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.
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.
Sandia National Laboratories' program in high-power fiber lasers has emphasized development of enabling technologies
for power scaling and gaining a quantitative understanding of fundamental limits, particularly for high-peak-power,
pulsed fiber sources. This paper provides an overview of the program, which includes: (1) power scaling of diffraction-limited
fiber amplifiers by bend-loss-induced mode filtering to produce >1 MW peak power and >1 mJ pulse energy
with a practical system architecture; (2) demonstration of a widely tunable repetition rate (7.1-27 kHz) while
maintaining constant pulse duration and pulse energy, linear output polarization, diffraction-limited beam quality, and
<1% pulse-energy fluctuations; (3) development of microlaser seed sources optimized for efficient energy extraction; (4)
high-fidelity, three-dimensional, time-dependent modeling of fiber amplifiers, including nonlinear processes; (5)
quantitative assessment of the limiting effects of four-wave mixing and self-focusing on fiber-amplifier performance; (6)
nonlinear frequency conversion to efficiently generate mid-infrared through deep-ultraviolet radiation; (7) direct diode-bar
pumping of a fiber laser using embedded-mirror side pumping, which provides 2.0x higher efficiency and much
more compact packaging than traditional approaches employing formatted, fiber-coupled diode bars; and (8)
fundamental studies of materials properties, including optical damage, photodarkening, and gamma-radiation-induced
darkening.
Gamma-radiation-induced photodarkening has been observed and characterized in a suite of Yb-doped, Er-doped
and Yb/Er co-doped optical fibers. Significant reduction in the optical transmission of the fibers under passive (not
pumped) conditions was observed for wavelengths across the infrared spectrum. In general, it was found that the co-doped
fiber tested showed the strongest radiation resistance whereas the Er-doped fibers tested exhibited the greatest
radiation sensitivity. A dependence on dose rate was also observed in all 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.
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).
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