KEYWORDS: Optical communications, Light scattering, Scattering, Clouds, Monte Carlo methods, Receivers, Transmitters, Performance modeling, Water, Signal to noise ratio
QinetiQ, in association with Frazer-Nash Consultancy and Dstl, have developed a new numerical model of optical communications where part of the transmission is through water. The model (called Optical Communications Underwater Model or OCUM) finds the signal from air platform to sea platform (and vice versa) or between underwater platforms including the effects of scattering within the water, the refraction at the sea surface and transmission through cloud. The effects of scattering are found through Monte Carlo simulation before parameterizing the results to be used in subsequent calculations. The background light is also included from the sun to obtain the signal to noise ratio which is then used to find the analytical and numerical (via a message transmission simulation) bit error rate. This paper shows some of the details of the model and the approaches taken to obtain the transmission efficiency and performance. Some basic results will be presented to demonstrate the utility of the model.
An investigation of the use of hollow-core photonic bandgap (PBG) fiber to transport high-power narrow-linewidth light is performed. In conventional fiber the main limitation in this case is stimulated Brillouin scattering (SBS) but in PBG fiber the overlap between the optical intensity and the silica that hosts the acoustic phonons is reduced. In this paper we show this should increase the SBS threshold to the multi-kW level even when including the non-linear interaction with the air in the core. A full model and experimental measurement of the SBS spectra is presented, including back-scatter into other optical modes besides the fundamental, and some of the issues of coupling high power into hollow-core fibers are discussed.
We review methods of combining beams using the Brillouin interaction. We analyze a two cell geometry that provides
both high gain amplification (G~300) simultaneously with high efficiency power conversion (η=90%). We consider
some configurations that could demonstrate this performance but note that their complexity makes them relatively
unattractive at this point in time.
We report on the phase locking of a fibre bundle laser based on a single frequency oscillator coupled into four fibre
amplifiers to provide a coherent beam of over 600 W. The oscillator was phase modulated to a width of up to 2 GHz to
increase the threshold for stimulated Brillouin scattering and then a fraction split off and frequency shifted to form a
reference beam. The oscillator output was amplified by end-pumped fibre amplifiers based on 20 μm core Yb doped
fibre to provide a power of up to 260 W per channel. The beams combined to form a coherent output with phase errors of
a twentieth of a wave, unaffected by the spectral broadening.
We report on a laser communications experiment over a kilometre optical range where we have used a retro-reflective
transponder incorporating an optical modulator based on silicon micro-electro-mechanical systems (MEMS) device. This
employs interference to provide modulation and relies on performing as a coherent array to modulate incident light in the
near-IR band (1550nm) over a wide angular range (120 degrees). Modulation is achieved by tuning a large array of
Fabry-Perot cavities via the application of an electrostatic force to adjust the gap between a moveable mirror and the
underlying silicon substrate.
The micro-mirrors have a strong mechanical resonance, and modulate light by adjusting the spacing between the micromirrors
and the substrate. We use a 'release and catch' technique to exploit the mechanical resonance, and we time the
motion of the micro-mirrors to be synchronised with the arrival of an interrogator pulse to ensure that the etalon spacing
provides the required modulation, whatever the angle of incidence.
We describe experiments over a one kilometre path where simple strings were sent at 200kbit per second. We also
discuss approaches to adapting the link to a given angle of incidence.
The development of a micro-opto-electro-mechanical system (MOEMS) technology employing interference effects to
modulate incident light in the near-IR band (1550nm) over a wide angular range (120 degrees) is reported. Modulation is
achieved by tuning a large array of Fabry-Perot cavities via the application of an electrostatic force to adjust the gap
between a moveable mirror and the underlying silicon substrate.
The optical design determines the layer thicknesses; however, the speed and power are determined by the geometry of
the individual moveable elements. Electro-mechanical trade-offs will be presented as well as a key innovation of
utilising overshoot in the device response in reduced pressure environment to reduce the drive voltage.
Devices have been manufactured in a modified polysilicon surface micromachining process with anti-reflection coatings
on the back of the silicon substrate. Measurements of individual mirror elements and arrays of mirrors at 1550nm show
excellent uniformity across the array. This enables good response to an incident signal over a wide field of view when
integrated with a silicon retroreflector in a passive optical tag. In conjunction with appropriate anti-stiction coatings,
lifetimes of over 100 million cycles have been demonstrated.
Key advantages of the modulator are that it is low cost being based on standard polysilicon micromachining; high speed
(>100kHz) and robust due to utilising a massively parallel array of identical compact devices; low power for portable
applications; and operates in transmission - allowing simple integration with a retroreflector in a passive tag for halfduplex
free-space optical communications to a remote interrogator.
Fibre amplifiers exhibit rapid time dependent phase fluctuations due to the environment and to thermal and other effects
associated with the pumping and lasing processes. We characterise these effects in a large mode area fibre amplifier
having an output power of 260W limited only by pump power. The amplifier retains its coherence even at the highest
available output power with negligible linewidth broadening. Phase fluctuations are characterised by a low-amplitude
power-independent jitter superimposed on a power-dependent drift due to heating. We also measure the phase
fluctuations in a COTS fibre preamplifier and find they are predominantly large amplitude periodic oscillations at
110Hz, probably induced by pump power fluctuations. The two amplifiers were combined in series to give a high gain
amplifier chain and actively phase stabilised to high precision (~&lgr;/37 rms) using a piezo-ceramic fibre stretcher
incorporated into a PC-based feedback loop.
KEYWORDS: Cameras, Facial recognition systems, Projection systems, 3D image processing, Sensors, 3D modeling, Databases, Stereoscopic cameras, Data modeling, 3D metrology
We describe a camera capable of recording 3D images of objects. It does this by projecting thousands of spots onto an object and then measuring the range to each spot by determining the parallax from a single frame. A second frame can be captured to record a conventional image, which can then be projected onto the surface mesh to form a rendered skin.
The camera is able of locating the images of the spots to a precision of better than one tenth of a pixel, and from this it can determine range to an accuracy of less than 1 mm at 1 meter. The data can be recorded as a set of two images, and is reconstructed by forming a 'wire mesh' of range points and morphing the 2 D image over this structure. The camera can be used to record the images of faces and reconstruct the shape of the face, which allows viewing of the face from various angles. This allows images to be more critically inspected for the purpose of identifying individuals. Multiple images can be stitched together to create full panoramic images of head sized objects that can be viewed from any direction. The system is being tested with a graph matching system capable of fast and accurate shape comparisons for facial recognition. It can also be used with "models" of heads and faces to provide a means of obtaining biometric data.
A retro-reflective communications system comprises a laser transmit/receive station and a remote retroreflector that can be switched between "on" and "off" states. The laser illuminates the remote station and a collection telescope directs the reflected light to an associated detector the output of which is interpreted as logic 1 or 0. Atmospheric turbulence affects the outgoing illumination beam, resulting in beam spreading and in fluctuations in the intensity (scintillation)1. The reflected beam undergoes further turbulence induced spreading and there is an enhancement in the intensity fluctuations. These fluctuations mean that the logic level of signals may be wrongly identified, leading to bit errors. The signal may fade below detectable levels for periods of time, leading to sections of the bit stream being lost. We develop a description of the intensity fluctuations in terms of the Gamma-Gamma distribution2, and incorporate the effect of "aperture averaging" associated with the retro-reflector and the collection aperture. We characterise signal-to-noise ratios and calculate bit error rates as a function of retro-reflector cross-section and contrast for a variety of ranges, turbulence levels and system configurations. We also identify the characteristic timescale over which the atmosphere causes changes of intensity and discuss the implications.
A summary is presented of some of the design criteria relevant to the realisation of silicon micromachined modulator arrays for use in free-space optical communication systems. Theoretical performance levels achievable are compared with values measured on experimental devices produced using a modified Multi-User MEMS Process (MUMPS). Devices capable of realising modulation rates in excess of 300 kHz are described and their optical characteristics compared with published data on devices based on multiple quantum well technology.
We describe a model of a Yb:glass fibre amplifier incorporating amplified spontaneous emission (ASE). The model is able to predict the output spectrum and power of spectral components in a fibre amplifier, and identifies the optimum configuration for efficiently extracting pump power at a given signal wavelength.
We describe the experimental study of phase locking of a four element phased array of fibres, in which the output brightness of the bundle is enhanced by phase locking of the individual elements, and steered by controlling the phase of each channel.
When a laser plasma is produced on a target, various electromagnetic phenomena can occur. These can produce substantial currents and voltages in nearby structures. The effects depend on the target material and morphology, the pressure and species of the atmosphere, and the nature of the laser pulse.
The following mechanisms are known to make a major contribution to electromagnetic signals detected near laser plasmas:
(1) UV plume causing transient high conductivity in semiconductor targets, and ionisation in buffer gasses;
(2) Laser plasma generating multi-GHz microwaves due to the generation of plasma waves;
(3) Space charge and current charge travelling through vacuum due to differences in the electron and ion velocities;
(4) Generation of transient magnetic fields that induce anomalous currents in conductors at the target point, and secondary induced current in nearby conductors.
Many of which were first reported in the 1970s, and in this report we review their relative contributions and identify regimes where each dominate.
We describe a compact laser beam profiler that records multiple images of a laser beam at different positions along the beam waist in a single shot. This is used to determine the laser beam parameter M2. It is known that if light is incident on a curved grating such as a section of a Fresnel zone plate, then the +1 and -1 diffracted orders are focused or defocused. By combining two curved gratings and a conventional lens, it is possible to build a system in which a mosaic of nine images are created, corresponding to nine separate object planes. These nine images can be processed and fitted to an appropriate beam profile. This type of system measures beam quality in real time without any moving parts, and has the potential to provide real time monitoring of pulsed lasers, lasers with dynamic spatial behaviour, and in systems where beam handling optics may produce transient aberrations. Unlike other 'single shot' techniques, this technique makes no assumptions about beam propagation, and produces a value of M2 based only on measurements of beam waists.
An adaptive optics system usually has three basic elements, a wavefront sensor, a deformable element, and a feedback scheme. Typically these components are a Shack-Hartmann sensor, a bimorph or segmented mirror, and a DSP solution for performing the necessary calculations. These components are expensive, and give rise to a complex optical and computational system. In this paper a novel implementation of an adaptive optics system will be discussed. The wavefront sensor is based on an IMP grating to measure the curvature of the incoming light. This sensor has been found to be robust to scintillation, so is applicable to horizontal propagation paths. An OKO technologies deformable mirror is used, and the feedback loop calculations run on a standard Pentium III computer using Windows 2000. Results from recent trials of the system correcting for errors over various horizontal propagation lengths will be shown. Additionally results using this system for laser beam propagation will also be discussed.
Considerable work is being undertaken to develop laser sources with increasingly high power and good beam quality. In all solid state lasers, especially cw ones, optical pumping causes heat to be generated in the gain volume which is conducted away via a surface. The associated temperature gradient produces refractive index variations which degrade the beam quality.
The feasibility of retrieving information from lidar return signals has been discussed in connection with salient features of the formation of multiple scattering for different lidar field-of-view angles (FOVA). However, the aspects of hydrosol sensing connected with the presence of the rough air-water interface make the lidar signal interpretation more difficult. Results of a statistical simulation by the Monte Carlo method are presented in the report for a water scattering layer homogeneous with depth. peculiarities of the waveforms of the lidar signals recorded during a flight above Lake Baikal (in November 1996) and the Atlantic Ocean to the West of Great Britain (in June 1997) are discussed. We estimate that the effect of multiple scattering compensates for the effect of wind-driven sea waves when FOV angle is about p/2 equals 3 - 4 mrad, and that these FOVA are optimal for sounding.
We propose a method of numerical simulation of an image formation in vision systems looking through the waved water surface. The simulation of radiation propagation through the waved sea surface and sea depth is realized in ray approximation. Waviness is simulated as evolving in the time random surface with given spectrum of elevations and dispersion relation. New values of angular, energy, and polarization parameters of each ray in the beam after its refraction and reflection at a given surface point are calculated by means of Fresnel formulae. Seawater medium is split in the layers with thickness chosen from the condition that water characteristics can be considered as constant and that multiple scattering of a ray can be neglected within a layer. Attenuation of beam energy corresponds to the length of the beam path in the layer, and probability of beam scattering at a random angle within a layer is determined by phase scattering function. We have obtained numerically instantaneous and averaged over different exposition times images of 2D self-luminous and located mira made up from spokes and situated at various depths for various values of wind velocities and seawater scattering coefficients. Influence of water and waviness parameters on the image quality has been analyzed.
The results of experiment on airborne laser sounding of coastal sea waters about Northern Scotland are described. The aircraft- laboratory of Institute of Atmospheric optics with Makrel-2 lidar was used. As a rule, the flights were carried out at height 300 M above water with average speed of 320 km/h. The pulse repetition rate of laser shots changed from 5 up to 25 Hz. The total length of flight lines above water surface has made about 8000 km. The processing of signals was made by a method of logarithmic derivative with statistical processing. The large attention was given to neutralization of influence of afterpulses, arising in photomultipliers, and especially strongly interfering to sounding of water.
Lidars are widely used for sensing of sea water turbidity and bottom depth. Our Makrel'-2 lidar has been used since the 1980s for water monitoring and searching for schools of fish over the Norwegian, Barents, and North seas. In the present paper, we discuss the results obtained with our lidar, placed onboard the Optik-E Antonov-30 aircraft- laboratory during our flight over Lake Baikal on November 2, 1996. Some results of lidar data processing are presented, which demonstrate the salient features of lidar return signals and the possibilities for lidar signal processing. The extinction index of the purest water found in the northern part of Lake Baikal was 0.12MINPLU0.01 m- (averaged over 600 pulses). In the region near the mouth of the River Selenga, the extinction index changed from 0.14 to 0.6 m-. The minimum and maximum bottom depths recorded with our lidar are estimated. We detected bottom depths as great as 40 m when the aircraft flew near Bol'shoi Ushkanii Island.
Mikhail Bogdanov, Sergei Dimakov, Alexander Gorlanov, Valery Irtuganov, Sergei Kliment'ev, Konstantin Lazunin, Irina Orlova, Andrew Scott, Vladimir Sherstobitov, Nataliya Sventsitskaya, Dmitrii Zhuk
We describe a number of experiments devoted to compensation for aberrations in the primary mirror of a telescope using a volume dynamic holography technique, where the writing and reading of the hologram are carried out independently. Three sets of experiments demonstrating holographic aberration correction have been performed. In all cases transmitting holograms were formed in photorefractive crystals such as BSO and SBN. In experiments with a small diameter primary mirror the image is formed by a multicolor point-like object. The system includes a unit which compensates for the hologram dispersion. The aberration correction of a six- segmented primary mirror of 150 mm in diameter is demonstrated in conditions where there is a pronounced difference in the wavelengths of the recording beams and that of the source observed (Δλ=0.12 μm). The experiments on observation an extended object of continuous spectrum (Δλ=0.35 μm) with a single primary mirror of 150 diameter have been also made. All these experiments indicate an efficient compensation of distortions of the primary mirror by the hologram.
A. Dudov, S. Buyko, A. Dolgopolov, V. Eroshenko, Gennady Kochemasov, Stanislav Kulikov, V. Novikov, A. Shkapa, Stanislav Sukharev, L. Zykov, Andrew Scott
For xenon at temperature t equals 19.3 degrees Celsius in a range of density 0.3 g/cm3 less than or equal to (rho) less than or equal to 1.0 g/cm3 SBS gain coefficient (g) and phonon lifetimes are measured. It is shown that at the densities higher than (rho) equals 0.5 g/cm3 SBS gain g levels out, reaching the value g approximately equals 100 cm/GW, and the phonon lifetimes decrease down to (tau) ph equals 25 ns with approaching to a critical point of Xe.
The process of Brillouin amplification when the input pump and signal beams have Gaussian spatial profiles is investigated. Two distinct regimes are considered. In the small signal regime, it is shown that the signal beam undergoes spatial beam narrowing as it propagates through the amplifier owing to the exponential dependence of signal beam gain on pump beam intensity. The relative change in spot size is found to depend on both the peak signal intensity gain and on the input signal to pump spot size ratio. In the large signal (depleted pump) regime, it is found that the signal beam becomes broadened if the pump depletion is significant and the output spot size depends on the input signal beam intensity. The theoretical predictions are verified experimentally using a Q-switched Nd-YAG laser to provide pulses of 30 ns FWHM duration and wavelength 1.06 micrometers , and using CCl2FCCIF2 as the nonlinear medium.
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