This PDF file contains the front matter associated with SPIE Proceedings Volume 75303, including the Title Page, Copyright information, Table of Contents, the Conference Committee listing and introduction.

Numerical simulations predict that the coherent backscattering noise of a fiber optic gyroscope (FOG) interrogated with a laser decreases when the laser's coherence length exceeds the length of the fiber loop. With a sufficiently coherent laser, this noise drops below the excess noise of the same FOG probed with broadband light. This new principle is paramount for the air-core FOG, which has a low sensitivity to the Kerr effect but exhibits stronger backscattering. It is also applicable to a conventional FOG. We provide evidence of this effect in an experimental FOG.

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

This talk discusses some sensing applications of hollow-core photonic bandgap fibers (HC-PBFs) including gyroscopes, hydrophones, gas/liquid detectors, long period fiber grating sensors, and in-fiber interferometric sensors. The development of HC-PBF components is also discussed.

The fabrication of different types of optical fiber sensors using nanofilms implementing different optical fibers based architectures will be explained. Thanks to the control on the nanometer scale of the thickness and composition of the nanofilms it is possible to fabricate very selective and sensitive sensors. It will be demonstrated that fiber optic sensors and nanotechnology have come together, in recent years, to provide new opportunities for sensing. The possibility of depositing sensitive nanofilms with thicknesses under the wavelength of the interrogating optical light signal, make necessary to re-invent many of the classical photonic sensors and devices. New phenomena and properties are appearing.

PHOSFOS (Photonic Skins For Optical Sensing) is a research project funded by the European Commission's 7^th Framework Programme. The project aims at developing a flexible and stretchable foil that integrates highly advanced optical fibre sensing elements as well as optical and electrical powering functionalities and read-out of the sensors. This skin-like foil can be wrapped around or attached to irregularly shaped objects or bodies and will allow quasi-distributed sensing of mechanical quantities such as deformation and pressure. The applications targeted can be found in the fields of structural health monitoring and healthcare.

We demonstrate for the first time true distributed sensing of scale and location of transverse loading along an extended length of polarisation maintaining [PM] fibre with high spatial resolution using Optical Frequency Domain Reflectometry [OFDR]. Transverse loading of PM fibres induces local and differential changes in refractive index in slow and fast axis, depending on scale and direction of load relative to the axes. The change in refractive index is detected by monitoring the change in local periodicity of the statically distributed Rayleigh scatter for each PM fibre axis simultaneously using OFDR.

A distributed strain measurement with millimeter-order spatial resolution was demonstrated in optical fibers based on Brillouin optical correlation-domain reflectometry (BOCDR) employing a tellurite glass fiber as the fiber under test. First, the dependences of Brillouin frequency shifts (BFSs) on strain and temperature in the tellurite fiber were investigated, and they showed negative dependences with coefficients of -0.023 MHz/με and -1.14 MHz/K, respectively. Then, using this tellurite fiber, the BFSs around 1-cm strain-applied section were successfully measured based on BOCDR with a nominal spatial resolution of 6 mm.

The various fiber Bragg grating(FBG)sensors such as FBG force rings, differential FBG displacement cells, FBG strain sensors and FBG temperature sensors had been used to monitor the strengthening and maintenance process of the continuous concrete beam bridges and the continuous concrete rigid frame bridges which are the part of Wuhan Second Yangtze River Bridge. In the strengthening and maintenance process, the tension force of the external prestressed tendons, the cracks change and intensity of cross sections had been monitored to insure the instruction safety, study the effect of strengthening and maintenance, and verify the design theories of strengthening and maintenance. Also the reference state criterion for long-term bridge health monitoring had been provided according to the monitoring results.

An effective-soil-pressure sensor for geotechnical applications based on Fibre Bragg Gratings is presented. The sensor simultaneous measures total soil pressure and pore pressure, allowing the calculation of the effective stress of soil. Calibration of the sensor using pressurised air demonstrated a pressure sensitivity of 2.02x10^-3 ± 2.84x10^-5nm/kPa and 1.87x10^-3 ± 6.88x10^-5nm/kPa for the total and pore pressure respectively. This corresponds to a pressure resolution of 4.95x10^-1kPa and 5.46x10^-1kPa for total and pore pressure using a 1pm interrogation system. Measurements undertaken in two types of soil demonstrated dependence of the total pressure sensitivity on soil density/stiffness. Pore pressure measurements agreed well with the preliminary calibration.

This paper presents the full-spectral measurement of fiber Bragg grating sensor responses during impact testing of composite laminates. The sensors are embedded in carbon fiber/epoxy laminates which are subjected to multiple low velocity impacts until perforation of the laminate occurs. Applying a recently developed high-speed interrogator, the Bragg grating sensor interrogation is demonstrated at 534 Hz over a 14.9 nm bandwidth. The measurement of the transient response of the grating sensors during impact reveals unique spectral signatures that could not be detected through peak-wavelength monitoring or post-impact full-spectral scanning of the sensors, including local relaxation of the laminate.

We report the development of a comprehensive safety monitoring solution for coal mines. A number of fibre optic sensors have been developed and deployed for safety monitoring of mine roof integrity and hazardous gases. The FOS-based mine hazard detection system offers unique advantages of intrinsic safety, multi-location and multi-parameter monitoring. They can be potentially used to build expert systems for mine hazard early detection and prevention.

This paper presents a novel concept of simultaneously measuring pressure and temperature using a silica optical fibre extrinsic Fabry-Perot interferometric (EFPI) pressure sensor incorporating a fibre Bragg grating (FBG), which is constructed entirely from fused-silica. The novel device is used to simultaneously provide accurate pressure and temperature readings at the point of measurement. Furthermore, the FBG temperature measurement is used to eliminate the temperature cross-sensitivity of the EFPI pressure sensor.

A simple system for sensing temperature in multiple zones based on a multi-wavelength Brillouin fiber laser ring is presented. Optical fiber reels are serially concatenated and divided in zones (one per sensing area). Setting the Brillouin lasing in each spool of fiber generates a characteristic wavelength that depends on the fiber properties and the temperature in the zone. Thus, it is possible to measure temperature independently and accurately through heterodyne detection between two narrow laser signals. The proposed sensor integrates the temperature along the whole spool of fiber in each zone. These real time measurements were successfully checked in our laboratory.

The interest in using optical sensors as replacements for typical electrical sensors in the oil and gas industry has increased as the potential benefits and reliability of such technologies become more established. Presented here are the results from initial tests using optical temperature sensors to monitor the effectiveness of subsea tree insulation to retain heat within the flow-loops of a Christmas Tree to prevent hydrate formation during system cool-down. Initial tests were carried out to compare the optical fibre sensor performance with traditional electrical thermocouples. Suggestions for further development and future tests are also presented and discussed.

Here, the fabrication and characterization of a novel optical fiber humidity sensor based on surface plasmon resonance (SPR) in the infra-red region is presented. Firstly, an indium tin oxide (ITO) coating is deposited onto a 200 μm core diameter optical fiber causing the shift of the SPR wavelength to the infra-red region. Then, the LbL method is used to deposit a polymeric coating onto the ITO layer. The variations in the external humidity originated changes in the thickness and refractive index of the polymeric coating and hence in the resonance.

Fibre Bragg gratings (FBGs) in polymer optical fibres (POFs) have been used to measure the strain in a woven textile. FBGs in both POFs and silica optical fibres were attached to a woven textile specimen, and their performance characterised. It was demonstrated that the POF FBGs provide improved strain transfer coefficients and reduce local structural reinforcement compared to silica FBGs and therefore make a more suitable proposition for textile monitoring.

Biophotonics is a new and highly interdisciplinary scientific discipline comprising the application of light (i.e. innovative photonic tools) in life sciences. It is non exaggerated to say Biophotonics is on the way to solve the most important problems in biomedicine. In particular Raman microspectroscopy allows one to derive detailed and specific information on a molecular level which other photonic methods methods can only provide by a limited extent. Here we will present latest results of our own research dealing with the application and development of innovative Raman spectroscopic techniques for biomedical applications.

Surface plasmon resonance (SPR) biosensors represent the most advanced label-free optical affinity biosensor technology. In the last decade numerous SPR sensor platforms have been developed and applied in the life sciences and bioanalytics. This contribution reviews the state of the art in the development of SPR (bio)sensor technology and presents selected results of research into SPR biosensors at the Institute of Photonics and Electronics, Prague. The developments discussed in detail include a miniature fiber optic SPR sensor for localized measurements, a compact SPR sensor for field use and a multichannel SPR sensor for high-throughput screening. Examples of applications for the detection of analytes related to medical diagnostics (biomarkers, hormones, antibodies), environmental monitoring (endocrine disrupting compounds), and food safety (pathogens and toxins) are given.

Optical sensing has played an important role in medicine since the 1980s mainly thanks to the invasive capabilities and unique performance of the optical fibres that have allowed measurements inside the human body, otherwise impracticable. But optical fibres are also becoming more and more important for minimally invasive and not-invasive applications.

This paper reports the development of a fiber-optic gas detection system capable of detecting three types of dissolved fault gases in oil-filled power transformers or equipment. The system is based on absorption spectroscopy and the target gases include acetylene (C₂H₂), methane (CH₄) and ethylene (C2H₄). Low-cost multi-pass sensor heads using fiber coupled micro-optic cells are employed for which the interaction length is up to 4m. Also, reference gas cells made of photonic bandgap (PBG) fiber are implemented. The minimum detectable gas concentrations for methane, acetylene and ethylene are 5ppm, 2ppm and 50ppm respectively.

Optical fibres are a platform of interest for biosensing due to their low cost, high robustness and long interaction length with their surroundings. However, the sensitivity of these techniques lags behind chemiluminescence and other techniques. Here we explore opportunities for improving the sensitivity of fibre-based biosensors by investigating the process by which biomolecules are immobilized onto the glass. Two immobilization technologies are compared: the well-known silane technology and the emerging polyelectrolyte method. This was done using a specific interaction model based on biotin-streptavidin. The number of specific binding sites as well as the induced losses within a microstructured fibre has been characterized.

We report the design and use of multi-channel fibre Bragg grating based manometry catheters with pressure sensors spaced at 1 cm intervals along its axis. The catheters have been tested in-vivo in both the human oesophagus and colon and have been shown to provide analogous results to commercially available solid state pressure sensors. The advantage of using fibre gratings comes from the ability to extend the number of sensor elements without increasing the diameter or complexity of the catheter or data acquisition system. We present our progress towards the fabrication of a manometry catheter suitable for recording manometric data along the full length of the human colon. Results from early phase equivalence testing and recent in-vivo trials in the human oesophagus and colon are presented. The colonic recordings were taken in basal and post-prandial periods of 2.5 hours each. The close axial spacing of the pressure sensors has identified the complex nature of propagating sequences in the colon in both antegrade (towards the anus) and retrograde (away from the anus) for the first time. By sub-sampling the data using data from sensors 7 cm apart the potential to misrepresent propagating sequences at wider sensor spacings is demonstrated and proposed as a potential reason why correlation between peristaltic abnormalities recorded using traditional catheters, with 7.5-10 cm spaced sensors, and actual patient symptoms remains elusive.

In this work, surface plasmon resonance (SPR) based optical fiber sensors are proposed by using a transparent conductive coating, Indium Tin Oxide (ITO) in our case, as the SPR supporting layer. The utilization of these ITO coatings instead of using a thin film of gold or silver shifts the plasmon resonance band to the infra-red region and allows the tunability of the SPR wavelength by adjusting the film fabrication parameters. The spectral response of these novel devices is characterized, achieving a sensitivity of 0.00025 refractive index units per nanometer, which opens the door for a wide range of applications.

OCT probes for endoscopy typically use a single optical fibre to deliver light to the measurement region. 2D imaging is achieved using mechanical scanning components at the probe tip. We are investigating an OCT system in which fibre imaging bundles replace the single probe fibre, allowing many thousand measurement points to be addressed without any mechanical scanning at the probe tip, over a region measuring a few millimetres in each dimension. This allows a small-diameter, electrically passive probe to be engineered using offthe- shelf scanning optics. Images from a single-fibre, conventionally-scanned probe and from a bundle-based probe are presented, using swept-source processing centred at 1330 nm.

We demonstrate a simple but highly sensitive hydrogen sensor based on palladium-coated long-period fiber grating (LPG) inscribed in low core index fiber, which induces higher order cladding modes. As palladium layer absorbed 4% of hydrogen gas, the dual resonant wavelengths of the higher order cladding mode (LP₀₈) are shifted to the opposite direction. The spectral sensitivity was much higher than those of other fiber-optic palladium-coated hydrogen sensors.

A stable, tunable, CW, narrow-linewdith ring distributed feedback (DFB) fibre laser fabricated in erbium-doped photonic crystal fibre is demonstrated. Wavelength tuning of this device using a piezoelectric transducer was utilized to probe the P11 molecular absorption band of acetylene (C₂H₂) with a resolution of 0.45pm. Furthermore, by applying an AC voltage this absorption band could be continually-swept, allowing square-wave modulation of the DFB output at repetition rate of 500Hz.

In this paper, the evaluation and testing of a fibre optic relative humidity sensor specifically designed for structural health monitoring is discussed in detail. The sensor is based on a polymer coated Fibre Bragg Grating (FBG) where with the increase of the moisture level, the polymer coating swells and thus applies strain to the FBG, causing a Bragg wavelength shift. The device is designed so that the temperature effect on the humidity sensor can be compensated by the inclusion of a second un-coated FBG packaged inside the same sensor probe. The sensor thus produced has been tested extensively and calibrated using both a humidity chamber and standard salt solutions. The probe has demonstrated a very good performance for relative humidity measurements.

This paper presents a Surface Plasmon Resonance (SPR) fiber sensor capable of monitoring the deposition of thin nm-scale polymer films. The sensor was used for in situ monitoring of the adsorption of 30 mono-layers of oppositely charged polyelectrolytes with an overall thickness of each individual monolayer less than 0.55 nm. The in situ monitoring of the formation of multi-layer structures was implemented through the excitation of SPR on the surface of a gold-coated fiber with a Tilted Fiber Bragg Grating (TFBG) written in the core of the optical fiber.

Fibre laser sources offer interesting possibilities for gas sensors since they can operate over an extended wavelength range, encompassing the near-IR absorption lines of a number of important gases but a major problem is that overtone absorption lines of gases in the near-IR are relatively weak. In order to enhance sensitivity, we present here a simple method of intra-cavity laser absorption spectroscopy (ICLAS) which makes use of the amplified spontaneous emission (ASE) already present within a fibre laser cavity. The ASE also provides a convenient broad-band source for the interrogation of several gases within the gain-bandwidth of the fibre laser. The key principle is based on adjusting the cavity attenuation to select an appropriate inversion level and hence flatten the erbium-fibre gain curve. Under this condition, the ASE undergoes multiple circulations within the fibre laser cavity, enhancing the effective path-length of a gas cell placed within the laser cavity. We have experimentally demonstrated the principle of operation with acetylene gas, using a simple erbium fibre laser system containing a 6cm path-length, fibre coupled, intra-cavity, micro-optic gas cell. For 1% acetylene gas, we have experimentally observed 16 absorption lines in the 1530nm region and a path length enhancement of ~60 has been demonstrated, transforming the 6cm micro-optic cell into an effective path length of ~3.5m. Apart from the OSA, all components are inexpensive and the system is very simple to construct and operate.

A limiting factor of tuneable diode laser spectroscopy (TDLS) with wavelength modulation spectroscopy (WMS) is the presence of background residual amplitude modulation (RAM) on the recovered 1^st harmonic signal. The presence of this background term is due to direct modulation of the source laser power. This work presents a novel method to optically remove the unwanted background, with the major benefit being that measurement sensitivity can be increased. The recently developed phasor decomposition method1 (PDM), is a near IR (NIR) TDLS analysis technique that is used with the addition of the new RAM nulling method to recover gas absorption line-shapes. The PDM is a calibration free approach, which recovers the gas absorption line-shape and the isolated 1^st derivative of the line-shape from the 1^st harmonic signal. The work presented illustrates and validates the new RAM nulling procedure with measurements examining the 1650.96nm absorption line of methane (CH₄) with comparisons to theory.

In this paper, research into the fabrication and characterization of a new fibre optic sensor for chloride ion detection is reported. This sensor is based on the fluorescence quenching of an acridinium dye, which is covalently bound to polymer substrates, in presence of chloride ions. The active material is formed as a powder and packed in a tablet form. Calibration data shows that this fibre optic sensor shows a sensitivity to chloride at concentrations larger than 0.1M.

The cladding mode resonances of a tilted fiber Bragg grating show very strong polarization splitting for modes near cutoff, as determined by the surrounding refractive index. Rotating the linear polarization of the core guided light results in amplitude changes that can exceed 10 dB, but only for a few modes. The wavelengths of these highly sensitive modes can then be used to determine the surrounding refractive index.

In this work, a novel CWDM (Coarse Wavelength Division Multiplexing) network that multiplexes intensity modulated optical fibre sensors is presented. These transducers are based on a sensing material that suffers reversible changes in its optical properties in presence of organic vapours. This compound is fixed onto cleaved ended single mode optical fibres pigtails using Electrostatic Self-Assembly method. As their response depends on intensity changes, a referencing signal is simultaneously generated to eliminate the effect of optical source power fluctuations. Thus, a parallel array of phaseshifted fibre Bragg gratings (PS-FBGs) is included to obtain the referencing signal. Looking forward to remote sensing applications, an Erbium doped fibre amplifier (EDFA) is included in the header of the network to reach up to 4 km of distance. Four new sensors have been multiplexed and exposed to saturated atmospheres of methanol and ethanol individually, showing different response and recovery times in each case. Finally, we checked that our sensors were able to distinguish between pure and kerosene polluted water probes in terms of response and recovery times as well.

We report an all-fiber hydrogen sensing system for low-temperature operation. The sensor consists of a fiber Bragg grating written in high-attenuation fiber and coated in Palladium. Heating the sensor with in-fiber light power greatly enhances sensitivity at low temperatures. A multi-functional infrared light source is used to provide both in-fiber heating and sensor monitoring. This technology promises a single fiber feedthrough solution for low temperature multipoint hydrogen leak detection.

The development of a rapid response time, plastic optical fiber (POF) based oxygen sensor for the measurement of oxygen partial pressure (pO₂) is described. The POF oxygen sensor is based on the luminescence quenching of a luminophore by oxygen and consists of PMMA plastic optical fibre and a polymer matrix medium that contains immobilized Pt (II) complexes. The performance of the POF oxygen sensors was evaluated using luminescence intensity measurement. In order to accurately determine the response time of the POF oxygen sensors, a test chamber was used to provide rapid step changes in the partial pressure of oxygen. The details of the sensor chemistry, experimental system and the results are presented.

In this study, we report the fabrication and engineering of an infrared fibre evanescent wave spectroscopic (FEWS) sensor using GeTeSe chalcogenide fibres, which manifests high temperature durability (glass transformation temperature at 260°C) and extended infrared transmission from 1.5 to 18 μm. We also report the methodologies adopted for the fabrication of fibres, including the purification of starting chemicals for high spectral purity, glass melting, quenching and annealing, fabrication of fibre performs. The designed FEWS sensor consists of a 40 cm unstructured GeTeSe fibre coupled with a Fourier transform infrared (FTIR) spectrometer and a Mercury-Cadmium-Tellurium (MCT) infrared detector, and the active sensing length of the fibre is 3 cm. In this paper, we demonstrate the sensing results with various analytes and investigate the sensitivity of this sensor.

An optical fibre sensor for determination of acetic acid is presented. The sensing probe is based on a fiber Bragg grating (FBG) Fabry-Perot cavity, coated with a thin film of sol-gel-PVP (polyVynil Pirrolidone) composite material. The polymeric thin film renders interferometric output sensitive to the presence of carboxylic acid species. Results show that the wavelength of the interferometric peaks change with acetic acid concentration, enabling its quantification. Coupling the fibre probe with a serrodyne modulated readout interferometer enables pseudo-heterodyne interrogation and the detection of acetic acid with a sensitivity of 1 deg/0.01% and a resolution of 0.2% v/v. The results demonstrate the potential of the proposed scheme to operate as a sensitive chemical sensor platform.

A new interrogation method based on Fibre Bragg Gratings (FBG) for Surface Plamon Resonance (SPR) sensors in the region of refractive indices of aqueous solutions is described. Two FBGs are selected with their Bragg wavelengths at opposite sides of the plasmon resonance peak. The response of the system can be made independent of the fluctuations of the optical power source, and the linearity and the sensitivity of the sensor are improved. The use of the spectral selectivity of gratings for the interrogation of SPR sensors in different configurations is also promising in terms of multiplexing, temperature referencing or multiparameter detection.

We demonstrate a new method that could make possible the mass production of symmetric and asymmetric corrugated long period fiber gratings (C-LPFG) by utilizing hot embossing and imprint lithography on polycarbonate (PC). The poly-dimethylsiloxane (PDMS) is imprinted on PC to create the periodic revealed region of fiber for hydrofluoric acid (HF) etching. The asymmetric C-LPFGs show some unique optical characteristics of two separate dips due to their asymmetric structures. After the torsion, the second dip would be gone and the first one would begin to shift toward shorter wavelength. It is assumed that the asymmetric C-LPFG behaves similar to the symmetric one with the torsion of a full circle. A C-LPFG was well packaged by fixing itself in the PC and having some open area close to the fiber to receive the liquid under test. The corrugated structures close to the fiber core were covered by the liquid that the packaged refractive index sensor has the sensitivity of 26.7 nm/RIU.

This paper presents a high-performance fiber-optic carbon dioxide (CO₂) sensor based on sol-gel matrix composed of n-octyltriethoxysilane (Octyl-triEOS)/tetraethylorthosilane (TEOS) doped with pH-sensitive fluorescent dye and silica particles. The sensor film consists of 1-hydroxy-3,6,8-pyrenetrisulfonic acid trisodium salt (HPTS, PTS-), silica particles, tetraoctylammonium cation (TOA+), and a tetraoctylammonium hydroxide (TOAOH) phase transfer agent (i.e. the base) immobilized within the sol-gel matrix. The sensor has a sensitivity of approximately 26 and exhibits a uniquely linear response for CO₂ concentrations in the range 0~100 %. The response time of the sensor is 9.8 s when switching from a pure N2 atmosphere to a pure CO₂ atmosphere and 195.4 s when switching from CO₂ to N₂.

A support vector regression (SVR) system is proposed for recognizing chemical compounds characterized by near infrared molecular absorption spectrometry. The results demonstrate SVR system has a high reliability in the trace gas detection field.

A static tandem Michelson interferometer configuration is reported for remote measurements of group delay change in a thermally modulated, optically dispersive BK7 glass sample. Using a superluminescent diode (SLD) to illuminate the interferometer, low-coherence measurement interferograms with signal-to-noise ratios as low as 16 dB were captured and subsequently processed using dispersive Fourier transform spectrometry (DFTS). Measurements of thermallyinduced delay change were made with < 2 fs root mean square error for optical path delay scans lengths of only 260 μm.

In this work, we present preliminary results concerning the development of a platform for biochemical sensing based on overlayered Long Period Gratings (LPGs) working in transition mode. The surrounding medium refractive index (SRI) sensitivity of the bare device was greatly enhanced by means of the modal transition phenomenon which is enabled in LPGs with a nanoscale polymer overlay of higher refractive index than the cladding. This strategy was pursued to develop a sensitive tool for biochemical detection in aqueous solutions. Dip-coating was used to deposit thin layers of Polystyrene (PS) with different thicknesses on the same LPG to test the performances of the device in different working points of its modified sensitivity characteristic. The PS overlay surface was modified through adsorption of biotinylated bovin serum albumin (BBSA). Adsorption dynamic of BBSA was observed by on-line monitoring the shift of the LPGs attenuation bands. In the same way also a subsequent steptavidin (SA) binding dynamic on the byotinilated sites of the adsorbed proteins was recorded. Several washing cycles between the steps showed a rather stable immobilization of the protein monolayers. Experimental results show that overlayered LPGs are among the most sensitive refractive index transducers to be employed in biochemical detection and that wide margins of further optimization exist.

We aim at composing a novel remote monitoring system of the water level and soil humidity under the natural environment with a hetero-core spliced fiber optic chemical sensor inserted into an existing communication line. This paper demonstrates the fusion characteristics of an optical communications and a humidity sensing by means of a hetero-core spliced fiber optic sensor that has been highly-vaunted from the practical perspective. However, this humidity sensor leaks light and attenuates the optical signal power of a communication wavelength by intent on sensing use. From the communications standpoint, this may lead to decrease intensity of optical signals sent to the receiver and increase a bit error. In other words, there may be a trade-off relationship between communications quality and measuring accuracy. We have proved the possibility of the co-existence of communication signals and measuring functions by finding out the effect of humidity sensing on the communications is the optical loss and modal dispersion.

Species selective vapour sensing has been demonstrated using a coated Long Period Grating with a period and coating thickness that gives rise to a dual resonant response. A calixarene coating with nanostructured features was deposited using the Langmuir-Blodgett technique. A nanoscale cavity in the calixarene molecule adsorbs the analyte molecule causing an increase in the density and thereby the refractive index of the coating. Preliminary vapour sensing experiments were performed and indicate the sensor's selectivity, demonstrating strong responses to toluene and benzene, while exhibiting lower sensitivity to hexane and cyclohexane.

An optical fiber refractometer based on the Fresnel reflection in the fiber tip and twin FBG interferometer located near the fiber tip for light intensity referencing is reported. A wavelength scanning DFB diode laser was used for RI sensor interrogation. Signal processing in the Fourier domain allows intensity-independent accurate measurements for liquids and gases with practically any RI. A resolution of 5 - 10 5^-5 was demonstrated experimentally using different liquids including an anti-reflection index-matching liquid. Simple construction and the use of low-cost components make it interesting for many applications.

We report the fabrication and characterization of tapered fibers covered with porphyrin monolayer films prepared by Langmuir-Blodgett (LB) deposition method. The studied molecule was 10 mol-% 5,10,15,20- tetrakis(pentafluorophenyl)porphyrin (PFP) entrapped in an octadecylamine (ODA) matrix. PFP molecules, deposited on plane glass surfaces, have relatively long fluorescence lifetime (~ 4 ns) together with high fluorescence efficiency. Therefore, such photoactive materials, example of which are PFP molecules, hold much promise for the development of chemical sensors and efficient light harvesting devices.

Combustible gases, such as propane and methane-based city gas, are widely used in many homes and factories. However, accidents caused by gas leakage have become a menace to everyday life. Therefore, we have developed a simple and safe optical sensor for detecting leaking gas, which consists of plastic optical fiber (POF) coated with swelling polymer as a sensitive cladding layer. In experiments, a gas concentration below 3% was detected for both propane and methane gas. However, it was also confirmed that this system is influenced by humidity. Therefore, further improvement of the POF gas-sensing system with a POF humidity calibrator is proposed and discussed in this paper.

Over the last few years, several novel quantitative phase imaging techniques have been developed for the study of biological cells. However, many of these techniques are encumbered by inherent limitations including 2π phase ambiguities and diffraction limited spatial resolution. In addition, subsurface information in the phase data is not exploited. We hereby present a novel quantitative phase imaging system without 2 π ambiguities, which also allows for subsurface imaging and cell refractometry studies. This is accomplished by utilizing simultaneously obtained shear-force topography information. We will demonstrate how the quantitative phase and topography data can be used for subsurface and cell refractometry analysis and will present results for a fabricated structure and a malaria infected red blood cell.

A two wavelength photometric fibre-optic reflectance sensor was developed for assessing abdominal organ perfusion. In vitro tests showed that reliable photoplethysmographic (PPG) signals and blood oxygen saturation (SpO₂) values were recorded when the separation between emitting and receiving fibres was at 3mm and the emitting current was between 20mA-40mA. In a clinical study, good quality PPG signals were obtained from the small and large bowel of a patient undergoing laparotomy. Abdominal organ SpO₂ were in good agreement with those obtained from a commercial device. These observations suggest that the fibre-optic sensor may be suitable for monitoring abdominal organ perfusion.

A novel fiber-optic phase-sensitive optical time-domain reflectometer (Φ-OTDR) distributed sensing system is proposed and demonstrated for long-distance intrusion monitoring. By using the bi-directional Raman optical amplification technology, even intrusion signal detection along the whole sensing distance of ~62km has been demonstrated, resulting in the longest and the most sensitive Φ -OTDR distributed sensing system reported so far, to the best of our knowledge. In addition, high-power single-mode fiber laser with ultra-narrow line-width and special optical fiber cable encapsulated by fiber reinforced plastic (FRP) are used to further enhance the intrusion detection sensitivity, respectively.

The most important challenge for distributed fibre-optic intrusion detection systems is to minimise the nuisance alarm rate without compromising the system sensitivity. Event classification and discrimination is a powerful tool which can be used to minimise nuisance alarm rates whilst maintaining optimum sensitivity and probability of detection. A novel event classification system using supervised neural networks together with a level crossing based feature extraction algorithm is presented for a fence-based fibre-optic intrusion detection system. Performance results are presented showing accurate classification of and discrimination between different intrusion and non-intrusion events such as fence-climbing, fencecutting, stone-throwing and stick-dragging.

A novel tunable fiber ring laser configuration with combination of hybrid Raman amplification and erbium-doped fiber amplification (EDFA) is proposed for realizing ultra-long distance (300km) fiber Bragg grating (FBG) sensor system. The experimental results show that a 300 km transmission distance is achieved with an optical SNR of 4 dB, which is the longest FBG sensing distance, to the best of our knowledge.

A novel self-referencing parameter for electro-optical WDM networks in reflective configuration for remote fiber-optic intensity sensors is reported and experimentally tested. In this configuration electrical delay lines are deployed at the processing unit, instead of long delay fiber coils in each measuring point; providing a more compact sensor-head and an easy-reconfigurable operation point. The measurement parameter is defined as the ratio between voltage values at the reception stage for different electrical phase-shifts and it is analyzed following the Z-transform formalism. Measurements validating the theoretical model are reported and self-referencing and crosstalk properties of the network are analyzed.

A novel fiber Bragg grating (FBG) hydrophone array is proposed in order to realize simultaneous multipoint underwater acoustic detection with thermal stabilization. The interrogation technique of the FBG hydrophone to be arrayed is based on an intensity-modulation scheme, in which a tunable laser is used for an optical source with its oscillation wavelength tuned to a slope of the reflection spectrum curve of the FBG. The light reflected at the sensing FBG is modulated by an underwater acoustic wave and offers a waveform signal as a sensor output. For fabricating the FBG hydrophone array, a multi-wavelength tunable fiber laser is constructed for providing multiple narrowband optical sources and the sensing FBGs are arranged in tandem using a wavelength-division multiplexing (WDM) technique. In the experiment, simultaneous three-point underwater acoustic detection and measurement of sound velocity in water have been successfully demonstrated.

A simple interrogation system for semi-distributed fiber Bragg grating (FBG) sensor array with a tunable pulsed laser is demonstrated. The pulsed light source is used to discriminate, in space or in time, the FBG position. The sensor array can then be constituted with low reflectivity FBG (<5%) and with the same wavelength (the FBGs have to be spatially separated). This improvement increases the capacity of the tunable pulsed laser system interrogation, which can reach up to 1000 sensors in one single fiber. The signals are measured with only one photodetection system, which makes detection less dependent of intensity variations and minimizes external influences on the circuit, such as variations in the environment temperature. A test approach has been assembled for the interrogation of five sensors in the same fiber, varying its reflectivity from 0.8% to 1.6% and with the same nominal wavelength.

A multipoint fiber optic refractive index sensor is presented, that is potentially capable of simultaneous measurement from many samples. The operation of this sensor is based on the reflection of light from a cleaved fiber end when the sensor is moved from one medium to another, while the sensor consists of two fiber couplers and an optical fiber Bragg grating (FBG). Experimental characterization of the sensor shows excellent agreement with numerical analysis and a precision of 0.002 and resolution of the order of 10-3. Through the use of multiple Bragg wavelengths measurement at multiple points is achieved.

This paper presents the application of a new erbium-doped fiber ring laser configuration for the multiplexing of intensity sensors. The proposed scheme exploits two separated branches to generate simultaneous emission at four different wavelengths. Fiber Bragg gratings are used to select the operating wavelengths within the erbium-doped fiber ring. The first branch presents a serial configuration to generate the reference lasers. The second branch exploits a tree configuration and includes the optical intensity temperature sensors. A comparison between both branches of the same setup is carried out, and their temperature and time stability is shown by varying the erbium doped fiber.

We present a novel technique for the calibration of a high speed FBG interrogation system which incorporates an Arrayed Waveguide Grating (AWG). It uses temperature tuning to wavelength sweep the AWG to generate calibration data and to determine the optimum temperature of AWG for the actual test. For ideal operation the FBG wavelengths under zero stimulus would lie midway between the International Telecommunications Union (ITU) grid wavelengths of the AWG. The attached FBG wavelengths cannot always be accurately prescribed since they vary during attachment, and with temperature. This technique reduces this constraint and enables different detector gains to be used on different channels for system optimisation.

The intensity-based measurement could be necessary to smart structures embedding multipoint fiber optic sensors for real-time monitoring such as the human motion in daily activity. Hetero-core fiber optic displacement sensor has great advantage to the intensity-based measurement because the sensitivities of hetero-core fiber optic displacement sensors change by different of the hetero-core insertion length and the transmission wavelength. This paper proposed the new approach for multipoint real-time measurement of hetero-core fiber optic intensity-based sensors. The bending optical loss characteristics of hetero-core fiber optic sensor were measured about four kinds of hetero-core insertion lengths at the two wavelengths, 0.85 μm and 1.31 μm. The bending displacement of two sensors on a single transmission line could be successfully detected by using two hetero-core fiber optic sensors to have the different hetero-core insertion lengths at the two wavelengths of 0.85 μm and 1.31 μm. The accuracies of two sensors, which had the hetero-core insertion lengths of 0.5 and 1.0 mm, with tandem connection were ±0.003 mm and ±0.003 mm of the displacement, ±2.87 % and ±0.63 % for the full-scale displacement of 2 and 0.5 mm, respectively.

In this work, we present and experimentally demonstrate a switchable Erbium-doped fiber laser for remote sensing applications. The laser uses four Fiber Bragg Gratings (FBGs) for wavelength selection and for temperature sensing and a 2x4 optical switch. By adjusting the switch combinations, the laser can be switched among the four different wavelength lasing configurations. Stable one- and two- wavelength oscillations were achieved based on the use of this device. An output power instability analysis with the temperature for two different multi-wavelength configurations, one of them using the 2x4 optical switch and the other using a 1x4 coupler, was performed, for a cavity length of 50 km. A comparison between the performances of these topologies is carried out, and their temperature sensitivity and stabilization time are shown.

In this paper, multipoint refractive index measurement is described using surface plasmon resonance (SPR) sensors based on hetero-core structured fiber optic technique. The sensor simply consists of two different core diameters fibers, which are connected by thermal fusion splicing, in order to deliberately leak the transmitted light wave into the cladding layer of the sensing fiber region. Chromium film and gold film were uniformly deposited around the cladding surface with a layer thickness of 5 nm and 45 nm, respectively, for SPR excitation. Multipoint measurement system consists of three SPR sensors, whose hetero-core insertion length are employed as 2-mm, located in a single transmission optical fiber and an optical time domain reflectometer (OTDR) to be used to measure refractive index. As a result, a hetero-core insertion length has trade off between sensitivity for refractive index and the number of sensors in tandem. The proposed multipoint refractive index measurement has been successfully demonstrated using three hetero-core SPR sensors and OTDR.

This paper shows a long-distance remote sensing system using a multiwavelength Raman laser. The sensor network is based on a 50 km of standard single-mode fiber (SMF) and is composed by a simple cavity based on a loop mirror and four Fiber Bragg Gratings (FBGs) arranged in a star configuration. FBGs are used for both the sensing function and the selection of the lasing wavelengths. The system is designed to be inherently resilient to fiber failures. The multiwavelength laser has been characterized for temperature measurements showing a good stability performance.

An in-line fiber F-P sensor is formed by splicing a section of hollow-core photonic bandgap fibers in between two single mode fibers. Such a F-P sensor can be used under high temperature of up to 600 °C, and has a low temperature sensitivity of ~1.05nm/°C, while it has a relatively high strain sensitivity of ~5.93nm/με. Moreover, this F-P sensor is insensitive to bend or transverse load. Furthermore, the long cavity length (>1cm) of the sensor makes it suitable for multiplexing. These characteristics would make this F-P sensor to be an excellent strain gauge for use in high temperature environments.

In this paper, an extrinsic optical fibre sensor (OFS) for the quantitative determination of dyes used in the textile industry is presented. The system proposed is based on absorption spectroscopy and multivariate calibration methods to infer the concentration of different textile dyes. The performance of the sensor has been successfully assessed using calibrated dyes, with a very good correlation between the multivariate calibration models and the predicted values. The sensor system here demonstrated could be used to predict the colour of dye mixtures during the dyebath and, therefore, reduce the manufacturing costs.

An extrinsic Fabry-Perot interferometric (EFPI) optical fiber hydrogen sensor based on palladium silver (Pd-Ag) film is designed for hydrogen leakage detection. The sensing mechanism of such a sensor is based on the mechanical stress that is induced in the Pd-Ag film when it absorbs hydrogen. The sensor system which is portable and suitable for field detection is formed by a conventional coupler, a low-power LED operating at 850 nm, and a high resolution miniature spectrometer. To obtain the absolute length of the EFPI air gap, a cross-correlation signal processing method is introduced.The sensor is suitable for monitoring concentrations of hydrogen below the lower explosive limit.

We proposed a new approach to measure the rotational angle and describe how the rotational positioning sensor could be devised arranging the hetero-core fiber-optic macro-bending sensors in terms of detecting the absolute rotational angle. The hetero-core fiber optic sensor has many advantages such as ability of macro-bend sensing with optical intensity-based measurement, single-mode transmission basis and independence of temperature fluctuation for external environment. Therefore, it is suitable that the rotational positioning sensor is fabricated with the hetero-core fiber-optic technique. We designed two types of the absolute rotational position sensor modules to convert the absolute rotational angle to the displacement. The result showed that the proposed rotational positioning modules were sufficiently sensitive to the given rotational angle with monotonic loss change characteristics. The hetero-core rotational positioning sensors were successfully perceptive with typical sensitivities approximately 0.77 and 0.71 dB in the rotational angle ranges of 60 - 360 and 60 - 180 degrees. The deviation of the module in the range of 60 - 180 degrees induced 1.74 % that corresponded to 2.13 degrees.

Strong regenerated gratings are reported, with a maximum grating strength exceeding (40-50) dB. Further annealing between 1000 and 1100°C leads to a stabilised grating ~18dB in strength. This suffers no further degradation at 1100°C for the period monitored, over 4 hrs.

We present an optical bend sensor based on a Bragg grating written in an eccentric core polymer optical fibre. The grating wavelength shifts are studied as a function of bend curvature and fibre orientation and the device exhibits strong fibre orientation dependence, wide bend curvature range of ± 22.7 m^-1 and high bend sensitivity of 63 pm/m^-1, which is 80 times higher than the reported sensor based on an offset-FBG in standard single mode silica fibre.

In this paper we propose the use of multi-longitudinal mode lasers, such as Fabry-Pérot (FP) lasers, for simultaneous distributed strain and temperature sensing based on hybrid Brillouin and Raman scattering. Fabry-Pérot lasers allow for high-power Raman intensity measurement and the simultaneous detection of the Brillouin frequency shift parameter for all FP longitudinal modes, by using a self-heterodyne detection scheme based on a multi-wavelength optical local oscillator. Experiments point out significant improvement in hybrid sensor performance, also suggesting further potential benefits when combining the use of FP lasers with optical coding techniques and/or optical amplification.

An acoustic vector sensor based on distributed feedback fibre laser technology is reported. The sensor is designed as an accelerometer that will sense acceleration induced by the difference in acoustic pressure across the sensor. Experimental measurements demonstrate that the sensor has excellent response characteristics and sufficient cross-axis discrimination to be a useful acoustic vector sensor.

A fibre optic Fabry-Perot sensing structure based on suspended-core fibre is presented. The interferometric structure is formed when a small length of suspended-core fibre with three or four holes is spliced to the end of a standard single mode fibre. The sensing structure is implemented firstly for strain and temperature measurement, and secondly for refractive index measurement. The interrogation of the sensing structure is obtained for the first implementation by the wavelength variation, and for the second implementation through the fringe visibility and through the analysis of the fast Fourier transform.

We investigate a simple and flexible multiwavelength light sources based on fiber Bragg gratings (FBGs) and a tunable chirped FBG. Using a simple multiwavelength fiber laser configuration with a specialty FBG and a tunable chirped FBG, we readily achieved simultaneous multiwavelength fiber lasers with a high extinction ratio of more than 50 dB over a 50-km distance.

A novel configuration has been developed to optimize the response of Brillouin echoes for distributed fibre sensing. Fully resolved measurements of the Brillouin frequency shift of a 5cm spot perturbation have been performed using a 500 ps (5cm) pulse width. The linewidth of the measured Brillouin gain spectrum remains comparable to the intrinsic linewidth for any pulse width. The high accuracy and inherent stability of the technique have been successfully verified.

We demonstrate frequency-shifted light storage in a photonics crystal fiber (PCF) via stimulated Brillouin scattering. Because of the ultra-small core of PCF, only 20 W of peak power is used for the write and read pulses to store and retrieve the data pulse at much lower peak power of 200 mW. A 3-nanosecond-long data pulse is stored for up to 11 ns with a maximum readout efficiency of 11% at 7 ns storage time using main acoustic mode; a polarization-independent frequency-shift stored pulse is realized by using the high-order acoustic modes with a shifted frequency of ~20GHz and a readout efficiency of 6% at 6ns storage time.

We show that the noise of long range optical frequency domain reflectometry (OFDR) after compensating for the phase noise of the optical source results from environmental acoustic perturbation rather than the compensation process. If both the auxiliary and measurement interferometers are insulated against acoustic noise, the phase noise compensation scheme works almost ideally even over a 20-km range, with at least eight-fold concatenation. Although the spatial resolution achieved by phase-noise compensated OFDR (PNC-OFDR) is influenced by the environmental acoustic perturbation, a resolution of ~10 cm was achieved over 20 km in a normal laboratory environment by insulating the auxiliary interferometer. This approach can be used for mid to long range network diagnosis with a narrow spatial resolution, which is unachievable with other technologies.

This article demonstrates how spatial resolution of distributed Brillouin sensing systems can be significantly enhanced by measuring the spatially resolved Brillouin gain spectra of an optical fiber in the frequency domain. We employ a novel signal processing method to overcome the known problem of spectral broadening that occurs at centimeter-range spatial resolutions. The method is based on an analytical description of stimulated Brillouin scattering in optical fibers for the case of harmonically modulated optical signals. From this analysis, the artifacts that degrade the measurement resolution were modeled; for the first time to our knowledge, undegraded gain spectra could be restored by means of deconvolution techniques adopted from digital image processing.

We demonstrate a 12-km differential pulse-width pair BOTDA using 40/50 ns pulses with DC-coupled detection. A spatial resolution of 1 m and a narrowband Brillouin gain spectrum of 33 MHz are obtained simultaneously compared with 88 MHz for 10 ns pulse. The experimental results also show that the differential Brillouin gain is improved by 6.8 times over direct Brillouin gain of 10 ns pulse, and temperature uncertainty for 40/50 ns pulses is 0.25 °C and while for 10 ns pulse is 1.8 °C.

We present recent work and progress on Spectrally Coded Multiplexing (SCM). SCM as a generic multiplexing technique adds new dimension of freedom and provides additional flexibility and channel capability needed for fibre and fibre grating based sensor systems. We show a few examples of our newly developed SCM techniques based on specially designed fibre gratings.

By now, fiber Bragg gratings (FBGs) represent a well assessed technology in both communications and sensing fields. In particular, thanks to their small signal bandwidth and wavelength encoded information, they allow simple measurements in reflection for sensing purposes and also easy multiplexing capability in realizing sensors arrays. Unfortunately, to make FBGs sensitive to surrounding refractive index (SRI), hosting fiber structuring is needed. In last years, also tilted FBGs (TFBGs) - intrinsically SRI sensitive structures - have been proposed as promising technological platform for several sensing applications. However, complex spectral features combined with the difficulty to be configured as quasi distributed or multi-point sensors network limit the practical exploitation of this assessed and mature technology. It would be extremely useful to merge the peculiar spectral characteristics of both grating types. To address this issue, here, we propose a hybrid cavity involving two unbalanced uniform FBGs written at both sides of a TFBG to form an all-fiber interferometer. The proposed configuration provides a wavelength gated reflection signal with interference fringes depending on the cavity features modulated by spectral dips associated to the wavelength dependent optical losses due to cladding mode coupling occurring along the TFBG. Such a structure preserves the advantages of uniform FBGs in terms of interrogation methods and allows the possibility to contemporarily measure multiple parameters.

Recently, we have reported a scheme for measuring Bragg-wavelength distribution inside a long-length fiber Bragg grating using synthesis of optical coherence function, which is one of the reflectometry techniques based on interferometer and heterodyne detection with an optical frequency-shifter and an electronic band-pass filter. However, the system did not have enough measurement range since it measured the distribution with only one grating. For extending the range, long-length FBG sensors are multiplexed and distribution in each grating is measured. In the process of multiplexing FBGs, a scheme for compensating the beat frequency shift due to the high speed sweep of the optical center frequency of the light source is introduced in the system.

We present an ultra-short distributed Bragg reflector fiber laser photowritten in Er/Yb co-doped fiber. The total length of the fiber laser is only 8.4 mm. The lasing threshold is less than 1 mW. The optical signal-to-noise ratio of the laser output is around 70 dB. The laser emits two orthogonal polarization modes. The electrical signal-to-noise ratio of the beat signal generated by the fiber laser is better than 70 dB.

We are going to present and demonstrated the feasibility of a novel sensor interrogation scheme that combines the advantages of the distributed and point sensor systems using a robust interrogation technique. The experimental results show the high linearity and stability of the measurements. Also, our proposed scheme uses a tunable source with optical elements that have very few losses. Thus, the number of sensors that can be allocated in one fiber is very high.

An improved layer peeling algorithm removes the constraint of equal segments length in a low-loss interferometric sensor array. Consequently, the construction of the sensor array is greatly simplified while its performance remains essentially unaffected. With unequal segments, the frequency response of the sensor array, which is based on a serial concatenation of High Birefringence (HiBi) fibers, becomes effectively non-periodic. The original version of the layer peeling algorithm was based on scanning one period of the frequency response and expanding it in the form of a Fourier series. This led to decreased sensing performance since in practice it is virtually impossible to fabricate an array of identical segments. In the non-periodic case, we show that broadening the scanning range, as well as windowing the measured frequency dependent Stokes vector, prior to transforming it to the time-domain, greatly reduce sensing errors. The new approach is demonstrated in a 14-segments array of HiBi fibers with a total length of 235m. Sinusoidal strain modulation at up to 65Hz is applied to one of the segments. The improved layer peeling algorithm reveals the induced signal in the perturbed segment with only little crosstalk in the signals extracted from the other segments.

The development of new techniques for writing and tailoring the properties of Bragg gratings has generated a suite of distinct grating types that are optimised for performance within different temperature windows. These cover gratings produced by recipes such as hypersensitisation, thermal processing and single and multiphoton writing. In this paper, we review four types of high temperature gratings that offer comprehensive coverage of temperature space for most applications of interest. Up to 1200°C novel processing methods allow standard silica-based optical fibres to be used. However, beyond these temperatures, optical fibres made from other materials, such as sapphire, need consideration.

Strain and thermal sensitivities of germanate and tellurite glass fibres were measured using a fibre Fabry-Perot (FFP) interferometer and fibre Bragg gratings (FBG). The strain phase sensitivity for germanate and tellurite fibre were 5900×10³ rad/m and 5600×10³ rad/m respectively at a central wavelength of 1540nm using FFP interferometer, which is consistent with the value of 1.22pm/με obtained for a germanate fibre FBG. The Young's modulus for germanate and tellurite fibre were also measured to be 58GPa and 37GPa. The thermal responses of germanate fibre were examined as 24.71 and 16.80 pm/°C at 1540nm and 1033nm wavelength using the FBG.

A new concept of optical fiber dynamic strain sensor has been experimentally characterized. It is based on two wave mixing by gain saturation in an erbium doped optical fiber. The main feature of this sensor is insensitivity to slow varying perturbations, which is of major interest for underwater acoustic applications for instance. Its multiplexing capabilities are also investigated.

We propose a simple method to self-compensate temperature which can affect the sensitivity of palladium-based hydrogen gas sensor. When a long-period fiber grating is fabricated in a double cladding fiber, the fundamental core mode is coupled to the inner cladding mode as well as the outer cladding mode. Since the inner cladding mode is insensitive to the external contact, it is independent to the thin palladium layer coated out of cladding surface. Thus, the spectrum corresponding to outer cladding modes reflects the reaction between hydrogen and palladium while that of the inner cladding mode indicates the ambient temperature only.

A two-axis inclinometer is created by affixing a glass capillary tube to the end of a multicore fiber, forming the proof mass of a fiber cantilever. As the sensor tilts, fiber Bragg gratings inscribed in individual cores of the multicore fiber experience strain and are interrogated interferometrically. A phase generated carrier demodulation technique is then used to measure the differential phases between cores. Within a measurement cycle, the instrument reports a maximum phase excursion of ±0.55 mrad, corresponding to a curvature uncertainty of ±3.7×10^-4 m^-1. The experimental calibration and performance of the sensor are presented.

We report a novel liquid level sensor based on a simple uniform fiber Bragg grating by monitoring both the short-wavelength-loss peaks and its Bragg resonance. The liquid level can be measured from the amplitude changes of the short-wavelength-loss peaks, while temperature can be measured from the wavelength shift of the Bragg resonance. Such a scheme has some advantages including robustness, simplicity, flexibility in choosing sensitivity, and simultaneous temperature measurement capability.

We present a solution to the simultaneous intrinsic point sensing of bi-directional magnetic field strength and temperature. Using a passive, retroactive method for polarisation selection based on a local chiral fibre device, a series of dual-wavelength fibre Bragg gratings written in polarisation-maintaining fibre may be interrogated for both measurands over long distance using linearly polarised light. Through simulation we identify and evaluate sources of error in the scheme in order to find the design that yields the optimal sensitivity and noise level, which for the multiplexing interferometric interrogator are 3.0 μrad/mT and 22 mT rms respectively at a rate of up to 10 kHz.

An in-fiber Bragg grating (FBG) dynamic strain sensing system based on a fiber ring laser configuration and the Eigenvector spectrum analysis method, is proposed and demonstrated. An ultrahigh dynamic strain resolution of 5x10^-5 nε /Hz^1/2 is achieved due to both the use of the supper white-noise suppression ability of the advanced spectrum analysis method and the high optical signal-to-noise ratio (SNR) (~65dB) of the sensing signal generated by the fiber ring laser scheme. This is the highest dynamic strain resolution achieved so far for a single FBG sensor at frequency of <100Hz, to the best of our knowledge.

A fiber optic sensor to detect acoustic emission is reported, which is based on a fiber taper incorporated in a fiber Mach- Zehnder (M-Z) interferometer as the sensing arm. Benefited from their micrometer sizes in transverse dimension, fiber tapers have higher sensitivity to the environmental vibrations compared with standard single mode fibers (SMFs). Under the same conditions, the thinnest fiber taper in this report with a diameter of 1.7 μm shows 22dB improvement in the signal to noise ratio (SNR) comparing that in SMF28. We also calculated the phase change of a fiber taper under longitudinal strain and the variation of phase changes with different diameters.

We report about a thermal regeneration of fiber Bragg gratings written in photosensitive fibers with nanosecond laser pulses. We observe a complex regenerative process in a photosensitive fiber without hydrogen loading which indicates a secondary grating growth in an optical fiber by thermal activation. This process is more temperature stable than the commonly known color center modifications. The writing conditions of such new grating types are investigated and the temperature behavior of these regenerated fiber Bragg gratings (RFBG) is analyzed. The main application options are foreseen in the field of high temperature sensor systems by combining the good spectral shape of a Type I grating with the high temperature stability of a Type II grating.

A miniature all-fiber in-line tip pressure sensor fabricated by 157nm laser pulses is proposed and demonstrated, for the first time to our knowledge. Such a pressure sensor can be operated under high temperature due to its all silica structure. The sensor has been tested for pressure sensing over 0MPa~60MPa, showing a pressure sensitivity of ~83.3pm/MPa, an excellent linearity of 99.998%, and a long-term stability of ±0.03MPa, respectively. Such a miniature fiber-optic pressure sensor could find important applications in automotive and aerospace industries, and down-hole monitoring, etc, due to its extreme compactness, capability for operation at high temperature, easy fabrication, excellent stability, good potential for mass-production.

In this work, interferometric sensors based on antiresonance reflecting waveguide (ARROW) fibers were developed and their performances were characterized for measurement of physical parameters, particularly strain and temperature. Two types of ARROW fibers were considered and signal demodulation was achieved using the white light interferometric technique. Application issues of sensing heads based on these fibers are also addressed.

A sensing head configuration and the necessary interrogation parameters to simultaneously measure strain and temperature using a single FBG are presented and demonstrated. By writing the FBG in a linearly etched fibre, we were able to use the information encoded in the peak wavelength and in the spectral width of the fibre Bragg grating. The spectral width of the grating depends uniquely on the applied strain and is temperature independent. An uncertainty of ±15.26 με and ±1.92 ºC was obtained in the experimental verification.

This manuscript presents a fully engineered fiber-optic magnetic field sensor based on the Lorentz force. The sensor comprises a gold coated optical fiber that is soldered to a current carrying aluminum ribbon. The ribbon vibrates in the presence of a magnetic field which is detected with fiber optic interferometry. The mechanical-Q of the mechanism is measured to be 200±10% which is greater than previous prototype designs by a factor between 2.7 and 11.3. This yields a higher resonant responsivity and lower thermo-mechanical noise.

The interaction of fibre Bragg grating sensors with ultrasonic acoustic waves in water is investigated in the near field region of a 1-3 piezocomposite transducer. Longitudinal and transverse strain coupling is studied using two different grating designs and high fidelity measurement of the emitted field was demonstrated.

A new Time Division Multiplexing (TDM) optical fiber interrogation system for distributed Fiber Bragg Gratings (FBGs), designated as Delayed Transmission/Reflection Ratiometric Reflectometry (DT3R), is presented. The Bragg wavelength shift of FBG, which varies according to temperature or strain, is estimated as the ratio of transmittance and reflectance of a slope filter, to which the reflected light from the FBGs is input. To obtain both simultaneously using a single detector, a definite propagation delay is given to either transmission or reflection path, both of which are added to each other and detected using a common photodiode. The reflection impulse response of the FBGs is analyzed using OTDR based on Pseudo-random Noise code (PN) correlation scheme. The hardware structure of an interrogator is simple, resulting in low cost. The FBG topology is a "bus": multiple FBGs branch from the through line started from the interrogator. Each FBG is distinguished by its time of flight between the interrogator and the FBG. Because of these schemes, the FBGs can all be identical. A breakdown of any FBGs has no effect on the other sensors; moreover, "hot swapping" is possible. Experimental evaluation results are presented herein.

A two core Gemini fiber structure is described with easily accessible cores and suitable for sensing applications. The cross-section of the fiber consists of independent circular cross-sectioned fibers, connected to each other by a narrow glass bridge. The Gemini fiber is mechanically rigid and thermally stable making it suitable for sensing applications with the advantage of allowing for easier coupling at the input and output extremes. A temperature independent bend sensor based on Bragg gratings in the Gemini fiber is demonstrated.

Experimental results of static pressure measurements from specially packaged π-phase shifted optical fibre Bragg gratings (FBGs) that were installed into a 2D, 3 element high lift wing are presented. The static pressure measurements from the fibre sensors are found to be in good agreement with theory. Further pressure calibrations are currently under investigation.