J.P. Dakin

Modeling and performance analysis of a fiber Bragg grating interrogation system using an acousto-optic tunable filter

The performance of a fiber grating interrogation system using an acousto-optic tunable filter (AOTF) is analyzed. An equivalent linear circuit is presented to describe the system, which tracks the wavelength of a fiber Bragg grating (FBG) sensor in a FBG sensor array. From the model, the tracking resolution of a shot-noise-limited system has been determined for both transmissive and reflective interrogation of FBGs. An optimum ratio between the bandwidths of the FBG and the AOTF has been derived. Experimental results of a tracking system are presented to validate the theoretical predictions. The theory should also be suitable for interrogation systems using other forms of tunable bandpass filters for tracking the FBG wavelength.

Distributed optical fiber sensors

This review paper covers the field of distributed optical fiber sensors, where measurements may be taken along the length of a continuous section of optical fiber. Such a feature greatly increases the information that can be obtained from a single instrument and hence the cost per sensing point can be more acceptable. The review does not attempt to cover all methods, but gives a selection of some of the more interesting theoretical concepts, describes the current status of research, and indicates where optical sensing methods are being applied in commercial instruments.

Real-time location of multiple time-varying strain disturbances, acting over a 40-km fiber section, using a novel dual-Sagnac interferometer

Updated results using a novel sensing architecture based on a Sagnac interferometer are presented and, for the first time, real-time separation and positioning of multiple disturbances has been realized. A 40-km long dual-Sagnac sensor was formed by spectral slicing of light from a single, broad-band erbium-doped-fiber super-luminescent source and wavelength division multiplexed (WDM) routing around the loop to form an inherently low loss system. Independent active phase biasing of each Sagnac was employed, allowing the use of a single optical detector. The effects of residual optical cross talk between the two Sagnacs has been accurately modeled, allowing resulting errors to be corrected. The new system has capability for narrow-band fast Fourier transform (FFT) analysis of detected disturbance signals, and hence their separation in the frequency domain. For audio-frequency excitation, an average positional resolution of 100 m over a 40-km length was achieved with a postdetection signal processing bandwidth of 8 Hz.

A low power ultra violet spectrophotometer for measurement of nitrate in seawater: introduction calibration and initial sea trials

A low power UV spectroabsorptiometer has been developed to measure the concentration of dissolved nitrate in seawater, in situ, with a rapid 1 Hz response. Measurements at a number of wavelengths in the region of 220 nm determine the concentrations of dissolved nitrate, sea salt and, possibly, dissolved organics, whilst a third channel at 300 nm acts as a reference channel to correct for light intensity changes related to lamp output and scattering by particles in seawater. This paper describes the first prototype UV nitrate sensor designed to evaluate and compare options for the light source, spectral filter and detector. The choice of components was based on a compromise between performance, cost, power consumption and convenience of use. The eventual instrument uses a xenon flashlamp light source, fused silica windows and lenses, a sample cavity, a grating spectrometer and UV enhanced silicon photodiode detectors. The power consumption of the sensor was typically from 3 to 4 W, depending on the selected repetition rate of the flashlamp source. Initial laboratory trials have shown the sensor to be unresponsive to changes in pressure allowing for continuous use to depths of up to 5000 m. Initial responses to changes in temperature have been eliminated by the adjustment of the sensors power supply and lamp flash rate. Calibration of the sensor has been performed under laboratory conditions and resulted in a second order linear fit (r2 > 0.99, p 0.21 µmol/l N03. Measurements made in situ were compared to concentrations of nitrate determined in water samples using an AAII type autoanalyser. These demonstrated the capability of the instrument to precisely determine nitrate concentrations in seawater.

Distributed grating sensors using low-coherence reflectometry

Distributed grating sensors have recently been interrogated with low-coherence reflectometry. Initial results have been enhanced using two new and versatile configurations. The first system tracks the wavelength using a closed-loop scheme, while the second system scans the distance using an open-loop approach. Arbitrary strain and temperature profiles along gratings have been examined with 300 /spl mu/m spatial resolution and 5.4 /spl mu//spl epsiv///spl radic/Hz accuracy. A theoretical model of the interrogation technique is derived and the predicted performance limits are examined experimentally.

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.

Detection of gases by correlation spectroscopy

This paper describes the detection of various common gases by means of Correlation Spectroscopy, employing a Complementary-Source-Modulation (CoSM) approach based on compact light-emitting diode (LED) sources. Theoretical results for the quantitative detection of O in air are presented, with the use of practical low cost LED sources.

Progress with optical gas sensors using correlation spectroscopy

The paper reviews recent progress on gas detection using real-time correlation spectroscopy. The general method relies on using a gas sample in a reference cell as a matched optical filter to detect, preferentially, similar absorption spectra in a measurement cell. All variations of the method have the advantage of excellent selectivity for gases with narrow line spectra, even when using broadband sources for illumination. They are also suitable for remote detection over optical-fibre leads. The recent progress is in two main areas. First, the earlier reported Stark modulation method has been extended to investigate a novel hygrometer. Secondly, we have developed a novel multi-line light source by combining a broadband optical source with a Michelson interferometer, where the latter contains a gas in one arm.

Gas sensors using correlation spectroscopy compatible with fibre-optic operation

This paper reviews methods for gas detection using real-time correlation spectroscopy. These methods involve using a gas sample in a reference cell as a matched optical filter to detect, preferentially, similar absorption spectra in a measurement cell. The methods all have the advantage of excellent selectivity, even using broadband sources, and are suitable for remote detection over optical-fibre leads. Our previously published methods using pressure and Stark modulation are reviewed. In addition, our recent theoretical treatment of the methods is presented. Finally, we describe new atmospheric-pressure measurements of gases using the method of phase-modulation spectroscopy.

Theory of dispersion in lossless multimode optical fibres

Using the ray propagation model generalized expressions for the impulse response of multimode fibres have been derived. The analysis has been applied to lossless fibres for pulses having both Gaussian and Lambertian spatial distributions as well as impulse and Gaussian temporal distributions. Detailed results are given for output pulse shapes and fibre dispersions for various configurations.