太赫兹量子级联激光器(THz QCL)的增益谱较宽,使用普通法布里-珀罗腔时一般为多纵模激射。二阶分布反馈THz QCL可利用表面辐射损耗消除模式简并,实现单纵模激射。采用耦合模理论对单面金属波导二阶分布反馈THz QCL基本参数的计算公式进行了理论推导,并研究了光栅结构参数对其耦合系数、阈值增益、光子密度及外微分量子效率等参数的影响。在以自然解理面为端面的波导上刻蚀占空比约为0.15、有源区深度为0.5 μm的光栅时,激光器的阈值增益较低,腔内光子密度分布相对均匀,能够实现单纵模激射。

The terahertz quantum cascade laser (THz QCL) has a wide gain spectrum, and the emission is often multiple-longitudinal mode when using the conventional Fabry-Perot cavity. A second-order distributed feedback THz QCL can eliminate the mode degeneracy to realize the single longitudinal mode lasing by means of surface radiation loss. Based on the coupled-mode theory, the computational formulas of the basic parameters for the second-order distributed feedback THz QCL with the single-side metal waveguide are deduced. The influences of the grating structure parameters on the coupling coefficients, threshold gain, photon density, and external differential quantum efficiency is investigated. When the grating with a duty cycle of about 0.15 and a depth in the active region of 0.5 μm is etched on the waveguide with a natural cleavage plane, the threshold gain is low, the photon density distribution within the cavity is uniform, and the single longitudinal mode emission can be achieved.