The free‐spectral range (FSR) of quantum cascade lasers (QCLs) emitting at 4.7 µm can be tuned through direct microwave modulation. The intrinsic short carrier lifetime of inter‐subband transitions, along with impedance matched electrical packaging, allows a high‐speed modulation up to more than 30 GHz. A significant broadening and flattening of the lasing spectrum is observed under radio frequency (RF) injection with frequencies close to the round‐trip cavity. An accurate analysis of the high resolution spectra of the laser shows a comb‐like regime for both free‐running and RF modulated QCLs, if the modulation frequency is within the locking‐range of the device. One of the main advantages of collecting high‐resolution mid‐infrared spectra, over the plain investigation of the beatnote in the microwave region, is the access to all the longitudinal modes and thus the accurate measure of the FSR over the whole optical spectrum. The use of high‐resolution spectroscopy provides an in‐depth and comprehensive analysis of lasing spectra under microwave modulations.

中文翻译：

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微波注入中红外量子级联激光器中的自由光谱范围的可调谐性

可以通过直接微波调制来调谐以4.7 µm发射的量子级联激光器（QCL）的自由光谱范围（FSR）。子带间转换固有的短载波寿命以及阻抗匹配的电气封装可实现高达30 GHz以上的高速调制。在射频（RF）注入下，观察到激光光谱的显着展宽和展平，其频率接近往返腔。如果调制频率在设备的锁定范围内，则对激光器的高分辨率光谱的准确分析表明，对于自由运行和RF调制的QCL来说，都呈梳状。收集高分辨率的中红外光谱的主要优势之一，是对微波区域中的节拍音进行简单的研究，可以访问所有纵向模式，因此可以在整个光谱范围内精确测量FSR。高分辨率光谱的使用可以对微波调制下的激光光谱进行深入而全面的分析。