An apparatus for output light with a wavelength tunable function, includes: a semiconductor laser diode (LD) including regions to determine an oscillation wavelength, the regions providing respective diffraction gratings and electrodes to control refractive index of the regions; a temperature controller for mounting the LD thereon to control a temperature of the LD; and a controller configured to vary the refractive index of the regions by providing control signals to the electrodes and another control signal to the temperature controller for varying the temperature of the LD to coincide the oscillation wavelength with a target wavelength.