Microstructured optical fibers can be precisely tailored for many different applications, out of which sensing has been found to be particularly interesting. However, placing silica optical fiber sensors in harsh environments results in their quick destruction as a result of the hydrolysis process. In this paper, the degradation mechanism of bare and metal-coated optical fibers at high temperatures under longitudinal strain has been determined by detailed analysis of the thermal behavior of silica and metals, like copper and nickel. We furthermore propose a novel method of enhancing the lifetime of optical fibers by the deposition of electroless nickel-phosphorous alloy in a low-temperature chemical process. The best results were obtained for a coating comprising an inner layer of copper and outer layer of low phosphorous nickel. Lifetime values obtained during the annealing experiments were extrapolated to other temperatures by a dedicated model elaborated by the authors. The estimated copper-coated optical fiber lifetime under cycled longitudinal strain reached 31 h at 450 °C.

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In this study, an innovative method is presented for preparing optical fibers for application in fiber optic sensors operating in harsh environments. It is shown, how to attach a metalcoated fiber electrolytically to a metal sensing element, as well as an electroless method for depositing a nickel protective layer on optical fibers. Additionally, the results of the reliability tests of these methods are presented. It is also shown, that by depositing an additional nickel protective layer, it is possible to slow down the oxidation process of the copper coating. Analysis of the conducted experiments allows us to predict, that the connection method investigated and the method of protecting the fiber are robust and may find application in industrial optical sensors.Index Terms-Metal-coated fibers, optical fiber applications, optical fibers, optical fiber sensors, reliability test of fibers, specialty optical fibers.

In this work we present an innovative method of connecting metal coated optical fibers with metal surfaces. The process is based on electrolytic reaction between copper and allows to obtain a robustand inflexible connection. Furthermore reliability tests of such fiber to metal joints have been performed, with the results of mechanical strength and temperature resistance tests presented. Additionally, as accelerated oxidation of copper at elevated temperatures is a major concern in long term temperature stability of the connection, we propose a method of slowing down the oxidation process with chemical nickel coating. Analysis of the obtained results allows us to predict that the investigated connection may find applications in various industrial optical sensors with special focus on harsh environments.