  Full phase control of THz emitting quantum cascade laser (QCL) combs has
recently been demonstrated, opening new perspectives for even the most
demanding applications. In this framework, simplifying the set-ups for control
of these devices will help to accelerate their spreading in many fields. We
report a new way to control the emission frequencies of a THz QCL comb by small
optical frequency tuning (SOFT), using a very simple experimental setup,
exploiting the incoherent emission of an ordinary white light emitting diode.
The slightly perturbative regime accessible in these condition allows tweaking
the complex refractive index of the semiconductor without destabilizing the
broadband laser gain. The SOFT actuator is characterized and compared to
another actuator, the QCL driving current. The suitability of this additional
degree of freedom for frequency and phase stabilization of a THz QCL comb is
shown and perspectives are discussed.
