# A Review of Variable-Beam Divergence Angle FSO Communication Systems
## 3 Existing Methods for Vbda
### Vary Beam Divergence by Adjusting the Focal Length F of the Collimating Lens
#### 3.2.3 Using Optical Wedge Pair  
The schematic diagram in Figure 19 shows the optical wedge pair used in the optical path, where the thickness of the pair is adjusted by adjusting the moving optical wedge to change the beam divergence angle. This method offers several advantages, including a wide range of divergence angle changes, low impact on the optical axis deflection during adjustment, and a large tolerance for lateral positioning errors. This method requires a tradeoff between the range of VBDA and wavefront aberrations. If the wedge angle is reduced, the range of VBDA will be narrowed. Conversely, if the wedge angle is increased, astigmatism will be introduced, causing the outgoing beam to become oval. Figure 17: **(a)** FSO terminal and its VBDA mechanism. **(b)** Prototype of FSO communication terminal principle [63]. Reprinted/adapted with permission from Ref. [63]. 2021, Elsevier. Figure 18: Schematic diagram of VBDA achieved by inserting a parallel plate. **(a)** Collimation state. **(b)** Large divergence angle state. [PERSON] et al. [67] used a set of optical wedge pair of the same size between two lens groups in the beam expander to achieve the function of VBDA. [PERSON] et al. [58] placed the wedge pair in front of the collimating lens and achieved continuous adjustment of the beam divergence angle between 38 \(\upmu\)rad and 300 \(\upmu\)rad (full width at half maximum (FWHM)), as shown in the green dotted box in Figure 20. Building upon this mechanism, [PERSON] et al. [68] designed and manufactured a FSO communication system with continuously adjustable beam divergence angle and optical power, also shown in Figure 20. To minimize the terminal SWaP, the beacon and signal beams (at different wavelengths) were amplified by the same HPOA, and the relative power level of each beam was dynamically controlled using an electronically controlled variable optical attenuator (VOA). In summary, we have thoroughly investigated various VBDA methods in FSO communication systems. The key parameters of all current VBDA mechanisms in Table 1, including the wavelength, range of beam divergence angles, power range, communication distance, data rate, and bit error rate (BER). Moreover, we have assessed the advantages and disadvantages of different VBDA methods and compiled them in Table 2. This comprehensive overview can help designers and researchers compare and evaluate the performance of different VBDA techniques for various FSO applications. By highlighting the benefits and limitations of each method, we aim to provide readers with a clear understanding of the trade-offs involved in selecting a particular VBDA technique. Figure 19: Schematic diagram of the VBDA method using wedge-pair. (**a**) Collimation state. (**b**) Large divergence angle state. Figure 20: Optical path diagram of FSO communication system with continuously adjustable beam divergence angle and optical power [58; 68]. Reprinted/adapted with permission from Ref. [58]. 2015, SPIE. Reprinted/adapted with permission from Ref. [68]. 2019, [PERSON]. Furthermore, we have presented Table 3, which summarizes the current studies that have applied VBDA technology to different link types, such as ground terminal platforms, air platforms, high-speed train (HST) platforms, underwater platforms, and satellite platforms. It is worth noting that each VBDA method described in this paper has its unique characteristics, and designers should choose the appropriate technique for the given FSO link situation and communication terminal conditions. Factors such as power consumption, beam quality, cost, and environmental robustness must be carefully considered when selecting a VBDA method. We hope that the comprehensive analysis and discussion in this paper will contribute to the advancement of VBDA technology and its successful integration into practical FSO systems.