High sensitivity optical fiber sensors for simultaneous measurement of methanol and ethanol

High sensitivity volatile organic compounds (VOCs) sensors based on a tapered small core single mode fiber (TSCSMF) and a microfiber coupler (MFC) are reported. The TSCSMF had a waist diameter of ∼5.1 μm and the MFC had a waist diameter of ∼1.9 μm each and both were fabricated using a customized microheater brushing technique. Silica based materials containing immobilized Nile red prepared by sol-gel method with two different recipes (recipe I and recipe II) are investigated. Initially recipe I based coating materials were applied to the surfaces of the TSCSMF and MFC. The experimental results show that the sensor based on an MFC shows much better sensitivities of −0.130 nm/ppm and −0.036 nm/ppm to ethanol and methanol than those of the TSCSMF based sensor. The corresponding minimum detectable concentration change of the MFC based sensor are calculated to be ∼77 ppb and ∼281 ppb to ethanol and methanol respectively. Both sensors are demonstrated fast response times of less than 5 min, while the recovery times varied from 7 min to 12 min. In addition, another TSCSMF based sample (∼7.0 μm) coated with a mixed layer of sol silica and Nile red prepared by recipe II was fabricated to achieve simultaneous measurement of ethanol and methanol, employing a second-order matrix approach.