A tunable laser source includes a gain chip, a collimating lens, a grating, a focusing lens and a wavelength selection device. The grating is positioned to be impinged by the collimated light to provide diffracted beams at different diffraction angles whose wavelengths disperse in accordance with the diffraction angles. Those diffracted beams are then brought to separate focal points on the wavelength selection device through the focusing lens. The wavelength selection device includes a light shield with a single slit formed thereon and a mirror. The single silt allows the mirror to reflect only one diffracted beam back to the gain chip so as to maintain the oscillation of the tunable laser source at the wavelength corresponding to the diffraction angle of the only one focused diffracted beam.