Electric field visualization system for antenna characterization at terahertz frequency based on a nonpolarimetric self-heterodyne EO technique

We demonstrate a nonpolarimetric self-heterodyne electrooptic (EO) field visualization system at terahertz (THz) frequency based on a 1.55 μm telecom technology. An optical intensity beat generated by mixing two frequency-detuned free-running lasers is used for both the generation and the detection of the RF signal. The frequency of the beat for the RF generation is shifted by an optical frequency shifter to realize coherent heterodyne measurement using free-running lasers. The amplitude and the phase of the RF signal is read out through the IF signal generated by heterodyning of the probe beam and the sideband generated by the RF signal to be detected. In our nonpolarimetric technique, the polarization state of the probe beam is maintained, therefore the sensitivity of the system is stable compared to the conventional system in which the RF signal is read out through the polarization state of the probe beam which easily fluctuates in the optical fiber and the EO crystal. Stable sensitivity improves the repeatability of the measurement which enables us to visualize the phase evolution of the radiated field. The phase evolution of the propagating THz field (126 GHz) radiated from a horn antenna is visualized.