Self-heterodyne terahertz spectrometer based on photodiodes

We demonstrate a self-heterodyne terahertz (THz) frequency-domain spectrometer in which commercially available uni-traveling-carrier photodiodes (UTC-PDs) are used as an optical-to-electrical (O/E) converter and a photonic THz mixer. Optical intensity beat generated by mixing two frequency-detuned 1.55 μm laser diodes is used as a source for the photomixing at the O/E converter and also used as a photonic local oscillator (LO) signal for the THz mixer. The amplitude and phase of the THz wave can be measured simultaneously without delay-line modulation, which improves system simplicity and decreases data acquisition time compared to the conventional homodyne system. We confirmed that the frequency response of the system calculated using the frequency characteristics of the output power of the UTC-PDs fits to the tendency of the measured response in the range of 150 GHz to 350 GHz.