the present invention provides a short resonator laser capable of high - speed operation with a low threshold value current and an element structure realizing a wavelength changeable laser having an excellent wavelength stability . a laser resonator waveguide is formed in such a manner that its lateral width is set to a wide value allowing lateral - multi mode at a part or the entire portion of the waveguide . this enables enhancement of a laser gain and reduction of electric resistance and thermal resistance while keeping the aforementioned characteristics of the short resonator laser . here , by using self - focusing effect as a result of multi - mode interference effect , it is possible to reduce the mode conversion loss in the laser resonator and the light intensity distribution at the laser emitting end becomes a single - hill lowest order mode . thus , it is possible to obtain a structure appropriate for connection with an optical fiber .