Current noise and photon noise in quantum cascade lasers

A model for the photon noise and the current noise in quantum cascade lasers is presented. It is shown that compared to diode lasers quantum cascade lasers exhibit much less photon-number squeezing even when the noise in the drive current is suppressed well below the shot-noise value. The theoretical model presented in this paper self-consistently accounts for the suppression of the current noise in electron transport in multiple quantum-well structures due to various electronic correlations. The nature of these electronic correlations is discussed. Mechanisms that make photon-number squeezing difficult to achieve in quantum cascade lasers are discussed. Scaling of the laser intensity noise and the current noise with the number of cascaded gain stages is also described. Direct-current modulation response of quantum cascade lasers is also studied, and it is shown that contrary to the predictions in the literature of terahertz modulation bandwidth for these lasers, bandwidth of almost all quantum cascade lasers that have been reported in the literature is limited by the inverse photon lifetime inside the laser cavity to tens of gigahertz.