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Latest revision as of 21:52, 11 May 2013

How a Laser Works

Mirrored from Lawerence Livermore Labs

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Background

The word "laser" stands for "light amplification by stimulated emission of radiation." Lasers are possible because of the way light interacts with electrons. Electrons exist at specific energy levels or states characteristic of that particular atom or molecule. The energy levels can be imagined as rings or orbits around a nucleus. Electrons in outer rings are at higher energy levels than those in inner rings. Electrons can be bumped up to higher energy levels by the injection of energy-for example, by a flash of light. When an electron drops from an outer to an inner level, "excess" energy is given off as light. The wavelength or color of the emitted light is precisely related to the amount of energy released. Depending on the particular lasing material being used, specific wavelengths of light are absorbed (to energize or excite the electrons) and specific wavelengths are emitted (when the electrons fall back to their initial level).

The ruby laser was the first laser invented in 1960. Ruby is an aluminum oxide crystal in which some of the aluminum atoms have been replaced with chromium atoms. Chromium gives ruby its characteristic red color and is responsible for the lasing behavior of the crystal. Chromium atoms absorb green and blue light and emit or reflect only red light.

For a ruby laser, a crystal of ruby is formed into a cylinder. A fully reflecting mirror is placed on one end and a partially reflecting mirror on the other. A high-intensity lamp is spiraled around the ruby cylinder to provide a flash of white light that triggers the laser action. The green and blue wavelengths in the flash excite electrons in the chromium atoms to a higher energy level. Upon returning to their normal state, the electrons emit their characteristic ruby-red light. The mirrors reflect some of this light back and forth inside the ruby crystal, stimulating other excited chromium atoms to produce more red light, until the light pulse builds up to high power and drains the energy stored in the crystal.

The laser flash that escapes through the partially reflecting mirror lasts for only about 300 millionths of a second-but very intense. Early lasers could produce peak powers of some ten thousand watts. Modern lasers can produce pulses that are billions of times more powerful.

Another characteristic of laser light is that it is coherent. That is, the emitted light waves are in phase with one another and are so nearly parallel that they can travel for long distances without spreading. (In contrast, incoherent light from a light bulb diffuses in all directions.) Coherence means that laser light can be focused with great precision.

Many different materials can be used as lasers. Some, like the ruby laser, emit short pulses of laser light. Others, like helium-neon gas lasers or liquid dye lasers emit a continuous beam of light. Our ICF lasers, like the ruby laser, are solid-state, pulsed lasers.

How the First Ruby Laser Works

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In contrast to an ordinary light source, a laser produces a narrow beam of very bright light. Laser light is "coherent," which means that all of a laser's light rays have the same wavelength and are in sync.

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1. High-voltage electricity causes the quartz flash tube to emit an intense burst of light, exciting some of the atoms in the ruby crystal to higher energy levels.

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2. At a specific energy level, some atoms emit particles of light called photons. At first the photons are emitted in all directions. Photons from one atom stimulate emission of photons from other atoms and the light intensity is rapidly amplified.

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3. Mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification.

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4. The photons leave through the partially silvered mirror at one end. This is laser light.

There are many types of lasers, including solid-state, gas, semiconductor, or liquid. The ruby laser is a solid-state laser. Solid-state lasers provide the highest output power of all laser types. The National Ignition Facility laser will also be a solid-state laser, but will use a special glass (rather than crystals of ruby) to amplify the initial laser pulses to very high energy levels. The NIF laser will be the most powerful laser in the world.