# 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
#### 3.1.2 Using Zoom Lens  
This method involves using the adjustable focal length of a liquid lens to introduce defocus and change the beam divergence angle. At present, there are three types of liquid lenses commonly used:  
Electro-wetting liquid lens. Figure 12a shows the cavity of this liquid lens, which contains two liquids of similar density and mutually insoluble. The liquid-liquid contact surface forms a refractive surface [41; 42], and the radius of curvature of the surface can be changed by voltage, thereby changing the focal length. The advantage of using it to change the beam divergence angle is that the principle is simple, the zoom range is large, and no mechanical displacement element is needed. Additionally, it can spontaneously form lenses with very high-surface quality. However, it also faces the challenge of difficulty in making the aperture larger. When the lens aperture is greater than 5 mm, the wavefront aberration caused by gravity becomes non-negligible [43]. Liquid crystal material liquid lens. By applying a voltage to a specially shaped electrode, the liquid lens generates a gradient-varying electric field which changes the refractive index and focal length of the lens [44; 45], as shown in Figure 12b. One advantage of it is that the liquid crystal layer can be made very thin, down to tens of \(\mu\)m. And the required driving voltage is small. However, the lens is usually used with a polarizer, which results in low light transmittance and makes it unsuitable for high-power laser applications. Mechanical hydraulic-driven liquid lens. Typically, this technique involves confining the liquid within a closed cavity using a stretchable elastic film [46; 47], as shown in Figure 12c. By applying external pressure to change the curvature radius of the elastic film, the focal length of the liquid lens can be changed. One advantage of it is its short response time. However, its disadvantage is that the rough surface of the elastic film would affect light transmittance, the uneven deformation of the film layer would reduce image quality, and the outer surface of the elastic film is prone to absorbing dust and other small particles. Overall, using a liquid lens to change the beam divergence angle has several advantages, including a short response time, a large variation range for the divergence angle, a simple structure, and the ability to reduce the mechanical actuator and terminal volume. However, it also faces the challenge of the difficulty of making the aperture larger[PERSON] Therefore, liquid lenses with larger apertures are best used in vertical and downward positions and are not suitable for rotation with a turntable. [PERSON] et al. [48] and [PERSON] et al[PERSON] [49] used liquid lenses to change beam divergence angles. [PERSON] et al. [50] used three mechanical hydraulic liquid lenses in combination with wide-angle fisheye lenses to achieve 2D beam scanning of \(\pm 75^{\circ}\), as shown in Figure 13. Tunable liquid lens 1 was used to maintain the beam divergence angle while tunable liquid lens 2 and tunable liquid lens 3 were decentered in the Y and X directions, respectively, acting as prisms to control the beam deflection angle. [PERSON] et al. [51]  
Figure 12: Working principal diagram of liquid lens. (**a**) Electro-wetting liquid lens. (**b**) Liquid crystal material liquid lens. (**c**) Mechanical hydraulic-driven liquid lens. also used three liquid lenses to achieve adaptive control of beam steering and divergence angle, as shown in the inset of Figure 14, called VFL system. The authors found that deterioration of the beam quality can be clearly observed when the liquid lens is aligned horizontally. Therefore, a vertical arrangement of the liquid lens was suggested. Later [PERSON] et al. [52; 53; 54] used the above system to perform an experimental demonstration of wavelength-division-multiplexing (WDM) transmission, as shown in Figure 14. The channel spacing of WDM is 0.8 nm, and the four communication wavelengths are 1549.2 nm, 1550 nm, 1550.8 nm, and 1551.6 nm, respectively.