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.
We characterize and compare the performance of various commercially available lasers in terms of their absolute
frequency stability, lineshape, linewidth, and frequency noise. The frequency stability, linewidth and lineshape are
evaluated using an 'optical ruler' - a carrier-envelope stabilized optical comb. The frequency noise is measured over an
extended spectral range starting from 2 Hz. The performed analysis gives data necessary when deciding which laser to
use in a particular application.
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.
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.
We present results obtained from the first all-fiber, lensless, optical correlation spectroscopy gas sensor for acetylene
(C2H2). In the reported sensing configuration, hollow-core photonic bandgap fiber (PBGF) is employed to contain all gas
samples required for optical absorption measurements. This sensor relies upon comparison of the absorption spectrum of
acetylene held in a 'reference gas cell' to that of a gas sample under test, which is contained in the 'measurement gas
cell'. Ingress of the test gas mixture into the measurement cell is achieved via femtosecond laser-machined micro-channels
running from the surface of the PBGF to its hollow core. Stable, lensless optical interrogation of the
measurement cell is guaranteed by means of arc fusion splices to standard (solid-core) single-mode fiber (SMF). The
reference cell is filled with acetylene at atmospheric pressure, and is permanently sealed at both ends by splices to SMF.
Therefore, being constructed entirely from optical fiber, both the reference and measurement gas cells are inherently
compact and coilable, and dispense with the need for lenses or other free-space optics for connection to the correlation
spectroscopy system. We quantify the acetylene concentration of various test gas mixtures and compare our sensor's
measured results with computer simulations.
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 silica's transparency window, and in particular in the 3μm wavelength region.
We describe a compact, pulsed, broadband source for sensing using ASE from Er3+ doped fibre driven by an amplitudemodulated
pump. Quiescent pumping halved rise time. The spectrum could be tailored with reflective fibre filters, eg. for C2H2 gas detection.
We present a comprehensive model of a CO2 correlation spectroscopy based gas sensor. Predictions of the sensor response for typical fiber optic-coupled systems are made, taking into account effects of noise in detected signals.
KEYWORDS: Spectroscopy, Modeling, Modulation, Carbon monoxide, Environmental monitoring, Statistical modeling, Light sources, Signal attenuation, Databases, Signal to noise ratio
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.
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.
Recent measurements are presented, using an improved, real-time, all-optical sensor for simultaneous measurement of dissolved oxygen and temperature. The sensor has a small Cr3+ - doped sapphire (ruby) thermal monitoring crystal mounted at the probe tip, to which is bonded a Ru2+-based oxygen-indicator membrane. The probe may be interrogated in real-time, using, for both temperature and oxygen monitoring, the same combination of blue LED, light source, optical filter set, photomultiplier detector and digital processor. The ruby crystal also provides a fluorescent intensity reference, for possible on-line self-testing of the interrogation hardware. By examining the relative intensities from ruby and the membrane, mechanical damage, detachment or photo-bleaching of the sensing membrane may also be recognised. Recent developments of our novel, Ti3+ - doped sapphire fluorescence-lifetime calibration probe are reported. These confirm that the fluorescence lifetime of the probe can be thermally controlled in a reliable manner. This calibration probe then allows multi-point calibration of Ru2+-chemical sensors.
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