Effects of Asymmetric Grating Structures on Output Efficiency and Single Longitudinal Mode Operation in $\lambda/4$-Shifted DFB Laser

We report on the design of the
$\lambda/4$
-shifted DFB laser that can provide the high output efficiency as well as the stable single-mode operation. In particular, we investigate the effects of the asymmetry in the grating structure on the lasing characteristics of the
$\lambda/4$
-shifted DFB laser theoretically and experimentally. The steady-state and mode-stability analyses are performed for the
$\lambda/4$
-shifted DFB lasers having the asymmetric phase shift (APS) and asymmetric coupling coefficient (ACC) grating structures by numerical simulations. Although the
$\lambda/4$
-shifted DFB laser fabricated with APS structure is found to be capable of providing high output efficiency, its lasing mode is relatively unstable due to the increased longitudinal spatial hole-burning. On the other hand, by using the ACC grating structure, we can significantly improve both the output efficiency and single-mode stability. To verify these simulation results, we fabricate both types of
$\lambda/4$
-shifted DFB lasers having APS and ACC grating structures. The measured data agree well with the simulated results. For example, when we fabricate the DFB laser with the ACC structure and coupling coefficient ratio of 0.55, the output power ratio between the front and rear facets of
${\sim}{2.5}$
and the side-mode suppression ratio (SMSR) of
${>}{\rm 50}~{\rm dB}$
can be achieved as long as the operating current is in the range of 60 mA
${\sim}{\rm 200}~{\rm mA}$
.