GaN-based laser diodes have been developed over the last 20 years making them desirable for many security and defence
applications, in particular, free space laser communications. Unlike their LED counterparts, laser diodes are not limited
by their carrier lifetime which makes them attractive for high speed communication, whether in free space, through fiber
or underwater. Gigabit data transmission can be achieved in free space by modulating the visible light from the laser with
a pseudo-random bit sequence (PRBS), with recent results approaching 5 Gbit/s error free data transmission. By
exploiting the low-loss in the blue part of the spectrum through water, data transmission experiments have also been
conducted to show rates of 2.5 Gbit/s underwater. Different water types have been tested to monitor the effect of
scattering and to see how this affects the overall transmission rate and distance. This is of great interest for
communication with unmanned underwater vehicles (UUV) as the current method using acoustics is much slower and
vulnerable to interception. These types of laser diodes can typically reach 50-100 mW of power which increases the
length at which the data can be transmitted. This distance could be further improved by making use of high power laser
arrays. Highly uniform GaN substrates with low defectivity allow individually addressable laser bars to be fabricated.
This could ultimately increase optical power levels to 4 W for a 20-emitter array. Overall, the development of GaN laser
diodes will play an important part in free space optical communications and will be vital in the advancement of security
and defence applications.
Gallium Nitride laser diodes fabricated from the AlGaInN material system is an emerging technology for laser sources from the UV to visible and is a potential key enabler for new system applications such as free-space (underwater & air bourne links) and plastic optical fibre telecommunications. We measure visible light (free-space and underwater) communications at high frequency (up to 2.5 Gbit/s) and in plastic optical fibre (POF) using a directly modulated GaN laser diode.
AlGaInN ridge waveguide laser diodes are fabricated to achieve single-mode operation with optical powers up to 100 mW at ∼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.
The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v., ~380nm, to the visible ~530nm, by tuning the indium content of the laser GaInN quantum well. We consider the suitability of AlGaInN laser diode technology for free space laser communication, both airborne links and underwater telecom applications, mainly for defense and oil and gas industries.
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 aircraft
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, ~140Mbps video data is streamed over the free-space link
to a BAE Systems helmet mounted display (Q-Sight™) worn by the pilot.
This paper describes recent progress in developing a wireless optical link between the fuselage of a cockpit and an
aviation helmet. Such a link is desired to replace the physical umbilical cable existing in current cockpit systems, for
reasons of potential bandwidth, immunity to EM interference, and freedom from physical constraints within the cockpit.
The link concept consists of multiple transmitters embedded in the cockpit fuselage, each sending video (or symbology)
data out in a cone of light over free space, which is detected by an array of receivers positioned on the helmet - the data
is then sent to the eyepieces or visor of the pilot (after any intermediate processing). The design is such that one of these
links is always maintained throughout possible movement of the head. In a recent proof-of-principle demonstration we
showed uncompressed, 100 Mbps video data streamed live from the fuselage of a cockpit simulator to an angled cluster
of silicon-based receivers mounted on the helmet, via a pair of ~1 Watt free-space lasers operating at 810 nm. Fast
Ethernet media converters were used here for convenience and cost. The bespoke optical and electrical link components
were developed in close collaboration with suppliers. The system performance arises from: the high dynamic range of
the receivers (up to 25 dB), which are equipped with optical antennae to magnify the optical gain; the high power of the
lasers; and the switching electronics used to control the signal path on the helmet. Future potential improvements to the
technology are discussed, with an indication of wireless link requirements for relevant BAE Systems applications.
In current laser countermeasure technology concepts where frequency conversion is required, each active component has
its own laser source. During this paper we show that by using microstructured fibre technology as a delivery system,
output in multiple wavebands can be efficiently generated at locations remote from the laser pump source. We
demonstrate that laser radiation (with specifications close to those currently on airframes) can be delivered without
significant spectral, temporal or modal degradation over lengths representative of that in an airframe. This fibre delivered
radiation is used as a pump source for active frequency conversion, generating tuneable laser output in the 2 μm, 3.5 μm
and 0.532 μm regions, i.e. in wavebands of interest to countermeasure applications.
A Nd:YVO<sub>4</sub> laser (λ = 1.064 μm) with 16 W of average power in a train of 15 ns pulses acts as the single pump source
for our system. Different types of microstructured fibre are assessed for high power delivery over lengths greater than
6.5 m. Three frequency conversion devices were constructed here to demonstrate the quality of the fibre-delivered
radiation - the devices are all based around periodically poled lithium niobate (PPLN) crystals and consist of two optical
parametric oscillators converting the pump source to wavelengths of ~2 μm and ~3.5 μm and a second harmonic
generator to double the frequency to 0.532 μm. The efficiencies of the frequency conversion sources are comparable
whether radiation is delivered through free space or by microstructured fibre.
In this paper we discuss recent work at the Advanced Technology Centre of BAE Systems on photonic technology, in particular photonic crystal fibres, applied to infra-red and electro-optic countermeasure systems. The use of Photonic Crystal fibres or holey fibres in countermeasure systems could significantly simplify platform integration by enabling remote location of laser sources, the generation of multiple wavelengths or continuum generation from a single pump source .The paper will describe the development of these fibres, drawing examples from recent civil collaborative research projects such as PFIDEL and LAMPS.
In this paper we seek to assess the potential impact of microstructured fibres for security and defence applications. Recent literature has presented results on using microstructured fibre for delivery of high power, high quality radiation and also on the use of microstructured fibre for broadband source generation.
Whilst these two applications may appear contradictory to one another the inherent design flexibility of microstructured fibres allows fibres to be fabricated for the specific application requirements, either minimising (for delivery) or maximising (for broadband source generation) the nonlinear effects.
In platform based laser applications such as infrared counter measures, remote sensing and laser directed-energy weapons, a suitable delivery fibre providing high power, high quality light delivery would allow a laser to be sited remotely from the sensor/device head. This opens up the possibility of several sensor/device types sharing the same multi-functional laser, thus reducing the complexity and hence the cost of such systems.
For applications requiring broadband source characteristics, microstructured fibres can also offer advantages over conventional sources. By exploiting the nonlinear effects it is possible to realise a multifunctional source for applications such as active hyperspectral imaging, countermeasures, and biochemical sensing.
These recent results suggest enormous potential for these novel fibre types to influence the next generation of photonic systems for security and defence applications. However, it is important to establish where the fibres can offer the greatest advantages and what research still needs to be done to drive the technology towards real platform solutions.
Microstructured fibers (MOFs) are among the most innovative developments in optical fiber technology in recent years. These fibers contain arrays of tiny air holes that run along their length and define the waveguiding properties. Optical confinement and guidance in MOFs can be obtained either through modified total internal reflection, or photonic bandgap effects; correspondingly, they are classified into index-guiding Holey Fibers (HFs) and Photonic Bandgap Fibers (PBGFs). MOFs offer great flexibility in terms of fiber design and, by virtue of the large refractive index contrast between glass/air and the possibility to make wavelength-scale features, offer a range of unique properties. In this paper we review the current status of air/silica MOF design and fabrication and discuss the attractions of this technology within the field of sensors, including prospects for further development. We focus on two primary areas, which we believe to be of particular significance. Firstly, we discuss the use of fibers offering large evanescent fields, or, alternatively, guidance in an air core, to provide long interaction lengths for detection of trace chemicals in gas or liquid samples; an improved fibre design is presented and prospects for practical implementation in sensor systems are also analysed. Secondly, we discuss the application of photonic bandgap fibre technology for obtaining fibres operating beyond silica's transparency window, and in particular in the 3μm wavelength region.
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 μm 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.
The requirement for realistic simulation of military scenarios arises from a dearth of suitable and accessible measured data. Furthermore, measurement campaigns are restricted by the trial locality and availability of appropriate targets. Targets located in and around tree-lines are of particular interest, as they present scenarios that conventional broadband sensor systems find problematic. Utilising the spectral component of scenes, through the use multi- or hyperspectral technologies, can be beneficial in detecting these difficult targets.
In this paper we describe the use of a Monte Carlo ray-tracing model (FLIGHT) to simulate forest scenes. This model is capable of calculating the interesting BRDF properties specific to forests. Targets are also incorporated in these simulations, and we describe contrast discrimination of the target from the background. This technique has application for targets in deep hide as well as at the forest edge (i.e., in a tree-line).
Assessment methods that can be applied to simulated hyperspectral imagery are investigated, to determine how realistic these scenes are in comparison to measurement. This is of key importance in ensuring that simulated imagery, as well as measured data, can be used to assess algorithmic techniques to detect and discriminate targets. Statistical assessment measures are discussed that utilise the spatial and spectral properties of the image.