Fluctuations of the laser characteristics and the effect of the index-coupling component in the gain-coupled DFB laser

The normalized oscillation frequency /spl delta/L and threshold gain /spl alpha/L of gain-coupled distributed feedback (DFB) lasers with an AR/AR (symmetric) structure vary significantly with a variation in the corrugation phase at the facets when the facet reflectivity |r| is large, even though the spatial-hole-burning-corrected yield is 100%. For example, the standard deviation for the /spl delta/L, /spl sigma/(/spl delta/L), increases from 0.018 to 0.40 when |r| is increased from 0.03 to 0.30. The magnitude of /spl sigma/(/spl delta/L) and /spl sigma/(/spl alpha/L) are small, however, for the gain-coupled, index-coupled, and complex-coupled DFB lasers when the |r| is kept small. For the HR/AR (asymmetric) structure, these two values are more than an order of magnitude larger than those for the AR/AR structure, except for the /spl sigma/(/spl alpha/L) for the index-coupled laser, even though yields of each of these lasers are relatively large. In the AR/AR structure, the contamination by the index-coupling and gain-coupling components always degrades the device characteristics because of the difference in the phase shifter and when the amount of contamination is small this degradation is particularly severe for the index-coupled laser. Several properties of the index-coupled laser with the HR/AR structure can be improved, however, by introducing the gain-coupling component.