Ultrahigh-speed multiquantum well-distributed feedback semiconductor lasers

Abstract

With regard to a 1.55-μm InGaAsP multiquantum well (MQW) distributed feedback semiconductor laser which is promising for its applications to high-capacity, long-distance optical fiber communication systems with transmission rate above 10 Gb/s, the relaxation oscillation frequency and nonlinear damping phenomena determining the high-speed capability and the amount of chirping determining the long-distance capability are examined both theoretically and experimentally. First, by optimizing the number of wells in the MQW active layer and the barrier layer thickness, the relaxation oscillation frequency has been increased to 1.6 ∼ 1.7 GHz/√mA, which is almost twice the conventional value. A distinct xL dependence of the chirping at 10 Gb/s modulation has been clarified and explained by the spatial hole burning effect in the axis direction. A minimum chirp width of 3.4 A has been obtained at 10 Gb/s large swing modulation, which is the world record. Furthermore, the physical mechanism of the nonlinear damping phenomenon has been considered from the dependences of the damping K factor and gain saturation coefficient on laser structure and oscillation wavelength and has been explained by the spectral hole burning theory. Finally, p-type doping of the MQW active layer has been shown to be effective as a means to increase the maximum modulation bandwidth which is limited by the damping phenomenon, and the modulation above 40 GHz has been shown to be possible.