Experimental Investigation of Long-Period Grating Transition Modes to Monitor CO2 in High-Pressure Aqueous Solutions

An experimental investigation of transition modes of long-period gratings (LPG) is reported. Coating LPGs with a material of high refractive index causes a reorganization of the cladding modes, leading to a substantial increase in sensitivity to changes in the surrounding refractive index (SRI). The sensitivity of a coated LPG to SRI over a given range of SRI is a function of coating thickness. In this study, the thickness of polystyrene coatings applied by dip coating was measured by scanning electron microscopy. The LPG sensors were used to monitor dissolution of liquid CO₂ in deionized water at high pressure, simulating use of the sensors to monitor geologically sequestered CO₂ in a deep aquifer.