A simplified analysis of direct intensity modulation of quantum cascade lasers

Although it has been reported that quantum cascade lasers (QCLs) can theoretically achieve terahertz direct intensity modulation (IM) bandwidth, simple explanation in terms of a second-order system model of interband lasers is unsatisfactory. This paper uses simplified rate equations to analyze IM of QCLs. For fast carrier removal rates, the IM response does not have the resonant behavior of IM response of interband lasers. An approximate formula for IM bandwidth is presented. The bandwidth tends to a constant value of the order of 100 GHz with increase in light power. For slow carrier removal rates, associated with large bias currents, a peak can appear in IM response for reasons entirely different from those for the peak in IM response of interband lasers. Slow removal rates can also raise the 3-dB frequencies to terahertz values. However usable terahertz bandwidth cannot be attained even theoretically and practical bandwidths may be much less than 100 GHz.