A brief summary is provided which outlines recent progress towards the development of infrared transparent fiber optic waveguides with intrinsic optical losses potentially in the 0.01 to 0.001 dB/km range. Among the topics discussed are the selection of candidate materials, problems arising from extrinsic absorp-tion and crystallization in these materials, the approaches to preform and fiber fabrication, and the relevant optical, mechanical and chemical properties of these glasses. An extensive bibliography on the subject is provided for those desire more details on this topic.
The search for improved glasses for optical fibers with a potential for losses lower than those of fused silica-based composition could be expedited if the relevant optical parameters could be calculated. This is particularly important for the magnitude and wavelength of the minimum in the loss curve, since these values have been experimentally established only for fused silica. Details are given of a way of estimating such data. A summary is also given of the way of calculating the wavelength where the material dispersion parameter is zero, and of the material dispersion slope at that wavelength. The availability of these parameters should expedite the search for new and improved glasses.
The intrinsic and extrinsic processes that limit light propagation at low powers in infrared optical fibers are described. A theoretical V-curve is constructed and specific absorption coef - ficients and vibrational absorption of various impurities are tabulated versus frequency. The limitation imposed by parametric and nonlinear processes at high powers is described. Values for the critical powers for these processes in typical infrared optical fibers are presented.
The necessity to take into consideration the absorption by free carriers in silver and thallium halides for the evaluation of minimal' optical losses in these materials and fibers from them in the range 2 T 25 μm is pointed out for the first time. It is shown that the absorption by free carriers exceedsfor 1- 2 orders the previous minimal loss evaluations and its value is found to be 10-1 - 10-2 dB/km.
On the example of fluoride glass fibers as one of the most promising waveguides for the mid-IR region, the criteria of soliton pulse propagation in mid-IR fibers are formulated. The characteristic properties of IR soliton dynamics in the condition of essentially strong linear and non-linear absorption in fibers are considered. The case of frequency modulation of the input laser pulses is also considered. It is shown that the phase modulation of the input laser pulses together with the linear absorption causes the high-order solitons desin-tegration into composing solitons.
Heavy metal fluoride glasses (HMFG) based on ThF4, which exhibits a large transparency domain from 0.2 pm to 8 pm, have been optimized in three directions. The devitrification tendency has been decreased by using InF3 as doping while maintaining the I.R. transparency in the 8 pm region. Purification by a RAP technique using CS2 has been applied to eliminate hydroxyle and oxygen contamination. The corrosion of HMFG by atmospheric moisture during the melting and annealing operations has been investigated.
Heavy metal fluoride glasses (HMFG) are potentially useful as optical components in a wide range of devices. Their utilization has so far been delayed mainly because of insufficient material purity and inadequate processing conditions. However, as the result of numerous research efforts these problems are gradually diminishing and it now seems likely that the ultimate limitations for use of HMFG components, at least in those applications where high optical transparency is not a prerequisite, will be imposed by more intrinsic instabilities of the glasses themselves. These include their strong tendency to crystallize on quenching and subsequent reheating, low mechanical and chemical durability and the possibility that they will undergo significant physical aging in situ. Experimental data relating to these problems have now been obtained and in the light of these we wish to present an assessment of their relative importance.
The kinetics of crystallization of a number of fluorozirconate glasses were studied using isothermal and dynamic differential scanning calorimetry, and X-ray diffraction. The addition of the fluorides LiF, NaF, AiF3, LaF3 to a base glass composition of ZrF4-BaF2 reduced the tendency to crystallize, probably by modifying the viscosity-temperature relation. ZrF4-BaF2-LaF3-A2,F3-NaF glass was the most stable against devitrification and perhaps is the best composition for optical fibers with low scattering loss. Some glasses first crystallize out into metastable --BaZr2F10 and ,.---BaZrF6 phases, which transform into the most stable a-phases when heated to higher temperatures. The size of the crystallites was estimated to be (≈ 600 Å from X-ray diffraction.
Heavy metal fluoride glasses are a recently synthesized class of non-oxide amorphous materials whose molecular structure, morphology and optical behavior is at variance from the more common and conventional oxide based glasses. The compositional flexibility of these heavy metal fluoride glasses is such that glasses with a broad range of optical transmission, refractive index and magneto-optic characteristics can be produced. This study will treat two specific classes of fluoride glasses; namely the fluorozirconates and fluorohafnates where the network formers are ZrF4 and HfF4, respectively, with modifiers being BaF2 and fluorides of rare earths, group-III elements or alkalis. These glasses exhibit high transparency over a frequency range spanning the mid IR to the near UV. This property alone makes them possible candidates for a wide variety of applications ranging from laser windows, infra-red transmitting windows to infra-red fiber optics. Attenuation of light in a glass is caused by several mechanisms such as Rayleigh scattering which dominates below 2000 nm; whereas at longer wavelengths, absorption features, for example those of the OH-ion dominate; and beyond 5000 nm multi-phonon absorption is many orders of magnitude larger than the Rayleigh scattering or the OH-absorption. The primary goal of this study is to focus on the intrinsic Rayleigh scattering of these glasses. The principles of quasi-elastic and inelastic light scattering in glasses will be presented and discussed in great detail. The application of Rayleigh and Brillouin light scattering to various heavy metal fluoride glasses has resulted in valuable information on understanding the possible scattering mechanisms in these glasses. In addition, Brillouin scattering measurements allowed the calculation of the elastic and elastooptic (Pockels') coefficient of the same fluoride glasses. The full implications that the scattering behavior has on the possible fiber optic waveguide application of heavy metal fluoride glasses will be discussed. The physical significance of the elastic constants and elastooptic coefficients will also be considered on the basis of existing theoretical models.
Raman scattering and infrared absorption spectra in the multiphonon region have been studied for fluoro-zirconate glasses containing varying amounts of oxygen, transition metal (iron), and rare earth (cerium) impurities. It is shown that, for sufficiently small impurity concentrations, it is possible to separate intrinsic Raman scattering from extrinsic scattering and luminescence processes. It is found that it is generally more difficult to separate intrinsic and extrinsic processes in the multiphonon absorption region.
A ZBLAL fluoride glass produced at NRL was characterized mechanically both in bulk and fiber forms. Fracture toughness measurements gave Ki values of 0.38 ± 0.05 MPa.m1/2, about 1/2 that of Si02 glasses. Flexural strength of machined bars of the current ZBLAL glasses was about 60MPa, about 80% of similarly finished silicate glasses. While the fluoride glass has lower strength than Si02 glass, it had much less slow crack growth, which should give it a longer useful life over a range of stresses. Although the average tensile strength of fibers from initial fiber drawing trials was only about 170 MPa, fractographic examinations indicated that the useful strength of this fiber could be raised considerably with improved processing.
A new approach to the fabrication of bulk samples of heavy metal fluoride glasses with high optical quality is described. The method makes use of a sealed, modified crystal growth furnace coupled with careful attention to raw materials preparation and handling. A series of 12 fluorozirconate glasses and 2 fluorohafnate glasses prepared via the method were examined in terms of optical and physical properties, and compared with samples made by more conventional approaches. Glasses fabricated using the new technique consistently exhibit low levels of both light scattering and bulk OH content, along with very reproducible hardness, thermal parameters and UV and IR edge absorption behavior.
For attainment of 0.01-dB/km absorption losses in the 2-4 micrometer range, iron, cobalt, nickel, and copper concentrations in fluoride fibers must lie in the 0.2-10 range. (1 PPB = 1 mol/109 mol glass.) Commercial grades of zirconium salts tend to form colloids and to contain hundreds or thousands of times more than the tolerable concentrations. We have found filtration to be partially effective for colloid removal. Working with rela-tively concentrated (2 molar, about 17% by weight) solutions of Zr salts, we have introduced about 7000 PPB each of Fe, Co, Ni, and Cu. We have determined that all of these metals can readily be removed, by laboratory-scale ion-exchange methods, to meet the absorption stan-dards for 0.01-dB/km absorption.
Fluoride glass fibers of raising numerical aperture have been drawn from preforms. The increase in refractive index difference between core and cladding has been obtained by lowering the concentration of aluminium in the core. The evolution of the numerical aperture of these fibers has been measured with an apparatus specially constructed. The numerical aperture decreases slowly with increasing wavelength. In high numerical aperture fibers, N.A passes through a minimum at 2.9 μm which can be due to a difference in OH- absorption of core and cladding. Fibers with a low aluminium content in the core have lower losses at high wavelength than others, although their average level of losses-is higher.
Chalcogenide glasses containing elements like As, Ge, Sb and Se have been prepared. A new technique of preparing the raw material and subsequently drawing fibers has been devel-oped in order to avoid the forming of oxygen compounds. The fibers have been drawn by cru-cible and rod method from oxygen free raw material inside an Ar atmosphere glove box. The fibers drawn to date with air and glass cladding have a diameter of 50-500 pm and length of several meterd. Preliminary attenuation measurements indicate that the attentuation is better than 0.1 dB/cm and it is not affected even when the fiber is bent to 2 cm circular radius. The fibes were testes a CO laser beam and were not damaged at power densities below 10 kW/2cm2 CW &100 kw/cm using short pulses 75 n sec. The transmitted power density was 0.8 kW/cm2 which is an appropriate value to the needed for cutting and ablation of human tissues.
This paper reviews the possibilities of using flexible infrared transmissive wave-guides for high power and single mode transmission. Under appropriate conditions, more than 95% transmission through several meters of waveguide with power levels the order of kWatts is possible.
Several kinds of dielectric-coated metallic waveguides such as germanium-coated nickel or silicon-coated aluminum hollow waveguides are fabricated by using a method based on rf sputtering, plating, and etching techniques. Transmission losses of 0.34 dB/m are achieved for straight waveguides at 10.6 μm. Measurements of bending losses of several kinds of hollow waveguides are conducted, which reveal that dielectric-coated metallic waveguides have smaller bending losses than conventional metallic waveguides. Edge-guided modes or whispering gallery modes in circular metallic waveguides are also observed. A design theory for reducing bending losses of circular dielectric-coated waveguides has been presented by using a new theory' different from that used previously.2 Theoretical and experimental results have been compared.
Single crystal fibers of thallium, silver and cuprous halides have been fabricated by passing polycrys-talline extruded fibers through a small submillimeter melt zone, converting them to single crystal fibers. The lowest loss measured in a single-crystal fiber was 6.6 dB/m at 10.6 μm for AgBr, 620 μm in diameter and 80 cm in length.
Scattering due to crystalline imperfections and roughness of the core surface limits the transmission loss of KRS-5 polycrystalline fiber severely, compared to its theoretical limit, in the middle IR wavelength region. Reduction of the loss by an annealing is described in this paper. The lowest loss was 0.12 dB/m at 10.6μm after an annealing at 150 °C for 2 hours. The accuracy of the measurement was better than 0.04 dB/m. To investigate the mechanism of the annealing effect, the change in the X-ray diffraction curve caused by the annealing was measured. The results indicate an improvement in the crystallinity by the annealing, which can be considered to be a main factor in the reduction of the transmission loss.
The versatility of the laser heated pedestal growth method for producing a wide range of compounds in single crystal fiber form is discussed along with some potential applications. Some preliminary results on the growth of single domain LiNb03 crystals, and crystals of BaF2 and CaF2, sapphire and several laser materials are discussed.
Long wavelength (λ > 22 pl) lead-salt diode lasers are useful for spectroscopy studies and also for long distance fiber-optical communications. Double heterojunction diode lasers have now been fabricated using a new material system, Pbl-x EuxSeyTel-y. These structures were grown lattice-matched to (100) oriented PbTe substrates by molecular beam epitaxy. Laser operation up to 190 K pulsed, 147 K CW, has been attained with up to 1 mW single mode output power. The growth of single quantum well lead-chalcogenide diode lasers will be described. The threshold current of these quantum well lasers increases relatively slowly with temperature, yielding CW operation up to 174 K (at 4.41 μm wavelength), and pulsed operation up to 241 K (at 4.01 μm). To achieve single Nide operation, a simple technique has been developed for the fabrication of lead-salt C (cleaved-coupled-cavity) diode lasers. The improvement in spectral purity and the reduction in threshold current of these coupled cavity lasers will be discussed.
Lead Salt Tunable Diode Lasers (TDL's) operating in the mid infrared region (2.6 - 30 microns) are uniquely suited sources for infrared fibers in the specific role of testing and evaluation of attenuation and dispersion losses of the materials. This unique capability results from the fact that 1) source areas of 25 microns diameter or less are easily achieved, 2) inherently TDL's are high speed devices (160 picosecond risetimes have been measured), and 3) very high photometric accuracies have been realized, and are routinely realized in practice; this permits accurate low loss measurements to be accomplished. A system which has been developed to perform these measurements will be described along with other data substantiating these performance characteristics.
The non-linear index of refraction of a compound semiconductor is calculated below the fundamental absorption edge as a function of incident laser field intensity, frequency and basic material parameters such as band gap energy, effective electron mass, heavy hole mass, spin orbit splitting energy and lattice constant. No adjustable parameters are involved. Theoretical results are obtained from a quantum mechanical calculation of the dielectric constant of a compound semiconductor. Comparison of theory with experimental results for the ternary compound Hgl-xCdxTe and binary compound InSb is discussed.