How to spare the phase modulator in fast-sweeping coherent cw THz systems

A new scheme for rapid coherent cw THz spectroscopy is presented achieving full phase information without active phase modulation. Through photomixing, the beating of two lasers drives the generation of the THz field. Due to the interferometric character of this technique, fast sweeping of one laser combined with a static delay-path results in continuous phase and amplitude modulation of the signal. A quadrature lock-in amplifier running at this modulation frequency detects amplitude and phase while reducing noise. This scheme allows for fully phase sensitive spectroscopy without the need of a phase modulator or an amplitude modulator.