Underwater optical communication has recently become the topic of much investigation as the demands for underwater data transmission have rapidly grown in recent years. The need for reliable, high-speed, secure underwater communication has turned increasingly to blue-light optical solutions. The blue-green visible wavelength window provides an attractive solution to the problem of underwater data transmission thanks to its low attenuation, where traditional RF solutions used in free-space communications collapse. Beginning with GaN laser diodes as the optical source, this work explores the encoding and transmission of digital data across underwater environments of varying turbidities. Given the challenges present in an underwater environment, such as the mechanical and optical turbulences that make proper alignment difficult to maintain, it is desirable to achieve extremely high data rates in order to allow the time window of alignment between the transmitter and receiver to be as small as possible. In this paper, work is done to increase underwater data rates through the use of orbital angular momentum. Results are shown for a range of data rates across a variety of channel types ranging in turbidity from that of a clear ocean to a dirty harbor.
Space division multiplexing of optical beams has recently been demonstrated for improving the bandwidth of optical communication links. This paper will explore the use of space division multiplexing utilizing blue lasers for potential undersea applications. Experimental results will be shown for optical vortices utilizing a range of charge numbers corresponding to various Orbital Angular Momentum states.
The recent development and refinement of Gallium nitride (GaN) semiconductor devices has produced both blue light emitting diodes (LEDs) and laser diodes, which provide an efficient means to obtain high emission powers in the blue spectral range. Such sources have potential applications in both imaging and communication systems. However, many applications require precise control over the spectral emission from these devices and the current blue laser diodes lack this ability. In this paper, we demonstrate a method to control the spectral emission from GaN blue laser diodes. We present the simulation and subsequent fabrication of a guided-mode resonance filter (GMRF) that can be used to lock the output wavelength of a GaN blue laser diode. Successful locking of the emission wavelength with respect to fluctuations in the surrounding environment addresses challenges associated with communication systems. Our experiment uses an optical cavity with a GaN blue laser diode source and an on-axis narrowband GMRF fabricated for 445.2 nm. Based on spectral drift of the diode emission caused by an increase in input current, experimental measurements were taken with the GMRF installed to verify wavelength locking capability.