Ed Austin

Microstructured fibres for sensing applications

Microstructured fibers (MOFs) are among the most innovative developments in optical fiber technology in recent years. These fibers contain arrays of tiny air holes that run along their length and define the waveguiding properties. Optical confinement and guidance in MOFs can be obtained either through modified total internal reflection, or photonic bandgap effects; correspondingly, they are classified into index-guiding Holey Fibers (HFs) and Photonic Bandgap Fibers (PBGFs). MOFs offer great flexibility in terms of fiber design and, by virtue of the large refractive index contrast between glass/air and the possibility to make wavelength-scale features, offer a range of unique properties. In this paper we review the current status of air/silica MOF design and fabrication and discuss the attractions of this technology within the field of sensors, including prospects for further development. We focus on two primary areas, which we believe to be of particular significance. Firstly, we discuss the use of fibers offering large evanescent fields, or, alternatively, guidance in an air core, to provide long interaction lengths for detection of trace chemicals in gas or liquid samples; an improved fibre design is presented and prospects for practical implementation in sensor systems are also analysed. Secondly, we discuss the application of photonic bandgap fibre technology for obtaining fibres operating beyond silicas transparency window, and in particular in the 3μm wavelength region.

Highly Scalable Amplified Hybrid TDM/DWDM Array Architecture for Interferometric Fiber-Optic Sensor Systems

We present a distributed amplified hybrid dense wavelength division multiplexing (DWDM) and time division multiplexing (TDM) array architecture for large scale interferometric fiber-optic sensor array systems. This architecture employs a distributed Erbium doped fiber amplifier (EDFA) scheme to decrease the distribution loss among multiplexed wavelengths, and employs TDM at each wavelength to increase the total number of sensors that can be supported. The first experimental demonstration of this system is reported including results which show the potential for multiplexing and interrogating up to 4096 sensors using a single telemetry fiber pair with good system performance. The number of interrogation sensors could be further increased by increasing the number of wavelength channels. These architectures would be of great importance in the application of systems requiring very large number of sensors with limited telemetry cabling.

Theoretical analysis of a methane gas detection system, using the complementary source modulation method of correlation spectroscopy

Results from simulations of the response of a methane gas sensor using optical correlation spectroscopy (CoSp) are presented. Predictions of the sensor response, signal/noise performance and detection sensitivity are made for a typical fibre optic-coupled system. Spectral absorption data of the gases is obtained from the publicly available HITRAN database. Emphasis is placed on the effects on the detection sensitivity of varying (a) the optical filter characteristics, i.e. center wavelength and bandwidth, and (b)the concentration (partial pressure) of CH4 a theoretical noise-limited detection limit below 1 ppm is predicted. The cross-sensitivity to water vapor is derived and compared to that which would occur with a conventional broadband absorption method, i.e. one that would not exhibit the same selectivity as the CoSp method. This work is important for predicting the responsivity, sensitivity and crosstalk performance of practical CoSp gas detection systems.

Optoelectronic systems for addressing Ru oxygen sensors: their design optimization and calibration process

The paper describes research at Southampton University, aimed at optimizing the design of fibre-remoted dissolved-oxygen sensors, using immobilized fluorescent Ru2+ indicators. The design and construction of two types of fluorescence lifetime monitoring units, one type using phase-delay-monitoring and the other using photon-counting, is described. Results from a detailed theoretical study of a photon-counting RLD fluorescence lifetime sensor are presented, with specific attention to noise aspects. By numerical modeling of an analytical solution, the optimum time-window boundaries for the photon-counting system are identified. A surprising result is that the signal/noise can actually be improved by not using photon counts from all of the exponential decay, but leaving a time-gap in the measurement improves lifetime accuracy. Our previously reported Ti3+ - doped sapphire fluorescence-lifetime calibration probe is described, and a new method for RLD interrogator verification using the probe is demonstrated.

High performance architecture design for large scale fibre-optic sensor arrays using distributed EDFAs and hybrid TDM/DWDM

A distributed amplified dense wavelength division multiplexing (DWDM) array architecture is presented for interferometric fibre-optic sensor array systems. This architecture employs a distributed erbium-doped fibre amplifier (EDFA) scheme to decrease the array insertion loss, and employs time division multiplexing (TDM) at each wavelength to increase the number of sensors that can be supported. The first experimental demonstration of this system is reported including results which show the potential for multiplexing and interrogating up to 4096 sensors using a single telemetry fibre pair with good system performance. The number can be increased to 8192 by using dual pump sources.

500km remote interrogation of optical sensor arrays

We present remote measurements from a large-scale interferometric optical sensor system, using a 500km optical transmission link between interrogator and sensor array, 3 times longer than the longest reported so far to our knowledge. A phase noise floor of -80dB re 1 rad·Hz-0.5 peak was achieved (equivalent to 1 mPa·Hz-0.5). 256 sensors may be interrogated via the link using a single fibre pair, making the system highly suitable for remote interrogation of large scale sensor arrays for applications such as seismic and acoustic sensing. Eight amplified 125km spans using standard Corning SMF-28 single mode fibre form the transmission link.

Theoretical modelling studies of gas-sensing systems using correlation spectroscopy

Monitoring the concentration of gaseous O2, CO2 and CH4 is needed for many environmental, medical and industrial applications. We model the COSM method of correlation spectroscopy, where two broadband light sources are intensity modulated in antiphase, the first being directed via the measurement cell after first passing through the reference sample, the second being more directly-coupled. The subsequent difference in fractional attenuation in the measurement cell indicates the concentration of target gas in this cell. Using data from the HITRAN database, comprehensive analyses are presented to predict the optical modulation index and the signal to noise ratio at the detector, as a function of optical filter properties, and for various gas temperatures and pressures (concentrations). The predicted detection sensitivities are presented for each gas.

Investigation of the peak power enhancement available from a surface emitting GaAlAs near-infrared light emitting diode by cooling and pulsing

Light emitting diode (LED) light sources are required for many illumination and optical sensing applications, due to their compact dimensions, low cost and low power consumption. The optimization of optical power output in a specific wavelength band is often of critical importance. We are interested in using LEDs for optical sensing systems, particularly for gas sensing. We have conducted a very comprehensive engineering study to show that, by packaging a commercial surface emitting LED in a cooled heat-sunk package and optimizing the drive current for this lower temperature, the power from the LED may be substantially improved, particularly when pulsed at low duty cycle. We have found that, even with moderate cooling using a Peltier heat pump, the intensity in our required spectral band was improved by a factor of 5 compared to that attainable under normal drive conditions. We have also verified that almost all spectral changes in the LED were due to temperature changes in the emitting chip, rather than from other effects related to injection current, such as internal optical gain.

High performance fibre-optic acoustic sensor array using a distributed EDFA and hybrid TDM/DWDM, scalable to 4096 sensors

An amplified 16 channel dense wavelength division multiplexing (DWDM) array architecture is presented for interferometric fibre optic sensor array systems. This architecture employs a distributed Erbium doped fibre amplifier (EDFA) scheme to decrease the array insertion loss, and employs time division multiplexing (TDM) at each wavelength to increase the number of sensors that can be supported. The first experimental demonstration of this system is reported including results which show the potential for multiplexing and interrogating up to 4096 sensors using a single telemetry fibre pair with good system performance.

Microstructured optical fibers for gas sensing: design, fabrication, and post-fab processing

Air/silica Microstructured Optical Fibers (MOFs) offer new prospects for fiber based sensor devices. In this paper, two topics of particular significance for gas sensing using air guiding Photonic Bandgap Fibers (PBGFs) are discussed. First, we address the issue of controlling the modal properties of PBGFs and demonstrate a single mode, polarization maintaining air guiding PBGF. Secondly, we present recent improvements of a femtosecond laser machining technique for fabricating fluidic channels in PBGFs, which allowed us to achieve cells with multiple side access channels and low additional loss.