Patent 8005122 was granted and assigned to NEC Corporation on August, 2011 by the United States Patent and Trademark Office.
A mode-locked laser includes a two-electrode semiconductor laser (3) including a saturable absorber section (31) and gain section (32), and an optical waveguide (4) formed outside the gain section (32) of the two-electrode semiconductor laser. The optical waveguide (4) includes a ring resonator (43) having a resonator length which is an integral fraction of the total resonator length of the mode-locked laser, another ring resonator (44) having bandpass filter characteristics whose bandwidth is narrower than a gain bandwidth, and a reflective surface (46) which reflects, toward the two-electrode semiconductor laser (3), a laser beam having arrived from the two-electrode semiconductor laser (3) via the first and second ring resonators.