Enhanced modulation bandwidth for strain-compensated InGaAlAs-InGaAsP MQW lasers

Strain-compensated (SC) multiple-quantum-well (MQW) lasers were designed using tensile-strained InGaAlAs barrier layers in order to enhance the modulation bandwidth of MQW lasers at 1.55 μm. The design scheme simultaneously ensures the pseudomorphic growth of a large stack of highly strained wells, a uniform hole injection into large number of wells, a large conduction band discontinuity to suppress the carrier overflow effect, and a large differential gain by suppressing the band mixing effect. The SC-MQW structures were processed into a mushroom stripe laser structure to obtain a low parasitic capacitance. The number of wells and the cavity length were optimized to maximize the modulation bandwidth. Both the relaxation oscillation and RC cutoff frequencies increased with reducing the cavity length, and a maximum 3-dB modulation bandwidth of 30 GHz was obtained at a short cavity length of 120 μm for 20-well SC-MQW lasers. Moreover, a high internal quantum efficiency and large differential gain were obtained for the SC-MQW lasers with well numbers of up to 20 as a result of the reduced carrier transport and overflow effects. The differential gain, gain compression factor, and K factor were evaluated experimentally from the modulation characteristics and compared to the theoretical calculation based on the spectral hole burning theory. The observed experimental results were well explained by the model using the identical intraband relaxation times typically used for 1.55-μm bulk lasers