W. J. Thomes

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).

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.

Investigation of radiation-induced photodarkening in passive erbium-, ytterbium-, and Yb/Er co-doped optical fibers

The deployment of optical fibers in adverse radiation environments, such as those encountered in a low-Earth-orbit space setting, makes critical the development of an understanding of the effect of large accumulated ionizing-radiation doses on optical components and systems. In particular, gamma radiation is known to considerably affect the performance of optical components by inducing absorbing centers in the materials. Such radiation is present both as primary background radiation and as secondary radiation induced by proton collisions with space-craft material. This paper examines the effects of gamma radiation on erbium-, ytterbium-, and Yb/Er co-doped optical fibers by exposing a suite of such fibers to radiation from a Co-60 source over long periods of time while monitoring the temporal and spectral decrease in transmittance of a reference signal. For same total doses, results show increased photodarkening in erbium-doped fibers relative to ytterbium-doped fibers, as well as significant radiation resistance of the co-doped fibers over wavelengths of 1.0 - 1.6 microns. All three types of fibers were seen to exhibit dose-rate dependences.

Correlation of optical luminescence with radiation hardness in doped LiNbO 3 crystals

Transient ionizing radiation fields have been observed to cause substantial optical loss in undoped LiNbO 3 crystals operating at 1.06 microns. This loss is slow to recover and makes the selection of this material for Q-switch applications in radiation environments unfeasible. We have studied the effects of Mg doping on the radiation response of LiNbO 3 crystals and have investigated the optical luminescence of doped and undoped samples. Our results indicate a strong correlation between crystal defects, formed primarily during crystal growth, and the radiation-induced optical loss exhibited by these materials. These findings have enabled us to produce radiation-hard LiNbO 3 crystals for use in high gamma-field environments.

Vacuum-ultraviolet spectroscopy measurement of poly(methylphenylsilylene) photosensitivity

Photoinduced optical absorption changes in the vacuum-ultraviolet (VUV) spectral range have been measured in poly(methylphenylsilylene) thin films under varied excitation photon energies and local atmospheric environments. Spectral changes in resonances associated with both the linear chain Si–Si backbone and the side groups of the hybrid structure are consistent with the photodisruption of backbone topology. These effects are more pronounced under a higher energy photon exposure (5.10eV) resonant with the fundamental π‐π* transition of the phenyl moiety. An aerobic environment also favors more dramatic bleaching of VUV absorption in these materials. Finally, the present study enables a Kramers-Kronig analysis of absorption change from the visible to the VUV. These results do not adequately describe the photoinduced refractive index changes measured at 632.8nm via ellipsometry, indicating the presence of other contributions to the index modifications observed.

Polysilane-based thin films with high photosensitivity

The present work investigates the intrinsic photosensitivity of a family of poly(alkyl)(aryl)silanes and poly(hydridophenyl)silane for use in the development of photoimprinted waveguide devices. Limited testing of passive optical behavior (e.g. absorption, refractive index) and photosensitive response was performed for these materials in thin film form. It was determined that the materials exhibited dramatic photobleaching under 248 nm (KrF excimer laser) exposure. Based on a Kramers-Kronig analysis of the absorption changes, refractive index changes on the order of - 0.1 are estimated. Confirmation of this calculation has been provided via ellipsometry which estimates refractive index changes at 632 nm of -0.14 ± 0.01. In addition, embedded strips have been photoimprinted into the material to confirm waveguiding capacity of the films. Possible sources of photosensitivity in this material and its potential for application in various device configurations will be discussed.

Pulsed and Steady-State Radiation Effects on Single Junction Si and Multiple Junction GaAs Photocells

Si single junction photocells manufactured by Sandia National Laboratories and commercially available multiple junction GaAs photocells were tested in a pulsed high-dose mixed gamma-neutron environment. The Si photocells were also tested in steady state gamma environment at two different dose rates.

Photosensitive polysilane thin films for write-as-needed optical devices

The use of photosensitive materials for the development of integrated, refractive-index structures supporting telecom, remote sensing, and varied optical beam manipulation applications is well established. Our investigations of photosensitive phenomena in polysilanes, however, have been motivated by the desire to configure, or program, the photonic device function immediately prior to use. Such an operational mode imposes requirements on wavelength sensitivity, incident fluence and environmental conditions that are not typical of more conventional applications of photosensitive material. The present paper focuses on our efforts to understand and manipulate photosensitivity in polysilane thin films under different excitation wavelengths, local atmospheric compositions and thermal history in this context. We find that the photoresponse can be influenced through the control of such optical exposure conditions, thereby influencing the magnitude of the photoinduced refractive-index change attained.

Photo-control of nanointeractions.

The manipulation of physical interactions between structural moieties on the molecular scale is a fundamental hurdle in the realization and operation of nanostructured materials and high surface area microsystem architectures. These include such nano-interaction-based phenomena as self-assembly, fluid flow, and interfacial tribology. The proposed research utilizes photosensitive molecular structures to tune such interactions reversibly. This new material strategy provides optical actuation of nano-interactions impacting behavior on both the nano- and macroscales and with potential to impact directed nanostructure formation, microfluidic rheology, and tribological control.