Tunable light source and control method

A method for controlling an oscillation light in a wavelength variable light source comprising:

• a multi-mode interference waveguide (MMI waveguide) having an M×N port configuration (M is one or more integers, N is two or more integers);
• N reflection type delay lines connected to the N port side of the MMI waveguide respectively;
• and an optical gain waveguide connected to at least one port on the M port side of the MMI waveguide;
• the method comprising:
• detecting an intensity of light from two or more ports the M port side of the MMI waveguide, excluding the at least one port, at the oscillation wavelength of the oscillation light, the two or more ports not outputting the oscillation light generated by the optical gain waveguide;
• and
• generating a signal for controlling side mode suppression ratio (SMSR) of the oscillation light based on a magnitude relationship between intensities of the light from the two or more ports,
• wherein the signal is further determined based on a sum of intensities of each light from the two or more ports.