Dual-purpose optical fiber sensor: relative humidity and ammonia detection

Abstract

We propose a dual−purpose sensor to detect ammonia besides relative humidity with tapered multimode fiber (TMMF) as the sensor structure and silica gel as the sensing layer. In achieving the desired TMMF, we heated and pulled a few pieces of multimode fibers to obtain 9-mm long tapered regions with different diameters in the range of 4−40 µm. Then synthesized silica gel by sol-gel method and passed the tapered section of the TMMFs through drops of silica gel to form the sensing layers. Using the wavelength shift and output power reduction appearing in the fiber transmission spectrum, we determine the sensor response to the alteration of ammonia concentration and relative humidity in the environment. Our experimental results reveal that a decrease in the tapered waist gives rise to an increase in the sensor sensitivity. The same results show that the highest measured sensitivity is 14.8 pm/ppm (for ammonia) and 0.435 dB/%RH (for relative humidity) when we coat the tapered region of a TMMF of 4−µm waist diameter with silica gel. Moreover, the time responses for relative humidity and ammonia sensors are 10 and 32 s, and their corresponding recovery times are 8 and 19.5 s.