Multiplexed visible light communication systems using GaN-based sources

With the emergence of efficient semiconductor solid state lighting a new application space has emerged for communications, namely visible light communications (VLC). The high speed modulation capabilities of Gallium Nitride (GaN) based LEDs and laser diodes means these devices have the potential to supplement or replace existing radio wave standards such as Wifi, as well as creating new applications for optical communications such as underwater VLC. Given the ever increasing demand for information in modern society, it is desirable to continually increase the bandwidth capabilities of communication systems through both exploration of unused frequency spectrum, like the visible spectrum, and also the application of existing and developing multiplexing techniques. This thesis will focus on the investigation of quadrature amplitude modulation (QAM) based orthogonal frequency division multiplexing (OFDM) as applied to GaN based sources in various VLC systems. In this thesis investigation of complex modulation formats and advanced multiplexing techniques applied to novel m-LED devices has shown VLC system bandwidths of up to 655 MHz and data rates of up to 7.91 Gbit/s which, to the authors knowledge, is the highest data rate achieved using a single m-LED pixel for data transmission. The laser based VLC systems shown in this thesis have utilised both the simplest form of baseband modulation on off keying (OOK) as well as QAM based OFDM. These systems, at the time of publication, demonstrated the highest data rate achieved for each of these modulation types using commercially available devices. In addition to multiplexing using orthogonal frequencies the implementation of spatial multiplexing has become an area of great interest for free-space optical (FSO) communication links, particularly for its use in last-mile links within larger optical networks. Light carrying orbital angular momentum (OAM) has emerged as a potential candidate that could be utilised for multiplexing independent channels. The feasability of OAM multiplexing underwater has been investigated through analysis of inter-channel crosstalk for a set of 11 OAM modes propagating through 3 m of slowly flowing water, similar to that found in Oceanic conditions. At publication this was the first investigation of its kind where crosstalk effects induced by flowing water were measured

Laser Based Underwater Communication Systems

We report on recent progress in the field of visible light communications including direct modulation of blue laser devices at data rates beyond 10 Gbit/s, and the transmission of 2.5 Gbit/s OOK data through water. We also discuss the advantages of operating with single mode laser devices and matched filtering at the receiver in the context of applications with significant solar background. The system performance for two types of direct-detection receivers, a PIN detector and less conventional silicon Photomultiplier technology will be presented

A space division multiplexed free-space-optical communication system that can auto-locate and fully self align with a remote transceiver

Free-Space Optical (FSO) systems offer the ability to distribute high speed digital links into remote and rural communities where terrain, installation cost or infrastructure security pose critical hurdles to deployment. A challenge in any point-to-point FSO system is initiating and maintaining optical alignment from the sender to the receiver. In this paper we propose and demonstrate a low-complexity self-aligning FSO prototype that can completely self-align with no requirement for initial manual positioning and could therefore form the opto-mechanical basis for a mesh network of optical transceivers. The prototype utilises off-the-shelf consumer electrical components and a bespoke alignment algorithm. We demonstrate an eight fibre spatially multiplexed link with a loss of 15 dB over 210 m

15 Gb/s OFDM-based VLC Using Direct Modulation of 450 GaN Laser Diode

A record data rate for visible light communications (VLC) using a transistor outline (TO) packaged Gallium Nitride (GaN) laser diode is reported. Using a system 3 dB bandwidth of 1.4 GHz data transmission at 15 Gb/s is reported. This is achieved due to the use of orthogonal frequency division multiplexing (OFDM) in combination with a high system signal to noise ratio (SNR) and adaptive bit loading extending the effective bandwidth to 2.5 GHz. To the best of authors knowledge this is the highest reported data rate for single channel VLC

Recent progress in distributed feedback InGaN/GaN laser diodes

Laser diodes based on Gallium Nitride (GaN) are useful devices in a wide range of applications including atomic spectroscopy, data storage and optical communications. To fully exploit some of these application areas there is a need for a GaN laser diode with high spectral purity, e.g. in atomic clocks, where a narrow linewidth blue laser source can be used to target the atomic cooling transition. We report on the continuous wave, room temperature operation of a distributed feedback laser diode (DFB-LD) with high-order notched gratings. The design, fabrication and characterization of DFB devices based on the (Al,In) GaN material system is described. A single peak emission at 408.6 nm with an optical power of 20 mW at 225 mA and a side mode suppression ratio (SMSR) of 35 dB was achieved. Additionally, we demonstrate the use of a GaN DFB-LD as a transmitter in visible optical communications system. We also present results from a DFB-LD optimized for laser cooling of Sr+

Degradation of light carrying orbital angular momentum by ballistic scattering

Structured light can enhance the functionality of optical communication and sensing systems. Dense scattering environments such as those experienced in coastal water and foggy conditions result in degradation of structured optical fields. We present findings that characterize the degradation of the phase structure of ballistic scattered light carrying orbital angular momentum (OAM) propagated through a dense scattering medium over distances of up to 20 m. We present a numerical channel modeling approach that can predict the scattering behavior at extended distances, which indicates that there is a strong mode-dependent variance in cross talk from the interaction of beams that carry OAM with randomly displaced scattering particles. These results present an effect that could allow the use of OAM modes to enhance particulate size sensors and could potentially lead to the development of novel tools for monitoring particles in underwater or free-space optical channels

GaN-based distributed feedback laser diodes for optical communications

Over the past 20 years, research into Gallium Nitride (GaN) has evolved from LED lighting to Laser Diodes (LDs), with applications ranging from quantum to medical and into communications. Previously, off-the-shelf GaN LDs have been reported with a view on free space and underwater communications. However, there are applications where the ability to select a single emitted wavelength is highly desirable, namely in atomic clocks or in filtered free-space communications systems. To accomplish this, Distributed Feedback (DFB) geometries are utilised. Due to the complexity of overgrowth steps for buried gratings in III-Nitride material systems, GaN DFBs have a grating etched into the sidewall to ensure single mode operation, with wavelengths ranging from 405nm to 435nm achieved. The main motivation in developing these devices is for the cooling of strontium ions (Sr+) in atomic clock applications, but their feasibility for optical communications have also been investigated. Data transmission rates exceeding 1 Gbit/s have been observed in unfiltered systems, and work is currently ongoing to examine their viability for filtered communications. Ultimately, transmission through Wavelength Division Multiplexing (WDM) or Orthogonal Frequency Division Multiplexing (OFDM) is desired, to ensure that data is communicated more coherently and efficiently. We present results on the characterisation of GaN DFBs, and demonstrate their capability for use in filtered optical communications systems

MicroLED-pumped perovskite quantum dot color converter for visible light communications

The visible light communications properties of a microLED-pumped inorganic perovskite quantum dot color converter are reported. Free-space data communications at 364 Mb/s, using solely the color-converted light as the data signal optical carrier, is demonstrated

InGaN/GaN Laser Diodes and their Applications

Gallium nitride (GaN) laser diodes are becoming popular sources not only for lighting but for applications ranging from communications to quantum. This paper presents the use of a commercial, off-the-shelf laser diode, with an emission wavelength of 450 nm, for visible light communication, both in free space and for underwater scenarios. Data rates up to 15 Gbit/s have been achieved by making use of orthogonal frequency division multiplexing (OFDM). In addition, distributed feedback (DFB) lasers have been realised emitting at a single wavelength which lend themselves towards applications where high spectral purity is crucial such as atomic clocks or filtered free space transmission systems. These devices have the grating structure etched into the sidewall of the ridge and work is ongoing to measure the linewidth of these lasers with the intended application of cooling Sr+ ions