Underwater robots are widely employed in exploration and intervention missions in oceans, seas, rivers and lakes. Wireless data transmission in underwater environments is one of the biggest challenges presented in the development of robotic systems, since the use of Radio Frequency (RF) is practically unfeasible due to the high attenuation imposed by the environment. Underwater optical modems are great alternatives for solving this issue, as they allow for the establishment of communication links with MHz frequency band at distances of up to a hundred meters. The development of unidirectional devices is discussed and the system general functioning is evaluated, in a hyperbaric chamber, through the transmission of a stream of data, with emphasis on the maximum band reached and the measured Bit Error Rate (BER).