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This work presents a coherent beam combination (CBC) setup that consists of a four square tiled-aperture system with phase and tip-tilt control for each sub-beam. The system works at a wavelength of 1550 nm. Phase locking was performed with the stochastic parallel gradient descent (SPGD) algorithm, using the power reflected by a remote retro reflecting corner cube to a feedback detector located at the laser head. Experiments were performed for targets located at distances of 470 and 870 m during calm and partly cloudy conditions. The SPGD beam combining results are discussed and compared to optical wave propagation simulations. The optical wave propagation models that have previously been validated in a laboratory setting were extended to include atmospheric turbulence. In order to reproduce the experimental results it was important to include the propagation of the feedback signal from the retroreflector to the detector.

Laser radar 3D imaging has the potential to improve target recognition in many scenarios. One case that is challenging for most optical sensors is to recognize targets hidden in vegetation or behind camouflage. The range resolution of timeof- flight 3D sensors allows segmentation of obscuration and target if the surfaces are separated far enough so that they can be resolved as two distances. Systems based on time-correlated single-photon counting (TCSPC) have the potential to resolve surfaces closer to each other compared to laser radar systems based on proportional mode detection technologies and is therefore especially interesting. Photon counting detection is commonly performed with Geigermode Avalanche Photodiodes (GmAPD) that have the disadvantage that they can only detect one photon per laser pulse per pixel. A strong return from an obscuring object may saturate the detector and thus limit the possibility to detect the hidden target even if photons from the target reach the detector. The operational range where good foliage penetration is observed is therefore relatively narrow for GmAPD systems. In this paper we investigate the penetration capability through semi-transparent surfaces for a laser radar with a 128×32 pixel GmAPD array and a 1542 nm wavelength laser operating at a pulse repetition frequency of 90 kHz. In the evaluation a screen was placed behind different canvases with varying transmissions and the detected signals from the surfaces for different laser intensities were measured. The maximum return from the second surface occurs when the total detection probability is around 0.65-0.75 per pulse. At higher laser excitation power the signal from the second surface decreases. To optimize the foliage penetration capability it is thus necessary to adaptively control the laser power to keep the returned signal within this region. In addition to the experimental results, simulations to study the influence of the pulse energy on penetration through foliage in a scene with targets behind vegetation are presented. The optimum detection of targets occurs here at a slightly higher total photon count rate probability because a number of pixel have no obscuration in front the target in their field of view.

Time-correlated single-photon counting (TCSPC) is a laser radar technique that can provide range profiling with very high resolution. Range profiles of multiple surface objects and geometrical shapes are revealed using multiple laser pulses with very low pulse energy. The method relies on accurate time measurements between a laser pulse sync signal and the registration of a single-photon event of reflected photons from a target. TCSPC is a statistic method that requires an acquisition time and therefore the range profile of a non-stationary object (target) may be corrupted. Here, we present results showing that it is possible to reconstruct the range profile of a moving target and calculate the velocity of the target.

The hot exhaust gases from engines on helicopters are pushed down by the rotor in a turbulent flow. When the optical path of a laser beam or optical sensor passes through this region severe aberrations of the optical field may result. These perturbations will lead to beam wander and beam distortions that can limit the performance of optical countermeasure systems. To quantify these effects the Italian Air Force Flight Test Centre hosted a trial for the “Airborne platform effects on lasers and warning sensors” (ALWS) EDA-project. Laser beams were propagated from the airport control tower to a target screen in a slant path with the helicopter hovering over this path. Collimated laser beams at 1.55-, 2- and 4.6-μm wavelength were imaged with high speed cameras. Large increases in beam wander and beam divergence were found, with beam wander up to 200 μrad root-mean-square and increases in beam divergence up to 1 mrad. To allow scaling to other laser beam parameters and geometries formulas for propagation in atmospheric turbulence were used even though the turbulence may not follow Kolmogorov statistics. By assuming that the plume is short compared to the total propagation distance the integrated structure parameter through the plume could be calculated. Values in the range 10 -10 to 10 -8 m 1/3 were found when the laser beams passed through the exhaust gases below the helicopter tail. The integrated structure parameter values calculated from beam wander were consistently lower than those calculated from long term spot size, indicating that the method is not perfect but provides information about order of magnitudes. The measured results show that the engine exhaust for worst case beam directions will dominate over atmospheric turbulence even for kilometer path lengths from a helicopter at low altitude. How severe the effect is on system performance will depend on beam and target parameters.

Time-correlated single-photon counting (TCSPC) is a laser radar technique that can provide range profiling with subcentimetre range resolution. The method relies on accurate time measurements between a laser pulse sync signal and the registration of a single-photon detection of photons reflected from an object. The measurement is performed multiple times and a histogram of arrival times is computed to gain information about surfaces at different distances within the field of view of the laser radar. TCSPC is a statistic method that requires an integration time and therefore the range profile of a non-stationary object (target) will be corrupted. However, by dividing the measurement into time intervals much shorter than the total acquisition time and cross correlating the histogram from each time interval it is possible calculate how the target has moved relative to the first time interval. The distance as a function of time was fitted to a polynomic function. This result was used to calculate a distance correction of every single detection event and the equivalent stationary histogram was reconstructed. Series of measurements on the objects with constant or non-linear velocities up to 0.5 m/s were performed and compared with stationary measurements. The results show that it is possible to reconstruct range profiles of moving objects with this technique. Reconstruction of the signal requires no prior information of the original range profile and the instantaneous and average velocities of the object can be calculated.

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.

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.

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.

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.

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.

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.

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 ( C n 2 ) 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 C n 2 shows an exponentially decreasing function of distance from the wall. The exponential decay of C n 2 depends on the time of the day. The highest measured value of C n 2 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.

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 KTiOPO4 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. QC 100517

Laser beam propagation through adverse turbulent environments such as the region close to a jet engine exhaust need to be studied in order to predict performance degradations on airborne laser systems. The turbulent plume region may introduce severe perturbations which accumulate and cause beam degradation in terms of beam wander, intensity scintillations and beam broadening at longer ranges. Applications of interest with respect to laser beam propagation in jet engine plume environments include e.g. directed infrared countermeasures (DIRCM) and active imaging. By characterising and evaluating the perturbation effects schemes for compensation or avoiding performance degradation can be devised. The turbulence effects in the plume region occur by mixing of hot exhaust flow from the jet engine with surrounding ambient air causing spatial and temporal fluctuations in the refractive index. In comparison to atmospheric turbulence considerably shorter outer- and inner scales have been observed. Typical values of the structure constant within the plume region range from 10-10 to 10-9 m-2/3 making the turbulence several order in magnitude stronger in contrast to propagation through the atmosphere. Of importance in characterisation of the jet engine plume with respect to laser beam propagation are turbulent length scales, the extent of the turbid region, variation of the structure constant and temporal flow properties. In this paper reported experimental results and modelling approaches aimed for predicting laser beam propagation degradation in jet engine plume regions are reviewed. The results will be discussed in perspective of system performance.

Efficient laser sources in the 3 - 5 μm 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 μm Nd3+-laser to the mid-infrared. Multi Watt level output power in the 3-5 μm 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 μm 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 μm 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.

Airplane based laser systems for DIRCM, active imaging and communication are important applications attracting considerable interest. The performance of these systems in directions where the laser beam points close to or through the exhaust plume from the jet engines may be severely reduced. A trial to study these phenomena using a downscaled jet-engine test rig was carried out. The results on propagation of laser beams along and across the plume from these trials are presented. For laser beams propagation along the engine axis an OPO based source producing co-propagating laser beams at 1.52 and 3.56 μm was used. The beams were projected on a screen and imaged with separate IR cameras to study beam wander and spot degradation. Propagation across the plume was studied with a 532 nm laser projected on a screen and imaged by a high speed camera. The engine thrust and the distance between the engine nozzle and the laser beams were varied to study the effects of changing conditions. Scaling to full size engines and performance implications for DIRCM is discussed.

Range profiling with high resolution and accuracy can be accomplished using single-photon counting time-of-flight techniques. Detection of target surfaces with high resolution is of importance for several remote sensing applications. The use of laser pulses in the picosecond regime, single-photon avalanche photodiodes and acquisition electronics with high timing resolution provides the tools for improving the range accuracy. This paper gives examples of measured surface profiles and compares the results with simplified theoretical models. The results are discussed with respect to some applications.

Optical range profiling with high resolution can be accomplished using single-photon counting time-of-flight techniques. Detection of optical surfaces with high resolution is of importance for several remote sensing applications. The use of laser pulses in the picosecond regime, single-photon detectors and acquisition electronics with high timing resolution provides the tools for improving the range accuracy. In this work a system based on time-of-flight range profiling using time-correlated single-photon counting techniques is described. The system is intended to be used for optical range profiling in field experiments. Experimental results from measurements on test targets are presented. Schemes for extraction of range information from the experimental temporal profile and results from performance modelling are briefly discussed.

An optical communication link with a modulated retro-reflector in a cat's eye optics arrangement has been evaluated. A transmissive multiple quantum well modulator with a modulation bandwidth in excess of 10 MHz was placed in a wide field-of-view cat's eye optic system. A laser transceiver with one line and one broadband source around 1550 nm was used to interrogate the modulated retro-reflector and detect the reflected and modulated signal. Multi-level digital encoding was evaluated as a way to increase the information transfer rate in conditions with relatively high signal-to-noise ratio. The links were evaluated in indoor as well as outdoor conditions.

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 μ rad for the small turbojet engine studied. The PIB was calculated for beams with 80 μ rad 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.

Atmospheric turbulence can significantly degrade the performance of free-space optical communication links. Beam wander, intensity scintillations, beam broadening and angle of arrival fluctuations give rise to signal fading with communication channel drop-outs. In this work the performance of the potentially beneficial combination of a retro reflective link operating at 1535 nm in conjunction with a single-photon quantum key distribution link at 850 nm was studied. The general idea is that the weak light beam of the quantum channel can be steered and controlled by the 1535 nm signal. A dual-channel laser unit, with output beams at 850 and 1535 nm, was used to characterise turbulence effects along an atmospheric path close above ground. The laser beams were retro reflected from a corner cube and detected in the same laser unit. The 1535 nm beam was monitored with a position sensitive detector while the 850 nm beam (low intensity channel) was aligned with the 1535 nm beam and detected separately but along the same optical axis of the laser unit. Effects of beam wander, angular fluctuations and intensity scintillations were studied experimentally at different turbulence strengths and weather conditions. The experimental results are discussed with respect to theoretical models.

An increasing interest in lasers placed on aircrafts for active countermeasures and active imaging is observed. There remain unsolved issues regarding the propagation effects close to the jet engine exhaust and the possibilities of compensating them with adaptive optics. Laser beam propagation experiments parallel to the exhaust of a downscaled jet engine test rig have been performed. The experiments were carried out with nanosecond laser pulses at 1.6 and 3.5 μm wavelength. The laser spots were projected on a screen and the centroid motion were imaged by cameras. Root mean square magnitudes of the beam wander between 50 and 150 μrad were observed for different engine conditions and geometries. The 3.5 μm system had a frame rate of 607 Hz and could partly resolve the time variation of the beam wander. A correlation time (50 %) of 3.5 ms was observed for the beam wander. Deflections of several hundred μrad due to the average gradients in temperature and pressure were also found when the engine was turned on. In addition to beam wander intensity scintillations has been studied.

Atmospheric propagation experiments for active and passive EO systems were performed over a 2 and 8 km path. Single and double path propagation effects were studied using retroreflectors and hot point targets. The systems used include laser systems at 1.5 and 3.5 μm wavelength as well as imaging systems in the visible, 1-2 and 8-9 μm regions. A scintillometer operating at 0.8 μm wavelength was also used for the shorter path. Experimental data will be presented and evaluated concerning statistics. The results will be discussed mainly from a laser countermeasure point of view.

Atmospheric propagation degradation effects including attenuation, aerosol scattering and turbulence have a great impact on the performance of optical systems. Relevant military optical systems include active and passive imaging for target recognition, free-space optical communication and DIRCM/EOCM. This paper will report on experimental work including measurement of retro signals at 1.5 and in the 3-5 μm wavelength regions for evaluation of retro communication links and DIRCM performance. Imaging experiments using a range-gated system both in the active and passive mode at 1.5 μm, will also be carried along the same paths. A dedicated target box and test targets have been fabricated for mounting on a mast at 8 km from our laboratory. The box contains reflectors and receivers in different slots each of which can be opened by a telephone call. A heated target on top simulates a point target in the IR region. The test targets are aimed for the range-gated imaging system. Preliminary experimental data will be presented and discussed.

Understanding and predicting laser beam propagation effects in the atmosphere is of importance for laser countermeasures and related applications. Turbulence effects cause beam wander, beam broadening and intensity scintillations reducing e.g. the power in bucket and the tracking accuracy. Modelling laser beam propagation in turbulence using successive phase screens provides an efficient tool for performance predictions. In this work phase screens are used to model laser beam propagation over land and sea. Different phase screens generators utilising the Kolmogorov or von Karman spectra were considered. Critical parameters using phase screens include the number of screen applied along the propagation path, inner- and outer scale size, variations in the structure parameter and spatial frequencies. Effects such as beam wander, angle-of-arrival fluctuations and intensity scintillations are discussed. The simulated results are compared with experimental data recorded at different ranges, various turbulence strengths and for single- and double paths. A generic example describing laser countermeasure against an infrared homing missile in a naval scenario is presented.

Laser beam propagation in severe environment such as a jet engine exhaust may influence performance of e.g. laser countermeasure and active imaging systems located on airborne platforms. Beam propagation in close vicinity to the engine plume causes performance degradation due to beam wander and beam broadening effects. In this study beam propagation effects were studied experimentally using a downscaled jet engine. Beam wander and broadening effects were studied directing a 1.55 μm laser beam parallel to the jet exhaust. The beam centroid motion increased significantly during onset of the engine. Perturbation effects were registered imaging a blackbody source using a fast infrared camera. The geometrical set-up and thrusts were altered during the experimental registrations to examine various flow conditions. A comparison of experimental results with simplified theoretical models describing propagation close to jet engine plumes will be presented. Limitations in existing models will be discussed and methods for refinements proposed.

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.

A conventional 3D phase contrast acquisition generates images with good spatial resolution, but often gives rise to artifacts due to pulsatile flow. 2D cine phase contrast, on the other hand, can register dynamic flow, but has a poor spatial resolution perpendicular to the imaging plane. A combination of both high spatial and temporal resolution may be advantageous in some cases, both in quantitative flow measurements and in MR angiography. The described 3D cine phase contrast pulse sequence creates a temporally resolved series of 3D data sets with velocity encoded data.

Magnetic resonance imaging (MRI) studies of the heart have been used for some years, but there are few tools available to quantify cardiac motion. A method has been developed that creates an M-mode MRI image, analogous to the one used in echocardiography, to display motion along a line as a function of time. The M-mode image is created from MRI images acquired with an ordinary gradient echo cine sequence. In a cinematographic display of the images, a cursor line can be positioned in order to determine the orientation of the measurement. A resampling algorithm then calculates the appearance of the M-mode image along the cursor line. The MRI method has been compared to echocardiographic M-mode in a phantom study and by measuring mitral and tricuspid annulus motion in 20 normal subjects. The phantom study showed no significant differences between MRI and echocardiographic M-mode measurements (difference < 1 mm). The annulus motion exhibits a similar pattern using both methods and the measured amplitudes are in close agreement. M-mode MRI provides similar information to echocardiography, but the cursor line can be placed arbitrarily within the image plane and the method is thus not limited to certain acoustic windows. This makes M-mode MRI a promising technique for assessing cardiac motion.

The ESR lineshape of one electron spin coupled by anisotropic hyperfine interaction to a number of nuclei is calculated when at least two different configurations of the molecule interchange according to a given reaction scheme. The spectrum calculation includes also the anisotropic electron- zeeman and the nuclear quadrupole interaction. The lineshape, which simulates a nonsaturated CW experiment, is calculated numerically by a direct method and includes the nonsecular terms of the electron spin. The density matrix theory, combined with Liouville formalism, is used allowing both for an accurate treatment of spin dynamics as well as detailed description of the reorganization of the system during exchange [1]. The exchange is treated phenomenologically by a sudden-jump, markov model in which the steady-state populations are calculated prior to the lineshape and are used for the lineshape calculation. For a given overall molecular orientation each configuration (site) is characterized by a set of three strength constants and three Euler angles per interaction [2]. This basic system is used to simulate a single crystal case or is appropriately averaged to simulate a powder sample. The above theory is compared with two experiments.

In order to evaluate the performance of laser-based applications in maritime environments, laser beam propagation studies over the Baltic Sea during the period of about half a year have been carried out. A slightly slanted path with the laser system located 18 m above the sea level and corner cube targets and separate receivers placed on islands at 2.5-, 5.5-, and 16.5-km distance were used. From the registered laser signals irradiance fluctuation parameters for different beam offsets relative to the beam's center, temporal, and amplitude signal distributions, we derived the probability and mean time of fade. Results and statistics from single and double propagation paths are compared.