 emitter and the probe laser pulse (split from the beam splitter) passes collinearly into an electro-optic crystal. This incoming THz signal electric field is directly proportional to the birefringence (generated by the Pockels effect) of the EO crystals. The first order electro-optic effect rotates the polarization of the optical probe beam. This change in polarization is accounted for by a balanced detector placed behind a quarter wave plate (QWP) and a Wollaston prism. The QWP converts the linear polarization into a circular or elliptical polarization depending on the amplitude of the electric field components. The Wollaston prism/polarization sensitive beam splitter separates two orthogonal polarizations into two identical Wollaston EO crystal polarizer splitter Balanced detection Without THz ETH2(t) With THz Probe polarization FIGURE 12.11 Illustration of electro-optic sampling. 5G Wireless Communication System in Healthcare Informatics photodetectors. The difference between both photodetector signals determines the magnitude and phase of the THz signal. The higher the incoming THz signal at the EO crystal, the greater is the difference in the signal at the balanced photodetectors. This process can detect the phase of the THz signal as the optical pulse (~ 10s of fs order) which is much shorter than the THz pulse (~ ps order) while sampling it. This electro-optic sampling method is very sensitive to environmental noise, therefore, the measurement of the balanced photodiode signal through a lock-in amplifier is abso- lutely necessary to achieve a noiseless detection. 12.8.2 THz HETERODYNE DETECTOR To detect the weak and narrowband signals, a coherent detection technique such as heterodyne detection can also be used. The block diagram of a heterodyne detector to detect emitted terahertz waves is given in Figure 12.12. Nonlinear devices such as the Schottky diode, tunnel diodes, and bolometers are used in the mixers to combine the terahertz signal with the local signal. The high frequency signals are then filte