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The inner cladding mode in a photonic crystal fiber for temperature- and refractive index-independent strain sensing applications

Fiber Bragg gratings with strong resonance peaks for both Bragg and cladding modes are made in photonic crystal fiber modified with a germanium doped core. Experimental results for strain, temperature and refractive index sensitivities of fiber Bragg gratings are reported. We show the existence of an inner cladding mode that has the largest coupling from the core mode and that is insensitive to surrounding index changes. The core mode, inner cladding mode and outer cladding modes all have the same temperature sensitivity. By tracking the cladding mode resonances shifts relative to the core mode, a temperature and index insensitive strain sensor can be made.