Journal of Directed Energy, Volume 1, Number 4

A four-element fiber array has been constructed to yield 8 W of coherently phased, linearly polarized light energy in a single far-field spot. Each element consists of a 2-W, single-mode fiber-amplifier chain. Phase control of each element is achieved with a lithium-niobate phase modulator. A master laser provides a linearly polarized, narrow-linewidth signal that is split into five channels. Four channels are individually amplified by using polarization maintaining fiber power amplifiers. Frequency broadening of the signal is necessary to avoid stimulated Brillouin scattering. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically with a portion of each channel's signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative phase for each channel. Compensating adjustments are then made to each channel's phase modulator. The stability of the optical train is an essential contributor to its success. A state-of-the-art interferometer was built with mountless optics. A lens array was constructed by using nanopositioning tolerances, where each lens was individually aligned to its respective fiber to collimate its output and point it at a common far-field spot. This system proved to be highly robust and handled any acoustic perturbations.

The purpose of this theoretical study is to examine atmospheric effects on an optical propagation path using two coaxial beams, one a high-energy beam intended to modify the atmospheric channel for the other optical beam. The high-energy beam created local heating of the atmosphere that tends to reduce the effective structure parameter of the refractive index, thereby reducing some of the deleterious effects of turbulence-induced scintillation. For the high-energy beam we considered millimeter waves (63 and 183 GHz) and an infrared wave (10.6 µm) propagating through extended atmospheric turbulence and a thin random-phase screen. We found that the use of a coaxial high-power millimeter beam with an optical beam can reduce the effective refractive-index structure parameter C2n for the low-power optical beam in the atmospheric channel, but mostly for the initial portion of the propagation path. Greater reduction in C2n can be realized with the use of an infrared wave (10.6 µm) in place of a milimeter wave, because the absorption is greater than that of the millimeter beam. Overall, the most favorable atmospheric channel for the use of such coaxial beams appears to be one in which the atmospheric effects are limited primarily to a thin turbulent layer (phase screen) between the transmitter and receiver.

The challenge to high-power solid-state laser (SSL) heat rejection is the very high heat loads during lasing. A thermal energy storage (TES) system capable of rapid heat absorption followed by heat rejection over a longer period can reduce the thermal management system size by over a factor of 10. The most compact system approaches to thermal storage use phase change materials (PCMs) such as ice or paraffin. Because of use of latent heat of fusion of these materials, a large amount of energy can be stored in a small mass of the storage material. The major drawback with this approach is the relatively low thermal conductivity of PCMs (ice at 1.9 and paraffin at 0.15 W/m-K compared to copper at 386 W/m-K). A new PCM system design approach is proposed that uses a "carbon fiber composite" material manufactured by Energy Science Laboratories Inc. (ESLI), which dramatically increases the effective thermal conductivity of the PCM. The material consists of high conductivity carbon fibers, 2-5 mm long, bonded to a thin, flexible metal substrate. Water or paraffin is infiltrated into the carbon fiber structure, which can be constructed as circular tubes or flat plates. A high-heat-flux low-weight (HHF-LW) thermal storage system would consist of a number of plates in a heat exchanger configuration. The fluid used to cool the laser components is on one side of the heat transfer surface, and the recharge fluid from the chiller system is on the other side. The PCM material is "sandwiched" in between. The achievable value of keff (effective thermal conductivity) for the proposed HHF-LW depends on the density of fibers and the type of fiber used. With carbon fibers of thermal conductivity 1,100 W/m-K, a keff of ~100 W/m-K could reasonably be achieved. Preliminary analysis indicates that a keff of 60-80 W/m-K results in an optimum weight reduction in a TES system. A compact high-power thermal storage system using this technology can be fabricated to reduce the weight of the heat refection system for SSHCL (solid-state heat capacity laser) by 90%. The estimated weight of the TES for 100 kW SSHCL is 180 kg

At sufficiently high currents beam breakup (BBU) occurs in all accelerators. In recirculating accelerators, such as the energy recovery linacs used for high-power free-electron lasers (FELs), the maximum current has historically been limited by multipass, multibunch BBU, a form that occurs when the electron beam interacts with the high-order modes (HOMs) of an accelerating cavity on one pass and then again on the second pass. This effect is of particular concern in the designs of modern high-average-current energy recovery accelerators utilizing superconducting technology. In such two-pass machines rotation of the betatron planes by 90 deg, first proposed by Smith and Rand in 1980, is expected to significantly increase the threshold current of the multibunch BBU. The effect of rotation on the threshold current of the Jefferson Laboratory FEL Upgrade is being studied experimentally and with a newly developed four-dimensional tracking code. Several optical rotator schemes based on quadrupoles and solenoids are being evaluated for their performance in terms of the instability threshold current increase and their effect on FEL optics. Results of experiments and simulations are presented.

In the late 1970s DARPA initiated a program to develop the technologies necessary to fly a space-based laser weapon. The encompassing effort was know as Triad, and the three programs were the Alpha Program to develop the laser source, the Large Optics Demonstration Experiment (LODE) to develop the beam control, and the Talon Gold Program to develop the precision pointing capability. This paper provides an overview of the Alpha Program from its antecedents through its final, most successful, tests. Alpha was intended to develop a hydrogen fluoride (HF) laser to megawatt-class levels in a configuration compatible with space operation. We begin with a brief history of the development of HF [and deuterium floride(DF)] lasers up through the demonstration of megawatt-class performance with MIRACL, a DF laser designed for sea-level operation. We then cover the program goals, design, and hardware of the Alpha Program. The early tests led to some hardware modifications, following which Alpha achieved megawatt-class performance. We then discuss the diagnostics suite used with Alpha, which recorded the data that fed into a data review task under the Alpha Laser Optimization program. The results of this review allowed for the refinement of Alpha test operations and allowed the program to culminate with a series of highly successful tests. Although Alpha is now being dismantled, and no direct descendant is planned, what we have learned from the Alpha program will be very useful to future high-energy laser programs.

We begin with a discussion of the history of attempts to correct combustion-driven high-energy laser (HEL)beams; these attempts have not tended to be as successful as hoped. After discussing beam control tests on MIRACL, a megawatt-class deuterium fluoride (DF) laser, we consider the conclusions of a Beam Control Maturity Assessment Panel (BCMAP)--convened by the now-terminated Space Based Laser Integrated Flight Experiment program--concerning the history of HEL beam control. We then discuss the results of the Alpha LAMP Integration (ALI) tests and what was and was not learned from those tests. Next we evaluate what was known from the wavefront sensor available as part of the Alpha Laser diagnostics suite and from high-frequency subaperture intensity data available from a mode beating experiment performed with Alpha. Taken together, these sources defined the need for a high-bandwidth measurement of Alpha subaperture intensity and tilt behavior, the defining requirements for the MHz Intensity and Tilt Sensor (MITS).

The Air Force Institute of Technology's (AFIT's) Center for Directed Energy, sponsored by the High Energy Laser Joint Technology Office(JTO), has developed the High Energy Laser End-to-End Operational Simulation (HELEEOS) parametric one-on-one-engagement-level model. HELEEOS incorporates scaling laws tied to respected wave optics codes and all significant degradation effects to include thermal blooming due to molecular and aerosol absorption, scattering extinction, and optical turbulence. The HELEEOS model enables the evaluation of uncertainty in low-altitude, high-energy laser (HEL) engagement due to all major clear-air atmospheric effects. Atmospheric parameters investigated include profiles of temperature, pressure, water vapor content, and optical turbulence as they relate to layer extinction coefficient magnitude. Worldwide seasonal, diurnal, and geographical spatial-temporal variability in these parameters is organized into probability density function (PDF) databases using a variety of recently available resources to include the Extreme and Percentile Environmental Reference Tables (ExPERT), the Master Database for Optical Turbulence Research in Support of the Airborne Laser, the Global Aerosol Data Set (GADS), and the Directed Energy Environmental Simulation Tool (DEEST) in conjunction with Air Force Weather Agency MM5 numerical weather forecasting data. Updated ExPERT mapping software allows the HELEEOS operator to choose from specific site or regional surface and upper air data to characterize correlated molecular absorption, aerosol absorption and scattering, and optical turbulence by percentile. The PDF nature of the HELEOS atmospheric effects package enables realistic probabilistic outcome analyses that permit an estimation of the level of uncertainty in the calculated probability of kill (Pk).