▎ 摘　　要

In this letter, we develop two types of refractive index based sensors using single-mode fiber (SMF-28) to sense hazardous toxic gases in underground mines. The first sensor design is simply a graphene-oxide (GO) deposited long period grating (LPG) and second sensor consist of two LPGs with an interspacing distance of 1 cm (Delta l) acting as Mach-Zehnder interferometer (MZI). In addition, we also confirm GO deposition on the SMF using ultraviolet-visible spectroscopy, atomic force microscopy (AFM), and field emission scanning electron micrograph (FESEM). Our experiment results show that the sensitivity of two-LPGs based fiber optic sensor (SLPG-2 = 76.32 dB/RIU) is higher as compared to the single LPG sensor (SLPG-1 = 54.65 dB/RIU). Hence, reported graphene oxide deposited long period grating sensor possess better sensitivity and can detect maximum 1.52 x 10(-5) index variation in the optical telecommunication band.