# Evaluation of Optimal Visible Wavelengths for Free-Space Optical Communications
## 3 Methodology
### Experimental Setup  
The experimental platform was designed to emulate a short-range, point-to-point FSO link over a fixed transmission distance of 1 m. This specific distance was chosen to ensure experimental repeatability, minimize optical alignment errors, and eliminate environmental variables such as beam divergence and mechanical vibration. At this range, it becomes possible to isolate the intrinsic characteristics of each laser diode--such as threshold behavior, quantum efficiency, and modulation response--without interference from atmospheric or path-loss effects. While this configuration does not reflect full-scale FSO deployment, it serves as a baseline reference for diode-level performance under controlled conditions and provides a robust foundation for future, extended-range studies. The main components of the system are summarized below:  
* Laser Diodes: Three single-wavelength sources centered at 650 nm (red), 532 nm (green), and 405 nm (violet) were used as optical transmitters. * Control Unit: An Atmega328p microcontroller, interfaced with a Visual Basic for Applications (VBA) GUI in Microsoft Excel, enabled real-time adjustment of current and modulation frequency with high precision. * Photodetector: A silicon photovoltaic cell (VACTEC7-21G72S, Vactec Inc., St. Charles, MO, USA) acted as both the optical receiver and an energy harvester, converting incident light into electrical output. * Signal Conditioning: The output of the photodetector was amplified using a low-power ICL7611 operational amplifier (Renesas Electronics Corporation, Tokyo, Japan) to match the 0-5 V input range of the microcontroller's analog-to-digital converter. This modular hardware architecture enabled precise electrical control and standardized optical evaluation across all diode configurations, ensuring a fair comparison under identical test conditions. Figure 1 illustrates the block diagram of the complete system and the role of each subsystem in modulation, signal generation, detection, and data acquisition. The components of the experimental setup shown in Figure 1 are described as follows: PC with VBA/Excel Interface: Provides the user interface for setting diode parameters and recording measurements via USB communication with the microcontroller. AVR Microcontroller Unit (MCU): Executes control logic, generates digital signals, and orchestrates modulation and current settings based on user input. Port IO 2-4: Digital output pins controlling laser diode activation. Port IO 8-11: Digital outputs regulating modulation frequency selection. Port IO AD0-AD3: Analog outputs for driving current control via DACs. Digital-to-Analog Converters (DACs): Convert digital control signals to analog voltages for regulating frequency (via the VCO) and diode current (via the voltage regulator). Voltage-Controlled Oscillator (VCO): Produces square-wave modulation signals; frequency is controlled by analog input from the DAC. Duty Cycle Control: Adjusts the on/off ratio of the modulation waveform to optimize spectral efficiency and reduce noise susceptibility. Voltage Regulator (LM317T, Texas Instruments, Dallas, TX, USA): Converts DAC output voltage into a stable current source for laser diode operation. Output Amplifier (ICL7611): Amplifies the regulated signal to ensure diode operation within its optimal power range. Laser Diode Module: A switchable module housing three visible laser diodes (650 nm, 532 nm, 405 nm); only one diode operates during each test. Photovoltaic Detector: The VACTEC7-21G72S silicon photocell detects modulated optical signals and converts them into measurable voltage output, while also supporting energy harvesting. Operational Amplifier (ICL7611, Renesas Electronics Corporation, Tokyo, Japan): Amplifies the photocell output to a readable level for the MCU's analog input. This configuration enables repeatable, high-precision measurements of laser diode behavior under dynamic modulation and electrical input conditions.