The research reported herein includes the fabrication of a tunable optical fiber Bragg grating (FBG) fiber ring laser (FRL)¹ from commercially available components as a high-speed alternative tunable laser source for NASA Langley's optical frequency domain reflectometer (OFDR) interrogator, which reads low reflectivity FBG sensors. A Macro-Fiber Composite (MFC) actuator invented at NASA Langley Research Center (LaRC) was selected to tune the laser. MFC actuators use a piezoelectric sheet cut into uniaxially aligned rectangular piezo-fibers surrounded by a polymer matrix and incorporate interdigitated electrodes to deliver electric fields along the length of the piezo-fibers. This configuration enables MFC actuators to produce displacements larger than the original uncut piezoelectric sheet. The FBG filter was sandwiched between two MFC actuators, and when strained, produced approximately 3.62 nm of wavelength shift in the FRL when biasing the MFC actuators from -500 V to 2000 V. This tunability range is comparable to that of other tunable lasers and is adequate for interrogating FBG sensors using OFDR technology. Three different FRL configurations were studied. Configuration A examined the importance of erbium-doped fiber length and output coupling. Configuration B demonstrated the importance of the FBG filter. Configuration C added an output coupler to increase the output power and to isolate the filter. Only configuration C was tuned because it offered the best optical power output of the three configurations. Use of Plastic Optical Fiber (POF) FBG's holds promise for enhanced tunability in future research.

A fiber optic structural health monitoring (SHM) system was developed and deployed by the Iowa State University (ISU) Bridge Engineering Center (BEC) to detect gradual or sudden damage in fracture-critical bridges (FCBs). The SHM system is trained with measured performance data, which are collected by fiber optic strain sensors to identify typical bridge behavior when subjected to ambient traffic loads. Structural responses deviating from the trained behavior are considered to be signs of structural damage or degradation and are identified through analytical procedures similar to control chart analyses used in statistical process control (SPC). The demonstration FCB SHM system was installed on the US Highway 30 bridge near Ames, IA, and utilizes 40 fiber bragg grating (FBG) sensors to continuously monitor the bridge response when subjected to ambient traffic loads. After the data is collected and processed, weekly evaluation reports are developed that summarize the continuous monitoring results. Through use of the evaluation reports, the bridge owner is able to identify and estimate the location and severity of the damage. The information presented herein includes an overview of the SHM components, results from laboratory and field validation testing on the system components, and samples of the reduced and analyzed data.

Effects of intensity modulation of light source on fiber distributed sensors based on Brillouin optical correlation domain analysis (BOCDA) are analyzed by numerical simulation, and the results are compared with reported experimental results. We show that the shape of the Brillouin gain spectrum in the BOCDA system has a particular dependence on the optical spectrum of the light source, and that it can be controlled and tailored by a proper modification of the optical spectrum using the intensity modulation.

We investigate the influence of the intrinsic thermal stress in optical fibers on their Brillouin gain spectra (BGS). We apply our newly-proposed modal analysis based on two-dimensional finite element method (2D-FEM) to a single-mode optical fiber (SMF) and a PANDA polarization-maintaining fiber (PMF) that are drawn from preforms of the same design except the stress applying parts in PMF. We evaluate both the optical and the acoustic effective velocities and their corresponding fields, and then quantify the BGS of the fibers. The BGS of the two fibers are measured for comparison and the measured results are in good agreement with the calculated ones. The difference in the feature of BGS between the two fibers is found dominantly originated from the effect upon the acoustic velocity due to 2D intrinsic thermal stress in the PMF.

In recent years a paradigm in chemical manufacturing has emerged, numbering-up production instead of the traditional scaling-up. This new approach employs nanoliter to milliliter reactors that increase control of reaction pathways, product choice and yield. These small-scale reactors virtually eliminate mixing and heat transfer problems associated with large-scale reactors that often limit yield. The value of small-scale reactors is being recognized by the pharmaceutical industry where only small-scale synthesis is required until clinical trials are complete, at which time fullscale production needs to be accomplished in the shortest possible time. One of the most often used reaction steps during the synthesis of pharmaceuticals is protecting carboxylic acid groups by esterification. We have been developing Raman spectroscopy as a process analytical tool to monitor and control chemistry in such small-scale reactors. Here we present Raman spectra of the esterification of benzoic acid performed in a 5-mL batch reactor.

Bragg grating refractometers have been previously disclosed by other authors but sometimes suffer from temperature instability due to well know grating characteristics. In order to overcome this limitation, a temperature insensitive refractometer is developed by using a cladding mode resonance as a temperature reference, with the relative shift of the core mode resonance used to measure the refractive index of substances under test. Specifically, the device fabricated here produces a relative resonance shift of 1 pm for every 5×10^-4 of measured index change, with a temperature sensitivity less than 0.5 pm/°C.

Fiber optic-based chemical sensors are created by coating fiber Bragg gratings (FBG) with the glassy polymer cellulose acetate (CA). CA is a polymeric matrix capable of localizing or concentrating chemical constituents within its structure. Some typical properties of CA include good rigidity (high modulus) and high transparency. With CA acting as a sensor element, immersion of the gratings in various chemical solutions causes the polymer to expand and mechanically strain the glass fiber. This elongation of the fiber sections containing the grating causes a corresponding change in the periodicity of the grating that subsequently results in a change in the Bragg-reflected wavelengths. A high-resolution tunable fiber ring laser interrogator is used to obtain room-temperature reflectance spectrograms from two fiber gratings at two different wavelengths - 1540nm and 1550nm. The graphical representation from this device enables the display of spectral shape, and not merely shifts in FBG central wavelength, thereby allowing for more comprehensive analysis of how different physical conditions cause the reflectance profile to move and alter overall form. Wavelength shifts on the order of 1 to 80 pm in the FBG transition edges and changes in spectral shape are observed in both sensors upon immersion in a diverse selection of chemical analytes.

Conjugated polymers are unique materials for use in the development of chemical and biological sensors because of their widely tunable optical and electrical properties that allow them dual functionality as both the sensing element and the signal transducer. Furthermore, as optical photoluminescence based sensors, electroactive polymers are found to exhibit high sensitivity due to the ability of the analyte of interest to quench the photoluminescence of the entire polymer chain. In order to produce a more chemically robust thin film for use as a "solid-state" optical sensor, we succeeded in grafting various poly (3-alkyl-thiophene)s to optically transparent substrates such as glass, quartz, and ITO coated glass. This was accomplished by first grafting a thiophene monomer to the surface then chemically growing the films via oxidative polymerization. XPS studies indicated that each chemical step was accurately understood. The polythiophene growth, unaltered by sonication and tape peeling tests, was uniform across the substrate and could be directed by selective silanization of the substrate. Film thicknesses range from 20 to 200 nm and exhibit varying degrees of surface roughness, depending on the polymerization process. The reaction times and solvents were varied in order to optimize the desired film properties. The absorption and photoluminescence properties of the thin films compared well with literature on spun-cast polythiophene films, as did the electrical conductivities of the doped and undoped material. The photoluminescence intensities of the films are found to be unaffected by paraquat in water but are sensitive to trace amounts of ferric chloride in acetonitrile with measurable Stern Volmer constants.

Analogue Balanced Photo-detection has found extensive usage in high- sensitivity small signal applications e.g. coherent heterodyne detection. It is particularly effective for laser intensity noise removal. Nevertheless, the high cost of the commercially available analogue systems makes them unsuitable for multi-channel applications, such as fast tomography. In this paper a flexible, scalable, inexpensive and compact solution for multi channel digital balanced detection is presented. The proposed system has two components: an analogue front-end, comprising a differential photodiode amplifier for minimizing the external interference noise, and a digital balanced noise remover. The latter component initially calculates a balancing factor (BF) from the average power ratio of the signal and reference photocurrents, measured with the object removed from the signal path. Three digital balancing algorithms (DBAx) are considered for subsequent processing. In DBA1, BF is directly used in real-time ratiometric calculations. In DBA2, the BF is adjusted in real time by monitoring the window-averaged power of the received photocurrents. In DBA3, first the baseline is removed using differentiation and then ratiometric detection is performed. Using the digital alternative only one measurement of the reference beam is necessary for single-source, multi-channel detection systems. The data from multiple channels are processed in parallel by pipelined hardware, configured as a state machine. The proposed system leads to a fast optical computerized tomography system using digital balanced detection.

We report on a novel fiber based coherent detection system employing an optical preamplifier, a spectrum bandpass filter, and a time-domain filter. The time-domain filter, a synchronous time gate, reduces the in-band Amplified Spontaneous Emission (ASE ) beat noise, which cannot be achieved by the spectrum bandpass filter alone. In preliminary experiments with a 100 GHz bandpass filter, no degradation is observed from the optically preamplified coherent detection compared to pure coherent detection. With a 10 ns pulse width, 500 kHz repetition rate, and 10 pW input power, 2.78 dB and 1 dB signal-to-noise (SNR) improvement has been achieved, respectively, when 5% and 50% time gating duty cycle is used.

There are broad ranges of homeland security sensing applications that can be facilitated by distributed fiber optic sensors and photonics integrated wireless systems. These applications include [1]: Pipeline, (Monitoring, Security); Smart structures (Bridges, Tunnels, Dams, Public spaces); Power lines (Monitoring, Security); Transportation security; Chemical/biological detection; Wide area surveillance - perimeter; and Port Security (Underwater surveillance, Cargo container). Many vital assets which cover wide areas, such as pipeline and borders, are under constant threat of being attacked or breached. There is a rapidly emerging need to be able to provide identification of intrusion threats to such vital assets. Similar problems exit for monitoring the basic infrastructure such as water supply, power utilities, communications systems as well as transportation. There is a need to develop a coordinated and integrated solution for the detection of threats. From a sensor standpoint, consideration must not be limited to detection, but how does detection lead to intervention and deterrence. Fiber optic sensor technology must be compatible with other surveillance technologies such as wireless mote technology to facilitate integration. In addition, the multi-functionality of fiber optic sensors must be expanded to include bio-chemical detection. There have been a number of barriers for the acceptance and broad use of smart fiber optic sensors. Compared to telecommunications, the volume is low. This fact coupled with proprietary and custom specifications has kept the price of fiber optic sensors high. There is a general lack of a manufacturing infrastructure and lack of standards for packaging and reliability. Also, there are several competing technologies; some photonic based and other approaches based on conventional non-photonic technologies.

A system for interrogating fiber optic Bragg grating arrays at kiloHertz sampling rates with sub-microstrain resolution was presented recently. The system makes use of a tunable fiber Fabry-Perot filter for demultiplexing and a path-imbalanced Mach-Zehnder interferometer for wavelength conversion. The operationally-passive demodulation technique for the interferometer makes use of probing the 3x3 coupler at the interferometer output for its coupling parameters to execute the technique. In this work, we discuss the effects of how errors in determining these parameters translate into measurement error and harmonic distortion. We compare measured effects in the laboratory with predictive models to give error sensitivity metrics. We also consider two modes of sampling errors for such frequency-modulated systems and propose a generalized sampling criterion for minimizing harmonic distortion and measurement error.

Line scan cameras are used for rapidly monitoring a moving web or sheet of material. Lighting for line scan inspection should illuminate a long narrow rectangle, which is imaged onto the linear array of pixels in a line scan camera. This distributed light source should provide a uniform power density at the desired wavelengths. Tungsten halogen lamps and LED arrays can meet many of these objectives, but not in a highly directional beam with minimal thermal issues. We have developed a new distributed light source that is based on diffracting light from a highly blazed grating written in the core of a single mode fiber. The grating is blazed such that out-coupling is 90 degrees to the fiber axis. The fiber is bonded to a cylindrical optic that collimates the azimuthal power distribution. Connecting a single laser diode to the fiber can generate 1 milliwatt per square centimeter over a 10 cm by 0.5 cm rectangular region. Longer gratings and/or multiple segments can be connected to illuminate longer regions. The distributed power density, spatial uniformity, degree of collimation, and spectral bandwidth of these illuminated rectangles are reported. This highly directional distributed source will enhance the utility of line scan cameras in multiple applications.

Current underwater protection systems are complex expensive devices consisting of multiple electronic sensing elements. The detection and identification of divers and small submerged watercraft requires very high image resolution. The high price of an array of conventional piezoelectric transducers and associated electronic components makes this solution feasible for localized implementations, but the protection of large stretches of coastline requires a different approach. We present a novel multichannel sonar design that augments current active sonar transducers with a passive fiber-optic multichannel acoustic emission sensing array. The system provides continuous monitoring of the acoustic wave reflections emitted by a single projector, yielding information about the size and shape of approaching objects. A novel fiber hydrophone enclosure is utilized to dramatically enhance the sensor response to the sonar frequency, while suppressing out-of-band sound sources and noise. The ability of a fiber hydrophone to respond to acoustic emissions is based on established fiber Bragg grating sensing techniques. In this approach, the energy of an acoustic wave is converted into the modulation of the in-fiber optical transducer's optical properties. The obtained results demonstrate significant response of the designed fiber optic hydrophone to the incident acoustic wave over the frequency domain from 1-80 kHz. Our approach allows selective tuning of the sensor to a particular acoustic frequency, as well as potential extension of the spectral response to 300- 400kHz.²

Signal detection can present a major challenge for fluorescence based detection modalities when target is encountered together with intrinsically fluorescent (autofluorescent) components. Luminophores with long emission lifetimes (eg lanthanide chelates) afford a means to discriminate signal from short-lived autofluorescence through the use of Time-Gated Luminescence (TGL). We have recently synthesized a number of novel europium chelates and required an instrument to accurately compare luminescence lifetime, spectral output and emission intensity. The photophysical response of the chelates was captured using a lab-built time-gated luminescence analyser employing pulsed UV (360 nm) excitation from a high-power (~200 mW) LED. Chelate luminescence was detected using a R928 photomultiplier tube gated electronically into conduction shortly after the excitation pulse had ceased. The photomultiplier dynodes were configured in a novel switching arrangement using high-voltage field effect transistor (FET) devices driven by an optically isolated signal. Using this arrangement, the photomultiplier was gated off until LED excitation had fully extinguished whereon the tube was switched to full-gain within less than a microsecond. In the prototype instrument the strongly emitting europium line (⁵D₀→⁷F₂) was collected with high efficiency using epifluorescence optics. We used the instrument to compare two intensely luminescent europium chelates (BTOT and BHHT) using the analyser and report key photophysical parameters for both compounds.

Experimentally, in the open-top ridge waveguides, the sensitivities of core and cladding resonances to the surrounding medium's refractive index are different while the temperature sensitivities are similar. Based on these characteristics, a temperature insensitive refractometer has been proposed. To increase the sensitivity of these devices, a theoretical model is developed to investigate the performance of some potential waveguide and Bragg grating structures. Relationships between the waveguide core size, refractive index distribution, tilt angle of the Bragg gratings as well as the relative evanescent sensitivity of the core and cladding modes are examined. As a result, we find that sensitivity can be enhanced by decreasing the waveguide core size, making the effective index of the waveguide close to the expected refractive index of the analyte, and incorporating tilt in the Bragg grating structures. Furthermore, the inclusion of tilt also appears to reduce the grating's birefringence for the waveguide structure examined.

In this work, we demonstrate the possibility of planar waveguide photonic crystals to be used in general sensing proposes. The electromagnetic performance of photonic crystals obtained by connecting in cascade planar optical waveguides with high index contrast was analysed. In our case, the periodicity of the lattice is obtained in the substrate, instead of in the guiding region, as in conventional waveguide photonic crystals. The theoretical model involves a new generalized scattering matrix concept, together with the generalized telegraphist equations formulism and the modal matching technique. The implementation of the pattern waveguide photonic crystals was carried out by connecting abruptly planar optical waveguides. To get the periodic lattice, we use the substrate refractive index as lattice periodic parameter, and the waveguide lengths as lattice constant. Photonic band gaps and photonic windows were obtained. In all cases the power conservation was excellent. If a local defect is introduced in the PBG structure, an on state can be introduced in the gap. The local defect modifies the optical path, so that the PBG is broken, and the on state appears in the PBG interval. Besides, the on state wavelength can be tuned if the optical path of the defect is modified: changing the physical length or/and the refraction index of the defect. In this way, planar waveguide photonic crystals could be used for sensing applications when a specimen modifies refraction index lattice site. Sensing properties of planar waveguide photonic crystals, with single and double sensing channel, are demonstrated.

Many types of optical fibre sensor are based on either sensitive measurement of optical attenuation or on spectral measurements involving absorption at specific wavelengths or a shift in wavelength, for example, chemical analysis based on spectroscopy, or strain and temperature sensors based on fibre Bragg gratings. Measurements are normally performed under steady-state conditions. Here we propose a technique for optical sensors based on analysis of the transient output of fibre lasers containing an intra-cavity cell or sensor element which affects the attenuation or spectral characteristics of the cavity. We report a detailed theoretical and experimental study of the dynamics of erbium fibre ring lasers. Time domain analysis of the transient involves measurement of parameters such as build-up time and characteristics of the relaxation oscillations which are dependent on the cavity parameters. Spectral domain analysis involves monitoring the detailed optical spectrum during the build-up period. Due to the multiple circulations of light within the cavity during this period, monitoring the spectral evolution may provide high-sensitivity spectroscopic data on absorption lines of gases within an intra-cavity cell. A key challenge, however, is the experimental capture of mode evolution which requires high resolution spectra to be collected during the build-up period.

Because methane is an inflammable and explosive gas, it is indispensable to monitor methane. It is very useful to develop a portable methane detector in order for the application multiple purpose, such as coal mine safety monitoring. This paper gives a prototype design of a portable methane gas sensor with 1.65μm LED. Sensor is made by a 1.65μm LED and high responsivity photo receiver. The experimental result shows nearly linear at low methane concentration. And a prototype portable gas sensor has been demonstrated.

This document shows the theory and set-up of a non-contact measurement strain gauge, which measures translation and strain of a mechanically or thermally loaded specimen. The measurement gauge basically consists of a light source emitting a collimated monochromatic laser beam illuminating the specimen and two CMOS line- scan cameras, which are arranged symmetrically about the incident laser beam picking up speckled reflection. The cameras are recording the granular laser speckles in specific time-intervals and the subsequent images are processed by an algorithm¹ implemented in GNU C. As a result one obtains accurate information about changes in the state of strain and rigid body translation the specimen undergoes² . Furthermore experimental results are introduced. The dilatation of a piezo-stack, the elastic modulus of a thin copper wire and the elastic modulus of a soldering joint are investigated.

An unattended seismic sensor based on optical fiber Bragg grating (FBG) sensing technology is presented in this paper. One of the applications is its deployment in the battlefield remote monitoring system to detect the presence of personnel, wheeled vehicles, and tracked vehicles. The customized FBG sensor prototype is demonstrated which consists of two FBG sensor/demodulation grating pair attached on a spring-mass mechanical system. The sensor performance is evaluated in laboratory and the field tests were carried out in the shooting range using the conventional military Rembass-IIS/A sensor (remotely monitored battlefield sensor system II seismic acoustic sensor) as the benchmark. Personnel and a series of vehicles were used as targets. The experimental data of the field test show that the FBG sensor averaged a 30.20 % greater detection range than the Rembass-IIS/A sensor. It is hoped that the FBG sensor system will be a promising tool for real time monitoring system in the battlefield applications.