Lars Sjöqvist

Swedish Defence Research Agency, Tukholma, Stockholm, Sweden

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Publications (46)22.58 Total impact

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    ABSTRACT: The performance of electro-optical platform protection systems can be degraded significantly by the propagation environment around the platform. This includes aero-optical effects and zones of severe turbulence generated by engine exhausts. For helicopters rotor tip vortices and engine exhaust gases that are pressed down by the rotor airflow form the so called downwash phenomena. The downwash is a source for perturbations. A wide range of spatial and temporal fluctuations in the refractive index of air can occur. The perturbations from the turbulent flow cause detrimental effects on energy delivery, angle of arrival fluctuations, jam-code transmission, tracking accuracy and imaging performance in general. Therefore the effects may especially have a severe impact on the performance of laser-based protection systems like directed infrared countermeasures (DIRCM). The chain from passive missile detection and warning to obtaining an optical break-lock by the use of an active laser system will be influenced. To anticipate the installed performance of an electro-optical defensive aids suite (DAS) for helicopter platforms it is necessary to develop models for the prediction of the perturbations. Modelled results have to be validated against experimental findings. However, the data available in open literature on the effects of rotor downwash from helicopters on optical propagation is very limited. To collect necessary data and to obtain a first impression about the magnitude of occurring effects the European defence agency group (EDA) on “airborne platform effects on lasers and warning sensors (ALWS)” decided to design and perform a field trial on the premises of the Italian Air Force Flight Test Center in Pratica di Mare, Italy. ALWS is a technical arrangement under the Europa MoU among France, Germany, Italy, Sweden and the United Kingdom.
    No preview · Conference Paper · Oct 2015
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    ABSTRACT: In certain directed infrared countermeasure (DIRCM) situations, the laser beam path may have to pass close to the engine exhaust plume of the aircraft and models of plume turbulence are needed for DIRCM performance simulations. The jet engine plume was modeled using large eddy simulation (LES), providing time resolved information about the large scale turbulent eddies. The refractive index data from the LES calculations were integrated along the propagation path to produce time resolved phase screens for optical beam propagation. The phase screens were used to calculate laser beam parameters including beam wander and power-in-bucket (PIB). Numerical beam propagation resulted in a root-mean-square beam wander of 200 μrad for the small turbojet engine studied. The PIB was calculated for beams with 80 μrad and 2 mrad divergence having equal beam diameter when passing through the plume. For the beam with low divergence, the average PIB was reduced from 0.23 to 0.040, while the beam with wider divergence showed no significant reduction. In both cases, the plume introduced significant temporal variation of the instantaneous PIB. The beam wander is not affected by the divergence, but only depends on beam size. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE).
    No preview · Article · Aug 2015 · Optical Engineering
  • Markus Henriksson · Lars Sjöqvist
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    ABSTRACT: The scintillation index is a common measure of the effects of atmospheric turbulence. Using a photon-counting sensor, the integration time for each sample needs to be short enough to ensure that the intensity is constant during this time. Simultaneously, hardware limitations, including detector dead-time, restrict the count rate so that the number of counts in a single time segment is extremely low. The dead-time also introduces nonlinear effects. The variance calculation in the scintillation index formula is then dominated by quantization error, and the scintillation index is severely overestimated. We investigate two methods of correcting the scintillation index based on data from a time-correlated single-photon counting laser radar system. The first approach is based on the covariance calculation of the data and can be used for very low count rates and high temporal resolution. This method may also be useful in other cases where the variance of noisy, time-resolved data needs to be calculated. The second method is based on fitting the theoretical probability density function for the intensity fluctuation caused by propagation through turbulence to the experimental data. This method can take dead-time effects into account and be used for higher count rates. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)
    No preview · Article · Aug 2014 · Optical Engineering
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    Lars Sjöqvist · Markus Henriksson · Per Jonsson · Ove Steinvall
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    ABSTRACT: Time-correlated single-photon counting (TCSPC) range profiling and imaging provide high resolution laser radar data applicable in several optical remote sensing applications at short and long distances. Excellent range resolution, below centimetres, can be obtained and information about remote objects can be extracted from TCSPC range profiles. The present study describes a TCSPC range profiling system with subcentimetre range resolution applied for remote sensing of objects at short and longer ranges. Experimental results from interrogation of geometrical shapes, reflectance tomographic imaging and range profiling at longer distance in daylight conditions are presented.
    Preview · Article · Mar 2014
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    ABSTRACT: Detection and localisation of optical assemblies used for weapon guidance or sniper rifle scopes has attracted interest for security and military applications. Typically a laser system is used to interrogate a scene of interest and the retro-reflected radiation is detected. Different system approaches for area coverage can be realised ranging from flood illumination to step-and-stare or continuous scanning schemes. Independently of the chosen approach target discrimination is a crucial issue, particularly if a complex scene such as in an urban environment and autonomous operation is considered. In this work target discrimination strategies in optics detection are discussed. Typical parameters affecting the reflected laser radiation from the target are the wavelength, polarisation properties, temporal effects and the range resolution. Knowledge about the target characteristics is important to predict the target discrimination capability. Two different systems were used to investigate polarisation properties and range resolution information from targets including e.g. road signs, optical reflexes, rifle sights and optical references. The experimental results and implications on target discrimination will be discussed. If autonomous operation is required target discrimination becomes critical in order to reduce the number of false alarms.
    No preview · Article · Oct 2013 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: We have investigated the possibilities of using Computational Fluid Dynamics (CFD) simulations to characterize the impact of refractive index fluctuations in a jet engine plume on Directed InfraRed CounterMeasure (DIRCM) system performance. The jet plume was modelled using both Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) formulations of Navier-Stokes equations. The RANS calculations provided a time-averaged description of the refractive index and the turbulence strength. The more computationally intense LES model provided time resolved data on large scale turbulent eddies within the engine plume. The smaller structures are assumed to be isotropic and are modelled implicitly to reduce the computational demands to levels feasible for current computational hardware. The refractive index data from the CFD calculations was integrated along the optical propagation path to produce phase screens. For RANS data this approach provided time averaged aberrations, whereas for LES data the temporal variation of low spatial frequency aberrations were available for a short time sequence. Modal descriptions of the phase screens were investigated to allow study of temporal variation at longer time scales. Alternatively the structure parameter (Cn2) can be estimated and used to provide order of magnitude approximations for the optical effects. The generated phase screens were used to calculate laser beam system level quality parameters including beam wander, fidelity ratio and power-in-bucket. The paper focuses on method development, but examples of a jet plume simulation showing that the engine plume turbulence has a significant impact on DIRCM system functionality are presented.
    No preview · Article · Oct 2013 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: Snipers and other optically guided weapon systems are serious threats in military operations. We have studied a SWIR (Short Wave Infrared) camera-based system with capability to detect and locate snipers both before and after shot over a large field-of-view. The high frame rate SWIR-camera allows resolution of the temporal profile of muzzle flashes which is the infrared signature associated with the ejection of the bullet from the rifle. The capability to detect and discriminate sniper muzzle flashes with this system has been verified by FOI in earlier studies. In this work we have extended the system by adding a laser channel for optics detection. A laser diode with slit-shaped beam profile is scanned over the camera field-of-view to detect retro reflection from optical sights. The optics detection system has been tested at various distances up to 1.15 km showing the feasibility to detect rifle scopes in full daylight. The high speed camera gives the possibility to discriminate false alarms by analyzing the temporal data. The intensity variation, caused by atmospheric turbulence, enables discrimination of small sights from larger reflectors due to aperture averaging, although the targets only cover a single pixel. It is shown that optics detection can be integrated in combination with muzzle flash detection by adding a scanning rectangular laser slit. The overall optics detection capability by continuous surveillance of a relatively large field-of-view looks promising. This type of multifunctional system may become an important tool to detect snipers before and after shot.
    Full-text · Article · Oct 2013 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: Detection of optical assemblies is important in revealing threats arising from snipers or other weapons guided by optical means. Several approaches can be imagined using flood illumination or scanning laser techniques. One challenging problem in optics detection applications in urban environments, particular if an autonomous approach is chosen, is to reduce the false alarm rate. In this work a dual channel approach for optics detection using a narrow scanning rectangular laser beam is described. One channel is used for locating targets in the vertical direction while a second channel simultaneously determines the distance to the targets. An experimental system consisting of two channels operating at 0.8 micrometer wavelength was used to study the characteristics of different targets such as road signs, optical reflexes, rifle sights, optical references and backgrounds at different ranges and in different environments. Schemes for refining the target discrimination, reducing the false alarm rate and improving the performance are discussed using experimental results. A dual channel approach is suggested to improve capabilities in optics detection using a scanning rectangular laser beam.
    No preview · Conference Paper · Nov 2012
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    ABSTRACT: Tomographic signal processing is used to transform multiple one-dimensional range profiles of a target from different angles to a two-dimensional image of the object. The range profiles are measured by a time-correlated single-photon counting (TCSPC) laser radar system with approximately 50 ps range resolution and a field of view that is wide compared to the measured objects. Measurements were performed in a lab environment with the targets mounted on a rotation stage. We show successful reconstruction of 2D-projections along the rotation axis of a boat model and removal of artefacts using a mask based on the convex hull. The independence of spatial resolution and the high sensitivity at a first glance makes this an interesting technology for very long range identification of passing objects such as high altitude UAVs and orbiting satellites but also the opposite problem of ship identification from high altitude platforms. To obtain an image with useful information measurements from a large angular sector around the object is needed, which is hard to obtain in practice. Examples of reconstructions using 90 and 150° sectors are given. In addition, the projection of the final image is along the rotation axis for the measurement and if this is not aligned with a major axis of the target the image information is limited. There are also practical problems to solve, for example that the distance from the sensor to the rotation centre needs to be known with an accuracy corresponding to the measurement resolution. The conclusion is that that laser radar tomography is useful only when the sensor is fixed and the target rotates around its own axis.
    No preview · Article · Sep 2012 · Proceedings of SPIE - The International Society for Optical Engineering
  • Ove Steinvall · Lars Sjöqvist · Markus Henriksson
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    ABSTRACT: Photon counting techniques using direct detection has recently gained considerable interest within the laser radar community. The high sensitivity is of special importance to achieve high area coverage in surveillance and mapping applications and long range with compact systems for imaging, profiling and ranging. New short pulse lasers including the super continuum laser is of interest for active spectral imaging. A special technique in photon counting is the "time correlated single photon counting" (TCSPC). This can be utilized together with short pulse (ps) lasers to achieve very high range resolution and accuracy (mm level). Low average power lasers in the mW range enables covert operation with respect to present laser warning technology. By analyzing the return waveform range and shape information from the target can be extracted. By scanning the beam high resolution 3D images are obtained. At FOI we have studied the TCSPC with respect to range profiling and imaging. Limitations due to low SNR and dwell times are studied in conjunction with varying daylight background and atmospheric turbulence. Examples of measurements will be presented and discussed with respect to some system applications.
    No preview · Conference Paper · May 2012
  • Markus Henriksson · Lars Sjöqvist
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    ABSTRACT: A time-correlated single-photon counting (TCSPC) laser radar system can be used for range profiling of objects with high time resolution and dynamic range. A system setup is described and daytime outdoor measurements over distances up to 1 km are presented. The system has 114 ps full width half maximum system response, indicating a Rayleigh criterion resolution of two surfaces separated by 17 mm and much better with more advanced signal processing methods. The high dynamic range and time resolution allows measurement of distances between different optical surfaces in objects such as optical sights. The system thus has a potential use to classify objects and remove false alarms in an optics detection system. Effects of atmospheric turbulence and background radiation in daytime conditions are analyzed. A method for determining the scintillation index in noisy data using the temporal autocorrelation is described. System performance calculations based on measured data indicate that the performance necessary to detect characteristic features of optical sights and other retroreflecting objects may be found in reasonable dwell times well below 100 ms.
    No preview · Article · Oct 2011 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: Atmospheric turbulence effects close to ground may affect the performance of laser based systems severely. The variations in the refractive index along the propagation path cause effects such as beam wander, intensity fluctuations (scintillations) and beam broadening. Typical geometries of interest for optics detection include nearly horizontal propagation paths close to the ground and up to kilometre distance to the target. The scintillations and beam wander affect the performance in terms of detection probability and false alarm rate. Of interest is to study the influence of turbulence in optics detection applications. In a field trial atmospheric turbulence effects along a 1 kilometre horizontal propagation path were studied using a diode laser with a rectangular beam profile operating at 0.8 micrometer wavelength. Single-path beam characteristics were registered and analysed using photodetectors arranged in horizontal and vertical directions. The turbulence strength along the path was determined using a scintillometer and single-point ultrasonic anemometers. Strong scintillation effects were observed as a function of the turbulence strength and amplitude characteristics were fitted to model distributions. In addition to the single-path analysis double-path measurements were carried out on different targets. Experimental results are compared with existing theoretical turbulence laser beam propagation models. The results show that influence from scintillations needs to be considered when predicting performance in optics detection applications.
    No preview · Article · Oct 2011 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: The possible use of a simple methodology to study laser-beam propagation in a transonic flow and observe the aero-optical effects was investigated. The experimental setup consisted the DIRCM pod, the wind tunnel, and the image acquisition system. A steady-state CFD model of the experiment was developed using a commercial finite volume code. The numerical model found that the blockage caused by the pod results in some local supersonic flow with a consequent normal shock downstream. The main sources of potential discrepancy between the predicted beam wander and the measured values are found. These include the numerical model is limited in its resolution and the laser beam as it left the wind tunnel passed through a 10-mm-thick piece of optical glass and it was not possible to determine the effect this had on the laser path. The third source is that the simulation assumes that the inflow conditions to the wind tunnel are symmetrical about the x-z plane.
    No preview · Article · Jul 2011 · AIAA Journal
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    ABSTRACT: The exhaust from engines introduces zones of extreme turbulence levels in local environments around aircraft. This may disturb the performance of aircraft mounted optical and laser systems. The turbulence distortion will be especially devastating for optical missile warning and laser based DIRCM systems used to protect manoeuvring aircraft against missile attacks, situations where the optical propagation path may come close to the engine exhaust. To study the extent of the turbulence zones caused by the engine exhaust and the strength of the effects on optical propagation through these zones a joint trial between Germany, the Netherlands, Sweden and the United Kingdom was performed using a medium sized military turboprop transport aircraft tethered to the ground at an airfield. This follows on earlier trials performed on a down-scaled jet-engine test rig. Laser beams were propagated along the axis of the aircraft at different distances relative to the engine exhaust and the spatial beam profiles and intensity scintillations were recorded with cameras and photodiodes. A second laser beam path was directed from underneath the loading ramp diagonally past one of the engines. The laser wavelengths used were 1.5 and 3.6 mum. In addition to spatial beam profile distortions temporal effects were investigated. Measurements were performed at different propeller speeds and at different distances from exhaust nozzle to the laser path. Significant increases in laser beam wander and long term beam radius were observed with the engine running. Corresponding increases were also registered in the scintillation index and the temporal fluctuations of the instantaneous power collected by the detector.
    No preview · Article · Oct 2010 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: The exhaust from jet engines introduces extreme turbulence levels in local environments around aircrafts. This may degrade the performance of electro-optical missile warning and laser-based DIRCM systems used to protect aircrafts against heat-seeking missiles. Full scale trials using real engines are expensive and difficult to perform motivating numerical simulations of the turbulence properties within the jet engine exhaust. Large Eddy Simulations (LES) is a computational fluid dynamics method that can be used to calculate spatial and temporal refractive index dynamics of the turbulent flow in the engine exhaust. From LES simulations the instantaneous refractive index in each grid point can be derived and interpolated to phase screens for numerical laser beam propagation or used to estimate aberration effects from optical path differences. The high computation load of LES limits the available data in terms of the computational volume and number of time steps. In addition the phase screen method used in laser beam propagation may also be too slow. For this reason extraction of statistical parameters from the turbulence field and statistical beam propagation methods are studied. The temporal variation of the refractive index is used to define a spatially varying structure constant. Ray-tracing through the mean refractive index field provides integrated static aberrations and the path integrated structure constant. These parameters can be used in classical statistical parameterised models describing propagation through turbulence. One disadvantage of using the structure constant description is that the temporal information is lost. Methods for studying the variation of optical aberrations based on models of Zernike coefficients are discussed. The results of the propagation calculations using the different methods are compared to each other and to available experimental data. Advantages and disadvantages of the different methods are briefly discussed.
    No preview · Article · Oct 2010 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: Resonant output energy enhancement in a singly resonant nondegenerate type-I optical parametric oscillator with a volume Bragg grating output coupler is demonstrated. The resonances occur when the pump laser and parametric oscillator cavity length ratio is an integer or a fraction of small integers. Although the length resonances are similar to those observed in doubly resonant optical parametric oscillators, the physical mechanism is distinctly different. The resonances in the singly resonant oscillator are caused by correlation of the instantaneous power between the quasi-periodic multimode pump laser beam and the OPO signal.
    Full-text · Article · May 2010 · Optics Express
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    ABSTRACT: Resonant output energy enhancement in a singly resonant nondegenerate type-I optical parametric oscillator with a volume Bragg grating output coupler is demonstrated when there is a low fraction cavity length ratio between laser and OPO.
    Full-text · Article · Feb 2010
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    ABSTRACT: Optical measurements and tests of optical instruments are often performed through an opened window or from the roof of an elevated building. This can also be a common situation for free-space optical (FSO) communication systems. Wind friction in combination with solar heating of the wall and the ground will create increased turbulence in a boundary layer close to the wall. For an outgoing laser beam this thin region of strong turbulence causes beam wander, beam broadening and beam break-up. For imaging and detection systems angle of arrival fluctuations and image blurring may result. In an attempt to estimate the strength of the atmospheric turbulence in the layer at the wall the refractive index structure constant (Cn2) was measured with an ultra sonic anemometer as a function of distance from the wall. The measurements were performed at the lower part of a window that was open just enough to give space for the anemometer. The window was placed 10 m above ground in a 12 m high building, with brick wall below the window and wooden panel above the window. Measurements of the turbulence as a function of distance from the wall were performed during different times of the day to study the influence of sun heating of the wall. The measured average Cn2 shows an exponentially decreasing function of distance from the wall. The exponential decay of Cn2 depends on the time of the day. The highest measured value of Cn2 was approximately 3x10-11 m-2/3 near the wall. The influence of wall turbulence is discussed with respect to its influence on laser beam propagation.
    Full-text · Article · Sep 2009 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: Spectral requirements for the first stage OPO used to pump a tandem ZGP mid-infrared OPO are theoretically investigated. Based on these requirements we demonstrate a singly-resonant type-I OPO including periodically poled KTiOPO(4) and volume-Bragg gratings as output couplers. Singly resonant oscillation is demonstrated very close to degeneracy, where signal and idler spectra are not well separated. Investigations of the longitudinal mode spectrum and the idler spectrum with high resolution using a scanning Fabry-Perot interferometer show the essential role played by the phase correlations of the multi-longitudinal mode Q-switched pump laser in formation of the nonresonant idler spectrum.
    Full-text · Article · Sep 2009 · Optics Express
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    ABSTRACT: Efficient laser sources in the 3 - 5 mum wavelength range are needed for directed infrared countermeasures, but also have applications in remote-sensing, medicine and spectroscopy. We present new results on our tandem optical parametric oscillator (OPO) scheme for converting the radiation from a 1.06 mum Nd3+-laser to the mid-infrared. Multi Watt level output power in the 3-5 mum range at 20 kHz pulse repetition frequency is reported. Our setup uses a type I quasi phase-matched PPKTP crystal in a near degenerate OPO to generate 2.13 mum radiation. A volume Bragg grating resonant close to, but not exactly at the degenerate wavelength, is used as a cavity mirror to reduce the bandwidth and ensure singly resonant operation. Both signal and idler from the PPKTP OPO are used to pump a ZGP OPO generating high power radiation in the 3-5 mum region. Using this scheme for each pump photon it is possible to generate four photons for each pump photon, all in the interesting wavelength range, thus enabling high efficiency conversion.
    Full-text · Article · Oct 2008 · Proceedings of SPIE - The International Society for Optical Engineering