For many structural applications it is highly desirable to be able to measure two or more axes of strain at a single point. In many cases one dimensional strain measurements may be insufficient to fully characterize events or lead to erroneous predictions. This paper will provide an overview of the use of multi-axis fiber grating strain sensors to perform structural diagnostics. Examples of usage of multi-axis fiber grating strain sensors in a smart bearing cell for damage assessment of bridges and for adhesive joints associated with aircraft will be given as illustrations of these methods.

In civil engineering it is of interest to monitor long-term performance of structures made of new lightweight materials like glass or carbon fiber reinforced polymers (GFRP/CFRP). In contrast to surface applied optical fiber sensors, embedded sensors are expected to be better protected against rough handling and harsh environmental conditions. We report on two recently done fiber optical sensor applications in civil engineering. Both include structurally embedded fiber Bragg grating (BG) arrays but have different demands with respect to their operation. For the first application fiber BGs were embedded in GFRP rockbolts of 3 - 5 m in length either of 3, 8, or 22 mm diameter. The sensor equipped rockbolts are made for distributed measurements of boulder motion during tunnel construction and operation and should withstand strain up to 1.6%. Rockbolt sensors were field tested in a tunnel near Sargans in Switzerland. For a second application fiber BGs were embedded in CFRP wires of 5 mm diameter used for the pre- stressing cables of a 56 m long bridge near Lucerne in Switzerland. The permanent load on the cable corresponds to 0.8% strain. Due to the embedded sensors, strain decay inside the cable anchoring heads could be measured for the first time during loading and operation of the cables. For both applications mechanical and thermal loading tests were performed to assess the function of these new elements. Also, temperature and strain sensitivity were calibrated. Reliability studies with respect to stress transfer, fiber mechanical failure, and wavelength shift caused by thermal BG decay as well as monitoring results of both applications are presented.

In recent years growing interest has surrounded the development of fiber laser sensors (FLS). This is due to their ultra high sensitivity to temperature and strain as well as their ability to be multiplexed along a single fiber using WDM techniques. It is their extreme sensitivity that has led to them being considered as acoustic pressure sensors rather than standard fiber Bragg gratings. The work presented here describes the development of an array of FLS configured as hydrophones. We discuss the design of the single mode fiber laser used throughout our system; comparing examples based upon distributed Bragg reflectors (DBR) and distributed feedback (DFB). In addition we discuss both the theoretical and experimental acoustic sensitivity enhancements obtained by the application of an elasto-plastic coating to the FLS. The array configuration is described, as is the heterodyne interrogation scheme using an unbalanced Mach-Zehnder interferometer with WDM channel selection. Results from the measurement of the minimal detectable acoustic signal of a bare fiber laser are shown to be -69 dB re.Pa/(root)Hz at 1 kHz when using a 200 m path imbalanced readout interferometer. Further gains in the sensitivity due to the application of various coatings are reported, as is a full characterization of an array of fiber laser hydrophones. Finally we discuss the future research of the FLS, and the areas in which the technology is particularly applicable.

A fiber-optic system capable of measuring deformations, impacts and modes of vibration of a composite panel is presented. Wavelength-division signal de-multiplexing allows for the simultaneous interrogation of 8 strain sensors.

Bragg grating-based optical systems are important for both telecommunications and sensor applications. Work to data on the simulation of such systems has been concentrated upon using approximate methods such as the coupled mode theory (CMT). In this work, a combination of three numerical methods has been used, all of which are rigorous and at the same time computationally very efficient. The new approach presented here incorporates the finite element, the least squares boundary residual and the transfer matrix methods. Our simulated results show that the CMT could be adequate for Bragg grating devices in fiber, since perturbed refractive index change is small. However, for Bragg grating devices in semiconductors, CMT could generate less accurate results. Simulated results for various types of grating devices, such as uniform, chirped, apodized, phase-shifted, super-structures and sampled grating devices are presented.

We report on feasibility studies of two new approaches for simultaneous strain and temperature measurements with optical fiber Bragg gratings. In sensor applications fiber Bragg gratings cannot be thermally isolated. Therefore temperature has to be compensated or measured simultaneously with the measurand of interest. In strain sensing this is often done by measuring the wavelength response of a second Bragg grating attached to an unstrained sample specimen and comparing its response to response of the strained sample. This method is very accurate. However, often it is not possible or difficult to have unstrained samples, e.g., in rockbolt anchors with embedded fiber Bragg gratings. We propose two new simultaneous measurement concepts and compare them to known concepts. The first investigated method uses the different sensitivities to strain and temperature of the main peak of a Bragg grating in transmission compared to the back-reflected cladding modes. We will present first results of simultaneous strain and temperature measurements over a range from 2 degrees Celsius to 95 degrees Celsius up to 3500 micrometer/m in strain. This method leads to a combined resolution in temperature and strain of 4 degrees Celsius and 35 micrometer, respectively. The second method is based on two low reflectivity gratings. They are in close vicinity to each other in the fiber. Both have a small refractive index modulation of about 2 (DOT) 10^-4, but one was homogeneously post-illuminated and has an increased mean refractive index. We will present first results indicating that the additional UV-illumination changes temperature- and strain-sensitivity of the post-illuminated Bragg grating slightly.

Ultrahigh sensitivity fiber cavity etalon (FCE) sensors have been embedded in graphite-reinforced polymer tubes fabricated by two different methods: resin transfer molding (RTM) and standard autoclave curing, and FCEs have been embedded in autoclave-cured unidirectional flat laminates. Significant issues encountered in embedding the sensors include protecting the fiber egress during layup, curing, and breakout, survival of the butt-coupled splice between the cavity and lead fiber during composite cure, maintaining sensor location, and sensor reliability and response. Methods were successfully devised to overcome these obstacles.

A low-coherence optical fiber interferometric technique for simultaneous measurement of geometric thickness and group refractive index of highly dispersive materials is described. The technique, immune to the dispersion-induced asymmetry of the low-coherence interferograms obtained, overcomes some of the drawbacks associated with recently reported low-coherence approaches to this simultaneous measurement. The technique uses the experimental configuration of a tandem interferometer with the samples to be characterized placed in an air-gap in one arm of the measurement interferometer. Dispersion- insensitive measurements of the group delay imbalances in the measurement interferometer are made using dispersive Fourier transform spectrometry (DFTS). Sample thickness and group refractive index are calculated from these group delays which are unambiguously determined from the optical frequency dependence of the measured phases of the interferograms. Thickness measurements accurate to within 1 micrometer and group index measurements accurate to within one part per thousand have been achieved for BK7 and fused-silica glass samples in the thickness range 2000 to 6000 micrometers.

We have interrogated a commercially available (SOFO) fiber optic displacement sensor in a dynamic mode using microwave sub-carrier interferometry. The detection threshold is in the region of 50 nm and the linear dynamic range extends to several centimeters using a sub-carrier frequency around 900 MHz.

Standard meteorological sensors and sensor suites used for weather and environmental monitoring are currently based primarily on electronic instrumentation that is frequently susceptible to destruction and/or interruption from natural (e.g. lightning) and man-made sources of Electromagnetic Interference (EMI). The cost of replacement or shielding of these systems is high in terms of frequency of replacement and the incipient capital cost. Sensors based on optical fibers have been developed in sufficient variety as to allow the development of full meteorological instrumentation suitess based on individual or multiplexed optical fiber sensors. Examples of sensing functions which can be implemented using optical fibers include: wine speed (cup anemometers & Doppler lidars), wind direction (vanes & lidars), temperature, humidity, barometric pressure, accumulated precipitation and precipitation rate (fiber lidar). Suites of such sensors are capable of using little or no electronics in the environmentally exposed regions, substantially reducing system EMI susceptibility and adding functional capability. The current presentation seeks to explore options available in such meteorological suites and examine the issues in their design and deployment. Performance data on several newer fiber sensors suitable to meteorological use will be presented and discussed.

Based on an earlier version presented at OFS13 in Korea, a short review of distributed and multiplexed fiber grating sensor technology is given, followed by details of work at the University of Southampton, including aspects of temperature and strain discrimination and our own methods for multiplexed and distributed sensing. The paper concludes with a short discussion of the problems that should be avoided in order to construct viable systems for engineering requirements.

In 1988 the idea of measuring bile in the stomach and in the oesophagus via optical fibers was conceived and patented in collaboration with physicians from the University of Florence. The working principle is based on the spectrophotometric properties of the bile which contains some pigments with definite absorption properties. Bilirubin is the main pigment and it is characterized by an absorption peak in the blue region: therefore it is possible to detect optically the bile in the stomach by optically detecting bilirubin. The possibility of measuring bile reflux directly measuring the presence of bile represented a winning aspect in comparison with the traditional techniques (pH-metry, cholescintigraphy, bile acid assessment in aspirates); on the contrary the new technique had to overcome the traditional 'cultural' barriers constituted by the conservative attitude of clinicians concerning any innovative technology. The realization of the first laboratory prototype demonstrates the feasibility and validity of the proposed optical method. Then many years were necessary to arrive at the definitive and marketable product. The history of Bilitec 2000 is described, with the purpose to stress how a laboratory prototype is still very far from the market.

A new sensor head and imaging application with planar oxygen optodes is presented. It combines the versatility of the recently presented modular luminescence lifetime imaging system (MOLLI) and the oxygen measuring features of planar optodes to investigate the 2D-distribution of oxygen with a high spatial resolution. The marine sediments are settled by micro-organisms. They are characterized by steep gradients of solutes perpendicular to the interface. Oxygen as the most favorable electron acceptor plays an important role in these communities and exhibits steep gradients within distances of 100 micrometer to a couple of millimeters. Traditionally these gradients are commonly accessed by either oxygen microelectrodes or oxygen micro-optodes, that measure single oxygen depth profiles at spatial resolutions in the range of 50 - 5 micrometer. As the influences on the metabolism of these organisms are complex, profiles that are measured at one location exhibit a natural heterogeneity. To address this problem by using the potential of planar oxygen optodes first measurements with optodes fixed to an aquarium wall (Glud et al. 1996) based on luminescence intensity measurements have been published. They showed the large amount of information that can be gathered by this method. We further developed this approach by combining the planar optode with the capabilities of imaging fibers that form an endoscope. We developed a periscope type sensor head that can be independently applied in biological samples. It has an overall diameter of 2.5 mm and a calculated pixel resolution of 12 micrometer. The according measuring system compared to the published system has a different excitation light setup and corresponding endoscope optics to measure the lifetime based images. We present the adapted system, sensor head and the first results of an application.

We report on a reflection mode fiber optic oxygen sensor based on the ³O₂ quenching of the red emission from hexanuclear molybdenum chloride clusters. Measurements of the probe operating in a 0 - 20% gaseous oxygen environment have been obtained, which is suitable for biological and automotive applications. The luminescence signal increases with decreasing oxygen concentration in accordance with theory. We observe clearly resolvable steps in the sensor response for changes of 0.1% absolute oxygen concentration in the 0 to 1.0% range. The response time of the fiber probe is less than or equal to 30 s, limited at present by the gas mixing system. The sensor is fabricated by immobilizing the clusters in an oxygen permeable polymer at the end of an UV transmissive optical fiber. For a 1.5 m long fiber pumped at 325 nm, the auto-fluorescence is less than 4% of the reflected luminescence signal from 600 - 850 nm, enabling remote probing of oxygen in harsh environments. The present probe is thermally stable to 200 degrees Celsius, limited by the polymer matrix. Since the clusters are synthesized at 1000 degrees Celsius, immobilization of the clusters in an inert sol-gel matrix is an attractive alternative. This will enable oxygen sensing in gaseous environments at elevated temperatures as well as liquid environments with varying salinity and pH.

Intra-cavity spectroscopic measurements may be obtained with any material that has an absorption signature under the gain bandwidth of a fiber laser. The high sensitivity of the intra- cavity technique combined with the compact size of a fiber laser makes this approach very attractive for a wide range of applications. Experiments have been carried out with acetylene and a tunable erbium-doped fiber laser. Compared to regular absorption measurements, an enhancement in sensitivity limited mainly by laser noise is observed. Since intra-cavity spectroscopy is essentially a single beam technique, dual beam noise reduction is not possible with this arrangement. However, if we consider the fact that a single mode fiber can support two modes of polarization, we can create two lasers using the fiber as a common element for both optical cavities. The proposed fiber laser configuration uses a polarization beam splitter to create two independent cavities (x and y polarization) with the same noise, of which one cavity contains the absorber. This permits the convenient use of Balanced Ratiometric Detection in conjunction with sensitive intra-cavity absorption. Although tested using bulk devices, this fiber laser sensor can easily be implemented in an all- fiber version using off the shelf fiber optic components, which makes it a good candidate for compact field spectroscopic measurements.

The application of a novel sensing fiber with an inverted parabolic-index profile to the detection of refractive-index changes of a medium surrounding the fiber core is investigated. The detection experiments were made with a set- up consisting of a fiber connector-based coupler allowing the excitation of the sensing fiber by a light source of a relatively small size placed on the fiber axis in a preset distance from the fiber end face, and an optical cell containing a declad segment of the sensing fiber in an immersion whose refractive index could be varied in a known way by heating. Various light sources, such as outputs of multimode and single-mode fibers were tested as more or less accurate approximations to the ideal point light source. For overcoming problems with the profile imperfections in the central core region appearing in some of the produced fibers and originating from the fiber manufacturing technology, the excitation of the fiber by an inclined beam from a bevelled single-mode fiber was studied as well. For comparison, PCS fibers were used in the experiments, too. The results have shown that in an index range slightly below the index of silica, higher sensitivity can be achieved with the novel fiber than with PCS fibers. The possibility of shifting the measuring range down to a more practical lower index values by using inclined-beam fiber excitation and confinement of the index profile to the outer region at the core will be shown.

Lifetime-imaging has quickly become a standard tool for many applications in biology, medicine, and engineering. Luminescence lifetime is dependent on the chemical environment surrounding the lumiphore so lifetime images give scientists far more chemical information than traditional images. Most lifetime imaging systems require the sample to be brought to the sample stage of a fluorescence lifetime microscope or other lifetime imaging set-up. We are developing lifetime- based fiber-optic imaging systems and sensors for a variety of in-situ measurements. In this work, small diameter optical imaging guides are used to carry light from an excitation source to a remote, or in-situ, sample and also to return the resulting luminescence image through the same image guide to a time-resolved imaging detector. For example, we have developed lifetime-based fiber-optic water and oxygen imaging sensors with the sensing chemistry directly attached to the distal tip of an image guide. These sensors use a pulsed excitation source and a gated camera (ICCD) to collect the images. In addition, we are developing a dual-pulse lifetime imaging system for remote imaging without a gated detector. This paper will report on the current status of our lifetime- based fiber-optic imaging sensors, dual-pulse methods and potential applications.

Surface plasmon resonance is a widely used phenomenon in chemical and biochemical analysis. Recently, a novel design of a fiber optical SPR sensor for refractive index and chemical sensing was reported which is based on the wavelength resolved principle. In this paper we present a further improvement of the sensor performance. The main advantage of the sensor system is its easy adjustment to the application requirements due to the use of specific sensor tips and the modular configuration of the optics and optoelectronics. The sensor can be used for measurements of aqueous solutions directly in the medium. Details of the sensor configuration and the sensor response will be presented.

A novel fluorescence immunosensor using a tapered tubular optical waveguide probe for analyte detection has been developed based on magnetic focusing of paramagnetic microspheres. The new design features a tubular optical waveguide tapered at both ends with a tapered magnet embedded inside of the waveguide. The excitation light is injected from the middle part of the tubular waveguide and is guided to the front end where it illuminates paramagnetic particles attracted by the magnet. The associated fluorescent signal is collected by the optical probe and is guided to the distal end, where it is connected to the optical detection system. The waveguide thus serves multiple purposes: the front end of the waveguide serves as the optical probe while the rear end serves as the connector to the signal transmission fiber; the waveguide body serves as a holder for the magnet, a directional coupler for the excitation and a high split-ratio coupler for the fluorescent signal.

A fiber optic chemical sensor based on gold-island surface plasmon excitation is presented. The sensing part of the fiber is the end of the fiber onto which a thin layer of gold has been deposited to form a particulate surface. Annealing the gold reshapes the particles and produces an optical absorbance near 535 nm with the fiber in air. The optical absorption resonance of the gold particles is shifted if the fiber is immersed in a medium other than air. These resonance shifts are examined by transmission spectroscopy through the fiber. Experimental results for the sensitivity and dynamic range in the measurement of liquid solutions are in agreement with a basic theoretical model which characterizes the surface plasmon using nonretarded electrodynamics.

Lightly crosslinked monodisperse poly(vinylbenzylchloride-co- 2,4,5-trichlorophenylacrylate) microspheres were prepared by dispersion polymerization. Then microspheres were derivatized with various secondary amines to convert the vinylbenzylchloride to a pH sensitive amine. These microspheres were embedded in poly(vinylyalcohol) hydrogel to form a membrane. Optical sensitivity of this membrane for pH is based on refractive index changes accompanying swelling and shrinking of the polymer microspheres. The membrane appears turbid (opaque-white) because the refractive index of microspheres is greater than that of the hydrogel. 2,4,5- trichlorophenyl acrylate (TCPA) was added into the polymer formulation in order to introduce porosity and hydrophilicity to the microspheres. In the amination procedure, the large 2,4,5-trichlorophenol group is replaced by the smaller amine forming a more hydrophilic amide. This shortened the response time of the membrane from hours to a few seconds. The membrane is more turbid at short wavelengths. In addition, turbidity of the membrane increases as the pH of solution increases. There is a linear relation between microsphere concentration of the membrane and its turbidity. The range in which the membrane is sensitive to pH is from 6 to 8.5, with an apparent pK_a of 7.5.

In this work we have been developing fast response optical sensors for dissolved oxygen determination based on measuring the luminescence lifetime of a metalloporphyrin polymer. The sensor is produced by electropolymerization of the metalloporphyrin monomer units of platinum tetraphenylporphyrin (Pt-TPP), platinum octaethylporphyrin (Pt-OEP), palladium tetraphenylporphyrin (Pd-TPP) or palladium octaethylporphyrin (Pd-OEP). The polymerization process results in films which are in the region of micrometers thick. The Stern-Volmer quenching constants as determined from luminescence lifetime measurements for these sensors range from 0.90 (mg 1^-1)^-1) for Pt-TPP to 1.83 (mg 1^-1)^-1) for Pd-OEP. The response time of these sensors to a step change from an oxygen free to an oxygen saturated solution is in the millisecond region.

Historically, Raman spectroscopy has not been widely used for analytical quantitative analysis for a variety of reasons, most notably, its lack of sensitivity. In this manuscript, we will demonstrate our efforts to effectively develop high sensitivity Raman detection for both static and dynamic liquid analysis. A Raman detection cell compatible with the elution volumes of minibore and microbore liquid chromatographic separations has been developed. The detector is a combination of a liquid core waveguide and a commercially available Raman instrument. The waveguide cell enhances the sensitivity of a typical Raman measurement upwards of 150 times, without resorting to surface enhancement or resonance approaches. In addition, the enhanced sensitivity allows one to capture spectral information in 'real time' as materials elute from a column. An information rich data matrix results from the combination of a temporal separation with species specific spectral signatures. There are a number of unique optical parameters that allow us to achieve such enhancements. We will discuss the physical detector and optical cell design as well demonstrate the effects of absorption, refractive index and numerical aperture on the sensitivity of the Raman measurements. The resultant Raman detector is a very sensitive and selective alternative to many of the currently available detectors used for flow analysis (HPLC, LC, FIA).

The concept of waste water treatment from microelectronics fab. Processes is referring to waste water discharged from ultrapure water plant which can't be treated on recovery waste water section. These wastes concentrated contains organic and inorganic acids, alkalis, metals, cyanide, chromium and fluoride effluent from fab. processes. They will be canalized on qualities for treatment sections which permit the discharge of treated waste water as neutral with solids removal as compacted sludge. For management of waste water treatment plant we are using the fiber optic sensors as follows: level and flow control, signalization automatic pumps control and protection, solid control, leak detection a.s.o. The neutral quality of treated water has 'null' impact against of environmental system recommended for all semiconductors and microelectronics fab. processes.

The telecommunications industry is currently undergoing dramatic increases in the transmission capability, or 'bandwidth,' of both 'long-haul' and 'local area network' systems. These improvements are being enabled by advances in photonic and fiber optic technology that have increased the transmission capability by more than 100 fold over the past 8 years. This increased transmission capacity is being fueled by the near insatiable demand for internet data, video and other non-voice traffic over the telecommunications network.

Information from the reservoir can be used throughout its lifetime to make significant improvements to yield and operating costs. Traditional sensing methods, such as well logging using wireline techniques and installation of permanent gauges (both electronic and fiber optic sensors such as fiber Bragg grating sensors) either provide occasional snapshots of the reservoir or provide sensing systems which become dated and which usually do not survive over the lifetime of the reservoir. This paper reviews methods of instrumenting oil wells with sensing systems that can be upgraded throughout the lifetime of the reservoir. Applications shown include pressure sensors and distributed temperature sensing.

Distributed strain measurements on synthetic fiber ropes and cable constructions are reported. The tension members incorporate single mode fiber-optic sensors for strain measurement. Brillouin-amplification-based distributed strain measuring system has been the measurement technique of choice to interrogate the fiber sensors incorporated into a parallel yarn aramid rope. Initial results are presented which conclusively demonstrate the technical feasibility of the approach.

We present a novel concept for an acoustic fiber sensor array based on the Sagnac interferometer. Its characteristics and performance are analyzed theoretically and shown to be superior in several important areas to standard sensor arrays based on Mach-Zehnder (MZ) interferometers. In particular, the Sagnac sensor array (SSA) exhibits a stable bias point, a reduced conversion of source phase noise into intensity noise, a frequency response matched to the ambient ocean noise (of relevance for underwater acoustic sensing), and freedom from polarization-induced signal fading. The dynamic range of the SSA can also be easily designed to far exceed the dynamic range of a MZSA.

A novel design approach for a highly reliable buried intrusion detection sensor is described. The design involves the use of a low cost depolarized Sagnac fiber interferometer with a 'sensing loop' consisting of a delay line and buried fiber segment. The intrusion sensor is configured for an 'all fiber' remote deployment where active components (source, receiver, demodulator) are located separately and connected to the sensor through an insensitive fiber tether. A robust and cost effective buried sensor 'mat' design was developed. This design enabled high sensitivity as well as ease of deployment. Sensors were built and evaluated. Test results indicate an effective design.

A triangle-shape optical coherence function is synthesized, and is applied to construct a system for real-time stress- location measurement along an optical fiber. We utilize a polarization maintaining fiber (PMF) for the sensing element, and the coupling from one eigen polarization mode to the other by the stress is used as the sensing mechanism. The delay difference between the two eigen polarization modes linearly corresponds to the stress-position on PMF, which is measured as the degree of coherence linearly changing from 1 to 0, like a triangle. By extracting the coherence value, the stress point is figured out in real time.

A re-entrant fiber optic gyro (RE-FOG) that utilizes a ring resonator (RR) and a low-coherence length light source is designed and demonstrated. A reflector is placed on one end of input/output directional coupler to realize light beams propagating in CW and CCW directions. RE-FOG promises a higher sensitivity than the conventional Sagnac interferometer fiber optic gyro (I-FOG) without the disadvantages of a resonant fiber optic gyro (R-FOG). Theoretical considerations of the sensitivity and the resolution of RE-FOG are discussed. Experimental results (the constant bias of 0.2 deg/h and the variance of random drift rate better than 0.1 deg/h) are also presented.

Described in this paper is fiber optic gyroscopes for harsh environment applications. The fiber optic gyroscope consists of a super luminescent diode, a fiber coupler, a polarizing optical fiber coil, a detector and a signal-processing device. The peak wavelength of SLD is 1.3 micrometer. The multifunctional integrated optical circuit that includes a polarizater, a Y type junction coupler and a phase modulator, is manufactured with annealed proton exchange process. The polarizing optic fiber coil is fabricated by applying quadrupolar winding technology. The fiber optic gyroscope can work in high/low temperature, large acceleration, vibration, shock and other harsh environments. It is applied in aircraft inertial navigation systems and directional measuring system in oil-rigs.

The results of an investigation on the performance of a time- division addressed fiber optic gas sensor array using wavelength modulation of the DFB laser are reported. The system performance is found to be limited seriously by the extinction ratio of the optical switch used for pulse amplitude modulation. Formulas that relate the crosstalk level to the extinction ratio of the switch, the modulation parameters of the DFB laser and the optical path difference between sensing channels are derived. Computer simulation shows that a methane gas sensor array of 20 sensors with a detection sensitivity of 2000 ppm (10 cm gas cell) for each individual sensor may be realized using present commercial available single Mach-Zehnder type amplitude modulator (-35 dB extinction ratio). An array of 100 sensors with 100 ppm detection sensitivity for each sensor may be realized if a double Mach-Zehnder type amplitude modulator is used.

A novel scheme for measuring Brillouin gain spectrum distribution along an optical fiber is developed. This scheme utilizes direct frequency modulation of a laser diode for two purposes: generation of the pump and probe lightwaves, and position-selective excitation of stimulated Brillouin scattering. A laser diode is modulated with a rectangular-wave signal, so that the pump and probe lightwaves for Brillouin gain spectrum measurement are generated in time-division manner. A sinusoidal-wave signal is mixed with the modulation signal for spreading the spectra of the pump and probe lightwaves. Because of the spread spectra, stimulated Brillouin scattering occurs in a small section of the fiber where the two lightwaves are highly correlated. The Brillouin gain spectrum at the section is obtained selectively. The section to be measured is chosen by varying the period of the sinusoidal modulation. Experimental result of measurement of the Brillouin spectrum distribution with a spatial resolution of 70 cm is presented. As a comparison, another system is demonstrated, where the probe lightwave is generated by intensity modulation with LN modulator. Spatial resolution of about 6.5 cm is achieved.

Systems of multiplexed Bragg grating fiber-optic sensors are currently used for the mechanical strain measurements in abutments and bays of bridges, in hulls of buildings, vessels and aircraft. In many cases, however, the control of constructions elements elongation is inadequate for prevention of their mechanical damages, as far as other sorts of deformations (a bending, for example) can be important breaking factor. Hence special fibers sensitive element owning advantages of easy fabrication, absence of moved details, high sensitivity and ability to be combined in one quasi distributed sensor is developed in present work. The suggested sensitive element (SA) presents a place of optical fiber diameter local decreasing which is fabricated by elongation of heated section of fiber. When guided light propagates through this drag mark zone both insitivity of transmitted and backward Rayleigh scattered beams are strongly depends on the fiber curvature in zone of drag mark's neck. The drag mark optimal size and shape providing maximal efficiency and linearity of transformation of bending into intensity of optical signals modulation are defined. In mentioned optimum condition the bending angle sensitivity of SA reaches 2 dB/degree, which exceeds the bending sensitivity of original fiber having no irregularities onto 60 dB. It is demonstrated that by sequential arrangement of drag marks along fiber one can fabricate quasi distributed sensor. Hereby one can use the time domain reflectometry for dividing of signals of seven sensitive elements. Extreme simplicity and high sensitivity of suggested quasi-distributed sensor opens a perspective of its application in systems of monitoring of deformational processes in various objects.

A fiber optic wavelength modulation sensor based on the spectral shift of tantalum pentoxide thin films for absolute temperature sensing up to 650 degrees Celsius is described. A tantalum pentoxide single layer was deposited directly onto the cleaved end-face of a single mode optical fiber and was illuminated with a super luminescent diode (SLD) through an addressing fiber. Interference fringes of the film on reflection were obtained within the optical bandwidth of the SLD using an optical spectrum analyzer. The spectral shift versus temperature rise showed no turning points and the output was unambiguous, linear, monotonic and gave about 0.016 nm wavelength shift in the spectrum per 1 degree Celsius. A semi-empirical calibration procedure for reading absolute thermometric measurements is presented.

In white light interferometry, the measurement resolution is limited relatively by the coherence length of the light source, usually in tens of fringes. By the use of two wavelength combination and multi-wavelength combination source approaches, the equivalent coherence length can be decreased to a few fringe level and nearly one fringe level respectively. By employing a discrete fringe pattern transform technique, the equivalent coherence length can be further reduced to a sub-fringe level, representing a high potential in high precision white light interferometric measurement.

Two fiber-optic cure sensors based on the refractive index measurement are presented. An optical fiber distal end reflectometer and a stripped claddind fiber refractive index sensor, both made by an optical fiber with the core refractive index of 1.558, are tried and tested to be calibrated and provide quantitative information during cure. The fiber reflectometer is confirmed to be suitable to be calibrated during cure and may be possible to provide quantitative cure extent when hybrided with an Optic Time Domain Reflector (OTDR). The stripped cladding refractive index sensor is found to be unsuitable for the purpose of being calibrated during cure.

A new type of demodulator is described which uses direct sampling of the optical power to measure the sine and the cosine of the signal phase in the presence of a sinusoid modulation. The modulation sinusoid is also held at the optimum modulation depth and sampling phase.

A novel fiber optic vibrometer has been built and characterized. The vibrometer's optical design is based on the Sagnac interferometer. Due to the nature of the Sagnac's optical configuration, the optical phase shift induced by the surface being probed is differentiated, and therefore results in a measured optical phase shift that is directly proportional to the velocity of surface. The path matched Sagnac design eliminates the need for a coherent light source used in laser doppler vibrometers and offers great design flexibility for tuning the vibrometer's frequency response and dynamic range. A fiber optic Sagnac vibrometer was built and operated with a modulation frequency of 100 kHz. The vibrometer's dynamic range exceeds 70 dB with a maximum velocity of greater than 10 m/s and a noise floor of less than 1 micron/s/(root)Hz. Experimental results demonstrating the vibrometers performance will be presented.

A twin-bridge fiber optic calorimeter has been built and is currently being tested at Los Alamos National Laboratory (LANL). The intrinsic optical phase shift in single mode optical fiber induced by changes in temperature is measured in both a measurement chamber and an identical reference chamber. The walls of each canister are surrounded by optical fiber configured in a Michelson interferometer arrangement driven by a 1319 nm diode pumped YAG laser. The interferometers have an optical path mismatch of 1.6 km and a temperature sensitivity of 471,000 radians/C. A digital demodulation scheme is used which produces a 32 bit phase word and tracks up to 3 X 10⁶ radians with a resolution of 10^-3 radians, giving the system a dynamic range greater than 10⁹. This dynamic range was used to match the 1.6 km optical path mismatch of the sample and reference Michelson interferometers to within 1 mm in order to minimize phase errors produced by frequency drifts of the optical source. Both interferometers are demodulated simultaneously at a rate of 83 kHz. The phase difference between the reference and sample interferometers is proportional to the temperature difference between the chambers and therefore measures the power produced by a sample placed in the measurement chamber. The calorimeter sensitivity was initially measured to be 151 radians/mW, however, the calorimeter sensitivity has subsequently been increased to 383 radians/mW by the addition of foam insulation around the thermals. With this sensitivity, the theoretical calorimeter resolution should be less than 20 nW, however, the observed long term optical phase drift of 18 radians limits the calorimeter resolution to 47 (mu) W. Nevertheless, the fiber optic calorimeter performance has been observed to exceed that of a state-of-the-art wire wound calorimeter of similar dimensions by a factor of 2. The optical system performance will be described along with calorimetric measurement results.

Herein describes a new light principle for measuring distances based on fiber optic technology. Measuring distances correlated with the amplitude of light signals in an optical fiber. Basics of this new measuring principle, main sources of errors, and distance resolution of this new method are herein described. The method's distance resolution is up to 0.01 - 0.001 Angstrom and theoretically is unlimited. Methods of experimental tests of distance resolution characteristics are described. The new measurement method is used for the construction of different sensors such as accelerometers, pressure sensors and microphones. The microphones based on the new measurement principle are produced industrially. Frequency range of the method is from DC up to 500 kHz. The new method may be used as a basis to construct a wide range of physical, chemical, bio-chemical, medical and other sensors.

A theoretical concept and its experimental realization of a fiber-optic Bragg grating strain sensor system to measure dynamic deformations in rock masses are presented. The system has been designed in order to monitor strain variations in the range of 10^-9 within a bandwidth of 0.1 to 2 kHz. First promising results from field experiments are shown where seismic signals have been detected, in comparison with conventional geophone registrations.

High concentrations of H⁺ and other chemical species can enhance the growth rate of sub-critical cracks in metals subjected to corrosive environments. High concentrations of H⁺ are thought to be the result of both metallic dissolution and bulk transport. The rate of mass transport of chemical constituents into the crack tip is proposed to be largely a function of the surface geometry of the crack. For this study, a fiber optic chemical sensor (FOCS) is developed to investigate the influence of crack size and crack surface roughness on the transport rate of hydrogen in an occluded environment. The FOCS combines pH sensitive fluoresceinamine attached by photopolymerization to the distal tip of a single optical fiber to non-invasively monitor pH in the occluded cell. This sensor geometry provides the benefit of real-time pH measurement at specific points in the crack tip environment. Chemically inert crack specimens have been used in order to investigate crack surface geometry affects without the added complexities of metallic dissolution. Varying crack sizes and roughnesses are considered and preliminary results for model geometries are presented.

The objective of this project was to design a monitoring system capable of detecting and quantifying tritium in-situ in ground and surface waters, and in water from effluent lines prior to discharge into public waterways. The design work was successfully completed with the predicted capability to detect tritium levels below 20 nanocuries per liter. The designed system was based on the detection of the low energy beta radiation from the radioactive decay of tritium using a special form of scintillating optical fiber directly in contact with the water to be measured. To support the design, laboratory tests were performed in several areas. Different types of scintillating fiber were tested to determine which would provide optimum system performance. The fibers contained a fluor material in a special cladding configuration which optimizes the absorption of beta radiation. The tritium detection system consists of an immersible sensor module containing the optical fiber and detection electronics as well as signal processing electronics. An umbilical cable is used to interconnect the components. The system design goals included optional permanent installation for routine water monitoring in wells, process and effluent lines or as a potential portable survey tool which could be moved from one location to another. Not all the design goals were met due to the large physical size of the immersible sensor module. Discussed in this paper are the design details of the in-situ tritium beta detector, the tests performed, and results obtained. The work was supported by U.S. Department of Energy (DOE) contract number DE-AC21-96MC33128.

This paper deals with two novel modifications of the inverted- graded index (IGI) fibers, namely with IGI fibers with a truncated inverted-linear refractive-index profile and with a truncated inverted parabolic refractive-index profile which is localized only to part of the core. Preforms for drawing the fibers were prepared by the MCVD method by using boron oxide for decreasing the refractive index of silica. Fibers were drawn from the prepared preforms and coated with polydimethylsiloxane polymeric cladding. Fabrication of the fibers and its relation to characteristics of the prepared fibers such as refractive-index profiles are described. The experimental sensitivity of the prepared IGI fibers to changes of the refractive index of the cladding was determined in immersion experiments under the excitation of the fiber with an inclined collimated beam. Results of ray-optics theoretical analysis of these sensing structures for the above excitation conditions are given, which show the possibility of further increasing the sensitivity to refractive-index changes of the optical cladding of IGI fibers even for claddings with a refractive index value of about 1.41 (commercial PCS fibers). The obtained theoretical and experimental results show higher evanescent-wave sensitivity of the two prepared novel sensing IGI fibers in comparison with standard PCS fibers.

This paper describes a new fiber-optic calcium ion sensor using the calcein derivative, calcein acrylamide. Calcein acrylamide (CA) is a fluorescent indicator with a sensitivity for calcium ions at pH greater than 10. Ca²⁺ binds with CA with a binding ratio of 2:1 ratio therefore the concentration range for Ca²⁺ sensing depends on the amount of indicator present. Typically, calcium can be detected in the range 0 to 20 micrometers ol 1^-1. The indicator can be covalently immobilized within a polymer matrix which itself is covalently immobilized onto a chemically modified optical fiber using photo-initiated polymerization. Three Ca²⁺ sensitive polymers are presented; CA in (1) poly-hydroxyethyl methacrylate (poly- HEMA), (2) poly-acrylamide and (3) poly-N-vinyl-pyrrolidinone (n-VP). The sensitivity of the immobilized CA are similar to that of CA in free solution.

A Quasi-distributed pH sensing system based on Optical Time Domain Reflectometry (OTDR) was developed to determine the spatial and intensity information from fluorescence signals coupled back into a single fiber. The evanescent wave due to a 488 nm light pulse from a N₂ pumped dye laser was used to excite a pH sensitive fluorophore and the emission around 515 nm was detected. A 3dB Y-coupler was used to convey excitation light to the sensing sites and to deliver emission signals from these sites back to a filtered photomultiplier tube. Data collection was performed using a HP 54540A 500 MHz digital scope and analysis was carried out using a dedicated Pentium 166 MHz PC. A reproducible laboratory fabrication process was developed to produce sensing sites at discrete intervals along the length of the fiber. A polishing process carefully removed the cladding at each sensing site and photo- polymerization was then used to covalently bind the fluorophore fluorescein with a co-polymer directly onto the site. The results show the sensors performance over a range of pH4-pH10 with a pKa value of 6.3. The present system was chosen to have sites 10 m apart, however, based on the propagation rate of 5 ns m^-1 for light in the fiber and 10 ns for the fluorescence lifetime, a resolution of approximately 1 m is possible.

The paper presents a method, which allow increase sensitivity of measurements with fiber optic chemical sensors based on absorbance indicators. This can be especially useful if weak absorbance indicator is utilized. The idea of the measurements is based on two wavelengths specific for the indicator. One wavelength is an analytical and the second -- can be called as a complementary i.e. the absorbance, at this wavelength, changes in opposite direction in comparison to analytical signal. The mathematical combination of these two signals gives enhancement of the sensor dynamic range.

A new refractive index microsensor based on surface plasmon resonance (SPR) for fine scale measurements in aquatic environments is presented. The local light conditions in marine sediments determine the activity of photosynthetic organisms. The light field can be investigated by scalar irradiance microprobes at a spatial resolution better than 100 micrometer but the refraction index still has to be assumed constant or measured by techniques with lower spatial resolution. It is well known from other microsensor measurements that the microenvironment strongly determines the metabolism of the organisms. Therefore we developed the new sensor to access the fine scale distribution of refraction index. A second parameter of interest is the salinity which can be assumed not to be constant in some biofilms. As the salinity is usually measured with a refractometer, we checked if we could measure salinity independent of the ambient optical conditions in the sediment with the new microsensor. The microsensor is formed by a multimode silica fiber where the plastic jacket and fiber cladding has been removed. The tip is then tapered and a gold layer is deposited. Due to the tapered geometry the diameter is decreased to achieve a higher spatial resolution for profiling applications. SPR is excited at the fiber tip by coupling polychromatic light into the fiber. The reflected light is measured with a spectrometer. A refractive index change of the fiber tips surrounding area causes a wavelength shift of the spectral intensity distribution. The effects of different tip geometries have been characterized and results are presented. An appropriate measuring system is proposed.

We discuss the results on development of very-high output power, wide and flat spectrum superluminescent diodes (SLD) and singlemode fiber pigtailed modules at different spectral ranges. It is shown that by optimizing of SLD design and using by nearly 100% laser diode-consistent SLD technology and structure, it is possible to obtain from SLDs the same outputs as that of fluorescent fiber-doped sources. Using of common approach to SLD design, singlemode fiber outputs up to 20 - 30 mW are realized at 680 - 980 nm spectral bands, with very flat Gaussian-like spectrum and extremely low residual Fabry-Perot modulation depth. Up to 10 mW power is realized at 1300 nm using the same approach. Further possibilities, like SLD spectrum broadening and others, are also discussed.

In this report we summarize the first, to our knowledge, results on complex analysis of long-term stability of medium power 820 nm band superluminescent diodes (SLEDs, or SLDs) and singlemode fiber pigtailed modules. Up to 100,000 h MTTF was estimated from accelerated and nominal operation tests of emitters and modules in constant current (ACC) mode; less than 1000 ppm changes of mean wavelength over 30,000 h of operation are predicted from these tests. It is sown that constant power (APC) driving may result in considerable lifetime increasing; lifetime estimations from tests results in APC lifetime 300,000 hours, that is comparable to laser diodes. Twice less wavelength changes, 500 ppm, were obtained in case of APC driving of SLDs.

We report development of SLD emitters with reduced dependence of SLD power on ambient temperature, and study of their performance parameters over very wide temperature range. SLDs were able to deliver up to 0.1 mW output power from SM fiber till +100 degC ambient temperature. Wavelength dependence on operation temperature was measured within range from -55 to +85 degC with the help of hermetic fiber pigtailed modules on the base of such SLDs. Parameters governing power stability of such modules are discussed. It is shown that SLD wavelength dependence on temperature may be linearly approximated with relative tolerance of less than 3 X 10^-3, second-order approximation by parabola results in less than 3 X 10^-4 relative mistake. It is found that experimental coefficients in wavelength-on- temperature approximations are close to fundamental bandgap wavelength dependency of AlGaAs.

Elliptically polarizing optical fiber has been fabricated. Measurements show an extinction of the lossy mode relative to the transmitted mode of around 10 dB/m. The preform is spun during drawing and the ellipticity of the transmitted polarization state is as expected from the measured beat length of the unspun fiber and the pitch length of the spun fiber. This fiber is expected to be useful for interferometric or laser-based electric-current sensing, and perhaps in other applications for an in-line polarizer.

A novel fiber optic sensor employing microbend modulator and small-displacement diaphragm with hard center is presented in this paper. The sensor is made up of isolation diaphragm, silicon oil, sensing diaphragm, microbend modulator, sensing optical fiber, reference optical fiber, supporting platform, adjusting bolt, front cover and back cover etc. It can be used to measure the pressure of gas or liquid. When the pressure is applied to the isolation diaphragm, it passes on it to the sensing diaphragm through silicon oil. Then the hard center of the sensing diaphragm produces small displacement. Thus the light in the sensing optical fiber between the microbend modulator is attenuated. By detecting the output light power, the relationship of the input pressure and relative output light power is obtained. The basic structure is given and the sensing mechanism is discussed. Experiments have shown a satisfactory result. The sensitivity and measuring range of the sensor can be modified by altering the mechanical wavelength of the microbend modulator, ratio of the teeth number of the two deformer plates or by changing the parameters of the sensing diaphragm. The sensor has the function of zero adjustment and overload protection.

The inherent temperature stability of a fiber voltage sensor is far from industrial requirements. Usually, a special channel for a temperature control is needed. Temperature- dependent birefringence of optical elements, such as a quarter-wave plate and a sensitive crystal, is the main source of temperature-induced drift of sensor parameters. To solve this problem we used a special back reflecting prism as a phase-retarding element, BTO crystal as a sensitive element, and a double-pass scheme. The double-pass scheme enables to diminish the negative role of the optical activity in the crystal, to increase an interaction length and, thus, to enhance the sensitivity of the sensor. The special back reflecting prism demonstrates temperature stability more than 20 times better, than a zero-order quartz quarter wave plate. This permits to decrease the temperature-induced drift of sensitivity. The sensor demonstrates temperature stability of (1.5% from -20 degrees Celsius to 60 degrees Celsius) and sensitivity of 0.145% per 1 V rms at 850 nm without using an additional temperature control channel.

Optical hydrophone arrays have reached a sufficiently advanced stage of development that prototype systems have been successfully deployed at sea, however, one of the remaining areas of development required for a full scale system is multiplexing gain. Many different multiplexing schemes have been proposed based on techniques such as time, code, frequency, coherence and wavelength division multiplexing which typically limit the number of sensors addressable to around 60. Described here is a demonstration interferometric hydrophone array using time and dense wavelength division multiplexing. The system uses a 3 wavelength fiber laser source, with a wavelength spacing of 1.6 nm, in a configuration where two of the wavelengths each interrogate a pulsed reflectometric array module with up to thirty-two time division multiplexed hydrophones. The fiber architecture uses optical add/drop multiplexers (OADM) to drop wavelengths from a telemetry fiber,and launch them into an array module and the return signals are re-combined with the telemetry fiber. The received optical signals are separated, detected and processed separately. The experiment demonstrates the potential to address in excess of 96 sensors through a single fiber pair with 32 hydrophones per wavelength. The measured level of array-sensor crosstalk is shown to be less than 72 dB.

The construction and performance of a two-sensor fiber-Sagnac- based sensor array is reported. The array is constructed in a folded configuration which passively desensitizes the delay coil to phase modulations. Experimentally, the delay coil desensitization is measured to agree with predictions. The sensor phase sensitivity is measured to be as low as 3 (mu) rad/(root)Hz and 0.8 (mu) rad/(root)Hz in a single- detection and a balanced-detection configuration, respectively.

In-fiber Bragg grating sensors have been used to study mechanical strain in optical fibers that have been terminated in ST connectors. Our findings indicate that terminated sensors experience a compressive strain whose magnitude depends on the cure profile of the epoxy encapsulant used in these connectors. These experiments demonstrate the viability of using in-fiber sensors to characterize fiber optic connector assemblies during and following termination. However, the stain state of the sensing environment is a complex one, so there is the challenge of reading out the correct strain from the sensor response. To address this problem, the T-matrix formalism is being utilized. A review of this method and examples of its use will also be presented.

Commercial and military launch vehicles are designed to use hydrogen as the main propellant, which is very volatile, extremely flammable, and highly explosive. Current detection systems uses Teflon transfer tubes at a large number of vehicle locations through which gas samples are drawn and the stream analyzed by a mass spectrometer. A concern with this approach is the high cost of the system. Also, the current system does not provide leak location and is not in real-time. This system is very complex and cumbersome for production and ground support measurement personnel. The fiber optic micromirror sensor under development for cryogenic environment relies on a reversible chemical interaction causing a change in reflectivity of a thin film of coated Palladium. The magnitude of the reflectivity change is correlated to hydrogen concentration. The sensor uses only a tiny light beam, with no electricity whatsoever at the sensor, leading to devices that is intrinsically safe from explosive ignition. The sensor, extremely small in size and weight detects, hydrogen concentration using a passive element consisting of chemically reactive microcoatings deposited on the surface of a glass microlens, which is then bonded to an optical fiber. The system uses a multiplexing technique with a fiber optic driver-receiver consisting of a modulated LED source that is launched into the sensor, and a photodiode detector that synchronously measures the reflected signal. The system incorporates a microprocessor (or PC) to perform the data analysis and storage, as well as trending and set alarm function. As it is a low cost system with a fast response, many more detection sensors can be used that will be extremely helpful in determining leak location for safety of crew and vehicles during launch operations.

A new technique is described for simultaneously obtaining chemical images in an imaging sensor with high spectral resolution over a wide spectral range. The technique is demonstrated using a combination CO₂/O₂ sensor, in three different sensor configurations. It is shown that it is potentially feasible to measure as many as 300 indicators on the tip of an image guide simultaneously at many different wavelengths.

This paper gives an overview of the primary issues of structural health and evaluation monitoring for civil structures, such as bridges, dams, buildings and roadways, and role that fiber optic sensors play in the monitoring efforts. Some of the quantities that need to be measured are displacement, velocity, acceleration, jerk, force, stress, strain, temperature, fracture, moisture, fatigue, and chemical state, i.e. corrosion. Fiber optic sensors have the capability to measure most, if not all, of these quantities. Fiber optic sensors exploit a variety of physical principles through which physical quantities are measured. The particular types of fiber sensors that will be discussed in this paper are: intensity-based, modal domain interferometric, Bragg grating, white light interferometric, and Brillouin backscatter. The operating principles and application results from field and laboratory studies are presented.

The international oilfield service industry has used a large variety of sensing devices for the exploration of oil and gas in the last 70 years. These include sensors that range in the measurement of nuclear information, resistivity, acoustic, seismic, E & M fields including most recently optical sensors. The environments are very demanding in temperature, pressure, and vibration which, in general, describes the difficulty in converting current laboratory or commercial sensors for the oilfield industry. Applications where optical sensors have been successfully used and the potential future of fiber optic sensors for this industry will be reviewed.