Terahertz quantum cascade lasers (QCLs) have narrow linewidths
(< 1 MHz) and electrical/thermal tuning ranges of several GHz. This combination allows high resolution laser absorption spectroscopy at frequencies between 1.9 and 5 THz.

At frequencies above 3 THz, the terahertz QCL is the only source capable of these measurements.  For additional, broader tuning, additional devices can be integrated in a simple, electronically controlled package. In a tunable diode laser spectroscopy configuration, a single-modeQCL  is operated in continuous-wave mode and electronically/thermally tuned across the absorption lines of low-pressure gasses in a gas cell.

A small AC signal is added to the QCL bias causing small frequency oscillations. As the laser is tuned across the absorption line, this oscillation is detected by a lock-in amplifier referenced to the frequency of the AC signal.  This results in a measurement of the first derivative of the absorption line. Alternatively the lock-in amplifier can be referenced to the first harmonic of the AC signal, resulting in a measurement of the second derivative of the absorption line (2f spectroscopy), as shown here with a methanol (CH3OH) gas cell.