Frequency noise reduction of delay-coupled quantum cascade lasers.

This work theoretically investigates the frequency noise and spectral linewidth characteristics of mutually delay-coupled quantum cascade lasers, which are operated in the stable locking regime. We demonstrate that the mutual injection significantly reduces the frequency noise at proper coupling phases. However, the relative intensity noise is insensitive to the mutual injection. Influences of the pump current, the linewidth broadening factor, the coupling phase, and the delay time on the frequency noise are discussed as well. In addition, it is found that the appearance of multiple compound laser modes can deteriorate the frequency noise performance of the lasers.