Temperature sensor based on self-interference of a single long-period fiber grating

A novel temperature sensor device based on a conventional long-period fiber grating but having an improved sensing resolution is presented. By forming a reflector at one cleaved end of the fiber embedding a long-period gratin g, a fine interference fringe pattern was obtained within the conventional broadband resonant spectrum of the grating. Due to the fine internal struct ure of the reflection spectrum of the proposed device, the accuracy in read ing the temperature-induced resonant wavelength shift was improved. The for mation of the self-interference fringe is analyzed and its properties are d iscussed in detail. The performance of the proposed device is analyzed by m easuring the resonant wavelength shift of the device placed in a hot oven u nder varying temperature. The rate of the fringe shift is measured to be 55 +/-1pm/degrees C. The rms deviation is 10 pm over a 100 degrees C dynamic r ange, which corresponds to 0.2 degrees C in rms temperature deviation. The thermal variation of the differential effective index of the fiber is calcu lated to be (0.3 +/- 0.1) x 10(-6)/degrees C by comparing the analytic calc ulations with the experimental results. The interference fringe shift is re vealed to be inversely proportional to the differential effective group ind ex of the fiber, which implies that the shifting rate strongly depends on t he type of fibers and also on the order of the involved cladding mode.