Dynamic Longitudinal Mode Spectral Behavior of Laser Diodes Under Direct High Frequency Modulation

The steady state longitudinal mode spectrum of semiconductor lasers has been extensively studied, and major observed features can be understood in terms of modal competition in a common gain reservoir. It was generally agreed that gain saturation in semiconductor lasers is basically homogeneous. Thus, a well-behaved index-guided laser should oscillate predominantly in a single-longitudinal mode above lasing threshold [41,42]. This has been verified extensively in semiconductor lasers of many different structures. It was also recognized that a single-mode laser will not remain single-mode during turn-on transients and high-frequency modulation. This can be predicted theoretically from numerical solutions of the multimode rate equations [43]. The optical spectrum of a semiconductor laser during excitation transient has been observed by many researchers [44–50]. It was generally observed that when a laser is biased at a certain dc current and excited by a current pulse, the relative amplitude of the longitudinal modes at the beginning of the optical pulse is essentially identical to the prepulse distribution. Depending on the laser structure, it will take ̃0.5–5 ns for the laser to redistribute the power in the various longitudinal modes to that corresponding to the CW spectrum at the peak of the current pulse.