Wide-Range pH Sensor Based on a Smart- Hydrogel-Coated Long-Period Fiber Grating

We report a fiber-optic sensor for pH measurement, which is a long-period fiber grating (LPFG) coated with smart hydrogel. When the pH of a solution that surrounds the hydrogel coating changes, the volume of the coating and hence its refractive index are changed, which results in a shift in the resonance wavelength of the LPFG. Our experimental sensor shows a sensitivity of ~0.66 nm/pH over the pH range from 2 to 12 and a response time of less than 2 s. The effects of the refractive index of the surrounding solution and temperature variations on the sensor are also measured.