Optical pH sensor based on fiber interferometer coated with intelligent hydrogel

In this paper, we propose an optical fiber pH sensor with homemade poly (acrylic acid-co-acrylamide) hydrogel coating based on Mach-Zehnder interferometer (MZI), which is designed via concatenating no-core fiber, few-mode fiber and no-core fiber in sequence. The fiber surface of the sensing region is silanizated to enhance the adhesion of the hydrogel onto the fiber. When the pH value of the solution varied, the amount of free carboxylic acid ions in the hydrogel changes. This change causes the hydrogel to expand or contract, resulting in a change in refractive index. Such a refractive-index change of the hydrogel alters the effective refractive index of the cladding layer of the NFN MZI, leading to a spectral shift of the MZI. The sensor can be employed to measure a wide pH range of 2 to 12. According to the relationship between the pH values and spectral dips, the corresponding sensitivities are -0.346 nm/pH and -64 pm/pH at the pH ranges of 2-5 and 5-12, respectively. Meanwhile, the coefficients of determination of the fitting lines are both large than 0.973, which indicated that the sensor possesses an excellent linearity. Moreover, the response time of is 21.78 s. The results suggest that the proposed sensor exhibits a good repeatability and stability and holds considerable potential in fields such as environmental monitoring and chemical analysis.