In long wavelength quantum well lasers the effective electron temperature (T e ) is often a strong function of the pump current and hence the T e correlates with the carrier concentration n in the active region. On the other hand, the material gain g in the active layer depends on both variables, g=g(n,T e ). We discuss a convenient way of analyzing this situation, based on considering the contours of constant gain g on the surface g(n,T e ). This is qualitatively illustrated with two model

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... ples involving quantum well lasers, the long-wavelength quantum well laser with current dominated by the Auger recombination and the unipolar quantum cascade laser. 1998 Optical Society of America OCIS codes: (140.5960) Semiconductor lasers; (230.5590) Quantum-well devices 2. V. B. Gorfinkel and S. Luryi, "Fundamental limits for linearity of CATV lasers", J. Lightwave Technol. 13, 252-260 (1995); http://www.ee.sunysb.edu/~serge/133.html 3. M. Silver, E. P. O'Reilly, and A. R. Adams, "Determination of the wavelength dependence of Auger recombination in long-wavelength quantum-well semiconductor lasers using hydrostatic pressure", IEEE J. Quantum Electron. 33, 1557-1566 (1997). 4. Z. Shi, M. Tacke, A. Lambrecht, and H. Böttner, "Midinfrared lead salt multi-quantum-well diode lasers with 282 K operation", Appl. "High power mid-infrared (OaPm) quantum cascade lasers operating above room temperature", Appl. Phys. Lett. 68, 3680-3682 (1996). 8. Vera Gorfinkel, Serge Luryi, and Boris Gelmont, "Theory of gain spectra for quantum cascade lasers and temperature dependence of their characteristics at low and moderate carrier concentrations", IEEE J. Quantum