# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 2 FSO Communication Channel Effects and Their Mitigation Techniques
### FSO Communication--Spatial Profile Challenges
#### 2.3.3 Wavefront Corrections  
In optical beams, during the propagation, phase disturbances are introduced due to fluctuations in the atmosphere. Phase compensation based on adaptive optics (AO) provides an effective solution for making the FSO communication system reliable [60]. Usually, heavy wavefront aberrations are caused by fluctuations in ground atmospheric turbulences [61]. These are commonly seen in the applications of astronomical imaging. AO is a better solution used to compensate for wavefront aberrations [62]. Li, M et al., in their study, used real-time wavefront correction AO techniques to evaluate the channel capacity of FSO links based on OAM, under atmospheric turbulence of different ranges [63]. Liu, W et al., in their paper, derived empirical models for the fluctuation of AOA and wavefront distortion. In their work, they used a deformable mirror (DM) and an FSM to compensate for the distortions in the wavefront [64]. The distortions in the wavefront were evaluated directly by using a wavefront sensor (WFS), a charge-coupled device  
Figure 6: Schematic of a PAT system in FSO communication.  
(CCD) camera or Shack-Hartmann WFS (SH-WFS). In all these sensor measurements, the wavefront aberrations were corrected using a deformable mirror (DM) or MEMS mirrors. However, a WFS-based measuring approach has significant drawbacks, especially for biological imaging using multiphoton microscopes and turbulence-induced optical paths in FSO communications [63]. To solve these limitations, AO systems without a wavefront sensor were presented and determined to be more appropriate for both FSOC and astronomic applications [65].  
Today's AO systems are attracting significant attention for both model-free and model-based optical beam phase aberration correction techniques. Figure 7 shows a conceptual configuration of AO-based FSO systems, where a Gaussian beam is transmitted using the laser source from a remote Tx point. As the transmitted beam travels through a turbulence-induced atmospheric channel, its wavefront is aberrated. An optical lens is used in the collection, and the collimation of the arriving beam and is made to fall on the DM. As the beam falls on the DM, it is reflected and is split into two paths: the propagating beam and the reflecting beam. The communication detector detects the propagating beam while the CCD camera receives the reflected beam. To drive the DM to make the aberration-restrained \(\Phi(r,\theta)\) accurate, a DSP controller is used to generate appropriate regulatory signals \(\Psi(r,\theta)\), which are based on the output of a CCD. This closed-loop process continues if the wavefront aberration remains. The correction of wavefront distortions in the incoming optical beam, i.e., \(\varphi(r,\theta)\) in Figure 7, improves the coupling efficiency and, consequently, the quality of communication. The performance capability of the AO system is determined by a metric known as \"Strehl ratio\" (SR) [65]. A higher SR means a better performing system. In Figure 7, the inserts (a) and (b) show the beam profile without and with the AO device, respectively. In-depth discussions on other control techniques of the measurement of wavefront aberrations, wavefront corrections, and control algorithms are available in [64; 65]. High-speed wavefront aberration measurement and correction is crucial in FSO communications because of high data rates. As mentioned in [63], present-day FSO-AO systems suffer from a poor rate of speed of wavefront correction, which invites the need for real-time high-speed (nano second), high-accuracy (prad) actuators and their controllers.