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Erbium-doped yttrium aluminum garnet (Er 3+ :YAG) rods were inserted inside undoped tubes and grown into single-crystal fibers of a diameter of 300 μm using the laser-heated pedestal growth technique. Growth at various rates resulted in radially graded distributions of Er 3+ dopant ions, as observed using laser-induced fluorescence imaging. Profiles of the refractive index were measured using cross-sectional reflectometry in a microscope. Dopant distributions and the corresponding index profiles were compared with thermal diffusion theory to determine the inter-diffusion coefficient of Y 3+ and Er 3+ ions at 2000°C, yielding an estimated value of D=(9.10±0.8)×10 -11   m 2 /s. This work constitutes a step toward controlled growth of fibers with high thermal conductivities, low Brillouin gain, and waveguiding properties required for high-power optical amplifier and laser applications.

The effect of misalignments of different optical components in the laser heated pedestal growth apparatus have been modeled using Zemax optical design software. By isolating the misalignments causing the non-uniformity in the melt zone, the alignment of the components was fine-tuned. Using this optimized alignment, low-loss YAG single crystal fibers of 120 μm diameter were grown, with total attenuation loss as low as 0.5 dB/m at 1064 nm.

Metal/dielectric-coated hollow glass waveguides (HGWs) have been studied extensively for the efficient transmission of radiation over a broad spectral range. In this study, a low-absorption optical polymer, cyclic olefin copolymer (COC), is investigated as a dielectric material for HGWs designed for the delivery of various IR lasers. Using established silver (Ag) plating techniques and a newly optimized polymer-coating procedure, Ag/COC HGWs with low attenuation coefficients are fabricated for operation at the following three wavelengths: 808 nm, 1.064 μm, and 2.94 μm. The spectral responses of the HGW designs are used to develop a film thickness dependency of the COC layer formation based on the concentration of the solution used in the polymer deposition procedure. Further, the attenuation coefficients of the HGWs are measured using the cutback method at the three wavelengths as a function of the curvature of the waveguide. In order of increasing operation, the attenuation coefficients are measured to be 0.549, 0.095, and 0.298  dB/m for the HGWs in the straight configuration. These experimental values for the straight attenuation coefficients are compared to theoretical values calculated using a ray transfer matrix approach and are found to be in good agreement.

This analysis explores the theory and design of dielectric multilayer reflection-enhancing thin film stacks based on high and low refractive index alternating layers of cadmium sulfide (CdS) and lead sulfide (PbS) on silver (Ag)-coated hollow glass waveguides (HGWs) for low loss transmission at midinfrared wavelengths. The fundamentals for determining propagation losses in such multilayer thin-film-coated Ag hollow waveguides is thoroughly discussed, and forms the basis for further theoretical analysis presented in this study. The effects on propagation loss resulting from several key parameters of these multilayer thin film stacks is further explored in order to bridge the gap between results predicted through calculation under ideal conditions and deviations from such ideal models that often arise in practice. In particular, the effects on loss due to the number of dielectric thin film layers deposited, deviation from ideal individual layer thicknesses, and surface roughness related scattering losses are presented and thoroughly investigated. Through such extensive theoretical analysis the level of understanding of the underlying loss mechanisms of multilayer thin-film Ag-coated HGWs is greatly advanced, considerably increasing the potential practical development of next-generation ultralow-loss mid-IR Ag/multilayer dielectric-coated HGWs.

This study involves the fabrication and characterization of improved quality silver (Ag)/polystyrene (PS) thin-film-coated hollow-glass waveguides for the low-loss transmission of terahertz radiation via modified dynamic liquid phase deposition techniques. High-quality PS thin films were deposited from aqueous PS solutions, and the spectral response of fabricated samples was measured from λ=1-100 μm. Fabricated samples exhibited highly defined spectral responses throughout this entire range indicative of PS films of excellent quality. The spectra of experimental samples were compared to the theoretical and bulk PS spectra in the near-IR and far-IR regions. The thickness of deposited PS thin films was found to depend on total sample length and to vary from approximately 10-16 μm for sample lengths ranging from 115 to 140 cm. Such PS film thicknesses are adequate for low-loss delivery from approximately 2-4 THz. Furthermore, film thickness was found to vary minimally along the waveguide length regardless of total sample length.

Silver/silver halide-coated hollow-glass waveguides (HGWs) are capable of low-loss, broadband transmission at infrared wavelengths with the advantage of optical response tunability through alteration of a number of key design parameters. Generally, the design of circular HGWs has primarily involved optimization of the waveguide bore size and deposited film structure in order to obtain the desired optical response, with the waveguide bore size being held constant as a function of length. In this study, the effects of HGW structures consisting of linearly tapered inner diameters on the optical response at infrared wavelengths are theoretically and experimentally investigated. Theoretical analysis involving numerical ray optics methods accounting for the dynamic nature of bore size, and consequently light propagation, along the waveguide length is presented and compared to experimental results in order to gain a deeper understanding of these atypical HGW structures.

Hollow glass waveguides, composed of Ag/AgI coatings, have been studied at 10.6 µm. The losses for different bore sizes equal the theoretical loss, which for the 700 µm bore guide was about 0.15  dB/m. The losses for the guides increase upon bending, varying linearly with increasing curvature. These hollow guides propagate a single mode when the bore size of the guide is approximately 30λ. In addition, the best single-mode transmission is obtained when the thickness of the glass wall is large. These smaller bores, thick wall hollow guides, can also be used to filter higher order modes from poor quality input laser beams.

Low- and high-refractive-index chalcogenide glasses are studied for their potential use in the fabrication of one-dimensional hollow Bragg fibers. The low-index glasses are based on the GeSe-glass systems with indices varying from 2.0 to 2.5, while the high-index glasses are formed from the AgAsSe glasses with indices ranging from 2.8 to 3.8. High-purity elemental starting materials are distilled and the surface oxides removed prior to mixing in a rocking furnace. The refractive indices of the AgAsSe glasses, measured using a CO2 laser reflectometer, were near 3.10 for the compositions most compatible with the low-index Ge20Se80 glass (n = 2.46). Spectral measurements show impurity absorption bands between 12 and 16 microm. The loss at 10.6 microm for the Ag25As40Se35 glass measured using CO2 laser calorimetry was 1.16 x 10(-3) cm(-1).

Hollow polycarbonate waveguides with thin-film coatings of Ag-AgI were fabricated by liquid-phase chemistry methods. These hollow waveguides, which have bore sizes ranging from 500 to 2000 microm and lengths as long as 2 m, are transmissive from 2 to more than 20 microm. The lowest loss of 0.02 dB/m was obtained for a straight 2000 microm bore guide at 10.6 microm. This is to our knowledge the lowest loss measured for any IR fiber at CO2 laser wavelengths. The bending losses were found to increase as 1/R, where R is the radius of the bend. These waveguides were able to withstand 18 W of CO2 laser input power for bore sizes greater than 1000 microm.

Hollow glass waveguides with bores tapered from 1000 to 500 microm and from 700 to 500 microm over a length of 2 m were coated with silver and silver iodide inner films. These waveguides were designed for low attenuation at the 10.6-microm CO2 laser wavelength. The straight losses, which were measured to be 0.8 and 1.6 dB/m, respectively, decreased when the guides were bent. A simple ray-trajectory model is presented to explain this unexpected behavior.

An all-fiber-optic infrared multispectral radiometer for measurements of temperature and emissivity of graybodies at near-room temperature was constructed. Different spectral regions in the radiometer were obtained by use of hollow glass waveguides (HGWs) as filters. Using HGWs instead of bulk filters was advantageous because each HGW can be used as two different spectral filters when a dual-band IR detector is used. In addition, HGWs are much cheaper than the bulk IR filters that are usually used in such applications. For one graybody with a mean emissivity of 0.71, the estimated mean errors obtained for sample temperature, ambient temperature, and sample emissivity for all measured temperatures were 0.50% (approximately 1.65 K), 0.48% (approximately 1.4 K), and 7.3% (approximately 0.052) respectively. For a second graybody with a mean emissivity of 0.8 the estimated mean errors were 0.35% (approximately 1.2 K), 0.48% (approximately 1.4 K), and 5.0% (approximately 0.04), respectively.

A 250-microm-bore size and 50-cm-length hollow-glass waveguide (HGW) coated with a thin film of silver has been used to transport laser pulses at 800 nm from a Ti:sapphire oscillator. The silver film is deposited by a liquid-phase process. The measured transmission of this silver-coated HGW is 95%, which is considerably higher than the transmission of HGWs with no coating, as reported by other groups. The image of the output beam taken at different distances from the fiber output shows that a single HE11 mode couples to free-space modes from the exit of the fiber. Considerable spectral broadening can be obtained with high-intensity femtosecond pulse when this waveguide is filled with argon. This HGW can be used for such applications as beam transport and optical-pulse compression.

The purpose of this research is to deliver free-electron-laser (FEL) pulses for intraocular microsurgery. The FEL at Vanderbilt University is tunable from 1.8 to 10.8 microm. To deliver the FEL beam we used a metallic-coated hollow-glass waveguide of 530-mum inner diameter. A 20-gauge cannula with a miniature CaF2 window shielded the waveguide from water. Open-sky retinotomy was performed on cadaver eyes. The system delivered as much as 6 x 10(5) W of FEL peak power to the intraocular tissues without damage to the waveguide or to the surgical probe.

Hollow glass waveguides are an increasingly popular fiber for the delivery of high-power IR laser radiation. At CO(2) laser wavelengths the measured and theoretical losses agree, but at the 3-microm Er:YAG laser wavelength the losses remain higher than expected. The reason for this is the surface roughness of the silver film used to form the first layer of the Ag/AgI thin-film structure. We found that the roughness of the silver film increases fivefold as silvering times increase from 5 to 80 min. This increased surface roughness produces a concomitant linear increase in the attenuation coefficient for the silver-only guides for wavelengths shorter than approximately 5 microm.

The Er:YAG laser-induced damage (LID) threshold and modal properties of single-crystal sapphire fibers grown by the laser-heated pedestal-growth method have been measured. The lowest loss (~0.4-dB/m) sapphire fibers produce little mode mixing and therefore deliver a near-single-mode output profile if the Er:YAG laser input beam profile is also nearly Gaussian. Normally, however, Er:YAG laser output beam profiles are multimode with numerous high-energy spikes. This leads not only to a multimode output from the fiber but also increased fiber loss that is due to higher-order mode coupling. The results of LID testing give a damage fluence of ~1.4 kJ/cm(2) for 300-mum core-only sapphire fibers at 2.94 mum.

Single-crystal sapphire fibers have been grown with the laser-heated pedestal-growth method with losses as low as 0.3 dB /m at 2.94 ?m. With the incorporation of a computer-controlled feedback system, fibers have been grown with less than +/-0.5 % diameter variation, or +/-1.5 ?m for a 300- ?m fiber. The losses in these fibers have been reduced further through a postgrowth anneal at 1000 degrees C in air, from 5.4 to 1.5 dB /m at 543 nm and from 0.4 -0.3 dB /m at 2.94 ?m. These fibers delivered 4.7 W at 10 Hz of Er:YAG laser power.

Flexible hollow glass waveguides with internal metallic and dielectric coatings have been used to deliver high-peak-power transversely excited atmosphere CO(2) laser energy. The straight guide loss is as low as 0.17 dB/m for 1000-mum-bore guides and 0.46 dB/m for 530-mum-bore guides propagating the HE(11) mode. The loss increases to 0.93 and 1.36 dB/m, respectively, when guides are bent to a radius of 0.25 m. The hollow glass waveguides have been used to deliver pulsed CO(2) laser energy successfully with a peak power of 0.7 MW and an energy of 350 mJ per pulse with a gas purge through the hollow core. The delivered average power is as high as 27 W. It is concluded that these waveguides are promising candidates for pulsed CO(2) laser delivery in medical and surgical applications.

A new type of hollow glass waveguide has been fabricated that transmits radiation from visible to infrared wavelengths with low loss. The broadband transmission is achieved with a structure consisting of two distinct core regions; a silica annulus for transmission of wavelengths from 0.3 to 2.0 μm and a hollow core for transmission from 2.0 to 12.0 μm. Losses in the silica core at 633 nm are 0.3 dB/m. Losses in the 575-μm bore hollow core at 10.6 μm are 0.6 dB/m. Bending loss is negligible for radiation transmitted in the solid silica core, whereas the hollow guide loss exhibits a 1/R dependence. The dual-core waveguide can transmit broadband radiation, is rugged and flexible, and therefore, is a good candidate for medical or sensor applications.

Hollow glass waveguides are used to deliver free electron laser (FEL) energy for applications in medicine and laser surgery. The hollow guides, optimized for the delivery of 6.45-μm FEL radiation, exhibited losses for the 1000-μm bore as low as 0.39 dB/m when the guide was straight and 1.75 dB/m when bent to a radius of 25 cm. Hollow glass guides are flexible, and their broadband capability provides an ideal fiber optic for the tunable FEL.

Flexible hollow glass waveguides with bore diameters as small as 250 µm have been developed for 3-µm laser delivery. All the guides exhibit straight losses between 0.10 and 1.73 dB/m, and the loss increases to between 2.4 and 5.1 dB/m upon bending 1 m of the guides into 15-cm-diameter coils. This behavior is shown to depend strongly on the launch conditions and mode quality of the input beam. The waveguides are capable of efficiently delivering up to 8 W of Er:YAG laser power with proper input coupling, and they are suitable for use in both medical and industrial applications.

Hollow-sapphire and metal-dielectric-coated hollow-glass waveguides have been used to deliver CO(2) laser power for industrial laser applications. The transmission, bending loss, and output-beam properties of these waveguides are described. The bore sizes of the hollow-sapphire waveguides were 1070 and 790 μm, and the hollow-glass waveguide had a bore of 700 μm. The waveguides ranged in length from 1.1 to 1.5 m. The sapphire waveguides were bent to 90°, and the hollow-glass waveguides were bent into a full 360° loop. We delivered a maximum of 1.8 kW through the 1070-μm-bore sapphire waveguide and 1.0 kW through the hollow-glass waveguide. All the hollow waveguides incorporated a water jacket to prevent overheating.

Solgel alumina films were prepared by use of the Yoldas process and were characterized optically and microstructurally. From nitrogen adsorption and electron microscopy, we determined that the material was highly porous, with pores and crystallites of the order of tens of nanometers in size. The infrared transmission and reflectance of the films were measured, and dispersion curves were calculated as a function of firing temperature by extracting the film optical constants from the reflectance and thickness data. The use of this material in a hollow waveguide structure for the delivery of CO(2) laser radiation for surgical applications is discussed. Calculated waveguide losses indicate that solgel-based alumina is a good candidate material for this application.

Hollow glass waveguides with a 250-µm i.d. have been fabricated with a liquid-phase deposition technique that uses silica tubing as a base material. The losses of the 250-µm-bore guide measured at CO(2) laser wavelengths are as low as 2.0 dB/m. The straight losses for the hollow guides are in good agreement with theoretically predicted losses as a result of the nearly ideal structure of the guides. It is also shown that the guides have low bending losses, a nearly pure-mode delivery, and good high-power laser transmission. By proper design of the dielectric thickness, the guide is also able to deliver Er:YAG laser energy with a low loss of 1.2 dB/m for the 320-µm-bore waveguide. Because the hollow glass waveguide is very flexible and robust, it is quite suitable for medical applications.

A rapid and sensitive technique for quantitatively examining the three-dimensional light distribution from optical-fiber diffusers for photodynamic therapy applications was developed. We have used this system to evaluate the longitudinal and azimuthal uniformity of the light output from cylindrical diffusing tips and have determined a standard detector configuration for assessing the optical quality of these diffusers. As an example of the sensitivity and the accuracy of this method, the effect of microbending of the fiber on the output intensity distribution, as well as the effect of the choice of laser used, was investigated and was seen to have a significant impact on the uniformity of these diffusers.

Calcium aluminate glass has a refractive index less than 1 at 10.6 µ, and therefore it is a good candidate for a hollow fiber for the transmission of CO(2) laser energy. We have drawn hollow calcium aluminate glass fibers with inner diameters ranging from 380 to 500 µ. The loss for our 500-µm inner-diameter hollow glass fibers measured at 10.6 µm is 8.6 dB/m.

The attenuation coefficients for hollow sapphire and lead-based, oxide glass waveguides are calculated using an approximate formalism from Marcatili and Schmeltzer [Bell Syst. Tech. J. 43, 1783-1809 (1964)]. These results are compared with an exact calculation, and the agreement is found to be excellent for bore diameters greater than 100 µm. Although the hollow lead glass waveguides have measured losses in agreement with theory, the hollow sapphire waveguides have losses greater than those calculated for the lowest-order mode. This excess loss is due to roughness of the inner surface of the sapphire. The hollow dielectric waveguides are also shown to preserve polarization and the single mode as do some of the hollow metallic waveguides.

High peak power transverse excitation atmospheric CO(2) laser pulses are transmitted through flexible hollow sapphire waveguides with losses of 0.84 dB/m and no damage to the waveguides.

Single-crystal sapphire fibers are produced by the laser heated pedestal growth technique. The fibers have attenuation coefficients of less than 2 dB/m at the Er:YAG laser wavelength of 2.94 µm and are used to deliver over 600 mJ of Er:YAG laser energy. Mechanical testing of these fibers and the sapphire fibers produced by the edge-defined, film-fed growth technique results in a measured 0.4% strain to failure when testing is done under a 4-point load. Teflon-FEP (perfluorinated ethylene propylene) is applied to sapphire fibers as a cladding. The cladding is extremely effective in preventing leakage of energy from the fibers into absorbing environments that may surround the fiber.

The optical absorption has been measured at DF and HF wavelengths in a wide variety of transparent materials that show promise for use as windows on high powered chemical lasers. These measurements, which were made using DF-HF chemical laser calorimetric methods, will be discussed in terms of surface and bulk contributions to the total absorption as well as in terms of the total absorption measured for each type of material studied. Materials studied include the alkaline earth fluorides, alkali halides, Ge, Si, ZnSe, MgO, Yttralox, and A1(2)O(3).

We present an analysis of laser calorimetric data that deduces both the bulk and the surface absorption in a single run. The method involves use of long rod geometry combined with an analytical solution of the heat equation for the temperature distribution in a sample that is heated both internally and on the surfaces. Bulk and surface absorption coefficients, heat transfer coefficient, and thermal diffusivity appear as parameters; the last is treated as known, and the thermal rise curve is fitted to the three others. The solution obtained is valid at all points and times, and measurement of the temperature during and after laser heating at different points therefore narrows the possible fit considerably. Examples illustrating the method are presented for ZnSe, CaF(2) NaF:Li, NaCl, KBr, and KC1 at 2.7 microm, 3.8 microm, and 10.6 microm. Surface absorption is found to be dominant in all cases.

A systematic study was made of thin-film absorption at 2.8microm and 3.8 microm for a wide variety of transparent coating materials. The extinction and absorption coefficients were measured using HF/DF chemical laser calorimetry in single-layer films of PbF(2), LiF, YbF(3), NaF, ThF(4), As(2),S(3), As(2),Se(3), Si, MgO, ZnSe, and Al(2),O(3), on CaF(2), substrates. The lowest absorbing film materials were the As glasses, ZnSe, and some fluorides. In these cases, high-purity starting material led to the lowest loss films.

The optical performance of antireflection coatings on alkaline earth fluoride window materials at DF chemical laser wavelengths is presented. Reported coating absorption losses are less than 0.01% per surface. Those material properties that are important for this application are discussed, and theoretical coating designs are compared with experimental results.

The requirements for low-loss optical materials for use on excimer lasers have stimulated the investigation of optical absorption in a variety of highly transparent materials at visible and uv wavelengths. To provide information over a wide spectral range at low absorption levels ( approximately 10(-5) cm(-1)), laser calorimetric and wavelength modulation spectroscopic techniques were used. Blending these two methods provided, for the first time, spectral information well below the usual levels of absorption measured in studies of the Urbach tail.

Polycrystalline alkali halide fibers were fabricated using a hot extrusion technique. Core-clad fibers of KBr/KCl had losses as low as 0.1 dB/m at 10.6 microm, but the average loss for 8OO/100O-microm fiber was 0.69 dB/m. The salt fibers were coated with Teflon to minimize surface fracture from microcleavage cracks and to protect the fiber from contamination. The maximum output power we have obtained from the fiber is 67 W of cw CO(2) laser power. This corresponds to a power density of 13.3 kW/cm(2). The 100O-microm salt fiber has been bent into a 12-cm diam circle with a 5% reduction in transmission.

Adiabatic laser calorimetry, which is the most widely used method for studying the absorption coefficients of low-loss materials, can be adapted to study both the bulk and surface absorption by using a long rod sample geometry. In the limiting case of small heat losses, calculations of the thermal rise curves obtained in laser calorimetry indicate that two regions of constant slope can be expected. The first of these can be identified with the bulk absorption coefficient only and the second with the sum of the surface and bulk absorptions. Experimental data illustrating this two-slope behavior are presented.

Traditionally, the optical absorption in thin dielectric films has been obtained by comparing the absorption in coated and uncoated substrates. A new technique has been developed in which the absorption of the film is obtained directly without resorting to comparison or difference methods. This method relies on the separation of surface and bulk heat that results from laser calorimetric measurements on long, bar-shaped samples. Single layer films of As(2)S(3), CaF(2), ThF(4), and PbF(2) were deposited on a KCl bar, and the absorption coefficient of each film was extracted from the slopes of the temperature-time curves.