PT Journal
AU Rabanus, D
   Graf, UU
   Philipp, M
   Ricken, O
   Stutzki, J
   Vowinkel, B
   Wiedner, MC
   Walther, C
   Fischer, M
   Faist, J
TI Phase locking of a 1.5 terahertz quantum cascade laser and use as a local oscillator in a heterodyne HEB receiver
SO Optics Express
PY 2009
BP 1159
EP 1168
VL 17
IS 3
DE QCL heterodyne; 300 uW at 1.5 THz; HEB mixer
AB We demonstrate for the first time the closure of an electronic phase lock loop for a continuous–wave quantum cascade laser (QCL) at 1.5 THz. The QCL is operated in a closed cycle cryo cooler. We achieved a frequency stability of better than 100 Hz, limited by the resolution bandwidth of the spectrum analyser. The PLL electronics make use of the intermediate frequency (IF) obtained from a hot electron bolometer (HEB) which is downconverted to a PLL IF of 125 MHz. The coarse selection of the longitudinal mode and the fine tuning is achieved via the bias voltage of the QCL. Within a QCL cavity mode, the free-running QCL shows frequency fluctuations of about 5 MHz, which the PLL circuit is able to control via the Stark–shift of the QCL gain material. Temperature dependent tuning is shown to be nonlinear, and of the order of -16 MHz/K. Additionally we have used the QCL as local oscillator (LO) to pump an HEB and perform, again for the first time at 1.5 THz, a heterodyne experiment, and obtain a receiver noise temperature of 1741 K.
ER