Scientific examinations of paintings are routinely carried out in major galleries and museums to assist in conservation treatment and as part of technical or art historical examinations. Care is taken to examine the paintings non-destructively as far as possible. However, in order to study the paint and varnish layers, it is still currently necessary to take tiny samples of a painting to examine the cross section of a small area of the painting under a microscope. In an attempt to solve this problem we evaluate the potential of optical coherence tomography (OCT) in providing high resolution information about paint layers. Two OCT systems have been assembled, operating at 850 nm and 1300 nm, each using two single mode in-fiber couplers. Both systems can produce A (reflectivity profile in depth), T (lateral reflectivity profile), B (cross section image) and C-scans (constant depth image). Using superluminiscent diodes, a depth resolution better than 9 microns is achieved. We present results of applying OCT to sample panels and paintings. We show that infrared OCT is capable of non-destructive examination of paintings in 3D, which shows not only the structure of the varnish layer but also the paint layers. The OCT images present better microscopic tomography of the surface of the varnish and paint layers than any system currently employed in the examination of paintings. OCT could also be used for accurate measurement of the thickness of the varnish layer on a painting.

New miniature extrinsic Fabry-Perot interferometric (MEFPI) optical fiber sensors with a size of 125μm in diameter are presented, which are ideal for applications where the operation space is highly restricted. The temperature sensor can work up to 800°C with a sensitivity of 0.46nm/°C. The pressure sensor exhibited a sensitivity of about 0.36nm/psi. The sensitivities of the pressure and temperature sensor can be controlled with high precision during fabrication. In addition, their Fabry-Perot cavity lengths can be controlled with a resolution of several nanometers, which provides excellent flexibility in sensor design and signal demodulation. The sensors are composed entirely of fused silica, which is very reliable, biocompatible, corrosion resistant and immune to electromagnetic interference (EMI).

Fully-distributed optical-fibre sensing (FDOFS) systems are developing rapidly and are offering significant advantages for measurement functions in a variety of structural applications, especially in the oil industry, the power supply industry, the aerospace industries and civil engineering construction. Polarization techniques are well established in FDOFS, and in the analysis of polarization-mode dispersion (PMD) for optical-fibre telecommunications. However, a major problem has been that of determining, from one end of the fibre, the distribution of the full polarization properties of a monomode optical fibre, along its length, with some specific spatial resolution. This paper will present a new technique for providing this full information, and thus for measuring the distribution of any parameter, external to the fibre, which can modify its polarization behaviour. As a result, for example, it becomes possible to measure simultaneously the distribution of a strain field comprising the longitudinal and the two transverse components of direct strain, plus the transverse shear strain. The technique comprises an extension of polarization-optical time domain reflectometry (POTDR) [16], and necessitates on-line processing. Details of the physical principles, the algorithms and the polarimetry will be presented, together with some early results illustrating the measurement accuracies which can be achieved.

A conventional SMF-28 was used to conduct localized pipe-wall buckling monitoring in a section of energy pipe with 2,667 mm (105 in) in length and 762 mm (30 in) in diameter by a coherent probe-pump based distributed Brillouin fiber sensor with 15 cm spatial resolution. The locations of pipe-wall buckling have been found by measuring the strain distributions along the outer surface of the pipe. However the sensing fiber (SMF-28) was broken when the bending load increased above 1335 kN (300 kips), which caused the sensing fiber experienced more than the compressive strains of -8,084 με. In order to get strain data after pipeline buckling happens, a high strength sensing fiber with carbon coating instead of conventional acrylate coating should be used. The Brillouin measurement on the carbon coating single-mode fiber has Brillouin frequency shift of v_B 12.479 GHz at wavelength of 1320 nm and room temperature. The measured Brillouin bandwidth Δv_B is equal to 66 MHz. The central Brillouin frequency shows a strong dependence on strain with 1.510 GHz shift at 2.5% elongation. The excellent linearity of the central frequency v_B on strain is confirmed and the strain coefficient was measured as 16.21 με/MHz. Its strain-stress relation keeps linearity up to 2.5% elongation, which is much bigger than that of SMF-28.

Fiber optic sensors having their entire length as the sensing elements for chlorine or hydrogen sulfide are reported here. The chlorine fiber consists of a silica core and a chlorine-sensitive cladding, and the hydrogen sulfide fiber has a hydrogen sulfide sensitive cladding. Upon exposure to the corresponding challenge gas, the cladding very rapidly changes color resulting in attenuation of the light throughput of the fiber. A one-meter portion of the chlorine sensor fiber responds to 10 ppm chlorine in 20 seconds and to 1 ppm in several minutes. The attenuation after 10 minutes of exposure is very high, and is dependant on both chlorine concentration and fiber length. A ten-meter portion of the hydrogen sulfide sensor fiber responds to 100 ppm hydrogen sulfide in 30 seconds and to 10 ppm in 1 minute. The high sensitivity suggests that the propagating modes of the light interact strongly with the cladding, and that these interactions are massively increased (Beers Law) due to the extended sensor length. This approach will supersede the current method of having a collection of point-detectors to cover large areas.

Large-scale acoustic fiber sensor arrays consisting of hundreds of hydrophones distributed along kilometers-long fiber buses are required for applications such as undersea oil exploration. Sagnac-based Sensor Arrays (SSAs) exhibit attractive performance; however the acoustic wave incident on the buses generates a pickup signal that can swamp the signals from the hydrophones. We propose, model, and demonstrate a simple technique for reducing this unwanted pickup by periodically inserting null sensors in the buses instead of hydrophones, so the pickup signal is measured at different locations along the buses. The signal from the null sensor (pickup only) is then subtracted from the signal seen by the adjacent hydrophone (true signal and pickup) to recover the true hydrophone signal. This paper describes in detail the signal processing steps used to perform this recovery. Applied to an experimental two-rung SSA, this technique produced a -18.6 dB pickup suppression for a pickup amplitude as large as 0.44 rad and signal amplitudes up to 0.44 rad, and -15 dB for a signal as large as 0.88 rad. These values are limited by the accuracy of the 8-bit data acquisition and/or electronic noise. With a low-noise 12-bit data acquisition, the pickup suppression for small signal amplitudes is predicted to be -35 dB. This work makes headway towards practical SSAs.

The manufacturing process has a huge impact on the characteristics of the all optical fiber sensors array. By automating the manufacturing of fiber Bragg gratings, FBG arrays with much larger count of sensing points, stronger mechanical strength, tighter optical parameters tolerances and enhanced reliability are produced in a cost effective manner. Such fiber Bragg grating arrays are now commercially available with both acrylate or polyimide coating widening the range of applications for FBG sensors to larger scale of services for strain and temperature in a distributed configuration.

We have reported recently a multiplexed fiber Bragg grating (FBG) strain sensor by using the technique of synthesis of optical coherence function. By modulating the optical frequency of the light source in a sinusoidal waveform, the coherence function is synthesized into a series of periodical peaks in the meaning of time- integration. Using one of the coherence peaks as a measurement window, and sweeping it along a string of FBGs by adjusting the repetitive frequency of the sinusoidal modulation waveform, we can selectively pick up the reflection as interference signal from any one FBG from the string. Therefore, the FBGs are resolved spatially; they are not necessarily different to each other in Bragg wavelength. By sweeping the center frequency of the light source in a sawtooth waveform, the shape of the FBG reflection spectrum can be obtained, and thus the amount of the strain applied to the FBG can be estimated. Up to date, 100-Hz interrogation speed was achieved with this method, and the measurement range is limited to within the coherence length of the light source. In this presentation, novel methods are proposed to enhance the interrogation speed and the measurement range further. The performance-limiting factors on the interrogation speed and the measurement range are evaluated. It is found that the detected interference signal appears at a certain frequency shifted from the heterodyne beat due to the sweeping of the center frequency. By observing at the shifted frequency, 1-kHz interrogation speed and measurement range beyond coherence length of the light source are achieved.

Many applications of fiber Bragg grating (FBG) sensors take advantage of the serial multiplexibility of these sensing devices, but the measurement application is typically restricted to a single measurand (e.g. strain). This paper discusses the use of FBG-based sensors employed as multi-parameter sensing elements where a single instrumentation system can be utilized to provide measurements of strain, temperature, pressure, acceleration, etc. FBG-based strain, temperature, and acceleration sensors are serially multiplexed to monitor a lathe cutting process. Issues regarding serial multiplexing, wavelength spacing, and sensor transducer design are discussed along with experimental examples of various sensor transducers. An experimental case of serially multiplexed acceleration, strain, and temperature sensors is shown to demonstrate the multi-parameter sensing capability of FBG sensors.

Fiber Bragg gratings with grating planes tilted at small angles relative to the fiber axis couple light both to backward propagating core modes and cladding modes. The resonant wavelengths for these mode couplings depend differentially on external perturbations. Using the core mode back reflection resonance as a reference wavelength, the relative shift of the cladding mode resonances can be used to selectively measure perturbations affecting the region outside the cladding independently of temperature. We have measured a relative wavelength shift lower than 0.4 pm/degree in conventional single mode fiber while the sensitivity to external changes in refractive index can be larger than 300 pm per % index change. Experimental results on the bending selective sensitivity (relative to uniform axial strain) are also reported.

The demand for high safety and reliability standards for aerospace vehicles has resulted in time-consuming periodic on-ground inspections. These inspections usually call for the disassembling and reassembling of the vehicle, which can lead to damage or degradation of structures or auxiliary systems. In order to increase aerospace vehicle safety and reliability while reducing the cost of inspection, an on-board real-time structural health monitoring sensing system is required. There are a number of systems that can be used to monitor the structures of aerospace vehicles. Fiber optic sensors have been at the forefront of the health monitoring sensing system research. Most of the research has been focused on the development of Bragg grating-based fiber optic sensors. Along with the development of fiber Bragg grating sensors has been the development of a grating measurement technique based on the principle of optical frequency domain reflectometry (OFDR), which enables the interrogation of hundreds of low reflectivity Bragg gratings. One drawback of these measurement systems is the 1 - 3 Hz measurement speed, which is limited by commercially available tunable lasers. The development of high-speed fiber stretching mechanisms to provide high rate tunable Erbium-doped optical fiber lasers can alleviate this drawback. One successful approach used a thin-layer composite unimorph ferroelectric driver and sensor (THUNDER) piezoelectric actuator, and obtained 5.3-nm wavelength shift. To eliminate the mechanical complexity of the THUNDER actuator, the research reported herein uses the NASA Langley Research Center (LaRC) Macro-Fiber Composite (MFC) actuator to tune Bragg grating based optical fibers.

The application of MEMS and nanotechnology (MNT) to the field of structural health monitoring (SHM) is a fairly recent development. The recent change in this focus for MNT has been driven by the need to expand the applications for much of the technologies that were developed in the late 1990s. In addition, many companies desire to expand beyond their target high volume market segments of automotive, wireless communications, and computer peripherals, since these market segments were not as lucrative as first predicted. Most of the aerospace structural health monitoring developmental activity has been sponsored by agencies of the U.S. Government, which serves to pace the examination of these newer technologies to some degree. With that said, efforts are underway by companies such as Acellent Technologies and Blue Road Research to explore various MNT structural health monitoring approaches. The MNT under test include embedded piezoelectric sensors, MEMS accelerometers, time domain region sensors, and topical and embedded single and multi-axis fiber optic Bragg grating sensors. The promise of MNT for the SHM market segment is very enticing. The many wireless communication developments and miniaturization developments of the past five years is very attractive to the SHM community, especially those that are able to reduce the cost and complexity of integration. The main challenge for the community is one of selective integration. That is, certain pieces may be appropriate for SHM systems and certain pieces may not be. The better companies will chose wisely and put forth an approach that can be seamlessly integrated into the larger structure. For over a decade, Blue Road Research has been developing technologies aimed at structural health monitoring of both composite and non-composite parts, through the use of single and multiaxis fiber optic Bragg grating sensors. These sensors are 80 to 120 microns in diameter making them smaller than the diameter of a human hair. Multiaxis fiber optic sensors are able to measure pressure, temperature, axial and transverse strain, chemical properties, corrosion, as well as transverse strain gradients. This technology is easily embedded in between the various layers of the composite structure, during manufacture, without compromising the structural integrity, in order to verify manufacturing parameters during the cure cycle and well as monitor the on-going condition of the composite structure throughout its life time. This paper reviews some of the technical work that has been accomplished during the past two years; specifically the embedding of fiber optic sensors into various composite structures in order to be able to conduct in situ non-destructive evaluation of the curing process and the service life of the component. The fiber optic technology has been developed to the point that it is at a TRL of 6.

In this paper, we report the development of a new bonding agent and method for the surface mounting of optical fiber Bragg grating (FBG) strain and temperature sensors for use in high temperature environments--where there is a presence of water, moisture, dust, susceptibility to corrosion and/or elevated temperatures up to 800°C. To ensure a stable reflectivity response of FBGs and their survival at elevated temperatures, we are using chemical composition gratings (CCGs). The refractive index modulation in these gratings is caused by a chemical change, which results in a higher activation energy and stable behavior up to 1000°C. Samples of CCGs were successfully encapsulated and mounted onto metal shims. The packaged sensors were tested for strain (+/- 1000με) and temperature (to +400 °C) response. The encapsulated sensors display a linear response with an increase in the temperature sensitivity of the FBG, with a factor of ~ 28.34pm/°C, and a strain gauge factor of 1.7pm/με.

Long-period fiber gratings (LPFGs) have recently been utilized as optical bend sensors by observing changes in their transmission spectra as the fiber is bent. One such spectral change reported is the "splitting" of each attenuation notch into two. To date, explanations given for this apparent notch splitting have proven unsatisfactory. In this communication, we show that the apparent notch splitting is due neither to a splitting of degenerate cladding modes nor to the phase-matching condition being satisfied at multiple wavelengths for a given cladding mode. In contrast, bending causes new notches to be formed at nearby wavelengths as a result of coupling to asymmetric cladding modes that are not coupled to in a straight UV-induced LPFG. With increased bending, these new notches' central wavelengths shift in the opposite direction as the original notches, thus causing the apparent splitting of the latter. We use a numerical analysis to show that the cladding modes of a fiber undergo large spatial changes when the waveguide is bent. These changes allow coupling in a bent fiber between modes with differing azimuthal symmetry even with a uniform UV-induced index perturbation. All of the primary experimental effects published thus far are well-described with this analysis. This improved understanding of bent LPFGs will be important for the development of devices and is also relevant whenever there is interaction with the cladding modes in a curved optical fiber.

We report on a new class of novel optical fiber structures, designed for use in harsh environments typical of Oil and Gas Applications. Specifically, we focus on fiber designs that alleviate the effects of hydrogen ingression and its associated darkening of optical fibers in harsh environments. We demonstrate theoretically, how a carbon coated optical fiber structure consisting of an array of randomly or systematically placed voids running along the length of the fiber, can lead to significantly reduced hydrogen ingression effects. The array of voids can be of arbitrarily varying shapes and sizes, along the length of the fiber. We derive an equation describing the increase in the fiber lifetime as a function of the average cross-sectional fraction of voids in the fiber. Fiber darkening effects are predicted to decrease by factors of as much as x10, for moderately low fraction of voids in the fiber cross-section. Theoretical predictions are confirmed experimentally by performing ingression tests in a hydrogen test chamber with on-line monitoring capability, simulating down-hole temperatures and pressures. Additional geometric factors, such as fiber diameter, that may also be optimized to further improve the hydrogen ingression resistance of fibers are discussed; in this vein a new larger form-factor fiber, different from the standard 125um fiber is proposed. Finally, the lifetime predictions greater than 5-10 years obtained for such void-filled optical fibers in typical down-hole conditions make them extremely attractive candidates for use in Oil and Gas applications such as well monitoring and logging.

Whispering Gallery Modes (WGMs) in microsphere ring resonators enable excellent sensitivity due to the high Q-factor (> 10⁶) that significantly increases the light-matter interaction. The analytes attached to the sphere surface change the local refractive index, leading to a spectral shift in the resonances of the WGM. A practical microsphere-based sensor must detect minute changes in the refractive index. In addition, the microsphere surface should be functionalized for subsequent binding of bio/chemical molecules. Both functionalization and binding processes should be monitored in order to better anchor the captured molecules and to acquire quantitative and kinetic binding information. In this work, we have carried out a series of experiments towards developing highly sensitive fused silica microsphere based sensors. A fiber prism is used to couple the light from a tunable diode laser to the sphere. A fluidic well is built to allow for injection and withdrawal of samples. The sensor sensitivity of refractive index is characterized by using the mixture of water and alcohol. It is shown that our system is able to detect changes in refractive index as low as 10^-7. We further monitor the kinetics of layer deposition when the sphere surface is functionalized with silane solution. Finally we monitor the protein binding to and peptide cleavage from the functionalized microsphere. Our results should lead to highly sensitive microsphere bio/chemical sensor arrays with applications in biomedical sciences, environmental monitoring, and drug discovery.

A novel integrated optical planar waveguide platform for absorption-based biosensing is demonstrated. The platform integrates surface ion-exchanged channel waveguides with one-step UV patterned sol-gel structures to define the probing regions. Cytochrome c protein was utilized to characterize the device performance. Spectroscopically specific attenuation of approximately 2 dB in the guided signal occurred at 532nm for 1.4 cm long probing region. The estimated level of detection is about 1 pmol/cm² of surface adsorbed cytochrome c. The proposed structure enables environmentally stable, compact, and inexpensive sensing devices that can be applied to a wide range of biological and chemical species.

In this paper, new types of hydrogen fiber optic sensors are reported. Our sensors consist of multimode optical fiber with a thin palladium film deposited on a suitable length of the fiber core, and operate through evanescent field interactions. Different sensor configurations are discussed and experimental results for different hydrogen concentration are presented. It is shown that these kind of sensors are suitable for detecting hydrogen concentrations form 0.5% to 3% with high sensitivity and fast time response.

Optical fiber laser sources with switching capabilities are of interest in applications such as WDM fiber communications systems and optical fiber sensors. In particular, fiber sensor arrangements based on switched optical bridges would benefit with the development of a polarization switched optical source. Such an arrangement could lead to the realization of compact and low noise sensing configurations. We have studied the performance of a fiber laser in which polarization switching is achieved through intra-cavity devices. Besides using bulk optical components within the laser cavity, we have evaluated the switching characteristics with an all-fiber version of the proposed laser configuration. This was realized by means of an intra-cavity electro-optic polarization switch. The effects of intra-cavity losses and length of the gain medium on switching speed of the fiber laser have been experimentally studied. Whereas only low switching frequencies (a few hundred Hz) are attained with the first configuration, the all-fiber arrangement yields switching frequencies limited by the relaxation oscillations of the fiber laser. A simple arrangement for a polarization switched fiber laser is thus demonstrated using commercially available optical fiber devices. The application of this fiber laser source in switched optical bridges is discussed, and the potential features of this type of arrangement are evaluated. Other interesting features observed with this arrangement, such as anti-phase population dynamics, are also discussed.

We propose a fiber optics detector of new design able to perform human voice registration. The device is in principle a phase detector based on the well known effect of light diffraction. The light is highly diffracted at the end of the fiber. The diffraction pattern obtained depends strongly on the phase changes that the light can bear during its propagation throughout the fiber. An acoustic field impinging over the fiber can induce such a phase changes due to the photo-elasticity effect. The effect accumulates along the fiber during light propagation affecting the diffraction pattern when the light leaves the fiber at the exit end. If we located a small aperture near the exit end of the fiber and a small semiconductor detector behind this aperture we can register changes of the intensity proportional to these phase changes. The detected signal can be filtered from DC components and then amplified before sending it to a sound reproduction system. The sensitivity of the detector is high enough to register the weak acoustic field of a human voice. The system can also detect ultra and infrasound. We report detection of ultrasound of frequency up to several MHz. Besides acoustic field the detector can register any kind of perturbation able to affect the phase of the propagating light beam.

The structure and performance of an all polarization-maintaining fiber optic accelerometer is described in this paper. The all polarization-maintaining fiber optic interferometer structure can avoid instability of polarization. With phase generated carrier technology, the signal fading caused by random phase-shift in the interferometer is eliminated. The experimental results show that the stable acceleration signal is attained and the measured resonance frequency of the system is 950 Hz. The on-axis acceleration sensitivity is 656 rad/g. The minimum measurable acceleration reaches 15ng/√Hz . The measured frequency response curve is in good agreement with that of theoretical analysis. The experimental results indicate that the accelerometer is useful for research and industrial applications.

In this weigh-in-motion (WIM) research, we introduce a novel design of WIM system based-on fiber Bragg grating (FBG) technologies. The novel design comes from the idea using in-service bridge as the weigh scale. While vehicles traveling over the bridge, the weights can be recorded by the strain gauges installed on the bridge abutments. In this system, the bridge beam is replaced by a piece of steel plate which supports the weight of the traveling vehicle. Four steel tubes are attached firmly at the corners of the plate serving as the bridge abutments. All weights will be finally transferred into the tubes where four FBGs are attached and can record the weight-induced strains by shifting their Bragg wavelengths. Compared with other designs of fiber-optic WIM systems, this design is easy and reliable. Especially it's suitable for heavy vehicles because of its large capacity, such as military vehicles, trucks and trailers. Over 40-ton load has been applied on the system and the experimental results show a good repeatability and linearity under such a large load. The system resolution has been achieved as low as 10 kg.