# Transmitters and Receivers for High Capacity Indoor Optical Wireless Communication
## 2 Beam-Steering for Optical Wireless Communications Transmitter
### Transmitter Experimental Setup  
Our experimental setup implements an Anritsu pulse pattern generator to create an OOK (On-Off Keying) pseudorandom binary sequence (PRBS) data stream. This signal drives a high-bandwidth intensity modulator, which is powered by a C-band laser operating at 10 dBm optical power. The topology of the experiment is illustrated on Figure 3a, and its implementation on Figure 3b. To achieve sufficient signal strength, the modulator's output is amplified to 13 dBm using an Amonics erbium-doped fiber amplifier (EDFA). The amplified beam maintains eye safety standards, resulting in free-space power of 9 dBm after accounting for the system losses. This losses are due to the 1 dB launching loss at the fiber tip and a 3 dB reflection loss from the silicon micro-lenses, as this elements do not have anti-reflection (AR) coatings. While our experimental setup could compensate for these reflection losses through increased amplification, systems with strict power constraints would benefit significantly from AR coatings on the fiber and micro-lenses to preserve transmission power. The beam steering mechanism consists of a pair of commercial bimorph piezoelectric actuators that manipulate the EDFA output fiber along the X and Y axes. For optimal beam formation, the fiber tip is precisely positioned in the focal plane of the fixed micro-lens, with actuator control provided by variable power supplies operating in the range of 0 to +25 V. To achieve precise beam characteristics, we employ a tapered lensed fiber that forms a 2 um waist spot at the lens's focal plane, resulting in a well-collimated beam with a divergence half-angle of 3.2\({}^{\circ}\). The silicon lenses were design to achieve a 30\({}^{\circ}\) with a 20 um displacement from optical axis. This parameters were selected based on the limitations of the given actuators [8]. We used the paraxial matrix equation [11] to determine the appropriate focal length. The Equation (1) allows for estimate calculation of ray propagation through the optical system. \[ \begin{bmatrix}x_{1}\\ \theta_{1}\end{bmatrix} = \begin{bmatrix}1&0\\ -\frac{1}{7}&1\end{bmatrix}\begin{bmatrix}x_{0}\\ \theta_{0}\end{bmatrix}  \tag{1}\]  
where \(x_{0}\) and \(x_{1}\) represent the initial and final ray heights, \(\theta_{0}\) and \(\theta_{1}\) the initial and final ray angles, and \(f\) is the focal length of the lens. This Equation (1) depends on the paraxial approximation where \(sin(\theta)\approx\theta\) and \(cos(\theta)\approx 1\). Consequently, it holds up to 10\({}^{\circ}\), after  
Figure 3: Experimental System Topology (**a**): Experimental setup showing key components (**b**): (1) C-Band Laser Source, (2) 40 GHz Amplitude Modulator, (3) Power Source, (4) EDFA (Erbium-Doped Fiber Amplifier), (5) Actuators, (6) Silicon micro-lenses, (7) Adjustable Fiber Collimator, (8) Multimode Fiber Photo Detector, (9) Pattern Generator and Error Detector, and (10) Oscilloscope. having an increasing error. Assuming that the center ray has a initial incidence angle of \(\theta_{0}=0\), and ignoring the translation of the output beam as it does not influence the steering angle we get the following simplified Equation (2):  
\[\theta_{1}=-\frac{x_{0}}{f} \tag{2}\]  
Through our initial calculations, we determined the lens radius to be 94.8 \(\upmu\)m. To validate and refine this design, we utilized Ansys Zemax Opticstudio 2023, for ray-tracing. The iterative process of ray tracing allowed us to further optimize the lens size, adjusting the radius to 100 \(\upmu\)m and a focal length of 40.2 \(\upmu\)m. These silicon lenses are fabricated in our clean room using a combination of photolithography, dry etching, and thermal reflow. Resulting in silicon lenses have a radius of curvature of 102.6 \(\upmu\)m, Figure 4. This increased size provides a larger area for beam movement, enabling greater steering angles.