Transport Effects On Modulation Characteristics Of Quantum Well Lasers

Abstract

Strained Quantum well lasers were theoretically expected to have very high modulation bandwidths, around 90 Ghz, owing to their high differential gain compared to bulk double heterostructure DH lasers. Experimentally it was found that multiple quantum well lasers have modulation bandwidths lower than half the expected value, while single quantum well SQW lasers have bandwiqths even less. This reduction in bandwidth is suggested to depend on several factors, the non-zero capture time of the carriers in the well, the emission time of carriers out of the well and the transport effects of • carriers specially holes, in wide undoped separate confinement heterostructure SCH barriers. A model is constructed which takes into consideration the above three factors simultaneously. This model can also account for the effect of placing the QW away from the center SCH to study the relative effect of the transport of holes against electrons. The model is based on continuity equations for electron and hole currents, and the rate equations which describe photon and carrier dynamics with the gain compression included. A linearized small signal single mode solution for these coupled equations is obtained analytically. The frequency response results• are analyzed and compared to published experimental work. The results show that the capture time, although very small around 1ps, cannot be neglected as it is multiplied by large weighting factor that depends on the width of the SCH, and affects both the -3 dB modulation bandwidth BW and the damping coefficient of the laser response. On the other hand the transport affects only the BW and rather less dominantly and can be neglected for small SCH width. The model also predicts that damping depends on the ratio of capture to emission times, in accordance with previous work.