In-Line Mach–Zehnder Interferometer and FBG With Smart Hydrogel for Simultaneous pH and Temperature Detection.

A composite optical fiber sensor based on an in-line Mach–Zehnder interferometer (MZI) with fiber Bragg grating was proposed. The MZI was fabricated by two standard single-mode fibers and one section of photonic crystal fiber. And the sensing region was coated on hydrogel to measure pH and temperature. When the pH around the sensor changes, the swelling (shrinking) of the hydrogel results in the variations of the refractive index, which causes variations in the transmission spectrum. Due to resonance wavelengths of the two structures that experience different responses to the ambient pH and the temperature, these parameters can be measured simultaneously. The experimental sensor can indicate the pH change at the wide measurement range (2–12), and the pH and temperature sensitivities are −0.27 nm/pH and 0.088 nm/°C, respectively. Because of the wide pH measurement range and excellent stability, the proposed sensor has a broad application of the biochemistry measurements. [ABSTRACT FROM AUTHOR]