Fig. 3: QCL device characteristics and measured system response.
From: Free-space optical communications at 4 Gbit/s data rate with a terahertz laser
a Spectrogram of THz QCL emission as a function of drive current. The color scale indicates the amplified mixer-intermediate frequency (IF) power derived from each QCL mode. b CW L--I--V characteristics of the QCL in the absence of modulation measured at a temperature 19 K. Red: THz power vs current showing sub-linear rise with mode-related steps and a peak before roll-over; Black: voltage vs current. Gray band marks the current range used for direct modulation, where the differential efficiency is approximately constant. c The full system’s end-to-end (QCL, Schottky barrier diode detector, bias-tees, and amplifiers) S21 amplitude response, including the cascaded frequency response of all the electrical and optoelectronic devices in the setup, measured with a vector network analyzer (VNA). The red trace shows the amplitude response of the complete system. The gray trace shows the system’s noise floor, recorded under the same configuration with the QCL off.
