Recent work has been done to study the transmission properties of infrared fibers (sapphire, fluoride and chalcogenide) and hollow reflecting waveguides for a range of cw and pulsed infrared lasers. Performance of these devices for medical applications has also been studied. Work has been focused on lowering costs and improving performance as well as on fabrication of novel handpieces that increase safety and improve energy transfer for cosmetic therapy. Evanescent wave coupling to tissue for diagnostic and therapeutic applications is also being evaluated.

This review summarizes the potential and perspectives of mid- infrared optical sensing schemes with respect to the most promising fields of application and novel technological developments in this research area. Main attention is paid to sensors utilizing the principle of evanescent wave spectroscopy for the signal generation. With the availability of mid-infrared transparent fiberoptic materials, access to the mid-infrared spectral region from 2 - 20 micrometer is enabled, which provides molecule-specific information for qualitative and quantitative analysis. Novel mid-infrared light sources, such as quantum cascade lasers, the introduction of microfabrication techniques for miniaturized, mid-infrared optical components and advanced recognition layers based on molecularly imprinted polymers will be discussed as well as the potential and application of mid- infrared sensors in biology/biochemistry, clinical analysis and environmental/process monitoring.

Environmental regulations are driving development of cleaner spark ignition, diesel, and gas turbine engines. Emissions of unburned hydrocarbons, NOx, and CO can be affected by the characteristics of the mixing of the fuel with air in the engine, and by the amount of exhaust gas recirculated to the engine intake. Fiber optic sensors have been developed that can measure the local fuel concentration in the combustion chamber of a spark ignition engine near the spark plug. The sensors detect the absorption of 3.4 micrometer radiation corresponding to the strongest absorption band common to all hydrocarbons. The sensors have been applied to both liquid and gaseous hydrocarbon fuels, and liquid fuels injected directly into the engine combustion chamber. The sensors use white light sources and are designed to detect the absorption throughout the entire band minimizing calibration problems associated with pressure and temperature broadening. Other sensors can detect the concentration of CO2 in the engine intake manifold providing time-resolved measurement of exhaust gas recirculation (EGR). Proper EGR levels are critical for achieving low engine-out emissions of NOx while maintaining acceptable engine performance.

As part of a European research project in the field of marine science and technology, efforts have been made to develop a portable sensor system for chlorinated hydrocarbons in seawater. This novel analytical tool for marine real-time, in- situ pollution monitoring consists of a robust, miniaturized FT-IR spectrometer in a sealed aluminum pressure vessel and a suitable fiber-optic sensor head attached to one of the container end plates. The signal generation is based on fiber evanescent wave spectroscopy, an application of the internal reflection spectroscopy principle. The sensor head is coated with a hydrophobic polymer to enrich hydrophobic analytes from the seawater matrix and to protect the fibers from corrosion by aggressive seawater constituents. This real-world application imposes a number of restrictions on the system, originating from both, engineering considerations and physico- chemical limitations. Various sensor layouts, e.g. a fiber coil, have been developed and tested in order to find a sensor head geometry with optimal sensitivity and operating stability under these harsh conditions.

Chalcogenide glass fibers based on sulphide, selenide, telluride and their rare earth doped compositions are being actively pursued at the Naval Research Laboratory (NRL) as well as world-wide. Great strides have been made in reducing optical losses using improved chemical purification techniques, but further improvements are needed in both purification and fiberization technology to attain the theoretical optical losses. Despite this, current singlemode and multimode chalcogenide glass fibers are enabling numerous applications. Some of these applications include laser power delivery, chemical sensing, scanning near field microscopy/spectroscopy, and fiber IR sources/lasers and amplifiers.

The good performances of Te-As-Se based glass fibers allow using them in evanescent wave spectroscopy. These IR optical fibers have been developed in order to optimize their response when they are used as evanescent wave chemical sensors. The diameter of the sensitive part of the fiber can be reduced by tapering the fiber during the drawing process or by chemical polishing. In using a FTIR spectrometer associated with a MCT detector, it was possible to evaluate the sensitivity of such sensor. The influence on the analyzed liquid IR signatures has been tested versus the fiber diameter, the immersed fiber length and the liquid concentration. The high flexibility of thin fibers allows the achievement of a detection probe that enables to follow in situ, real-time and on-line chemical or biological reaction.

Efforts have been underway for several years at AMI to develop methods to fabricate infrared imaging bundles from chalcogenide glass fibers. Bundles have been formed using fibers made from an As-Se-Te glass (C1) and from As₂S₃ glass (C2). Most have been made using 100 micrometer core fibers, clad and unclad. Bundles have contained as few as 100 active fibers and as many as 3000. Lengths have ranged from 1 to 10 meters. Methods of construction will be discussed. Evaluation results will be presented. Images formed using infrared cameras sensitive at 1.4 micrometer, 3 - 5 micrometer and 8 - 12 micrometer, will be shown.

With the requirements of the remote control outside CVD clean room environment it is recommended to have a FTIR spectrometer separated from the clean room. However to transmit mid-IR light from FTIR to CVD chamber in free space is out of question due to the high loss (absorption of water and CO₂ etc.). Naturally, the mid-IR fiber with the full spectrum transmission range (5000 to 500 cm^-1) will provide the solution. Unfortunately none of the mid-IR fiber can cover such a broad range with low loss, unless a few different kinds of fibers are used together. A mid-IR fiber bundle consisting of two silver halide and six zirconium fluoride fibers was designed and fabricated. The transmission of this bundle shows the broad spectrum coverage of 5000 to 500 cm^-1, which is required for mid-IR FTIR spectrometer in monitoring the gas concentrations in a CVD chamber. The possibility of using this fiber bundle for remote monitoring and control of chemical process in a CVD chamber will be discussed and some experimental results will be presented.

Chalcogenide glasses (ChG) based on As, S and Se are transparent in the infrared and have found applications in bulk, planar and fiber waveguide optical components. Due to their recent use in planar channel waveguide devices, a study to assess how structural variations imposed by processing conditions (film deposition) lead to changes in linear and nonlinear optical properties, is ongoing in our group. High resolution, near infrared (NIR) ((lambda) equals 840 nm) Raman spectroscopy has been employed to characterize changes in bonding between bulk glass specimens and glass in planar form. To obtain spectroscopic and spatially resolved information on chemical bonding, a microscope attachment has been constructed and is characterized as to its spatial resolution. Measurements are presented on single layer films prepared using processing and illumination conditions such as those used in fabricating waveguide components. These data are discussed in comparison to spectra obtained on bulk glass materials.

The solubility of rare earths at a level of 0.5 mol% in traditional heavy metal oxide glasses based on bismuth, lead and gallium is limited to the largest cations Pr, Nd and Sm. Within the families of these oxide glasses, a higher level of doping was achieved in this work with a heavy metal oxide glass containing germanium as a fourth component. Spectroscopic studies on the Nd³⁺ doped glasses revealed that the fluorescence peak for the 1.3 micrometer transition occurred at a relatively long wavelength of about 1360 nm. Decay time constants for the ⁴F3/2 level were considerably shorter than in other host materials.

Tellurite glasses doped with Er³⁺ and Tm³⁺ are investigated for broadband amplifiers in the third telecommunications window. Fluorescence spectra and lifetimes of Er³⁺ and Tm³⁺ in tellurite glass were measured. Stimulated emission cross-sections were calculated using the McCumber method for Er³⁺ and the Judd-Ofelt analysis for Tm³⁺. The obtained emission parameters are compared with those in other glass hosts. The potential advantages of tellurite glass as amplifier host are discussed.

Fiber lasers offer several advantages over bulk solid-state lasers because they can achieve both high efficiency and fair output power. Still, the use of those silica fiber lasers is limited to very few particular applications like broadband ASE source and pulsed fiber lasers. But, since non-oxide fibers open a broad wavelength range not accessible via rare-earth doped silica fiber nor semiconductor lasers, several niches should be available. In this paper, a comparative study of performances and commercial readiness of both oxide and non- oxide fiber lasers will be done. Effectively, non-oxide fiber laser developers are confronted to several fundamental (photo- induced loss) and technical challenges (splicing, moisture, handling in general). For example, the availability of the right pump laser wavelength lags behind any serious commercial applications. Fortunately, efficient up-conversion process helps access visible to UV wavelength range with commercial IR and near-IR pumps. Also, optimization and prediction of the performance must rely almost solely on experimental validation because the numerical simulation of non-oxide glass is very complex. In particular, for up-conversion lasers, one must consider and more important, know both the emission and absorption cross-sections of 5 to 10 energy levels. Nevertheless, we will review some promising applications coming from sensors system, RGB visible sources, telecommunication applications and some special LIDAR systems that can use double-clad fiber geometry for more efficient pumping and higher power output.

Variations of the lead germanate glasses composition were made and the effects in glass transition temperature, crystallization behavior and UV-VIS-IR transmission were studied. Reducing the lead content and partial substitution of oxides by fluorides causes a blue shift of the optical bandgap of bulk material of about 15 nm. However, the fluoride incorporation into the lead germanate glasses leads to a variation of sample composition by the evaporation of fluorides at temperatures around 1200 degrees Celsius. Spectral and time resolved fluorescence investigations on Tm³⁺ and Yb³⁺ doped lead germanate glasses were carried out. The samples show a weak red (650 nm), intensive blue (480 nm) and near-infrared (800 nm) emission under 976 nm excitation. A comparison of thulium fluorescence lifetimes in different glass hosts is given. The fiber drawing procedures of unstructured and multimode fibers and the loss behavior of the core and cladding glasses are described. Unstructured fibers show a minimum loss of about 0.3 dB(DOT)m^-1 at 580 nm.

We present a model for 455-nm thulium-doped fluorozirconate fiber lasers co-pumped at 645 nm and 1064 nm. Twelve radiative transitions are accounted for in our model, along with cross- relaxation and cooperative upconversion processes. Blue laser output power is computed using a rate equation analysis. Relevant spectroscopic data used in our model are given, including cross-section measurements that we have performed. The results of our simulation show a good agreement with previously published experimental data. The importance of cross-relaxation processes is discussed. The dependence of output laser power on fiber length, output mirror reflectivity, and pump powers is also addressed.

Hollow fibers with the internal silver and polymer layers have been developed for delivery of high energy pulses of infrared lasers. We adopted a cyclic olefin polymer (COP) as the dielectric layer. Since this polymer exhibits no strong absorption in the Er:YAG (2.94 micrometer) or Nd:YAG (1.06 micrometer) wavelength region, it is appropriate for inner dielectric of hollow fibers. We fabricated the COP-coated hollow fibers by using a liquid-flow, polymer coating technique. We report our recent results on transmission properties of the polymer-coated fibers for Er:YAG and Nd:YAG lasers.

IR transmitting hollow waveguides are an attractive alterative to solid-core IR fibers. Hollow waveguides are made from silica glass or plastic tubing which has highly reflective coatings deposited on the inside surface. These guides have losses as low as 0.1 dB/m at 10.6 micrometer and may be bent to radii less than 5 cm. For laser-power delivery applications the hollow glass guides have been shown to be capable of transmitting up to 1 kW of CO₂ laser power. In some power delivery applications it is necessary to have a distal tip configured to bend and/or concentrate the light for more efficient ablation. Curved tips (2 cm in length) are shown to increase the loss of a 1-m long straight guide from 22 to 34%. New research is described on the fabrication of coherent hollow glass bundles for IR imaging. The number of guides in the bundle is currently less than 20 but the results indicate that the hollow bundle can be coated to achieve an identical spectral response for each individual guide.

We developed a launching coupler for infrared hollow fibers transmitting Er:YAG laser light. The coupler consists of a short piece of tapered-hollow waveguide and a single lens at the entrance. We fabricate the tapered waveguide by stretching a Pyrex glass tube in a cylindrical furnace. By controlling the temperature and the stretching time, the shape of the paper is precisely controlled. Next a silver layer and a polymer layer are formed upon the inside of the tapered waveguide by using a liquid-phase coating method. Experiments performed using Er:YAG laser light showed the low-loss property and the high-power resistivity of the tapered waveguide. Also it was shown that the lensed-taper coupler effectively removed the influence of misalignment between the input laser beam and the hollow fiber.

Large-bore polystyrene-Ag-coated hollow glass waveguides with inner diameters of 3.6 mm and 8 mm had been produced for transmission of solar energy, the waveguides were fabricated by liquid phase deposition method, over 6 W focused solar radiation had been transmitted by an 8mm-thick 1m-long waveguide. Although transmission losses of the waveguides were high at the moment, they were promising for transmitting high- power solar energy after being improved.

We report on some physical properties of candidate Ga-La-S compositions pertinent both to being able to extrude fiberoptic preforms, and pull fiber, as well as giving the potential for low optical loss and waveguiding for the finished fiber. For Ga-La-S glasses of Ga/La equals 70/30 and 65/35, each at two oxide contents; 0.65 wt%[O] and 2.95 wt%[O]: viscosity/temperature relations between ca. 10⁸ Pas to 10^11.3 Pas have been measured by indentation viscometry using a thermo-mechanical analyzer, and refractive indices at 632.8 nm and 3390 nm have been measured by the minimum deviation method using small glass prisms. In addition, characteristic temperatures and absorption spectra are reported. On the basis of the measured properties we have selected a core/clad pair and prepared a rod-in-tube preform by one-step extrusion using high viscosity in the region of 10⁸ Pas. Rod with good surface finish, and close to 2.5 mm diameter with a variation of less than 0.01 mm over lengths of 300 mm has been extruded. The extrusion of rods of small diameter is being investigated to enable monomode rod-in-tube preforms to be produced without the need for a stretching operation with the inherent risk of crystallization. Tube of 12 mm OD and 2.5 mm ID proved a close fit to the extruded rod.

A representative of heavy metal oxide glasses, i.e., a PbO- Bi₂O₃-Ga₂O₃ glass, was investigated to identify the network structure of the glass and the electronic transition properties of rare-earth ions doped. X-ray absorption spectroscopic analyses showed that gallium forms GaO₄ tetrahedral units with an average Ga-O bond length of approximately 1.87 A. Lead forms both PbO₃ and PbO₄ polyhedra, but the fraction of PbO₄ decreases with decreasing PbO content. Bismuth in glasses constructs BiO₅ and BiO₆ polyhedra, which have a similar coordination scheme of the (alpha) -Bi₂O₃ crystal. Formation of three-coordinated oxygens is necessary to compensate shortage of oxygens to be two-fold coordinated. These glasses exhibit a relatively good thermal stability as well as the lowest phonon energy among oxide glasses, and thereby enhance numerous fluorescence emissions that are quenched in the conventional oxide glasses. Magnitudes of multiphonon relaxation are the lowest among oxide glasses and comparable to those of fluoride glasses. Fluorescence emission characteristics of Pr³⁺: 1.3 micrometer and Er³⁺: 2.7 micrometer were discussed in detail. In addition, influence of OH^- on the Nd³⁺: 1.3 micrometer emission was analyzed. Further research efforts on impurity minimization and fiberization may realize a new oxide-based fiber-optic host.

For the first time crystalline silver halides optical fibers with optical losses lower than 50 dB/km in broad region from 9 to 14 micron were fabricated by an extrusion process. The optical loss mechanism essential to lowest absorption in silver halides materials and fibers as intraband absorption by free holes in valence band are proposed. The crystalline fibers with Rayleigh type (lambda) ^-4 optical scattering were obtained. The IR region near 13 micron in AgBrI fibers with optical losses less than 10 dB/km was discovered. Non aged, stable IR polycrystalline silver halides optical fiber cables with losses, lower than 1 dB/m in region from 3 to 20 microns are presented. Various applications of developed ATR and reflection remote FTIR sensors, based on low losses silver halides fibers are discussed.

We are currently investigating two infrared glasses for active applications. Gallium lanthanum sulphide (GLS) glass is investigated as a potential host material for rare-earth doped mid-infrared fiber lasers. We have fabricated gallium lanthanum sulphide glass by melt quenching and drawn it into fibers using the rod-in-tube technique. Fluoroaluminate glasses (ALF) are being prepared in planar form by spin coating and clad waveguides have been achieved. The quality of waveguides from both these materials is gradually being improved as methods to eliminate transition metals and other impurities, understand crystallization and reduce the imperfections at the core/clad interface are developed. Although initially motivated by the demand for a practical 1310 nm amplifier, interest has now extended further into the infrared. We describe recent progress in these glasses, their properties and applications.

The present paper describes a technique for enhancing the signal of fiber-optic evanescent wave spectroscopy (FEWS) in the infrared region, by flattening the sensing part of the fiber. FEWS is a novel method for measuring the absorption spectra of chemicals in contact with a segment of an optical fiber. It enables remote in-situ measurements using an optically closed system. The feasibility of FEWS for detecting chemical substances in air, water or biological liquids was proven. Theoretical computations and simulations found that the absorption signal is inversely proportional to the thickness of the fiber. These results were experimentally verified. A technique for flattening a central part of silver- halide (AgCl_xBr_1-x) fibers was developed. Fibers of thickness down to 50 (mu) were produced, and tests proved their enhanced evanescent wave spectroscopy performance. This will be important, in particular, for water or soil pollution monitoring, where extremely small quantities of pollutants have to be detected in a solution.