Researchers led by Prof. Sheng-Cai Shi, Purple Mountain Observatory, Chinese Academy of Sciences, report  on the investigation of a 3.7 THz phase-matched third-order distributed  feedback (DFB) quantum cascade laser (QCL) as the local oscillator (LO)  of a superconducting hot electron bolometer (HEB) heterodyne receiver. Exploring  the terahertz (THz) frequency range is of particular importance for  many research fields such as astrophysics and atmospheric science since  this frequency range holds a large number of fine structure and  molecular lines providing important information of fundamental  astrophysical and atmospheric processes. At frequencies above 1.5 THz,  superconducting hot electron bolometer (HEB) mixers are commonly used  and have been demonstrated with a superior sensitivity up to 5.3 THz.  However, the lack of suitable LO sources at frequencies above 2 THz  seriously limits practical applications of superconducting HEB mixers.  

             BY with ZHANG Wen       

                 Fig.  Calculated (left) and measured (right) beam pattern of the third-order DFB QCL.       

  Due to the incorporation  of the third-order Bragg gratings, the beam pattern of the QCL was found  much improved. Beam pattern measurements show that the QCL has a narrow  beam with ~12×18  degree divergence, which is in good agreement with simulations by  considering the gratings as an array of phased linear sources. Using  this QCL as LO, we measured the noise temperature of a log-spiral  antenna coupled superconducting HEB mixer at 3.7 THz. The lowest  double-sideband (DSB) receiver noise temperature is about 4000 K, and  reduced to 1100 K after correcting the optical losses. This result shows  that phase-matched third-order DFB QCLs are very promising device as a  LO in THz heterodyne receivers.

  The  work by W. Miao, W. Zhang, Z. Lou, Y. Ren, S.L. Li, K. Zhang, Q. J.  Yao, T.Y. Kao, Q. Hu, and S. C. Shi (corresponding author), accepted  by General Assembly and scientific symposium (URSI GASS)

has been published online：

 (http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=6930015&abstractAccess=no&userType=inst).