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Semiconductor lasers subject to optical injection exhibit both stable locking regime and unstable locking regime. Inside the stable locking regime, various laser performances can be improved. Outside the stable locking regime, the lasers produce rich nonlinear dynamics. However, very few dynamics are known when the QCLs are subject to optical injection. This work reports the measured dynamics of a QCL subject to optical injection, both within and outside the stable locking regime. In the experimental setup, the master QCL is injected into the slave QCL through a unidirectional isolator. Both QCLs are single-mode distributed feedback lasers, and emit around 2182.5 /cm. The lasing threshold of the master laser is 385 mA and the threshold of the slave laser is 425 mA. The detuning frequency between the two lasers is adjusted by tuning the pump current of the master laser, and the frequency tunability is -776.5 MHz/mA. In the experiment, the slave laser is fixed at 435 mA with an output power of 2.9 mW. The slave laser is stably locked by the master laser when decreasing the pump current from 429.2 mA down to 426.1 mA, which corresponds to a detuning frequency from -0.85 GHz up to +1.42 GHz. Meanwhile, the injection ratio slightly reduces from 4.24 dB down to 4.02 dB. Therefore, the stable locking frequency range is as large as 2.27 GHz. Within the stable locking regime, the optical power rises almost linearly with increasing detuning frequency. Interestingly, the injection-locking boundary exhibits clear hysteresis phenomenon when increasing the pump current of the master laser, where the stable-locking range shrinks to 1.03 GHz. Outside the stable-locking regime, the QCL mostly produces period-one oscillations. Besides, we also observe quasi-periodic oscillations and spiking pulsations, although the occurrence of both dynamics is rare.
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