## 2 Experimental Scheme  
Underwater wireless optical communication consists of the transmitter, channel path, and receiver side. The proposed scheme is shown in Figure 1 below:  
The experiment scheme consists of the transmitter, medium, and receiver. At the transmitter side, a current driver (ILX Lightwave LDC3722, ILX Lightwave Corporation, Bozeman, US) drove the current to the blue laser diode (OSRAM PL450 TO-38, OSRAM Licht AG, Munich, Germany). The laser diode (LD) used collimation to focus the light and a cooler to control the laser temperature. The light was transmitted through a 1.5 m \(\times\) 0.3 m \(\times\) 0.3 m size water tank. Secondly, the medium was seawater with different types of salinity. The heater, fan, and water pump were placed inside the tank to vary the temperature and water condition. A digital thermometer showed the temperature measurement inside the water tank. Then, an optical mirror reflected the laser beam and expanded the transmission distance to 3 m. In the receiver part, the focus mirror lens and avalanche photodiode (APD 210, Menlo System GmbH, Martinsried, Germany) were used to receive the laser beam. Then, the information signal was evaluated using a bit error rate test (BERT). The modulation in this experiment used NRZ-OOK. This experiment used a pseudorandom binary sequence 7 with a repetition period of 127 values. The equation can be found in N = 2\({}^{\mathrm{k}}-1\), where 'k' indicates the size of random data for each sequence. For a maximum length sequence, where N = 2\({}^{\mathrm{k}}-1\), the duty cycle is \(\frac{1}{2}\). A bias tee was used to superimpose a modulation current onto the laser diode DC-supply current, thereby modulating the output power of the laser. The modulation speed was 1.25 Gbps, and the signal generator could modulate up to 2.5 Gbps. Due to limitations in our laboratory, the APD frequency could only receive  
Figure 1: UWOC experimental scheme for environmental measurement. modulations up to 1.25 Gbps. In the demodulation part, we used a Photodetector (Menlo System GmbH, Martinsried, Germany) APD 210 to change the optical signal to an electrical signal. The BERT measurement was used to compare the modulated signal transmitted and the demodulated signal received by the photodetector. The relationship between salinity, mass, and volume is shown in the equation below:  
\[\text{Salinity}=\frac{\text{Mass}}{\text{Volume}}, \tag{1}\]  
As shown in Equation (1), the salinity value is mass (g) divided by volume (L). When the salt was dissolved in the tap water, salinity increased. Therefore, the amount of mass added to a particular volume of water increases the salinity of the water. The authors did measure using a hydrometer first to see the density in the hydrometer scale. Since there may have been undissolved salt inside the water tank, the extra salt could have drowned in the bottom of the water tank. The hydrometer scale of seawater was around 1.025 at 25 \({}^{\circ}\)C and 37 salinity in parts per 1000 [17]. The volume was obtained by multiplying the width 0.3 m \(\times\) length 1.5 m \(\times\) height 0.12 m; the result was 54 L.  
Based on Table 1, sea salt was added to vary the environment of the water in this experiment. In our experiment, the salinity of water, based on the hydrometer scale, was 1.03 by adding 45.56 g/L sea salt into the tap water. The water tank was filled with salt water, and a hydrometer was used to measure water salinity, as shown in Figure 2. Typically, the salinity increases from the ocean's surface to deep water; thus, the saltwater density increases as seawater depth increases at a constant temperature (R1Q3). Based on the halocline or a vertical salinity gradient, low-salinity water floats on top of high-salinity water. When the density increases, the amount of salt in the water, or salinity, also increases. Many factors may change the seawater density. Whether the polar ice is melting or frozen can affect salinity. The sea salt was evenly distributed.