Distributed fiber-optic vibration sensors receive extensive investigation and play a significant role in the sensor panorama. Optical parameters such as light intensity, phase, polarization state, or light frequency will change when external vibration is applied on the sensing fiber. In this paper, various technologies of distributed fiber-optic vibration sensing are reviewed, from interferometric sensing technology, such as Sagnac, Mach-Zehnder, and Michelson, to backscattering-based sensing technology, such as phase-sensitive optical time domain reflectometer, polarization-optical time domain reflectometer, optical frequency domain reflectometer, as well as some combinations of interferometric and backscattering-based techniques. Their operation principles are presented and recent research efforts are also included. Finally, the applications of distributed fiber-optic vibration sensors are summarized, which mainly include structural health monitoring and perimeter security, etc. Overall, distributed fiber-optic vibration sensors possess the advantages of large-scale monitoring, good concealment, excellent flexibility, and immunity to electromagnetic interference, and thus show considerable potential for a variety of practical applications.

https://www.mdpi.com/1424-8220/16/8/1164/pdf

Distributed Fiber-Optic Sensors for Vibration Detection

The design and preliminary performance results of a fiber-optic (FO) Michelson interferometric hydrophone are described.

Fiber-Optic Hydrophone

Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.

OFDM for Optical Communications

The constant miniaturization of hardware and an increase in power efficiency, have made possible the integration of intelligence into ordinary devices. This trend of augmenting so-called non-intelligent everyday devices with computational capabilities has led to the emergence of the Internet of Things (IoT) domain. With a wide variety of applications, such as home automation, smart grids/cities, and critical infrastructure management, the IoT systems make compelling targets for cyber-attacks. In order to effectively compromise these systems, adversaries employ different advanced persistent threat (APT) methods, with one such sophisticated method, being botnets. By employing a plethora of infected machines (bots), attackers manage to compromise the IoT systems and exploit them. Prior to the appearance of the IoT domain, specialized digital forensics mechanisms were developed, in order to investigate Botnet activities in small-scale systems. Since IoT enabled botnets are scalable, technologically diverse and make use of current high-speed networks, developing forensic mechanisms capable of investigating the IoT Botnet activities has become an important challenge in the cyber-security field. Various studies have proposed, deep learning as a viable solution for handling the IoT generated data, as it was designed to handle diverse data in large volumes, requiring near real-time processing. In this study, we provide a review of forensics and deep learning mechanisms employed to investigate botnets and their applicability in the IoT environments. We provide a new definition for the IoT, in addition to a taxonomy of network forensic solutions, that were developed for both conventional, as well as, the IoT settings. Furthermore, we investigate the applicability of deep learning in network forensics, the inherent challenges of applying network forensics techniques to the IoT, and provide future direction for research in this field.

Forensics and Deep Learning Mechanisms for Botnets in Internet of Things: A Survey of Challenges and Solutions

Distributed sensing systems can transform an optical fiber cable into an array of sensors, allowing users to detect and monitor multiple physical parameters such as temperature, vibration and strain with fine spatial and temporal resolution over a long distance. Fiber-optic distributed acoustic sensing (DAS) and distributed temperature sensing (DTS) systems have been developed for various applications with varied spatial resolution, and spectral and sensing range. Rayleigh scattering-based phase optical time domain reflectometry (OTDR) for vibration and Raman/Brillouin scattering-based OTDR for temperature and strain measurements have been developed over the past two decades. The key challenge has been to find a methodology that would enable the physical parameters to be determined at any point along the sensing fiber with high sensitivity and spatial resolution, yet within acceptable frequency range for dynamic vibration, and temperature detection. There are many applications, especially in geophysical and mining engineering where simultaneous measurements of vibration and temperature are essential. In this article, recent developments of different hybrid systems for simultaneous vibration, temperature and strain measurements are analyzed based on their operation principles and performance. Then, challenges and limitations of the systems are highlighted for geophysical applications.

A Review of Hybrid Fiber-Optic Distributed Simultaneous Vibration and Temperature Sensing Technology and Its Geophysical Applications

For many years, passive optical networks (PONs) have received a considerable amount of attraction regarding their potential for providing broadband connectivity to almost every citizen, especially in remote areas where fiber optics can attract people to populate regions that have been abandoned. Error-free connectivity without dropouts can offer new opportunities to communicate, earn money and enjoy cultural events. Transmission speeds are multigigabit with distances of a few tens of kilometers; these specifications were previously reserved for high-speed and long-haul backbone networks. PONs can also support a new class of applications, such as accurate time transfer or distributed fiber sensing and follow new trends in open networking. An outline of past and current standards and standards that have been proposed for the latest generation of multigigabit PONs is provided.

/pdf/passive-optical-networks-progress-a-tutorial-2pqx20qpe9.pdf

Passive Optical Networks Progress: A Tutorial

A sensing system utilizing a standard optical fiber as a distributed sensor for the detection and localization of mechanical vibrations is presented. Vibrations can be caused by various external factors, like moving people, cars, trains, and other objects producing mechanical vibrations that are sensed by a fiber. In our laboratory we have designed a sensing system based on the Φ-OTDR (phase sensitive Optical Time Domain Reflectometry) using an extremely narrow laser and EDFAs.

/pdf/distributed-fiber-optic-sensor-for-detection-and-1udmk1sd01.pdf

Distributed Fiber-Optic Sensor for Detection and Localization of Acoustic Vibrations

The infrastructure essentialities for accurate time and stable frequency distribution are presented. Our solution is based on sharing fibers for a research and educational network carrying live data traffic with time and frequency transfer in parallel. Accurate time and stable frequency transmission uses mainly dark channels amplified by dedicated bidirectional amplifiers with the same propagation path for both directions of transmission. This paper targets challenges related to bidirectional transmission, particularly, directional nonreciprocities.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-56/issue-2/027101/Joint-accurate-time-and-stable-frequency-distribution-infrastructure-sharing-fiber/10.1117/1.OE.56.2.027101.pdf

Joint accurate time and stable frequency distribution infrastructure sharing fiber footprint with research network

Simultaneous transmission of accurate time, stable frequency, data, and sensor system over one fiber with ITU 100 GHz grid

The article deals with the Transmission Convergence Layer in 10 Gigabit passive optical networks. There is a description of conditions, under which the Optical Network Units (ONU) pass through, when is activated. Additionally, this article focuses on the processes of connecting ONUs to the Passive Optical Networks (PONs). The simulations demonstrate that as the number of simultaneous connections to ONU increase, longer wait time for each additional connection becomes. This also solves the collision conditions, that might arise in communication and the Equalization Delay (EqD). Finally, the article deals with the influence of a refractive index on timing relationships. The speed up of the frame transfer between Optical Line Terminal (OLT) and ONUs has been proven.

Tomas Horvath

Papers

Passive Optical Networks Progress: A Tutorial

Distributed Fiber-Optic Sensor for Detection and Localization of Acoustic Vibrations

Joint accurate time and stable frequency distribution infrastructure sharing fiber footprint with research network

Simultaneous transmission of accurate time, stable frequency, data, and sensor system over one fiber with ITU 100 GHz grid

Transmission convergence layer in XG-PON