Synchronization of mutually delay-coupled non-identical quantum cascade lasers

The rate equations for two delayed coupled quantum cascade lasers (QCLs) are investigated analytically in the limit of weak coupling and small frequency detuning. Our analysis is motivated by the observations of Bogris et al (IEEE J. Sel. Top. Quant. 23, 1500107 (2017)) who found higher sensing performances when one laser is operating close to threshold. We show that the previously determined coupled delayed Adler equations fail to describe this particular limit and derive equations for a single mode optically injected laser where the delay is no more present into the arguments of the dependent variables.