Reflection filter device and wavelength-tunable laser device

A wavelength-tunable laser device comprising:

• a laser resonator including a diffraction grating, a ring-shaped waveguide, two arms, a dual-branch portion, and a reflection filter device, each of the two arms being optically coupled to the ring-shaped waveguide, the dual-branch portion including a light input/output port and two branch ports, the light input/output port being configured to allow input and output of light, the two branch ports being configured to allow output of the light input from the light input/output port, the light being split into two, the two arms being connected to the dual-branch ports, respectively, at least one of the two arms being equipped with a phase adjuster and a refractive index changing unit arranged along the phase adjuster, the reflection filter device being optically coupled to the diffraction grating on the light input/output port;
• a gain portion disposed in the laser resonator, wherein:
• the phase adjuster and the refractive index changing unit are arranged in the arms and separated from the gain portion,
• the diffraction grating generates a first comb-shaped reflection spectrum,
• the reflection filter device generates a second comb-shaped reflection spectrum having a peak with a full width at half maximum narrower than a full width at half maximum of a peak of the first comb-shaped reflection spectrum and having a wavelength interval different from a wavelength interval of the first comb-shaped reflection spectrum, and
• the diffraction grating and the reflection filter device are configured such that one of peaks of the first comb-shaped reflection spectrum and one of peaks of the second comb-shaped reflection spectrum are overlappable on a wavelength axis;
• a first refractive index changing unit configured to change a refractive index of the diffraction grating;
• and
• a second refractive index changing unit configured to change a refractive index of the ring-shaped waveguide, wherein one of peaks of the first comb-shaped reflection spectrum and one of peaks of the second comb-shaped reflection spectrum are overlapped on a wavelength axis by using at least one of the first refractive index changing unit and the second refractive index changing unit.