论文标题
量子级联激光器中的谐波频率梳：时间域和频域理论
Harmonic frequency combs in quantum cascade lasers: time-domain and frequency-domain theory
论文作者
论文摘要
最近在量子级联激光器（QCL）中发现了谐波频率梳子，其中激光模式与数十几个自由光谱范围的时期分开。在连续抽水下如何形成和稳定的谐波梳子存在一场持续的辩论。在这里，我们通过时空域数值模拟重现了QCL中激光的谐波状态，并表明相应的光波具有频率调节（FM）性质，并且电子群体中的基本节拍被抑制了。为了了解谐波状态的形成和稳定性背后的物理学，我们开发了频域线性化的分析理论，该理论分析了单模式激光相对于谐波状态形成的稳定性以及所得和谐状态的不稳定性。我们的分析表明，单个激光模式的不稳定性如何导致谐波侧模式的增长。与AM增益相比，两种侧模式之间的耦合导致FM和振幅调节（AM）波的相反增益，FM增益得到了增强。还研究了边带增益光谱对组速度色散的依赖性。虽然单个激光模式的不稳定性并不能证明谐波状态可以是自动的，但我们在存在三种强谐模式的情况下分析了边带不稳定性增益，并证明谐波状态可以稳定且自我支持。
Harmonic frequency combs, in which the lasing modes are separated by a period of tens of free spectral ranges from each other, have been recently discovered in quantum cascade lasers (QCLs). There is an ongoing debate how the harmonic combs can be formed and stable under continuous pumping. Here we reproduce the harmonic state of lasing in QCLs by space-time-domain numerical simulations and show that the corresponding optical wave has a frequency-modulated (FM) nature, with the fundamental beat note in the electron population being suppressed. To understand the physics behind the formation and stability of the harmonic state, we develop a frequency-domain linearized analytic theory which analyzes the stability of single mode lasing with respect to the harmonic state formation and the instabilities of the resulting harmonic state. Our analysis shows how the instability of a single lasing mode can result in the growth of harmonic side modes. The coupling between the two side modes leads to opposite gains for FM and amplitude-modulated (AM) waves, and the FM gain is enhanced as compared to the AM gain. The dependence of the sideband gain spectrum on group velocity dispersion is also studied. While the instability gain of a single lasing mode does not prove that the harmonic state can be self-starting, we analyze the sideband instability gain in the presence of three strong harmonic modes and demonstrate that the harmonic state can be stable and self-supported.
