Influence of blue-shifted energy in the spectrum for 1.5 μm AlInGaAs/InP MQW DFB lasers on dispersion penalty in 25 Gbit/s systems

Chirp or wavelength shifts in distributed feedback (DFB) lasers have been the limiting factor in high bit rate transmission systems. Furthermore, dynamic chirp in direct modulation schemes results in pulse broadening because of the chromatic dispersion in the fiber leading to higher power penalties for the link. It is thus crucial to be able to unambiguously quantify the chirp component at the chip level with a quick and inexpensive method. Towards this, we present results of a set of measurements done to understand the extent of correlation between the observed chirp and the system's dispersion penalty. Strained MQW DFB lasers fabricated in the AlInGaAs/InP system were used. The ratio of the conduction to the valence band offsets in the AlInGaAs/InP system is larger than in the InGaAsP/InP system. This is expected to provide better electron confinement minimizing leakage. Coupled with a MQW structure, uniform hole distribution across wells is possible thus enabling higher bandwidth. Different facet coating combinations were also included to provide a rich subset of the device dynamic properties for this study. The tested devices had a lasing wavelength between 1520-1550 nm. We have experimentally shown the existence of a correlation between the transient chirp in the leading edge of the light pulse and the observed dispersion penalty under 2.5 Gbit/s PRBS modulation. This offers a cost-effective solution to screen chips at the chip level without incurring costs associated with module level components. It is important to note, however, that this is a necessary but not sufficient screen due to issues like mode partition noise and timing jitter, which affect the overall power penalty.