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A new polarization-independent tunable spectral filter based on the combination of a single isotropic acoustooptic cell and a pair of conventional diffraction gratings is suggested and investigated. The filter features narrow bandwidth, frequency shift compensation, and low polarization dependence. Using standard AMTIR-1 chalcogenide glass acoustooptic modulator with frequency 40-77 MHz and ruled diffraction gratings with 600 grooves/mm, a 3-dB bandwidth of 1.1 nm, and a tuning range of 16 nm were obtained. Tuning range can be increased by the use of acoustooptic cell with increased center frequency or diffraction grating of a lower dispersion. Inserting the filter inside a laser cavity, continuous-wave tunable generation of Er-doped fiber laser with maximum output power 3 mW was realized. When a Fabry-Perot etalon (0.166-nm free spectral range, 40% reflectance) was inserted into laser cavity, the linewidth was observed to be less than 0.05 nm.