Wavelength filter and wavelength tunable light source

A wavelength tunable light source comprising:

• a semiconductor optical amplifier configured to emit light;
• and
• a wavelength filter configured to perform wavelength filtering on the light entered from the semiconductor optical amplifier, wherein
• the wavelength filter includes:
• a splitter configured to split the light input from the semiconductor optical amplifier and output first split light and second split light;
• a first optical waveguide that propagates the first split light from the splitter to a first optical coupling unit;
• a second optical waveguide that propagates the second split light from the splitter to a second optical coupling unit;
• a first ring-shaped optical waveguide around which the first split light from the first optical coupling unit circulates;
• a second ring-shaped optical waveguide, around which the second split light from the second optical coupling unit circulates, having an optical path length shorter than the first ring-shaped optical waveguide;
• and
• a third optical waveguide that propagates light between a third optical coupling unit facing the first optical coupling unit in the first ring-shaped optical waveguide and a fourth optical coupling unit facing the second optical coupling unit in the second ring-shaped optical waveguide, and
• a first optical path length between the splitter and the first optical coupling unit in the first optical waveguide is shorter than a second optical path length between the splitter and the second optical coupling unit in the second optical waveguide.