Abstract:Based on the three-level rate equations, the relative intensity noise and modulation characteristics of free-running operation, master-slave injection and optically mutual-injected Terahertz Quantum Cascade Lasers(THz-QCLs) are studied theoretically. In the case of free-running operation, the spontaneous emission noise of THz-QCLs exhibits white noise characteristics at low frequencies. Unlike conventional semiconductor lasers, there is no resonance peak corresponding to relaxation oscillation in the low frequency region. Master-slave injection can effectively reduce the noise of THz-QCLs for nearly 20 dB. It is an effective and practical technology to achieve low noise of THz-QCLs. For optically mutual-injected THz-QCLs,even in the phase-locked region, the noise is significantly higher than that in the case of free operation. In addition, compared with direct modulation scheme, the modulation bandwidth can be greatly increased by using optical injection phase-locking scheme.