Overview of radiation induced point defects in silica-based optical fibers

This paper will review recent progress in understanding the behavior of optical fiber waveguides when they are exposed to ionizing radiation. Not only have the growth and recovery of the radiation-induced attenuations been thoroughly characterized, in some cases the defect centers which cause these absorptions have been identified, and means for reducing the radiation sensitivity of the fibers have become apparent. The behavior of the radiation-induced loss is described in terms of parameters such as fiber composition and dopants, fiber structure, wavelength and intensity of the light source, temperature, total dose, time after irradiation, dose rate, and radiation history.

Optical fiber waveguides in radiation environments, II

The output signal of an organic scintillator probe consists of a scintillation signal and Cerenkov and fluorescence radiation (CFR) signal when the probe is exposed to a mega-voltage photon or electron beam. The CFR signal is usually unwanted because it comes from both the scintillator and light guide and so it is not proportional to the absorbed dose in the scintillator. A new organic scintillator detector system has been constructed for absorbed dose measurement in pulsed mega-voltage electron and photon beams that are commonly used in radiotherapy treatment, eliminating most of the CFR signal. The new detector system uses a long decay constant BC-444G (Bicron, Newbury, OH, USA) scintillator which gives a signal that can be time resolved from the prompt CFR signal so that the measured contribution of prompt signal is negligible. The response of the new scintillator detector system was compared with the measurements from a plastic scintillator detector that were corrected for the signal contribution from the CFR, and to appropriately corrected ion chamber measurements showing agreement in the 16 MeV electron beam used.

https://iopscience.iop.org/article/10.1088/0031-9155/47/8/313/pdf

A temporal method of avoiding the Cerenkov radiation generated in organic scintillator dosimeters by pulsed mega-voltage electron and photon beams.

In this topical review, the recent progress on radiation-hardened fiber-based technologies is detailed, focusing on examples for space applications. In the first part of the review, we introduce the operational principles of the various fiber-based technologies considered for use in radiation environments: passive optical fibers for data links, diagnostics, active optical fibers for amplifiers and laser sources as well as the different classes of point and distributed fiber sensors: gyroscopes, Bragg gratings, Rayleigh, Raman or Brillouin-based distributed sensors. Second, we describe the state of the art regarding our knowledge of radiation effects on the performance of these devices, from the microscopic effects observed in the amorphous silica glass used to design fiber cores and cladding, to the macroscopic response of fiber-based devices and systems. Third, we present the recent advances regarding the hardening (improvement of the radiation tolerance) of these technologies acting on the material, device or system levels. From the review, the potential of fiber-based technologies for operation in radiation environments is demonstrated and the future challenges to be overcome in the coming years are presented.

https://iopscience.iop.org/article/10.1088/2040-8986/aad271/pdf

Recent advances in radiation-hardened fiber-based technologies for space applications

The effect of variations in the Ge, B, P, and OH content on the radiation response of Ge-doped silica-core optical fiber waveguides has been measured at 0.82 and 1.3 microm. It has been established that variations in neither OH content nor Ge concentration affect the response of these fibers. B doping increases the loss at short times following pulsed irradiation, whereas P suppresses the intense Ge-related transient absorption at both 0.82 and 1.3 microm while inhibiting long-term recovery. Since there is a minimum in the induced loss near 1.1-1.2 microm in P-doped fibers, the damage at 1.55 microm is significantly higher than that at 1.3 microm.

Compositional effects on the radiation response of Ge-doped silica-core optical fiber waveguides.

Radiation-induced effects in low loss optical waveguides have been studied to determine parameters affecting the use of these materials in plasma diagnostic instrumentation operating in severe radiation environments. Radiation-induced luminescence of fibers was shown to result from the Cerenkov process occurring within the fibers. Both spectral data and angular dependence of luminescence were used to verify the Cerenkov effect. Absolute magnitude of luminescence data were obtained. Different fiber types were shown to exhibit comparable luminescence output. Radiation-induced absorption was measured immediately following a 40 ns radiation pulse at wavelengths of 600 nm and 800 nm. Saturation effects in absorption were observed as well as changes in fiber recovery vs time as a function of radiation dose. Step- and graded-index profile fibers were shown to exhibit comparable absorption. Fibers tested included plastic clad silica fibers, as well as graded- and step-index germanium-doped and boron-doped fibers. Plastic clad silica fibers exhibited much less radiation-induced absorption than any other fiber type. The data presented allow optical fiber data links to be designed for use in severe radiation environments where data transmission is required during, or shortly after, the radiation pulse.

Transient Radiation Effects in Low-Loss Optical Waveguides

A technique based on measurement of time dispersion in a δ‐function Cerenkov pulse has been developed for determining the attenuation coefficient α (λ) of low‐loss optical fibers. The time domain amplitude distribution of the transmitted polychromatic pulse is transformed to the wavelength domain by accurate measurement of the differences in arrival times of spectral bands isolated by interference filters. Since the Cerenkov pulses are generated in the fiber by a short burst of relativistic electrons, α (λ) can be measured over a wide and continuous range of path lengths without breaking the fiber. The technique has been demonstrated over the 500–800‐nm spectral region for fiber lengths up to 800 m.

A technique for measuring optical fiber transmission using the time dispersion of Cerenkov radiation

Optical fiber cables have been successfully used for 100-MHz analog data transmission during an underground nuclear test at the Nevada Test Site. Two 700-m Corning Corguide cables were used to provide thirteen single fiber data channels from the vicinity of the underground detonation, 350 meters below ground level, to recording instrumentation, 350 meters from the downhole shaft. No fiber performance degradation was observed during the extensive procedures used to seal the shaft. These procedures included backfilling the shaft with layers of sand and gravel, as well as poured epoxy plugs. Techniques were developed for internal sealing of the Corguide cable to prevent any possible radioactive gas flow through voids within the cable. The effects on optical fibers of intense, pulsed neutron and gamma irradiation were studied. Specialized tools, including a system for location of faults or breaks in the optical fibers, were developed. The success of this first test will allow consideration of fiber optic cables for future nuclear tests as well as for other applications involving extremely rough handling in field environments.

/pdf/fiber-optic-utilization-at-the-nevada-test-site-1d18f93f51.pdf

Fiber optic utilization at the Nevada Test Site

An optical time domain reflectometer is described which combines in one instrument the basic capability to analyze several important fiber characteristics. The device uses the polarization properties of light to enable high sensitivity fault detection close to the fiber input end; its probe pulse temporal characteristics and high gain photodetector in combination provide excellent discontinuity location resolution in long lossy fibers, and give an indication of fiber dispersion at large bandwidths.

/pdf/a-fiber-optic-time-domain-reflectometer-7lod2m9ojt.pdf

P. B. Lyons

Papers

Transient Radiation Effects in Low-Loss Optical Waveguides

A technique for measuring optical fiber transmission using the time dispersion of Cerenkov radiation

Fiber optic utilization at the Nevada Test Site

A Fiber-Optic Time Domain Reflectometer