Laser Weapons

Lasers at the Starfire Optical Range.

Lasers are common in science fiction as futuristic weapons. Lasers project light at damaging intensities onto a target. To be effective, this light must be focused through optics (laser beams do not travel as perfect columns! Focusing makes the beam converge to a tight spot on the target). The ability to focus is limited by diffraction and, in an atmosphere, thermal blooming; and energy can be lost from the beam due to atmospheric absorption, scattering, and other phenomena. The color (wavelength, frequency, or per-photon energy) of the light you choose can be very important for your ability to get the light to the target.

Lasers can be expected to be exceptionally accurate. Depending on the power, energy, and intensity you can deliver to the target, they could be limited to relatively shallow surface burns or could drill deeply enough to reach vital equipment or organs deep inside the target. You can defend against lasers with armor or smoke, but mirroring will not work against higher intensity lasers.

Certain colors of lasers can cause unintended blindness to bystanders or friendlies, including all visible colors of light. Use with caution.

Introduction

Lasers are remarkable tools, with a wide range of applications across all realms of technology - communications, medicine, remote sensing, computing, data storage - but of course you don’t care about any of that. No, what draws us to places like this is using lasers to blow stuff up! So what is a laser, anyway? What makes it good at blowing stuff up, and why can’t it blow stuff up even better?

When we think about some new technology, we often default to thinking about it the same way as some analogous technology we are already familiar with. Early writers imagined airplanes as giant cruising air battleships rather than jet fighters. Early robots were very often metal humans that could talk and sense and interact with their environment rather than mechanical arms carrying out rote motions in factories or disk-shaped vacuum cleaners. In the same way, we often default to thinking about laser weapons as just like bullet-shooting guns. But they are not. Laser weapons, even laser guns, will be shaped by their own physics and design constraints into machines that optimize their potential and very often this leads to appearance and behavior that will be very different from the bullet guns that we expect.

If you look at the usual definitions of a laser, they will say that a laser is a device that uses stimulated emission to produce monochromatic, unidirectional, coherent light. All of this is mostly correct, but there are exceptions to each of these cases. Real life gets complicated!

• Stimulated emission is a process where an atom, molecule, or other quantum system in an excited (energized) state can be made to transition to a lower energy state by being hit by a quantum unit (or particle) of light (a photon), and in the process produce a second photon that is identical in every way to the first. Stimulated emission is a useful method to make directed beams of light, but there are machines called lasers that produce laser-like light without stimulated emission.
• Monochromatic means the light the laser produces is only one color (or wavelength, frequency, or photon energy … any one of these terms suffices to specify the color). In fact, monochromatic means that the laser is exactly one color - but this is unphysical. Every real phenomena has some finite distribution of colors, even if this distribution is so narrow that it can be hard to detect and can be neglected for many purposes. Most lasers are very close to only one color. But a number of lasers, especially those made for extremely high powered pulses, produce beams with a wide spectral range of colors.
• Unidirectional means all the light is going in one direction. As we will see later, this is ultimately not possible. The wave nature of light makes light that seems to originally be going in the same direction eventually diverge. In fact, it is often helpful to have the light coming out of a laser be slightly diverging or converging, and adjust that later with lenses or mirrors.
• Coherent (or, for our purposes, transversely coherent - there’s also longitudinal coherence but we don’t need to worry about that here) means that if you look across the beam at any given distance along the beam, all the light will be pointing or wiggling the same way. It is this property that allows lasers to be focused and directed so well. Loss of coherence means your beam can’t focus as well as it potentially could. Even incoherent light can be focused to some extent by forming an image - witness using a magnifying lens to focus sunlight - but coherent light can be focused much better.

Fundamentally, a laser is a tool that amplifies light. But even if we could, we wouldn’t want to amplify just any old light. No, we want to amplify light that has only those properties we want. For laser combat, that usually means light that will be focused on the target after going through the appropriate optics.

Technically, what happens is that to make it efficient to amplify light, we put the light we are trying to amplify into a cavity, a structure that allows light of a certain kind to resonate (or bounce back and forth a very large number of times with little loss). The material, substance, or device that amplifies the light is called the gain medium. This gain medium is placed so that the resonating cavity light goes through it, getting amplified with each pass.

The ways that the light can bounce around inside of a cavity are called its modes. A given mode will have only one color, and will have a specific relative intensity and direction profile that depends on the geometry of the cavity. The light in a given mode might be converging (focused) or diverging (defocused), but that can be altered later with optics. Any single mode will be fully coherent. The gain medium will usually only allow certain colors to be amplified, and thus only certain cavity modes can be excited. What we want to do is arrange the cavity so that only the modes that are most useful to us are amplified. When we do this, we can make beams with a minimum divergence that we can focus to destructive power on our enemies.

The Nature of Light

Lasers emit light. So to understand lasers we need to understand light. Lasers and the electromagnetic spectrum goes over some fundamental properties of light, and how the different colors of light impact what your laser can do.

Credit

Author: Luke Campbell