Single wavelength interrogated refractive index sensors based on leaky mode couplings

A novel long-period fiber grating (LPG) refractive index sensor is presented for the measurements of ambient refractive indices which are higher than that of the fiber cladding. As the measurement parameter, the transmission power of the core mode is interrogated at a single wavelength where the core mode and a leaky mode are phase-matched. Firstly, the couplings of the core mode to leaky modes in the novel structure are analyzed by using complex coupled-mode theory, and then classified into three cases analogous to those in the damped oscillations. The power evolutions of the core mode in the couplings are thus intuitively understood. Based on these, for the first time, we demonstrate, with optimized design parameters the transmission power of the core mode is rather sensitive to the change of a higher ambient refractive index at resonant wavelengths. Then we focus on two optimization objectives. One is to enlarge the operational range while keeping given sensitivity, the other is to enhance the sensitivity within a given operational range. Finally, we demonstrate the operational range of the LPG refractive index sensor can be from 1.46 to 1.7 with a sensitivity of similar to 10(-4), while for a given operational range from 1.455 to 1.465, the sensitivity can be similar to 10(-6), if assuming the dynamic range of the power is less than 30dB and the measurement resolution is 0.01dB for both cases.