Evaluation of in-situ optical loss of polyimide-coated optical fiber under hydrothermal environments

When distributed fiber optic sensing (DFOS) devices are deployed in a geothermal borehole, the optical fiber as the key element, must be able to withstand the harsh environment because the temperature of the geothermal boreholes is higher than 250 degrees C in most cases. Understanding the deterioration behavior of optical fibers in hydrothermal conditions higher than 250 degrees C is important for predicting the lifetimes of DFOSs in operation.Geophysical equipment using optical fiber sensing was developed for in-situ measurement of the optical loss of optical fibers under hydrothermal conditions, and a survey method was established. Using this optical fiber sensing, we characterized the deterioration behavior of single-mode polyimide-coated optical fibers. Our experiments with carbon / polyimide-coated fibers under subcritical water from 250 to 350 degrees C revealed that the polyimide layer and silica cladding easily dissolved in water in that environment.

Automated summary

DFOS devices deployed in geothermal boreholes require optical fibers that can withstand high temperatures and understanding the deterioration behavior at these temperatures is crucial for predicting device lifetimes.