CRITERION OF EXTERNAL FEEDBACK SENSITIVITY IN INDEX-COUPLED AND GAIN-COUPLED DFB SEMICONDUCTOR-LASERS TO BE FREE FROM EXCESS INTENSITY NOISE

Sensitivities of index-coupled as well as gain-coupled DFB semiconduct or lasers to external optical feedback are theoretically investigated in this work. The conditions of getting free operation from excess int ensity noise in these two types of DFB lasers are obtained. It is foun d that the sensitivity of external feedback for these two types of DFB laser depends significantly on the structure such as the internal gra ting, the facet reflection, and the phase relation between the grating and the facet. The condition of making a DFB laser less sensitive to external optical feedback is to adjust the lasing wavelength to the Br agg wavelength in both index-coupled and gain-coupled structures. A ph ase-adjusting mechanism such as the lambda/4 shift region in the index -coupled structure is effective to avoid the external feedback.