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Temperature-dependent chromatic dispersion measurement using long-period fiber grating

A novel method for measuring the temperature-dependent chromatic dispersion of a fiber is proposed, which is based on the interference fringe formed by a pair of long-period fiber gratings (LPGs). The effective index difference between the fundamental core mode and a cladding mode was obtained from the interference fringe of the LPG pair. The order of the involved cladding mode and the core size could be obtained from the general parameters of the fiber; the numerical aperture and the cut-off wavelength. Using the obtained cladding mode order and the Sellmeier equation of silica material for the cladding index, the refractive index of the fiber core was obtained by comparing the measured effective index difference with the calculated one. Since the temperature-dependent Sellmeier equation of the cladding material is already known, the temperature-dependent refractive index of the core can be obtained. Measured refractive index of the core is fitted with a linear combination of the well-known Sellmeier equations of germanium and silica. The concentration of germanium is used as the fitting parameter. The calculated chromatic dispersion is well matched with the one measured with a conventional instrument within 0.2 ps/km/nm in a spectral range of from 1.3 mm to 1.6 mm at room temperature.