D. C. Meister

Ion energy and angular distributions in inductively coupled radio frequency discharges in argon

We report measurements of the energies and angular distributions of positive ions in an inductively coupled argon plasma in a Gaseous Electronics Conference Reference Cell. Use of two separate ion detectors allowed measurement of ion energies and fluxes as a function of position as well as ion angular distributions on the discharge centerline. The inductive drive on our system produced high plasma densities (up to 1012/cm3 electron densities) and relatively stable plasma potentials. As a result, ion energy distributions typically consisted of a single feature well separated from zero energy. Mean ion energy was independent of rf power and varied inversely with pressure, decreasing from 29 to 12 eV as pressure increased from 2.4 to 50 mTorr. The half‐widths of the ion angular distributions in these experiments varied from 5° to 9°, or equivalently, the transverse temperatures varied from 0.18 to 0.29 eV with the distributions broadening as either pressure or rf power was increased.

Gamma Radiation Effects in Yb-Doped Optical Fiber.

Determination of the radiation response of doped-fiber laser materials, systems and components to relevant ionizing radiation fluxes is central to the prediction of long-term fiber-based laser performance/survivability in adverse and/or space-based environments. It is well known that optical elements that are placed into orbit around the Earth experience harsh radiation environments that originate from trapped-particle belts, cosmic rays, and solar events. Of particular interest to optical materials is the continuous flux of gamma photons that the materials encounter. Such radiation exposure commonly leads to the formation of color centers in a broad range of optical materials. Such color center formation gives rise to changes in optical transmission, loss and luminescent band structure, and, thus, impacts long-term optical device performance. In this paper we will present the results of our investigation of gamma-radiation-induced photodarkening on the passive optical transmittance of a number of ytterbium- (Yb-) doped optical fibers. We will discuss the evolution of the optical response of the fiber across the 1.0 to 1.6 micron wavelength window with increasing gamma exposure. Results indicate that these fibers exhibit reasonable radiation resistance to gamma exposures typical of a 5-year, low-earth-orbit environment. Maximum transmittance losses of less than 10% were observed for total gamma exposures of 2-5 krad (Si). In this paper we will present the results of our investigation of gamma-radiation-induced photodarkening on the optical transmittance of a number of ytterbium- (Yb-) doped optical fibers. We will discuss the evolution of the optical response of the fiber across the 1.0 to 1.6 micron wavelength window with increasing gamma exposure. Results indicate that these fibers exhibit reasonable radiation resistance to gamma exposures typical of a 5-year, low-earth orbit environment. Maximum transmittance losses of less than 10% were observed for total gamma exposures of 2-5 krad (Si).

Spectrally Resolved Transmission Loss in Gamma Irradiated Yb-Doped Optical Fibers

Yb3+-doped silicate fibers are commonly employed in optical systems utilizing fiber lasers and amplifiers. Deployment of such materials and systems in space-based and other adverse radiation environments requires knowledge of their response to fluxes of ionizing radiation. This paper reports the results of gamma radiation exposures on a suite of passive, modern, highly Yb3+-doped aluminosilicate fibers. Of interest are the effects of total dose and dose rate as well as the development of radiation-induced absorption across a broad spectral window (1.0-1.7 mum). Results indicate that these fibers exhibit reasonable radiation resistance to gamma exposures typical of a five-year low-Earth-orbit environment. Maximum transmittance losses of less than 10% in the 1.0-1.7-mum spectral region for total gamma exposures of 2-5 krad (Si) were observed. In addition, it was found that the dependence of transmittance on radiation dose generally followed a power law that was dependent on dose rate.

Ionizing Radiation Effects in Single-Crystal and Polycrystalline YAG

The effect of ionizing-radiation fields on optical materials is predominantly seen in changes in the transmission properties of the materials. Both transient and permanent photodarkening can occur in passive and active optical elements as a result of exposure to either short-pulsed or steady-state radiation. In laser materials, such as YAG, such induced optical loss can result in significant degradation of the lasing characteristic of the material, making its selection for optical device applications in radiation environments unfeasible. In the present study, the effects of ionizing radiation on the optical response of undoped and 1.1%Nd-doped single-crystal and polycrystalline YAG have been investigated. Both room temperature and elevated temperature studies have been performed. An evaluation of the data shows some potential advantages of single-crystal over polycrystalline YAG in applications involving radiation exposure of the materials

Temperature and dose-rate effects in gamma irradiated rare-earth doped fibers

Rare-earth-doped fibers, such as Er3+- and Yb3+-doped aluminosilicates can be advantageous in space-based systems due to their stability, their high-bandwidth transmission properties and their lightweight, small-volume properties. In such environments the effect of ionizing-radiation on the optical transmission of these fibers is of paramount importance. For the present work, gamma-radiation experiments were conducted in which un-pumped Yb3+ and Er3+ doped sample fibers were irradiated with a Cobalt-60 source under different dose-rate and temperature conditions. In-situ spectral transmittance data over the near IR was monitored during the irradiations for total doses of up to tens of krad (Si). It was found that there was a dose-rate dependence in which higher rates resulted in more photodarkening. Higher temperatures were not found to significantly affect the rate of photodarkening at the dose rates used.

Gamma-radiation-induced photodarkening in actively pumped Yb3+-doped optical fiber and investigation of post-irradiation transmittance recovery

Fibers doped with rare-earth constituents such as Yb3+ and Er3+, as well as fibers co-doped with these species, form an essential part of many optical systems requiring amplification. This study consists of two separate investigations examining the effects of gamma-radiation-induced photodarkening on the behavior of rare-earth doped fibers. In one part of this study, a suite of previously irradiated rare-earth doped fibers was heated to an elevated temperature of 300°C and the transmittance monitored over an 8-hour period. Transmittance recoveries of ~10 - 20% were found for Er 3+- doped fiber, while recoveries of ~5 - 15% and ~20% were found for Yb3+- and Yb3+/Er3+ co-doped fibers, respectively. In the other part of this study, an Yb3+-doped fiber was actively pumped by a laser diode during a gamma-radiation exposure to simulate the operation of an optical amplifier in a radiation environment. The response of the amplified signal was observed and monitored over time. A significant decrease in amplifier output was observed to result from the gamma-radiation exposure.

Radiation-induced optical response of single-crystal and polycrystalline YAG

Exposure of optical materials to transient-ionizing-radiation fields can give rise to transient and/or permanent photodarkening effects. In laser materials, such as YAG, such induced optical loss can result in significant degradation of the lasing characteristic of the material, making its selection for optical device applications in radiation environments unfeasible. In the present study, the effects of ionizing radiation on the optical response of undoped and 1.1% Nd-doped single-crystal and polycrystalline YAG have been investigated. In the undoped materials it is seen that both laser materials exhibit significant loss at the 1.06 μm lasing wavelength following exposure to a 40 krad, 30 nsec pulse of gamma radiation. In the undoped single-crystal samples, the transmission loss is initially large but exhibits a rapid recovery. By contrast, the undoped polycrystalline YAG experiences an initial 100% loss in transmission, becoming totally opaque at 1.06 μm following the radiation pulse. This loss is slow to recover and a large residual permanent photodarkening effect is observed. Nd-doping improves the optical response of the materials in that the radiation-induced optical loss is substantially smaller in both the polycrystalline and single-crystal YAG samples. Preliminary results on the radiation response of elevated-temperature samples will also be reported.

Structural defect control and photosensitivity in reactively sputtered germanosilicate glass films

The optical performance of refractive index structures induced in photosensitive (PS) glasses ultimately depends on the index modulation depth attainable. In germanosilicate materials, the photosensitive response is linked to the presence of oxygen-deficient germanium point defect centers. Prior efforts to increase PS in these materials, e.g., hydrogen loading, rely on a chemical reduction of the glass structure to enhance the population of oxygen deficient centers and thus increase the saturated refractive index change. We have previously reported the development of highly photosensitive, as-deposited germanosilicate glass films through reactive atmosphere (O2Ar) sputtering from a Ge/Si alloy target. The present work details our investigation of the effect of substrate temperature during deposition on the material structure and propensity for photosensitivity. Using optical absorption/bleaching, Raman, electron paramagnetic resonance (EPR) and selective charge injection techniques we show that the predominate defect states responsible for the PS response can be varied through substrate temperature control. We find that two regimes of photosensitive behavior can be accessed which exhibit dramatically different UV-bleaching characteristics. Thus, the corresponding dispersion of the refractive index change as well as its magnitude can be controlled using our synthesis technique. Tentative defect models for the photosensitive process in materials deposited at both ambient temperature and at elevated substrate temperatures are presented.

Permanent and transient response of Nd:YAG and Cr:YAG to ionizing radiation

Increased reliance on optical components in harsh radiation environments requires a deeper understanding of the radiation-induced behavior of common optical elements. Of particular interest is the impact of ionizing radiation on both optical transmission and absorption. The present work focuses on an examination of the optical response of singlecrystal Nd:YAG, Cr:YAG and co-doped Nd:Cr:YAG to pulses of gamma-radiation. In-situ, transient optical behavior was observed by measuring the transmittance of the materials at 1064 nm before, during and after exposure to 30-60 krad (Si) of pulsed gamma radiation. The gamma-radiation-induced response of the Cr-doped materials was seen to exhibit exceptional radiation resistance as compared to the undoped YAG and to the Nd-doped materials. Furthermore, the addition of Cr3+ into the Nd:YAG crystal matrix was seen to greatly improve the radiation resistance of the laser materials. Both transient and permanent effects will be discussed.

Impact of Ionizing Radiation on the Optical Properties of YAG Laser Materials

Exposure of optical materials to transient-ionizing-radiation fields can give rise to transient and/or permanent photodarkening effects. In laser materials, such as YAG, such induced optical loss can result in significant degradation of the lasing characteristic of the material, making its selection for optical device applications in radiation environments unfeasible. In the present work, the ionizing-radiation response of Nd:YAG laser rods of varying composition and microstructure are examined. The optical properties of the materials are examined using a variety of optical spectroscopies and observations are correlated with the results of the ionizing-radiation studies. It is found that radiation damage in these materials is strongly influenced by the material microstructure.