A method of generating a non-zero frequency spacing between two laser diodes, using four-wave mixing, mediated by injection-locking in order to potentially realise THz radiation is presented. Four distinct regimes of operation are observed, including a periodic response, where the oscillations are simply related to the relaxation oscillations of the system; a periodic response with multiple frequency components, where there is a harmonic relationship between the fundamental frequency and the other frequency components; a chaotic response, where the apparent frequency components are not harmonically related; and finally the locking condition between the lasers, where a continuous wave operation is apparent. A clear linear relationship between the frequency offset and the detuning frequency while operating under the locking condition is observed, and there is a clear transition in behaviour on either side of the locking region, where the system shows a nonlinear behaviour. For higher
injection rates, locking is only observed at either end of the locking region, and from this arises a so-called generalised locking region.