The Why Files The Why Files -- whyfiles.org

Laser nears 50. Long live the laster!
POSTED 17 JULY 2008

Key laser figure dies; laser lives on

 Purple rectangular cube is hit with a beam of light, blasting orange sparks into the air

An ultra-fast "femtosecond laser" fires pulses as short as 20 quadrillionths (20/1,000,000,000,000,000) of a second; here, it is used to blast material from a sample for analysis. A "sluggish" nanosecond laser would also heat the material that was released, muddying the results.

Image: Lawrence Berkeley Laboratory

Last month, William Bennett, a retired Yale University physicist who played a big role in the invention of the laser, died. Bennett, with others at Bell Laboratories, a private research Mecca in New Jersey, described the first gas laser in 1961, one year after the solid laser was first tested.

From a clunky tube that mainly fascinated pocket-protected people, the laser and the coherent beam of light it makes have certainly come a long way. Lasers were quickly adopted by scientists for spectroscopy -- the study of how light interacts with matter -- and eye surgery, after doctors noticed that a laser beam could pass unmolested through the front of the eye, then cauterize a bleeding retina in the back.

But the initial uses provided only the palest image of what this tight beam of organized, one-color light could do. Here's a by-no-means-exhaustive list of what lasers do today:

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 Zap single cells in biology labs to learn what they do;

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 Form pulses of light that carry telephone and Internet signals in fast, high-capacity fiber-optic networks;

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 Engrave, cut, weld and heat materials in labs and factories;

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 Mimic radar to measure weather and catch speeders;

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 Read and write CDs and DVDs;

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 Make "freeze-frame" images of chemical reactions in fractions of a nanosecond; and

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 Produce the red beams in bar-code scanners in supermarkets and warehouses.

How a photon (particle of light) is made
Two illustrations show arrow moving letter E and a small blue sphere from lower line to upper line.  Illustration 2 has yellow sphere given off when arrow returns blue sphere to lower line

All light begins when an atom's electron drops down to a lower energy level. First, the electron must absorb some energy ("E"), raising to a higher orbit (1). When the electron returns to its original orbit, it releases a photon (2). In a laser, these photons have a nearly identical color and are coherent, meaning their waves are synchronized.

Adapted from Lawrence Livermore National Laboratory

The Why Files failed to find an estimate of the total size of the laser industry, perhaps because lasers have become ubiquitous. About $6 billion worth of lasers are sold each year. But this sum is dwarfed by the industries that depend on laser technology: for example, global sales of music on optical disks were $15.9 billion in 2007, and U.S. consumers spent $23.4 billion to rent and buy movies on DVD.

None of these numbers include the price of laser research or the value of the infrastructure that supports a few small sectors of the economic landscape, like the fiberoptic telephone and Internet networks.

Without the laser, in other words, you might not be reading this.

Fundamental parts of the laser
Magenta tube held between two flat mirrors, beam of light runs through center of tube, yellow arrows indicate direction of laser beam

1. Gain medium: A liquid, solid, plasma or gas that emits light when stimulated by pumping energy; held inside optical cavity.

2. Pumping energy: External energy that enters the gain medium and stimulates light emission.

3. High reflector: A highly reflective mirror at one end of the laser's optical cavity.

4. Output coupler: A less-reflective mirror that allows some light to leak out, forming the laser beam.

5. Laser beam: Coherent light that passes through the output coupler.

Image: Wikipedia

How lasers work

Lasers are now made in a vast range of materials and designs, but they share some basic components:

To understand what these parts do, let's start with the acronym: LASER = Light Amplification by Stimulated Emission of Radiation. When molecules absorb energy, electrons move to higher, more energetic orbits. When the electrons fall back to their original orbit, a particle of light (a photon) gets emitted. These photons, trapped inside a pair of mirrors, bounce back and forth, where they are continually joined by more stimulated radiation. Eventually, the photons become strong enough to escape through the weaker mirror.

Black and photo of a man with white shirt sleeves rolled up, bent over large contraption of glass tubes, dials and mirror in a lab

A Bell Laboratory scientist studies the relationship of power output to cavity length in a helium-neon laser.

Photo ca.1963-1964, courtesy AT&T Archives and History Center

The escaping light -- the laser beam -- is a stream of light with properties quite distinct from the more familiar, chaotic light we get from conventional sources, like lava lamps and stars. A laser beam:

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 Can be held within a tight column, so it fans out extremely slowly with distance;

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 Is "coherent," meaning the peaks and troughs of the light waves move in unison;

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 Contains more or less one "color," or wavelength, of electromagnetic radiation; and

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 Can be continuous or pulsed, with precise control over timing and intensity.

Together, these unusual characteristics lie at the root of the phenomenal utility of lasers in science, industry, computers, telecommunications and manufacturing.

Copper penny towers over a small, round lens set on a brass plug with two connectors in the back

About 733 million diode lasers were sold in 2004, far outstripping sales of other laser types. Diode lasers are found in optical drives, laser printers, bar code scanners, and in fiber-optic communication.

Image: Wikipedia

No lightweight invention

Since the first laser was operated in 1960, at Hughes Research Laboratory in California, a huge variety of gas- and solid-state lasers have been invented. Carbon-dioxide lasers are used to cut steel, while semi-conductor ("diode") lasers are used in CDs, DVDs, laser printers and fiberoptics. Lasers have moved from a lab curiosity tended by highly-educated physics lab-dwellers to tiny, highly reliable solid-state devices that can operate untended for months or years at a time.

The future is now

Almost 50 years after they jolted the world of physics, laser science and technology are still gaining traction:

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 The world's most powerful array of lasers, 192 big-as-a-buffalo light sabers -- is scheduled to zap a tiny target to ignite controlled nuclear fusion in 2009.

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 NASA is trying to communicate with distant spacecraft with a laser, which could dwarf the data-transmission capacity of existing, radio-based communication. (Due to their shorter wavelength, lasers can carry hundreds of thousands of times more information than radio waves.) A test instrument, slated to reach Mars in 2010 aboard the Mars Telecommunications Orbiter, will carry a 5-watt laser that can be pointed at a receiver over a distance of a couple hundred-million kilometers.

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 By 2012, the Pentagon plans to launch optical-based satellites that would likewise use lasers to boost bandwidth, to help operate drone aircraft for remote-control warfighting.

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 Rapid pulses of laser light may speed up hard disks in a decade or so, according to one report.

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 The newest frontier in short-pulse lasers is the attosecond laser, which fires in fractions of a femtosecond. This is fast: One femtosecond is to one second as one second is to about 31 million years...

Could lasers help with climate predictions?

Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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