Control of resonant peak depths of tunable long-period fiber gratings using overcoupling

We present a method to reduce changes in the resonant peak depth of a long-period fiber grating (LPFG) as the resonant band is tuned by varying the external refractive index. We theoretically analyze the effects of the initial coupling strength on the peak depth change as external refractive index is varied. By controlling the initial coupling strength, it is experimentally demonstrated that an optimum peak depth can be obtained over a range of operating wavelengths that will maximize the sensitivity and stability of LPFG based sensors and tunable filters.

Research highlights► Peak depth dependence of LPFGs on the external refractive index is analyzed. ► Controlling the initial coupling strength maximizes the resonant coupling. ► Overcoupling technique enables an optimum peak depth in the most sensitive operating range.