World Wide Directed Energy Weapons
The production of lasers in the modern scientific world is fairly new. The first laser was developed in the 1960s and represented the beginning of a drastic change in how the military viewed warfare. The late 1970s and 1980s, too, marked a busy time period for developing lasers into possible weapon systems. All branches of the military and industry were striving to master high power levels, beam control, and adaptive optics.
The word "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. Lasers are finding ever increasing military applications -- principally for target acquisition, fire control, and training. These lasers are termed rangefinders, target designators, and direct-fire simulators. Lasers are also being used in communications, laser radars (LIDAR), landing systems, laser pointers, guidance systems, scanners, metal working, photography, holography, and medicine.
The primary wavelengths of laser radiation for current military and commercial applications include the ultraviolet, visible, and infrared regions of the spectrum. Ultraviolet radiation for lasers consists of wavelengths between 180 and 400 nm. The visible region consists of radiation with wavelengths between 400 and 700 nm. This is the portion called visible light. The infrared region of the spectrum consists of radiation with wavelengths between 700 nm and 1 mm.
Laser radiation absorbed by the skin penetrates only a few layers. In the eye, visible and near infrared radiation passes through the cornea, and is focused on and absorbed by the retina. It is the wavelength of the light that determines the visible sensation of color: violet at 400 nm, red at 700 nm, and the other colors of the visible spectrum in between. When radiation is absorbed, the effect on the absorbing biological tissue is either photochemical, thermal, or mechanical: in the ultraviolet region, the action is primarily photochemical; in the infrared region, the action is primarily thermal; and in the visible region, both effects are present. When the intensity of the radiation is sufficiently high, damage to the absorbing tissue will result.
In 1960, the very first laser (a ruby laser) was built, producing minimal power. This event was followed by many other laser technology developments. The first chemical laser, hydrogen fluoride (HF), was built in 1965, producing 1 kW. It was then that DoD became interested in researching and developing a more powerful laser for weapon applications. Subsequently, in 1968, the Defense Advanced Research Projects Agency (DARPA) Baseline Demonstration Laser produced 100 kW, and the Navy-ARPA Chemical Laser (NACL) produced 250 kW in 1975.
Solid-State Lasers (SSLs) uses a solid lasing medium, such as a rod made up of glass or crystal, or a gem, like the ruby laser. Along with the rod or host material is an active material, such as chromium, neodymium, erbium, holmium, or titanium. Chromium is the active material used in ruby lasers. Neodymium is the active material in the most widespread applications. A flash lamp, arc lamp, or another laser carries out the optical cavity pumping to achieve population inversion and stimulate the laser beam. The Neodymium Yttrium-aluminum garnet (Nd:YAG) laser is a popular SSL. It operates at a 1064.5-nm wavelength and can be pulsed wave or CW. A great advantage of these lasers is that the wavelength and pulse duration can be varied considerably.
A chemical laser uses chemical reaction to create population inversion in the lasing medium. One example is the Mid-Infrared Advanced Chemical Laser (MIRACL) developed in the mid-1980s. The MIRACL is a continuous-wave, mid-infrared (3.8- µ) laser. Its operation is similar to a rocket engine in which a fuel (ethylene, C2H4) is burned with an oxidizer (nitrogen trifluoride, NF3). Free, excited fluorine atoms are among the combustion products. Just downstream from the combustor, deuterium and helium are injected into the exhaust.
Gas lasers consist of a gas filled tube placed in the laser cavity. A voltage (the external pump source) is applied to the tube to excite the atoms in the gas to a population inversion. The light emitted from this type of laser is normally continuous wave (CW). One should note that if Brewster angle windows are attached to the gas discharge tube, some laser radiation may be reflected out the side of the laser cavity. Large gas lasers known as gas dynamic lasers use a combustion chamber and supersonic nozzle for population inversion.
Free electron lasers have the ability to generate wavelengths from the microwave to the X-ray region. They operate by having an electron beam in an optical cavity pass through a wiggler magnetic field. The change in direction exerted by the magnetic field on the electrons causes them to emit photons.
Modern fiber lasers are powered by electricity, making them highly mobile and supportable on the battlefield. Fiber lasers use optical fibers as the gain media. In most cases, the gain medium is a fiber doped with rare earth elements—such as erbium (Er3+), neodymium (Nd3+), ytterbium (Yb3+), thulium (Tm3+), or praseodymium (Pr3+)—and one or several laser diodes are used for pumping. Optical fibers have been used in industry, specifically for telecommunications to transport information via light. With developing technology, optical fibers have become high-energy, powerful laser energy sources.
Fiber lasers have proven to have much benefit over traditional Solid-State Lasers (SSLs). They are rugged and do not require a clean room to operate or maintain, as most other laser systems do. They also are extremely efficient; however, they cannot operate well in all weather conditions. This type of laser is easy to mount due to the flexible fibers.
The physical processes affecting the propagation of high-power laser beams in the atmosphere are complex and interrelated. These processes include diffraction, molecular/aerosol scattering and absorption, turbulence produced by air density fluctuations, thermal blooming, and others. Fortuitously the fiber laser wavelength, l = 1.075 µm, is near a narrow water vapor transmission window centered at l = 1.045µm.
The two main nonlinear propagation problems encountered by high power lasers in the atmosphere are gas breakdown and thermal blooming. For total laser power levels less than ~100 kW, and depending on the transverse airflow and atmospheric absorption, thermal blooming effects can usually be neglected.
The influence of particulate matter on gas breakdown and also on thermal blooming is particularly severe near sea level. The presence of particles in the air path of a high power laser beam can dramatically influence the propagation of the beam. The particles are vaporized and ionized by a high power laser beam. The plasma balls ["fireflies"] generated not only attenuate the laser radiation, but they also act as scattering centers with completely different scattering characteristics than the original particles. The hot vapor can also act as a thermal source which could cause self-induced thermal distortion of subsequent laser pulses.
Current high energy laser (HEL) weapon systems primarily consist of continuous wave (CW) laser sources with output powers in the kilowatts. These kilowatt-class CW laser systems predominantly engage targets via absorption of light; either causing the target to burn and melt or overwhelming optical sensors with high intensities. Thanks to the emergence of diode and fiber laser technology, these laser systems have grown increasingly ruggedized to the point they have been integrated onto platforms ranging from ground to sea.
CW lasers provide solutions to many problems but due to their fundamental different natures, lasers with pulse widths in the range of femtoseconds provide unique tactical capabilities due to their rapid discharge of enormous power. While most CW lasers simply melt targets, USPL systems are able to neutralize threats via three distinct mechanisms: ablation of material from the target, the blinding of sensors through broadband supercontinuum generation in the air, and the generation of a localized electronic interference used to overload a threat’s internal electronics. Even the propagation of light from a USPL system holds unique advantages.
The sheer amount of intensity in a terawatt pulse laser is able to cause a non-linear effect in air resulting in a self-focusing filament. These filaments propagate without diffraction, providing a potential solution to the negative impact turbulence has on beam quality when propagating a conventional CW laser system.
Differences in lethality as well as propagation mechanisms makes USPL technology one of particular interest for numerous mission sets. Over the last two decades, femtosecond lasers have gone from requiring dedicated buildings at national laboratories to sitting on academic optics tables across the country. These USPL advancements, while promising, still have many hurdles to overcome in SWaP, relevant operating environments, and consistent mass manufacturing.
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