Direct gigabit modulation of injection lasers - Structure-dependent speed limitations

Samples of a large variety of quaternary 1.3- and 1.5-μm injection lasers were directly modulated at bit rates up to 2 Gbit/s using a drive circuit known to have a flat response up to speeds more than twice this high. The resulting optical signals were detected and analyzed for eye degradation and bit error rates. It was found that some lasers were capable of modulation at rates exceeding 2 Gbit/s without degradation while others exhibited significant eye closure at bit rates as low as 300 Mbit/s. A clear correlation was found between high-speed capability and the absence of a current blocking junction. That is, the lasers capable of the fastest modulation were of the simplest geometries: ridge guide and oxide stripe. Buried heterostructure (BH) and buried crescent (BC) lasers both showed significant speed limitations. The modulation bandwidths of the various lasers were also investigated using small-signal sinusoidal modulation and the measured half-power bandwidths were consistent with the large-signal modulation results. The shape of typical modulation transfer characteristics is consistent with a simple rolloff with products of ≳ 400 ps for buried crescent lasers, ∼ 150 ps for buried heterostructure lasers, and < 60 ps for ridge guide lasers. These results apply also to samples which had been processed to form cleaved-coupled-cavity (C³) lasers for mode stabilization.