Continuous wave terahertz spectrometer with 2 THz bandwidth and update rates of 24 Hz

Time-domain spectroscopy (TDS) is the most prominent technique for fast acquisition of broadband terahertz (THz) spectra with update rates of several ten Hz up to kHz. However, fast tunable continuous-wave (cw) laser sources enable rapid acquisition of broadband THz signals without the well-known drawbacks of THz-TDS systems: mechanical delay lines and femtosecond pulse lasers. In this work, we make use of a fast tunable laser to demonstrate coherent continuouswave THz spectroscopy with unprecedented speed and bandwidth. The system features three different modes of operation exploiting both broad spectral bandwidth and high frequency resolution. In broadband mode, 2 THz-wide spectra with 800 MHz resolution can be acquired at a continuous update rate (UR) of 24 Hz. To our knowledge, this is the highest update rate of a broadband, phase-sensitive cw THz spectrometer. In high-speed mode, 200 GHz wide spectra are acquired with 800 MHz resolution at an UR of 120 Hz, ideal for high-speed spectroscopy of absorption lines. In high-resolution mode, frequency steps of 20 MHz and a scan range of 200 GHz allow for high-resolution gas spectroscopy. In broadband and high-speed mode, the peak dynamic range exceeds 65 dB for single shot measurements. More than 100 dB peak dynamic range and a 3 THz bandwidth are obtained after 7 min. averaging in the broadband mode. Due to its high update rates, in combination with high bandwidth and flexible operation modes, this system paves the way for industry-scale non-destructive testing based on cw THz technology.