A device structure whereby an excellent tunable laser does not require a highly complicated wavelength control circuit, and delivers a high output, and excels in wavelength stability; and a structure of an optical module incorporating the laser; and a method for manufacturing the laser and the optical module. The wavelength of the laser is varied continuously with the single electric signal in the tunable wavelength range by setting the laser resonator length, the electric driving conditions of the laser at the time of tuning the wavelength, and the operating temperature to be in appropriate ranges, respectively. The device structure whereby improved laser gain, decreased electric resistance, and reduced heat resistance is attained by setting the waveguide width, particularly at part of or all parts of the laser resonator waveguide, to a wide width that allows multiple transverse modes to be sustained and setting both the width and the laser resonator length to appropriate values, respectively. By employing a self-image-formation effect resulting from the multi-mode interference effect, mode conversion loss in the laser resonator is reduced and, connection with an optical fiber is facilitated because the light intensity distribution at an emitting facet of the laser becomes a single-peaked lowest-order mode.