The U.S. Navy is constantly predicting future threats and contemplating new weapon technologies to counter them. One of those technologies is directed-energy (DE) weapons. Conventional weapons rely on the kinetic energy of projectiles. High-energy lasers (HELs), one type of directed-energy weapon, work in a fundamentally new way, using electromagnetic radiation to damage or destroy enemy assets.

Picture of the U.S. Navy Laser Weapons System (LaWS) onboard the USS Ponce.

High-energy lasers offer several advantages over conventional weapons, including the delivery of energy at light speed, a low cost per shot, virtually unlimited magazines, ability to rapidly engage several targets, and the ability to operate discreetly. Despite the many advantages, DE weapons also have some disadvantages to conventional weapons, including a high susceptibility to performance degradation in challenging atmospheric conditions.

The technology of adaptive optics (AO) has proven useful in mitigating the effects of the atmosphere on light beam propagation in the field of astronomy, where AO is often used to improve image resolution. Likewise, AO shows promise in improving HEL performance. To better understand how much adaptive optics can improve HEL performance and facilitate sound decision making on whether or not to integrate adaptive optics with HELs, the effect of adaptive optics on laser beam irradiance was simulated and compared to cases without adaptive optics. Many simulations have been created that accomplish this by using numerical diffraction methods. Uniquely, adaptive optics were incorporated into a scaling law code to facilitate large parameter studies.

Directed-energy weapons include all weapons that project electromagnetic radiation. High-energy lasers are one of the most promising DE weapons and have already undergone some field testing. All conventional lasers use the process of stimulated emission to emit a coherent light beam.

Stimulated emission describes the process of a photon of light perturbing an excited electron, causing the electron to drop to a lower energy state and emit a coherent photon of the same frequency, polarization, and direction as the initial perturbing photon. For this process to amplify light, electrons must be energized into the excited state, creating population inversion. The material containing the excited electrons is called the gain medium and can be made from a variety of materials. A fundamental component of lasers is the optical cavity, which is the arrangement of mirrors that surrounds the gain medium and provides the feedback loop for stimulated emission to occur.

Laser designs have used several methods to achieve population inversion. Chemical lasers use chemical reactions while gas lasers typically use an electric current or optical pumping to excite gases. In weapons research, a common chemical laser is the deuterium fluoride laser, which creates fluorine atoms through combustion and combines deuterium and helium atoms with the fluorine atoms to create a stable population inversion. A common gas laser is the CO2 laser, which achieves population inversion through burning a hydrocarbon and expanding the hot gas through nozzles.

Perhaps the most common designs in use today are solid state lasers (SSLs), which use solid state materials as the gain medium. Because the applications for solid state lasers are so broad and numerous, the technology has matured rapidly over the past few decades. The ubiquity of conventional solid state lasers make them the most ready asset for near- term implementation of DE weapons on the battlefield. Fiber lasers are SSLs that use optical fibers as the gain medium, making them more rugged and efficient than previously developed SSLs. Slab lasers are a particular form of SSLs that use slabs for the gain medium, generally defined as large aspect ratio rectangular cross-section materials. Slab lasers offer a potential technological advantage over traditional rod lasers in their power-scalability.

A free electron laser (FEL) uses a relativistic electron beam from a particle accelerator as the gain medium. The electrons oscillate between a series of alternating permanent magnets, emitting light in the process. The emitted light is captured inside a resonator cavity that is similar to the mirror cavity of a conventional laser. Notably, the FEL does not require the use of a fixed gain medium with its associated heating and distortion issues, so FELs have the potential to be scaled to high output power. Additionally, FELs can be tuned to specific wavelengths and produce high quality light beams.

This work was done by Donald Puent for the Naval Postgraduate School.For more information, download the Technical Support Package (free white paper) here under the Photonics category. NPS-0007

This Brief includes a Technical Support Package (TSP).
Integration of Adaptive Optics Into High-Energy Laser Modeling And Simulation

(reference NPS-0007) is currently available for download from the TSP library.

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