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An external-cavity semiconductor laser with a 50-dB sidemode suppression ratio and a 0.05-nm spectral bandwidth is realized by using a Bragg grating in a multimode fiber (MMF). With the MMF, a coupling efficiency of 90% and a slope efficiency of 0.12 mW/mA are obtained, compared to a coupling efficiency of 50% and a slope efficiency of 0.05 mW/mA with a standard single-mode (SM) fiber. The alignment tolerance for the MMF is also improved in contrast to the SM fiber. The lasing wavelength can be stably locked at one of the Bragg wavelengths of the fiber grating in large ranges of injection current and temperature under the static and dynamic states.