Christopher A. Rice

Methodology for comparing worldwide performance of diverse weight-constrained high energy laser systems

The Air Force Institute of Technologys Center for Directed Energy has developed a software model, the High Energy Laser End-to-End Operational Simulation (HELEEOS), under the sponsorship of the High Energy Laser Joint Technology Office (JTO), to facilitate worldwide comparisons across a broad range of expected engagement scenarios of expected performance of a diverse range of weight-constrained high energy laser system types. HELEEOS has been designed to meet JTOs goals of supporting a broad range of analyses applicable to the operational requirements of all the military services, constraining weapon effectiveness through accurate engineering performance assessments allowing its use as an investment strategy tool, and the establishment of trust among military leaders. HELEEOS is anchored to respected wave optics codes and all significant degradation effects, including thermal blooming and optical turbulence, are represented in the model. The model features operationally oriented performance metrics, e.g. dwell time required to achieve a prescribed probability of kill and effective range. Key features of HELEEOS include estimation of the level of uncertainty in the calculated Pk and generation of interactive nomographs to allow the user to further explore a desired parameter space. Worldwide analyses are enabled at five wavelengths via recently available databases capturing climatological, seasonal, diurnal, and geographical spatial-temporal variability in atmospheric parameters including molecular and aerosol absorption and scattering profiles and optical turbulence strength. Examples are provided of the impact of uncertainty in weight-power relationships, coupled with operating condition variability, on results of performance comparisons between chemical and solid state lasers.

Design and test of military cockpits

The IT revolution is dramatically affecting modern military cockpit design. Modern displays add multiple dimensions that contribute to being able to provide more information to the pilot. The abundance of information now available to the military pilot can easily overwhelm him. Whereas automating cockpit functions reduces the pilot workload, automation reduces the flexibility that the military pilot requires. Automation complicates the flight test of the cockpit. With automated tasks, the pilot is in a monitoring role where contemporary workload scales are ineffective for measuring pilot workload. Development of a pilot workload rating system that incorporates the cognitive pilot workload will improve cockpit designs. While efforts continue to focus on improving cockpit automation and the use of modern electronic display systems, reducing the effects of fatigue on the pilot will prevent pilot performance from being degraded and will result in an overall improvement in capability.

Profiling of atmospheric turbulence along a path using two beacons and a Hartmann turbulence sensor

The Hartmann Turbulence Sensor (HTS) is an optical system capable of estimating several atmospheric turbulence parameters, such as Greenwood frequency, Fried’s coherence diameter and inner scale of turbulence. It primarily comprises of a 40 cm Meade telescope, a 32 x 32 Shack- Hartmann lenslet array, and a high-speed camera. The HTS estimates the turbulence parameters by measuring the local tilts of the aberrated wavefront coming from a laser source and incident at the pupil plane of the telescope. At the Air Force Institute of Technology (AFIT), a technique has been developed to measure the distribution of turbulence along an experimental path using the HTS and two laser sources of the same wavelength. By measuring the variances of the difference in wavefront tilts due to the two sources sensed by a pair of Hartmann subapertures with varying separations, turbulence information along the path can be extracted. The method relies on deriving a set of weighting functions, each weighting function dipping to zero at a range where the two sensing paths from the beacons to the subapertures intersect, thus canceling out the effect of turbulence at this location on the differential tilt signal. The analytical expression for the path weighting functions has been derived here. The technique has been applied to experimental data collected over a 500 m grassy path and the profiling results have been compared to a co-located scintillometer. This work will eventually aid in obtaining a better understanding of turbulence in the lower atmosphere and how it varies with height.

CW 3µm lasing via two-photon pumping in cesium vapor with a 1W source

We report the first CW lasing from two-photon pumping via a virtual state. Pulsed and the CW lasing of the 3096 nm 72 P1/2 to 72 S1/2 line are observed from degenerate two-photon pumping of the cesium 72 S1/2 to 62 D3/2 transition. High intensity pulses excite over 17 lasing wavelengths. Under lower intensity CW excitation, 3 μm lasing is still observed with efficiencies of 0.7%. CW experiments utilized a Cs heat pipe at 150 °C to 270 °C, and a highly-focused, single pass, Ti-Sapphire pump with no aid of a cavity. Unlike normal DPALS, this architecture does not require buffer gas, and heat is released optically so a flowing system is not required. Both suggest a very simple device with excellent beam quality is possible. This proof of concept can be greatly enhanced with more optimal conditions such as non-degenerate pumping to further increase the two-photon pump cross section and the addition of a cavity to improve mode volume overlap. These improvements may lead to an increase in efficiencies to a theoretical maximum of 14%. Results suggest two-photon pumping with diodes is feasible.

Excitation of higher lying energy states in a rubidium DPAL

The spontaneous emission in a cw rubidium diode dumped alkali laser (DPAL) system was analyzed. The fluorescence from higher lying states decreases with additional buffer gas. The intermediate states (7S, 6P, 5D) decay more slowly with buffer gas and scale super-linearly with alkali density. A detailed kinetic model has been constructed, where the dominant mechanisms are energy pooling and single photon ionization. It also includes pumping into the non-Lorentzian wings of absorption profiles, fine structure mixing, collisional de-excitation, and Penning ionization. Effects of ionization in a high powered CW rubidium DPAL were assessed.

Excited argon 1s5 production in micro-hollow cathode discharge arrays for use as potential rare gas laser sources

The diode-pumped rare gas laser (DPRGL) has been suggested as a potential high-gain, high-energy laser which requires densities on the order of 1013 cm−3 at pressures around 1 atmosphere for efficient operation. Argon 1s5 number densities have been measured in micro-hollow cathode discharges with electrode gaps of 127 and 254 μm and hole diameters from 100-400 μm. The dependency of the metastable argon (1s5) density on total gas pressure, electrode gap distance and hole diameter were explored. The measured densities were all in the range of 0.5 − 2 × 1013 cm−3 with the 400 μm hole diameters being the lowest.

Efficient non-linear two-photon effects from the Cesium 6D manifold

We report several non-linear process that occur when two-photon pumping the cesium 6D states. Cesium vapor possess some of the largest two-photon pump cross sections in nature. Pumping these cross sections leads to strong amplified spontaneous emission that we observe on over 17 lasing lines. These new fields are strong enough to couple with the pump to create additional tunable lines. We use a heat pipe with cesium densities of 1014 to 1016 cm-3 and 0 to 5 Torr of helium buffer gas. The cesium 6D States are interrogated by both high energy pulses and low power CW sources. We observe four-wave mixing, six-wave mixing, potential two-photon lasing, other unknown nonlinear processes, and the persistence of some processes at low thresholds. This system is also uniquely qualified to support two-photon lasing under the proper conditions.

Efficiency measurements for a digital-holography system

This paper compares efficiency measurements to predictions for a digital-holography system operating in the off-axis image plane recording geometry. We use a highly coherent 532 nm laser source, an extended Spectralon object, and an Si focal-plane array to perform digital-holographic detection, which provides access to an estimate of the complex-optical field and is of utility to long-range imaging applications. In the experiments, digitalholographic detection results from the interference of a signal beam with a reference beam. The signal beam was created from the active illumination of the extended Spectralon object and the reference beam from a local oscillator which is split off from the master-oscillator 532 nm laser source. To compare efficiency measurements to predictions, an expression was developed for the signal-to-noise ratio, which contains many multiplicative terms with respect to total-system efficiency. In the best case, the measured total efficiency was 15.2% ± 5.8% as compared to the predicted 16.4%. The results show that the polarization and the fringe-integration efficiency terms play the largest role in the total-system efficiency.

Characterizing atmospheric turbulence over long paths using time-lapse imagery

In recent times, there has been a growing interest in measuring atmospheric turbulence over long paths. Irradiance based techniques such as scintillometry, suffer from saturation and hence commercial scintillometers have limited operational ranges. In the present work, a method to estimate path weighted Cn2 from turbulence induced random, differential motion of extended features in the time-lapse imagery of a distant target is presented. Since the method is phase based, it can be applied to longer paths. The method has an added advantage of remotely sensing turbulence without the need for deployment of sensors at the target location. The imaging approach uses a derived set of path weighting functions that drop to zero at both ends of the imaging path, the peak location depending on the size of the imaging aperture and the relative sizes and separations of the features whose motions are being tracked. For sub-aperture sized features and separations, the peaks of the weighting functions are closer to the target end of the path. For bigger features and separations, the peaks are closer to the camera end. Using different sized features separated by different amounts, a rich set of weighting functions can be obtained. These weighting functions can be linearly combined to produce a desired weighting function such as that of a scintillometer or that of r0. The time-lapse measurements can thus mimic the measurements of a scintillometer or any other instrument. The method is applied to both simulated and experimentally obtained imagery and some validation results with a scintillometer is shown as well.

Global tilt removal on a Hartmann turbulence sensor

A Hartmann Turbulence Sensor (HTS) was used to quantify the atmospheric turbulence along a 1 km near ground-level path. This study examines the effect of removing the average tilt over all subapertures from each subaperture in the data analysis. The HTS captures a laser beam projected along a path of interest with a telescope; a lenslet array in the detector system breaks the beam up into 700 subapertures spread across the telescope pupil, and then forms images of the laser source from each of these subapertures onto a fast camera. Turbulence along the path induces tilts in the laser wavefront which are captured as centroid motion of the many laser spots in the camera image. This motion is used to quantify the turbulence. The raw spot positions contain undesired image motion due to telescope motion and vibration. This motion can be removed from the data by subtracting the average centroid motion of all subapertures from each subaperture. This subtraction changes the data in other ways, and this detail must be included in the analysis. The result can be exactly represented as weighted sum over the differential tilt variances between subapertures pairs. The tilt-removed variance averaged over all the subapertures is shown to be one-half of the average variance over all subaperture pairs. This work also resolves some discrepancies in previous results involving the expected variances of these differential tilts.