Free-space Optical Transceivers Research Articles

Advancing LWIR FSO communication through high-speed multilevel signals and directly modulated quantum cascade lasers.

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.

Performance Analysis of Multi-Hop Hybrid FSO/mm Wave Communication System for Next-Generation Wireless Networks

Next-generation wireless networks are facing increasing demand for high data rates, low latency, and seamless connectivity. To address these challenges, a multi-hop hybrid communication system integrating Free Space Optics (FSO) and millimeter wave (mm Wave) technologies for backhaul communication is proposed. This system combines the advantages of FSO, such as high bandwidth and low latency, with the robustness and reliability of mm Wave technology. The multi-hop architecture enables the formation of a network of interconnected nodes, providing improved coverage and flexibility. Each node is equipped with FSO and mm Wave transceivers, allowing for seamless handovers and adaptive routing. For the proposed system, analytical formulas for outage probability (OP) and average bit error rate (ABER) are derived and validated using simulations. The Monte Carlo simulations provide evidence of enhanced performance in the proposed Multi-Hop Hybrid FSO/mm Wave (MHFM) system compared to the FSO system.

Beam Size Optimization for High-Altitude Platforms to Ground Links in FSO Communications

Free-space optical (FSO) communication has been used in practice mainly for short-distance transmission because it requires light of sight (LoS) between the transmitter and receiver. For long-distance communication, to avoid terrestrial obstacles, high-altitude platforms (HAPs) flying at stratosphere are used to carry intermediate FSO transceivers which relay data through several hops from the source to the destination stations. A HAP can communicate with a large ground area if its FSO transceiver projects a wide beam onto the ground. However, an excessively large beam makes the FSO transceiver consume a lot of energy. This study investigates the problem of finding individual optimal beam sizes for FSO transceivers on HAPs so that the total cost of the HAP network, including the amortization, energy, and maintenance costs, is minimized. An optimization algorithm was proposed and implemented. The simulation results show the network designed by the algorithm achieves a nearly optimal number of HAPs, leading to a low network cost.

High Spectral Efficiency Long-Wave Infrared Free-Space Optical Transmission With Multilevel Signals

This study explores the potential of long-wave infrared free-space optical (FSO) transmission that leverages multilevel signals to attain high spectral efficiency. The FSO transmission system consists of a directly modulated-quantum cascade laser (DM-QCL) operating at 9.15 μm and a mercury cadmium telluride (MCT) detector. To fully understand the system, we conduct measurements on the DM-QCL chip and MCT detector and assess the overall amplitude response of the DM-QCL, MCT detector, and all electrical components. We apply various signals, including on-off keying (OOK), 4-level pulse amplitude modulation (PAM4), 6-level PAM (PAM6), and 8-level PAM (PAM8) to maximize the bit rate and spectral efficiency of the FSO transmission. Through a two-dimensional sweeping of the laser bias current and MCT detector photovoltage, we optimize the transmission performance. At the optimal operation point, the FSO system achieved impressive results which are up to 6 Gbaud OOK, 3.5 Gbaud PAM4, 3 Gbaud PAM6, and 2.7 Gbaud PAM8 signal transmissions, with a bit error rate performance below 6.25% overhead hard decision-forward error correction limit when the DM-QCL operates at 10°C. We also evaluate the eye diagrams and stability of the system to showcase its remarkable transmission performance. Our findings suggest that the DM-QCL and MCT detector-based FSO transceivers offer a highly competitive solution for the next generation of optical wireless communication systems.

Adaptive Transceiver Design for High-Capacity Multi-Modal Free-Space Optical Communications With Commercial Devices and Atmospheric Turbulence

In this paper, to achieve higher robustness against atmospheric turbulence for high-capacity free-space optical (FSO) communications, an adaptive multi-modal FSO transceiver has been designed and experimentally demonstrated. We show that based on the dynamically estimated channel state information, modulation formats and power for different transmit modes can be adaptively allocated at the transmitter side. Meanwhile, at the receiver side, we show that the most suitable multi-input/multi-output decoder can be selected and employed to meet the requirement of forward error correction at the minimal expense of power consumption. By employing time-division multiplexed transmitter and receiver emulation and a spatial light modulator for turbulence emulation, an aggregate data rate of 590 Gbit/s/wavelength has been achieved when suffering from strong atmospheric turbulence, verifying the feasibility of the proposed adaptive transceiver over a turbulent FSO link. Moreover, to demonstrate practical applicability, all key devices such as transponder, multiplexer, and demultiplexer are commercially available in this work.

Latency Aware Transmission Scheduling for Steerable Free Space Optics

Free space optics (FSO) has been demonstrated as a secure and high-speed solution for long distance and line-of-sight wireless communications. Applying FSO as fronthaul/backhaul communications between base stations (BSs) and the gateway can significantly increase the fronthaul/backhaul link capacity. Traditionally, the gateway has to be equipped with multiple FSO transceivers, each of which is used to communicate with a BS by establishing a dedicated FSO. In this paper, we propose to use a steerable FSO system, where the gateway is equipped with a steerable FSO transceiver to communicate with multiple FSO transceivers at different BSs in a time division multiplexing manner. Applying the steerable FSO system can reduce the number of FSO transceivers at the gateway, and thus reduce the capital cost of implementing an FSO based fronthaul/backhaul network. We formulate the transmission scheduling problem in the steerable FSO system to optimize the active time for each FSO link associated with the steerable FSO transceiver such that the overall delay of transmitting a packet from geo-distributed BSs to the steerable FSO transceiver at the gateway is minimized, while the latency requirements of the BSs are met. We propose the laTency aWare transmIssion Scheduling for sTeerable FSO (TWIST) algorithm to efficiently solve the proposed problem.

High-Speed 9.6-μm Long-Wave Infrared Free-Space Transmission With a Directly-Modulated QCL and a Fully-Passive QCD

Free-space optics (FSO) in the mid-infrared (mid-IR) contains rich spectral resources for future ultrahigh-speed wireless communications yet is currently under-exploited. Two atmospheric transmission windows at the mid-IR, namely, the mid-wave IR (MWIR, 3–5 μm) and the long-wave IR (LWIR, 8–12 μm), show great potential in supporting free-space communications for both terrestrial and space application scenarios. Particularly, the LWIR signal with a longer wavelength has high intrinsic robustness against aerosols’ scattering and turbulence-induced scintillation and beam broadening effects, which are the main concerns hindering the wide deployment of practical FSO systems. In this context, high-bandwidth semiconductor-based mid-IR FSO transceivers will be desirable to meet the requirements of low energy consumption and small footprints for large-volume development and deployment. Quantum cascade devices, including quantum cascade lasers (QCLs) and quantum cascade detectors (QCDs), appear promising candidates to fulfill this role. In this work, we report a high-speed LWIR FSO transmission demonstration with a 9.6-μm directly-modulated (DM)-QCL and a fully passive QCD without any active cooling or bias voltage. Up to 8 Gb/s, 10 Gb/s, and 11 Gb/s signal transmissions are achieved when operating the DM-QCL at 10 °C, 5 °C, and 0 °C, respectively. These results indicate a significant step towards an envisioned fully-connected mid-IR FSO solution empowered by the quantum cascade semiconductor devices.

Free‐space optical communication assisted emergency communication scheme against multiple fiber link failures in low‐edge‐connected optical networks

AbstractThe optical network may become disconnected under fiber link failures if its edge connectivity is not greater than the number of link failures, this will make the transmissions of many services to be interrupted. In the optical network with lengths of fiber links from several kilometers to more than 10 km, free‐space optical (FSO) links can provide considerable capacity for the transmissions of large‐bandwidth services. Using FSO links to establish the emergency communication path for uninterrupted transmission can be seen as a viable scheme. In this paper, we propose the FSO communication assisted emergency communication (FSOC‐EC) scheme against multiple fiber link failures in the low‐edge‐connected optical network. Using FSO links and fiber links to establish emergency communication paths can achieve a quick recovery for communication. The layout of FSO transceivers can be seen as a classic NP‐hard problem. To reduce capital expenditure (CAPEX), we propose an integer linear programming (ILP) model and a heuristic algorithm for the layout of FSO transceivers. And to improve the protection success rate (PSR) of services, we propose an ILP model and a heuristic algorithm to pre‐allocate the wavelengths on FSO links. We consider the adverse influence of rainfall on FSO links and propose the corresponding schemes to alleviate this on FSOC‐EC. Simulation results show that adding available FSO links in different locations can alleviate the adverse influence of rainfall and increase the network connected rate (NCR) under multiple fiber link failures, and FSOC‐EC can greatly improve PSR.

Survivable free space optical mesh network using high-altitude platforms

Free space optical (FSO) communication refers to the information transmission technology based on the propagation of optical signals in space. FSO communication requires that the transmitter and receiver directly see each other. High-altitude platforms (HAPs) have been proposed for carrying FSO transceivers in the stratosphere. A multihop HAP network with FSO links can relay traffic between ground FSO nodes. In this study, we propose an end-to-end switching model for forwarding traffic between massive pairs of ground FSO nodes over a HAP network. A protection mechanism is employed for improving the communication survivability in the presence of clouds, which may break the line of sight (LoS) between HAPs and ground nodes. We propose an algorithm to identify a set of necessary HAPs and to design the HAP network topology in integrating the protection mechanism. The design aims to a network with the minimal equipment cost. The results demonstrate that, even though networks with survivable capacity use more resources, they are not necessary much more expensive than those without survivability in terms of equipment, i.e., HAPs and FSO devices, and in terms of wavelength resource utilization.

Direct Modulation and Free-Space Transmissions of up to 6 Gbps Multilevel Signals With a 4.65-$\mu$m Quantum Cascade Laser at Room Temperature

A roadmap for future wireless communications is expected to exploit all transmission-suitable spectrum bands, from the microwave to the optical frequencies, to support orders of magnitude faster data transfer with much lower latency than the deployed solutions nowadays. The currently under-exploited mid-infrared (mid-IR) spectrum is an essential building block for such an envisioned all-spectra wireless communication paradigm. Free-space optical (FSO) communications in the mid-IR region have recently attracted great interest due to their intrinsic merits of low propagation loss and high tolerance of atmospheric perturbations. Future development of viable mid-IR FSO transceivers requires a semiconductor source to fulfill the high bandwidth, low energy consumption, and small footprint requirements. In this context, quantum cascade laser (QCL) appears as a promisingtechnological choice. In this work, we present an experimental demonstration of a mid-IR FSO link enabled by a 4.65-μm directly modulated (DM) QCL operating at room temperature. We achieve a transmission data rate of up to 6 Gbps over a 0.5-m link distance. This achievement is enabled by system-level characterization and optimization of transmitter and receiver power level and frequency response and assisted with advanced modulation and digital signal processing (DSP) techniques. This work pushes the QCL-based FSO technology one step closer to practical terrestrial applications, such as the fixed wireless access and the wireless mobile backhaul. Such a QCL-based solution offers a promising way towards the futuristic all-spectra wireless communication paradigm by potentially supporting the whole spectrum from the MIR to the terahertz (THz).

Multi-element full-duplex FSO transceiver design using optimum tiling

With ever-growing demand of high-speed mobile data and in vision of smart cities, optical wireless, a.k.a. free-space optical (FSO), communication deem to be a critical technology due to its significantly faster data transfer rate, higher security, lower costs, and reduced power usage. These beneficial effects have led to interest in exploring FSO technologies for mobile platforms such as drone-based cell tower to provide Internet services to remote areas, or even for wireless communications on the ground with jammed radio channels as in battlefields. Direct line-of-sight (LOS) is required to establish secure directional FSO communication (FSOC) links which are highly susceptible to random and erratic movements of the mobile nodes as well as the turbulence in the free-space medium. The performance of FSOC links can be improved by designing multi-element tiling of the laser-based transceivers which is capable of in-band full-duplex (IBFD) communication. In this work, we propose a genetic algorithm framework to explore optimized multi-element FSO transceiver tiling patterns to ensure maximal signal-to-interference and noise ratio (SINR) and minimize the effects of vibration of the mobile platform and atmospheric turbulence.

Adaptive optical beam alignment and link protection switching for 5G-over-FSO.

Free-space optics (FSO) convey an enormous potential for ultra-high-capacity seamless fiber-wireless transmission in 5G and beyond communication systems. However, for its practical exploitation in future deployments, FSO still requires the development of very high-precision and robust optical beam alignment. In this paper, we propose two different methods to achieve tight, precise alignment between a pair of FSO transceivers, using a gimbal-based setup. For scenarios where there is no information about the system, a black-box artificial intelligence (AI)-based method resorting to particle swarm optimization (PSO) is presented, enabling to autonomously align the system with a success rate above 96%, converging from a blind starting position. Alternatively, for scenarios with partial information about the FSO system, we propose a tailored custom algorithm, with a success rate of 92%, but with a ∼4 × reduction on the alignment time. The automatic alignment is then validated in a 5G-like fiber-FSO scenario, transmitting a 16 × 400 MHz signal and achieving a maximum bit-rate of 30 Gbps. Moreover, we propose the implementation of a fail-safe mechanism with a backup FSO receiver, thereby providing an extra degree of robustness towards temporary events of strong degradation on the FSO channel or line-of-sight (LOS) interruption.

A Point-to-Multi-Point Tracking System for FSO Communication

Free space optics (FSO) is considered primarily for establishing point-to-point (PtP). However, due to the increasing demand for mobile platforms and mechanisms to establish point-to-multipoint (PtM) connections, this research proposes and implements a novel PtM tracking mechanism for FSO communication. The alignment mechanism consists of two tracking systems, i.e., coarse tracking and fine-tuning. The coarse tracking is responsible to locate FSO transceivers placed at different positions by using magnetometer sensors connected to the cloud. On the other hand, the fine-tuning mechanism overcomes high-frequency amplitude oscillation or jitter with a displacement range of 8 cm. The link is continuously monitored and if the link is lost or not established after a pre-determined time, then the reference line-of-sight (LoS) angle is updated in the cloud to provide an alternate connection. The stability of the closed-loop system is analyzed in terms of root locus and step response. The proposed PtM system is experimentally validated with FSO terminals located at different locations and distances over indoor and outdoor environments. The proposed method achieved an alignment time of less than 5 s for ten degrees of misalignment.

Bidirectional MDRZ Downstream and NRZ OOK Upstream SS-WDM RoFSO Communication System

Abstract A cost-effective spectrum sliced wavelength division multiplexing radio over free space (SS-WDM RoFSO) full duplex communication system architecture has been proposed and analyzed. For the downlink, a 10 Gbits/s high spectrally efficient and amplitude maintaining modified duobinary return to zero (MDRZ) modulated the filtered slices from the light source. The free space optical (FSO) channel was modeled by the well-known Gamma-Gamma model and was further subjected to moderate rain attenuation conditions. Four FSO transceivers were used to mitigate against laser beam scintillation, geometrical losses and rain attenuation. For uplink, a reflective semiconductor optical amplifier (RSOA) was used both as a modulator and an amplifier. Its amplitude squeezing effect was used to erase the information in the MDRZ modulated downstream seeding wavelength for uplink transmission of 10 Gbits/s NRZ OOK signals. At a bit error rate (BER) of 1.262 × 10 − 9 $1.262\times 10^{-9}$ , our proposed architecture revealed that 10 Gbits/s symmetric SS-WDM RoFSO can achieve 4.167 km of free space transmission. Power penalty of approximately 0.2 and 3 dB was achieved for downstream and upstream, respectively, which assures the feasibility of the proposed architecture. Such a system is meant to co-exist with the forthcoming fifth generation long-term evolution urban systems.

Reducing the Impact of Handovers in Ground-to-Train Free Space Optical Communications

The growing deployment and advanced development of high-speed train (HST) systems, coupled with the reliance on and demand for constant Internet connectivity anytime anywhere, have necessitated the imminent provisioning of broadband Internet services in HSTs. Ground-to-train free space optical (FSO) communications suffer from frequent handovers due to high mobility of HSTs, thus shortening the connection time between the train and ground, and greatly impacting passengers' user experience. To provision high-speed Internet services in HSTs, we propose the rotating transceiver scheme to mitigate the impact of handover processes with steerable FSO transceivers in a ground-to-train communications system. Owing to the rotating feature of the scheme, the on-roof transceiver of a train can maintain a reliable link with a base station (BS) for a longer time, and remarkably reduce the number of handover processes, thus provisioning higher data rates of the system. Meanwhile, since the separation distances between BSs are extended, the total number of BSs required to be deployed along the track is also reduced, thus reducing the deployment cost for service providers. Moreover, a new handover method is designed to mitigate the impact of each handover delay, especially on live streaming applications. Finally, the performance improvement of the proposed scheme over the existing fixed transceiver one is demonstrated via simulation results.

Sum rate–based relay selection strategy in FSO systems

SummaryWe present a relay selection strategy for bidirectional free‐space optical (FSO) communication systems. A parallel relayed FSO network consisting of multiple decode‐and‐forward protocol based two‐way relays (TWRs) is considered. Using the proposed relay selection strategy, one TWR is selected to establish an FSO communication link between two non‐line‐of‐sight FSO transceivers. As the two‐way communication process is characterized by achievable sum rate of the system, the relay selection strategy is designed such that the achievable sum rate is maximum. Considering a Gamma‐Gamma distributed optical channel with path loss and pointing errors, we present a mathematical framework for the proposed relay selection strategy.

Topology Design and Cross-Layer Optimization for FSO Mesh Networks Impaired by Atmospheric Turbulence and Misalignment Fading

In this paper, we design and optimize freespace optics (FSO) mesh networks over atmospheric turbulence and misalignment fading channels. We propose a heuristic algorithm to choose the best sites to install the FSO transceivers and also the best topology for a given traffic matrix in a region. The algorithmaims to use as few FSO links as possible since this allows us to reduce the network costs. In addition, the algorithm takes into account the influence of turbulence, misalignment, and noise by choosing links with a low bit-error rate (BER). Besides the heuristic, we also propose an optimal integer linear programming model for solving the same problem. The simulation results show that the proposed heuristic runs very fast, even with a large number of choices of FSO sites. Therefore, it is practical to use the heuristic to quickly design a restoration communication network for replacing a regular one affected by a disaster. The average BER of all FSO links of the designed network also meets the requirement of an end-to-end BER threshold.

On Divergence-Angle Efficiency of a Laser Beam in Free-Space Optical Communications for High-Speed Trains

We compare narrow and wide laser beams in a free-space optical (FSO) communications system for high-speed train communications and analyze the tradeoffs among the received power, coverage area, and the complexity of the acquisition-tracking-pointing (ATP) mechanism. We propose to employ a divergence angle of a wide beam in the range [0.07 $^{\circ }$ , 2.002 $^{\circ }$ ] to relax the steering speed of the fast steering mirror, which is a major component of the ATP mechanism in an FSO transceiver. In addition, we overcome the negative effects of train vibrations by using a proposed divergence-angle range for a wide beam. The proposed range of divergence angles provides a large link range and effective coverage length, and contact time as compared to a narrow beam.

Provisioning Internet Access Using FSO in High-Speed Rail Networks

The growing deployment and advanced development of high-speed train (HST) systems, coupled with the reliance on and demand for constant Internet connectivity any time any where, have necessitated the imminent provisioning of broadband Internet services in HSTs. Train-to-ground communications mostly suffers from frequent handoffs due to the high mobility of the trains. To provision high-speed Internet services in HSTs, we propose a novel solution named Free-space OptiCs Utilization in high-Speed trains (FOCUS). Our solution takes advantage of the high optical bandwidth of free space optics (FSO) to provide high data rates. FOCUS aims to improve the trainto- ground communications system by deploying on-roof FSO transceivers. To minimize the number of handoff processes, the control unit inside the train forwards each request to go through the proper transceiver. The proposed mechanism decreases the packet loss ratio and increases network performance in high-speed train communications.

GPS-Free Maintenance of A Free-Space-Optical Link Between Two Autonomous Mobiles

Free-Space-Optical (FSO) communication has the potential to provide optical-level wireless communication speeds. It can also help solve the wireless capacity problem experienced by the traditional RF-based technologies. Despite its capacity advantages, FSO communication is prone to mobility. Since the FSO transceivers are highly directional, they require establishment and maintenance of line-of-sight (LOS) between each other. We consider two autonomous mobile nodes, each with one FSO transceiver mounted on a movable head capable of rotating 360 degree. We propose a novel scheme that deals with the problem of automatic maintenance of LOS alignment between the two nodes with mechanical steering of the FSO transceivers. We design protocols to maintain an FSO link between the mobiles satisfying a minimum received power or signal-to-noise ratio (SNR). We also present a prototype implementation of such mobile node with FSO transceivers. The effectiveness of the alignment protocol is evaluated by analyzing the results obtained from both simulations and also experiments conducted using the prototype. The results show that, by using such mechanically steerable transceivers and a simple auto-alignment mechanism, it is possible to maintain optical wireless links in a mobile setting with nominal disruption.