The effect of spatial hole burning on mode proliferation has been shown both theoretically and experimentally to be important for frequency comb operation in quantum cascade lasers (QCLs). It is therefore of great interest to investigate the frequency comb operation from integrated ring cavities where this effect is absent. Here we report a mid-infrared quantum cascade laser based on a buried heterostructure ring cavity that exhibits unidirectional lasing with a frequency comb spectrum displaying a predominantly ${{\rm sech}^2}$ profile over a bandwidth of $\sim{10}{^{ - 1}}$. A single beat note with 3 dB linewidth of $ \lt {900}\;{\rm Hz}$ was observed over the last 85% of the whole current dynamic range. To further verify the frequency comb operation, a multiheterodyne experiment was carried out using a standard Fabry–Pérot QCL comb as a reference laser. This measurement allowed the retrieval of the spectral phases, indicating an operation with strong amplitude modulation.

中文翻译：

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环形量子级联激光器的中红外频率梳

理论上和实验上都表明，空间空穴燃烧对模式扩散的影响对于量子级联激光器（QCL）中的频率梳操作很重要。因此，非常感兴趣的是从不存在这种影响的集成环形腔中研究频率梳的工作情况。在这里，我们报告了一种基于埋入异质结构环形腔的中红外量子级联激光器，该激光器显示出单向激射，其频率梳频谱在$ \ sim {10的带宽上显示出主要的$ {{\ rm sech} ^ 2} $轮廓} {^ {-1}} $。3 dB线宽为$ \ lt {900} \ ;; {\ rm Hz} $的单个拍子音符在整个当前动态范围的最后85％处观察到。为了进一步验证频率梳的工作，使用标准Fabry-PérotQCL梳作为参考激光器进行了多外差实验。该测量允许检索频谱相位，表明具有强幅度调制的操作。