Based on rigorous coupled mode theory, a theoretical model is established for studying the optical characteristics of
long-period fiber grating (LPFG) coated with the sensing thin films. The vector components of the electric field and the
local intensity curves for the lowest order cladding mode are plotted to study the field distribution of cladding mode. It is
found that the transverse field components of HE11 cladding mode are approximately 102 times larger about than the
longitudinal field components, and the low order HE modes have a larger proportion of intensity localized in the core
than the low order EH modes, just like the double-clad LPFG. Further, the influences of the sensing film optical
parameters and grating structure parameters on the attenuation peak of transmission spectra are analyzed. Data
simulation shows that the sensitivity to the refractive index of sensing films is predicted to be more than 10-7. The
optimal design parameters of the LPFG film sensor for higher sensitivity are ascertained by plotting the contour of the
sensor sensitivity. Experimentally, the sol-gel derived SnO2 film LPFG was prepared, and a preliminary gas-sensing test
for detection of C2H5OH was performed. The results indicate that the LPFG film sensor with structure optimization has
higher sensitivity, and the detection sensitivity is available to 10-1ppm on the condition of optimum optical parameters.
With the advantages of both film sensors and fiber sensors, the coated LPFG sensor has a wide and promising application
prospect in process analytical chemistry, environmental monitoring, and biochemical sensing.