Tunable laser

A wavelength tunable laser comprising a filter region having a wavelength selection function on light from a gain region, wherein

• the filter region is a Sagnac interferometer that functions as a loop mirror, and includes two ring resonators,
• the ring resonator has two optical couplers, and first and second curved waveguides connecting the two optical couplers,
• each of the two optical couplers is configured to receive input of the light from the gain region through an input-output port, to split the light into light of a resonant peak and light except light at a resonant peak wavelength, to couple the light of the resonant peak to a bar port of the input-output port, and to couple the light except the light at the resonant peak wavelength to a cross port of the input-output port, and
• the first curved waveguide connects the bar ports of the input-output ports of the two optical couplers, and the second curved waveguide connects the cross ports of ports, of the two optical couplers, that the first curved waveguide is connected to, the wavelength tunable laser comprising
• inside a loop of the ring resonator, two radiation waveguides that are connected to the cross ports of the input-output ports of the two optical couplers and discard the light except the light at the resonant peak wavelength, wherein
• the two radiation waveguides each have a structure that radiates, inside the loop of the ring resonator, the light except the light at the resonant peak wavelength to a front surface or a back surface of a substrate of the ring resonator,
• the two radiation waveguides have a bending radius smaller than that of the first and second curved waveguides, and
• the structures of the two radiation waveguides that radiate are located on an end of their respective radiation waveguide that is opposite the cross ports and are aligned parallel to each inside the loop of the ring resonator.