Abstract
<jats:p>Terahertz-frequency quantum cascade lasers (THz QCLs) are promising sources for transmitters in free-space links above 1 THz, where conventional electronics are power-limited and photonic approaches become increasingly complex. However, existing THz QCL communication systems have not fully exploited spectrally efficient high-order modulation, partly due to limitations of the Fabry–Perot metal–metal laser waveguides typically employed, which have low output power and poor beam quality. Here, we demonstrate a spectrally efficient THz QCL communication link enabled by a single-mode 2.7 THz third-order distributed-feedback (DFB) QCL. This architecture preserves the high-speed modulation capability of metal–metal waveguides while improving the emission properties, yielding a threefold increase in output power and a one order-of-magnitude enhancement in power coupled into the receiver. The QCL is directly modulated with orthogonal frequency-division multiplexing (OFDM) using quadrature-amplitude modulation (QAM). Using a room-temperature Schottky-barrier diode receiver and offline digital signal processing, we achieve 32-QAM OFDM transmission over a 4 GHz bandwidth. A peak net bit rate of 15.84 Gb/s is obtained, corresponding to a spectral efficiency of 3.96 bit/s/Hz. These results show that improved transmitter design combined with multi-carrier modulation enables THz QCL links to move beyond binary formats, providing a practical route toward high-capacity communication above 1 THz.</jats:p>