Refractometric optical fiber platforms based on reflective long-period fiber gratings

We present a comparative assessment of several refractometric optical fiber platforms based on reflective long-period fiber gratings (RLPGs), which was fabricated by combining the long-period gratings with the different fiber optic reflectors such as fiber optic retroreflector, Faraday rotator mirror, and Sagnac fiber loop mirror. In the experiment the refractive index (RI) of liquid was measured with RLPGs. It was found that the reflection spectrum remained the resonant dip without any interference fringes and the resonant wavelength appeared obvious blue shift with the increase of external RI. The spectral depth was reduced about 20 dB after a fiber optic reflector was configured. The simulation result of the resonance wavelength change with the refractive index of the liquid was also given. Meanwhile, the effect of ambient temperature has been considered. The variation in temperature often makes the resonant wavelength changes almost without changing the intensity of resonant dip. The refractometric optical fiber platforms with the configuration of RLPGs had several advantages such as longer sensing distance, reflective operation mode and non-interference fringe. Combined with the advantages, the sensor structure can not only be applied to measure the RI of transparent gas and liquid, but also be widely used to the measurement of toxic chemical liquid based on the fiber characteristic of resistance to hostile environments, especially far away from the toxic source.