In standing-wave lasers, spatial hole burning induces a static grating of the population inversion, enabling multimode operation with several independent lasing modes. In the presence of a mode-locking mechanism, these modes may become correlated, giving origin to a frequency comb. Quantum cascade lasers, owing to their ultrafast gain dynamics, are ideally suited to achieve comb operation. Here we experimentally demonstrate that the modes of a quantum cascade laser frequency comb coherently beat to produce time-dependent population inversion gratings, which spatially modulate the current in the device at frequencies equal to the mode separation and its higher harmonics. This phenomenon allows the laser to serve as a phased collection of microwave local oscillators and is utilized to demonstrate quadrature amplitude modulation, a staple of modern communications. These findings may provide for a new class of integrated transmitters, potentially extending from the microwave to the low terahertz band.

中文翻译：

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激光频率梳中与时间有关的种群反转光栅

在驻波激光器中，空间空穴燃烧会引起粒子反转的静态光栅，从而使多模式操作具有几种独立的激光模式。在存在锁模机制的情况下，这些模式可能会变得相关，从而使频率梳成为起点。量子级联激光器，由于其超快的增益动态特性，非常适合实现梳状操作。在这里，我们通过实验证明，量子级联激光器频率梳的模式连贯地跳动，以产生与时间有关的种群反转光栅，该光栅在空间上调制器件中电流的频率等于模分离及其高次谐波。这种现象可以使激光充当微波本地振荡器的相控集合，并被用于演示正交幅度调制，现代通讯的主要内容。这些发现可能会提供一种新型的集成发射机，可能从微波扩展到低太赫兹频段。