Several different fiber Bragg gratings (FBGs) were exposed to accumulated high doses of gamma rays (up to 80 MGy) and neutrons (5*1018/cm3) in a research grade nuclear reactor. The FBG peak wavelengths were measured continuously in order to monitor radiation induced shifts. Gratings inscribed with IR femtosecond pulses through a phase mask showed the smallest shifts (around 20 pm), while under identical conditions point-wise inscribed femtosecond gratings and a UV inscribed grating showed shifts of around 100 pm and 400 pm respectively. The different responses to irradiation are attributed to the various inscription techniques inducing gratings whose refractive index modulation is derived from different physical modifications of the fiber material.
In this work, we demonstrate optomechanical measurements of radiation induced alterations of the acoustic velocity in a fluoroacrylate polymer coating of a silica optical fiber. The optomechanical measurement is based on forward Brillouin scattering initiated in the fiber core which stimulates acoustic waves that reach the fiber coating. The measurement may serve as an additional metric to quantify the dose of ionizing radiation to which the fiber was exposed. We have demonstrated that the stiffness of the coating increases following gamma irradiation, as measured by the time of flight of radial acoustic waves through the coating. The measurement was performed on few meters long fiber, but can be extended to a spatially distributed analysis in longer fibers. The tests showed a linear dependence of the acoustic time-of flight on the overall dosage of gamma irradiation. The time of flight decreased by as much as 15% following exposure to 180 Mrad from a 60Co source. In a follow-up study, we found that the stiffness of the previously exposed fiber coatings continued to slowly increase over months, after extraction from the radiation field. These results reveal the vulnerability of the specific coating to ionizing radiation and the potential complexities involved with dosimetry.
In order to test and compare the suitability of various radiation hardened optical fibers, we have established a test site in the 5MW research grade open pool reactor located at Soreq Nuclear Research Center, Israel. Several commercial fiber samples were coiled inside a specially designed apparatus which was lowered into the reactor core. The input and output legs of each fiber coil were prepared sufficiently long to extend outside the reactor pool, and were attached to a source (input) and detector (output). The transmission/attenuation could then be measured during, and in between, operations of the reactor. Since the reactor does not operate continuously, we were able to monitor in real time both transmission changes under very high radiation rates (approx. 0.5 MGy/hr) and doses (about 20 MGy) of gamma rays and neutrons, as well as recovery processes after each reactor shut down. Results are presented comparing the RIA and recovery kinetics of different commercial radiation hardened fibers under identical exposure/relaxation cycles. This study also examined effects on fibers with different coating materials and a fiber with inscribed Bragg gratings.
A concept for fiber optic measurement of strain in rotating structures where the fiber cannot access the central rotation axis is described. Various interrogation techniques are considered, and the use of a fast spectrometer-based interrogator is preferred. An automated algorithm for optical alignment while the structure is rotating is described.
Fiber Bragg Grating (FBG) sensors may probe ultrafast changes in pressure caused by shock waves propagating in solid and liquid media impacted by high velocity projectiles. The FBG spectra are measured using an optical system comprising economically priced electro-optical components offering 5 nsec temporal resolution and 0.8 – 1.6 nm spectral resolution. We present results showing evolution of 5 kBar shock wave pressure in approx. 100 nsec, as well as the dependence of the FBG response on the physical length of the sensor (1mm and 0.1mm), and on the relative orientation between the FBG axis and the shock wavefront.
FBGs respond to external pressures in ways that reflect both the strain-optic effect and the geometrical variations, both induced by the applied pressure. While the response to static isotropic pressure is quite straight forward and intuitive, the response to anisotropic shock waves is much more complex and depends also on the relative orientation between the fiber and the shock propagation direction. We describe and explain experimental results for both cases.
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