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Many applications, such as fiber Bragg grating characterization, wavelength division multiplexing, etc. require tunable laser sources with one emission line in a certain wavelength range. Generally, such sources are laser systems combined with tunable filters. In this work an all-fiber laser tunable by a filter composed of two Sagnac interferometers in series is described. The filter loops were constructed of high birefringence fibers (HiBiFi) and the filter was thermally tunable with a single transmission maximum in the Erbium gain spectrum. The results showed that for interferometers with 14 cm HiBiFi length a bandwidth reduction of the transmission peak, a single-line laser emission, a tuning sensitivity of approx. -0.8 nm/degC and a spectral full-width at half-maximum bandwidth of 0.03 nm in the range of 1538-1580 nm were obtained.