Experimental investigation of intensity noise in injection locked mid-infrared quantum cascade lasers

The intensity noise properties of injection locked mid-infrared quantum cascade lasers are experimentally investigated. The injection locking is realized below and near the threshold of the free running slave laser, resulting in an efficient technique to achieve low noise operation. It is found that below threshold the locking characteristics (locking range shape and bandwidth) are different in comparison to those above threshold. Furthermore, an alternative injection locking realization is also investigated: injection locking into longitudinal side modes of the slave laser. Here, similar characteristics were observed, however, with the potential to achieve even higher relative intensity noise reduction suppression with respect to the quantum noise limit. The measurements are confirmed by numerical simulations with a travelling-wave model which takes into account the multi-mode spectrum of the slave laser and the spectral profile of the material gain. The experiments give the perspective for the achievement of the relative intensity noise reduction of the slave laser of up to 10 dB (above threshold) and up to 20 dB (below threshold) in comparison to the free running slave laser noise level.