The utilization of downhole optical cables has significantly enhanced the efficiency and reliability of oilfield production operations; however, the challenging high-temperature and high-pressure conditions prevalent in oil-gas fields markedly reduce the service lifespan of these optical cables. This limitation severely impedes their application and further development in subterranean environments. In this study, a qualitative analysis was conducted on the structural materials utilized in two types of optical cables to identify these materials and assess the high-temperature tolerance and aging resistance properties of the optical fibers incorporated within. It was discovered that hydrogen infiltration into the subterranean optical cables predominantly accounts for their operational failure. To address this issue, an optical loss testing platform was established, facilitating the execution of a high-temperature and high-pressure hydrogen permeation aging experiment on the optical fibers, allowing for the evaluation of the hydrogen resistance capabilities of the two types of optical fibers. The findings from this study provide a theoretical foundation and methodological guidance for the optimization of optical fibers, aiming to enhance their durability and functional performance in adverse environmental conditions encountered in oil-gas field applications.

An in-fiber liquid microsphere whispering gallery mode (WGM) resonator is proposed and experimentally demonstrated. The in-fiber resonator is fabricated by creating an air cavity in tangential contact with the fiber core using femtosecond (fs) laser micromachining and fusion splicing. The air cavity is then filled with high index liquids through a laser drilled channel to form a liquid microsphere. The size and location of the microsphere can be controlled by using different fabricating parameters, while higher Q factors can be achieved with smaller coupling distance. The spectrum of the liquid sphere WGM resonator can be tuned by the index value of the liquid, and the resonant peaks shift toward longer wavelength as the refractive index (RI) of the liquid increases, with a sensitivity of 602.3 nm/RIU in the index range of 1.584∼1.688. On the other hand, the liquid microsphere resonator exhibits high temperature sensitivity due to the high thermo-optic coefficient of the liquid. Temperature sensitivity of -360.9 pm/°C is obtained with a resonator with sphere diameter of 49 μm and liquid RI of 1.692. Such a device can be an attractive candidate for sensing in high index environments such as oil industry.