1.  Field of the Invention
The present invention relates to a distributed feedback (DFB) semiconductor laser and an optical integrated semiconductor device. 
2.  Description of the Related Art
A known semiconductor laser is described in “Wavelength Tunable DFB LD with High Coupling Coefficient Gratings” written by Nobuhiro Nunoya et al.,  The collected papers of The Institute of Electronics, Information and Communication Engineers (IEICE) General Conference, C-4-20, p. 282, 2007.  This semiconductor laser includes two distributed feedback (DFB) regions having a grating, and a phase shift region.  One of the DFB regions, the phase shift region, and the other DFB region are arranged in that order in the optical axis direction of this semiconductor laser.  In this DFB laser structure, the two DFB regions sandwich the phase shift region.  These DFB regions and the phase shift region are optically coupled.  When a distributed feedback operation is performed in the two DFB regions, the lasing wavelength can be changed within a stopband of the grating formed in the DFB regions by injecting a current into the phase shift region.  The principle through which the lasing wavelength is changed will now be described in the following.  In the semiconductor laser having the above structure, when a current is injected into the phase shift region through an electrode for wavelength tuning provided in the phase shift region, the refractive index of the phase shift region changes.  As a result, the optical length of the phase shift region changes.  Accordingly, when the amount of phase shift is adjusted by changing the current injected into the phase shift region, the lasing wavelength changes within the stopband.