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In this paper, the characterization of a thermally tunable fiber optic filter is presented. It consists of two Sagnac interferometers (SIs) connected in series. Each SI contains a high-birefringence fiber (HiBiFi) segment with different length in the loop, which generates a reduction in the bandwidth of the filter transmission peak. In a previous work, a filter composed by two SIs with equal HiBiFi segments connected in series was reported. An experimental bandwidth reduction of approximately 31%, compared with a filter composed by a single SI was obtained. Using the configuration proposed in the present work, a reduction of approximately 52% in the range of 1533 to 1586 nm and a sensitivity of -1.27 nm/degC was obtained.