The present study originated in the lack of research into achieving underwater total internal reflection (TIR) via the acousto-optic effect. The uniqueness of this technique exists in the fact that it is based on a high sound pressure level which induces a localised change in refractive index of seawater sufficient to achieve total internal reflection within the communication channel. Different transducer systems for generating the pressure wave have been investigated and take the form of a wave which may be either a standing wave, or a novel beamforming technique. The former is based on an array of transducers and with an acoustic mirror at the receiver in order to establish the standing wave. The alternative approach relies on the high intrinsic directionality of a novel beamformer where an annular transducer array is examined as an acoustic source. In this paper, the main characteristics of the acoustic optic waveguide will be presented. This will include both sound and light propagation in the ocean, TIR, novel beam propagation, the refractive index of water as a function of the externally applied acoustic pressure, and the acoustic technology. The modelled results, the limitations imposed by the challenging medium, and the system requirements required to obtain an Underwater Wireless Acousto-Optic Waveguide (UWAOW) will be also addressed.
There is a growing requirement to transfer large volumes of data between underwater platforms. As seawater is transmissive in the visible band, underwater optical communications is an active area of interest since it offers the potential for power efficient, covert and high bandwidth datalinks at short to medium ranges. Short range systems have been successfully demonstrated using sources with low directionality. To realise higher data rates and/or longer ranges, the use of more efficient directional beams is required; by necessity, these must be sufficiently aligned to achieve the required link margin. For mobile platforms, the acquisition and tracking of each node is therefore critical in order to establish and maintain an optical datalink. This paper describes work undertaken to demonstrate acquisition and tracking in a 3D underwater environment. A range of optical sources, beam steering technologies, and tracking sensors have been assessed for suitability. A novel scanning strategy exploiting variable beam divergence was developed to provide robust acquisition whilst minimising acquisition time. A prototype system was assembled and demonstrated in a large water tank. This utilised custom quadrant detectors based on Silicon PhotoMultiplier (SiPM) arrays for fine tracking, and a Wide Field of View (WFoV) sCMOS camera for link acquisition. Fluidic lenses provided dynamic control of beam divergence, and AC modulation/filtering enabled background rejection. The system successfully demonstrated robust optical acquisition and tracking between two nodes with only nanowatt received optical powers. The acquisition time was shown to be dependent on the initial conditions and the transmitted optical power.
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.
A remote laser timing system has been developed for use by the British Cycling team. Five optical Timing Gate Units
(TGU) have been installed around the track at the Manchester Velodrome. Each TGU can identify and monitor multiple
cyclists during training sessions. Lap and split times can be measured as well as the speeds of individual cyclists passing
each gate. The system allows coaches to concentrate on observing the cyclists' technique rather than manually capturing
their times. It has resulted in more effective and efficient training sessions that have helped cyclists improve their
performance. This paper will describe the design issues encountered, as well as the optical and signal processing
solutions. Example results obtained from training sessions will be presented.
We describe the development of flexi-circuit addressed, high resolution small aperture Bimorph Deformable Mirror
(BDM) suitable for a satellite to ground optical communications system. In the first phase, the base of an existing BDM
was modified to enable integration onto a commercially available tip-tilt stage. Evaluation demonstrated that there was
no reduction in performance as a result of the integration. In the current phase, a 100 element BDM with a 10mm optical
aperture suitable for integration onto a bespoke tip-tilt platform has been designed and assembled. The results of initial
tests are presented.
We present the results of a study on Dual-Stage Deformable Mirrors using Zonal Bimorph Deformable Mirror (ZBDM)
technology. A high density 'tweeter' DM has been assembled onto a lower density, high dynamic range 'woofer' DM to
generate an integrated mirror which offers both high resolution and dynamic range simultaneously. Such a device has the
potential to significantly simplify the design of astronomical Adaptive Optics (AO) systems. The latest developments are
presented, including the fabrication of a small scale demonstrator.
KEYWORDS: Mirrors, Sensors, Adaptive optics, Actuators, Wavefront sensors, Deformable mirrors, Turbulence, Signal to noise ratio, Wavefronts, Control systems design
We present a novel method for integrating a wavefront sensor into a deformable mirror. This development should
simplify the design of laser and electro-optic systems, and lead to smart mirrors which need no external control systems.
In operation, a small fraction of the incident light is transmitted through the mirror coating and is sampled by a Hartmann
Mask. Options include open loop, traditional closed loop or fully integrated operation whereby the wavefront sensor is
used to provide direct feedback to the mirror actuators, enabling automatic alignment or phase conjugation.
The Zonal Bimorph Deformable Mirror (ZBDM) is a new concept of adaptive mirror. It exploits the benefits normally
associated with bimorph mirrors, namely simple rugged construction, low capacitance, and cost effectiveness, but in a
significant departure from classical, edge supported bimorphs each element is supported from underneath. This results in
a localized (zonal) response that should enable the device to be scaled up to large aperture, multi-1000 element devices.
Crucially, the combination of continuous support coupled with the use of flexi-circuit interconnect promotes the
assembly of a high density 'tweeter' onto a lower density, high dynamic range 'woofer' to generate an integrated, dualstage
deformable mirror which can deliver both high resolution and high dynamic range simultaneously. Such a device
has the potential to significantly simplify the design of AO systems. We present the progress made on the development
of the ZBDM as part of a collaborative programme funded by the UK Science and Technology Facilities Council.
Imaging devices are very attractive as sensors on modern airborne platforms and there is a continuing trend toward widespread employment of imaging either alone or in combination with complementary technologies. In the civil domain, modern silicon CCD and CMOS image sensors are becoming extremely small, so that the package size of commercial miniature cameras is becoming dominated by the image forming optics, even if the latter is only a structure supporting a pinhole. We have previously reported a biologically inspired, near IR, imaging system that offers a wide field-of-view, thanks to the use of a multi-aperture sensor based on micro-optics which can be used to observe simultaneously in different directions. In this paper we describe our recent work on the design, assembly and test of a novel, all solid state camera that exploits spherical lens technology to provide a robust, wide field-of-view camera. We have also considered active fluidic lenses that can operate in the mid infra-red in order to provide additional functionality.
The Zonal Bimorph Deformable Mirror (ZBDM) is a new concept of adaptive mirror. It exploits the benefits normally
associated with bimorph mirrors, namely simple rugged construction, low capacitance, and cost effectiveness, but in a
significant departure from classical, edge supported bimorphs each element is supported from underneath. This results in
a localised (zonal) response and enables the device to be scalable up to large aperture, multi-1000 element devices.
Crucially, the combination of continuous support coupled with the use of flexi-circuit interconnect promotes the
assembly of a high density 'tweeter' deformable mirror (DM) onto a lower density, high dynamic range 'woofer' DM to
generate an integrated, dual-stage deformable mirror which can deliver both high resolution and high dynamic range
simultaneously. Such a device has the potential to significantly simplify the design of astronomical adaptive optics (AO)
systems. We present the progress made on the development of the ZBDM as part of a collaborative project funded by the
newly formed UK Science and Technology Facilities Council.
There is a need for faster processing hardware to provide modern radar systems with advanced capabilities such as
multiple hypothesis tracking, real-time clutter removal and space-time adaptive beamforming (STAP) for jammer
nulling. One approach that may help to meet this need is to use analogue methods in parts of the signal processing chain
using optoelectronics. The vector-matrix multiplier is a powerful optical processing architecture that potentially offers
very large gains in computation speed, but has not so far become commercially successful. This paper reports
investigation of a novel arrangement for this type of processor that is aimed at improving the prospects for
commercialisation, using guided-wave and micro-optic components. This approach will assist miniaturisation of the
processor and improve ruggedness and scalability to large matrices.
Imaging devices are very attractive as sensors in modern airborne platforms and there is a continuing trend toward
widespread employment of imaging either alone or in combination with complementary technologies. In the civil
domain, modern silicon CCD and CMOS image sensors are becoming extremely small, so that the package size of
commercial miniature cameras is increasingly being dominated by the image forming optics, even if the latter is only a
structure supporting a pinhole. Recently, there have been demonstrations of ultra-flat, extremely light weight sensors
working in the visible region of the spectrum. Similar ideas for cameras developed to operate in the infra-red could help
to drastically reduce the size, weight and cooling requirements of imaging, also offering substantial cost reductions. In
addition, designs providing wide field-of-view can potentially eliminate the need for sightline steering hardware. This
paper describes work on a biologically inspired imaging system offering a wide field of view, thanks to the use of a
multi-aperture sensor based on micro-optics which can be used to observe simultaneously in different directions. Results
from a near-infrared, narrowband demonstrator are reported.
Adaptive Optics (AO) is a critical underpinning technology for future optical countermeasures, laser delivery, target illumination and imaging systems. It measures and compensates for optical distortion caused by transmission through the atmosphere, resulting in the ability to deploy smaller lasers and identify targets at greater ranges. AO is also well established in ground based astronomy, and is finding applications in free space optical communications and ophthalmology.
One of the key components in an AO system is the wavefront modifier, which acts on the incoming or outgoing beam to counter the effects of the atmosphere. BAE SYSTEMS ATC is developing multi-element Deformable Bimorph Mirrors (DBMs) for such applications.
A traditional bimorph deformable mirror uses a set of edge electrodes outside the active area in order to meet the required boundary conditions for the active aperture. This inflicts a significant penalty in terms of bandwidth, which is inversely proportional to the square of the full mirror diameter. We have devised a number of novel mounting arrangements that reduce dead space and thus provide a much improved trade-off between bandwidth and stroke. These schemes include a novel method for providing vertical displacement at the periphery of the aperture, a method for providing a continuous compliant support underneath the bimorph mirror, and a method for providing a three point support underneath the bimorph. In all three cases, there is no requirement for edge electrodes to provide the boundary conditions, resulting in devices of much higher bandwidth. The target is to broaden the use of these types of mirror beyond the current limits of either low order/low bandwidth, to address the high order, high bandwidth systems required by long range, horizontal path applications. This paper will discuss the different mirror designs, and present experimental results for the most recently assembled mirrors.
This paper reports the development of novel retroreflectors for use in free-space optical communication systems. It will be important for the retroreflectors to have a very wide field of view to make such systems practicable and affordable. Corner cube retroreflectors present a practical means of meeting the requirement for a wide field of view, but require use of materials with very high refractive index. Practical measurements on initial samples of high index corner cubes have shown encouraging optical performance. The measured results approximately confirm predictions of the variation of reflection efficiency with the angle of incidence. Retroreflectors based on graded-index, spherical (GRIN-sphere) lenses potentially offer an alternative with valuable technical advantages over the use of high-index corner cubes, if such lenses can be fabricated with a suitable combination of optical quality, size and relative aperture. The key property of GRIN-sphere lenses is that they can in principle suppress the most problematic feature of sphere lenses, that is, their strong spherical aberration. Predictions for practical graded-index sphere lens structures show valuable potential for improvement compared to uniform sphere lenses, including diffraction limited optical performance over significant fractions of the lens aperture.
Adaptive Optics (AO) is a critical underpinning technology for future laser delivery (including free-space optical communications), target illumination and imaging systems. It measures and compensates for optical distortion caused by transmission through the atmosphere, resulting in the ability to deploy smaller lasers and identify targets at greater ranges.
One of the key components in an AO system is the wavefront modifier, which acts on the incoming or outgoing beam to counter the effects of the atmosphere. BAE SYSTEMS Advanced Technology Centre is developing multi-element bimorph deformable mirrors for such an applications.
Our initial designs were based on a standard construction and exhibited a resonant frequency of 1kHz with a maximum stroke of ±20μm for an active aperture of 50mm. These devices were limited by the necessity to have a 'dead space' between the inner active area and the mirror boundary; this ensured that both the requirements for the stroke and the fixed boundary conditions could be met simultaneously. However, there was a significant penalty to pay in terms of bandwidth, which is inversely proportional to the square of the full mirror diameter. In a series of iteration steps, we have created novel mounting arrangements that reduce dead space and thus provide the optimum trade-off between bandwidth and stroke. These schemes include supporting the mirror from underneath, rather than at its edge. As a result, models of 60mm active diameter mirrors predict a resonance in excess of 5kHz, combined with a maximum stroke greater than ±40μm. This paper will discuss a number of different mirror designs and present experimental results for recently assembled devices.
This paper presents power budget predictions for retroreflective, free-space optical communication systems and examines options for producing retroreflectors with very wide field of view. Power budgets containing data representative of practical conditions show that operational ranges of many kilometres can be expected. Novel graded-index (GRIN) spherical retroreflectors have been examined in comparison with other types of retroreflector, and it is shown that they can offer technical advantages if they can be fabricated with suitable sizes and focal lengths. It is also shown that realisable high-index corner cube reflectors may be technically acceptable for this application.
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