Terahertz (THz) coherent detectors are crucial for the stabilization and
measurement of the properties of quantum cascade lasers (QCLs). This paper describes the exploitation of intra-cavity sum frequency generation to up-convert the emission of a THz QCL to the near infrared for detection with fiber optic coupled components alone. Specifically, a low cost infrared photodiode is used to detect a radio frequency (RF) signal with signal-to-noise ratio of approximately 20dB, generated by beating the up-converted THz wave and a near infrared local oscillator. This RF beat note allows direct analysis of the THz
QCL emission in time and frequency domains. The application of this technique for QCL characterization is demonstrated by analyzing the continuous tuning of the RF signal over 2 GHz, which arises from mode tuning across the QCL’s operational current range.