结合实验和理论计算研究了太赫兹二级分布反馈量子级联激光器中的模式竞争和功率特性.研究表明, 激光器在整个动力学范围内均稳定地工作在横向及纵向的基模.横向基模的产生原因是脊条两侧的吸收边界有效提高了高阶横模的损耗.纵向基模的产生原因主要是谐振腔内基模与高阶纵模的电磁场分布交叠较大, 并且频率接近, 从而有效避免了增益在空间和频域的烧孔效应.此外, 激光器的辐射效率随分布反馈光栅长度的增加而减小, 导致只有在特定的光栅长度才能获得最大的输出功率.该工作有助于高性能单模太赫兹激光器及锁相激光器阵列的研制.

We investigated experimentally and theoretically the mode competition and output power characteristics in terahertz second-order distributed feedback quantum cascade lasers. The experiment results demonstrated that the lasers operate stably on the fundamental transverse/longitudinal mode during the whole laser dynamic range. High-order transverse modes are suppressed due to the absorbing boundaries set along the laser ridge. In addition, the fundamental mode and the high-order longitudinal modes are very close in frequency, and their electromagnetic fields have strong overlap. The spectral or spatial hole burning effect is therefore effectively avoided, leading to a stable operation of the fundamental longitudinal mode. The experimental results also show that the slope efficiency of output power decreases with the length of the distributed feedback grating. As a result, only in certain grating length, the output power reaches its maximum. Our work provides experimental and theoretical supports for the studies of high performance THz lasers and the phase locked laser arrays.