PROBLEM TO BE SOLVED: To provide a direct-modulation semiconductor laser capable of obtaining a favorable eye opening.;SOLUTION: The laser includes a first semiconductor laser 128 for outputting a first signal light 117 and a second semiconductor laser 127 for outputting a second signal light 113 including a wavelength absorbed into an active layer 122 of the first semiconductor laser 128 independently of each other, wherein the first semiconductor laser 128 and the second semiconductor laser 127 are serially disposed such that the second signal light 113 is incident upon the active layer 122, an NRZ signal current 129 of a transmission rate X1 is input, a sinusoidal signal current 111 of a frequency X2 using a one-bit time width of the transmission rate X1 as one cycle is input, and the phases of the NRZ signal current 129 and the sinusoidal signal current 111 are controlled to situate a local maximum point of the intensity of the second signal light 113 which will have a sinusoidal wave at the center of the time width of each bit of the first signal light 117.;COPYRIGHT: (C)2011,JPO&INPIT
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