A tunable laser, for continuously changing a laser output wavelength within a specific wavelength range, comprising: a reflective semiconductor optical amplifier, SOA, a grating codirectional coupler, and a reflective microring resonator, wherein: the grating codirectional coupler and the reflective microring resonator are both formed on a silicon base; an anti-reflection film is disposed on a first end surface of the reflective SOA, the first end surface of the reflective SOA being coupled to a first waveguide of the grating codirectional coupler; a second waveguide of the grating codirectional coupler is coupled to the first waveguide; a first grating is disposed on the first waveguide; a second grating disposed opposite to the first grating is disposed on the second waveguide; the first grating and the second grating constitute a narrow-band pass filter; and the second waveguide is connected to the reflective microring resonator, wherein narrow-band means that a filtering spectrum of the narrow-band pass filter has only one main peak in the specific wavelength range whereas a filtering spectrum of the reflective microring resonator is comb-shaped and that the filtering spectrum of the narrow-band pass filter is such that only one main peak with a maximum transmittance exists in a total filtering spectral line generated by light passing through the narrow-band pass filter and the reflective microring resonator. a reflective semiconductor optical amplifier, SOA, a grating codirectional coupler, and a reflective microring resonator, wherein: the grating codirectional coupler and the reflective microring resonator are both formed on a silicon base; an anti-reflection film is disposed on a first end surface of the reflective SOA, the first end surface of the reflective SOA being coupled to a first waveguide of the grating codirectional coupler; a second waveguide of the grating codirectional coupler is coupled to the first waveguide; a first grating is disposed on the first waveguide; a second grating disposed opposite to the first grating is disposed on the second waveguide; the first grating and the second grating constitute a narrow-band pass filter; and the second waveguide is connected to the reflective microring resonator, the grating codirectional coupler and the reflective microring resonator are both formed on a silicon base; an anti-reflection film is disposed on a first end surface of the reflective SOA, the first end surface of the reflective SOA being coupled to a first waveguide of the grating codirectional coupler; a second waveguide of the grating codirectional coupler is coupled to the first waveguide; a first grating is disposed on the first waveguide; a second grating disposed opposite to the first grating is disposed on the second waveguide; the first grating and the second grating constitute a narrow-band pass filter; and the second waveguide is connected to the reflective microring resonator, wherein narrow-band means that a filtering spectrum of the narrow-band pass filter has only one main peak in the specific wavelength range whereas a filtering spectrum of the reflective microring resonator is comb-shaped and that the filtering spectrum of the narrow-band pass filter is such that only one main peak with a maximum transmittance exists in a total filtering spectral line generated by light passing through the narrow-band pass filter and the reflective microring resonator.