To provide a complex coupling type distributed feedback system semiconductor laser, keeping high single wavelength oscillation characteristics and making outgoing light from the end surface of an oscillator asymmetric in the fore direction and the rear direction while suppressing holeburning due to the introduction of a phase shift.
When the length of a first grating 18 before a phase shift region 20 is shown by L1 and the length of a second grating 19 behind the region 20 is shown by L2, for examples, the complex coupling factor of the first grating is defined as k1=ki1+ikg1. In this case, ki1 shows a refractive index coupling factor, and kg1 shows a gain coupling factor. When the complex coupling factor of the second grating is defined as k2=ki2+ikg2, ki1≠0, kg1≠0, ki2≠0, kg2≠0 absolute value K1×L1> absolute value k2×L2>0. In this case, ki2 shows the refractive index coupling factor, and kg2 shows a gain coupling factor. When αth-sh shows the threshold gain of a mode generated in a stop band due to a phase shift and αth_ex shows the threshold gain of a mode outside but nearest to the stop band, the size of the absolute value of K1 and L1 as well as the absolute value k2 and L2 are adjusted so as to satisfy αth_sh>αth_ex.
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Tadahiro Mitsuishi
Yasuyuki Tanaka