太赫兹量子级联激光器光束特性分析

Analysis of Terahertz Quantum Cascade Laser Beam

用有限元法计算太赫兹量子级联激光器激光模式的阈值增益.结果表明:接触层厚度和掺杂浓度对阈值增益的影响远远大于波导宽度和激射波长;接触层厚度较小(大)和掺杂浓度较低(高)时,TM1(TM0)模的阈值增益较小.在此基础上,用矢量衍射理论分析岀射光束的远场特性,得到光束的远场光斑基本是椭圆;x方向的远场散射角随波导宽度或激射波长的增加分别线性减小或增加,尽管对应的接触层厚度和掺杂浓度不同,但TM0和TM1模x方向的远场散射角相同;另外,还得到y方向远场散射角不受波导宽度或少受激射波长的影响.在阈值增益和光束质量方面,TM1模都优于TM0模.

Threshold gain of laser mode in terahertz quantum cascade laser is calculated with finite element method. It shows that contact layer thickness and doping density have far greater impact on threshold gain than waveguide width and lasing wavelength. As thickness and density becoming smaller （greater）, threshold gain of TM1 （TM0 ） mode becomes smaller. Far-field characteristics of the beam are analyzed using vector diffraction theory. The beam spot is basically elliptical. With increase of waveguide width or lasing wavelength, far-field divergence angles at x direction decrease or increase linearly. Though the contact layer thickness and doping density for TM0 mode are different from that for TM1, their far-field divergence angles at x direction are same. Moreover, far-field divergence angles at y direction are unaffected by waveguide width, and less affected by lasing wavelength. In terms of threshold gain and beam quality, TM1 is more desirable than TM0 mode.