Phase locking and spectral linewidth width of a two-mode terahertz quantum cascade laser
A. Baryshev (SRON–Netherlands Institute for Space Research)
J. N. Hovenier (Kavli institute of nanoscience Delft, TU Delft - QN/Fysics of NanoElectronics)
A. J.L. Adam (Kavli institute of nanoscience Delft, TU Delft - ImPhys/Adam group)
I. Kašalynas (Kavli institute of nanoscience Delft)
J. R. Gao (SRON–Netherlands Institute for Space Research, TU Delft - QN/Fysics of NanoElectronics)
T. O. Klaassen (Kavli institute of nanoscience Delft, TU Delft - QN/Afdelingsbureau)
B. S. Williams (Massachusetts Institute of Technology)
S. Kumar (Massachusetts Institute of Technology)
Q. Hu (Massachusetts Institute of Technology)
J. L. Reno (Sandia National Laboratories, New Mexico)
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Abstract
We have studied the phase locking and spectral linewidth of an ∼2.7 THz quantum cascade laser by mixing its two lateral lasing modes. The beat signal at about 8 GHz is compared with a microwave reference by applying conventional phase lock loop circuitry with feedback to the laser bias current. Phase locking has been demonstrated, resulting in a narrow beat linewidth of less than 10 Hz. Under frequency stabilization we find that the terahertz line profile is essentially Lorentzian with a minimum linewidth of ∼6.3 kHz. Power dependent measurements suggest that this linewidth does not approach the Schawlow-Townes limit.