Description: Semiconductor-based light emitting diodes can be used for photon emission as well as for detection of photons. In this paper, we present a fair comparison between off-the-shelf power Light emitting diodes (LEDs) and a silicon photodetector with respect to their spectral, temporal, and spatial properties. The examined LED series features unexpected good sensitivity and distinct optical bandpass characteristic suitable for daylight filtering or color selectivity. Primary application is short range optical underwater communication, but results are generally applicable.

Description: Underwater swarm robotics is an emerging topic.Compared to individual autonomous vehicles, high-capacity com-munication links are required between the mobile agents. In thistutorial, suitable communication technologies are studied, withemphasis on LED-based underwater optical wireless communica-tions. A comprehensive overview about challenges, advances, andpractical aspects of underwater swarm robotics employing opticalwireless communications is provided. The tutorial includes thefollowing topics: (1) Channel modeling fundamentals; (2) Physicallayer transmission techniques for underwater optical wirelesscommunications; (3) Data link layer aspects and hybrid trans-mission schemes; (4) Ambient light and interference suppression;and (5) Realization aspects. Finally, suggestions regarding futurework are given. The tutorial is intended for readers with abackground or interest in electrical and information engineering.

Description: The number and complexity of subsea applications is constantly increasing, as are the demands on data traffic. Fast underwater optical wireless communication (UOWC) represents a solution for shorter distances. This thesis focuses on the adaption of this technology to the challenging conditions in the channel. Natural water features a narrow spectral window of least attenuation in the blue to green regime. In conjunction with the spectral properties of the LEDs emission and the detectors responsivity, this results in a threefold relationship. In order to enable an energy-efficient system design in coastal waters and to bypass the green-gap in LEDs, the use of converted-green LEDs is proposed and investigated. The effect and the constraints of optical filtering to suppress natural ambient light is examined, a model is developed, and simulations for optimization are carried out. Since the signal-to-noise ratio is very important in an UOWC system, in-depth noise analyses are performed, comprising different detector types and various operating conditions. LEDs can be used as photodetectors, and this is examined for single-color power LEDs in terms of a potential dual-use in UOWC. An unexpected high responsivity and a distinct bandpass characteristic is discovered, and a potential for low-cost applications is clearly identified. A comprehensive survey of housing and port technologies is given and the optical path is analyzed. The total characteristics of assemblies in various configurations are determined by underwater goniometric measurements and compared to calculations. The exact prediction of the link budget relies on individual system attenuation coefficients ksys, but these exhibit many dependencies. In order to generate first spectral curve sets of these coefficients, extensive Monte Carlo simulations of the underwater light field are conducted. Additionally, a method for experimental determination of ksys is developed and field tests are conducted.