Showing papers by "E. J. Friebele published in 1993"

Radiation-induced defects in glasses: Origin of power-law dependence of concentration on dose

Abstract: We propose, and verify in the case of a Ge-doped-silica-core optical fiber, a general explanation for the power-law dependencies on dose frequently observed for the concentrations of radiation-induced defect centers in insulating glasses. This insight permits detailed prediction of the postirradiation recovery curves, given just the empirical exponent of the power law, 0

Survivability of optical fibers in space

Abstract: The survivability of optical fibers for data bus and gyroscope applications in the natural space radiation environment has been analyzed using radiation-induced loss data of single mode, multimode, and polarization-maintaining fibers. Since it is virtually impossible to simulate the dynamic conditions of space, extrapolations have been made from measurements at dose rates, temperatures, and total doses different from those onboard spacecraft. The anticipated degradation of most Ge-doped silica core fibers and all pure silica core fibers appears to be well within allowable margins in fibers for data bus applications, while the radiation sensitivity of polarization-maintaining fibers could result in a significant decrease in fiber gyro performance.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Thermal stability of photoinduced gratings and paramagnetic centers in Ge- and Ge/P-doped silica optical fibers

Abstract: The electron-spin-resonance spectra of paramagnetic defects in Ge/P-doped silica fibers prepared with second-harmonic generation (SHG) are compared with those induced in Ge- and Ge/P-doped silica preforms by 5-eV photons. Among the defects observed, the thermal stability of Ge E′-type centers is similar to that of photoinduced Bragg grating efficiency but not of SHG efficiency. The isochronal anneal curves of the Ge(1) and Ge(2) centers in Ge/P-doped silica correlate well with SHG efficiency, but the agreement is not nearly so good in silica containing only Ge. Only the thermal behavior of photoinduced Ge Ed1′ centers is similar to that of SHG efficiency in both materials, albeit only in the initial stages of annealing.

Radiation-induced coloring of erbium-doped optical fibers

Abstract: The radiation-induced coloring of erbium-doped optical fibers (EDF's) is reported. The radiation hardness of the EDF's is observed to be strongly dependent on composition. The implications for erbium-doped fiber amplifier (EDFA) performance is modelled.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Thermal stability of self-organized gratings and defects in Ge- and Ge-P-doped silica core fibers

Abstract: Thermal stabilities of photoinduced paramagnetic defects in second-harmonic (SHG) generation Ge-P-doped silica fibers and in Ge- and Ge-P-doped silica preforms are studied by electron-spin-resonance and compared with that of the reported self-organized gratings. Our data suggests that the charge trapping sites for the electric-field-induced SHG are Ge(1), Ge(2) and Ge E' d1 centers. The defects responsible for the Bragg gratings are Ge E'-type centers. Thermal darkening in Ge-doped silica core fibers reported is likely due to the thermal induced Ge E' d1 center.

Considerations for producing single-pulse fiber Bragg gratings

Abstract: The discussion of Bragg fiber gratings (FBGs) has expanded sharply in the last six years. FBGs were given versatility by the results at the United Technology Research Center when the technique of side-writing was pioneered (1989), which made the Bragg wavelength independent of the writing laser. The promise of economical production of these devices was advanced by the generation of FBGs with a single laser pulse at the Naval Research Laboratory (1992), and this result was expanded by the University of Southhampton in achieving saturated reflectivity with a single-pulse. The basis for manufacturing these excitingly versatile devices is near the point of supporting a new area of applications and industry. The intent of this article is to briefly discuss the conditions and requirements for the practical production of FBGs in support of the technology of distributed sensing.

Photosensitivity of rare earth-doped glasses

Abstract: Rare earth-doped glasses exhibit high initial photosensitivity but their response saturates at relatively modest values of (Delta) n (approximately 5 X 10 -7 ), which greatly limits their usefulness for device applications. In the context of our model, saturation results from either exhaustion of photosensitive rare earth sites, trap sites, or through competition between two photon creation and one photon bleaching processes. In this paper we report the results of new experiments designed to further elucidate the photosensitivity process with specific emphasis on the saturation mechanisms(s). Based on these new experimental results we present a refinement of our earlier model.

Fiber Bragg reflectors by single excimer pulse

Abstract: In-fiber Bragg reflectors produced with coherent, transverse UV illumination promise to be versatile optical elements for spectral selection and distributed sensing. These fiber devices were generated by repetitive exposures to a moderate intensity, frequency-doubled dye laser under holographically stable conditions until sufficient periodic index modulation had accumulated to produce a strong reflection. Fluences in the range of 1000 J/sq cm were delivered over the course of many seconds to minutes to obtain an index modulation depth of about 1 x 10 exp -4. In an alternate approach, we used single about 1 J/sq cm pulses from a line-narrowed KrF excimer to produce gratings with a modulation depth of about 2 x 10 exp -5, and more recently have achieved values above 1 x 10 exp -4. An unanticipated benefit of the single-pulse preparation method appears to be the markedly enhanced thermal stability of the grating structure, as compared to that reported for gratings produced with multiple exposures.

Erbium-doped fiber ring laser strain sensor

Abstract: We describe a fiber ring strain sensor configuration which incorporates an active fiber gain section to allow lasing within the ring. Analysis of the beating between modes of this ring laser provides a measure of the optical path length in the ring, and thus strain applied to the fiber. The sensor provides an output frequency which can be tracked to determine the integrated strain over the length of the fiber ring. Results are presented showing the sensor response up to a fiber strain of approximately 4000 micro-strain.

Evaluation of fast-neutron radiation effects in optical fibers for fusion reactor diagnostics

Abstract: A simple CCD-camera spectrometer was deployed at the Los Alamos Spallation Radiation Effects Facility to characterize fast neutron irradiation effects in several silica-based optical fibers over the wavelength range /spl sim/450-1100 nn. The experimental arrangement allowed optical loss spectra to be developed from remotely recovered frame grabs at various times during irradiation without necessary resort to "cut back" methods. Data recorded for a pure-silica-core/F-doped-silica-clad fiber displayed a peculiar artifact which is described and mathematically modelled in terms of leaky modes propagating in an optical cladding which is substantially less susceptible to radiation-induced optical attenuation than is the core. Evidence from optical time-domain reflectometry supports the postulate that mode leakage into the cladding may be due to light scattering from the tracks of ions displaced by the 14-MeV neutrons. These results suggest that fibers with fluorine doping in the core, as well as in the cladding, would be relatively resistant to radiation-induced attenuation in the UV-visible spectral region. >