Gary Allwood

Optical Fiber Sensors in Physical Intrusion Detection Systems: A Review

Fiber optic sensors have become a mainstream sensing technology within a large array of applications due to their inherent benefits. They are now used significantly in structural health monitoring, and are an essential solution for monitoring harsh environments. Since their first development over 30 years ago, they have also found promise in security applications. This paper reviews all of the optical fiber-based techniques used in physical intrusion detection systems. It details the different approaches used for sensing, interrogation, and networking, by research groups, attempting to secure both commercial and residential premises from physical security breaches. The advantages and the disadvantages of the systems are discussed, and each of the different perimeter protection methods is outlined, namely, in-ground, perimeter fence, and window and door protection. This paper reviews the progress in optical fiber-based intrusion detection techniques from the past through to the current state-of-the-art systems and identifies areas, which may provide opportunities for improvement, as well as proposing future directions in this field.

Optical Fiber Bragg grating based intrusion detection systems for homeland security

This paper describes the use of optical Fiber Bragg grating (FBG) sensors for use in various intrusion detection systems for homeland security. We show that a FBG sensor can be used effectively as an embedded in-ground acoustic sensor, sensitive enough to detect the acoustic emissions associated with walking on a concrete surface. Also, the FBG can be used as an in-ground pressure switch for intrusion detection through temporary flooring materials, such as tiles and wooden laminate. In addition, we verify the use of FBGs as in-fence perimeter breach detectors. Finally, we show how an FBG can be used as a reed switch for use in intrusion detection systems for doors and windows. The combination of the different intrusion detection techniques illustrate the versatility of FBGs in security applications, showing this single technology can be used to form a complete intrusion detection system for homeland security. Furthermore the paper details the progress made towards a real-time in-ground sensor network for advanced security applications.

Power over Fibre: Material Properties of Homojunction Photovoltaic Micro-Cells

A comparison of the conversion efficiency from optical power to electrical power for three common material homojunction photovoltaic micro-cells was performed. The device widths were varied as a function of incident wavelength such that optimum power conversions were determined whilst under illumination of monochromatic light. GaAs is the most effective material as optimum devices can be fabricated as thin as 15um thick with conversion efficiencies as high as 59%. However, GaAs is extremely expensive and has a limited wavelength response. Although Ge has the lowest conversion efficiency of 36%, it is the only material simulated that is responsive under illumination of long wavelengths above 1.0um, and may be particularly useful for specific applications as it is efficient at both 1310nm and 1550nm, where the attenuation in silica fibres is minimal. Si is a commercially viable material for the use as a photovoltaic power converter (PPC) with conversion efficiencies as high as 43% at 980nm. Lasers at this wavelength are extremely cheap to produce, as well as the cost of Silicon PPCs being minimal.

Fiber bragg grating sensors for mainstream industrial processes

This paper reviews fiber Bragg grating sensing technology with respect to its use in mainstream industrial process applications. A review of the various types of sensors that have been developed for industries such as power generation, water treatment and services, mining, and the oil and gas sector has been performed. A market overview is reported as well as a discussion of some of the factors limiting their penetration into these markets. Furthermore, the author’s make recommendations for future work that would potentially provide significant opportunity for the advancement of fiber Bragg grating sensor networks in these mainstream industries.

Distributed sensing, communications, and power in optical Fibre Smart Sensor networks for structural health monitoring

With distributed optical fibre sensors, a single source, a single detector, and a single fibre can be used for up to 1000 fibre Bragg grating sensors. However, this multiplexing architecture is not robust. Damage to any of these individual components can render the entire sensing system useless. To achieve a robust structural health monitoring system, this type of multiplexing cannot be utilised. To overcome the lack of robustness associated with multiplexing optical fibre sensors together, intelligence along with sensors needs to be distributed around a structure. Distributed Optical Fibre Smarts Sensing (DOFSS) represents a sensing architecture for the structural health monitoring of robust aerospace vehicles. The distribution of intelligence around the structure means that communications and power for the network are a significant consideration. Since optical fibre will be utilised for the sensing, then these “wired” links, can easily be utilised for power. The optical fibre links could also be utilised for the distribution of power around the sensor network. In this work, we investigate the distribution of sensing, communications, and power for DOFSS.

In-Ground Optical Fibre Bragg Grating Pressure Switch for Security Applications

In this study, a fibre Bragg grating (FBG) was embedded beneath three common flooring materials acting as a pressure switch for in-ground intrusion detection. This is achieved using an intensiometric detection system, where a laser diode and FBG were optically mismatched so that there was a static dc offset from the transmitted and reflected optical power signals. As pressure was applied, in the form of a footstep, a strain induced wavelength shift occurred that could then be detected by converting the wavelength shift into an intensity change. The change in intensity caused a significant change in the DC offset which behaved as on optical switch. This switch could easily be configured to trigger an alarm if required. The intention is to use the FBG sensor as an in-ground intrusion detection pressure switch to detect an intruder walking within range of the sensor. This type of intrusion detection system can be applied to both external (in soil, etc) and internal (within the foundations or flooring of the home) security systems. The results show that a persons footstep can clearly be detected through solid wood flooring, laminate flooring, and ceramic floor tiles.

A comparison of silicon and germanium photovoltaic power converters for power over fibre

In this study, we compare the practical implementation of both silicon and germanium Photovoltaic Power Converters (PPCs). Simulations have previously shown that silicon PPCs can produce up to 43% optical to electrical power conversion and germanium PPCs can produce conversion efficiencies as high as 22% when illuminated by 980nm light. Moreover, germanium can produce conversion efficiencies of up to 36% when illuminated by 1550nm light. Here, we compare these results to real power conversion efficiencies of off-the-shelf silicon and germanium photodiodes, producing 9.9% and 8.0% conversion efficiencies, respectively for 980nm. Furthermore, we show germanium produces conversion efficiencies up to 14.6% under illumination of 1550nm light. A discussion of the limitations is made. The results show there is a peak efficiency point corresponding to a specific input optical power. We also show that the power over fibre signal can be successfully combined with communications signals, using wavelength division multiplexing, and that the multiplexed signals can be separated without significant loss of signal, or power conversion efficiency. In addition, we investigate the affects of free space problems, such as divergence and misalignment, in both the lateral and longitudinal directions. As expected, optical alignment plays a significant role in producing maximum power conversion.

Monitoring of pressure in pipelines using externally-mounted fiber Bragg gratings

A non-intrusive helical fiber Bragg grating sensor design is proposed and tested for measuring a gas pipeline’s pressure and temperature response. A dual-FBG sensor proved to be an effective burst-detection and temperature sensor.

A Comparison of InGaAs and Ge photonic power converters for long wavelength power over fibre

Here, we present a comparison of the optical to electrical power conversion efficiency for off-the-shelf InGaAs and Ge photodiodes used as photonic power converters for power over fibre applications. The results show that, whilst the fill factor for the InGaAs photodiode is lower, the overall power conversion efficiency is better at long wavelengths due to the larger open circuit voltages.

Programmable logic controller optical fibre sensor interface module

Most automated industrial processes use Distributed Control Systems (DCSs) or Programmable Logic Controllers (PLCs) for automated control. PLCs tend to be more common as they have much of the functionality of DCSs, although they are generally cheaper to install and maintain. PLCs in conjunction with a human machine interface form the basis of Supervisory Control And Data Acquisition (SCADA) systems, combined with communication infrastructure and Remote Terminal Units (RTUs). RTUs basically convert different sensor measurands in to digital data that is sent back to the PLC or supervisory system. Optical fibre sensors are becoming more common in industrial processes because of their many advantageous properties. Being small, lightweight, highly sensitive, and immune to electromagnetic interference, means they are an ideal solution for a variety of diverse sensing applications. Here, we have developed a PLC Optical Fibre Sensor Interface Module (OFSIM), in which an optical fibre is connected directly to the OFSIM located next to the PLC. The embedded fibre Bragg grating sensors, are highly sensitive and can detect a number of different measurands such as temperature, pressure and strain without the need for a power supply.