# 108 m Underwater Wireless Optical Communication Using a 490 nm Blue VECSEL and an AOM
## 2 Design of the 490 nm Blue VECSEL  
At the core of a VECSEL system is a multi-quantum well (MQW) semiconductor gain chip. In the case of a blue VECSEL, the gain chip is composed of various layers, including the AlGaAs etch stop layer with a high-Al composition, the GaAs protect layer, the AlGaAs window layer with a high barrier, the active region, the distributed Bragg reflector (DBR), and the antioxidant GaAs cap layer that emits light at a wavelength of 980 nm. The active region consists of 15 compression-strained InGaAs quantum wells enclosed by GaAs/GaAsP barriers. The DBR is composed of 30 pairs of alternating high-Al (lower refractive index) and low-Al (higher refractive index) AlGaAs layers, designed to have a center wavelength of 980 nm and a high-reflectance bandwidth of 100 nm, with a reflectivity of 99.9% near the fundamental wave. The VECSEL chips are grown, microfabricated, and diced into small chips with dimensions of 4 mm \(\times\) 4 mm. The epitaxial end faces of the chips are then treated using titanium-platinum-gold metallization. The chip is soldered and bonded to a copper heat sink, along with a diamond. Subsequently, the GaAs substrate and etch stop layers are removed through a chemical wet etch process, exposing the active region of the chip. The chip assembly is then clamped to a water-cooled copper block for heat dissipation. The design of the laser-resonant cavity needs to take into account a variety of factors in order to obtain the best laser output effect. Common resonant cavities include straight and folded cavities. Given that the beam waist of a straight cavity is positioned on the chip, adopting a single flat concave mirror is not the most ideal solution for changing the optical field of the external cavity. As a consequence, straight cavities have a poorer frequency efficiency compared to folded cavity configurations. However, the folded cavity creates image dispersion and forms an oval output spot, making the dispersion angle difficult to manage. Consequently, the straight cavity architecture was utilized to ensure that the experimental spot shape remained round. The frequency-doubling crystal (LiB\({}_{3}\)O\({}_{5}\) (LBO)) within the cavity is positioned as close as feasibly possible to the gain chip without direct contact with the pump light, thereby resulting in a larger power density. Figure 1 shows the schematic and actual photographs of a 490 nm blue VECSEL. The blue laser used in the experiment has a straight cavity structure. Focusing the 808 nm pump source onto the chip surface through optical fiber coupling, with a 30\({}^{\circ}\) incident angle, generates a 980 nm fundamental frequency laser with a beam diameter of 400 \(\upmu\)m. A 10 mm LiB\({}_{3}\)O\({}_{5}\) (LBO) frequency-doubling crystal was positioned within the straight cavity and produced a 490 nm blue laser. The output mirror within the straight cavity is coated to achieve a transmittance of 0.1% at a wavelength of 980 nm and a transmittance of 98% at a wavelength of 490 nm. It is possible to achieve a high level of reflection for a 980 nm laser and a high level of transmittance for a 490 nm laser. The laser has a power conversion rate of 25% from pump light to blue light, and the highest power measured was close to 8 W. However, the experimental setup utilized a laser power of 100 mW, which considered factors such as the transmission distance of the laboratory tank, the detection threshold of the detector, the power consumption of the laser, and the thermal management. This decision not only met the requirements for communication performance, but also guaranteed the stability of the system and its long-term reliability. After constructing the optical cavity, the spectra and beam quality of the VECSEL were measured individually. Figure 2 illustrates the fundamental and frequency doubling light spectra of VECSEL measured using a spectrometer (HORIBA iHR320). The beam quality of the VECSEL is shown in Figure 3. The acquired M\({}^{2}\) factors, both in the horizontal and vertical directions, were 1.08.