# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 3 Hybrid RF-FSO Systems Techniques and Their Advanced Achievements
### Radio-over-FSO and Integrated FSO-RF Antenna Systems  
Radio signal transmission using an optical carrier wave can be achieved in three ways: intensity modulation, phase modulation, and intensity-phase modulation. The available FSO systems are exceedingly capable of performing the transmission of a radio signal on par with that of fiber-based optical links, with flexibility and added advantages. The synchronous broad-spectrum RF signal transmission over an FSO link is referred to as Ro-FSO communications. Figure 18 shows a simplified schematic block diagram of an Ro-FSO system that can be used between a base station and remote access points [80]. The Mach-Zehnder Modulator (MZM), pioneered by two physicists, Ludwig Mach and Ludwig Zehnder, can be used as an external modulator in light communications where the intensity/phase of the optical carrier wave is modulated by the RF signal. Because of the Pockels effect, the refractive index of the optical waveguide can be changed, thus resulting in amplitude, phase, and IQ phase modulations [81]. The Pockels effect, which is a linear electro-optic action, states that when an external electric field is applied to an optical medium, it produces birefringence. The MZM device uses this principle in which a beam divider is used to divide the incident optical beam into two paths (see Figure 19). The RF signal input is used to individually control the phase delay in the upper and lower paths in the MZM. Electric field tuning, i.e., the modulation of the optical signal is completed by moderating the intensity/phase value of the light beams propagating along two different paths with respect to the RF signal. The modulated beam(s) will either combine constructively or destructively, thus resulting in a phase-modulated optical signal at the output port. Thus, based on this process, the unmodulated optical beam's phase and amplitude are controlled. Note that a single integrated and compact MZM device with an RF signal applied to one of the arms which induced changes in the refractive index, can be used for the external modulation of the optical wave index [82]. The modulated optical signal at the output of the MZM is given as [82]  
\[E_{out}(t)=0.5E_{in}(t)\bigg{[}e^{i\pi\frac{E_{in}(t)}{V_{\pi}}}+e^{i\pi\frac{ E_{in}(t)}{V_{\pi}}}\bigg{]}, \tag{16}\]  
Figure 19: MZM with (**a**) single- and (**b**) dual-control arms. Figure 18: Simplified illustration of a Ro-FSO simplex system. where \(E_{1}(t)\) and \(E_{2}(t)\) are the electric signals at the lower and upper arms, respectively, and \(V\pi\) represents the bias voltage, which induces a phase shift of \(\pi\) radians between two arms. The constant 0.5 indicates that the input optical power \(E_{in}(t)\) is split into two with equal intensities. The generation of double-sided or multiple-sided moderated signals can also be carried out using a single-arm- or a double-arm-driven MZM. For a dual-drive MZM, the output is defined as [83]  
\[E_{out}(t)=0.5E_{in}(t)\bigg{[}e^{i(\pi\frac{E_{1}(t)}{V_{\pi}}+\frac{\pi}{4})}+ e^{i(\pi\frac{E_{2}(t)}{V_{\pi}}+\frac{\pi}{4})}\bigg{]}=E_{in}(t)cos\bigg{[} \frac{\pi E(t)}{V_{\pi}}+\frac{\pi}{4}\bigg{]}. \tag{17}\]  
Using Bessel function, Equation (17) can be expanded for the part of the signal at\(f_{c}\) of \(E_{out}(t)\) as  
\[E_{out}(t,f)_{f=f_{c}}=\frac{E_{in}(t)}{\sqrt{2}}\bigg{[}2I_{1}\bigg{(}\frac{ m\pi\tau}{V_{\pi}}\bigg{)}cos(2\pi f_{c}t)\bigg{]}. \tag{18}\]  
\(E_{out}(f_{c})\) is the amplitude modulation function for smaller values of m as well as for the first order linear Bessel function [35]. Phase modulation can also be completed using MZM by applying an equal amount of voltage to both the arms.