Gallium nitride (GaN) laser diodes (LDs) are considered for visible light communications (VLC) in free space, underwater, and in plastic optical fibers (POFs). A review of recent results is presented, showing high-frequency operation of AlGaInN laser diodes with data transmission rates up to 2.5 Gbit/s in free space and underwater and high bandwidths of up to 1.38 GHz through 10 m of plastic optical fiber. Distributed feedback (DFB) GaN LDs are fabricated to achieve single-frequency operation. We report on single-wavelength emissions of GaN DFB LDs with a side-mode suppression ratio (SMSR) in excess of 35 dB.

AlGaInN ridge waveguide laser diodes are fabricated to achieve single-mode operation with optical powers up to 100 mW at similar to 420 nm for visible free-space, underwater, and plastic optical fiber communication. We report high-frequency operation of AlGaInN laser diodes with data transmission up to 2.5 GHz for free-space and underwater communication and up to 1.38 GHz through 10 m of plastic optical fiber. (C) 2016 Society of Photo-Optical Instrumentation Engineers (SPIE)

Gallium-nitride based devices have been used for electronic and optical equipment for over 20 years now. In a world concerned with saving energy, the use of GaN-based solid-state lighting sources has gained interest for visible light communications (VLC), where a light-emitting diode (LED) or laser diode can be exploited, not only to provide illumination, but also communication simultaneously. Contemporary applications are expanding to include underwater and optical fiber communications, as well as the traditional free space domain. It has been shown that micro-LEDs have high modulation bandwidths [1] and can achieve fast data transmission rates, in particular by exploiting higher modulation techniques [2]. However, their performance is limited by the material carrier lifetime and hence laser diodes are considered to further this work. The laser diodes used in this work can give powers of up to 50-100 mW and emit at wavelengths between 421-429 nm. The frequency response of these devices was measured, with maximum -3 dB bandwidths in excess of 2 GHz acquired. One of these devices was used to conduct data transmission experiments in free space, and eye diagrams were achieved without any pre- or post-amplification at data rates up to 2.5 Gbit/s [3]. Error-free data transmission was confirmed by conducting bit-error rate measurements using a pseudo-random bit sequence (PRBS) of 2 7 -1 bits in length. However, further tests showed that data rates of 3.4 Gbit/s could be achieved. These are the fastest transmission rates achieved from a directly modulated laser diode without any higher order modulation schemes. These results not only show the potential of GaN laser diodes for high-speed free-space VLC over short distances but also their potential for use in fiber. High speed measurements were conducted through varying lengths of step-index plastic optical fiber (SI-POF). A different laser (from the same batch), emitting at a wavelength of 429 nm was used to conduct frequency response measurements through the fiber. Fiber lengths of 1 m, 2.5 m, 5 m and 10 m were tested in order to see the trend of bandwidth against fiber length. This device had a -3 dB bandwidth of 1.71 GHz in free space and could achieve error-free data transmission at 2.5 Gbit/s, like before. The maximum bandwidth values achieved for 1 m, 2.5 m, 5 m and 10 m of fiber were 1.68 GHz, 1.63 GHz, 1.62 MHz, and 1.1 GHz, respectively. This can be seen in Figure 1 (a).

We report on an investigation into optical alignment and tracking for high bandwidth, laser-based underwater optical communication links. Link acquisition approaches (including scanning of narrow laser beams versus a wide-angle ‘beacon’ approach) for different underwater laser-based communications scenarios are discussed. An underwater laserbased tracking system was tested in a large water flume facility using water whose scattering properties resembled that of a turbid coastal or harbour region. The lasers used were state-of-the-art, temperature-controlled, high modulation bandwidth gallium nitride (GaN) devices. These operate at blue wavelengths and can achieve powers up to ~100 mW. The tracking performance and characteristics of the system were studied as the light-scattering properties of the water were increased using commercial antacid (Maalox) solution, and the results are reported here. Optical tracking is expected to be possible even in high scattering water environments, assuming better components are developed commercially; in particular, more sensitive detector arrays. High speed data transmission using underwater optical links, based on blue light sources, is also reported.

This paper describes a prototype demonstration of a high bandwidth data link between the fuselage of an aircraf and a helmet mounted display. A single data receiver, powered by battery and equipped with a light-collecting optical antenna to increase optical gain, is worn on the body of the pilot, with a fast-modulated laser transmitter mounted in the pilot's seat area. The combination covered the expected range of body movement that a pilot typically undergoes during a flight. Uncompressed, similar to 140Mbps video data is streamed over the free-space link to a BAE Systems helmet mounted display (Q-Sight (TM)) worn by the pilot.

Coarse wavelength-division multiplexing (CWDM) systems offer significant advantages over the more conventional dense wavelength-division multiplexing (DWDM) networks for aerospace applications. In DWDM, the spacing between adjacent channels is typically <1nm, whereas the CWDM standard is 20nm channel spacing. By exploiting this WDM standard, the requirement for an optical source to transmit at a specific, well-defined wavelength is somewhat relaxed, enabling a degree of centre wavelength drift with temperature to be tolerated -the CWDM standard defines a 13nm filter bandwidth. This promises significant reductions in device cost, weight, volume and power consumption, since it may be possible to use un-cooled laser sources. However, this assumes that the CWDM filter technology is stable over the aerospace operating conditions. This paper looks at both source and filter CWDM devices and in particular their performance over the airframe (-55 to +125degC) and avionic (-40 to +85degC) temperature ranges. CWDM filters from two different COTS providers were studied, as well as several different commercial CWDM transceivers. In all cases the devices were at times subject to temperatures beyond those specified by the manufacturers.

This paper presents the design and construction of a photonic fibre pumped OPO. The photonic fibre is used to provide high-energy pump power to an optical parametric oscillator (OPO) from a remotely sited pump laser source. Delivery of the high power radiation required for these systems is not possible using conventional fibre, as the fibres would need to be highly multimode to handle the high intensities without damage. Photonic fibres are a disruptive technology for power transmission and light manipulation/control. The fibres have the potential for supporting high irradiation powers and can operate with robust singlemode guidance. The OPO is to be used to provide a 3-5 mum wavelength source for active sensor applications. The integration of high power lasers into air platforms for remote sensing applications would therefore be facilitated, as the fibre delivery would enable the laser to be sited remotely from the sensor head and open up the possibility of several sensor types sharing the same multi-functional laser. This could reduce the complexity and hence the cost of such sensors systems leading to the potential for an affordable, robust system for military platforms.