Nonlinear gain and output power characteristics of quantum-well sources

Abstract

The gain saturation in quantum-well heterostructures under the change in the populations of the subband levels is described in detail. The nonlinearity parameters are introduced and their change with the pump, radiation frequency, temperature, polarization, and the quantum-well level distribution is examined. The main attention is given to quantum-well heterostructure sources based on III-V compounds, including GaInAsSb, GaInAs, GaInAsP, GaAs, and GaInN, which emit in the spectral interval from the mid-infrared to visible light.For description of the emission dynamics in different quantum-well (QW) heterostructure systems (lasers, amplifiers, superluminescent diodes, photonic crystals, etc.) it is necessary to take into account saturation effects of the gain in the active region. The nonlinear gain is accompanied by nonlinear refraction and hole burning processes [1][2][3][4][5].In the paper, the gain saturation in QW heterostructures under the change in the populations of the subband levels is described in detail. The nonlinearity parameters are introduced and their change with the pump, radiation frequency, light intensity and polarization, temperature, and QW level distribution is examined. The main attention is given to QW heterostructure sources based on III-V compounds, which emit in the spectral interval from the mid-infrared to visible light.As a rule, the nonlinear gain is described in the phenomenological manner [6] and dependence of the nonlinearity parameter on the pump, frequency, temperature, and electron state structure is not considered. Generally, the gain saturation in QW heterostructures connected with the change in the population of the subbands depends on the semiconductor energy gap, temperature, and QW width in the active region. Two limit cases for analysis of the dispersion of the nonlinear gain can be took into consideration, i. e., the direct optical transitions and transitions with no the k-selection rule. The last approach is attributed qualitatively to spectral broadening effects [7]. For modeling the optical transitions in QWs and evaluating the nonlinearity parameter, it is possible to use the self-consistent two-band model [8,9].In the general case, the gain coefficient follows, in dependence on the radiation flux in the active region, the law [10] k = k 0 /(1+αS), where k 0 is the initial gain (including the light confinement in the QW source waveguide), α is the nonlinearity parameter, which depends on the frequency ν, quasi-Fermi level difference ΔF 0 , and sheet photon density S. There are three regions of the gain saturation. At low fluxes, k ≈ k 0 (1-α 0 S), where the initial nonlinearity parameter is α 0 . When the gain varies by a factor of two (k ≈ k 0 /2), it is worthwhile to use a middle value α 1/2 . At high powerful radiation the gain drops as k ≈ k 0 /α ∞ S, where the nonlinearity parameter is equal to α ∞ . Generally, the relation α 0 < α 1/2 < α ∞ is fulfillment. For the absorption saturation, the inverse relation between the parameters α 0 , α 1/2 , and α ∞ exists...