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 similar to 5.1 mu m and the MFC had a waist diameter of similar to 1.9 mu m 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 similar to 77 ppb and similar to 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 (similar to 7.0 mu m) 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.