The extra differential gain enhancement in multiple-quantum-well lasers

Abstract

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 ...

Low-Threshold, High-Power On-Chip Tunable III-V/Si Lasers with Integrated Semiconductor Optical Amplifiers

“…Nevertheless, this choice imposes a trade-off on the number of MQWs in the active region since the optimum III-V stack may differ for the laser and the SOA. From the laser's perspective, a relatively large number of MQWs leading to a high differential gain (a) and a comfortable gain margin may be desirable to target applications where directly modulated lasers are needed, or for high temperature operation [33]. On the other hand, the saturation power (P sat ) decreases with the number of MQWs, which is detrimental for the SOA [34].…”

The effects of carrier transport phenomena on the spectral and power characteristics of blue superluminescent light emitting diodes

“…where n w is the number of the quantum wells and N t is the total electron density in the SCH-MQW region defined as [10,31,32] …”

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“…The strain in a QW also increases the dierential gain, but it also increases the nonlinear gain coecient making the damping factor relatively unchanged (Ralston et al 1993). The heterostructure band oset in the QW should be large to obtain large dierential gain (Zhao et al 1992) and small damping factor, but too large a band oset will result in carrier trapping and injection heating which will lower the bandwidth (Tsai et al 1995). To enhance the high speed laser performance, the number, strain, and band oset of MQW devices should be optimized together with other parameters such as well size and doping (Uomi et al 1990).…”

“…It is generally believed that spectral hole burning and carrier heating are the two main causes of gain suppression in lasers (Rideout et al 1991;Zhao et al 1992;Girardin et al 1995). Hot carriers are produced as the laser is pumped harder.…”