An Optically Integrated Coherent Frequency-Domain THz Spectrometer with Signal-to-Noise Ratio up to 80 dB

A terahertz frequency domain spectrometer is implemented using two ErAs:GaAs photomixers in a highly compact configuration, utilizing all solid-state components and no moving parts. The system utilizes a single package integration of two 783 nm distributed feedback laser diodes with a high-resolution wavelength discriminator. Digital signal processing electronics provide precise frequency control and yield ~200 MHz accuracy of the THz signal frequency. Continuous mode-hop-free frequency sweeping is demonstrated with < 1 GHz resolution from 200 GHz to 1.85 THz. The coherent detection sensitivity is shown to be in good agreement with previous theoretical predictions and yields a signal-to-noise ratio of 80 dB/Hz at 200 GHz and 60 dB/Hz at 1 THz through a path length in air of one foot. These levels are more than 10 dB higher than previously measured. The spectrometer frequency resolution and dynamic range are suitable for applications involving analysis of chemical, biological, and explosive materials in solid-phase and gas-phase at atmospheric pressure; system performance is demonstrated via the transmission spectra of atmospheric water vapor.