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Multimode dynamics in quantum cascade lasers: From coherent instability to mode locking.

机译：量子级联激光器中的多模动力学：从相干不稳定性到锁模。

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摘要

Quantum Cascade Lasers (QCLs) are unipolar semiconductor lasers based on intersubband transitions in quantum wells. Since their invention in 1994, these lasers have undergone tremendous improvement, and have become the most prominent coherent light source in the mid-infrared and terahertz spectral ranges. However, the understanding of multimode regimes in QCLs is still in its infancy, and there has not been much effort toward generating ultrafast pulses from QCLs. The recent development of low loss, high power QCLs enables the study of those previously under-investigated aspects. This thesis can be divided into two main parts. In the first part, we study the multimode regimes in QCLs. We find that QCLs, because of their extremely fast gain recovery time, differ from diode lasers in multimode operation. While a saturable absorber can often lead to self mode-locking in lasers with long gain recovery compared to the cavity round-trip time, in QCLs it lowers the threshold of a coherent multimode instability, which is driven by the same fundamental mechanism of Rabi oscillations as the elusive Risken-Nummedal-Graham-Haken (RNGH) instability predicted 40 years ago. The main experimental signature of RNGH instability is a splitting corresponding to twice the Rabi frequency in optical spectrum. In QCLs this coherent instability is enhanced due to the large Rabi frequency compared to the relaxation rates. We have also shown that spatial hole burning, which is not so readily observable in diode lasers, also plays an important role in QCLs. Both experimental data and simulations based on Maxwell-Bloch equations are presented. In the second part of this thesis, we demonstrate active mode-locking in QCLs. The stable mode-locked pulse train was generated by actively modulating the pumping current of a small section on a QCL. Stable mode locking was confirmed by second-order interferometric autocorrelation measurements, and a FWHM of 3 ps and about 0.5 pJ per pulse were deduced from the autocorrelation traces. The system is also simulated using Maxwell-Bloch equations incorporated with a modulation term. We anticipate our results to be a big step toward a compact, electrically-pumped source generating ultrashort pulses in the mid-infrared and terahertz ranges.
机译：量子级联激光器（QCL）是基于量子阱中子带间跃迁的单极半导体激光器。自1994年发明以来，这些激光器已经取得了巨大的进步，并已成为中红外和太赫兹光谱范围内最突出的相干光源。然而，对QCL中的多模态的了解仍处于起步阶段，并且没有太多努力来从QCL中产生超快脉冲。低损耗，高功率QCL的最新发展使人们能够研究那些先前未充分研究的方面。本论文可以分为两个主要部分。在第一部分中，我们研究了QCL中的多模态。我们发现，由于QCL的增益恢复时间非常快，因此它在多模操作中不同于二极管激光器。尽管与腔往返时间相比，可饱和吸收体通常可以在激光器中实现自锁模，并且增益恢复时间较长，但在QCL中，它会降低相干多模不稳定性的阈值，这是由拉比振荡的相同基本机理所驱动的正如40年前所预测的难以捉摸的Risken-Nummedal-Graham-Haken（RNGH）不稳定性那样。 RNGH不稳定性的主要实验特征是分裂，对应于光谱中Rabi频率的两倍。在QCL中，与弛豫速率相比，由于大的Rabi频率，这种相干不稳定性得到增强。我们还表明，在二极管激光器中不容易观察到的空间空穴燃烧在QCL中也起着重要作用。给出了实验数据和基于Maxwell-Bloch方程的仿真。在本文的第二部分，我们演示了QCL中的主动锁模。通过主动调制QCL上一小部分的泵浦电流来生成稳定的锁模脉冲序列。通过二阶干涉自相关测量证实了稳定的模式锁定，并且从自相关迹线推导出了3 ps的FWHM和每个脉冲约0.5 pJ。该系统还使用带有调制项的Maxwell-Bloch方程进行了仿真。我们预计我们的结果将朝着紧凑的电泵源迈出一大步，该源将产生中红外和太赫兹范围的超短脉冲。

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  • 作者

    Wang, Christine Yi-Ting.;

  • 作者单位

    Harvard University.;

  • 授予单位 Harvard University.;
  • 学科 Physics Optics.
  • 学位 Ph.D.
  • 年度 2009
  • 页码 122 p.
  • 总页数 122
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 光学;
  • 关键词

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