# 108 m Underwater Wireless Optical Communication Using a 490 nm Blue VECSEL and an AOM
## 4 Results and Discussion
### Measured Performance of UWOC System  
To achieve a 108 m VECSEL-based UWOC link, we analyzed the BER performance at different received optical power levels over a specific transmission distance. We varied the time-slot frequency of the UWOC system from 10 MHz to 20 MHz, 50 MHz, and 100 MHz. The received optical power was attenuated at the receiving end using a series of neutral density filters. As the power decreased, the signal attenuation in the UWOC link became more pronounced. Thus, the study of BER versus received optical power is an important method for evaluating the minimum received optical power required for a VECSEL-based UWOC system to achieve a certain data rate over a certain transmission distance. Signals of different power levels were recorded at 60 m. The waveform recorded by the MSO illustrates the BER of PPM transmissions at different frequencies. The difference in communication performance between different PPM modulation orders and time slot frequencies were verified by varying the amount of received optical power. Figure 9a-d shows the BER of PPM modulation with different modulation orders at the time slot frequencies of 10 MHz, 20 MHz, 50 MHz, and 100 MHz. Respectively, the BER of the three modulation orders are quite close to each other under the same time-slot frequency. Following analysis, the BER of the 64 PPM was lower than that of the 4 PPM and the 16 PPM, corroborating our prior conclusions. Comparing the BER values for the three modulation orders in Figure 9a,b, the 10 MHz and 20 MHz time slot frequencies are approximately equal when the received optical power is the same, with values close to \(10^{-3}\). However, as shown in Figure 9c,d, the BER increases with increasing frequency. For the PPM signals with a time-slot frequency of 50 MHz, the received optical power is \(-19.58\) dBm when approaching the forward error correction (FEC) limit of \(3.8\times 10^{-3}\). At 100 MHz, the BER is already close to \(10^{-2}\). This is due to the decrease in signal modulation depth and the increase in SNR. We followed up with experiments using 64 PPM modulation and signals with a time-slot frequency of 50 MHz. In UWOC systems, water serves as the channel medium and plays a particularly significant role in determining the extent of the optical attenuation. Figure 9: Relationship between bit error rate (BER) and received optical power for different orders of PPM at 10 MHz (**a**), 20 MHz (**b**), 50 MHz (**c**), and 100 MHz (**d**) time slot frequencies; FEC means forward error correction. The attenuation of light in water channels is primarily caused by absorption and scattering phenomena [24]. When light enters an underwater channel, a portion of its power is absorbed by substances such as water, chlorophyll, and humus. Additionally, some of the power is scattered by suspended particles and dissolved salt [25]. The effect of light attenuation in an underwater environment can be described by [PERSON]'s law:  
\[P_{i}=P_{o}\times\mathrm{e}^{-cl} \tag{1}\]  
\(P_{o}\) represents the emitted optical power on the transmitter side before entering the water channel, while \(c\) denotes the attenuation coefficient and \(l\) represents the channel distance. Taking the example of a 108 m UWOC channel, we experimented by adding tap water to the tanks, and the attenuation coefficient of the laser was calculated by measuring the received optical power values at different distances. As shown in Figure 10, measurements were taken at intervals of 12 m. Based on the fitting curve, the attenuation of water in this experiment was 0.24 dB/m. Based on Equation (1), the attenuation coefficient can be determined by analyzing the received optical power at two different distances. The attenuation coefficient was therefore calculated to be 0.0549 m\({}^{-1}\). We tested underwater communication in tap water for a distance of 108 m. We also captured light spot and signal eye diagrams at various distances. As shown in Figure 11a-c, at 24 m and 48 m, the dispersion of the laser spot was still very small.