The linewidth of a conventional laser is due to fluctuations in the laser field due to spontaneous emission and described by the Schalow-Townes formula. In addition to that, in a semiconductor laser there is a contribution arising from fluctuations in the refractive index induced by carrier density fluctuations. The later are quantitatively described by the linewidth enhancement or alpha factor [C. H. Henry, IEEE J. Quantum Electron. 18 (2), 259 (1982), W. W. Chow, S. W. Koch and M. Sargent III, Semiconductor-Laser Physics, Springer-Verlag (1994), M.F. Pereira Jr et al, J. Opt. Soc. Am. B10, 765 (1993). In this paper we investigate the alpha factor of quantum cascade lasers under actual operating conditions using the Nonequilibrium Greens Functions approach [A. Wacker et a, IEEE Journal of Sel. Top. in Quantum Electron.,19 1200611, (2013), T. Schmielau and M.F. Pereira, Appl. Phys. Lett. 95 231111, (2009)]. The simulations are compared with recent results obtained with different optical feedback techniques [L. Jumpertz et al, AIP ADVANCES 6, 015212 (2016)].
Mauro F. Pereira, David O. Winge, Andreas Wacker, Louise Jumpertz, Florian Michel, Robert Pawlus, Wolfgang E. Elsaesser, Kevin Schires, Mathieu Carras, and Frédéric Grillot, "Nonequilibrium Green's functions theory for the alpha factor of quantum cascade lasers
(Conference Presentation)," Proc. SPIE 9934, Terahertz Emitters, Receivers, and Applications VII, 993408 (Presented at SPIE Nanoscience + Engineering: August 28, 2016; Published: 3 November 2016); https://doi.org/10.1117/12.2238619.5167075322001.
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