Public road infrastructure is developed all around the world. To save resources, ensure public safety, and provide longer‐lasting road infrastructure, structural health monitoring (SHM) applications for roads have to be researched and developed. Asphalt is one of the largest used surface materials for the road building industry. This material also provides relatively easy fiber optical sensor technology installment, which can be effectively used for SHM applications—road infrastructure monitoring as well as for resource optimization when road building or their repairs are planned. This article focuses on the research of the fiber Bragg grating (FBG) optical temperature and strain sensor applications in road SHM, which is part of the greater interdisciplinary research project started at the Riga Technical University in the year 2017. Experimental work described in this article was realized in one of the largest Latvian road sites where the FBG strain and temperature sensors were installed into asphalt pavement, and experiments were carried out in two main scenarios. Firstly, in a controlled environment with a calibrated falling weight deflectometer (FWD) to test the installed FBG sensors. Secondly, by evaluating the real‐time traffic impact on the measured strain and temperature, where different types of vehicles passed the asphalt span in which the sensors were embedded. The findings in this research illustrate that by gathering and combining data from calibrated FWD measurements, measurements from embedded FBG optical sensors which were providing the essential information of how the pavement structure could sustain the load and information about the traffic intensity on the specific road section, and the structural life of the pavement can be evaluated and predicted. Thus, it enables the optimal pavement future design for necessary requirements and constraints as well as efficient use, maintenance, and timely repairs of the public roads, directly contributing to the overall safety of our transportation system.

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.

Fiber Bragg grating (FBG) optical sensors are state-of-the-art technology that can be integrated into the road structure, providing real-time traffic-induced strain readings and ensuring the monitoring of the road’s structural health. By implementing specific FBG sensors, it is possible to detect each vehicle’s axle count and the induced strain changes in the road structure. In this study, FBG sensors are embedded at the top of the 240-mm-thick cement-treated reclaimed asphalt pavement mixture layer of the road (specifically, 25 mm deep within the road). Optical sensors’ signal interrogation units are used to measure the strain and temperature and collect data of the road’s passing vehicles, starting from passenger cars that have two axles and up to heavy trucks that have six axles. Passenger cars with 2 axles generate a typical (90% events) strain of 0.8–4.1 μm/m, the 2-axle minibus 5.5–8.5 μm/m, 2–3-axle trucks 11–26 μm/m, but 4–6-axle trucks 14–36 μm/m per each axle. A large number of influencing parameters determine the pavement design leading to the great uncertainty in the prediction of the strain at the boundary between the asphalt surface and cement-treated base layers. Real-time strain and temperature measurements help to understand the actual behavior of the pavement structure under an applied load, thus assisting in validating the proposed pavement design.

Market forecasts and trends for the usage of fiber optical sensors confirm that demand for them will continue to increase in the near future. This article focuses on the research of fiber Bragg grating (FBG) sensor network, their applications in IoT and structural health monitoring (SHM), and especially their coexistence with existing fiber optical communication system infrastructure. Firstly, the spectrum of available commercial optical FBG temperature sensor was experimentally measured and amplitude-frequency response data was acquired to further develop the simulation model in the environment of RSoft OptSim software. The simulation model included optical sensor network, which is combined with 8-channel intensity-modulated wavelength division multiplexed (WDM) fiber optical data transmission system, where one shared 20 km long ITU-TG.652 single-mode optical fiber was used for transmission of both sensor and data signals. Secondly, research on a minimal allowable channel spacing between sensors’ channels was investigated by using MathWorks MATLAB software, and a new effective and more precise determination algorithm of the exact center of the sensor signal’s peak was proposed. Finally, we experimentally show successfully operating coexistence concept of the spectrum-sliced fiber optical transmission system with embedded scalable FBG sensor network over one shared optical fiber, where the whole system is feed by only one broadband light source.

This article reports an implementation of a microsphere-based optical frequency comb (OFC) generator for substitution of individual laser arrays and simulates wavelength division multiplexed passive optical network (WDM-PON) based on this OFC generator. Our proposed generator is built based on silica microsphere, barely studied for fiber optical communication systems, and is a promising solution with photonic integration capability. Kerr OFC as a multiple light source containing more than 20 spectral carriers in the fundamental mode family in the C-band and beyond is demonstrated experimentally. Four of these OFC generator carriers with the highest peak power are studied in simulated 4-channel 393 GHz spaced WDM-PON. Additionally, we show this OFC as a source of optical carriers capable of providing data transmission over most utilized fiber types in modern optical communication systems, namely, single-mode fiber (SMF), non-zero dispersion-shifted fiber (NZ-DSF), and cut-off shifted fiber (CSF). We show through the simulations that error-free data transmission is possible, providing a total transmission capacity of 40 Gbit/s by using four OFC generated carriers.

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.