Tunable laser

A wavelength tunable laser comprising filter region having 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,
• the first curved waveguide connects or ports of the input-output ports of the two optical couplers, and the second cu connects the cross ports of ports, of the two optical couplers, that the first curve e is connected to,
• inside a loop of the ng resonator, two radiation waveguides are connected to the cross ports of put-output ports of two optical couplers and discard the light except the light at the resonant peak wavelengths, and
• the radiation waveguides are connected, inside the loop of the ring resonator, to a waveguide having an active layer identical to an active layer of the gain region for the laser.