Optical turbulence conditions at a mountain peak (North Oscura Peak, NM) have been calculated using two hot-wire anemometers. The anemometers (running in constant current mode, with a very low overheat ratio) measure temperature fluctuations. Combining the fluctuating temperature data with wind velocity data, local temperature and pressure, and invoking Taylor's hypothesis, the optical turbulence parameters can be calculated. These parameters include temperature structure parameter (C2t) and the refractive index structure parameter (C2n). The two probes are positioned at different elevations above the ground, thus the vertical optical turbulence gradient can be calculated. This relationship is used to calibrate an acoustic sounder. Optical turbulence data collected from the hot-wire anemometers as well as the acoustic sounder will be compared to meteorological events measured locally. Many days of data have bene collected and will be shown, of particular interest is the relationship between optical turbulence and solar radiation, as well as wind speed and direction. The diurnal relationship of the optical turbulence gradient will also be shown. As well as the effect of this parameter on the acoustic sounder calibration.
An Acoustic Sounder System has been installed on the side of the cliff at North Oscura Peak, WSMR to provide important refractive index structure parameter, Cn2 data for laser propagation tests. The acoustic sounder system records echo information that is used to provide 3D wind and optical turbulence profiles. The received signal is the product of the interaction of the transmitted acoustic pulse with the small scale atmospheric temperature variations. This information is displayed as a time-height display of the signal intensity. The frequency of the received signals are processed and converted into time histories of the horizontal wind field. The data from the Acoustic Sounder is calibrated with the hot-wire anemometer temperature structure parameter (Ct2) data, and meteorological data measured locally to produce the Cn2 profile. The design and location of the Acoustic Sounder System will be discussed along with the methodology of extracting the turbulence. Many days of data have been collected and representative data will be shown.
The optical turbulence conditions at a mountain ridge (North Oscura Peak, White Sands Missile Range, NM) were determined from observations of fine wire sensors and a sodar (sonic detection and ranging). Both instruments provided the temperature structure parameter (C2T) from which the refractive index structure parameter (C2n) was calculated using local measurements of temperature and pressure. The fine wire measurements were used to calibrate the sodar. Atmospheric measurements shown include wind speed and direction, temperature, and solar radiation sensed horizontally as well as parallel to the west-facing slope. Of particular emphasis is the relationship of the sodar observations to solar radiation and wind speed and direction. The results are explained in terms of the geometry of the site and the mountain-valley wind regime. Results are shown as average range profiles of C2n sensed at various zenith angles at different times of the day and as contours of C2n in a vertical plane oriented normal to the west-facing slope.
Atmospheric effects on normalized stellar irradiance fluctuations for the weak fluctuation regime are examined. Both monochromatic and polychromatic effects are considered. Calculations are performed using a Rytov theory which explicitly includes refractive effects arising from the polychromatic stellar source. The atmospheric turbulence along the path is specified in a spherical shell or `onion- skin' model using a vertical profile specified by either the Clear 1 model or actual thermosonde measurements. Finite optical bandwidth, receiver aperture size, and variable turbulent inner scale are also included. Good agreement is found between ground-based measurements and model results in the weak scintillation regime. It is also shown that refraction effects can significantly alter the wavelength dependence of scintillation as well as produce a leveling off and decrease of scintillation with increasing zenith angle that could be misinterpreted as saturation.
Accurate modeling of turbulence is required for investigating interactions between turbulence, blooming, and environmental and laser beam characteristics. A layered model propagates the optic field across each layer by Fresnel diffraction and a phase screen. Turbulence is modeled statistically with the Kolmogorov spectrum which goes to infinity as spatial frequency decreases. Below a cut-off frequency, the spectrum transitions to zero at a point corresponding to the largest eddies in the turbulence. Two different approaches for computing phase screens with this spectrum are considered. In the first approach, the phase screens are computed directly from the spectrum by inverse transformation. Gaussian random numbers are molded to the spectrum. A discrete 2D Fourier transform provides sample random phase screens. We show that the discrete Fourier transform cannot provide an accurate transform because of the character of the Kolmogorov spectrum. In the second approach, a covariance matrix is developed using structure functions. This avoids the need to compute a Fourier transform. The eigenvalues and eigenvectors are computed for the matrix. Eigenvalue weighted Gaussian random variables are premultiplied by the eigenvector array to generate phase screens. The advantages, disadvantages, and computational effort are discussed.
Presently the propagation models used to assess ABL performance employ an onionskin atmosphere. Onionskin atmospheres ignore the profound horizontal intermittency in Cn2 found in experiment. The problems associated with modeling optical turbulence over shallow slant paths using balloon and aerothermal point measurements are presented. We describe a method developed by the authors, which integrates the horizontal intermittency evident in the aerothermal data with the balloon profiles. This methodology takes randomly drawn segments of the aerothermal data to provide the intermittency along a smoothed balloon profile. An Independent and Identically Distributed (IID) process describes this methodology. To date all methods known by the authors utilize an IID process to account for intermittency. However, IID processes fail to fully appreciate the information contained in a continuous stream of aerothermal data. We present a methodology that utilizes time series analysis of horizontal aerothermal segments. The result of the time-series methodology is that the distribution of expected integrated turbulence levels is broader than those calculated previously. This suggests that in reality there will be a greater probability that a beam will experience sustained periods of extreme turbulence along the path, strong or weak, than would be predicted using an IID model.
Clouds can have a major impact on performance and mission opportunity of ground and airborne laser weapons. Database summaries and modeling techniques are presented for the physical and optical characteristics, frequency of occurrence, and probability of cloud-free line-of-sight (PCFLOS) of mid-level and high-altitude clouds. Emphasis is on subvisual and thin cirrus due to its predominance at mid- latitude. The statistical models presented are for top-level system engagement analyses, and to generate cloud realizations for more accurate simulation. The first portion of the presentation describes the requirements and limitations of current approaches and databases used for modeling. Criteria such as period-of-record, meteorological anomalies, spatial and temporal resolution, and measurement and data collection techniques, are discussed. The next part describes the effects of variations in cloud thickness, altitude, extinction, and scatter ratio on laser weapon operations and availability. This is followed by a practical, validated method for estimating PCFLOS as a function of elevation angle based on commonly-recorded, site-specific meteorological parameters.
This paper describes the development of a non-contact diagnosis system for analyzing the plasma density profile, temperature profile, and ionic species of a high energy laser-generated plasma. The system was developed by Physical Optics Corporation in cooperation with the U.S. Army Space and Missile Defense Command, High Energy Laser Systems Test Facility at White Sands Missile Range, New Mexico. The non- contact diagnostic system consists of three subsystems: an optical fiber-based interferometer, a plasma spectrometer, and a genetic algorithm-based fringe-image processor. In the interferometer subsystem, the transmitter and the receiver are each packaged as a compact module. A narrow notch filter rejects strong plasma light, passing only the laser probing beam, which carries the plasma density information. The plasma spectrum signal is collected by an optical fiber head, which is connected to a compact spectrometer. Real- time genetic algorithm-based data processing/display permits instantaneous analysis of the plasma characteristics. The research effort included design and fabrication of a vacuum chamber, and high-energy laser plasma generation. Compactness, real-time operation, and ease of use make the laser plasma diagnosis system well suited for dual use applications such as diagnosis of electric arc and other industrial plasmas.
The flowing gas system was almost always used for beam path conditioning in the past to reduce thermal blooming in the high energy laser (HEL) system. For an airborne system, however, system weight is important. The non-flowing medium was considered in order to keep the system weight down. This paper shows that the combination of high thermal diffusivity and low index of reflection of helium enables it to be used as the beam path medium at the non-flowing mode of operation. Wall temperatures have a significant effect on optical path difference (OPD) in the non-flowing beam tube, because there is no thermal boundary layer and the temperature gradient occupies the entire beam tube. The effect of wall temperatures on the OPD in the non-flowing helium system was evaluated analytically. Results show that the OPD in the non-flowing helium compares favorably to that in the flowing system with moderate flow velocity under the same wall conditions. In summary, results presented confirm that the non-flowing helium is indeed a viable alternative to the flowing systems used in the HEL systems in the past.
In this paper we present an overview of the laboratory configuration and provide details of the adaptive-optics and tracking hardware. Experimental results obtained using a variety of propagation scenarios are presented and compared with results from wave-optics simulations. In addition, we present results illustrating the impact of increasing beacon size and active illumination on system performance.
As a corollary to the USAF strategic Airborne Laser program, Boeing has been analyzing the use of the Chemical Oxygen Iodine Laser for tactical scenarios. These include its use in an airborne platform operating against low flying cruise missiles and miscellaneous ground targets, as well as on a mobile ground platform providing support defense against shorter-range rocket attack. Practical design concepts yielding high target lethality at significant ranges have been developed.
A Kalman Filter tracker algorithm is developed to increase track accuracy in the presence of scintillation. A simulation is developed which allows comparison of the Kalman Filter tracker to a Centroid tracker. This paper describes the development of the Kalman Filter tracker algorithm and the results of the simulation effort. The Kalman Filter Tracker formulation is described and sensitivity analyses are performed.
The Airborne Laser (ABL) system has extremely tight jitter requirements. Acoustic disturbances, such as those caused by the pressure recovery system of the high power laser, are a significant jitter source. Several technologies may be appropriate for reducing the acoustically induced jitter. The first choice for mitigation will be passive approaches, such as acoustic blankets. There is, however, some uncertainty whether these approaches will provide sufficient attenuation and there is concern about the weight of these approaches. A testbed that captured the fundamental physics of the ABL acoustically induced optical jitter problem was developed. This testbed consists of a flexure-mounted mirror exposed to an acoustic field that is generated outside a beam tube and then propagates within the tube. Both feedback and adaptive feedforward control topologies were implemented on the testbed using either of two actuators (a fast steering mirror and a secondary acoustic speaker located near the precision mirror), and a variety of sensors (microphones measuring the acoustic disturbance, accelerometers and microphones mounted on the precision optic, and an optical position sensing detector). This paper summarizes the results from these control topologies for reducing the acoustically induced jitter with some control topologies achieving in excess of 40 dB jitter reduction at a single frequency. This work was performed under an SBIR Phase I funded by the Air Force Research Laboratory Space Vehicles Directorate.
The UK's Defence Evaluation and Research Agency have been researching stabilized laser-pointing technology for a number of years. Several proof-of-principle generic laser pointing and beam steering platforms have been designed, built and tested for a range of applications. These platforms were developed as research tools to develop key laser pointing and target tracking technologies. The key research technologies examined include target tracking, sight-line stabilization, laser boresight and alignment, laser integration and system control processing. This paper describes typical techniques and technologies required to direct a laser beam to a target and maintain it on that target for the desired period of time. It covers some of the system aspect including laser integration and laser boresighting concepts and techniques.
The adaptive correction was frequently treated as straightening of wavefront, if deal with receiving of the deformed wave. For adaptive focusing of beams the correction was considered as predistortion of wavefront. However with infringement of a condition smoothness of wavefront, the situation varies. It is known, that dislocations of wavefront conterminous to points where instant meaning of intensity equal to zero, arise with distances approximately equal diffraction length. With presence of such points the wavefront of a reference wave cannot be determined as a smooth one-coherent surface, therefore efficiency of adaptive systems with flexible mirrors begins to be reduced. At the same time, the numerical experiment with model of a compound phase corrector has shown, that its efficiency practically does not change with transition in area of strong fluctuations of intensity. We have found out, that the efficiency of adaptive system with a compound corrector does not vary with transition from area weak fluctuations of intensity in area strong fluctuations.
One of the key elements of the Airborne Laser beam control system is its wavefront control subsystem. This subsystem provides compensation of local and atmospheric wavefront disturbances, which is essential in accomplishing the ABL mission. A critical performance driver for this subsystem is deformable mirror-to-wavefront sensor registration. To reduce overall sensitivity to misregistration and provide optimum wavefront control performance, the ABL system includes a built-in calibration system. This system is used in pre-flight checkout as well as during an automated calibration and alignment sequence conducted in-flight. The calibration system provides the ability to perform poke calibrations to update the estimation matrix. This paper provides a description of the methodology used for calibration of the ABL wavefront control subsystem and discusses the key features that provide this function.
We consider the performance of the wavefront reconstruction process when a Pyramid wavefront Sensor is used in a closed loop Adaptive Optics System. The Pyramid Sensor sensitivity in closed loop operations has been the subject of a first heuristic analysis showing that the sensor sensitivity is higher than that of a Shack-Hartmann sensor, at least when low order modes are considered. In this paper we evaluate the sensor accuracy by determining the closed loop reconstruction matrix. This is done using a diffractive analysis of the sensor behavior. Furthermore, knowledge of this matrix enables us to quantify the effect of error sources like sensor non linearity and photon noise on the reconstructed wavefront accuracy. Finally, a comparison of the performance of the Shack-Hartmann and Pyramid wavefront sensors is given.
Adaptive optics systems for the current generation of astronomical telescopes are being designed and developed at the present time with different goals and related complexities. In order to make the best use of such systems more and more projects are considering introducing at least one artificial reference star. It is a fact that for 8 m class telescopes a single artificial star will only give benefit in the last part of the NIR region of the spectrum (essentially H and K bands). To push the performances of such systems to shorter wavelengths and/or worse seeing it is required to have more than one reference star. It is known that the multi-reference, multi-conjugate adaptive optics systems (MCAO) could provide great advantages in mitigating the effect of the limited range of the artificial reference stars (the so-called cone effect) while in the mean time enlarging the corrected field of view of an AO system. We are currently approaching the problem of designing such a system from both practical and theoretical point of view and carefully comparing several possible solutions: multiple sodium laser guide stars, multiple Rayleigh back-scattering laser guide stars, hybrid system using both types of stars. The first type of stars has the advantage of having the highest range achievable today but currently the lasers required to produce this type of stars have a limited available power, high costs and low reliability. On the other hand the lasers required for Rayleigh guide stars do not suffer from these limitations but the ranges achieved are lower. Given these limitations we think that a system that would use either a constellation of Rayleigh stars or a hybrid with a single sodium star could be a good compromise between the various choices. In this paper we present some preliminary evaluation results of an MCAO system that would use a constellation of off-axis Rayleigh stars plus an on-axis Rayleigh or sodium star.
Two wave fronts are of interest in the AirBorne Laser, namely, the inbound wave front from the `beacon' and the outbound wave front of the high-energy laser. Since these two wave fronts propagate through different regions of the atmosphere and at different time instants, the spatial- temporal correlation between the Zernike polynomials' phase expansion coefficients representing these wave fronts must be determined. Using this correlation information, an underlying linear, stochastic, dynamical system that represents the atmosphere is identified. The wave front sensor and deformable mirror dynamics are also modeled. These models are then used in a Kalman filter which provides estimates of the outbound wave front's Zernike coefficients using measurements of the inbound wave front. A linear quadratic controller is also developed so that configuration, that is, atmospheric compensation, can be performed. The designed adaptive optics control system's performance is evaluated in a simulated experiment.
A novel integrated optic approach to the design and manufacture of an optical subsystems for a Range-Doppler imaging lidar is described and demonstrated. The approach uses hollow waveguides to guide light between system components which are integrated into a common substrate. The design, manufacture and operation of an eleven element subsystem which is compact, rugged and provides coherent mixing efficiencies in excess of 80% of the theoretical maximum, are discussed. The results of trials of the subsystem at the Army Missile Optical Range, involving measurement of Range-Doppler images of representative targets are described.
We study the effects of finite outer scale and inner scale of atmospheric turbulence on aperture averaging of optical scintillations. Analytical developments of the aperture- averaging factor have been limited somewhat because of mathematical complexities associated with the integral G equals $CINTb1(Dx)K(x)xdx. In part, this is due to the MTF model K(x) that characterizes circular aperture and the power spectrum of refractive index fluctuations. So, we derive a modified spectrum of refractive index fluctuations that features inner scale, outer scale and a high wave number bump. And we approximate the circular aperture with a Gaussian aperture model. The analytic expressions are obtained for the aperture-averaging factor associated with optical scintillations of unbounded plane waves in weak fluctuation regime based on a modified spectrum of refractive-index fluctuations and the Gaussian aperture model. This analysis is inclusion of the finite outer-scale and inner-scale are shown to be necessary for quantitative estimates. Our results obtained for the modified model turbulence spectrum significantly differ from those obtained for a Kolmogorov turbulence spectrum. In addition, interpolation expressions are obtained for the aperture- averaging factor associated with optical scintillations of a spherical wave in weak fluctuation regime.