(1) Low threshold current characteristics, (2) high single-mode characteristics, (3) high fr characteristics, (4) high-temperature characteristics, (5) to the wide wavelength range directly modulated light source beyond the modulation speed 10Gb / s that can be enabled, to provide a distributed feedback semiconductor laser extremely short gain-producing area. It is a distributed feedback semiconductor laser 1 which comprises a gain-producing region 30 to generate the gain of the laser beam, and a diffraction grating 13 formed in the interior of the gain-producing region 30. Reflectivity of the front surface 1a as well is set to 1% or less of the end faces of the two front and rear sandwiching the gain-producing region 30, the reflectivity viewed forward from the rear surface 1b side is set to 30% or more. Assuming that the length L of the gain-producing region 30, κ the coupling coefficient of the diffraction grating 13 and 100cm -1 above κ, L is the 150μm or less, it is set respectively. Assuming that gth = (internal loss αi + mirror loss αm) Δα, the threshold gain between modes gain difference, to use a combination of L and κ that Δα / gth is 1 or more.
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