Abstract
High resolution frequency study and phase-locking have been performed on
terahertz quantum-cascade vertical-external-cavity
surface-emitting-lasers at 2.5 THz and 3.4 THz. A subharmonic diode
mixer is used to downconvert the THz signal to a gigahertz intermediate
frequency (IF). Feedback from reflections at the mixer are observed to
have a strong influence on the free-running QC-VECSEL frequency
stability as a result of efficient coupling to free-space compared to
more typical ridge waveguide lasers. Instabilities in feedback result in
free-running linewidths of tens of MHz, and at times >100
MHz. The QC-VECSEL IF signal is phase locked to a 100 MHz reference
using the bias on the device as a means of error correction. Between
90-95% of the QC-VECSEL signal is locked within 2 Hz of the multiplied
RF reference, and amplitude fluctuations on the order of 1-10% are
observed, depending on the bias point of the QC-VECSEL. The bandwidth of
the locking loop is ~1 MHz. Many noise peaks in the IF
signal corresponding to mechanical resonances in the 10 Hz-10 kHz range
are observed. These peaks are generally -30 to -60 dB below the main
tone, and are below the phase noise level of the multiplied RF reference
which ultimately limits the phase noise of the locked QC-VECSEL.