronic components in achieving robust transmitter and receiver performance under controlled laboratory conditions. It addresses the major challenges and considerations necessary for transitioning these technologies from theoretical and experimental stages to practical deployment. In conclusion, this thesis not only enhances the understanding of MIRIM/DD techniques in FSO but also sets the stage for future research that could pave the way for widespread adoption of mid-infrared FSO technologies in and real-world applications, aiming at a transformative impact on global communications infrastructures.

We outline our recent experimental studies on free-space optical (FSO) communications enabled by directly modulated quantum cascade lasers (DM-QCL) operating in the mid-wave infrared (MWIR, 3-5 μm) and long-wave infrared (LWIR, 8-12 μm) regions. Our research compares the performance of unipolar quantum optoelectronic (UQO) devices, including the QCL, and various detector types, such as quantum cascade detectors (QCD) and quantum well-infrared photodetectors (QWIP), each with distinct characteristics. In the last section, we get into the FSO experimental setup and reported results in the atmospheric windows.

The sixth generation (6G) of mobile networks will enable novel services and applications but new challenges are expected to arise as well. As a result, novel implementation techniques need to be developed. Among these, distributed multiple-input multiple-output (D-MIMO) is a promising wireless technology that can provide higher coverage and spectral efficiency, both of which are essential to fulfill some of the new 6G requirements. To demonstrate the feasibility of DMIMO technology, we experimentally validate coherent joint transmission (CJT) in a D-MIMO network that uses analog radio-over-fiber (ARoF) fronthaul links and centralized processing, achieving signal-to-noise ratio (SNR) gains of up to 10.54 dB.