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We present a dual-wavelength single-longitudinal-mode erbium-doped fiber ring laser, incorporating a chirped fiber Bragg grating (CFBG) with a permanent π phase shift and a temporary π phase shift to form two passbands in the stopband of the grating. A thermal head induces the temporary phase shift which can move along the length of the CFBG to tune the position of the phase shift. This enables the wavelength spacing between the two passbands to be tunable thus allowing the microwave output frequency to be tunable when the two output lasing lines are heterodyned at the photodetector. Using this technique, microwave frequencies ranging from 6.88 to 36.64 GHz were obtained.