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Frequency stabilization, tuning, and spatial mode control of terahertz quantum cascade lasers for coherent transceiver applications.

机译：用于相干收发器应用的太赫兹量子级联激光器的频率稳定，调谐和空间模式控制。

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摘要

THz quantum cascade lasers (TQCLs) first demonstrated in 2002 are a promising source of THz coherent radiation for use as both transmitters and local oscillators in coherent heterodyne detection systems. However, present TQCLs have deficiencies which include lack of frequency tunability, as well as inadequate spatial and temporal coherence. In this thesis we have addressed these issues to demonstrate an improved 2.408 TQCL which operated as a transmitter in a coherent heterodyne detection imaging system. The 2.408 THz QCL used in this thesis was grown and fabricated by the Photonics Center (University of Massachusetts Lowell).;First, we showed that a short hollow Pyrex tube can act as a dielectric waveguide and transform the multimode, highly diverging TQCL beam into the lowest order dielectric waveguide hybrid mode, EH11, which then couples efficiently to the free-space Gaussian mode, TEMo0. This simple approach should enable TQCLs to be employed in applications where a spatially coherent beam is required.;Next, the tunability problem was addressed. A compact, tunable, narrowband terahertz source was demonstrated by mixing a single longitudinal mode, 2.408 THz, free running quantum cascade laser with a 2-20 GHz microwave sweeper in a corner-cube-mounted Schottky diode (SD). The sideband spectra were characterized with a Fourier transform spectrometer, and the radiation was tuned through several D2O rotational transitions to estimate the longer term (t ≥ several seconds) bandwidth of the source. A spectral resolution of 2 MHz in the CW mode was observed.;The temporal coherence of the TQCL was improved by assembling a simple analog locking circuit, which stabilizes the beat signal between the TQCL and a 2.40976 THz CO2 optically pumped molecular laser (OPL) line to 4 kHz full width at half maximum (FWHM). This is approximately a tenth of the observed long-term (t ∼ sec) linewidth of the OPL showing that the feedback loop corrects for much of the mechanical and acoustic-induced frequency jitter of the gas laser. The achieved stability is sufficient to enable the use of the TQCL as a transmitter in a coherent transceiver.;Finally, a coherent transceiver using the TQCL as a transmitter and an OPL as the local oscillator was used, with a pair of SD mixers in the receiver and reference channels, to acquire high-resolution inverse synthetic aperture radar (ISAR) images of fully illuminated targets, including scale models and concealed objects. Phase stability of the received signal was sufficient to allow coherent image processing of the rotating target (in azimuth and elevation). Several THz ISAR images with submillimeter resolution, obtained with the system, are presented. These images clearly illustrate the first use of a quantum cascade laser in a coherent detection system.
机译：太赫兹量子级联激光器（TQCL）于2002年首次展示，是有希望的太赫兹相干辐射源，可用作相干外差检测系统中的发射器和本地振荡器。但是，当前的TQCL具有不足之处，包括缺乏频率可调性以及空间和时间的连贯性不足。在这篇论文中，我们已经解决了这些问题，以演示一种改进的2.408 TQCL，该TQCL在相​​干外差检测成像系统中用作发送器。本文使用的2.408 THz QCL是由马萨诸塞州洛厄尔大学的光子学中心生产和制造的。首先，我们证明了一个短的空心Pyrex管可以充当介电波导，并将多模，高发散的TQCL光束转换为最低阶介电波导混合模式EH11，然后将其有效耦合到自由空间高斯模式TEMo0。这种简单的方法应可使TQCL应用于需要空间相干光束的应用中。接下来，解决了可调性问题。通过将单个纵向模式，2.408 THz，自由运行的量子级联激光器与安装在角c安装的肖特基二极管（SD）中的2-20 GHz微波扫频器混合，展示了一种紧凑，可调谐的窄带太赫兹源。用傅立叶变换光谱仪对边带光谱进行表征，并通过几个D2O旋转跃迁调整辐射，以估计光源的长期带宽（t≥几秒钟）。在CW模式下观察到2 MHz的光谱分辨率;通过组装一个简单的模拟锁定电路可以改善TQCL的时间相干性，该电路可以稳定TQCL和2.40976 THz CO2光泵浦分子泵（OPL）之间的拍频信号最大半高（FWHM）的最大宽度为4 kHz。这大约是所观察到的OPL的长期（t〜sec）线宽的十分之一，表明反馈回路校正了气体激光器在机械和声学上引起的大部分频率抖动。所获得的稳定性足以使TQCL用作相干收发器中的发送器;最后，使用了将TQCL用作发送器并将OPL用作本地振荡器的相干收发器，并在其中使用了一对SD混频器接收器和参考通道，以获取完全照明的目标的高分辨率逆合成孔径雷达（ISAR）图像，包括比例模型和隐藏物体。接收信号的相位稳定性足以允许对旋转目标进行相干图像处理（方位角和仰角）。该系统获得了几张亚毫米分辨率的THz ISAR图像。这些图像清楚地说明了量子级联激光器在相干检测系统中的首次使用。

著录项

  • 作者

    Danylov, Andriy.;

  • 作者单位

    University of Massachusetts Lowell.;

  • 授予单位 University of Massachusetts Lowell.;
  • 学科 Physics Optics.
  • 学位 Ph.D.
  • 年度 2010
  • 页码 133 p.
  • 总页数 133
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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