A resonant optical fiber gyroscope (RFOG) is an optical sensor that measures the angular rate of rotation proportional to the difference between the resonant frequency of the clockwise and counterclockwise optical paths generated by the Sagnac effect. A narrow linewidth laser with a continuously adjustable center frequency is a key element for developing RFOG, and the frequency noise characteristics of the laser directly affect the performance of this gyroscope. In this study, the frequency noise power spectral density of different types of lasers and the lasers before and after frequency locking was measured using an unbalanced Mach-Zehnder interferometer system based on a 3×3 coupler. The 1/f and white noise characterization coefficients of different types of lasers and the lasers before and after frequency locking were obtained. The study results can provide an important reference for selecting RFOGs with different accuracies and optimizing resonant frequency servo loops.