This PDF file contains the front matter associated with SPIE Proceedings Volume 7316, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

An increasing number of pipelines are constructed in remote regions affected by harsh environmental conditions where pipeline routes often cross mountain areas which are characterized by unstable grounds and where soil texture changes between winter and summer increase the probability of hazards. Third party intentional interference or accidental intrusions are a major cause of pipeline failures leading to large leaks or even explosions. Due to the long distances to be monitored and the linear nature of pipelines, distributed fiber optic sensing techniques offer significant advantages and the capability to detect and localize pipeline disturbance with great precision. Furthermore pipeline owner/operators lay fiber optic cable parallel to transmission pipelines for telecommunication purposes and at minimum additional cost monitoring capabilities can be added to the communication system. The Brillouin-based Omnisens DITEST monitoring system has been used in several long distance pipeline projects. The technique is capable of measuring strain and temperature over 100's kilometers with meter spatial resolution. Dedicated fiber optic cables have been developed for continuous strain and temperature monitoring and their deployment along the pipeline has enabled permanent and continuous pipeline ground movement, intrusion and leak detection. This paper presents a description of the fiber optic Brillouin-based DITEST sensing technique, its measurement performance and limits, while addressing future perspectives for pipeline monitoring. The description is supported by case studies and illustrated by field data.

Cross-borders smuggling tunnels enable unmonitored movement of people, drugs and weapons and pose a very serious threat to homeland security. Recent advances in strain measurements using optical fibers allow the development of smart underground security fences that could detect the excavation of smuggling tunnels. This paper presents the first stages in the development of such a fence using Brillouin Optical Time Domain Reflectometry (BOTDR). In the simulation study, two different ground displacement models are used in order to evaluate the robustness of the system against imperfect modeling. In both cases, soil-fiber interaction is considered. Measurement errors, and surface disturbances (obtained from a field test) are also included in the calibration and validation stages of the system. The proposed detection system is based on wavelet decomposition of the BOTDR signal, followed by a neural network that is trained to recognize the tunnel signature in the wavelet coefficients. The results indicate that the proposed system is capable of detecting even small tunnel (0.5m diameter) as deep as 20 meter.

One of the most important challenges of distributed fiber-optic intrusion detection systems is to minimize the nuisance alarm rate without compromising the probability of detection in a wide range of operating environments. This involves eliminating nuisance alarms caused by non-intrusion events such as torrential rain without compromising their sensitivity to intrusion events. An effective yet computationally non-intensive event recognition and discrimination technique is presented for eliminating rain-induced nuisance alarms. Results from real intrusion detection systems are presented showing the elimination of rain-induced nuisance alarms for torrential rainfall rates in excess of 4 inches/hr without any penalty to the simultaneous detection sensitivity of intrusion events.

We present Brillouin-tailored optical fiber for use in distributed temperature sensing systems. A manufactured fiber operates with two Brillouin modes that have been gain-equalized to within 0.5 dB, with frequency difference of 175 MHz. Some traditional distributed systems employ a heterodyne scheme to measure a temperature-dependent microwave frequency Stokes' shift (~11 GHz at 1530 nm) imparted by Brillouin scattering. Realizing an RF detection scheme for the temperature distribution may include the development of an optical fiber engineered to have two gain-equalized Brillouin frequencies. The two acoustic modes should respond differently to temperature variations, and thus the detection of their beat signal would provide temperature data. One approach investigated is to structure the core to have two or more dissimilar layers that are 'quasi-independent' such that their resulting Brillouin frequencies are far enough apart, and have a significantly dissimilar dependence on temperature. Gain equalization between these two modes results from the proper tailoring of the overlap integrals with the optical mode. Our best results were achieved through core-cladding Brillouin-gain equalization via the reduction of Brillouin gain in the core of a tailored fiber. A linear temperature dependence of -1.1 MHz/°C was measured for the beat frequency of a developed fiber.

Distributed Fiber Optic Sensing is a powerful technology with wide spread use in applications from down-hole oil & gas wells to environmental monitoring of streams. This paper will highlight some of the various technologies and applications. Recent advances in multi wavelength Raman systems will also be discussed.

We outline a design procedure and show some resulting Brillouin-tailored optical fibers for use in fiber-based sensor systems and networks. Brillouin scattering can be utilized, for example, in distributed temperature sensors. However, it can also be detrimental, such as when high power per-unit-bandwidth is propagated over long distances. Based on models we have developed, several Brillouin-tailored fibers were designed and manufactured, each potentially targeting a unique application. For example, a fiber was designed and fabricated that operates with two Brillouin frequencies that have been gain-equalized to within 0.5 dB, and a frequency difference of 175 MHz. Temperature sensing may be accomplished by the direct detection of the beat frequency between the two Stokes' lines. In another example, an optical fiber was designed and fabricated with eight acoustic modes that have all been Brillouin gain-equalized to within 3 dB, over a frequency span of nearly 1 GHz. In another example, fiber optic sensors that require the propagation of narrow linewidth signals over longer distances are limited by Stimulated Brillouin Scattering (SBS). A fiber has been designed and fabricated that has a Brillouin-threshold that is increased by > 10 dB relative to traditional fibers with the same optical mode size.

Hollow core fiber optics enable gas phase Raman spectroscopy with relatively low power laser excitation sources. A Raman sensor for gaseous fuel analysis is demonstrated using silver coated capillary optical fiber as the sample cell and as the signal collection optic. Using laser powers with as little as a few milliwatts excitation power, the majority species of natural gas and syngas are readily detected, as well as oxygen and nitrogen in a single sensor system. Exchange rates in the capillary optical fiber are high enough to enable optical analysis in sub-second response time for real time sensing and control. Because this one sensor system simultaneously detects and resolves all the component species, real time feedback to the combustion control system of fuel content and properties is enabled.

In recent years we have demonstrated the ability to analyze Rayleigh scatter in single- and multi-mode fused silica fibers to deduce strain and temperature shifts, yielding sensitivity and resolution similar to that obtained using Fiber Bragg Gratings. This technique employs scanning laser interferometry to obtain high spatial resolution Rayleigh scatter spectral information. One of the promising aspects of using Rayleigh scatter for distributed sensing is that the technique should work for any fiber that exhibits discernable Rayleigh scatter. We now demonstrate that distributed sensing with mm-range spatial resolution in off-the-shelf plastic multi-mode optical fiber is feasible. We report temperature and strain sensitivity, and comment on measurement range and hysteresis level. Distributed Rayleigh scatter sensing in plastic optical fiber may offer a valuable alternative to sensing in fused silica fibers because of plastic's low cost and differing mechanical and chemical properties.

Silica-based fiber Bragg gratings (FBG) sensors are versatile devices that are typically fabricated using UV laser exposure. As most standard optical fiber polymer coatings are highly absorbing in the UV, grating inscription typically requires the removal and reapplication of the protective coating by either chemical or mechanical means. Optical fiber stripping and recoating are time-consuming processes that can seriously degrade the mechanical integrity of the fiber. For high temperature sensor applications (> 200 °C), the optical fiber is coated in polyimide which is resistant to chemical attack. Invasive and hazardous techniques for its removal are required such as hot sulphuric acid stripping. In this paper, results of FBG inscription directly through the polymer coating of standard optical fiber with a femtosecond infrared laser and a phase mask are reviewed. Significant grating reflectivities are achieved along with improved mechanical reliability and performance at elevated temperatures. The only example of direct FBG inscription through polyimide coatings for high temperature stable grating sensors will also be presented.

The radiation sensitivity of Bragg gratings written with a femtosecond IR laser was measured for the first time. Type I-IR and type II-IR gratings were written into hydrogen loaded as well as unloaded fibers of distinctly different radiation sensitivity with the intention to find extremely radiation resistant gratings for temperature or stress measurements in radiation environments, as well as very radiation sensitive ones for radiation dose measurements. With a highly radiation-hard F-doped fiber we found a radiation-induced wavelength shift between about 3 and 7 pm after a dose of 100 kGy. These are the lowest shifts observed so far. In such fibers it is very difficult to write gratings with an UV laser. However, gratings made of the highly radiation-sensitive fibers only showed shifts of about the same size as those made of the quite radiation-insensitive Corning SMF-28e fiber. This was already observed with UV laser gratings written in such fibers.

Inscription of Bragg grating structures is reported in inexpensive multimode borosilicate fibers using femtosecond pulse duration 800 nm infrared radiation and a phase mask. Thermal annealing of the gratings up to 700 °C reveals a behavior similar to Type I-IR gratings made in silica fiber with ultrafast infrared radiation. A portion of the index modulation of the grating is stable up to 500 °C. Below 100 °C, the wavelength shift of the Bragg grating is characterized by ~ 12 pm/°C slope. Above 300 °C, the wavelength shift is ~ 5 pm/°C.

In order to fully calibrate hydrocodes and dynamic chemistry burn models, initiation models and detonation models of high explosives, the ability to continuously measure the detonation velocity within an explosive is required. Progress on an embedded velocity diagnostic using a 125 micron diameter optical fiber containing a chirped fiber Bragg grating is reported. As the chirped fiber Bragg grating is consumed by the moving detonation wave, the physical length of the unconsumed Bragg grating is monitored with a fast InGaAs photodiode. Experimental details of the associated equipment and data in the form of continuous detonation velocity records within PBX-9502 are presented. This small diameter fiber sensor has the potential to measure internal detonation velocities on the order of 10 mm/μsec along path lengths tens of millimeters long.

The polymer polydimethylsiloxane (PDMS), which is used as a cladding layer in waveguide-based optical components, is sensitive to some organic compounds. Absorption of organic compounds by PDMS results in changes to the polymer's refractive index and absorption spectrum. In this work, a compact and highly sensitive organic compound sensor based on an evanescent coupling of a Bragg grating ridge waveguide into a PDMS top cladding layer is proposed. The sensor is an open-top Ge-doped SiO₂ ridge waveguide possessing a photoinduced Bragg grating in the waveguide core that has a cladding overlayer of PDMS. When an analyte is applied to the top of the waveguide, changes to the refractive index and absorption of the PDMS layer result in shifts to the Bragg resonance of the core grating. The birefringence, and temperature sensitivity of the sensor are examined.

An optical fiber pressure sensor based on fiber Bragg grating (FBG) and metal bellows is presented in this paper. Due to the lower spring rate of metal bellows, the sensitivity is improved to 48pm/kPa. The relationship between Bragg wavelength and the applied pressure is derived. Experimental data indicates that there is good linear relation between the Bragg wavelength shift and the applied pressure. This sensor can be utilized in low pressure measurement.

We compare the sensitivity of two configurations of coupled resonator optical waveguide (CROW) gyroscopes proposed by others to conventional optical gyroscopes. In both cases, we demonstrate that for equal device footprint and loss, neither of these CROW gyroscopes configurations is more sensitive than its conventional counterpart. In all cases, loss ultimately limits the maximum rotation sensitivity. The fact that light travels more slowly (i.e., with a greater group delay) in a CROW than in a fiber therefore has no effect on sensitivity. The only benefit slow light does have is that it reduces the device length requirement, or equivalently it increases the sensitivity per unit length. However, we show that this improvement is quantitatively the same as in an RFOG. These conclusions are not limited to these two CROW configurations or to rotation sensing, but applicable to any measurand that modifies the phase of the signal(s) traveling in the resonators.

We present a novel highly sensitive biochemical sensor based on a Bragg grating written in the cladding region of a submicron planar Si/SiO₂ waveguide. Owing to the high refractive index contrast at the Si/SiO₂ boundary the TM modal power is relatively high in low refractive index sensing region, leading to higher sensitivity in this configuration [1]. Waveguide parameters have been optimized to obtain maximum modal power in the sensing region (P_Se) and an optimum core width corresponding to maximum sensitivity is found to exist while operating in TM mode configuration, as has been shown in Fig. 1. It has been found that operating in TM mode configuration at optimum core width the structure exhibits extremely high sensitivity, ~ 5×10^-6 RIU - 1.35×10^-6 RIU for the ambient refractive indices between 1.33 - 1.63. Such high sensitivities are typically attainable for Surface Plasmon Polariton (SPP) based biosensors and is much higher than any non SPP based sensors. Being free from any metallic layer or bulky prism the structure is easy to realize. Owing to its simple structure and small dimensions the proposed sensor can be integrated with planar lightwave circuits and could be used in handy lab-on-a-chip devices. The device may find application in highly sensitive biological/chemical sensing areas in civil and defense sectors where analyzing the samples at the point of need is required rather than sending it to some centralized laboratory.

The invention of optical fiber and semiconductor lasers in the 1960s opened up a cornucopia of applications, notably as a medium of carrying light signals for communications and sensing applications. Optical fibers provide a fundamental improvement over traditional methods offering lower loss, higher bandwidth, immunity to electromagnetic interference (EMI), lighter weight, lower cost, and lower maintenance. By applying a UV laser to "burn" or write a diffraction grating (A Fiber Bragg Grating-FBG) in the fiber it became possible to reflect certain wavelengths of light, which used together with an interrogation analyzer (spectral analyzer) precise sensing measurements could be taken. The recent developments of optoelectronics components in the optical telecommunications field have dramatically enhanced the capabilities of many components, such as: light sources, fibers, detectors, optical amplifiers, mux/demuxes, switches, etc. As a result, numerous applications are now available for monitoring strain, stress and pressure in harsh environments. Examples of current and planned deployments will be presented.

Fluoride glasses are the only materials that transmit light in a continuous fashion from ultraviolet up to 8 μm in the mid-infrared region, and can be drawn into high quality optical fibers. In fact fluoride glass fiber technology is the second most mature, beside silica based fiber technology. Fluoride glasses have experienced extraordinary development for more than 25 years. This development was motivated in the beginning by their outstanding optical properties, especially the minimum theoretical attenuation which is 0.01 dB/km between 2 and 3 μm. High quality optical fibers are now commercially available, with attenuation ranging from 5 to 30 dB/km, and mechanical strength ranging from 50 to 100 kpsi depending on fiber diameter. The fluoride glass transmission window is from 0.25 μm to 8 μm without any absorption peaks, while the resulting fiber transmission window can be from 0.3 μm to 4.5 μm for standard fiber and from 0.3 μm to 6 μm for the extended window fiber. In this paper we will present mechanical and optical properties of current fluoride glasses and fibers, as well as high power transmission results.

A spectrograph's design, e.g. the opto-mechanical system beginning at the entrance slit, and ending at the back focal plane position, directly impacts system level performance parameters including the height of the useable aperture, spatial and spectral resolving power, optical throughput efficiency, and dynamic range. The efficiency and integrity of both spatial and spectral input image reproduction within the entire back focal plane area is an often overlooked parameter leading to unnecessary acceptance of sacrificed system level performance. Examples of input images include the slit apertured area of a scene captured by a camera lens, a single optical fiber core located within the entrance aperture area, or a linear array of optical fiber cores stacked along the spatial height of the entrance aperture area. This study evaluates the spectral and spatial imaging performance of several aberration corrected high reciprocal dispersion retro-reflective concentric, as well as aberration corrected Offner imaging spectrographs which produce minimal degradation over a large focal plane. Ray trace images and pixilated area maps demonstrating spatial and spectral reproduction accuracy over the entire back focal plane are presented.

Most radiometers are directionally sensitive. Measuring optical radiation in a given environment is typically done using a collection aperture pointing in the direction of the optical source. The collection aperture has a limited field of view, and the collection efficiency decreases as the angle from direct line of sight increases. Thus, radiometers typically have a limited solid angle for viewing sources. This paper describes a model of an omnidirectional, multi-channel, rotating radiometer that provides a framework for acquiring spatially comprehensive radiometric data from an environment. By exploiting the spatial diversity of multiple collection apertures in multiple directions, sources from all directions are measured via three-dimensional scanning. As the radiometer rotates, data are collected that denote the radiant flux seen by each collection aperture as a function of time. These waveforms are used to determine the directions and magnitudes of electromagnetic sources in the environment without requiring a priori knowledge about the directions of specific sources.

It has been over 30 years since the first fiber optic Sagnac interferometer was demonstrated by Vali and Shorthill in 1976 and the invention of the closed loop fiber optic gyro by Udd and Cahill in 1977. In these years the Sagnac interferometer in the form of the fiber optic gyro became and remains perhaps the most successful fiber optic sensor development. However it is not the only application of the fiber optic Sagnac interferometer and this paper is a personal tour of some other applications that include its usage for acoustic, strain, vibration, distributed sensing, intrusion detection and intrusion prevention. This paper is not intended to be a compressive review of the fiber optic Sagnac interferometer, instead it is a brief overview of a personal effort to develop fiber optic sensors and intrusion resistant communications systems based on this amazing interferometer with the help of friends at McDonnell Douglas, Blue Road Research, Columbia Gorge Research and a great deal of input from researchers worldwide.

A high-speed, swept-laser interferometric interrogation approach is introduced. Dynamic measurements of weak Fabry-Perot or Fizeau type interferometers with gap ranges of 50 um to 1 mm up to 70 Ksamples per second are demonstrated and discussed. Displacement resolution is < 1 pm/rt-Hz. This has application with MEMS and FPPI type sensors.

A novel temperature-independent multi-mode fiber (MMF) lateral strain sensor based on a core-offset interferometer is presented and demonstrated experimentally. Slightly misaligning a splice between an MMF and a single-mode fiber (SMF), high extinction ratio of the interferometer based on SMF-MMF-SMF structure can be obtained. When the lateral strain is applied to a short section of the MMF, the extinction ratio of the interferometer will decrease accordingly while the interference phase remains almost constant. Temperature variation only leads to shift in the transmission power spectrum of the interferometer and does not affect the extinction ratio. Experimental results show that there is a good quadratic relationship between the lateral strain and the extinction ratio. The proposed strain sensor has the advantages of temperature-independency, high extinction ratio sensitivity, good repeatability, low cost, and simplicity in structure.

A novel lateral force sensor based on a core-offset multimode fiber interferometer with intensity-based interrogation technique is reported. An offset between the cores of the single-mode fiber and multimode fiber is made to produce high extinction ratio. When a lateral force is applied to a short section of the multimode fiber, the extinction ratio decreases with the interference phase almost unchanged. In addition to serving as a sensing head, the multimode fiber can also act as a filter to realize lateral force measurement by determining the power change from a power meter. Experimental results show that the power ratio change has a linear relationship with respect to the applied lateral force, and the resolution of the sensor configuration is about 0.01 N.

A 1550 nm DWDM planar external cavity laser (ECL) is demonstrated to provide low phase/frequency noise, narrow linewidth, and low RIN. The cavity includes a semiconductor gain chip and a planar lightwave circuit waveguide with Bragg grating, packaged in a 14-pin butterfly package. This planar ECL laser is designed to operate under vibration and in harsh environmental conditions. The laser shows linewidth ≤ 2.6 kHz, phase/frequency noise comparable with that of long cavity fiber lasers, RIN ≤ -147dB/Hz at 1kHz, and power ≥ 10mW. Performance is suitable for various high performance fiber optic sensing systems, including interferometric sensing in Oil and Gas, military/security and other applications, currently served mostly by costly and less reliable laser sources.

Direct bonding between two epitaxy-ready (EPI polished) sapphire wafers is demonstrated as the basis for an all-sapphire pressure sensor. Through chemical processing, hydrogen pre-bonding, and a final high-temperature bakeout, the two single-crystal wafers are directly bonded without the use of any adhesive or intermediate layer. Dicing across the edge of the structure and inspection of the diced pieces with a scanning electron microscope (SEM) indicates a successful direct bond. Control of the bonding wave generates an air bubble sealed between the two bonded sapphire wafers. Optical interference-based measurements of the bubble height and shape at pressures from 0 to 60psig prove that the bubble is sealed by the bonded wafers and demonstrate the potential for sapphire direct bonding as a means of constructing an all-sapphire pressure sensor. Since the structure contains no adhesives, such an all-sapphire sensor is ideal for pressure sensing in extremely harsh, high-temperature environments, potentially operating at temperatures over 1500°C.

Optical fibers with various protective coatings were submerged in liquid nitrogen to 77°K then tested for mechanical and optical reliability. It was found that while all the fibers maintained strength after low-temperature exposure, the optical response varied depending on the protective coating. The optical attenuation observed for some fiber samples is due to axial shrinkage of the coating, which then leads to an elevated microbending loss. The behavior of the fiber coating at temperatures below the glass transition temperature is discussed.

The performance evaluation of a low-cost macro-bend fiber based temperature sensor is examined in this paper. The temperature sensor is based on a macro-bend singlemode fiber loop employed in a ratiometric power measurement scheme and has a linear characteristic with temperature at a fixed wavelength and bend radius. The sensor head consists of a single turn of a bare bend sensitive singlemode fiber with an applied absorption coating. The temperature of the sensor head is varied up to 75 °C and the linearity of the response is studied with different applied absorption coatings. The impact of stress on the sensor is investigated by applying external forces to the sensor and an estimation of magnitude of the stress induced variation in the ratio of the system is determined. The proposed temperature sensor, based on a macro-bend fiber, has a wide range of applications such as in composite materials processing.

When used to detect extreme temperatures in harsh environments, warning devices have been placed at a distance from the "danger zone" for several reasons. The inability to mix electricity with flammable, caustic, liquid or volatile substances, the limited heat tolerances exhibited by most light sources, and the susceptibility of light sources to damage from vibration, have made the placement of a warning light directly within these harsh environments impossible. This paper describes a system that utilizes a beam of light to provide just such a warning. This system can be used with hard-wired or wireless sensors, side-light illumination, image projection and image transfer. The entire system may be self-contained and portable.

We present a prototype fiber-optic temperature, pressure and skin friction sensor suite for possible use in a deep water petroleum pipeline containing corrosive, multi-phase flow with solid particulates in a pressure and temperature gradient. The sensor suite is designed to measure shear stress ranging from 0-1,000 Pa, temperature from 0-175 Deg C and pressure from 5-69MPa. Each transducer was individually tested over the entire performance range. The integrated sensor package was tested over a limited temperature, pressure and shear range due to facility limitations.

We present progress on advanced optical antennas, which are compact, small size-weight-power units capable to receive super wideband radiated RF signals from 30 MHz to over 3 GHz. Based on electro-optical modulation of fiber-coupled guided wave light, these dielectric E-field sensors exhibit dipole-like azimuthal omni directionality, and combine small size (<< λ_RF) with uniform field sensitivity over wide RF received signal bandwidth. The challenge of high sensitivity is addressed by combining high dynamic range photonic link techniques, multiple parallel sensor channels, and high EO sensing materials. The antenna system photonic link consists of a 1550 nm PM fiber-pigtailed laser, a specialized optical modulator antenna in channel waveguide format, a wideband photoreceiver, and optical phase stabilizing components. The optical modulator antenna design employs a dielectric (no electrode) Mach-Zehnder interferometer (MZI) arranged so that sensing RF bandwidth is not limited by optical transit time effects, and MZI phase drift is bias stabilized. For a prototype optical antenna system that is < 100 in³, < 10 W, < 5 lbs, we present test data on sensitivity (< 20 mV/m-Hz^1/2), RF bandwidth, and antenna directionality, and show good agreement with theoretical predictions.

Scintillating materials, able to convert energy of ionizing radiation into light in the visible-UV interval, are presently used in a wide class of applications such as medical imaging, industrial inspection, security controls and high energy physics detectors. In the last few years we studied and developed a new radiation sensor based on silica-glass fiber-optic technology. In its simplest configuration such device is composed by a short portion (about 10 mm) of scintillating fiber coupled to a photomultiplier through a suitably long passive silica fiber. In this work, we present new results concerning the characterization of silica based Ce and Eu doped fibers glasses obtained by a modified sol-gel method and drawn by a conventional drawing tower for optical fibers. The radio-luminescence of Eu doped fibers is rather weak; moreover it displays a marked sensitivity increase during subsequent irradiations, preventing the use of such fibers in dosimetry. On the other hand Ce-doped fibers show very high radiation hardness, signal stability and reproducibility, and high sensitivity to radiations with energies from 10 keV to several tens of MeV. Numerous tests with photons (X and gamma rays), electrons, and protons have already been successfully performed. At the early stage of its market introduction it is the smallest radiation sensor, also compared to MOSFET and diode technology and it appears to be the ideal choice for in vivo measurements in medical field or remote sensing.

We characterized the responses of different types of rare-earth doped fibers (Yb, Er and Er/Yb) to various types of radiations like UV, gamma-rays, X-rays and protons. The understanding of the radiation-induced effects in this class of optical fibers is necessary as they are possible candidates for use as part of fiber-based systems like gyroscopes that will have to operate in space environment. For all types of irradiations, the main effect is an increase of the linear absorption of these waveguides due to the generation of point defects in the core and cladding. We characterize the growth and decay kinetics of the radiation-induced attenuation during and after irradiation for various compositions of optical fibers. In this paper, we particularly investigate the relative influence of the rare-earth ions (Er, Yb or Er/Yb) and of the glass matrix dopants (Al, P, ...) on the optical degradation induced by ultraviolet laser exposure at 5 eV. This has been done by using a set of five prototype optical fibers designed by iXFiber SAS to enlighten the role of these parameters. Additional spectroscopic tools like confocal microscopy of luminescence are also used to detect possible changes in the spectroscopy of the rare-earth ions and their consequence on the functionality of the active optical fibers.

Common sub-millimeter particle impact phenomena range from zero to thousands of joules of impact energy. The physics of impacts are associated with a wide variety of physical phenomena, including the generation of heat, light, and sound. Although higher energy impact events may result in vaporization of the impacted material and other easily detectable effects, lower energy level impacts of interest may occur with little obvious physical effect. Preliminary research with capacitative sensors provided encouraging results for detecting low-energy impacts. However, vibration within the sensor mounting structure interfered with the detection of impact events. Research on triboluminescent phosphors indicated that a thin layer of material could be used to form the basis of an optical sensor to detect small particle impacts without interference from structural vibrations. A ZnS:Mn phosphor was used as the basis for developing a triboluminescent fiber optic sensor to detect small particle impact events. Detection of impacts is accomplished by detecting the optical pulse that is generated by the abrupt charge separation caused by the particle impact within the phosphor. Laboratory-based experiments were performed to capture the operational characteristics of the sensor. The data are used to study the characteristic response, sensor repeatability, and spatial homogeneity of the detection surface. Tests were also performed to identify the energy detection boundary and to assess environmental survivability. Results of these tests are reported in this paper.

A multi-channel surface-enhanced Raman scattering (SERS) probe based on a multi-core photonic crystal fiber (PCF) is investigated. The multi-core fiber contains seven hollow core PCFs arranging in a compact hexagon pattern that one PCF is in the center with six other surrounded, and the total diameter including the protective jacket is less than 200μm. The seven PCFs can guide light respectively, providing seven channels for sensing. The excitation light is coupled into each core from one end (measuring-tip) of the fiber while the sample entrance is at the other end (probing-tip). Analyte solution mixed with the silver nanoparticles enters each core via the capillary effect, and the silver nanoparticles serve as the SERS substrate. The excitation light transmitting in each hollow core can interact directly with the analyte and the silver nanoparticles in the air cores along the fiber axis. The SERS signal scattered by the sample propagates through the fiber back to the measuring-tip; then couples out of the fiber into the Raman spectrometer. Comparing to a single core PCF SERS probe, the multi-core probe not only takes the advantages of high efficiency of light usage and large interaction space for SERS in each channel, but also is more robust and could provide multi-data. Basing on the seven data of the analyte from the seven channels, an accurate average result could be achieved with less instability. Different concentration Rhodamine 6G solutions have been used as test samples, and the multi-channel sensing idea has been demonstrated by the proof-of-concept experiments.

We report an in-reflection photonic crystal fiber (PCF) interferometer which exhibits high sensitivity to different volatile organic compounds (VOCs), without the need of any permeable material. The interferometer is compact, robust, and consists of a stub of PCF spliced to standard optical fiber. In the splice the voids of the PCF are fully collapsed, thus allowing the excitation and recombination of two core modes. The device reflection spectrum exhibits very regular interference pattern which shifts differently when the voids of the PCF are infiltrated with VOC molecules. The volume of voids responsible for the shift is around 500 picoliters whereas the detectable levels are in the nanomole range.

We report on compact and simple refractive index sensors suitable for measuring indexes in the 1.320-1.432 range with high resolution. The devices are based on modal interference and consist of a stub of large-mode area photonic crystal fiber spliced to standard single mode fiber. In the splice regions the voids of the holey fiber are fully collapsed which allows the coupling and recombination of core and cladding modes. The devices are robust and highly stable over time. The interference patterns are observed in a broad wavelengths range. The devices operate in both reflection or in transmission mode.

Numerical optimization of photonic crystal fiber (PCF) structures for refractive index sensors based on long period gratings inscribed in PCFs has been performed. The optimization procedure employs the Nelder-Mead downhill simplex algorithm. This direct-search method attempts to minimize a scalar-valued nonlinear function of N real variables (called the objective function) using only function values, without any derivative information. An inverse design approach utilizes the objective function constructed using desired sensing characteristics. For the modal analysis of the PCF structure a fully-vectorial solver based on the finite element method is called by the objective function. The dispersion optimization of PCFs is aimed at achieving a high sensitivity of measurement of refractive index of analytes infiltrated into the air holes for the refractive index and probe wavelength ranges of interest. We have restricted our work to the index-guiding solid-core PCF structures with hexagonally arrayed air holes.

The electronic tunability of ferroelectric liquid crystal filled photonic crystal fibers is experimentally demonstrated in the wavelength range of 1500 nm - 1600 nm. The tunability is achieved by applying electric field onto the ferroelectric liquid crystal infiltrated photonic crystal fiber. Tuning of the fiber propagation properties is achieved due to re-orientation of ferroelectric liquid crystal molecules on the application of the applied electric field. Such fibers could find applications in the fabrication of fast, low loss, cost effective and highly efficient in-fiber tunable devices to be used in the telecom wavelength range.

We report on a novel all-fiber refractometer sensor based on multimode interference (MMI) effects. The operating mechanism is based on the self-imaging phenomena that occur in the multimode fiber (MMF) section, which basically replicates the field at the input of the MMF on the output of the MMF for a specific wavelength. However, the longitudinal position of this image is highly dependent on the MMF diameter (D), since there is D² dependence on the longitudinal position of this image. For the refractive index measurement a section of no-core multimode fiber, whose cladding is air, is surrounded by the liquid sample. The liquid sample now works as the cladding medium and as a result of the Goes-Hanchen shift the effective width (fundamental mode width) of the No-Core fiber is increased. As a result, the maximum coupling resulting from the imaging phenomena occurs at a different wavelength, and this can be used to measure the refractive index of the liquid. Using this scheme we can achieve a resolution on the order of 1x10^-5 for a refractive index range from 1.333 to 1.434. The device was used here to measure refractive index in liquids, but can also be applied for measuring concentration of liquids. These sensors are promising and attractive in chemical and biotechnological applications because of their high sensitivity, immunity to electromagnetic interference, and compact size.