A new method of measuring the waveguide dispersion factor and the thermo-optic coefficient of long-period fiber gratings

Based on the Mach-Zehnder effect between the core mode and the cladding modes, the interference fringes are formed by a pair of cascaded long-period fiber gratings (CLPFGs). Theoretical analyses show that the spectral spacing and the wavelength of these fringes are functions of the waveguide dispersion factor γ, which is a characterizing parameter to LPFG and with theoretical and applicational significance. By measuring the characteristics of the transmission spectra of CLPFGs, the absolute value of γ can be obtained. At the same time, the thermo-optic coefficient of effective refractive index difference between core and cladding modes, μ, can also be obtained by measured the temperature sensitivity of these fringes. In the experiments, ∣γ∣ and μ were measured by this method to be 0.874 and 4.08 × 10−5 °C−1, respectively, for LPFGs with period of 450 μm and with a HE14 resonant peak at 1554 nm.