Design of long-period fibre grating refractometric sensors with linear transfer function by a genetic algorithm

We describe a new method for the design of optical fibre refractometric sensors based on non-uniform Long-Period Fibre Gratings (LPFGs). An essential part of this method is a specialized genetic algorithm with the properly designed operators that perform the theoretical synthesis of a necessary nonuniform LPFG profile. Such a profile yields the linear frequency response of the sensor to the refractive index of the surrounding medium. A distinctive feature of the designed LPFG is a linear variation of the core and cladding refractive index along the LPFG length. We also present a new mathematical model of nonuniform LPFG, which allows one to take into account the effect of LPFG sections of an average refractive index lower and higher than that of the surrounding medium. A numerical example illustrates the characteristics of the synthesised LPFG.