量子级联激光器中的低阈值RNGH不稳定性
Low-threshold RNGH instabilities in quantum cascade lasers
摘要：
对量子级联激光器（QCL）和激光二极管（LD）中的低阈值多模不稳定性进行了理论研究。以前，低阈值Risken-Nummedal-Graham-Haken（RNGH）不稳定性在QCL的几个实验研究中有报道。到目前为止，它们被归因于载流子数的诱导光栅和可能存在于这些法布里-珀罗激光器的单片单节腔中的内置可饱和吸收特性的综合效应。在这里，我们表明，低阈值RNGH QCL的不稳定性发生由于在增益介质中诱导的载波相干光栅，而不是由于腔内饱和吸收。我们发现，具有几毫米长腔的QCL表现出间歇性的RNGH自脉动，而在短腔QCL中，腔长为100 μ m或更小的规则的自脉动是可能的。我们研究了短腔QCL中RNGH自脉动的瞬态行为，并发现了类似于合作超辐射的特征。我们的研究结果打开了一个实用的方法，实现超短脉冲生产制度在中红外光谱范围。将同样的方法应用于半导体激光二极管（LD），我们解释了基于量子阱增益介质的LD中不存在RNGH自脉动，而实际建立的在LD中达到超快相干发射区的方法是在LD腔中加入单独接触的可饱和吸收体部分。
英文摘要：
A theoretical study on low-threshold multimode instabilities in quantum cascade lasers (QCLs) and laser diodes (LDs) is presented. Previously, low threshold Risken-Nummedal-Graham-Haken (RNGH) instabilities were reported in several experimental studies of QCLs. They were attributed so far to the combined effect of the induced grating of carrier population and of a built-in saturable absorption feature that may be present in the monolithic single-section cavity of these Fabry-Pérot lasers. Here we show that low-threshold RNGH instabilities in QCLs occur due to a carrier coherence grating induced in the gain medium and not due to intracavity saturable absorption. We find that QCLs with a few mm long cavity exhibit intermittent RNGH self-pulsations while regular self-pulsations are possible in short-cavity QCLs, with the cavity length of 100 um or smaller. We examine a transient behavior to RNGH self-pulsations in short-cavity QCLs and find features that resemble cooperative superradiance. Our findings open a practical way of achieving ultra-short pulse production regimes in the mid-infrared spectral range. Applying same approach to semiconductor laser diodes (LDs) we explain the absence of RNGH self-pulsation in LDs based on a quantum well gain media, while practically established method for reaching the ultrafast coherent emission regimes in LDs is to incorporate a separately contacted saturable absorber section in the LD cavity.
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