| 摘要 |
PROBLEM TO BE SOLVED: To solve a problem, in a conventional DFB laser element having a diffraction grating disposed on the surface, that it is difficult to satisfy two contradictory conditions: to set a normalized coupling coefficient KL to a suitable value and to achieve good high speed modulation characteristics. SOLUTION: A main diffraction grating and a sub-diffraction grating are coupled with wave-guided light in such a manner that propagation in a second-order transverse mode of the wave-guided light propagating in an optical waveguide when both the main diffraction grating and the sub-diffraction grating are disposed is suppressed more than propagation in the second-order transverse mode of the wave-guided light propagating in the optical waveguide when only the main diffraction grating is disposed. A coupling coefficient of a basic transverse mode of the wave-guided light and the sub-diffraction grating is smaller than that of the second-order transverse mode and the sub-diffraction grating. The main diffraction grating has the same period as that of the sub-diffraction grating. The main diffraction grating and the sub-diffraction grating have a periodic refractive index distribution in the propagation direction of the wave-guided light. The refractive index distribution of the sub-diffraction grating has a phase shift of 180° from that of the main diffraction grating. COPYRIGHT: (C)2011,JPO&INPIT
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