Optical fibers for telecommunications are designed to transmit light in a relatively benign and protected environment. The design aims to ensure minimal levels of attenuation, optical non-linearity, and other detrimental effects caused by external perturbations. However, for distributed sensing in harsh environments, the waveguide needs to be optimized as a sensing media and the coating on the optical fiber needs to provide mechanical protection at elevated temperatures. In this paper, we will review our work in three critical aspects of the optical fibers for sensing in harsh environments: waveguide design, coating thermal stability and mechanical strength at elevated temperatures.
As optical fiber is being used in much harsher environments than traditional telecommunications (e.g. distributed temperature sensing at elevated temperatures) understanding its mechanical properties at high temperatures is urgently needed. As a continuation of our previous work on high temperature strength of silica optical fiber, we report our results in fatigue behavior of polyimide coated silica optical fiber at 300°C in this paper. Fiber fatigue is the degradation in strength caused by a stress dependent chemical reaction between water vapor and the surface of the silica glass. In contrast to the published data on degradation in mechanical properties of silica optical fiber at elevated temperatures, our observations indicate a negligible decrease in strength along with unchanged, n-value, (fatigue resistant factor) at 300°C. To determine the n value, we tested tensile strength of the fiber using four different strain rates while the subject under test was at 300°C. The results indicate that the polyimide coating on the silica glass fiber continues to serve as an effective water vapor barrier at 300°C. These results will be compared with data available for room temperature performance of this silica/polyimide combination and possible failure mechanisms will be discussed.
High temperature mechanical strength and reliability of optical fibers have become important subjects as optical fibers
are increasingly used for harsher environments. Theories and models of fiber mechanical properties established for
traditional telecommunications applications may need to be validated for applications at elevated temperatures. In this
paper, we describe the test setup for high temperature tensile strength of fiber and report initial results of dynamic tensile
strength of polyimide coated optical fiber at 300 and 350ºC for different heating time intervals. The results are compared
with room temperature strength data, data available in the literature, and our earlier work on thermogravimetric analysis
(TGA) weight loss of the polyimide coating and the observations on surface morphology at elevated temperatures.
Interesting observations are discussed and possible explanations are proposed.
Large core silica optical fibers are often used for delivering high power laser for medical applications. It has been
observed that optical fibers that are transmitting high power laser light may fracture when bent and that the polymer
cladding or coating of the fiber plays a significant part in determining the fiber strength. In this work, we examine the
fiber performance in bending under high laser power after the fiber is treated at high humidity and high temperature, a
condition encountered commonly in medical applications, such as in an autoclave.