 CW-laser beams can be varied by tuning the laser cavity length, the optical grating structures, and the operating temperature. The optical sources used for photo mixers are generally the laser diodes, and the system is compact in structure with less complexity and light weight. The geometry of the photo mixer also affects the photocurrent and optical to terahertz conversion efficiencies. The dipole photo mixer gives high carrier density, high photocurrent, and better conversion efficiencies. But in this geometry, the elec- tric field distribution is not uniform. Hence, to attain a uniform electric field distribu- tion, interdigitated-like structures can be incorporated. This geometry has higher conductance as well as higher gap capacitance between the electrodes which helps to generate a high THz electric field. Moreover, it has much less screening effect compared to the dipole PCA. 12.5.3 THE PHOTOCONDUCTIVE ANTENNA Photoconductive antennas (PCA) are made of semiconductor substrate material with the metal electrodes placed on the top of the substrate, a laser source, and a DC bias voltage. In earlier literature, they are referred to as Auston switches as they were first discovered by Auston in 1984, similar to the Hertzian dipole structure. The THz PCA works with the principle of photoconduction. An ultrashort laser pulse with photon energy higher than the photoconductive material bandgap is excited on the THz PCA. The photocarriers are created and then bias voltage is applied to the electrodes. The photocarriers are accelerated towards the corresponding electrodes by the biased electric field and generate the photocurrent which drives the antenna to radiate the terahertz waves as represented in Figure 12.2. To generate the pulsed THz system, the PCA can be used both as an emitter and detector which is based on the ultra-fast optical techniques. 5G Wireless Communication System in Healthcare Informatics Si lens Femtosecond laser pulse radiation FIGURE 12.2 Biased THz emitter PCA. In RF/MW antennas, the substrat