This PDF file contains the front matter associated with SPIE Proceedings Volume 9641 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

An overview of remote sensing activities at the Institute of Maritime Technology conducted in above and underwater applications will be reviewed. The above water activities will focus on electro-optic sensing, while underwater activities will focus on environmental characterization. A brief introduction of the defence research institute will be followed by a review of remote sensing activities for the following purposes; infrared signature characterization (extended and point targets, grey-body and spectral emitters, ship wakes), persistent surveillance (harbours, on ships, panoramic systems), atmospheric and maritime environment characterization (above and underwater e.g. sea/air temperature profiles, atmospheric MTF, turbulence). A short review of recent joint international trials conducted at the institute will be given. This includes a small targets trail, the False Bay Atmospheric Experiment (FATMOSE) and the First European South African Transmission Experiment (FESTER), which is currently underway. The small surface target trail focused on the electro-optic signature characterization of small surface targets (including wakes) and sea, land and sky backgrounds. FATMOSE focused mainly on characterization of atmospheric effects that influence electro-optic sensor performance, while FESTER will extend on above research efforts to also include the radio frequency spectrum (RF) and a significant oceanographic characterization effort in support of FESTER above water activities. The factors which make the False Bay location interesting for remote sensing electro-optic and environmental characterization experiments and the interesting results that are expected from the FESTER experiment will be discussed.

Radiation created by stimulation and recombination/deactivation of atoms and molecules in the higher earth atmosphere is called nightglow. This nightglow can be found in the spectral range from the ultraviolet up to the thermal infrared, with a maximum in the shortwave infrared (SWIR). During moonless nights the illumination in the SWIR is by an order of magnitude higher than the visual one. Within the last years the SWIR sensor technology improved to a level of using the nightglow for night vision applications. This necessitates understanding of the highly variable illumination levels created by the nightglow and the performance assessment of the SWIR detectors in comparison to the image intensifiers respectively Si focal plane array detectors. Whereas the night illumination levels for the visual are standardized, corresponding ones for the SWIR are missing. IOSB started measuring and comparing night illumination levels and camera performance in both spectral ranges based on continuous illumination measurements as well as recording imagery of reflectance reference targets with cameras and analyzing the resulting signal-to-noise ratios. To date the number of illumination measurements are not yet statistically sufficient to standardize the levels, but at least allowed a first comparison of the two technologies for moonless night, clear sky conditions. With comparable F-number, integration time and frame rate, the SWIR sensors available in Europe were found to be inferior to the visual technology. An improvement of at least one magnitude would be necessary to ensure similarity between SWIR and visual technologies for all environmental conditions.

We have performed a series of experiments in order to simultaneously validate several devices and methods for measurement of the path-averaged refractive index structure constant ( 𝐶_𝑛 ²). The experiments were carried out along a horizontal urban path near the ground. Measuring turbulence in this layer is particularly important because of the prospect of using adaptive optics for free-space optical communications in an urban environment. On one hand, several commercial sensors were used: SLS20, a laser scintillometer from Scintec AG, BLS900, a largeaperture scintillometer, also from Scintec, and a 3D sonic anemometer from Thies GmbH. On the other hand, we measured turbulence strength with new approaches and devices developed in-house. Firstly, an LED array combined with a high-speed camera allowed for measurement of 𝐶_𝑛 ² from raw- and differential image motion, and secondly a two-part system comprising a laser source, a Shack-Hartmann sensor and a PSF camera recoded turbulent modulation transfer functions, Zernike variances and angle-of-arrival structure functions, yielding three independent estimates of 𝐶_𝑛 ². We compare the measured values yielded simultaneously by commercial and in-house developed devices and show very good agreement between 𝐶𝑛 2 values for all the methods. Limitations of each experimental method are also discussed.

A recent field trial in the Northern German littoral area of the Baltic Sea yielded a dataset of visibility, meteorological parameters, aerosol size distributions, as well as transmission over a horizontal path of 1344 m. The experimental results are compared to simulations using the MODTRAN (moderate resolution atmospheric transmission) model, that was run with the rural and Navy Aerosol Model, (NAM) in various configurations. Best results were obtained when MODTRAN was tuned with the measured visibility values. When NAM was used without visibility tuning, MODTRAN tended to overestimate the transmission in low-visibility conditions, which was attributed to the presence of a non-maritime aerosol fraction.

It is generally admitted that the relative location of an aerosol between an observation device and the observed scene will have an influence on the detected image quality. These effects are usually classified under the label “shower curtain effect” (SCE). The usual formulation describing it is as follows: an observer standing away from a shower curtain can detect the presence of a person standing just behind it whereas the opposite is not true. Starting from a discussion of experimental results which seemed to invalidate the SCE, we show that it is not the only mechanism at work and that thorough analysis of the measurement setup is required before reaching such conclusion. We base our discussion on four cases, two of them of the passive detection type, the two others being of the active type. We also show that the ratio of scattered to unscattered light at the detector is of utmost importance. We show this by further developing our model [10] of the point spread function (PSF) of the receiver. This model allows the discussion of the SCE in the frequency domain in terms of the cuton and cutoff frequencies of the receiver. In the end, we show that the apparent paradoxical results we had found cannot actually be placed under the “shower curtain effect” denomination because: 1-) the amount of unscattered light captured is higher than the amount of scattered light, and 2-) the receiver cuton frequency is much higher than the aerosol cutoff frequency rendering most mechanisms of the shower curtain effect ineffective.

Operation under degraded visual environment (DVE) presents important strategic advantages. 3D mapping has been performed under DVE and good quality images have been obtained through DVE with active imaging systems. In these applications, the presence of fog clouds degrades the quality of the remotely sensed signal or even renders the operation totally impossible. In view of making the active imaging method more robust against dense fog, the use of polarimetry is herein studied. Spherical particles typical of fog do not depolarize incident polarized light in the backscattering (180°) direction. So, in principle, there should be less dazzling caused by aerosols for active imaging systems operating using the secondary polarization. However, strong depolarization still occurs at angles close to 180°. The greater the ratio of size to wavelength, the closer to 180° will the depolarization occur. When the cloud optical depth is small, the major scattering events seen by an active camera are the single backscattering events. However, when the optical depth of the cloud is higher than 1, multiple scattering becomes more important and causes depolarization due to the backscattering around 180°. The physics of this process will be discussed. Experimental results supporting the analysis will be presented. Those experimental results were obtained under controlled environment using the DRDC-Valcartier aerosol chamber. The experimental method herein proposed is based upon the use of ICCD range gated cameras wherein gate width and gate location may be varied on the fly. The optimal conditions for the use of these devices in view of obtaining the best image contrast are experimentally studied and reported in this paper.

The growing demand for high-speed broadband communications with low orbital or geostationary satellites is a major challenge. Using an optical link at 1.55 μm is an advantageous solution which potentially can increase the satellite throughput by a factor 10. Nevertheless, cloud cover is an obstacle for this optical frequency. Such communication requires an innovative management system to optimize the optical link availability between a satellite and several Optical Ground Stations (OGS). The Saint-Exupery Technological Research Institute (France) leads the project ALBS (French acronym for BroadBand Satellite Access). This initiative involving small and medium enterprises, industrial groups and research institutions specialized in aeronautics and space industries, is currently developing various solutions to increase the telecommunication satellite bandwidth. This paper presents the development of a preliminary prediction system preventing the cloud blockage of an optical link between a satellite and a given OGS. An infrared thermal camera continuously observes (night and day) the sky vault. Cloud patterns are observed and classified several times a minute. The impact of the detected clouds on the optical beam (obstruction or not) is determined by the retrieval of the cloud optical depth at the wavelength of communication. This retrieval is based on realistic cloud-modelling on libRadtran. Then, using subsequent images, cloud speed and trajectory are estimated. Cloud blockage over an OGS can then be forecast up to 30 minutes ahead. With this information, the preparation of the new link between the satellite and another OGS under a clear sky can be prepared before the link breaks due to cloud blockage.

We report results from numerical simulations of laser beam propagation through atmospheric turbulence. In particular, we study the statistical variations of the fractional beam energy hitting inside an optical aperture placed at several kilometer distance. The simulations are performed for different turbulence conditions and engagement ranges, with and without the use of turbulence mitigation. Turbulence mitigation is simulated with phase conjugation. The energy fluctuations are deduced from time sequence realizations. It is shown that turbulence mitigation leads to an increase of the mean energy inside the aperture and decrease of the fluctuations even in strong turbulence conditions and long distance engagement. As an example, the results are applied to a high energy laser countermeasure system, where we determine the probability that a single laser pulse, or one of the pulses in a sequence, will provide a lethal energy inside the target aperture. Again, turbulence mitigation contributes to increase the performance of the system at long-distance and for strong turbulence conditions in terms of kill probability. We also discuss a specific case where turbulence contributes to increase the pulse energy within the target aperture. The present analysis can be used to evaluate the performance of a variety of systems, such as directed countermeasures, laser communication, and laser weapons.

We present the results of experimental investigation of measuring the wavefront distortions, accumulated during propagation of the bi-chromatic (0.53 and 1.06 μm) radiation propagation along the in-door atmospheric path by the pair of Shack-Hartmann wavefront sensors. The wavefront distortions for two wavelengths are compared, and the correlation between these distortions is revealed.

Bidirectional ground-satellite laser links suffer from turbulence-induced scintillation and phase distortion. We study how turbulence impacts on coherent detection capacity and on the associated phase noise that restricts clock transfer precision. We evaluate the capacity to obtain a two-way cancellation of atmospheric effects despite the asymmetry between up and down link that limits the link reciprocity. For ground-satellite links, the asymmetry is induced by point-ahead angle and possibly the use, for the ground terminal, of different transceiver diameters, in reception and emission. The quantitative analysis is obtained thanks to refined end-to-end simulations under realistic turbulence and wind conditions as well as satellite cinematic. Simulations make use of the reciprocity principle to estimate both down and up link performance from wave-optics propagation of descending plane waves. These temporally resolved simulations allow characterising the coherent detection in terms of time series of heterodyne efficiency for different system parameters. We show Tip/Tilt correction on ground is mandatory at reception for the down link and as a pre-compensation of the up link. Good correlation between up and down phase noise is obtained even with asymmetric apertures of the ground transceiver and in spite of pointing ahead angle. The reduction to less than 1 rad² of the two-way differential phase noise is very promising for clock transfer.

Laser Guide Star Adaptive Optical systems become an established technique at telescope facilities with large apertures. At these aperture diameters, such as 8m class telescope facilities, the finite distance and vertical extend of an artificial excited guide star result in perspective elongation, which produces errors in wave-front reconstruction and could influence the performance of adaptive optical systems seriously. In this paper, we shall briefly introduce and explain the effect of the perspective elongation, and show some results of theoretical simulation and experiment. First of all, we analyzed how the perspective elongation of sodium LGS changes, and gave the results of simulation which indicated the relation between the perspective elongation and some related parameters. The aberration caused by the elongation was analyzed, and the possibility of aberration correction was discussed. Based on the results of the theoretical simulation, we designed an experiment to observe the perspective elongation. A transmitting and receiving system has been set up. The system consisted of a 300mJ sodium LGS laser, a telescope with an aperture diameter of 450mm, a beam expander with an aperture diameter of 200mm, a LGS detecting device, etc. Based on the pulsed laser and the mobile LGS projector, we operated the experiment at different distance between the telescope and the laser projector. A series of elongated images, corresponding the distance from 5m to 30m, was obtained. The analytic results of the image data agreed with the theoretical simulation. Based on the experimental data, we deduced the aberration of wave-front at 30m separation. According to theoretical simulation of the perspective elongation, the effects including the aberration of wave-front could be achieved, which had been partially verified in the experiment. We suggest that one could improve the reconstruction accuracy in a sodium or Rayleigh LGS adaptive optical system by eliminating the influence of the perspective elongation in advance.

The performance of large ground-based optical telescopes is limited due to wavefront distortions induced by atmospheric turbulence. Adaptive optics systems using natural guide stars with sufficient brightness provide a practical way for correcting the wavefront errors by means of deformable mirrors. Unfortunately, the sky coverage of bright stars is poor and therefore the concept of laser guide stars was invented, creating an artificial star by exciting resonance fluorescence from the mesospheric sodium layer about 90 km above the earth’s surface. Until now, mainly dye lasers or sumfrequency mixing of solid state lasers were used to generate laser guide stars. However, these kinds of lasers require a stationary laser clean room for operation and are extremely demanding in maintenance. Under a development contract with the European Southern Observatory (ESO) and W. M. Keck Observatory (WMKO), TOPTICA Photonics AG and its partner MPB Communications have finalized the development of a next-generation sodium guide star laser system which is available now as a commercial off-the-shelf product. The laser is based on a narrow-band diode laser, Raman fiber amplifier (RFA) technology and resonant second-harmonic generation (SHG), thus highly reliable and simple to operate and maintain. It emits > 22 W of narrow-linewidth (≈ 5 MHz) continuous-wave radiation at sodium resonance and includes a re-pumping scheme for boosting sodium return flux. Due to the SHG resonator acting as spatial mode filter and polarizer, the output is diffraction-limited with RMS wavefront error < λ/25. Apart from this unique optical design, a major effort has been dedicated to integrating all optical components into a ruggedized system, providing a maximum of convenience and reliability for telescope operators. The new remote-pumping architecture allows for a large spatial separation between the main part of the laser and the compact laser head. Together with a cooling-water flow of less than 5 l/min and an overall power consumption of < 700 W, the system offers a maximum of flexibility with minimal infrastructure demands on site. Each system is built in a modular way, based on the concept of line-replaceable units (LRU). A comprehensive system software, as well as an intuitive service GUI, allow for remote control and error tracking down to at least the LRU level. In case of a failure, any LRU can be easily replaced. With these fiber-based guide star lasers, TOPTICA for the first time offers a fully engineered, off-the-shelf guide star laser system for groundbased optical telescopes providing convenient, turn-key operation in remote and harsh locations. Reliability and flexibility will be beneficial in particular for advanced satellite and space debris tracking as well as LIDAR applications.

An adaptive focusing of a laser beam through a turbulent atmosphere is investigated. We consider the reference source with a random position (fluctuations of the position of the center of gravity) on the object onto which laser radiation is focused. The problem of focusing of coherent optical radiation through the atmosphere can arise in practice when energy is transported to a sufficiently large remote object by laser radiation. The reference source can be formed by radiation reflected from the object to which coherent laser radiation should be focused. The methods used for determining the phase on the basis of coherent and non-coherent reference radiation were described early. The analytical calculated are performed. Distributions of the mean intensity of the field of a coherent laser beam focused in a turbulent medium by means of adaptive phase correction with the use of a point reference source with random center are calculated. The laser beam focused with the use of adaptive correction by the phase conjugation algorithm and the reference source is a spherical wave with a fluctuating radiation center. The distribution of the mean intensity of the focused coherent radiation beam is studied on the basis of the Huygens–Fresnel principle. The approach considered here can be used in cases with both coherent and non-coherent illumination. In this work, we made an attempt to estimate the efficiency of adaptive focusing of coherent laser radiation from the viewpoint of energy concentration and transfer. The Strehl ratio is used as the parameter of efficiency. Results of adaptive focusing with moving and motionless reference sources are compared. It is demonstrated that a point reference source with a random position of the center can be an effective tool for adaptive focusing of radiation beams.

Spectral imaging systems lead to enhanced sensing properties when the sensing system provides sufficient spectral resolution to identify materials from its spectral reflectance signature. The performance of diffraction gratings provides an initial way to improve instrumental resolution. Thus, subsequent manufacturing techniques of high quality gratings are essential to significantly improve the spectral performance. The ZEISS unique technology of manufacturing real-blazed profiles and as well as lamellar profiles comprising transparent substrates is well suited for the production of transmission gratings. In order to reduce high order aberrations, aspherical and free-form surfaces can be alternatively processed to allow more degrees of freedom in the optical design of spectroscopic instruments with less optical elements and therefore size and weight advantages. Prism substrates were used to manufacture monolithic GRISM elements for UV to IR spectral range. Many years of expertise in the research and development of optical coatings enable high transmission anti-reflection coatings from the DUV to the NIR. ZEISS has developed specially adapted coating processes (Ion beam sputtering, ion-assisted deposition and so on) for maintaining the micro-structure of blazed gratings in particular. Besides of transmission gratings, numerous spectrometer setups (e.g. Offner, Rowland circle, Czerny-Turner system layout) working on the optical design principles of reflection gratings. This technology steps can be applied to manufacture high quality reflection gratings from the EUV to the IR applications with an outstanding level of low stray light and ghost diffraction order by employing a combination of holography and reactive ion beam etching together with the in-house coating capabilities. We report on results of transmission gratings on plane and curved substrates and GRISM elements with enhanced efficiency of the grating itself combined with low scattered light in the angular distribution. Beside of the results of straylight measurement the actual results on improving efficiency and lowering the polarization sensitivity for transmission gratings will be discussed on theoretical simulations compared to measured data over the entire wavelength range.

Correction of atmospheric turbulence effects on images involves mainly mitigation of distortion (“de-warping”) and removal of image blur. One of the approaches for correcting atmospheric blurring involves the use of deconvolution. The ill-posed nature of the problem and the number of unknowns makes this problem hard to solve. This is why methods like blind deconvolution can be too time-consuming for real-time application. Additionally, an optimal parameter input is also often required (which requires interaction from an operator). Our ultimate goal is to perform an autonomous, software-based turbulence correction in real-time. This requires both very fast point-spread function (PSF) estimation and a deconvolution method. In this work we study new efficient ways to describe and estimate the PSF in anisoplanatic conditions.

In mid- to long-range horizontal imaging applications it is quite often atmospheric turbulence which limits the performance of an electro-optical system rather than the design and quality of the system itself. Even weak or moderate turbulence conditions can suffice to cause significant image degradation, the predominant effects being image dancing and blurring. To mitigate these effects many different methods have been proposed, most of which use either a hardware approach, such as adaptive optics, or a software approach. A great number of these methods are highly specialized with regard to input data, e.g. aiming exclusively at very short exposure images or at infrared data. So far, only a very limited number of these methods are concerned specifically with the restoration of RGB colour video. Beside motion compensation and deblurring, contrast enhancement plays a vital part in many turbulence mitigation schemes. While most contrast enhancement techniques, such as Contrast Limited Adaptive Histogram Equalization (CLAHE) work quite well on monochrome data or single colour frames, they tend to amplify noise in a colour video stream disproportionately, especially in scenes with low contrast. Therefore, in this paper the impact of different colour spaces (RGB, LAB, HSV) on the application of such typical image enhancement techniques is discussed and evaluated with regard to suppressing temporal noise as well as to their suitability for use in software-based turbulence mitigation algorithms.

It is suggested to reconstruct the phase screens with the use of stacked-actuator deformable mirror and to compensate for the introduced distortions by the bimorph mirror. The reproducing and correction results are presented in the paper. The problems of the reconstruction and compensation are discussed.

The analytical expressions for wave structure function of plane and spherical waves are derived both in the viscous dissipation and inertial range. Due to previously research, there is a discrepancy between theoretical results and the experimental datum in viscous dissipation range. In this paper, only considering the inertial range, taking plane waves for example, we give a comparison of results of WSF calculated by the analytical formula obtained in this paper and the numerical calculations of the definition at the fixed parameter (i.e., the generalized exponent α), it can be seen that the two results are in agreement with each other exactly. Based on non-Kolmogorov power spectrum, new characteristics for wave structure function (WSF) have been found for plane and spherical wave models when the different ratio of inner scale l₀ and outer scale of turbulence L₀ is obtained. In outer scale assumed finite case (i.e., L₀ =1m), WSF obtains the maximum when α approximates to 3.3 both for plane and spherical wave models. In outer scale assumed infinite case (i.e., L₀ = ∞), the WSF can be sorted into three parts, including two rapid-rising regions (i.e., 3.0 < α < 3.3 and 3.8 < α < 4.0 ) and one gently rising region (i.e., 3.3 < α < 3.8 ).Further, the changes of scaled WSF versus the ratio of separation distance and inner scale ( p/ l₀ ) are investigated under mentioned above conditions for two models. In L₀ = 1m case, both for plane and spherical waves, the value of α determines the bump position of WSF. In L₀ = ∞ case, the bump of scaled WSF disappears when the generalized exponent has large values. The changings of scaled WSF monotonically increase as α increased when the generalized exponent is larger than11/3 for two models. Besides, the properties of spherical waves are similar to plane waves, except which the values of WSF and the scaled WSF are smaller than plane ones.

It is suggested to measure non-axis parabolic mirrors aberrations and try to compensate for them. For the aberration prediction the computer modeling is used. For the aberration measurement Shack-Hartmann wavefront sensor is applied. For the aberrations compensation a digital algorithm is used.

The image quality of optical remote sensing satellite is affected by the atmosphere, thus the image needs to be corrected. Due to the spatial and temporal variability of atmospheric conditions, correction by using synchronous atmospheric parameters can effectively improve the remote sensing image quality. For this reason, a small light spaceborne instrument, the atmospheric synchronous correction device (airborne prototype), is developed by AIOFM of CAS(Anhui Institute of Optics and Fine Mechanics of Chinese Academy of Sciences). With this instrument, of which the detection mode is timing synchronization and spatial coverage, the atmospheric parameters consistent with the images to be corrected in time and space can be obtained, and then the correction is achieved by radiative transfer model. To verify the technical process and treatment effect of spaceborne atmospheric correction system, the first airborne experiment is designed and completed. The experiment is implemented by the "satellite-airborne-ground" synchronous measuring method. A high resolution(0.4 m) camera and the atmospheric correction device are equipped on the aircraft, which photograph the ground with the satellite observation over the top simultaneously. And aerosol optical depth (AOD) and columnar water vapor (CWV) in the imagery area are also acquired, which are used for the atmospheric correction for satellite and aerial images. Experimental results show that using the AOD and CWV of imagery area retrieved by the data obtained by the device to correct aviation and satellite images, can improve image definition and contrast by more than 30%, and increase MTF by more than 1 time, which means atmospheric correction for satellite images by using the data of spaceborne atmospheric synchronous correction device is accurate and effective.

For its better spectral response characterization, higher quantum efficiency and signal-to-noise ratio, camera is more and more used in atmospheric parameters measurement lidar. Camera lidars retrieval atmospheric parameters by analyzing the light column images acquired by the cameras and objectives through gathering the backscatter light of the laser beam. Lidars of this kind usually have higher spatial resolution and better real time performance. However, because of its limited depth of field (DOF), the measurement accuracy of the area out of the DOF is influenced by optical defocus in different degree. In the meantime, it is also not suitable for portable equipments for using small relative aperture receiving objective. Based on improving the design of the receiving objective, a new design scheme is proposed in this paper about improving the optical receiving system of the camera lidar. This scheme can improve the measurement accuracy of the area out of the DOF in traditional structure by using large DOF, large relative aperture offaxis objective and the special using mode of the camera. The optical receiving system designed according to this scheme is more compact and is especially suitable for portable instrument. Furthermore, the relation among the focus length, the distance between laser and objective and the installation angle is also analyzed in this paper. The formula is given at the same time. This scheme is carried out in camera lidar system in laboratory and the results are satisfactory.