# Transmitters and Receivers for High Capacity Indoor Optical Wireless Communication
## 2 Beam-Steering for Optical Wireless Communications Transmitter
### State of the Art for Narrow Beam-Steering  
Several techniques have been investigated for optical beam steering, each with distinctive characteristics and limitations, such as: Polarization gratings employ liquid-crystal imprinted with gratings to deflect circular polarized beams [4]. The gratings create a diffraction pattern that is switchable depending on the state of the liquid-crystal. Each grating has a fixed steering angle, but can be stacked to give a greater FoV. Although these systems benefit from low power consumption, their functionality is limited by fixed steering angles determined by the number of stacked gratings; they are also constrained by the circular polarized light for side lobe suppression. Arrayed waveguide grating routers implement a passive steering mechanism by selecting outputs from a fiber array positioned in a lens's focal plane [2]. This approach enables efficient multi-beam handling but faces limitations due to discrete steering angles and reliance on tunable laser sources. Meta-surfaces use sub-wavelength resonators to manipulate a reflected beam wavefront [5]; these nanoscale structures offer compact form factors and rapid steering capabilities. However, their application is restricted by narrow optical bandwidth operation, one-dimensional steering constraints, and beam squint effects, as the resonators are optimized for specific wavelengths and directions. Optical phased arrays achieve beam steering through coordinated phase manipulation in multiple emitting elements [6]. By adjusting the phase shifts across the coupling grating array, the overall wavefront can be shaped, allowing for beam steering through constructive and destructive interferences. While this approach provides an integrated solution, it presents significant fabrication and control complexities, particularly for two-dimensional steering applications that require wavelength tuning. Spatial light modulators enable beam steering by modulating the refractive index of pixels in a two-dimensional matrix [7]. These devices offer flexibility in wavefront shaping for multiple beams, through careful design of the screen hologram, but are inherently limited by pixel size, pitch, and refresh rate constraints. The proposed beam steering approach employs MEMs cantilevers with piezoelectric transducers (PZTs) [8], as illustrated in Figure 2, offering a simpler solution for optical wireless communications. By manipulating the light source within a lens's focal plane, the system achieves precise beam steering with remarkable flexibility. A key innovation lies in the integration of silicon micro-lenses with piezoelectric benders, which provides superior optical characteristics through silicon's high refractive index and transparency at C-Band wavelengths. The method's unique design ensures that optical properties remain nearly unchanged, as only the fiber tip is moved. Silicon micro-lenses magnify this movement, increasing achievable steering angles to effectively cover a large wireless cell. Experimental implementations have demonstrated impressive steering capabilities, including angles of 51.3\({}^{\circ}\) horizontally and 22.4\({}^{\circ}\) vertically, with wavelength independence across the infrared spectrum and continuous beam movement [9]. Despite its potential, the approach presents challenges such as asymmetric steering ranges and mechanical coupling between actuators. Each user beam requires individual piezoelectric transducers and control circuits, as seen in Figure 2b. This system offers a promising solution for indoor optical wireless communications, particularly where dynamic user tracking and flexible beam management are critical. As shown in Table 1, while the current implementation doesn't support multi-beam operation, it demonstrates a robust 10 Gbps OOk throughput. Although the throughput is lower than the Spatial Light Modulator (224 Gbps) and Arrayed Waveguide Grating (112 Gbps) solutions, the system offers unique advantages that compensate for this limitation.