Graham Wild

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.

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.

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.

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.

Spatial Performance of Acousto-Ultrasonic Fiber Bragg Grating Sensor

In this letter, we present results for the spatial performance of a fiber Bragg grating (FBG) sensor to continuous-wave acousto-ultrasonic (AU) signals. The FBG AU sensor is an intensity sensor, using a transmit reflect detection system. The AU sensor was used to receive actively generated continuous-wave ultrasonic signals from a PZT transducer. We present results showing the received signal strength as a function of longitudinal, lateral, and angular separation in small aluminum panels. Measurements were taken for distances of less then 100 mm and at angles from 0 to 90° between the sensor and the transducer. These results show no direct dependence between the received signal strength and the spatial separation, in the range considered. Only variations due to interference were observed.

An Intensiometric Detection System for Fibre Bragg Grating sensors

We present an intensiometric detection system for fibre Bragg grating sensors. The system is designed to directly detect intensity modulated signals, or to convert spectrally modulated signals into intensity modulated signals.

A Fibre Bragg Grating sensor as a receiver for acoustic communications signals

A Fibre Bragg Grating (FBG) acoustic sensor is used as a receiver for acoustic communications signals. Acoustic transmissions were generated in aluminium and Carbon Fibre Composite (CFC) panels. The FBG receiver was coupled to the bottom surface opposite a piezoelectric transmitter. For the CFC, a second FBG was embedded within the layup for comparison. We show the transfer function, frequency response, and transient response of the acoustic communications channels. In addition, the FBG receiver was used to detect Phase Shift Keying (PSK) communications signals, which was shown to be the most robust method in a highly resonant communications channel.

Simulation of optical delay lines for Optical Coherence Tomography

We present an analytical model which can be used to simulate different optical delays in time domain Optical Coherence Tomography (OCT). Its primary purpose is to compare the conventional moving optical delay line to quasi-stationary and stationary optical delay lines.

Numerical modelling of interrogation systems for optical fibre Bragg grating sensors

There are a number of interrogation methods that can be used in optical Fibre Bragg Grating (FBG) sensing system. For very high frequency signals interrogating the sensor signal from an FBG is limited to two intensiometric methods, edge filter detection and power detection. In edge filter detection, a broadband light source illuminates an FBG, the reflected spectrum is then passed through a spectral filter. In power detection, a narrowband light source with a wavelength corresponding to the 3dB point of the FBG is filtered by the FBG itself. Both methods convert the spectral shift of the FBG into intensity signals. These two categories each have a number of variations, all with different performance characteristics. In this work we present a numerical model for all of these interrogation systems. The numerical model is based on previous analytical modelling, which could only be utilised for perfect Gaussian profiles. However, interrogation systems can make use of non Gaussian shaped filters, or sources. The numerical modelling enables the different variations to be compared using identical component performance, showing the relative strengths and weakness of the systems in terms of useful parameters, including, signal-to-noise ratio, sensitivity, and dynamic resolution. The two different detection methods can also be compared side-by-side using the same FBG. Since the model is numerical, it enables real spectral data to be used for the various components (FBG, light source, filters). This has the added advantage of increasing the accuracy and usefulness of the model, over previous analytical work.