The ability of crystalline fibers extruded from the solid solutions of silver halides has been investigated from the point of view of their practical using in the flexible cables for laser power delivery. The critical characteristics of crystalline fibers have been compared with alternative types of fibers promising for the powerful laser fiber systems.
The spectral window of mixed AgClxBr1-x (0≤x≤1) crystals and polycrystalline fibers was investigated as a function of composition. Both visible edge, resulting from electronic transitions, and infrared edge due to multiphonon processes, behave as the one-mode (amalgamation) type of mixed crystals, shifting continuously with composition. The extruded polycrystalline optical fibers preserve the spectral window of silver-halide crystals. Deviations which slightly broaden the window in the infrared regime are explained by the fibers small-grain structure.
A new type of halide glasses, based on coordination compounds of the chlorides, bromides and heavy metals have been proposed. The formation of the halide glasses and their property will be discussed on the basis of suggested sheme of cluster compound formation. The paper also deals with the fabrication of silver and thallium halide fibers with axial-symmetric index profile by extrusion;the starting billet has the same cross-sectional profile.The resultant step-index and quasi gradient -index profile fibers had optical losses from 0.4 to 1.0 dB/m at 10.6 μm and good mechanical properties. Also we studied the mechanical properties of new waveguides extruded from AgBr-AgI and KRS-13-AgI solid solution crystals and numerically calculated the optical losses induced by silver colloides in AgHal-materials. The UV-resistance of silver halide fibers of different composion have been investigated. This losses depend on composion and state of fiber.
The intrinsic loss processes in mixed silver halide fibers and their dependence on the halide concentration are studies in this work. This includes the electronic band-gap absorption, multiphonon absorption, and Rayleigh scattering. In addition, we study the halide concentration dependence of the refractive index and the static dielectric constant in silver halide mixtures. As a result, we obtain numerical values for the predicted minimum loss, the wavelength of minimum loss, and the optical window of the mixtures as a function of the halide concentration in the fiber.
The last 20 years has witnessed a worldwide concentrated effort to provide low absorption optical fibers for visual or very near infrared applications. Tremendous progress has been made. Fiber optic telephone communications over great distances is a reality. However, in the infrared region beyond 3μm wavelength where high intensity gaseous lasers emit, the silicate glass fibers can not be used because of absorption. To provide extremely low absorption fibers useful at longer wavelengths, research programs throughout the world have concentrated their efforts to provide fibers made from fluoride glasses. Again, much progress has been made but much more will be required before the level of quality reaches that attained by oxide glasses. Even at that, fluoride glasses are not useful beyond 4μm because of absorption.
Sapphire single-crystal fibers are very promising for applications in high power laser delivery systems operating in the mid-infrared, e.g. the Er:YAG line at 2.9 μm. The fibers are nontoxic and chemically resistant. Also, they are mechanically strong and can be bent to a radius of less than 10 mm without breaking. Fibers with diameters of 110 μm and lengths of up to 1 m were grown by the laser-heated pedestal growth method. The minimum loss of 0.5 dB/m was measured in the near infrared at 1064 nm. An absorption band centered at 400 nm resulted in losses of up to 20 dB/m. Absorption loss at the Er:YAG line is 0.88 dB/m for a fiber grown in an atmosphere of pure oxygen. A damage threshold higher than 1.2 kJ/cm2 was measured for 110 μs long pulses, making tissue ablation feasible with fibers several meters long.
Current heavy metal fluoride glass fiber fabrication techniques are mostly based on the casting of preforms. In these techniques the quenching rate of the glass is limited by the square of the radius of the preform, typically 0.5 cm. This results in crystallization and hence enhanced light scattering, which is the main obstacle presently in achieving very low optical losses (~ 10-2 dB/km). We describe a novel glass (preform core) rod fabrication technique in which the rod is made with axial deposition of thin (<< 0.1 cm) layers of glass. This increases the quenching rate significantly compared to the casting techniques. In this new technique a bait rod is rapidly driven into a fluoride glass melt and withdrawn after the tip is wetted with a thin layer of glass. An approx. 10 cm long Zr-Ba-La-Al-Na fluoride rod (0.4 cm diameter) was fabricated with this technique.
New Tellurium halide glasses denominated TeX glass have been investigated in the Te-Se-I and Te-Se-Br systems in order to qualify these vitreous materials versus their tendency to devitrify, the value of their glass temperature Tg and the position of the band gap absorption edge. While the multiphonon edge is located around 20 μm for all the glass compositions, it is noted that the band gap edge is very depending on the Se content. The first Tex glass fibers have been prepared from standard starting materials and it is verified that the lowest attenuation value around 5 dB/m is located in the 10-11 μm region.
A power delivery system capable of delivering high energy densities of infrared radiation at 2.94μm is required for the application of the Erbium YAG laser in the medical industry. Conventional silica fibers have high intrinsic absorption coefficients in this spectral region, making them unsuitable for this application. Alternatively, heavy metal fluoride glasses possess very low intrinsic loss at this wavelength, and also offer transparency in the visible part of the spectrum. These glasses are very hygroscopic, however, and the fundamental OH stretching absorption band occurs in the spectral region of interest. In this work heavy metal fluoride glass samples were prepared with a variety of processing conditions and compositions. The techniques of laser calorimetry and integrated scatter measurement were employed to obtain the absorptive and scattering components of the total optical attenuation at 2.94μm independently. Absorption was found to dominate over the scattering loss at this wavelength. Comparison of laser calorimetry and scattering data with IR spectrometer data indicates that the OH absorption peak is a transparency limiting factor in this spectral region for the best optical quality samples. Of the glass samples prepared for this work the minimum absorption coefficient obtained at 2.94μm was 0.0075cm-1, and the minimum scattering loss measured at this wavelength was 1.14 x 10-5cm-1. These minima were both measured on an aluminum fluoride based glass sample.
The variations of thermal expansion coefficient (TEC) and viscosity with refractive index modifier dopants have been measured. When PbF2 replaces BaF2 there is an increase in TEC accompanied by a decrease in both T5 and viscosity at fibre pulling temperature whereas when PbF2 replaces NaF there is a drop in TEC with a complementary increase in Tg and viscosity at fibre pulling temperature. Similarly when HfF4 replaces ZrF4 the TEC decreases while Tg and viscosity at fibre pulling temperature increase. The expression αTg2 = constant holds well for glasses not containing Li and allows the magnitude and direction of changes in TEC to be predicted from changes in Tg. The activation energy for viscous flow at small dopant levels varied little indicating that the magnitude and direction of viscosity changes can be predicted from Arrhenius behaviour and Tg differences. The stresses in a fibre resulting from expansion mismatch and those mechanically induced during drawing as a result of viscosity mismatch are small for common core clad structures. The thermal and mechanical stresses are generally of the same magnitude and opposing sign; they therefore do not enhance or degrade fibre strength.
A totally anhydrous process has been developed for the preparation of high purity zirconium tetrafluoride for use in low loss fluoride glass applications. The ZrF4 purityis 99.99997% with respect to all transition elements (excluding HO based on analysis by spark source mass spectrometry (SSMS) and graphite furnace/atomic absorption spectroscopy (GF/AA). The only transition elements detected by these techniques were Fe, Ni and Cr, while Co and Cu were consistently below the detection limits. The anhydrous nature of the process, which is strictly maintained by the choice of reactants, affords product with very low oxide and hydroxide content. Total oxygen concentrations of less than 10 ppm have been measured by the inert gas fusion technique. A ZBLAN glass composition prepared using this ZrF4 showed extremely low UV absorption having an absorption constant of 1 cm-1 at 198 nm. ZrF4 from this process was also used in a ZBLAN glass fiber whose minimum optical loss was measured at 6.3 dB/km over 150 meters of fiber. The process is straightforward to scale up and has also been demonstrated to be useful for the preparation of HfF4, BaF2, A1F3 and LaF3.
The extinction coefficients of SeH and SH groups were measured for Se, As-Se and As-S glasses. The As-S and As-Se glass fibers were drawn from the melt with low optical losses: As-Se fiber - 76 dB/km at 4.4μm and 96 dB/km at 5.2 μm, As-S fiber - 44 dB/km at 2.5μm. The mechanical strength of fibers was also measured.
Direct application of the experience gained in preparing optical fibers for visual or very near infrared use has not produced good results in the far IR, 8-llμm. Joint efforts between suppliers of infrared transmitting (chalcogenide) glasses and those versed in the production of silicate glass fibers have met with only modest success. Perhaps oxide glass fiber methods are not compatible with the production of chalcogenide glasses. Separation of the glass production from the fiber production across organizational lines is another handicap preventing free flow of information. After participating in two such programs, Amorphous Materials concluded that a successful program would require that both activities be carried out together. This paper reports the results of efforts at Amorphous Materials to produce fibers in a manner compatible with chalcogenide glass production. Areas emphasized and discussed are: (1) Selection of glass composition from the standpoint of glass quality and fiber properties, (2) Fiber production designed to preserve bulk glass quality, (3) Fiber evaluation results, (4) Low level absorption glass production.
Various tellurium containing chalcogenide glasses from the systems Ge-Se-Te, I-Se-Te and Ge-(As,I)-Se-Te were synthesized and infrared transmission spectra collected from them. The iodide containing glasses tended to have higher long wavelength absorption edges and lower oscillation strengths of the absorption bands produced by hydride impurities than glasses containing no iodine. It was further demonstrated that it is possible to fabricate chalcohalide glasses containing large amounts of germanium. The presence of halogen likely reduces or eliminates hydride contamination through removal as hydrogen halide gas but has no effect on oxide impurities.
Chalcogenide glass fibers, transparent in the mid IR region, will soon find a wide range of applications, including military uses such as thermal imaging, sensing and tracking and remote spectroscopy. We have developed chalcogenide glasses, optical fibers and bundles, and hollow-core fibers suitable for some of these applications. Herein we describe the As2Se3 chalcogenide fibers and bundles with glassy core and TFE cladding, with a fiber diameter of 30-1000 μm. The optical loss, measured by CO2 laser and FTIR spectrometer in the temperature range of -197° C to 100° C is less than 1 dB/m in the 3-5 μm range and less than 10 dB/m at 10.6 μm. At longer wavelengths the attenuation is much higher. The fibers maintain their transparency in the temperature range of -40° C to 70° C (military standard range).Optical properties of hollow-core, chalcogenide glass cladding fibers were examined. Preceded by a theoretical background, experimental results concluding far-field patterns and bending effects are described. Hollow-core fibers are found to guide only a small number of modes.
Hollow plastic tubes covered inside with films of metal or metal and dielectric materials were shown to transmit CO2 laser light with high yields. There was only a small influence on the transmission of light by the radius of curvature on the tubes or the input energies. Output energy intensity measurements have shown that, under certain conditions, whispering gallery modes could exist inside the fiber, which gives low attenuation and negligible variations of transmission with bending. This makes this type of fibers suitable for medical applications especially in surgery.
A hollow waveguide made from alumina, ceramic tubing has been developed for use in delivering CO2 laser power in laser surgical applications. This hollow fiber is rigid and can deliver in excess of 70 watts of power with spot sizes less than 1 mm. The output beam is nearly TEMoo. with a full divergence angle of less than 3°. The attenuation of the hollow fibers varies from 0.4 to 1.5 dB/m depending on bore size. Using a special coupler, we attach the waveguides to the articulated arm of a CO2 laser. The fiber is then inserted into an endoscope for delivering power into the body.
Following the initial discovery of fluorozirconate glasses in 1974 it was immediately recognised that this glass system had a number of important differences to the more conventional oxide glasses. Firstly their extended IR transmission gave them potential as IR optical fibres with intrinsic losses well below those available in silica. Indeed a considerable amount of work has already been directed at this application . A second important property that was soon recognised their potential as laser host glasses and early measurements of fluorescence spectra from rare earth ions confirmed their usefulness in this area. More recently it has been recognised that "active fibres" made by doping with rare earth ions may play an important role in telecommunications, both as sources and in-line fibre amplifiers . A logical step therefore would be to investigate active fluoride fibres doped with rare earths and the work presented in this paper describes recent work in this area. The dopants studied are Nd, Ho and Er which have been made to lase at different wavelengths over the range 1.0 to 3.0 um.
Trivalent rare earth doped lasers in fluorozirconate glasses and fibers that lase between 2 and 3 μm are reviewed. There have been a large number of laser-fiber optic systems below 2pm developed for clinical microsurgery at a variety of sites. The required flexibility of the fiber optic waveguide varies with the clinical use, such as: intraocular (through a small diameter rigid tube), endoscopically accessible pulmonary and gastric mucosa (through a port of a fiber-optic endoscope of intermediate flexibility), and intra-arterial (as an integral part of a flexible catheter, which in the case of the coronaries must be very flexible so as to negotiate abrupt bends and bifurcations without damage to the vessels). Laser energy absorbed by tissue is capable of coagulation of tissue (denaturation of structural proteins), melting of fatty deposits or other structures (solid or gel to liquid phase transitions), as well as direct breakage of chemical bonds by high energy photons. It is of general interest to develop a pulsed laser system transmitted through flexible fiber optics that is capable of precise ablation of targeted tissue with minimal damage to the remaining tissue. Ideally, the device should be able to ablate any tissue because of the general absorptive properties of tissue, and not a specific chromophore such as melanin or hemoglobin, the concentration of which varies widely among tissues. Two obvious ubiquitous chromophores have been widely discussed: 1) proteins and nucleic acids whose high concentration and absorption coefficients lead to strong tissue absorption in the ultraviolet and 2) water whose strong infrared absorption bands have been widely utilized in CO2 laser surgery. Non-linear absorption occurring at very high power densities (~1 GW/cm2) has been shown to be very effective for non-invasive ocular (an optically transparent field) microsurgery at the image plane of a slit lamp, but this approach appears impractical in fiber optic systems because of similar non-linear damage mechanisms within the fiber.
A simple fiber-optic chemical sensor which may be used to detect in-situ and in real time concentrations and chemical transformation of liquids and pastes is reported. The sensor, comprised of a tunable IR diode laser source and a cladding-free multimode IR fiber, operates on the principle of evanescent field absorption spectroscopy. The sensor's response is theoretically analyzed and experimentally verified.
Transmission efficiencies, damage thresholds and bend radii have been determined for six fluorozirconate fibres. Using pulsed radiation at 2.94μm, average powers up to 3.5W have been delivered with an efficiency of 65%. After taking into account reflection losses, maximum efficiencies near to 100% have been measured at low powers. Minimum damage thresholds of 3.7MW/cm2 have been observed and bend radii are approximately 25mm for a 300μm core.
A silver halide infrared fiber-optic evanescent wave spectroscopic technique for in-situ monitoring of chemical processes and surface analysis is described. Samples are spread onto a fiber contained in a teflon-lined cell. Attenuated total internal reflectance (AIR) measurement with a Fourier transform infrared (FTIR) spectrometer yields spectra at various stages of a process (for example, the monitoring of adhesive curing and coupling agent polymerization). Changes in known spectroscopic features may be recognized in films as thin as a monolayer. The advantages and limitations of this surface analysis technique are discussed.
Fourier transform infrared (FT-IR) spectrometry has general utility for chemical analysis. However, these analyses are limited by the requirement of the sample being accessible to the spectrometer and often undergoing some amount of preparation before analysis. This restriction has kept FT-IR from many potential applications, in particular, medical and industrial process control. The ability to make measurements at a remote site or as a reaction occurs offers a significant advance in infrared analysis. We have used a mid-IR fiber to transmit radiation outside of the spectrometer, to the sample, and then to the detector. A section of the fiber, with the protective cladding removed is used as the sampling device. In this declad region the fiber, acting as an internal reflectance element, contacts the sample and provides the chemical information for analysis. Two examples of the use of an optical fiber will be discussed. The first demonstrates the ease in which a liquid sample can be measured. The second demonstrates the use of a fiber to monitor the progress of a curing reaction in thermoset composite materials where the fiber was imbedded in the matrix. In addition to discussing the analytical utility; the instruments, fiber optic sampling, and optical characterization of the fibers will also be presented.
Interaction of GHz and MHz radiation with CO2 laser propagation in a silver halide fiber using sBs based phonon coupling is furthet investigated. The external signal serves to both probe and enhance laser generated sBs phonons in the fiber. Efficient coupling of microwave radiation into the fiber is accomplished by placing the fiber in a hollow metallic waveguide, designed and constructed to transmit the dominant mode in the 0.9-2.0 GHz band. MHz radiation is conveniently coupled into the fiber using the guided microwave radiation as carrier. Phonon emissions from the fiber under CO2 laser pumping are first established on a spectrum analyzer; low frequency generators ale then tuned to match these frequencies and their maximum interaction recorded. Such interactions are systematically studied by monitoring the amplitude and waveform of the reflected and transmitted laser pulse at various power levels and frequencies of the externally coupled radiation. A plot of reflected laser power versus incident laser power reveals a distinct sBs generated phonon threshold. Variouslaunch directions of the GHz and MHz radiation with respect to the direction of laser propagation are realized to verify theory governing sBs interactions. The MHz radiation and its associated phonons in the fiber are convenient tools for probing sBs related phenomenon in infrared fibers.
During the past 10 years several publications described pilot units of metal halide infrared fibers. The reason such fibers are not yet available commercially is based on the particular properties of the unorganic metal halide salts used for the fabrication of these fibers. The paper analyse a complete fiber unit for medical application of CO2 Laser inside the human body, and summarise problems in commercialisation of metal halide infrared fibers.
The design of a two-color pyrometer with infrared optical fiber bundles for collection of the infrared radiation is described. The pyrometer design is engineered to facilitate its use for measurement of the temperatures of small, falling samples in a microgravity materials processing experiment using a 100-meter long drop tube. Because the samples are small and move rapidly through the field-of-view of the pyrometer, the optical power budget of the detection system is severely limited. Strategies for overcoming this limitation are discussed.
Over the last 25 years since their invention, CO2 lasers have slowly and steadily evolved as one of the most mature laser systems for various technological applications. It remains a predczninant source of radiation for applications in the strategic longer wavelength infrared atmospheric window range. The successful application of CO2 lasers will require optical canponents capable of operating at these laser waveiengths. A CO2 laser detector is an important component among these, and is the subject of this paper. In the earlier period after their invention, it was difficult to find suitable detectors for CO2 laser wavelengths. Available quantum detectors required cooling with liquid helium. Thermal detectors lacked the sensitivity and speed of photon detectors. However, the advantage of thermal detectors has been their room temperature operation.
In present work we report about development of semiconductor lasers and high-speed photodiodes based on GaInSbAs/GaAlSbAs solid solutions for 2.0-2.5 μm spectral range, which operate at the room temperature. Characteristics of.the lasers of several types are described. Among them there are pulsed lasers with output optical power reaching 1 W, cw lasers operating at room temperature at the wavelength of 2.0-2.2 μm, and pulsed lasers for the wavelength up to 2.5 μm. Properties of the developed photodetectors are also presented. There are described high-speed (τ<0.5ns) p-i-n photodiodes with quantum efficiency of 0.6 (without antire-flection coating) and avalanche photodiodes with separated absorbtion and multiplication regions (SAM APD), characterizing by multiplication factor of 20-30 at room temperature.