This PDF file contains the front matter associated with Proceedings of SPIE Volume 6619, including the Title Page, Copyright information, Table of Contents, Conference Committees listing, Introduction, and Conference Sponsors listing.

Recent technical advances in fiber optic sensor technology have brought fiber sensors into the mainstream. Using a wide variety of sensing elements, and interrogation techniques, these devices are finding applications in fields from power line management to homeland security. A variety of fiber sensor technologies, applications, and markets are discussed.

For a long time electric power was taken as a natural unlimited resource. With globalisation the demand for energy has risen. This has brought rising prices for fossil fuels, as well as a diversification of power generation. Besides conventional fossil, nuclear plants are coming up again. Renewable energy sources are gaining importance resulting in recent boom of wind energy plants. In the past reliability and availability and an extremely long lifetime were of paramount importance. Today this has been added by cost, due to the global competition and the high fuel costs. New designs of power components have increased efficiency using lesser material. Higher efficiency causes inevitably higher stress on the materials, of which the machines are built. As a reduction of lifetime is not acceptable and maintenance costs are expected to be at a minimum, condition monitoring systems are going to being used now. This offers potentials for fibre optic sensor application.

Photonic sensing technology and in particular the field of fiber optic sensing, has entered a period of dynamic market growth. Studies of anticipated world demographic shifts and climate change projections suggest areas in which significant opportunities for the use of the technology exist and will continue to grow. In this paper, the general currents and trends in photonic/fiber optic sensor technology will be discussed with a focus upon fiber optic biosensors. In 2004, the fiber optic biosensor market was estimated to be in the $6-8B range, while this year, its value is projected to be in the $7-11B range, an annual growth rate of between 6 and 10% during the period. The market is substantial with continued growth expected over the next 5 years at least. It is large enough and growing fast enough to offer a significant opportunity for penetration by appropriate fiber optic biosensor products.

Over the last few years, optical fiber sensors have seen increased acceptance and widespread use. Among the multitude of sensor types, FBG based sensors, more than any other particular sensor type, have become widely known and popular. Given their intrinsic capability to measure a multitude of parameters such as strain, temperature, pressure, chemical and biological agents - and many others - coupled with their flexibility of design to be used as single point or multi-point sensing arrays and their relative low cost, make of FBGs ideal devices to be adopted for a multitude of different sensing applications and implemented in different fields and industries. However, some technical hurdles and market barriers need to be overcome in order for this technology - and fiber sensors in general - to gain more commercial momentum and achieve faster market growth such as the need for industry standards on FBGs and FBG-based sensors, adequate packaging designs, as well as training and education of prospective customers and end-users.

Different experience from fibre sensor applications on-site has revealed the need of guidelines for developers, manufacturers, suppliers as well as users. Although best knowledge and well-founded experience are available in the fibre sensor community, a lack of technical instructions for all persons involved can lead to unreliable measurement results or even damage of sensing components. In contrast, manufacturing and application procedures according to generally binding technical rules will ensure long-term stable sensor systems and reliable measurement results. These rules have to include definitions of particular terms to describe the sensor and sensor system characteristics as well as methods to validate and use sensors properly. Because FBG sensors are the most popular sensing techniques in very different fields, important aspects to structure a set of technical recommendations will be discussed, especially for the use of FBG sensors in experimental stress analysis and for structure monitoring.

In the late 1970s one of the first applications identified for fibre-optic sensing was the fibre-optic hydrophone. It was recognised that the technology had the potential to provide a cost effective solution for large-scale arrays of highly sensitive hydrophones which could be interrogated over large distances. Consequently both the United Kingdom and United States navies funded the development of this sonar technology to the point that it is now deployed on submarines and as seabed arrays. The basic design of a fibre-optic hydrophone has changed little; comprising a coil of optical fibre wound on a compliant mandrel, interrogated using interferometric techniques. Although other approaches are being investigated, including the development of fibre-laser hydrophones, the interferometric approach remains the most efficient way to create highly multiplexed arrays of acoustic sensors. So much so, that the underlying technology is now being exploited in civil applications. Recently the exploration and production sector of the oil and gas industry has begun funding the development of fibre-optic seismic sensing using seabed mounted, very large-scale arrays of four component (three accelerometers and a hydrophone) packages based upon the original technology developed for sonar systems. This has given new impetus to the development of the sensors and the associated interrogation systems which has led to the technology being adopted for other commercial uses. These include the development of networked in-road fibre-optic Weigh-in-Motion sensors and of intruder detection systems which are able to acoustically monitor long lengths of border, on both land and at sea. After two decades, the fibre-optic hydrophone and associated technology has matured and evolved into a number of highly capable sensing solutions used by a range of industries.

Since the appearance of fiber optic sensors in the late 1970s, fiber optic sensing has been considered one of the most promising technologies to be used in structural health monitoring for aerospace applications. The irruption of advanced composites in space and military applications along this decade, and the posterior generalization in airliners along the 1980s was considered as an opportunity for these sensors, due to their up to date unique capability to be embedded in this materials with moderate intrusivity. The appearance of fiber Bragg gratings in late 1980s seemed to confirm an easy incorporation to the aerospace market. However, today, the use of fiber optic sensors on board of aircrafts is still little more than symbolic, and the obstacles for this slow application are still important: qualification of sensors and demodulator for on board operation, demonstration of reliability of measurements done by bonded or embedded sensors, demonstration of health monitoring capability in real scenarios, and demonstrate cost-effectiveness.

This contribution reviews the present state of the art in the development of surface plasmon resonance (SPR) (bio)sensor technology, discusses emerging trends, and presents recent results of research into SPR biosensors at the Institute of Photonics and Electronics, Prague. The developments discussed in detail include a high-performance SPR sensor for parallelized observation of biomolecular interactions, a miniature fiber optic SPR sensor for localized measurements, and sensing based on localized surface plasmons on gold nanoparticles.

Three-dimensional microfabrication of photostructurable glass by femtosecond (fs) laser direct writing is demonstrated for manufacture of photonic biochips. The fs laser direct writing followed by annealing and successive wet etching can fabricate the hollow microstructures, achieving a vareiety of microfluidic components and microoptical components in a glass chip. The optical waveguide written by the fs laser direct writing without the annealing and the successive etching can be integrated into these microfluidic and microoptical circuits. Such an integrated microchip is of great use for biochemical analysis and medical inspection based on photonic sensing. Furthermore, as one of the interesting and important applications, the microchips fabricated by the present technique are applied for inspection of living microorganisms.

Chiral fiber gratings are produced in a microforming process in which optical fibers with noncircular or nonconcentric cores are twisted as they pass though a miniature oven. Periodic glass structures as stable as the glass material itself are produced with helical pitch that ranges from under a micron to hundreds of microns. The geometry of the fiber cross section determines the symmetry of the resulting structure which in turn determines its polarization selectivity. Single helix structures are polarization insensitive while double helix gratings interact only with a single optical polarization. Both single and double helix gratings may act as a fiber long period grating, coupling the core and cladding modes. The coupling is manifested in a series of narrow dips in the transmission spectrum. The dip position is sensitive to fiber elongation, twist and temperature, and to the refractive index of the surrounding medium. The suitability of chiral gratings for sensing pressure, temperature and liquid levels is investigated. Polarization insensitive single helix silica glass gratings display excellent stability up to temperatures of 600°C, while a pressure sensor with dynamic range of nearly 40 dB is demonstrated in polarization selective double helix gratings.

Molecularly imprinted polymers (MIPs), human-made polymers capable of recognizing a particular molecule in the presence of others due to the selective cavities of the material, have been successfully applied to the development of chromatographic and solid phase extraction methods. They have also been applied to the development of electrochemical, piezoelectrical and optical sensors. In parallel with the classification of biosensors, MIP-based devices can work according to two different detection schemes: (1) affinity sensors ("plastic-bodies") and, (2) catalytic sensors ("plastic-enzymes"). In the first case the change in a characteristic optical property, most frequently fluorescence, of the analyte or of the polymer is monitored, upon their mutual interaction. Alternatively, a fluorescent analogue of the target analyte can also be used to develop sensors based on competitive assays (MIAs). Optimization of the polymer composition and, in particular, a proper choice of the nature of the functional monomers involved in the polymerization process, is critical to prepare materials able to selectively interact with the analyte in aqueous media and with the fast kinetics required for analytical applications. Moreover, a rational design of fluorescent analogues of non-naturally fluorescent templates or of fluorescent monomers able to change its property upon interaction with the analyte, is also a bottle neck for wide application of this recognition elements in optical sensing. In this paper we present several approaches to address these issues namely the optimization of MIP composition and the design and synthesis of novel fluorophores for the analysis of antibiotics and mycotoxins in real samples.

As fiber optic distributed scattering sensing systems are providing innovative solutions for the monitoring of large structures, the comparison of the different techniques and solutions is difficult because of the lack of standardized specifications and the difficulty associated to the characterization of such systems. The article presents a tentative definition of performance specifications and qualification procedures applicable to fiber optic distributed sensing systems aiming at providing clear guidelines for their design, specifications, qualification, application and selection.

Large scale manufacturing of nano and micro-optics, both free-space and planar waveguide optics, has many technical challenges. Supporting technologies required for manufacturing of different types of optical devices and modules in nano and micro scales are being developed. As fabrication technology to support nano and micro optics improves, challenges associated with metrology of optical feature in nano and micro scales become more apparent. In this paper some examples of research projects at the University of North Carolina at Charlotte in these areas are presented.

The Layer-by-Layer Electrostatic Self-Assembly (ESA) method has been successfully used for the design and fabrication of nanostructured materials. More specifically, this technique has been applied for the deposition of thin films on optical fibers with the purpose of fabricating different types of optical fiber sensors. In fact, optical fiber sensors for measuring humidity, temperature, pH, hydrogen peroxide, glucose, volatile organic compounds or even gluten have been already experimentally demonstrated. The versatility of this technique allows the deposition of these sensing coatings on flat substrates and complex geometries as well. For instance, nanoFabry-Perots and microgratings have been formed on cleaved ends of optical fibers (flat surfaces) and also sensing coatings have been built onto long period gratings (cylindrical shape), tapered fiber ends (conical shape), biconically tapered fibers or even the internal side of hollow core fibers. Among the different materials used for the construction of these sensing nanostructured coatings, diverse types such as polymers, inorganic semiconductors, colorimetric indicators, fluorescent dyes, quantum dots or even biological elements as enzymes can be found. This technique opens the door to the fabrication of new types of optical fiber sensors.

We review exceptional properties of the photonic crystal fibres enabling sensing applications of this new class of fibres. First, the sensing capabilities of highly birefringent index guided fibres are discussed. This includes dispersion characteristics of phase and group modal birefringence in different fibre structures, and sensitivity of these parameters to hydrostatic pressures and temperature. We demonstrate that index guided and photonic bandgap holey fibres of specific construction can be used as wide-band fibre-optic polarizer. We also show that combining of geometrical and stress effects makes it possible to design the holey fibres with either zero phase or group modal birefringence at virtually any given wavelength. Finally, different designs and performance of PCFs suitable for gas sensing are overviewed.

A selection of fiber-optic and micro-optic devices is presented designed and tested for monitoring the quality and safety of typical foods, namely the extra virgin olive oil, the beer, and the milk. Scattered colorimetry is used for the authentication of various types of extra virgin olive oil and beer, while a fiber-optic-based device for UV-VIS-NIR absorption spectroscopy is exploited in order to obtain the hyperspectral optical signature of olive oil. This is done not only for authentication purposes, but also so as to correlate the spectral data with the content of fatty acids that are important nutritional factors. A micro-optic sensor for the detection of olive oil aroma is presented. It is capable of distinguishing different ageing levels of extra virgin olive oil. It shows effective potential for acting as a smart cap of bottled olive oil in order to achieve a non-destructive olfactory perception of oil ageing. Lastly, a compact portable fluorometer is experimented for the rapid monitoring of the carcinogenic M1 aflatoxin in milk.

A single fibre Bragg grating is used to discriminate between temperature and strain by exploiting the core-cladding mode coupling of a tilted fibre Bragg grating (TFBG). The core and cladding modes exhibit different thermal sensivities, while the strain sensivities are approximately equal. Monitoring the core-core mode coupling resonance and the core-cladding mode coupling resonance within the spectrum of a single TFBG allows the separation of the temperature and strain induced wavelength shifts.

An irradiation test was performed for polymethylmethacrylate plastic optical fibers under gamma-ray irradiation in order to use the fiber in low-level radiation environments. Under gamma-ray irradiation at a high dose rate, only a large radiation-induced transmission loss at wavelengths less than 700 nm was observed. Under irradiation at a low dose rate, the loss was small and other two characteristic effects were observed. One is a radiation-induced loss that has peaks in the near infrared wavelength range, and the other is an improvement in the optical transmission over an entire wavelength range. The peak intensity depends on the absorbed dose of the gamma ray. Therefore, it is considered that the absorbed dose can be measured using the peak intensity. Irradiation tests were performed by changing the configuration of the fiber sample to observe the effect of a bending loss. It was found that the improvement in the optical transmission is dependent on the configuration of the irradiation sample and that the bending loss can be recovered by gamma-ray irradiation at a low dose rate.

Thermo-luminescence behavior of fused silica (SiO₂) core optical fibers was studied in the air under the &ggr; ray irradiation. The thermo-luminescence peak at 1390nm, being attributed to the oxyhydrate (OH) in the fused silica had an expected temperature dependence of the intensity and could be used as a temperature monitor. A partially replaced core optical (PARCO) fiber that consists of a fiber doped with high content of OH inserted into a pure silica fiber was proposed to realize a spatial resolution.

We present a fiber mode converter incorporated fiber-loop ring-down system for strain measurement. The sensing system consists of a fiber mode converter and a typical fiber-loop ring-down system including a pulsed VCSEL laser diode at 850 nm, multimode fiber couplers, multimode fiber loop, photo-receiver and high-speed data acquisition and signal processing. The bending multimode fiber-loop ring-down strain sensor is demonstrated with a resolution of 0.28 &mgr;&Vegr; over a range of 767 &mgr;&Vegr;.

In situ monitoring of gratings' characteristics during continuous gamma irradiation to a dose of 9 MGy revealed by an order of magnitude weaker Bragg wavelengths drift as compared to gratings written in Ge-doped silica-core fibres.

The work presented describes the development of a novel integrated optical sensor system for the simultaneous and online measurement of the colour and temperature of food as it cooks in a large-scale microwave and hybrid oven systems. The integrated probe contains two different sensor concepts, one to monitor temperature and based on Fibre Bragg Grating (FBG) technology and a second for meat quality, based on reflection spectroscopy in the visible wavelength range. The combination of the two sensors into a single probe requires a careful configuration of the sensor approaches in the creation of an integrated probe design.

We have measured the phase sensitivity to temperature of a fibre Fabry-Perot (FFP) formed by splicing multimode tellurite (TeO₂) glass fibre to singlemode silica fibre. The free spectral ranges of two FFPs of different lengths were consistent with the values expected from independent determinations of the core refractive indices. The phase sensitivity was 89.3 ± 0.3rad m^-1 K^-1 at a central wavelength of 1536 nm, compared with 99.8 rad m^-1 K^-1 for silica fibre.

A technique based on Fresnel reflection at the tip of a fiber optic probe is used to investigate local void fractions in bubbly flows. The fiber optic system with multiplexing scheme is designed for demonstrating the feasibility of measuring local void fractions in bubbly flows. Processing the signal acquired from the fiber optic system, local void fractions can be obtained. Linear regression with least square method is applied to analyze data. The correlation coefficient of 0.94 indicates that the technique seems to be suited for measuring local void fractions in the range from 2% to 20%.

We report an accelerometer based upon a simple fibre cantilever constructed from a short length of multicore fibre (MCF) containing fibre Bragg gratings (FBGs). Two-axis measurement is demonstrated up to 3 kHz.

The concept of highly sensitive fibre optic displacement sensor is presented. It is based on macrobending step index polymer optical fibre with multiple overlapping surface-side imperfections. The theoretical principles of optical power losses in such fibres due to multiple curvatures are based on a surface that serves as a model for an imperfect layer and has the same optical properties. The sensing element was tested using two different types of roughness topologies and their combination. The various angles of imperfections (90 and 45 degrees) placed on the fibre's core were evaluated. We discuss some experimental results that confirm our prediction of considerable dependence of the output signal on the changing imperfections caused by bending the fibre. It was found that sensitivity to bending of the angular imperfections could be further increased by implementing multiple overlapping imperfections.

Hollow glass micro-spheres, first used to make fibre optic sensors for high hydrostatic pressure, have been interrogated using a high-precision CCD spectrometer, to give far better precision than earlier. It is found that these simple, low-cost micro-sensors have excellent sensitivity to both static and dynamic pressure, and have the advantage of being hermetically sealed. Many other application areas are foreseen for these low-cost sensors.

The benefits of an Optical Torque Transducer (OTT) based on optical fibre Bragg sensors for applications in any environment prone to electro-magnetic and temperature influences are outlined. By using an optical Rotation Coupler (RC) the transducer avoids slip rings or telemetry systems. The OTT enables the measurement of both torque and temperature free from electro-magnetic interference (EMI) in situations conventionally very difficult to make these measurements. OTT torque measurements compare favorably with that of a dynamometer.

A multi-wavelength fiber-optic confocal position sensor, employing a diffractive optical element (DOE), is described. The DOE was designed with the aim of enhancing the chromatic dispersion of the optics, and thus improving the measurement range of the technique. A proof-of-principle experiment is presented, yielding a five-fold enhancement in the dispersion and thus in measurement range in excellent agreement with design simulation.

A cascaded long period grating Mach-Zehnder interferometer is used to monitor the change in refractive index of a UV cured epoxy resin over a cure cycle. Fourier techniques are used to calculate the phase shift and frequency spectral amplitude of the associated fringe pattern during the cure. The results are compared with the refractive index change during cure calculated using a Fresnel reflection based technique.

The effects of temperature on high concentration Erbium-doped fibers are characterized using parameters of transcendental equation model. The intrinsic parameters (intrinsic saturation power, excited-state lifetime and linear absorption coefficient) of six Erbium doped and Erbium codoped with Lanthanum fibers have been measured for different temperatures. The temperature dependence of intrinsic parameter has been compared respect to Erbium concentration and Lanthanum-Erbium concentration ratio.

We study the origin of antisymmetric perturbation of the fiber in arc-induced long-period gratings that couple the core mode into the antisymmetric cladding modes. We demonstrate that this perturbation is caused by a temperature gradient in the fiber, which is induced, in turn, by a temperature gradient in the arc discharge.

This work presents a temperature independent strain/load sensor using a highly birefringent photonic crystal fibre loop mirror. The length of the sensing head is 38 centimetres and its corresponding wavelength spacing between two interferometer minima is 8 nm. The obtained strain and transverse load sensitivity were 1.21 pm/&mgr;&Vegr; and 0.37 nm/Kg/mm, respectively, while is insensitive to temperature (0.3 pm/°C).

This paper presents a low cost fiber optic obstacle sensor. The detector is primarily developed for the use in applications like the electric windows in vehicles. Sensor relies on flexible and all polymer design and it based on micro bend losses in plastic optical fiber (POF). When designing the sensor high priority was given to the low cost components and materials that are already in use in the automotive industry. The durability tests were executed on the detector prototypes by continuously pressing on the same place of the sensing area, with a force of 50 N, at temperatures near to the maximal operating temperature of the POF. The results show that the obstacle is clearly detected after more than 10.000 continuous presses. The sensor solution presented in this paper can be used in other applications where obstacle presence detecting is required like are electric doors on buses, other power driven automatic doors, In-Circuit Testing devices (ICT), security sensors for windows, etc.

By using a simple technique of UV laser irradiation at various regions along a chirped phase mask, the responses in the region of 2/3 of the Bragg wavelength (i.e. ~1030 nm) and the Bragg wavelength (~1535 nm) are investigated experimentally and compared. The variation of the wavelength of both transmission dips (in the region of the 2/3 of the Bragg wavelength and the Bragg wavelength) were proportional to the increase in periodicities of phase mask. The ratios of these wavelengths, for the irradiation of each phase mask section, showed less than 0.7% variation compared with the value of 2/3, confirming that features at ~1030 nm are the 3^rd harmonic of a grating having the phase mask periodicity.

A chirped fiber Bragg grating was used to measure the non-uniform strain profile of a notched aluminum specimen used to simulate a cracked structure. The specimen was subjected to tensile tests that produced regions of non-uniform strain near the notches. Analysis of power reflectance spectra from the grating, through the use of an integration method, enabled the strain profile near the notches to be determined. Unlike other intragrating sensing methods, this method did not require a disturbance hypothesis to be postulated. The strain profile results from this intragrating sensor were in reasonable agreement with predictions from modeling conducted using the finite element method.

This paper summarises ongoing research conducted at the Cooperative Research Centre for Advanced Composite Structures (CRC-ACS) on the durability to fatigue loading of optical fibre sensors, which were surface mounted or embedded in fibre reinforced plastic composites. The objective of the research is to quantify the parameters affecting the fatigue performance of such optical fibres. Signal degradation of surface mounted and embedded Bragg grating sensors has been studied for a number of configurations. For some configurations, preliminary results indicate significant signal degradation on tension loaded optical fibres after 50,000 cycles strained to 4000 &mgr;&Vegr; and this degradation has been linked with fibre cracking. No such cracking was observed in compression loaded optical fibres. The durability of the bonding at various interfaces of the optical fibre / composite system has been investigated for various polyimide coated fibres. After one million cycles to 3000 &mgr;&Vegr;, 150 &mgr;m diameter optical fibres exhibited significant disbonding in the coating / cladding interface of the optical fibre. Special 52 &mgr;m diameter optical fibres also exhibited significant disbonding but to a lesser degree than that in the larger fibres. Stripped fibres exhibited no disbonding at the optical fibre to composite interface.

The stress in porous silicon during exposition to a liquid is investigated by an approach based on Raman scattering. When the porous silicon structure is exposed to isopropanol or ethanol, a reversible blue shift of the Raman spectra is observed. The blue shift of Raman scattering is ascribed to the contraction induced by the liquids that fill the pores.

We present a novel type of optical strain gage. The strain gage consists of a thin polyimide foil with an integrated optical circuit. The strain sensing elements are optical microresonators. The optical response to strain of these microresonators is a wavelength shift of the resonance wavelength. The optical circuit includes several of these resonators to measure strain in different directions. The strain sensor is read-out using a single-mode optical fiber. Because the different microresonators in the optical circuit have different resonance wavelengths, they can be read out using the same fiber. Our strain sensor is some kind of a cross between electrical resistance foil gages and fiber Bragg grating (FBG) sensors. It is a thin foil device, with a thickness of a few tens of micrometers, but it is an optical device and can be read out in a similar way as FBG sensors. We present the working principle, fabrication and first experimental results.

A new approach to simultaneously interrogate orthogonal axes of single Fiber-Bragg-Gratings (FBGs) and FBG-Fabry Perot resonator sensors fabricated in linearly highly birefringent (HiBi) fibre is presented. Novel interrogation techniques of single Fiber-Bragg-Gratings (FBGs) and FBG-resonator sensors are presented. For a single FBG, we combined a laser-modulation technique to an electronic feedback loop that keeps the source always frequency locked to one peak of the sensor's reflected spectrum. Two different lasers, with orthogonally-polarized states, were adopted to monitor simultaneously both the "fast" and "slow" FBG peaks. The corresponding correction signals from the servo-loop outputs can be interpreted as strain or temperature induced on the FBG. Detection limits ranging from 1 n&Vegr;/&sqrt;Hz to 100 n&Vegr;/&sqrt;Hz, for axial dynamic and static deformations, respectively, and of 0.025 °C/&sqrt;Hz for temperature variations, are expected. A similar approach was developed for sub-pε resolution interrogation of an optical resonator made of a high-reflectivity FBG-pair, using the Pound-Drever-Hall (PDH) stabilization method..

The very low thermal expansion coefficient of silica at cryogenic temperature prevents the use of Fibre Bragg Grating (FBG) sensors for high resolution temperature monitoring in cryogenic environments involving liquid gases or space applications. To overcome such limitations sensors have been coated with different metals to improve the measurement sensitivity in the very low temperature region, i.e. 4.2-35 K. Various coatings have been deposited by electrowinning on the external fibre surface after aluminium pre-coating. Full characterization of this new type of sensor is described in the paper.

A level and flexible quasi-distributed liquid sensor based on the changes in the light transmittance in a plastic optical fiber cable is proposed. The measurement points are constituted by small areas created by side-polishing on a curved fiber and the removal of a portion of the core. These points are distributed and adapted on each full-turn of a spire of fiber built on a cylindrical tube vertically positioned in a tank. The changes between the refractive indexes of air and liquid generate a signal power proportional to the position and level of the liquid. The sensor system was successfully demonstrated in the laboratory and experimental results of three prototypes with 10, 8, and 5 measurement points and with bend radius of 5mm are presented in this paper.

We report strain characterization of birefringent fiber Bragg gratings fabricated in hydrogen-free photosensitive Ge/B codoped PS1250/1500 fiber by high-intensity femtosecond 264 nm pulses. These anisotropic FBGs demonstrate high PDL value of 8 dB but, unlike the FBGs inscribed in PM fibers, show shape preservation under strain of [0, 700 &mgr;&Vegr;] by interrogation based on broad-band source and optical spectrum analyzer.

CdTe Quantum Dots (4 nm of diameter) have been successfully deposited on the inner part of hollow core fibers using the Layer-by-Layer Electrostatic Self-Assembly method. The architecture of the sensor consists on a short section of a hollow core fiber tapered at both ends and spliced to standard multimode optical fibers. Taking advantage of the dependence on temperature of the green fluorescent emission of the Quantum Dot sensitive nanofilms, optical fiber sensors were fabricated and experimentally demonstrated.

A fibre Bragg grating (FBG) pressure sensor, based upon the transverse loading of half its length, is proposed and demonstrated. When a transverse load is applied to half of the length of an FBG, a second Bragg peak is generated in the reflection spectrum, which separates from the initial Bragg peak. The new peak exhibits a red shift in wavelength as the load is increased. A new data analysis technique, which improves the minimum detectable pressure is discussed. A normalised pressure sensitivity of 5.73x10^-3 ± 5.23x10^-5 MPa^-1 was obtained. The technique offers the opportunity to measure independently pressure and temperature using a single FBG sensor element.

This paper presents an optical fibre sensor which can access the refractive index of the external liquid absorptive media. The sensor employs an evanescent-field intensity-type transparent dielectric detection element of hemispherical form. For this detection element, the effect of absorption of the external media on the measurement results was quantified. In order to reduce the measurement errors caused by finite absorption of the external media, we incorporated an absorption-sensitive optical cell in series with the detection element in the sensor. We describe the new measurement procedure that allows one to reduce the errors caused by the finite absorption of the external media.

Here we present a fiber optic seismic waves sensor based on in-fiber Bragg gratings. Fiber Bragg Grating sensors have been demonstrated to have very high sensitivity to dynamical strain in the sub-micro-strain range and very extended dynamical response from static to very high frequency. The seismic sensing system is based on the integration of three FBGs dynamical strain sensors in a mechanical structure acting as an inverse pendulum. Polar symmetry of the mechanical system and 120° placement of the FBG sensors guarantee a directional capability of the seismic sensor. Design, manufacturing and preliminary dynamical testing of the seismic sensor are discussed.

The paper concerns the results of ORMOSIL layer's applied in an optical fibre ultraviolet sensor Nuclear Magnetic Resonance investigation. The layer which substituted for optical fibre's cladding in device's sensing part acted as a matrix for a coumarin's derivative. The dye is able to convert ultraviolet into visible radiation which could be easily detected. Thanks to the fact that the refractive indices of ORMOSIL layers may be controlled and reach a value characteristic for the fused silica glass a satisfying sensitivity of the device can be achieved. However it may be expected that both the structure and the composition of ORMOSIL matrices could influence the organic dye's behaviour. The aim of the experiment presented in the paper was to prove a relation between the sol's condensation time and a composition of the obtained gel's structure. The obtained NMR spectra gave the evidence of non-condensed -Si-O-C₂H₅(H) moieties existence in the solid gels. Moreover, even a 42-days-long process does not lead to the total condensation of ethoxysilanes' molecules. It is probable that the remaining ethoxy or silanol groups interact with transducer's molecules as well as influence the refractive index of the gel layer.

This paper presents an optical approach to estimate the thickness of biofilms on metals in natural aqueous environment. An in-situ fibre optic sensor is held above the sample surface, to offer real time information continuously by measuring the spatially resolved profiles of scattered light in biofilms. The optical results correlate well with the biofilm thickness which is measured using a microscope and a microcapillary with a protruding thin, long flexible wire from the top of the microcapillary. The biofilm thickness is less than 120 &mgr;m on test material after 20 days. Microscopic image analysis also provides the evidence to confirm micro colony formation by rod and cocci bacterial species. The species Klebsiella are found to be dominant on carbon steel. It is observed that measured optical intensity is proportional to the concentration of organisms.

We report an all-optical fiber hydrogen sensor based on absorption changes of evanescent fields caused by an annealed Pd/Au thin film. The sensor consists of a small piece of standard single-mode fiber (SMF) coated with a Pd/Au thin film sandwiched between two multimode fibers (MMFs). Due to core diameter mismatch the SMF cladding guides light. When the device is exposed to hydrogen the layer refractive index diminishes and causes attenuation changes of the evanescent fields. Adding gold to palladium allows the fabrication of fast, durable, and reliable sensors suitable for the detection of hydrogen concentration below the critical 4%.

In this work, the surprising sensing performances of opto-chemical sensors based on SnO₂ particles layers against chemical pollutants either in air and water environment, at room temperature, are reported. The Electrostatic Spray Pyrolysis (ESP) method has been used to deposit the sensing coatings upon the distal end of standard fibers. This technique allows the fabrication of SnO₂ layers composed of micron and sub-micron dimensions able to locally modify the profile of the optical near-field collected in the close proximity of the fiber tip. Such layers morphology leads to strong surface interactions between sensing coatings, analyte molecules and the evanescent contribute of the field, resulting in an excellent sensors sensitivity against chemical pollutants, even at room temperature.

This paper describes a multipass absorption sensor based on an integrating sphere. The sphere has an internal coating which is highly reflective (over 95%) in the near infrared region and this allows the detection of carbon dioxide (CO₂) gas at 2 &mgr;m. CO₂ was detected using a light emitting diode as the emitter and a photodiode as the detector. A two inch (50.8 mm) diameter integrating sphere was used as an absorption gas cell. A method of calculating the effective path length of the integrating sphere is also presented. The latter is shown to be dependant on the reflectance of the sphere's internal surface, the sphere's port fraction and the level of attenuation of the optical signal due to the gas present in the sphere. Effective optical path lengths of 40 cm at the 2 &mgr;m region are reported. Experimental results demonstrating the detection of CO₂ using a two inch diameter integrating sphere are presented and these are compared to simulation results based on a CO₂ absorption over a 40 cm path length at 2 &mgr;m.

Results are presented for a transmission based optical fibre hydrogen sensor using a thin film layer of Palladium as the sensing element. The thin palladium film was deposited onto a glass substrate via thermal vacuum evaporation. The sensor was tested by exposing the palladium film to 5% Hydrogen gas in a Nitrogen atmosphere. The absorption and desorption of hydrogen causes the optical properties of palladium to change depending on the concentration of hydrogen present in the atmosphere. Using a deuterium/halogen light source in conjunction with a UV/VIS spectrometer the changes in the optical transmittance in the visible spectra of the Pd film was monitored for 5% hydrogen in a nitrogen atmosphere. A comparison of different wavelengths in the VIS spectrum is presented. The response and recovery time for the sensor is shown to be 4-5 seconds and 35 seconds respectively. The sensor is capable of responding to changes of up to 15% and the sensor shows good repeatability to continuous exposure cycles to 5% hydrogen using nitrogen as the carrier and recovery gas.

The development of an Ultra Violet (UV) Differential Optical Absorption Spectroscopy (DOAS) fibre-optic sensor for the monitoring of nitric oxide gases is described in this paper. Experimental results describing the operation of this sensor with cylinder gases are presented. These experimental results are compared with existing published spectroscopic absorption measurements. The sensor was developed to operate within an exhaust environment and demonstrate a low susceptibility to interferences from other gases present. A LabVIEW program was created to interrogate the highest absorbing wavelength for nitric oxide and calculate the concentrations present before outputting them to the user. The lower limit of detection for the sensor was found to be 5ppm with response times of 3.4 seconds.

We demonstrate the use of tilted fibre gratings to assist the generation of infrared surface plasmon resonances with short propagation lengths, resulting in a high index resolution of 2×10^-5 in the aqueous index regime. It was also found that the resonances could be spectrally tuned over 1000nm at the same spatial region with high coupling efficiency (in excess of 25dB) by altering the polarisation of the light illuminating the device.

Hydrogen detection is of great interest in many fields such as hydrogen concentrations around the explosive limit of 4% or in areas where strong electrical fields are present. Optical sensors have the advantage that they are not affected by these conditions. A comparison of three thin film sensors made of Pd, Pd Ni and Pd Ni / PTFE to detect H₂ is made. The double-sided samples are purpose built by plasma sputtering, with a thickness of 4nm on each side. It is shown that the signal of a pure Pd deposit is strong but it cannot resist higher H₂ concentrations. The Pd Ni deposit has shown good results and is able to resolve even higher concentrations of 10% H₂ but cannot resist high concentrations of H₂ over many cycles. The new Pd Ni / PTFE deposit can be used many times for concentrations up to 100% without any damage and can detect concentrations of 1-100% of H₂. Furthermore the response time for deposits of different thickness is tested.

Orthodontic forces are measured using a polymer photonic crystal fibre sensor. Transversal pressure deforms the fibre structure proportionally to the applied load causing light to leak out. The characterization for transversal pressure demonstrates linear behaviour within the studied load range 0.09 to 4.7 N. For the orthodontic measurements the sensor is placed between the orthodontic appliance and only one tooth. Loads ranging over 0.98 to 8.82 N, simulating extra oral appliances, are applied over the orthodontic system at the first molar region. The surface of the tooth experiences forces ranging from 0 to ~0.63 N compatible with forces required for dental movement.

A multi-channel fibre optic OSL dosimeter system is developed by the CEA LIST for quality control in cancer radiation therapy, in the framework of the European Integrated Project MAESTRO.

The major achievements in the field of optical sensors in the past two decades have remained mostly limited to the laboratory demonstrations. There are very few examples of optical sensors, which have been reduced to practice, and have established themselves in major markets. The main bottleneck in this field is the issue of manufacturability. In this paper we present optical sensors based on slotted multimode interference waveguides. We show that the sensitivity increases proportionally to the number of slots. The sensor can be tuned to highest sensitivity in the refractive index ranges necessary to detect protein-based molecules or other water-soluble chemical or biological materials. The material of choice is a sol-gel (ORMOCER) matrix that after completion of the process becomes mostly glass and it is highly stable. Sensors made with this technology are suited to high volume manufacturing.

In this work, the feasibility to exploit optoelectronic chemo-sensors based on cadmium arachidate (CdA)/single-walled carbon nanotubes (SWCNTs) composites for detection of chemical pollutants both in air and water environments has been investigated. The nanocomposite sensing layers have been transferred upon the distal end of standard optical fibers by the Langmuir-Blodgett (LB) technique. Single wavelength reflectance measurements (&lgr;=1310 nm) have been carried out to monitor chemicals concentration through changes in the optical length of the Fabry-Pérot (FP) cavity induced by the interaction of the sensitive layer with the analyte molecules. The preliminary experimental results evidence the good potentiality of these fiber optic nanosensors to detect toluene and xylene at ppm level both in air and water environments at room temperature.

Medical applications represent a unique chance of expansion for the optical fiber sensors (OFS) market that was confined so far mostly in niche applications where higher technological costs were justified by OFS distinctive advantages. Single use medical devices integrating OFS could however generate a significant growth for this type of technology. Thanks to cost reductions derived from the success of optical fiber used in the telecom industry, it is now possible to produce competitive disposable OFS for clinical environment. Cost reduction is nevertheless not the only challenge for this type of application: materials bio-compatibility and sterilization resistance, packaging issues, design considerations for end-user acceptance and operational simplicity, technology reliability including connectivity and sensor performances, manufacturing process monitoring and outstanding quality control, are among few of the problems that have to be considered to address correctly the complex medical market with successful disposable OFS devices. With a clear understanding of the needs and challenges of clinical applications, it is easier to respond to this reality and to offer commercially suitable solutions.

The potential impact of optical fibre sensors embedded into medical textiles for the monitoring of respiratory movements in MRI environment is presented. Preliminary investigations of the feasibility of sensing respiration movements with a POF OTDR are reported. In addition, a macro-bending sensor based on a periodic design is demonstrated and successfully implemented using a narrow fabric production process. It allows monitoring human abdominal breathing movements with a very simple monitoring set-up.

Although many optical fiber sensors have been proposed for chemical, environmental or biological measurements, it seems that the potential of this kind of devices has not been fully exploited. In this paper we discuss the possibilities of doubly-deposited uniform-waist tapered fibers (DL-UWTs) for the development of new sensors that can become a new standard in the field of Surface Plasmon Resonance (SPR) based sensors and contribute to the extension of the range of application of fiber technology to the mentioned fields. We also compare different configuration and evaluate some relevant features of DL-UWTs, as the possibility of fully independence of polarization or the excitation of multiple plasmons.

We present a polymer optical fibre sensor to sense skin moisture and droplet formation when sweating occurs. The sensor used evanescent field attenuation, by exploiting a moisture sensitive cladding with moisture indicator (fluorescein) contained within a porous cladding (HEMA). The sensor was designed to be comfortable to wear and unobtrusive, hygienic, with sterilised interchangeable sensing elements. It had maximum sensitivity between 98% and 100% humidity, and response time of 24 seconds.

A liquid-core air-clad microstructured fiber has been developed for determination of ethanol concentration in aqueous solutions by Raman spectroscopy. The sensor shows a linear response and a low ethanol concentration solution has been characterized using the calibrated sensor. The configuration used for light and liquid injection is stable and robust, making the sensor useful for on-line measurements.

Hollow-core photonic crystal fibres have a high potential for gas sensing applications, since large light-gas interaction lengths can be effectively attained. Nevertheless, in order to enhance effective diffusion of gas into the fibre hollow-core, multi-coupling gaps are needed, which raise coupling loss issues that must be evaluated prior to the development of practical systems. In this communication we present a study on the coupling losses dependence on lateral and axial gap misalignment for single-mode fibre and two different types of hollow-core photonic crystal fibres. In addition, experimental results on the splicing of these fibres are also presented.

The Electrostatic Self Assembly Layer-by-Layer (ESA) technique has been already used for the fabrication of optical fiber pH sensors, the properties of the resultant pH sensitive coatings has been found to be strongly dependent on the fabrication parameters, especially on the pH of the polyelectrolyte solutions used for the deposition of the coatings. In this work the resulting properties of the sensitive coatings have been optimized by changing the parameters of fabrication. Techniques as AFM, for studying the thickness and morphology of the sensing films, and spectroscopy, for studying the composition of the films, were used for the characterization of the devices. Optical fiber pH sensors with response from pH 3 to pH 7 were successfully fabricated.

The generation of a continuous-wave pumped supercontinuum source at 1.3 &mgr;m is described. The device makes use of a tunable Yb-doped fiber laser, a cascade of fiber Bragg-grating mirrors and a concatenation of standard silica fibers with stepwise decreasing dispersion. The generated supercontinuum spans from 1280 to 1513 nm, shows and average output power of 1.34 W and exhibits >0 dBm/nm spectral power density over 200 nm.

An all-optical scheme for simultaneous determination of oxygen and temperature is presented. A ruthenium complex immobilized in a non-hydrolytic sol-gel matrix is used as oxygen sensor. Temperature information is provided by CdSe quantum dots immobilized in the same material. While the dye luminescence is quenched by oxygen and temperature, the nanocrystals luminescence depends only on temperature. Results presented demonstrate that the combined luminescence response allows to simultaneously assess both parameters using a single optical fiber system.

Poly(p-xylene tetrahydrothiophenium chloride) was successfully deposited on tapered ends of optical fiber by means of the Layer-by-Layer Electrostatic Self-Assembly method. A posterior thermal curing at low temperatures (around 100°C) is necessary for the synthesis of the conjugated polymer poly (p-phenylene vinylene), (PPV). The fluorescence of this polymer has shown dependence to pH. Optical fiber pH sensors were experimentally demonstrated for the working range from pH 3 to pH 8.

The aim of this study has been the introduction and calibration of a novel Relative Humidity meter, based on optical sensors. It consists of a psychrometer, whose temperature measurements are performed by means of two Fiber Bragg Grating sensors, working as dry-bulb and wet-bulb thermometers. Their preliminary calibration permits to convert Bragg wavelengths into temperature values with high repeatability and accuracy: hence, the local relative humidity can be evaluated, using thermal gradient and dew point temperature, by a psychrometric table implemented as analytic function inside a processing algorithm. The proposed sensor presents all the typical advantages of the optical devices, especially the immunity to electromagnetic disturbances and to corrosion, allowing to measure humidity in aggressive environments as meteorological off-shore stations, marine structures and vehicles.

Novel fiber-optic biosensors have been developed for the analysis of pesticides in water based on Chlorophyceae microalgae immobilized into a porous silicone layer as recognition element, and on measurements of the photogenerated O₂ as chemical transducer. The inhibition of O₂ production by the photosynthetic green algae in the presence of the pesticide (simazine) was used as the biological signal. Luminescent thin films and a dedicated optoelectronic unit based on emission phase-shift measurements provide the tools for the sensitive O₂ measurements. Fluctuation analysis allows selection of sensitive and resistant microalgae mutants without genetic manipulation for maximum biosensing selectivity.

A pH sensor based on optical fibres was developed. The analysed sample is the interstitial fluid drawn from the adipose tissue by means of a microdialysis catheter. The pH sensing layer is constituted by a pH indicator, phenol red, directly immobilised onto the internal wall of a glass capillary. The interrogation of the glass capillary is performed by an optoelectronic unit, developed in our laboratory, which makes use of a light emitting diode at 590 nm as source and a photodiode as detector. A suitable animal model was developed and in-vivo tests on pigs were carried out.

In the present work an optical platform is described for the interrogation of a multichannel array for chemical and biochemical parameters. A fluorescent sensing layer is immobilised on the internal wall of the channel and the emitted light travels along the thickness of the channel and is detected with an optical fibre connected with a spectrum analyzer.

The present paper is concerned with the development of an optical system capable of interrogating an array of silicon microcantilevers. The use of a laser line-generator and of a CCD camera allows the simultaneous interrogation of the eight cantilevers of the chip without movable parts. The resolution achieved in terms of displacement of each cantilever end is better than 7.5 nm. As an example of the operation, the system has been characterized as a refractometer by filling the cell with liquids having different index of refraction.

A long period grating (LPG)-based optical fiber sensor, coated with polyvinyl alcohol, has been developed for relative humidity (RH) measurement over the range from 33% RH to 97% RH, for which the grating response has been monitored. The fiber optic sensor using the LPG has been designed to monitor the change in resonance loss which is then related to the humidity measured through calibration of the sensor with a series of materials of known refractive index. Results on the calibration and use of the sensor are reported and considered in this work, in light of other grating-based methods for humidity measurement.

The surface plasmon resonance (SPR) technique is an optical method that can be used to measure the refractive index of organic nano-layers adsorbed on a thin metal film. Although there are many techniques for measuring biomolecular interactions, SPR-based techniques play a central role in many current biosensing experiments, since they are most suited to sensitive and quantitative kinetic measurements. Here we give some results from the analysis and numerical elaboration of SPR data regarding the flow of different solutions with refractive indexes in the range of interest (1-1.4). After a brief discussion of the principles of SPR and of waveguide fabrication technique, we give a description of system setup and some results regarding the real time waveguide output intensity monitoring to measure the interaction between the gold thin film and the analyte.

An alternative approach to signal recovery in tunable diode laser spectroscopy with wavelength modulation is proposed and investigated. Measurements of transmission spectra of methane in nitrogen are compared with equivalent data in the HITRAN data base and curve fitting of HITRAN data to that measured have been used to obtain gas concentration and pressure. The results validate the new technique and demonstrate that absolute measurements of rotation / vibration gas line transmission functions, gas concentration and gas pressure can be made accurately without the need for calibration.

Two versions of opto-electronic signal processors applied to demodulating the output signal of a White Light Interferometry (WLI) based optical sensor, designed to measure ac high voltages, are presented. Both processors incorporate schemes to compensate optical power fluctuations caused by changes of attenuations and losses in components of the optical fiber sensor systems. The strategy adopted in the first signal processor is based on the precise detection of the peaks and valleys values present in the output signal of a modulated recover interferometer, which signal contains desired information about the ac high-voltage applied to a specially designed Pockels cell used as sensor interferometer. The second signal processor uses a reference sample of the optical power delivered in the output of the sensor interferometer to normalize the output signal of an unmodulated recover interferometer. Prototypes for both signal processors were developed and subjected to tests conducted to measure ac high voltages up to 110 kV_pp and to verify the stability of the measurements under conditions of optical power attenuations up to -8.66 dB introduced in the link.

This paper presents the first results on the use of the pre-stress to improve the performance of a magnetic field sensor based on a Terfenol-Fiber Bragg Grating integrated device. Indeed, since the pre-stress modifies the response of the magnetostrictive material, this should allow to fit the performances of the sensor to different operative conditions or to exploit also the stress as a control variable to optimize the design of the sensor.

We describe, first to our knowledge, optical resonances of Transverse Electric (TE) and Transverse Magnetic (TM) wave propagations in dielectric micro-circular-cylinder. New Asymptotic approaches have been developed based on TE and TM waves. Size parameter (pi times diameter divided by wavelength of the light) is phenomenal to demonstrate Whispering Gallery Mode (WGM) in dielectric circular cylinder. The developed expressions for size parameter for both TE and TM waves are very simple and can be used to characterize the resonances in dielectric micro-circular-cylinder. Asymptotic expressions have been developed based on Electro-Magnetic (EM) wave theory derivations which are mathematically robust than existing approaches presented in the literatures, and can be used to develop optical sensors by characterizing resonances in dielectric micro-circular-cylinders. The solutions are shown to be very accurate for large size parameters.

We evaluate the possibility to use a low-birefringence fibre for current sensing application in harsh environmental conditions as expected in quasi steady plasma burning reactor. Experimental results show that the Faraday rotation of the low-birefringence fibre is not significantly degraded by a gamma irradiation upto 5.4 MGy and when the fibre is kept at ambient temperature.

The DFB fiber laser strain sensor is shown to provide an improvement in the minimum detectable length change by a factor of 275 at 2 kHz for an applied strain when compared with an interferometric sensor interrogated by an equivalent DFB laser. This corresponds to a strain resolution enhancement by a factor of 18, if the strain is applied over a 10cm length of fiber. The ability of the fiber laser sensor to operate in its thermodynamic noise limit is shown to be the primary reason for this enhanced sensitivity. Thermodynamically limited noise performance is demonstrated with three interrogation methods.

The paper presents new results about identification near-field seismic rotational events by a fibre-optic rotational seismometer. The Sagnac interferometer with 2&sgr; sensitivity equal to 4.27 10^-8 rads^-1 has been used as rotational seismometer, which can detect rotational events in a direct way. The theoretic investigation of obtained results show that such events can be treatment as the seismic rotational waves. The paper presents the experimental results obtained simultaneously by a fibre-optic and a standard rotational seismometers installed in the Ojcow seismic observatory. The main advantage of such an approach is a possibility to recognize the velocity of seismic rotational waves, which is different from the velocities of other seismic waves.

A new type of stable interferometric fiber optic sensor based on the amplitude modulation (AM) technique is applied to the differential distance measurements. By synthesizing interferograms from two interferometers, output signal shows an AM waveform in the optical frequency domain. By extracting the low frequency interval, the imbalance of optical path differences (OPDs) can be extracted. This unique sensor system is described in detail with some experimental results.

In this work, the LPG-assisted fibre Michelson modal interferometer is studied as a sensing structure for environmental refractive index, temperature and liquid level when coherence addressing and heterodyne interrogation are considered. The effects on measurand sensitivity of the order of the cladding mode excited by the LPG, of the degree of etching of the sensing fibre and of the fibre type used are investigated.

In this work it is presented a novel in-fibre modal interferometer based on a non-adiabatic biconical fused taper that couples light between the cladding and the core, combined with the Fresnel reflection at the fibre end. It is observed that the returned light from this fibre structure shows a channelled spectrum similar to that of a two-wave Michelson interferometer. The application of this device as a fibre optic flowmeter sensor is demonstrated.

A novel Mach-Zehnder interferometer based on a fibre multimode interference structure combined with a long period fibre grating is proposed. The multimode interference is achieved through the use of a multimode fibre section spliced between two single-mode fibres, with a length adjusted to couple a fraction of light into the cladding modes. A LPG placed after the multimode fibre couples light back into the fibre core completing the Mach-Zehnder interferometer. This novel configuration was demonstrated as a bending sensor.

We describe experiments of fiber coils using different lengths, coil diameters, and configurations wound on a innovative winder. Geometric and polarimetric analyses of coils and effects on Sagnac area and bending induced birefringence are examined.

Ultrasound is a valuable tool for the detection of damage in structures and the characterisation of material properties. Its detection is conventionally done by piezoelectric transducers, however fibre optic sensors can operate over a greater range of frequencies and also yield information on the direction of wave propagation. The interaction between fibre sensors and ultrasound both demonstrates the integrating features of intrinsic fibre optic sensors and presents new opportunities in ultrasonic detection, offering enormous diversity in polar and frequency response. This paper summarises the interaction mechanisms between ultrasound and fibre sensors and confirms their functional flexibility. We use these results to demonstrate the practical use of these sensors to detect and locate damage in a sample.

This paper describes the design of an all-fibre multiwavelength erbium doped fibre laser operating at room temperature. By using a Sagnac loop filter, multiple wavelength channels are obtained with 0.8nm spacing in the C-band and simultaneous multiwavelength operation is achieved through an intracavity sinusoidal phase modulator. A polarization controller inside the filter is helpful in controlling the multiwavelength operation. The effects of modulation frequency, modulation index and pump power on the multiwavelength operation are experimentally investigated. The fibre laser has potential applications in fibre Bragg grating sensors and fibre optic spectroscopy.

Optical fiber sensors have shown great potentials for aerospace applications. But two issues need to be addressed before these applications can be realized. One is how to reliably implement optical sensors in the air vehicles. The other is the need of compact, low weight sensor interrogation systems. We propose to use planar lightwave circuits (PLC) to address the second issue. In this article, we report some of our results on the development of both echelle diffractive gratings based sensor interrogator and arrayed waveguide gratings based sensor interrogators. Both approaches offer miniaturized solutions for the development of optical fiber sensor interrogation systems.

The multimode interference structure is conventionally used as a splitter/combiner. In this paper, it is optimised as an edge filter for ratiometric wavelength measurement, which can be used in demodulation of fiber Bragg grating sensing. The global optimization algorithm-adaptive simulated annealing is introduced in the design of multimode interference structure including the length and width of the multimode waveguide section, and positions of the input and output waveguides. The designed structure shows a suitable spectral response for wavelength measurement and a good fabrication tolerance.

We report the implementation of a low-cost high-resolution WDM interrogation system operating around 800nm region with bandwidth up to 60 nm and resolution of 13 pm by utilising a tilted fibre Bragg grating as an out-coupling device and a CCD-array detector. The system has been evaluated for interrogating fibre Bragg grating based strain, temperature sensors, giving sensitivities of 0.6 pm/&mgr;&Vegr; and 5.6 pm/°C which are in good agreement with previously reported values. Furthermore, the system has been utilised to detect refractive index change of sample liquids, demonstrating a capability of measuring index change as small as 10^-5.

An experimental comparative study between two wide-band wavelength-tunable erbium doped fiber ring lasers (EDFRLs) with the same active fiber length but with two different pumping configuration (forward unidirectional pumping and symmetric bidirectional pumping) is reported in this paper. Both fiber lasers cover almost the whole C-band and L-band with a single setup laser. The signal wavelength can be tuned in a wide range of 60 nm with the two presented lasers. Nevertheless, experimental results verify that a higher output power is obtained with the bidirectional pump configuration.

In this work, the experimental analysis of the response of nanocoated Tapered Long Period Gratings (TLPGs) to Surrounding Refractive Index (SRI) changes is presented. The modal transition is demonstrated to take place in this kind of devices and to be an effective method to improve their SRI sensitivity. The dip-coating method was carried out by an automated system and showed to be a reliable technique for the deposition of conformal coatings. A comparison with traditional UV-written LPGs revealed that a thicker overlay is necessary for the TLPG to tune the transition region in the same SRI range.

This work numerically and experimentally investigates the spectral modifications due to non-uniform azimuthally symmetric high refractive index (HRI) nano-coatings deposited on long-period fiber gratings (LPFGs). First, a uniform overlay was deposited on a LPFG by using electrostatic self-assembling technique, which guarantees a fine control on the coating thickness. Successively, UV (&lgr;=193nm) laser micromachining was applied to locally and selectively remove the coating with high spatial resolution and preserving the azimuthal symmetry of the structure. As the overlay removal was performed starting from the middle of the grating, strong modifications of the LPFG transmission spectrum occurred. Phase-shift phenomenon and selective fringes generation in correspondence of all the attenuation bands can be observed. The two-effects occurring are ruled by the longitudinal length of the uncoated region and the overlay features (thickness and optical properties). The wavelength selectivity, combined with the effects of the HRI coatings on the cladding modes distribution, can be successfully applied for the development of advanced and high performances sensing devices.

This paper presents nanogrinding as an alternative technique for the fabrication of optical fiber micro profiles. Grinding of these lenses is carried out on a nanogrinding machine (NGM) specially built for this purpose. Cleaved single mode optical fibers with core diameter of 9 &mgr;m and outside diameter of 125 &mgr;m were used. Optical fiber endfaces with a multitude of micro profiles were successfully made. This includes conical lenses, tapered lenses, D-shaped endfaces, and fibers with oblique endfaces. The results are presented in the form of optical microscopy and scanning electron microscopy images of the produced lenses.

An arbitrary FBG spectrum can be obtained as the addition of the contribution of some concatenated subFBGs written without phase jump using the same phase mask. An adaptive filter is used in order to obtain, from the desired spectrum, the parameters of the different subFBGs reproducing the required response.

In this work, we report on the investigation of in-fiber photonic devices based on microstructured chirped fiber Bragg gratings (CFBGs) for sensing applications. The basic device here analyzed consists in a CFBG with single or multiple defects obtained by a deep and localized stripping of the cladding layer along the grating structure. The effect of each local thinning along the grating length, properly exploited, basically enables the formation of one pass-band channel within the pristine grating bandwidth and correspondently in one stop-band out of the pristine grating bandwidth. In addition, due to spatial encoding in chirped gratings, each channel spectral position exclusively depends on the features of its own defect (length, depth of the defect and surrounding refractive index) in a well defined location along the grating. Thus, the spectral properties of each channel are not affected by defects located elsewhere along the grating structure, enabling the possibility to develop independent multipoint sensors by exploiting a single grating device.

In this work, a comparative study has been carried out to investigate the effects of cladding stripping on the external refractive index sensitivity in tapered and UV long period gratings. Here, wet chemical etching combined with microscopic analysis allow us to identify the experimental dependence of the surrounding refractive index (SRI) sensitivity on the cladding radius for both grating types. The experimental results reveal that although in both cases a sensitivity enhancement is achieved by reducing the cladding thickness, the tapered devices offer substantially a greater sensitivity gain in respect to UV written devices.

In this work, we propose a novel fabrication method as technological assessment for the development of microstructured fiber Bragg gratings (FBGs) tailorable for specific applications. Micro-structured FBG (MSFBG) relies on the localized stripping of the cladding layer in a well defined region in the middle of the grating structure leading to the formation of a defect state in the spectral response. The key feature of this class of devices is the strong dependence of the defect state generated in the grating spectrum on the optical properties of the medium surrounding the device. In fact, this property enables the development of tunable devices as well as in fiber refractometers and chemical sensors. One of the main drawbacks of these devices is the fabrication process requiring a well precise control of the stripped region dimensions. Here, we demonstrate that the optimization of this device is possible by adopting a fabrication process based on polymeric coatings and UV laser micromachining. These method combined with wet chemical etching allows the easy prototyping of MSFBGs with the desired features for specific applications.

Recent work in the fabrication of self assembled quantum dot (QD) detectors for optical fiber sensors is reported in this paper. The ability to develop the QD based devices and materials via the electrostatic self-assembly (ESA) process has been demonstrated. The QD precursor nanocluster materials involved in ESA have been designed and synthesized to proper size, stabilized in an aqueous-based solution, and functionalized to allow self-assembly. The optical fiber sensor instrumentation has been developed to monitor the reflected optical power with the buildup of the QD layers on the fiber endface during the ESA process. The results are confirmed by observing the effects of low-finesse QD Fabry-Perot interferometric cavities formed via such processes on the ends of optical fibers. The photocurrent-voltage characteristics show a diode-like behavior with linear photocurrent in the reverse bias and nonlinearity in the forward bias. It is suggested that fast response times can be achieved due to the high carrier mobilities that arise in part due to structure of the materials formed via the solution-based ESA process.

In this work, an experimental analysis on the spectral effects induced by depositing a uniform high refractive index (HRI) thin coating on weakly tilted fiber Bragg gratings (TFBGs) is carried out. First, two weakly TFBGs presenting different tilt angles were fabricated. Successively, by using dip-coating technique, two different coating thicknesses were deposited on each of them. The transmission spectra of the obtained structures were collected as the surrounding refractive index (SRI) changed in the range 1÷1.47. The coated gratings present some differences in their spectral evolution, that can be opportunely exploited. The coated configuration could improve some aspects of previous demodulation techniques exploited in the case of bare TFBGs. Moreover, an important effect of HRI coatings is to enhance the SRI sensitivity of the spectral position of the dips corresponding to cladding modes coupling, leading to a different interrogation method for sensing applications.

We use a figure-eight fibre laser with a photonic crystal fibre NALM (nonlinear amplifying loop mirror) to produce a moderate supercontinuum from 1440 nm - 1680 nm in highly nonlinear dispersion shifted fibre. We also observe a 743 nm band in the fluorescence of the erbium doped amplifier outside the figure-eight fibre laser.

We present a multiwavelength fiber source based on cascaded of four-wave mixing in two semiconductor optical amplifiers followed by further four-wave mixing in an optical fiber enhanced by Raman amplification. The multiwavelength source is generated by two initial frequencies detuned 200 GHz and referenced in the absorption lines of the acetylene ¹²C₂H₂, which sweep in frequency keeping the detuning of the lasers constant. With this configuration, we have achieved a high resolution source with a spectrum of 36 channels centered with adjustable peaks separation. The source can be employed to interrogate a fiber Bragg grating sensors network and in gas spectroscopy applications.

The thermal drift of the characteristic wavelength of a Fiber Bragg Grating photowritten in the core of an 18-hole-microstructured fiber is significantly reduced by inserting a liquid of suitable refractive index into its holes. The maximum sensitivity is reduced, and the spectral range of variations is divided by a factor of 8, over a temperature range larger than 20°C. Such passive FBG temperature compensation technique is of great interest for applications involving accurate sensing free of thermal effects.

In this paper, the design of resonant cavity enhanced photodetectors, working at 1.55 micron and based on silicon technology, is reported. The photon absorption is due to internal photoemission effect over the Schottky barrier at the metal-silicon interface. A comparison is presented among three different photodetectors having as Schottky metal: gold, aluminium or copper respectively. In order to quantify the performance of photodetector, quantum efficiency including the image force effect, as a function of bias voltage is calculated.

We describe a new method for the design of optical fibre refractometric sensors based on non-uniform Long-Period Fibre Gratings (LPFGs). An essential part of this method is a specialized genetic algorithm with the properly designed operators that perform the theoretical synthesis of a necessary nonuniform LPFG profile. Such a profile yields the linear frequency response of the sensor to the refractive index of the surrounding medium. A distinctive feature of the designed LPFG is a linear variation of the core and cladding refractive index along the LPFG length. We also present a new mathematical model of nonuniform LPFG, which allows one to take into account the effect of LPFG sections of an average refractive index lower and higher than that of the surrounding medium. A numerical example illustrates the characteristics of the synthesised LPFG.

In this paper report on the design of a 2D PBG filter in polymeric material. The filter is a Fabry-Perot cavity having a self-sustained membrane configuration. A deep parametric analysis has been carried out for improving the performance, taking also into account the fabrication tolerances Best performance in terms of lateral confinement have been obtained in case of square lattice. As for materials, polystyrene shown best in terms of refractive index value, length of the photonic crystal structure and attenuation value in the band gap. The filter can be used either in sensing applications or in telecommunication field.

In this work, Hollow-core Optical Fibers (HOF) functionalized with Single Walled Carbon NanoTubes (SWCNTs) are proposed for volatile organic compounds (VOCs) detection. The sensing probe is composed by a piece of HOF with a termination coated and partially filled by SWCNTs. The infiltration of the SWCNTs inside the HOF holes has been accomplished by means of the Langmuir-Blodgett technique. Reflectance and far field transmission characteristics have been carried out within the HOF bandwidth. Finally the sensing capability of the proposed sensors has been investigated by exposure in a proper designed test chamber to traces of toluene. The experimental results obtained demonstrate the success of the SWCNTs partial filling within the HOF holes and the sensor capability to perform VOCs detection with a good sensitivity and fast response times.

The effective index, fundamental mode width, numerical aperture are some important parameters of a fibre. The behaviour of these parameters for a Modified Total Internal Reflection (MTIR) Photonic Crystal Fibre (PCF) is studied when strain is applied. We chose a range of normalized frequency (&Lgr;/&lgr;) and a range of d/&Lgr; and we analyzed the behaviour for different values of strain. The sensibilities of the studied parameter are obtained in the chosen range. Finally, the region of maximum influence of the strain is observed.

A series of low-contract photonic band-gap (PBG) fibers were fabricated by filling the holes of a commercial air-silica hollow-core PBG fiber with different refractive index liquids. The PBGs and the transmission characteristics of these fibers were investigated theoretically and experimentally. An increase in the refractive index of liquid filling the holes causes blue-shift of the PBG and a narrow down of the PBG width, which may be exploited for sensitive refractive index measurement.

We describe some interesting features that arise when slow and fast light structures are introduced in interferometers. With different configurations, one can obtain a spectral sensitivity enhancement of the interferometer or an enhanced robustness to laser frequency drifts. Furthermore, we describe simple practical ways to implement slow and fast light media in practical interferometers. We believe that these ideas may have interesting implications in optical sensing.

We propose a novel configuration for a Brillouin distributed sensor based on Brillouin optical time domain analysis. This new configuration eliminates many intensity noise issues found in previous schemes. Resolution of 3.5 m all over a 47 km single-mode fibre was achieved and resolution down to 30 cm in a few kilometre fibre. Noise reduction makes possible measurements with a 16 times averaging.

In this paper author proposes the novel application of fully distributed intrinsic fiber optic sensors. The system is devoted to real time long range high resolution surface road traffic monitoring. Conception of this innovative application is described and system requirements in the view of distributed fiber optic sensors are analyzed. Two suitable sensing principles are reviewed and their properties relative to real time recognition of traffic participants are investigated.

Distributed fiber sensing based on Brillouin gain scattering (BGS) principle is a useful way to develop devices capable to measure temperature and/or strain in optical fibers. In these distributed sensors, spatial resolution is a topic of special interest in the distributed fiber sensing field. The influence of the probe-pulse shape in the interaction between the pulsed light and the continuous wave laser in a pump-probe system. This study has the purpose of improving the spatial resolution of the measurement without losing stability in the BGS is presented. Also it is showed how the backscattering Brillouin gain is affected by inducing variations on the final value of the BGS intensity. Theoretical analysis of the probe pulse in the Brillouin shift and intensity values using triangular, sinusoidal and saw tooth shapes around the phonon lifetime (~10ns) are presented; and also considerations and conclusions are explained.

The performance of distributed temperature sensor systems based on spontaneous Raman scattering and coded OTDR are investigated. The evaluated DTS system, which is based on correlation coding, uses graded-index multimode fibers, operates over short-to-medium distances (up to 8 km) with high spatial and temperature resolutions (better than 1 m and 0.3 K at 4 km distance with 10 min measuring time) and high repeatability even throughout a wide temperature range.

Exploitation of integrated-optic capabilities in terms of compactness and low-cost are demonstrated in distributed temperature sensing experimentation. Design and characterization of an integrated-optic circuit in SiON technology with high index contrast suitable for Raman-based fiber sensor are presented.

We present the use of swept wavelength interferometry for distributed fiber-optic temperature measurements in a Nuclear Reactor. The sensors consisted of 2 m segments of commercially available, single mode optical fibers. The interrogation technique is based on measuring the spectral shift of the intrinsic Rayleigh backscatter signal along the optical fiber and converting the spectral shift to temperature.

An amplified optical fiber double ring network for the wavelength division multiplexing (WDM) of fiber Bragg grating (FBG) sensors is demonstrated experimentally. The network is inherently resilient to fiber failures due to the simultaneous interrogation of all the sensors using both rings. Furthermore, power transparency at the rings is obtained by means of Raman amplification. We optimize the performance of the network with the launched pump power that shows low levels of amplifiers' noise. We also demonstrate how the topology allows the received powers from the sensors to be equalized.

A system for the interrogation of fiber Bragg grating sensors using a strain-tuned EDF laser with linear cavity is described. An optical switch is spliced to one end of the laser cavity and connects one of two high-strength draw-tower fiber Bragg gratings (DTG). The gratings are simultaneously tuned by a stretching device and act as the end reflector of the laser cavity. By applying a ramp signal to the actuator synchronized to the optical switch, the laser signal sweeps over two different wavelength intervals, depending on the connected DTG. This approach represents a hybrid wavelength-time domain interrogation for multiplexed sensors and doubles the number of sensors that may be addressed when compared with single DTG scanning. In addition, the use of the DTG allows a fivefold increase in the strain tuned wavelength interval over standard fiber Bragg gratings. An example application is demonstrated where the temperature in an electrical motor is measured during the machine operation.

The concept of nested fibre optic long period grating (LPG) based interferometers is introduced. A number of in-series, identical LPGs may be used to form a set of nested, multiplexed Mach-Zehnder interferometers that may demodulated and demultiplexed by virtue of a Fourier analysis of the optical spectrum. The concept is demonstrated by the use of three LPGs to form a nested set of interferometers.

The aim of this work is to demonstrate the efficiency of fiber Bragg grating sensors to be used for in situ railway monitoring and train tracking applications. In the specific case, FBGs (Fiber Bragg Gratings) sensors have been bonded to rails in order to perform two different kinds of measurements: dynamic strain to analyze the characteristic frequency response of the rail and train tracking (speed and rail deformation when loaded by running trains). The efficiency of the sensing system has been verified in terms of significance of the information retrieved by the sensing data resolution and the high speed response. The obtained results confirm the real possibility to adopt fiber optic sensors based on FBG technology as excellent devices to ensure multipoint monitoring of railway structures taking advantages of the typical peculiarities of FBG such as long distance interrogation, easy multiplexing, electromagnetic interferences immunity.

Dual-channel fiber ring down force sensor based on the principle of time division multiplexing is descried. The system has the capability to interrogate more than two micro bend sensors by using more than one fiber delay line to control the time of flight of the pulse. The dual-channel fiber ring down force sensor with one fiber delay coil is demonstrated with high force sensitivities of 183.4ns/N and 163.6ns/N, respectively. Our system has also good linear responses.

In this paper, we propose an improvement of the Chirped-Pulsed Frequency Modulation (C-PFM) FBGs reading technique [8] as supported by new experimental results. The C-PFM technique, which was basically translated from its counterpart in the field of radar signal analysis, exploits the intensity modulation of the probe signal (the light traveling along the fiber in our case) by means of a sinusoid with a linearly variable frequency and a train of pulses, to improve the spatial resolution of the acquisition system. The response discrimination of the FBG sensors is achieved thanks to an infiber linear filter and a novel adaptive numerical filtering as it will be better explained in the following. Using a peculiar time window shaping (Blackman) of the light pulse we intended to improve the cross-talk features of the reading technique.

This work presents an optical fiber laser used as a sensor; the system consists of a laser formed by two Fabry-Perot type coupled cavities, two gratings as references and two more gratings as sensors.

Because of the characteristic of wavelength encoding, fiber Bragg grating (FBG) has the advantages of immunity to light power fluctuation, variation in polarization and connecting loss, so it has high sensing precision. However, for the demodulation of FBG, wavelength signal is usually converted to electric amplitude signal. By measuring the amplitude signal, the sensing result is obtained. It is well know that the amplitude signal is easy to be disturbed in sense. For this reason, amplitude demodulation limits the effect of wavelength encoding of FBG. This paper presents a novel method of counting wavelength demodulation for FBG sensors using a high birefringent fiber (HBF) loop mirror. This demodulator has simple structure, high precision, low cost and convenient to use. The resolution of the loop mirror device with 30 meter long of HBF is 0.067 nm. This counting wavelength demodulating method has the significance for widespread practical application of FBG sensors.

This paper presents the construction and working principles of a reflectometric intelligent fiber-optic sensor used for liquid examination. Unlike other well-known fiber optical sensors which use information from optical wavelength variations, the proposed system uses time domain data. The sensing element consists of a length of optical fiber and a short section of optical capillary and works on the reflection intensity basis. The reflected signal level depends on the optical construction of the sensor element. The changes of the monitored signal are caused mainly by variation in light propagation conditions at the interfaces of liquid and gaseous phases. The physical effects involved are Fresnel reflection, local numerical aperture variation and liquid lenses formation. It is possible to call out across the changes of those effects by introducing a measuring procedure which includes submerging, submersion, emerging and emergence of the sensing head from the examined liquid or by local heating of the liquid sample.

Design and characterization of a new generation of single photon avalanche diodes (SPAD) array, manufactured by STMicroelectronics in Catania, Italy, are presented. Device performances, investigated in several experimental conditions and here reported, demonstrate their suitability in many applications. SPADs are thin p-n junctions operating above the breakdown condition in Geiger mode at low voltage. In this regime a single charged carrier injected into the depleted layer can trigger a self-sustaining avalanche, originating a detectable signal. Dark counting rate at room temperature is down to 10 s^-1 for devices with an active area of 10 μm in diameter, and 10³ s^-1 for those of 50 &mgr;m. SPAD quantum efficiency, measured in the range 350÷1050 nm, can be comparable to that of a typical silicon based detector and reaches the values of about 50% at 550 nm for bigger samples. Finally, the low production costs and the possibility of integrating are other favorable features in sight of highly dense integrated 1-D or 2-D arrays.

A fiber ring down with a weak long period grating for temperature and strain sensing is proposed and demonstrated. The sensing devices consist of two matched fiber Bragg gratings, a section of single mode fiber, a weak long period grating, a pulsed laser source and a photodiode. The LPG incorporated in fiber ring cavity is used as sensing element, the induced resonance wavelength shift of which will cause an additional loss in the cavity. As a result, the amplitude of applied temperature variation or strain can be obtained by simply measuring the fiber ring down time. The proposed sensing system has advantages of easy configuration and rapid response.

A new spectral processing technique designed for its application in the on-line detection and classification of arc-welding defects is presented in this paper. A non-invasive fiber sensor embedded within a TIG torch collects the plasma radiation originated during the welding process. The spectral information is then processed by means of two consecutive stages. A compression algorithm is first applied to the data allowing real-time analysis. The selected spectral bands are then used to feed a classification algorithm, which will be demonstrated to provide an efficient weld defect detection and classification. The results obtained with the proposed technique are compared to a similar processing scheme presented in a previous paper, giving rise to an improvement in the performance of the monitoring system.

A data processing method for hyperspectral images is presented. Each image contains the whole diffuse reflectance spectra of the analyzed material for all the spatial positions along a specific line of vision. This data processing method is composed of two blocks: data compression and classification unit. Data compression is performed by means of Principal Component Analysis (PCA) and the spectral interpretation algorithm for classification is the Spectral Angle Mapper (SAM). This strategy of classification applying PCA and SAM has been successfully tested on the raw material on-line characterization in the tobacco industry. In this application case the desired raw material (tobacco leaves) should be discriminated from other unwanted spurious materials, such as plastic, cardboard, leather, candy paper, etc. Hyperspectral images are recorded by a spectroscopic sensor consisting of a monochromatic camera and a passive Prism- Grating-Prism device. Performance results are compared with a spectral interpretation algorithm based on Artificial Neural Networks (ANN).

This paper reports on an optical fibre based sensor system to detect the occurrence of premature browning in ground beef. Premature browning (PMB) occurs when, at a temperature below the pasteurisation temperature of 71°C, there are no traces of pink meat left in the patty. PMB is more frequent in poorer quality beef or beef that has been stored under imperfect conditions. The experimental work pertaining to this paper involved cooking fresh meat and meat that has been stored in a freezer for, 1 week, 1 month and 3 months and recording the reflected spectra and temperature at the core of the product, during the cooking process, in order to develop a classifier based on the spectral response and using a Self-Organising Map (SOM) to classify the patties into one of four categories, based on their colour. The combination of both the classifier and temperature data can be used to determine the presence of PMB for a given patty and can thus be used for Quality Control by food producers.

The relative intensity noise in sinusoidally modulated fiber optics gyroscopes is analyzed in order to determine which is the value of the minimum detectable signal by the demodulator electronics circuits. The analysis is conducted for both the well known lock-in amplifier demodulator and the mean value demodulator (MVD). Numerical results show that, in intensity noise-limited fiber optics gyroscopes, the MVD allows for the demodulation of smaller signals when compared to the lock-in amplifier.

We describe experiments aimed at assessing the applicability of fibre Bragg grating sensors to distributive tactile sensing. Strain signals from flexible surfaces instrumented with Bragg grating sensors are processed using neural networks so as to obtain the location, shape and orientation of objects placed on the surfaces.

This paper presents a novel approach that uses Artificial Neural Networks - ANN to extend the measurement range of Fiber Bragg Gratings - FBG interrogators based on fixed narrow band filter demodulation. Interrogators with fixed spectral filters use only one edge of the filter to demodulate the signal. The system proposed uses narrow band FBG's filters, where the entire filter bandwidth is applied to demodulate the signal. Furthermore, the approach has possibility to concatenate n filters, obtaining a measuring range n×bandwith_F, where bandwith_F is the bandwidth of a demodulating FBG filter. The great advantage of this method relies on the use of ANN to combine these signals, mitigating the ambiguities created in the both edges of each FBG filters, and generating a continuous linear output measuring range. Despite of this proposed demodulation method can be used for strain measurements, yet it is in the temperature sensing that the method offers better contribution. Wide temperature measuring ranges are common in the petrochemical industry, such as distillation columns (0 to 700°C), or in power plants to measure the temperature of transformer windings (-30 to 150°C). This paper presents theoretical results of a temperature measurement system in power transformers application.

A multi-point fibre optic temperature sensor network integrated inside a power transformer for continuous monitoring of hot-spots on windings, cellulose insulations and oil, is demonstrated and tested. The temperature sensors are based on proprietary encapsulated fibre Bragg grating (FBG) sensors and the optical interrogation unit uses a special designed narrowband high power broadband fibre source. The fibre optic sensing network is integrated into a 440 MVA power transformer having 12 temperature sensing points, distributed over several physical locations inside de transformer (windings, cellulose insulators, magnetic circuit and cooling oil entrance and exit).

The influence of temperature in the measurements of surrounding refractive index using long-period fiber gratings is studied for room temperature variations. For temperature changes close to 2°C it is verified wavelength shifts lower than 0.1 nm for the grating immersed in air and as high as 1.1 nm for a hydrocarbon sample whose refractive index is 1.4530. Sensitivity evaluations of long-period fiber gratings and evanescent fiber sensors require a temperature system control that keeps the temperature constant. Otherwise, it should be considered compensation systems that consider not only the temperature changes but also the refractive index and the thermal-optical coefficient of the materials under analysis.

An effort to develop a miniaturized multichannel optical fiber Bragg grating sensor interrogator was initiated in 2006 under the Small Business Innovative Research (SBIR) program. The goal was to develop an interrogator that would be sufficiently small and light to be incorporated into a health monitoring system for use on tactical missiles. Two companies, Intelligent Fiber Optic Systems Corporation (IFOS) and Redondo Optics, were funded in Phase I, and this paper describes the prototype interrogators that were developed. The two companies took very different approaches: IFOS focused on developing a unit that would have a high channel count and high resolution, using off-the-shelf components, while Redondo Optics chose to develop a unit that would be very small and lightweight, using custom designed integrated optical chips. It is believed that both approaches will result in interrogators that will be significantly small, lighter, and possibly even more precise than what is currently commercially available. This paper will also briefly describe some of the sensing concepts that may be used to interrogate the health of the solid rocket motors used in many missile systems. The sponsor of this program was NAVAIR PMA 280.