Semiconductor Lasers by Junji Ohtsubo

Author:Junji Ohtsubo
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

Fig. 8.41Numerical plots of the spatio-temporal output power of a ten-stripe laser array for two different injection currents. a I = 34 mA: periodic state and b I = 44 mA: chaotic state. The threshold current for each stripe is I th = 36 mA. The stripe width is w = 5.0 μm and the stripe separation is s = 5.8 μm [after Merbach et al. (1995); © 1995 APS]

8.6 Quantum-Dot Semiconductor Lasers

8.6.1 Theory of Quantum-Dot Semiconductor Lasers

Quantum-dot semiconductor laser is an important light source for high-speed data communication applications, since it is insensitive to temperature variations and optical feedback, and provides features of high modulation bandwidth and low chirp. The device structure of a quantum-dot laser is almost the same as common narrow-stripe edge-emitting semiconductor lasers. However, in the active layer, small quantum dots as small as nano-meter size are fabricated, usually by a self-assembled method of the crystal. A common quantum-dot semiconductor laser is not a single layer device, but several thin quantum-well layers with quantum dots are piled up in the active region. An electron and a hole are captured in a single dot and behave like zero-dimensionally confined particles with a fixed energy state. Thus, the light emitted from a quantum-dot semiconductor laser shows a high coherence state. The crystal growth, the device characteristics, and their theoretical treatments have been given in a book of Sugawara (1999). Figure 8.42 is an example of a self-formed quantum-dot structure of InAs in an active region of GaAs (Shoji et al. 1997). Figure 8.42a show a plane view of quantum dots. It is desirable that quantum dots stand in a line on the wetting quantum-well layer and the size of each quantum dot should be the same. However, the control of the crystal growth is very difficult. In spite of irregularities of the size and the position, the laser can emit a high quality beam. The grown quantum dots in Fig. 8.42b are 20 nm in diameter and 5 nm in height. The areal coverage is about 10%.

Fig. 8.42Example of a self-assembled quantum-dot structure. a Plane view and b cross-section view [after Shoji et al. (1997); © 1997 IEEE]

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