Strong signal analysis of optical nonlinearities in single-quantum-well and double-heterostructure lasers

Based on the numerical strong signal solution of the density-matrix formalism, the nonlinearities of the gain and the refractive index are discussed for both bulk double-heterostructure (DH) and single-quantum-well (SQW) InGaAs lasers. The results show that for both structures the nonlinear gain is well approximated by the recently proposed analytical expression and the modified two-level system approximation up to a range of photon density discussed herein, and the refractive index could either increase or decrease with the photon density depending on the wavelength detuning from the gain peak. The results of a new analytical expression for nonlinear refractive index is in qualitative agreement with the numerical ones; however, significant quantitative difference occurs between these two model results for high photon density values. Due to the more important nonlinear gain in SQW structures, the linewidth enhancement factor increases more rapidly with increasing photon density in these structures than in DH structures.<>