Optical noise properties of mutually coupled quantum cascade lasers

This work theoretically investigates the optical noise characteristics of mutually-coupled quantum cascade lasers, which is achieved through the small-signal analysis of a set of rate equations with Langevin noise sources. It is shown that the stable locking range of the mutually-coupled lasers is on the order of several GHz. Within the stable locking range, the inphase mutual injection hardly changes the relative intensity noise of the lasers. In contrast, the frequency noise and the spectral linewidth of the coupled lasers can be reduced by about 10 dB.