Folke Berglund

Single- and double-path 10.6-μm laser link measurements over sea water

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 beams 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.

Using an eyesafe military laser range finder for atmospheric sensing

Laser Rangefinders are well established components in various electro-optical fire control systems. Recent range finders are often operating at eye safe wavelengths around 1.5 μm which extend their utility. One such extension is the use of the sensor for atmospheric sensing based on the measured backscatter signal. The present paper investigates the use of an eye-safe laser rangefinder at 1.5 μm to obtain information on atmospheric attenuation at various paths in the atmosphere. This knowledge can in turn be used in combination with atmospheric and target/background models to estimate the performance of other EO sensors like TV and thermal imagers beside the laser range finder itself. Such information can be of great value both for estimating own sensor capabilities at a given moment as well as estimating the threat capability. One obvious example is ship defense where it is difficult to obtain visibility along a variable atmosphere especially in darkness. The paper will describe the experimental equipment and the results from measurements of atmospheric backscatter along various atmospheric paths. The backscatter curve is used to evaluate the extinction. This extinction values are compared with those deduced from a point visibility meter and from echo measurements against two similar nets positioned at 2 ranges from the sensor. The results indicated a good correspondence between these results. Finally the results are illustrated in a system perspective by estimating the performance for thermal IR and other EO sensors.

Using an eye-safe laser rangefinder to assist active and passive electro-optical sensor performance prediction in low visibility conditions

Abstract. Laser rangefinders are used in various electro-optical (EO) fire control systems. They often operate at eye-safe wavelengths around 1.55  μm, which extends their utility. The paper investigates the use of a modified eye-safe laser rangefinder at 1.55  μm to obtain information on atmospheric attenuation and couple that information to the performance of active and passive EO sensors with an emphasis of lower visibility conditions. Such information can be of great value both for estimating own sensor capabilities at a given moment as well as estimating the threat capability. One obvious example is ship defense where it is difficult to obtain visibility along variable and slant atmospheric paths, especially in darkness. The experimental equipment and the results from measurements of atmospheric backscatter along various atmospheric paths are presented. The backscatter curve is used to evaluate the extinction. These extinction values are compared with those deduced from a point visibility meter and from echo measurements against two similar nets positioned at two ranges from the sensor. TV and IR images of test targets along a 1.8 km path close to sea surface in the Baltic Sea were collected in parallel with the lidar. A weather station and a scintillometer collected weather and turbulence parameters. Results correlating the lidar attenuation with the imaging performance will be given.

Laser dazzling impacts on car driver performance

A growing problem for the Police and Security Forces has been to prevent potentially hostile individuals to pass a checkpoint, without using lethatl violence. Therefore the question has been if there is a laser or any other strong light source that could be used as a warning and dazzling device, without lethal or long term effects. To investigate the possibilities a field trial has been performed at a motor-racing track. A green CW laser with an irradiance on the eye of maximum 0.5 MPE, as defined by ICNIRP [1] and the ANZI standard [2], was used as a dazzle source. Ten drivers have been driving with dipped headlights through a course of three lines with orange cones. In every line there has been only one gate wide enough to pass without hitting the cones. The time through the course, the choice of gates and the number of cones hit have been measured. For every second trial drive through the track, the driver was exposed to the laser dazzler. The background illuminances ranged from a thousand lux in daylight to about ten millilux in darkness. The protective effect of the sun-visor of the car was investigated. The drives visual system was carefully examined before and after experimental driving and a few weeks after the experimental driving to verify that no pathological effects, that could potentially be induced by the laser exposure, pre-existed or occurred after the laser exposure. An analysis of variance for a within subjects design has been used for evaluation. It was found that green laser light can have an obvious warning effect in daylight. Dazzling does reduce the drivers ability to make judgments and manouver the car in twilight and darkness. A sun-visor can reduce the glare and give the driver an improved control, but that perception can be unjustified. No damage to the visual system was observed.

Coordinated multi-wavelength laser system propagation experiments

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.

Laser propagation through turbulence over land and sea (Invited Paper)

This paper will describe single and double path laser link measurements over land and sea. The laser system consisted of a CO2 laser, a pointing and tracking head, a quadrant laser receiver, a 3-5 μm IR-camera, a TV camera and a laser range finder. For the naval scenario the laser system was placed in a building 18 m above water and corner cube targets and a single path receiver were placed on islands at 2.5, 5.5 and 16.5 km. For the land scenario the laser system was placed in a building about 13 meters above ground looking at targets and a single path receiver at 2 km range. Together with the laser registrations, separate instruments such as a scintillometer and a weather station were recording the meteorological parameters. The analysis contains evaluation of temporal and amplitude signal distributions, probability and mean time of fade and tracking performance. Results from single ended and double ended paths will be compared. Different ways of using this database for turbulence simulation and laser system performance predictions also in other wavelength regions will be discussed as well as the impact on applications including laser imaging, free-space laser communications and directed infrared countermeasures.

Multi-wavelength laser propagation experiments

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.

Imaging and laser profiling for airborne target classification

Passive optical imaging for long range target classification has its practical limitations due to the demand on high transverse sensor resolution associated with small pixel sizes, long focal lengths and large aperture optics. It is therefore motivated to look for 1D laser range profiling for target classification which can preserve high resolution in the depth domain. Laser range profiling is attractive because the maximum range can be substantial, especially for a small laser beam width. A range profiler can also be used in a scanning mode to detect targets within a certain sector and can also be used for active imaging when the target comes closer and is angular resolved. Although the profiling may by itself be sufficient for target classification the discrimination capabilities among a group of anticipated targets candidates may be uncertain due to uncertainty in the target aspect angles, atmospheric effects and sensor limitations. It is therefore motivated to look at a sensor fusion approach in which the profiling data is combined with imaging data even when these data have a rather low resolution. Example of both simulated and experimental data will be investigated and analyzed for target classification purposes.

Passive and active EO sensing of small surface vessels

The detection and classification of small surface targets at long ranges is a growing need for naval security. This paper will present an overview of a measurement campaign which took place in the Baltic Sea in November 2014. The purpose was to test active and passive EO sensors (10 different types) for the detection, tracking and identification of small sea targets. The passive sensors were covering the visual, SWIR, MWIR and LWIR regions. Active sensors operating at 1.5 μm collected data in 1D, 2D and 3D modes. Supplementary sensors included a weather station, a scintillometer, as well as sensors for positioning and attitude determination of the boats. Three boats in the class 4-9 meters were used as targets. After registration of the boats at close range they were sent out to 5-7 km distance from the sensor site. At the different ranges the target boats were directed to have different aspect angles relative to the direction of observation. Staff from IOSB Fraunhofer in Germany and from Selex (through DSTL) in UK took part in the tests beside FOI who was arranging the trials. A summary of the trial and examples of data and imagery will be presented.

Predicting laser beam propagation in a naval environment

Directed infrared countermeasure (DIRCM) systems are being developed in several configurations ranging from open loop, closed loop (CLIRCM) and lately for preemptive use (PIRCM). Critical performance parameters, from an atmospheric propagation point of view, include attenuation as well as turbulence effects on target detection, tracking and the capability to deliver enough power to the target (power in bucket). In order to evaluate the performance of DIRCM and other laser applications in a naval environment we have performed laser beam propagation studies over the Baltic Sea during June to October 2003. The experimental data were used to derive atmospheric attenuation, irradiance fluctuation parameters for different beam offsets relative to the beam centre, temporal and amplitude signal distributions, probability and mean time of fade and tracking performance. Results from simultaneous single and double-ended paths were obtained. This paper will shortly summarize the results from this campaign, and discuss and exemplify how the results can be used for prediction of DIRCM system performance.