单模太赫兹量子级联激光器阵列：实现稳定锁相的结构与方法

Phase-locked array of single-mode terahertz quantum cascade lasers (THz-QCLs) is a promising terahertz coherent source with high output power, whose potential application includes terahertz spectroscopy, and so on. However, to realize stable phase-locking among multi-lasers is still an open key issue. In this program, we study the structures and solutions for stable phase-locking among multi single-mode THz-QCLs, by using the technologies of finite differential time domain calculations, molecular beam epitaxy, and semiconductor micro-fabrication. We will propose a “THz-QCL/double metal waveguide/coupling grating” hybrid structure, which makes THz wave efficiently coupled into the double metal waveguide, and enables precise measurements of the loss and gain features of the THz-QCL and double metal waveguide. We will then design and realize phase-locked arrays of second-order distributed feedback THz-QCLs by optically interconnecting the lasers via Y-type double metal waveguides. By tuning the structure parameters and separately setting the bias in different regions of the array, we will flexibly control the loss and gain in the Y-waveguide and THz-QCLs, and let the fundamental laser mode only be excited and propagated in the array. We then exploit the diffraction affect in the Y-waveguide, which will force the lasers operate in phase. Such strategy will finally provide a stable phase-locking among multi single-mode THz-QCLs, resulting in a significant improvement of output power and beam directionality, as well as keeping the coherence of the single-mode emission. Our research will accelerate the development of high power terahertz coherent source.

单模太赫兹量子级联激光器的锁相阵列是实现大功率太赫兹相干光源的理想途径，可应用于光谱分析等领域。已有研究尚未解决多个激光器之间锁相的稳定性。本课题结合有限差分时域计算、分子束外延与微细加工等技术，研究实现该激光器阵列稳定锁相的结构与方法。首先，提出“量子级联激光器/双金属波导/耦合光栅”复合结构，突破太赫兹波难以进入厚度小于衍射极限的双金属波导的瓶颈，揭示激光器和双金属波导中损耗与增益的特性及其偏压调控规律。由此，提出基于Y型双金属波导光互连的单模量子级联激光器锁相阵列。通过调节阵列的结构并分段施加不同偏压，实现Y型波导与激光器中损耗与增益的灵活调控，达到只激发并传播激光器基模的效果。进而利用Y型波导中电磁场的干涉效应，使阵列中各激光器具有稳定且相同的相位，最终实现稳定的锁相激光器阵列，保持单模激光的相干性并显著提高激光功率与光束准直性。本课题的完成有助于推动大功率太赫兹相干光源的发展。

本项目发展并完善了太赫兹量子激光器（THz-QCL）材料外延生长、器件研制和表征技术。提出了一种新的测量THz-QCL损耗和增益特性的实验方法，阐明了有源区掺杂以及偏压对损耗和增益的影响机制。提出并实现了基于倏逝波耦合的THz-QCL锁相激光器阵列，阐明了阵列中模式竞争的机理并获得实验验证。提出并实现了基于导波耦合的THz-QCL锁相激光器阵列，指出利用器件结构的对称性以及导波的相位可以稳定地控制阵列中各激光器的相位关系。在实验上通过导波耦合实现了多达8个THz-QCL的锁相激光器阵列，并稳定可控地工作在同相或者反相超模，在保持单模激射的同时显著提高了激光功率和光束准直性。本项目还在国际上首次提出并实现了具有“主控振荡—功率放大”谐振腔的太赫兹量子级联激光器（THz-MOPA-QCL），提出了描述该激光器中增益饱和效应的模型并获得实验验证，建立了优化该激光器的设计规则。该激光器实现了液氮工作温度下153mW的脉冲峰值功率，单模激射的边摸抑制比达到23dB，光束发散角仅为5.5° x16°，激光器综合性能处于国际先进水平。本项目所研制的高性能激光器已提供给国内多家科研机构，用于太赫兹光谱和成像等方面的应用研究。合计销售激光器6套，合同总额为45.5万元。发表SCI论文9篇，国际学术会议邀请报告3篇，申请国家发明专利4项，培养7名研究生和1名博士后。.通过项目实施，发展了THz-QCL的研制技术；加深了对激光器损耗和增益特性，以及锁相耦合机制的理解；发展了新型THz-QCL谐振腔结构，获得大功率单模太赫兹激光，综合性能处于国际先进水平。切实推动了我国太赫兹领域的研究与应用。