Experimental Validation of a Reflective Long Period Grating Design Methodology

In this work, we present an experimental demonstration of our previously published modeling work on reflective long period grating (LPG). To provide the practical realization of the modeling work, we coat a long segment of fiber both in the tail length and the end facet beyond the gratings with silver to invert the transmission mode LPG to reflection mode LPG. We then measure the LPG characteristics in both the transmission and reflection mode and validate our findings from modeling work. We further build temperature and refractive index (RI) sensors and demonstrate temperature sensing from 21 °C to 191 °C with similar temperature sensitivity coefficients of 54.4 ± 2.9 pm/°C and 53.2 ± 2.6 pm/°C for transmission and reflection mode LPG, respectively whereas same RI sensitivity coefficient of 370 ± 2.2 nm/RIU

An Ultrasensitive Long-Period Fiber Grating-Based Refractive Index Sensor with Long Wavelengths

The response of a novel long-period fiber grating (LPFG) with a period of 180 µm to a surrounding refractive index (RI) was investigated. The results displayed that, with the increase in RI of the surrounding media of cladding glass in the grating region, the resonant peak located at 1336.4 nm in the transmission spectrum gradually shifts towards a shorter wavelength, while the resonant peak located at 1618 nm gradually shifted towards a longer wavelength. Moreover, the resonant peak at 1618 nm is much more sensitive to the surrounding RI than that of the one at 1336.4 nm. Compared with the conventional LPFG and other types of wavelength-interrogated RI sensors, such as ring resonators, surface plasmon resonance sensors, and Fabry–Perot interferometric sensors, this novel LPFG possesses a higher sensitivity, which achieved 10,792.45 nm/RIU (RI unit) over a RI range of 1.4436–1.4489

Understanding the Radiation Effects on Fiber Optic Sensors

In this dissertation, the effects of radiation (gamma, neutron or mixed gamma and neutron) on optical fiber sensors are studied and new techniques for real-time measurement of radiation-induced macroscopic changes in optical fibers are presented. It is crucial among the research and development efforts in the nuclear energy field to conduct experiments in Advanced Test Reactor (ATR) to support lifetime extension, novel fuels and materials development, better fuel management, and enhanced safety of existing as well as future nuclear power plants (NPP). Due to their unparalleled and unique advantages over traditional sensors, optical fiber sensors are deemed potential candidates for their use in nuclear environments. However, optical fibers are susceptible to high levels of ionizing radiation emitted by fission reactors which are characterized by the highest levels of gamma dose, high flux of neutrons and potentially high temperatures depending on location in a reactor core. It is essential to accurately determine the information related to physical parameters such as temperature, pressure, and strain in nuclear environments for the safety of the existing and future NPPs. This dissertation starts with inverting a transmission mode long period grating (LPG) to reflection mode using a novel and cost-effective metal coating method since transmission mode LPG limits it applications in tight spaces or in nuclear fields. To understand the metal coating and metal coverage effects on the reflection spectrum of LPG, modeling work was performed, and it was validated by experimental work. We have shown that the sensitivity of LPGs to physical parameters in both transmission and reflection modes are almost the same. Next, we have modeled the radiation effects on different fiber optic sensors, proposed empirical models, and performed numerical analysis to understand the effects of nuclear environments on fiber optic sensors. We analyzed the real-time data from fiber Bragg gratings (FBGs) exposed to high neutron fluence and high temperature environments within the ATR at Idaho National Laboratory (INL). We have found that incoming radiation significantly drifts the characteristic signal of FBGs, leading to a temperature measurement error when FBGs are dedicated to temperature sensing. It is well known that neutron and gamma irradiation compacts silica optical fibers, resulting in a macroscopic change in the refractive index (RI) and geometric structure. The change in RI and linear compaction in a radiation environment is caused by three well-known mechanisms: (1) radiation induced attenuation (RIA), (2) radiation induced compaction (RIC), and (3) radiation induced emission (RIE). While RIA degrades the signal strength by creating different types of color centers in the silica fiber, RIC alters the density, and hence RI by displacing the host material atoms. However, Kramers-Kronig relation states that absorption, and hence the RIA, also modifies the RI of the silica fiber. Apart from RIA and RIC, other phenomena such as temperature, dose rate, stress relaxation, and dopant compositions exchange may change the RI. To overcome these problems, we have proposed an effective technique to measure the change in RI and compaction of optical fiber due to any specific phenomena the fiber is subjected to, including RIC, RIA, dopant diffusion, temperatures, dose, dose rate, etc. By knowing the individual contribution of RI and fiber length to the signal drift, it is possible to reduce the radiation induced signal drift in optical fiber sensors and provide accurate information regarding the temperature inside a radiation environment. Fission gas detection in nuclear environments is another important aspect that needs to be focused on. Pressure induced by fission gases during irradiation may lead to loss of coolant accident (LOCA), which can cause severe damage to the NPPs. We have modeled and fabricated optical fiber-based sensors to enable real-time monitoring of fission gases, which allows understanding the implications of fission gas release during an accident, important for safe and high performance