Quantum Cascade lasers (QCLs), relying on intersubband transitions in semiconductor quantum well structures, show very short carrier lifetimes of the order of picoseconds . As a consequence, relaxation oscillations remain over-damped up to modulation frequencies of several tens of GHz , enabling efficient amplitude modulation of the gain medium in this frequency range. These properties make QCLs ideally suited for RF-injection-locking. We demonstrate that the round-trip frequency of THz QCLs, as observed from the beatnote, can be injection-locked by RF-modulating the bias current. Within a certain locking range we observe mutual phase-locking of approximately 20 longitudinal modes for significantly lower RF-powers than in previous studies . Apart from injection-locking, we demonstrate beatnote control by means of an external cavity.
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