# 108 m Underwater Wireless Optical Communication Using a 490 nm Blue VECSEL and an AOM
###### Abstract  
Advanced light sources in the blue-green band are crucial for underwater wireless optical communication (UWOC) systems. Vertical-external-cavity surface-emitting lasers (VECSELs) can produce high output power and good beam quality, making them suitable for UWOC. This paper presents a 108 m distance UWOC based on a 100 mW 490 nm blue VECSEL and an acousto-optic modulator (AOM). The high-quality beam, which is near diffraction-limited, undergoes relatively small optical attenuation when using a conventional avalanche photodiode (APD) as the detector and employing 64-pulse position modulation (PPM). At the time-slot frequency of 50 MHz, the bit error rate (BER) of the UWOC was 2.7 \(\times\) 10\({}^{-5}\). This is the first reported AOM-based UWOC system with a transmission distance over 100 m. The estimated maximum transmission distance may be improved to about 180 m by fully utilizing the detection accuracy of the APD according to the measured attenuation coefficient of the blue VECSEL used. This type of UWOC system, composed of a high-beam-quality light source and a conventional detector, make it more closely suited to practical applications. underwater wireless optical communication; vertical-external-cavity surface-emitting laser; PPM; AOM 202424609. 24609.  
communication system, although underwater wireless optical communication is not as good as acoustic wave communication in terms of transmission distance, the optical signal has good directionality, the multipath effect is usually much smaller than in acoustic waves, and its bandwidth advantage can transmit more information in a shorter time [4; 5]. The advantages of UWOC can provide more efficient and consistent communication assistance for marine activities. In the future, UWOC will encourage the development of maritime communication technology alongside other communication techniques. The light source plays a critical role in UWOC systems. Light-emitting diodes (LEDs) and laser diodes (LDs) have been employed as light sources for UWOC systems. LEDs have a wide field of view and are easier to align for transmission and reception [6]. LDs have a small beam-scattering angle to allow for longer transmission links and high transmission rates. Therefore, they are widely used in most high-speed and long-distance UWOC systems [7; 8; 9]. The common modulation methods in UWOC systems are direct modulation and external modulation. Direct modulation is the direct input of a signal into the driver circuit of a light source, thus changing the output characteristics of the beam. Many studies related to direct modulation have been reported, but due to the limited output power of directly modulated lasers, they have mainly focused on low-power experiments [10; 11; 12; 13]. Acousto-optic modulator (AOM), as an external modulation, has been used in free-space optical communications due to its high extinction ratio, low driving power, and good temperature stability [14]. We believe that an acousto-optic modulator can be used for UWOC to achieve a better modulation bandwidth and a higher optical power, but this would require a more competitive light source [15]. In recent years, there has been substantial improvement in Vertical-external-cavity surface-emitting lasers (VECSELs) technology, principally owing to the adjustable external cavity design that can be modified to satisfy varied needs. Moreover, the performance of VECSELs in terms of producing high output power and good beam quality has received substantial attention from both the academic and industrial sectors [16; 17; 18]. Consequently, VECSELs can be designed with variable exterior cavities to fit different underwater circumstances, showing tremendous promise for UWOC. Most UWOC systems use the intensity modulation/direct detection (IM/DD) technique because of the simplicity and low complexity of its implementation.