Phase locking and spectral linewidth of a two-mode terahertz quantum cascade laser

We have studied the phase locking and spectral linewidth of an ˜2.7THz quantum cascade laser by mixing its two lateral lasing modes. The beat signal at about 8GHz 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 10Hz. Under frequency stabilization we find that the terahertz line profile is essentially Lorentzian with a minimum linewidth of ˜6.3kHz. Power dependent measurements suggest that this linewidth does not approach the Schawlow-Townes limit