# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 2 FSO Communication Channel Effects and Their Mitigation Techniques
### FSO Communication--Spatial Profile Challenges
#### 2.3.4 Random Focus Spot Mitigations  
Atmosphere is considered as a non-homogenous and non-isotropic medium, where the phenomena of turbulence is mainly caused by a difference in temperature distribution, spatially and temporally. This gradient distribution of temperature causes the distribution of refraction index which is also vary with space and time. The distribution of temperature leads to the conservation laws of thermodynamics phenomena, where the flow of air in  
Figure 7: Sensor-less AO system for wavefront aberration correction: inner plot (**a**) is without AO while (**b**) is with AO. the atmosphere will break the laminar state into a turbulence state. The phenomena are indicated by the Reynolds number, where turbulence eddies of the atmosphere obey the minimum value of 105, to change the state from laminar into turbulence. Thus, the distribution of the refraction index is no longer in the steady state but has changed drastically into a random distribution. For this reason, atmosphere as the media of optical propagation can be pointed out as the media of optical turbulence, where the distribution of the refraction index becomes a random distribution in space and time. Regarding whether the medium of the atmosphere is in a state of optical turbulence, the interaction of optical propagation in free space such as refraction, absorption, diffraction, and scattering also becomes random. As can be well understood, the media of optical turbulence is a natural process and it cannot be avoided nor can be neglected. FSO as the UOC scheme in free space faces those turbulence effects, as depicted in Figure 8, which is a major problem. The aforementioned major processes along the optical propagation are stochastic processes and could happen simultaneously. Thus, the degradation of performance could become worse if these major processes exist along the optical propagation and are received by the photodiode (PD). Regarding the stochastic process of optical turbulence, there is an opportunity to involve an optical method to reduce or minimize the turbulence effects into optical propagation. Generally, there are two schemes of signal reception in FSO; the first is the direct-detection (DD) method and second is the fiber-detection (FD) method [66; 67]. DD is commonly used as the final signal detection in the Rx of FSO. DD receives an optical signal directed by a receiver lens, whereas FD is commonly used as the signal amplification method in order to enhance the signal strength. FD is frequently implemented in the scheme of a relaying network, where a node has a role to amplify an optical signal forward and deliver it into next node of relaying, over several kilometers of optical propagation. FD is also implemented with an optical amplifier such as an Erbium-doped fiber amplifier (EDFA), praseodymium-doped fluoride fiber amplifier, fiber Raman-amplifier or semiconductor optical amplifier [68; 69; 70]. In Ref. [71], an optical method was developed to suppress the noise modulation instead of improving the signal quality input into the PD. The optical method is called spatial filtering, as shown in Figure 9, where a pinhole is used before the optical signal is received by the PD. The result of this method has been successfully reported in filtering spatial noise modulation and suppressing scintillation as well.