This paper reviews recent developments of IR fiber technology in Japan. Fabrication techniques as well as optical characteristics for fluoride fibers are described from the view point of realizing ultra-low-loss fibers aimed at telecommunication applications. A minimum loss of 3 dB/km is achieved for Zr-based fluoride fibers, whose minimum loss is predicted to be less than 0.01 dB/km based on the precise evaluation for intrinsic loss factors. Chalcogenide glass fibers are developed not for telecommunication but for sensing application. The development of other halide crystal and glass fibers is at a basic research level.

Extrinsic losses in long wavelength glass fibers (2-10 µm), which are related to the waveguide structure, must be reduced to ultra low levels to utilize the intrinsic low loss levels(10^-3-10^-2 dB/km). Two important loss mechanisms are microdeformation (microscopic bends and core diameter fluctuations) and radiative loss due to macrobending. Using a coupled mode and a spotsize theory, microdeformation losses are calculated for single mode, dispersion shifted, heavy metal halide glass fibers. Very low microdeformation losses can be achieved with modest values of the refractive index difference at long wavelengths for short to intermediate perturbation correlation lengths. For long correlation lengths it may be difficult to achieve very low losses at wavelengths > 3 microns. The macrobending loss calculation results determine the lower limit of the refractive index differences.

Heavy metal halide glass fibers have the potential of 0.001-0.01 dB/km optical loss in the 2-10 μm region. System considerations for very long distance transmission applications require these fibers to be single mode and dispersion shifted. Waveguide propagation characteristics have been calculated for step and triangular refractive index profiles for three representative halide glasses whose minimum loss wavelenghts span the 2-10 μm region. Modest relative index differences can be used to offset material dispersion, which decreases with increasing wavelength. These designs are consistent with low macrobending and microdeformation loss requirements.

We have investigated the material, waveguide and total dispersion in a fluorozirconate glass and found that the total dispersion can be kept below 3ps/nmkm over a broad wavelength region (1,4-3,9μm) in a step-index single-mode fiber structure.

The use of infrared transmitting fiber optics (IRFO) will improve or make feasible many radiometric systems. However, the characteristics of existing IRFO are relatively unknown. Santa Barbara Research Center, a subsidiary of Hughes Aircraft Company, is in the process of collecting and characterizing fiber optics that transmit infrared radiation. Data collected for various fiber types are presented and anticipated minimum requirements for IRFO test plans are discussed. Specifically, spectral loss and angular response measurements for KRS-5 and other metal halides, arsenic germanium selenide (AsGeSe), germanium antimonide selenium (GeSeSb), zirconium fluoride and fused silica fibers are presented. Fused silica data are included because some samples showed low losses for radiation wavelengths beyond 2 μm. Cut-on and/or cut-off wavelengths and other structures are discussed for each curve presented. Measurement apparatus and methods are described. We conclude with a description of current and anticipated tests of IRFO spectral loss, angular response, stability, radiation hardness, cyclic flex limits and high and low temperature limits.

The CO₂ laser is widely used in a diverse array of medical applications. Infrared ("IR") fibers which can deliver CO₂ laser energy to the surgical site through flexible endoscopes and catheters will allow the surgeon to perform new medical procedures which are less invasive and more economical than conventional surgery. Fiber specifications depend on the specific application, and must be developed based on true medical needs. In general, characteristics such as medium power transmission (up to 30 W), small bend radius (down to a few millimeters), and low cost ($30-50per meter) will be required for less invasive surgical procedures. The IR fiber remains a technically and economically important component of future medical laser systems.

Recent advances in preform processing techniques and purification have resulted in fluoride glass fibers having a minimum loss of 0.9 dB/km at 2.55 microns. This value was confirmed by separate measurements of scattering and absorption loss.

An estimation has been made of the minimum loss that might be expected in ZrF. based IR fibre taking into account extrinsic absorption losses as well as the intrinsic loss mechanisms associated with the IR edge and Rayleigh scattering. The results suggest that an overall loss of approximately 0.03 dB/ km might be expected at 2.56 μm, a factor of three higher than the intrinsic loss and a factor of seven lower than overall loss in a typical silica fibre.

Among numerous glass forming systems, only a few of them give glasses stable enough to be drawn into fibers. Fluoro-zirconate glasses which have been extensively studied provide industrial infra-red transmitting fibers with optical losses well lower than 1 dB/m between 1 and 4 μm, and transmission losses lower than 5 dB/km have been observed on experimental fibers. Experiments carried out on fluoroaluminate and fluorozincate glasses result in fibers of several dB/m in optical attenuation. Wavelength independent scattering which is the main source of losses arises from bulk crystallization while surface defects dramatically reduce fiber strength. Control of surface devitrification and optimization of glass composition are the major elements of further development.

In order to develop infrared optical fiber systems in nuclear media, studies are made to know the behavior on line of fluoride glass optical fibers under irradiation. the increase of induced loss and the influence of the dose rate are given at 2.4 microns. Cycles of rela-xation at room temperature and y ray exposure allows an important bleaching and an unaffected kinetic of losses. Characterization of defects created by y radiation on bulk of ZBLA glass is carried out by means of electron spin resonance (ESR). A linear kinetic of ESR signal with dose is observed and possible models for defects are discussed.

Silver chloride and silver bromide materials of pure and mixed compostions were extruded into 1 mm fibers for use in propagating 10.6 μm laser radiation. Insertion loss and scattered light flux was measured incrementally along the fiber length. Absorption losses were estimated from the difference between extrapolated input power and the integrated scattered power; these were not significant at the relatively low power levels employed (<50mW). Fibers having insertion losses less than 4db/m exhibited a relatively constant amount of scattered light flux with the distance from the launching face and a scattered attenuation coefficient which increases linearly with this distance. Major losses were attributed to internal defects and strain within the fibers. The surface roughness - a function of the material hardness - was found to be a minor source of scattering losses. Fibers measuring insertion losses greater than 8-10 db/m always exhibited a scattered flux which falls off exponentially with distance for the launching face and a constant scattered attenuation coefficient. Internal strain appears greater in these cases.

Silver halide crystals of composition AgC1_0.95Br._0.05 were extruded to form infrared transmitting fibers of 0.9 mm diameter and 1.5 meters in length. Microscopic examination reveals an initial fine grain structure with a grain size of the order of 1 pm. Thermal treatment at 200°C following extrusion results in rapid grain growth. Infrared spectral transmission measurements of the fibers show absorption features at 3-3.2 μm, 6.1 μm and 12-13 μm attributed to water and bands at 6.8 and 7.1 pm attributed to COI molecules. These features are superimposed on a scatter loss which has a λ^-2 dependence. Transmission measurements at 10.6 μm with a CO₂ laser show a linear dependence of power output on power input for levels from 1 to 20 watts incident on the fiber face. For a variety of power input geometries, the power output distribution was independent of the input conditions.

KRS-5 polycrystalline fibers having a core-cladding structure have been fabricated by complex extrusion. The core material is KRS-5 (T1Br-TlI), and the cladding material is KRS-6 (T1Br-T1C1). Through thermal treatment after fiber fabrication, the refractive index profile is changed to the graded index (GI) type. The attenuation loss of these core-cladding GI fibers is 0.2 dB/m at about 10.6 μm (lauching NA = 0.05). Fibers with this core-cladding structure will enable wide applications using far infrared wavelength light, for example in CO₂, laser processing machines and spectroscopy equipment, because of their ease in handling.

Single-crystal sapphire (α-Aℓ₂0₃) fibers are potentially useful in a wide variety of optical applications, particularly those involving high-power or high temperature light-guiding, over the wavelength range from 0.24 μm in the ultraviolet to 4.0 μm in the mid-infrared. These fibers are routinely grown at rates of up to 8 mm/min, and with diameter stability of better than 0.5% rms under feedback control. Recent measurements on unclad 150 μm diameter fibers show fundamental mode scattering losses of about 0.3 dB/m in the visible and less than 0.07 dB/m at 3.39 μm.

Traveling zone, Bridgman and Czochralski methods of crystal growth have been successfully adapted to produce single crystal fibers of nonlinear materials. In addition, a new structure was developed for nonlinear fiber systems by embedding a glass fiber into a single nonlinear crystal. Conditions necessary for non-critical phase-matching for such a glass-core/crystal-clad hybrid were calculated.

We have considered the mechanism of plastic deformation of T1 and Aq halide crystals under extrusion of polycrystalline fibers. The influence of the extrusion parameters on the structure as well as on the optical and mechanical properties of the fabricated fibers is examined.

Zinc Selenide (ZnSe) infrared optical fibers 33 cm long x 1 mm in diameter have been fabricated. The measured attenuation coefficient of these fibers at 10.6 pm ranges from 0.3 dB/m to 4.0 dB/m depending on the surface quality. The minimum bend radius of a number of uncladded fibers was measured to be 73 + 8 cm. The measured transmission of uncladded fibers does not change when the fibers are bent to a radius of 115 cm. When the fibers are cladded with TFE heat shrinkable tubing, the transmission is unchanged for straight fibers. However, the attenuation of these cladded fibers increases by a factor of 1.25 when they are bent to a radius of 85 cm. The minimum bend radius of the cladded fibers is equal to that of the uncladded fibers within the uncertainty of the measurement. ZnSe can efficiently transmit radiation over a wide range of wavelengths (0.6^-14 μm). Therefore, these fibers have potential applications in catheters and endoscopes for CO₂ laser surgery, in probes for remote optical measurements of hostile environments and in various military infrared optical systems.

Liquid chlorides and gaseous hydrides have been decomposed by low pressure, plasma-enhanced chemical vapour deposition (PECVD) to give Ge-Se and Ge-S glasses. Their glass transition temperatures were obtained by thermal analysis and their compositions were examined by infrared spectroscopy and energy dispersive X-ray analysis. Results from the prototype deposition system are being used to develop a higher pressure PECVD process.

Chalcogenide glass fibres have been developed for transmission in the 8-12 µm waveband. The bulk properties of a range of selenide and telluride glasses have been studied and compositions chosen to optimise transmission and glass stability. The role of major impurities and the procedures used to minimise them are described. Selected compositions have been drawn into long lengths of polymer coated fibre with diameters controlled accurately up to 500 µm. Current fibres transmit with losses of 5-10 dB m^-1 in the 8-12 µm waveband. Approximately 4 dB m^-1 of this loss is attributable to scattering. Mechanical testing has shown that fibre strength is determined by internal defects. Their elimination should enable fibres to sustain bend radii of the order of one centimetre, and at the same time should reduce minimum losses to below 1 dB m^-1.

Telluride based chalcogenide glass fibers with improved optical mechanical and thermal properties are presented. These fibers are manufactured by using a plastic-glass multiple co-drawing technique. The loss in the 6-11 micron range is between 0.2 and 3 dB/m.

Chalcogenide glass optical fibers with optical losses of 0.3-1 dB/m in the 3.5-7 I'm wavelength region have been obtained. The fibers optical properties as well as the dependence of their optical losses on low temperature and time have been investigated.

Chalcogenide hollow fibers were made from As-Se, As-Se-Te or Ge-Sb-Se cylindrical raw material. Theoretical investigation had shown that by using metallic cladding, losses due to bendings could be reduced. The optimum conditions for minimum loss were calculated for the ratio between TE and TM modes. Experimentally, fibers were drawn with an attenuation of 8 dB/m. It was shown that by using plastic for cladding, fibers with less fragility and high elasticity were achieved.

Glass core and hollow core chalocogenide fibers were fabricated and coated with Teflon cladding for protection. The dimensions of the hollow core fiber were 30-600μm OD, 20-500μm ID and up to tens of meters in length. Attenuation measurements were performed with a lead salt tunable diode laser as well as with a CO₂ laser. The power loss due to bending for a fiber with a hollow core, and the far field power distribution of this kind of fiber, are also reported. Spectral dependence of the fibers in the region 10^-13 μm was obtained using diode lasers. The advantages of glass core and hollow core fibers are discussed.

A computer-based measurement system capable of measuring the attenuation, NA, dispersion, and OTDR in IR optical fibers is described. A review of measurement theory is presented. The hardware and software elements of the measurement system are discussed. The system operates from 2.8-14.0 μm using Pb-salt laser diodes. 500-MHz bandwidth HgCdTe detectors and a waveform digitizer are used in the dispersion and OTDR measurements. The optical layout and computer interface is described. Measurement technique and data from chalcogenide optical fibers is presented.

Most current research on infrared glasses has concentrated on non-oxide systems which transmit to beyond 7 μm. There are still applications in the 2 to 6 μm range which can best he filled by oxide glasses which generally have better physical and chemical properties than the oxide-free materials. Of all glass systems, silicates have the largest compositional area which provides very stable glasses with good physical and chemical properties. Unfortunately, the IR absorption edge extends only to between 4 and 9 μm. This absorption edge can be optimized by minimizing silica content by dilution with oxides whose cation-oxygen bond vibrates with lower frequency. Transmission can be extended even farther by replacing Si⁺⁴ as the network former with other network forming cations with weaker bonding, such as Ge⁺⁴, Sb⁺³, or Te⁺⁴. Properties and stability to crystallization of these glasses, as a rule, become poorer as IR transmission improves. By far, the best transmission of any oxide glass belongs to a relatively new class of glasses based on networks formed by bismuth and/or lead oxides. They transmit out to R to q pm, have expansion coefficients (29°-200°C) from 83x10^-7/°C to 112x10^-7/°C, and refractive indices as high as 2.2 at 4 μm. They have sufficient stability to be cast into 8cm x Rcm x 1.9cm slabs.

With the advent of new optical materials a host of services can be provided through optical device technology which include voice, data, and visual communication. The realization of low loss optic fibers provide the option of long distance optical communication and the growth of information processing through computer technology by optical means is bound to occur. Heavy metal fluoride glasses have proved to be excellent hosts for both rare earth and 3d transition metal ions. At present the potential of the heavy metal fluoride glasses for ingrared transmission light guides is unexcelled with ultralow losses of the order of l0 db/km a distinct possibility if trace impurities can be removed. One potential complication of the utilization of these materials is the effect of radiation damage. Irradiation could produce defects detrimental to long range optical communication or laser action. Therefore a detailed study of irradiation effects in these materials is important. The purpose of this paper is to review some of the excellent work that has been done in this area and to provide a background on radiation effects in heavy metal fluoride glasses.

A new glass system, comprised of CdF₂-LiF-AlF₃-PbF₂, is described. The properties suggest that this may provide an alternative to fluorozirconates. Its potential as a "practical" glass is discussed.

Shear viscosity data for a glassforming ZrF₄-BaF₂-LaF₃-A1F₃ composition covering the range from the highly fluid melt down to the glass transition (10^-1 to 10¹³ P) have been collected from five sources. The viscosity temperature dependence is highly non-Arrhenius and cannot be described by three parameter expressions such as the Fulcher equation. The four parameter Cohen-Grest equation, however, does give a good fit to the data, possibly allowing interpolation in the range of intermediate viscosity important for fiber drawing where data is currently lacking. The viscosity data are compared with crystallization temperatures obtained by DSC during heating and cooling at 10K/min.

A general method for producing high purity semiconductor materials has been developed and successfully applied to the production of chalcogenide glasses. One glass composition designated AMTIR-I, a Ge-As-Se glass, has been produced at the rate of 4,000 lbs per year over the last several years. The method combines element purification, compounding and casting into one operation. Absorption levels based on actual quality control data obtained over the last 3-4 years will be presented. Factors affecting the absorption will be discussed. Application of the method to produce a low absorption chalcogenide glass suitable for fiber fabrication will also be discussed. In addition, specific results attained for AMTIR-2, an As-Se glass, will also be presented. The general method has been applied to the preparation of large plates of cadmium tellu-ride. A three zone furnace system is used to vaporize, condense and compound near stoichiometric quantities of 5-nines grade cadmium and tellurium. A controlled directional freeze results in circular plates up to 25 cm in diameter which are free of voids or gross inclusions and contain large areas of single crystal. A heat treating process under cadmium and tellurium vapor has been developed which results in a 100 fold improvement in absorption over non-heat treated material. Absorption coefficients less then 0.002 cm^-l have been observed for 2.5 to 25μm wavelength on samples ranging up to 2 cm in thickness. The attainment of near theoretical transmission, as expected, is dependent upon the sample thickness, anneal temperature, pressure of cadmium or tellurium used in the anneal, and length of time of treatment. Details of the pro-cedure will be presented.

This paper describes some aspects of the fluoride fibre optical transmission system experiments in progress at British Telecom Research Laboratories (UK). Initially, midinfra-red receiver designs based on InAs photovoltaic detectors are described . Preliminary measurments made at 1.52 μm indicate a sensitivity of -31.2 dBm with 34 MBit/s, non-return-to-zero, pseudo-random binary data. This is believed to be the first error-ratio characterization of an optical receiver with high responsivity in the 2-4 μm wavelength region. Further work will allow a complete mid-infra-red transmission system to be constructed. The initial design will use Zirconium Tetrafluoride fibre produced at BTRL and an optical source based on a 3.39 μm HeNe laser externally-modulated by a Ge Bragg cell. Results obtained at 34 MBit/s will be reported at the conference.

Significant improvements in the performance and availability of mid-infrared transmitting optical fibers have taken place over the past several years. The ultimate goal is the development of a new generation of ultra-low loss fiberoptic communication systems to someday replace the existing silica fiber technology. Concurrent with these fiber improvements have been developments in the field of mid-in-frared semiconductor laser sources, specifically tunable lead-salt diode laser (TDL) technology. TDL's represent the only commercially available semiconductor laser technology which gives access to the important 3-30μm spectral region. In this paper, issues of performance and reliability of currently available TDL device structures will be discussed. New, more advanced device structures particularly well suited to fiberoptics technology will also be presented.

The development of the novel heavy metal fluoride fibers with the potential for substantially reduced losses over silica fibers presents the need for sources and detectors that are specifically designed for the 2 to 4 pm mid-infrared spectral region. The III-V quaternary In_xGa_1-xAsySb_1-y is a promising material in this regard, with an energy gap range of 1.8 to 4.2 μm. 1.1quidphase epitaxy has been used to grow epitaxial layers on InAs substrates. Compositions with energy gaps near 2 μm and near 3.5 μm have been obtained.