We investigate in what turbulent conditions and propagation geometries conventional adaptive optics (AO) can provide improvement in laser applications. Characterizing the optical aberrations is essential to provide the input required for a conventional AO system. In this parametric study we characterize these aberrations by numerically propagating a beacon from an effector into the far field and back using a split-step method involving turbulent phase screens. The beacon’s aberrated field at the location of the effector is sensed assuming a perfect wavefront sensor and subsequently used to pre-correct the effector for its turbulent propagation into the far field. In the far field, beam metrics such as spot size, Strehl ratio and power-ratio-in-the-bucket (PRIB) with and without AO correction applied to the effector are investigated. By varying propagation geometries and turbulent conditions, the dependence of the beam metrics on the propagation scheme is analyzed in detail. Additionally, the dependence of the beam metrics on assumptions in the AO system such as number of Zernike modes taken into account in the correction are studied. The results can be used to identify when AO should be considered given the broader operational context in which a laser system is expected to operate and give insight in accompanying AO considerations.
We present a new analysis of laser propagation experiments carried out with the Laser Propagation Testbed (LPT) developed by TNO. A major goal of these experiments is to validate and improve atmospheric propagation models that are essential to applications such as laser communication, high energy laser weapon systems and remote sensing. The data were obtained during a field campaign with a 1W 1556 nm laser beam deployed over a 3.6 km maritime path in The Netherlands. The measurements consist of intensity profiles of the propagated laser beam and local meteorological and atmospheric conditions (visibility, refractive index structure parameter and aerosol data) obtained during a ten day period under varying weather conditions. We use the locally measured atmospheric conditions and numerical weather prediction to constrain a turbulent laser propagation model developed by TNO, and compare the results with the time series measurements of the laser beam profile.
TNO has expanded its 30 kW HEL research facility with the capability to monitor specular and diffuse reflections of the laser beam. A capture screen and high-speed camera focus on dynamic specular reflections, while 15 individually placeable probes monitor the diffuse component under different angles. This paper introduces the reflection measurement capability and discusses the behaviour of steel and aluminium coupons under high-energy laser irradiation. Laser-material interaction was found to be rather predictable in thermal behaviour up to the perforation event. Reflections, however, showed a highly dynamic pattern, varying in magnitude and direction and depending on bulk material, material surface condition, phase state of the material (solid or liquid) and geometry. The difficulty of assessing proper stand-off distances for laser safety is illustrated.
Fielding TNO’s LPT (Laser Propagation Testbed) for the first time in an international trial, a 3.6 km optical propagation path was created in a maritime environment between the city of Den Helder and the island of Texel in the Netherlands. Using a 1 W, 1556 nm laser beam, atmospheric propagation was investigated by capturing the resulting beam profile on a capture plate, which was imaged with a high-speed SWIR camera. Meteorological conditions were monitored using standard meteo stations, two visibility meters, two scintillometers and aerosol equipment. Over two weeks of measurements, propagation conditions varied from windy with clear, blue skies to significantly limited visibility. In this paper, the setup is introduced and a first discussion of the relation between beam behaviour and meteorological conditions is presented.
A 1.55µm Laser Propagation Testbed (LPT) has been deployed over a 3.5 km stretch of open water between the port of Den Helder and the island of Texel in The Netherlands. The laser intensity and beam profile have been measured after propagation and reflection off a capture plate. Supplementing data was provided by large-aperture scintillometers, aerosol counters, visibility meters and standard meteorological equipment. The LPT will be presented, as well as a selection of data to demonstrate the potential of this setup. The LPT is expected to validate and verify laser propagation models and thereby to contribute in the field of atmospheric propagation, optical communication and high energy laser systems.
Infrared imaging of the sea surface is used for many purposes, such as remote sensing of large oceanographic structures, environmental monitoring, surveillance applications and platform signature research. Many of these studies rely on determining the contrast of a target feature with its background and therefore benefit from accurately predicting the signature of the underlying sea surface background. We here present a model that synthesizes infrared spectral images of sea surfaces. This model traces explicitly the behaviour of the sea wave structure and light propagation. To self-consistently treat spatial and temporal correlations of the clutter, geometrical realizations of sea surfaces are built based on realistic sea wave spectra and their temporal behaviour is subsequently followed. A camera model and a ray tracer are used to determine which parts of the sea surface are observable by individual camera pixels. Atmospheric input elements of the model, being sky dome, path radiance and transmission, are computed with MODTRAN for a chosen atmosphere.
Using seasonally averaged meteorological and spectrally resolved aerosol profiles extracted from a maritime environment, this paper investigated how the resolution of the vertical profiles influences the 3-5μm and 8-12μm average transmittance and integrated path radiance computations conducted by MODTRAN in high-elevation scenarios. First, the minimum altitude to which the atmosphere should be defined in order to accurately determine the transmittance and path radiance along vertical and slant paths was investigated by recursively removing vertical layers until the relative changes in the transmittance and path radiance became smaller than those due to instrument uncertainty. Once this minimum height was found, the vertical resolution in the atmosphere below the minimum altitude was systematically varied. The suitability of several gradient-based criteria has been investigated to determine the optimal discretization of the vertical profiles. The results indicate that, depending on the quantity to be calculated, vertical discretizations based on the gradient in either the pressure, temperature or humidity serve as optimal discretizations in maritime high-elevation scenarios. Moreover, the followed methodology demonstrates how to adaptively implement a vertical resolution in a generic atmosphere, which generates crucial knowledge in supporting signature and sensor performance modelling for high-elevation scenarios.
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