Dependence of the Refractive Index of a Coating on a Long-Period Fiber Grating on the Initial Coupling Strength

The dependence of the refractive index of a coating on the transmission characteristics of a long-period fiber grating (LPFG) on the initial Coupling strength between core and cladding modes is discussed. The coupling strength is modified by changing the UV-induced refractive index, and the transmission of a LPFG varies like Cos(2)(kappa L), where K, and L are a coupling coefficient and a grating length, respectively. Since the cladding modes coupled from the guided core mode are interfaced with the external environment directly, the coupling strength and the transmission characteristics of a LPFG can be changed readily by the refractive index of a coating material, which induces a variation of both the resonant wavelength and the peak depth. The dependence of the LPFG on the variation of the ambient; index of a coating material is changed by using the initial coupling strength of the grating. We fabricate two types of LPFGs with different coupling strengths and measure their transmission characteristics with changing ambient index. The transmission peak depth of the overcoupled LPFG increases as the ambient refractive index is changed from 1.5 to 1.7. Compared with the sensitivity of the 1st saturated LPFG to the ambient index change, the total of the overcoupled LPFG is improved by changing the coupling strength