Abstract-By accounting for the unavoidable thermal population of injected carriers in the optical confining layers we find that the use of multiple quantum wells (MQW) as active region actually leads to an extra increase in differential gain. Specifically, the maximum differential gain increases with the number of wells in the quantum-well structures. The transparency current density in the MQW structures does not scale as the number of quantum wells. These conclusions are at variance with presently accepted theory and of major implications for the design of high-speed, low-threshold semiconductor lasers.UANTUM-well (QW) lasers have become the main contenders for high frequency ( > 10 GHz) modulation. This is due mostly to the predicted differential gain enhancement in the QW lasers compared to equivalent bulk devices [ 11. However, high-speed modulation experiments to date show no improvement in the unstrained single quantum-well (SQW) structure lasers [2], [3]. In conventional QW laser structures, a separate confinement heterostructure (SCH) is used to obtain optical confinement of the optical field and electronic confinement of the injected carriers. A reexamination of the theory shows that the neglect of carrier population in the optical confining layers (CL) is not justified in the typical structures used in conventional QW lasers. An inclusion of the CL population in the analysis leads to computed differential gain in room temperature SQW lasers which is lower than that in their bulk counterparts [4]. This reduction is due to the finite (and appreciable) Fermi occupation factor for the states near the (energy) bottom of the CL and to the large density of such states compared to the QW. The expectation of improved dynamics of QW lasers is significantly affected by the carrier population of the higher energy states (state-filling) in the QW structures [5].To date, it is well known that above transparency the differential gain decreases as the carrier density increases in QW structures because of the sublinearity in the gain versus carrier density dependence. This sublinearity is attributed to the flat feature of the two-dimensional step-like density of states in the QW structures. For a given value of modal gain, the differential gain increases as the number of quantum wells ( Nqw) increases because the carrier density associated with one single QW decreases with the increase of Nqw. This is Q The physical reason for the extra increase in differential gain in MQW structures is the following. In a SQW laser the need to obtain a sufficient optical gain (to overcome the losses) forces the Fermi energy Ef to rise toward the top of the quantum well as the pumping level is increased. The confining layer states, say at energy Eel, whose occupation is determined by the factor exp [ -(Ec/ -E f ) / k T ] are thus more heavily populated. This increase in the CL carrier density contributes negligibly to the increase in gain because of the large nonresonant nature of these transitions. This leads to reduced differential ...
Indexing terms: Semiconductor lasers, High-speed optical techniques A record high current modulation efficiency of SGHz/J(mA) has been demonstrated in an ultralow threshold strained layer single quantum well InGaAs laser.
Abstract-The carrier distribution functions in a semiconductor crystal in the presence of a strong optical field are obtained. These are used to derive expressions for the gain dependence on the carrier density and on the optical intensitythe gain suppression effect. A general expression for high order nonlinear gain coefficients is obtained. This formalism is then used to describe the carrier and power dynamics in semiconductor lasers above and below threshold in the static and transient regimes.
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