Fig. 1. Schematic setup and principle of operation of the optoelectronic FMCW spectrometer.

a Schematic drawing of the optoelectronic swept delayed self-heterodyning FMCW THz setup. Note that the system is fully fiber-coupled without any free-space optics. EDFA erbium-doped fiber amplifier, 3 dB 3 dB coupler, L_Tx, L_Rx optical fiber path length in the Tx and Rx arm, respectively, including an optional static fiber delay; L_THz THz path length, Tx fiber-coupled THz emitter, Rx fiber-coupled THz receiver, TIA transimpedance amplifier, DAQ data digitization and acquisition system. The colored arrows highlight the Tx path and Rx path, and indicate the path lengths relevant for the total delay time that arises from the path length asymmetry. b When the frequency f of the swept laser is tuned at constant velocity ν_S, a frequency offset f₀ = T df/dt occurs for a delay time T.