Steven Hinckley

Acousto-Ultrasonic Optical Fiber Sensors: Overview and State-of-the-Art

This paper gives a review of acoustic and ultrasonic optical fiber sensors (OFSs). The review covers optical fiber sensing methods for detecting dynamic strain signals, including general sound and acoustic signals, high-frequency signals, i.e., ultrasonic/ultrasound, and other signals such as acoustic emissions, and impact induced dynamic strain. Several optical fiber sensing methods are included, in an attempted to summarize the majority of optical fiber sensing methods used to date. The OFS include single fiber sensors and optical fiber devices, fiber-optic interferometers, and fiber Bragg gratings (FBGs). The single fiber and fiber device sensors include optical fiber couplers, microbend sensors, refraction-based sensors, and other extrinsic intensity sensors. The optical fiber interferometers include Michelson, Mach-Zehnder, Fabry-Perot, Sagnac interferometers, as well as polarization and model interference. The specific applications addressed in this review include optical fiber hydrophones, biomedical sensors, and sensors for nondestructive evaluation and structural health monitoring. Future directions are outlined and proposed for acousto-ultrasonic OFS.

Characterization of crosstalk between CMOS photodiodes

Abstract The crosstalk in CMOS photodiodes has been measured, at two wavelengths of 543 and 633 nm, by an experimental structure containing several types of photodiodes with varying dimensions. The role of the design of the junction in reducing crosstalk is studied. The measurements indicate that to reduce crosstalk it is essential to optically shield the gap between junctions and to reverse bias the adjacent junctions. Crosstalk is significantly reduced in double-junction photodiodes, but at the cost of lower quantum efficiency. The results indicate that with properly designed layout, the crosstalk may be small. However, there is a tradeoff between small crosstalk, reduced fill factor and quantum efficiency.

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.

A transmit reflect detection system for fiber Bragg grating photonic sensors

Traditional Fibre Bragg Grating (FBG) sensing systems acquire data about the measurand via the spectral response of the FBG. Edge filter methods are also used in the acquisition of data from FBGs. In edge filter systems, the spectral shift in the FBG due to the measurand is converted into an optical power change. This optical power change can then be easily measured using conventional optoelectronic devices. We demonstrate the use of a Transmit Reflect Detection System (TRDS) for Fibre Bragg Grating (FBG) sensors. The TRDS is in essence a dual edge filter detection method. In conventional edge filter detection schemes, the reflected portion of the incident spectrum is monitored to determine the change in the measurand. In the TRDS, both the transmitted and reflected portions of the input spectrum, from a narrow band light source, are utilised. The optical power of the transmitted and reflected signals are measured via two separate photoreceivers, where each generates a single edge filter signal. As the spectral response of the FBG shifts due to the measurand, the transmitted power will increase, and the reflected power will decrease, or vice versa. By differentially amplifying the transmitted and reflected components, the overall signal is increased. This results in improved sensitivity and efficiency of the photonic sensor. In this work, the FBG sensor and TRDS are used in the measuring and monitoring of temperature, force and strain. As such, results are presented for the FBG TRDS for all of the measurands.

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.

A Transmit Reflect Detection System for Fibre Bragg Grating Acoustic Emission and Transmission Sensors

A Transmit Reflect Detection System (TRDS) for a Fibre Bragg Grating (FBG) acoustic emissions and transmissions sensor is demonstrated. The TRDS utilises both the transmitted and reflected signals from the FBG. A narrow line width laser source is tuned to one of the 3 dB points of the FBG. The transmitted and reflected signals are detected, via two photoreceivers. The difference between the transmitted and reflected signals is then taken, doubling the total received signal. The TRDS improves the sensitivity and efficiency of the FBG acoustic emission and transmission sensor. The TRDS and FBG sensor was used to successfully receive actively generated ultrasonic signals, including acoustic communications signals, and to detect acoustic emissions. We include results for the sensitivity, frequency response, and transient response of the FBG sensor, and the acoustic emissions for low velocity impacts, and a lead pencil break test.

Fiber Bragg Grating Sensors for Acoustic Emission and Transmission Detection Applied to Robotic NDE in Structural Health Monitoring

Distributed acoustic emission sensors are used in structural health monitoring (SHM) for the detection of impacts and/or strain, in real time. Secondary damage may result from the initial impact or strain. This damage may include surface pitting, erosion, or cracking. This type of damage may not be detectable by the SHM system, specifically in passive fiber optic based sensing systems. The integration of non-destructive evaluation (NDE) by robots into SHM enables the detection and monitoring of a wider variety of damage. Communicating via acoustic transmissions represents a wireless communications method for use by NDE inspection robots to communicate with an integrated SHM system that does not require any additional hardware, as piezoelectric transducers are commonly used in the NDE of materials. In this paper, we demonstrate the detection of both acoustic emissions and transmissions with a fiber Bragg grating (FBG) sensor. The acoustic communications channel comprises of a piezoelectric transmitter, an aluminum panel as the transmission medium, and a FBG receiver. Phase Shift Keying was used to encode the acoustic transmissions. Results for the frequency and transient response of the channel are presented.

Modelling of device structure effects on electrical crosstalk in back illuminated CMOS compatible photodiodes

Standard CMOS fabrication processes provide the means to realize the further development of back illuminated photodiode arrays for imaging systems. We have simulated crosstalk effects in a back illuminated CMOS compatible photodiode array, and compared this effect with that predicted for front illuminated arrays, using a two dimensional simulation model. It was found that the crosstalk in back illuminated arrays is generally greater than that for front illuminated arrays with identical structure, although this effect can be reduced by decreasing the thickness of the array. The n-well junction depth had little effect on the crosstalk predicted for the back illuminated case.

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.

Pixel structure effects on crosstalk in backwall illuminated CMOS compatible photodiode arrays

CMOS imaging arrays in back-illuminated mode provide a means to realize photodiode arrays for high resolution imaging systems, provided crosstalk effects can be reduced to the level of those observed in front-illuminated arrays. In this study, we have simulated the crosstalk in back-illuminated and front-illuminated arrays as a function of different structural configurations, including the presence of biased guard electrodes in single junction photodiodes, or the development of double junction photodiodes. The results obtained show that significant crosstalk suppression can be achieved in back-illuminated arrays for these structures. The physical mechanisms responsible for electrical crosstalk, and its subsequent suppression, are explained using an absorption volume proportion concept.