Arc-induced Long Period Gratings in standard and speciality optical fibers under mixed neutron-gamma irradiation

Andrei Stancălie,Petrişor Bleotu,Stefania Campopiano,Flavio Esposito,Agostino Iadicicco,Dan Sporea,Rajeev Ranjan

doi:10.1038/s41598-017-16225-4

Abstract

In this paper, for the first time, the effects of mixed neutron and gamma flux on the spectral and sensing responses of Long Period Gratings (LPGs) are thoroughly analyzed. Six LPGs written by means of Electric Arc Discharge (EAD) technique in standard and speciality fibers, including radiation-hardened ones, were tested. The EAD technique was chosen because it enables the writing of gratings both in standard and not photosensitive fibers. The experiments have been carried out in a “TRIGA” pulsed nuclear reactor and the LPGs were irradiated by a gamma-ray dose-rate of 9 Gy/s and a mean 1.2∙1012 n/(cm2s) neutron flux. Real time monitoring was performed for a comparative investigation of LPGs’ response, in terms of radiation sensitivity and wavelength shift. Experiments show that LPG in a radiation-resistant fiber exhibits resonant wavelength shift higher than LPG in standard fiber. The changes of temperature sensitivity due to radiation were experimentally established by comparison of pre- and post-radiation characterization, indicating that radiation effects induce a slight increase of the temperature sensitivity, except for the LPG in pure-silica fiber. Theoretical and numerical analysis was combined with experimental data for evaluation LPGs’ parameters changes, such as refractive index and thermo-optic coefficient, after exposure to radiation.

Highlights

Over the years, great efforts have been made in order to enhance the sensitivity of in-fiber sensors to external parameters like temperature, strain, humidity, pressure and so on, with the aim to enlarge their field of application
The wavelength shift recorded by Fiber Bragg Gratings (FBGs) is related to the induced changes in the core effective refractive index, whereas for Long Period Gratings (LPGs) it is related to the difference between core and cladding effective refractive indices
On this line of argument, the authors were focused on the experimental and numerical investigation of the gamma radiation induced effects on LPGs fabricated in different fibers and by means of various techniques, studying both their resonant wavelength shift as function of radiation dose and their temperature sensitivity changes, as both are two known effects caused by irradiation[2]

Summary

Introduction

Great efforts have been made in order to enhance the sensitivity of in-fiber sensors to external parameters like temperature, strain, humidity, pressure and so on, with the aim to enlarge their field of application. Until now there is no systematic investigation on LPGs behaviour under irradiation, mostly due to the high irradiation costs On this line of argument, the authors were focused on the experimental and numerical investigation of the gamma radiation induced effects on LPGs fabricated in different fibers and by means of various techniques, studying both their resonant wavelength shift as function of radiation dose and their temperature sensitivity changes, as both are two known effects caused by irradiation[2]. LPGs are an important subject to investigate, and, to the best of our knowledge, these sensing devices were never tested before under neutron irradiation For these reasons, in the present paper we propose a wide experimental and numerical comparative investigation, concerning the effects on six LPGs written by EAD technique in different fibers, when exposed to a mixed gamma-neutron radiation field, at the nuclear reactor operated by the “Nuclear Research Institute ICN” (Mioveni, Romania). A comparison between the gamma-neutron induced changes in gratings fabricated in fibers with different composition is provided

Methods

Results

Conclusion