Thursday, September 30, 2010

Steph’s Science Corner: the lasers you know, love, and absolutely fail to appreciate

laura linney congo

Hi there, tech fans. Welcome to the new official science briefing of Charge Shot!!!

Some of you may recall that several months ago I made a brief and glorious appearance on After The Jump, during which I made an attempt explain how Blu-ray technology fits into the world of modern optical data storage. It landed me a lovely shout-out on the podcast title, but that also could have been a result of my juvenile abuse of the silly naming conventions that modern science trusts its community to take seriously, even when these acronyms and phrases are too hilarious not to be immature about (see: Highest Occupied Molecular Orbital).

This gave me an idea. I like to understand how modern technology works. I like to learn. I like to write. And I have an unimpressive bachelors degree in a scientific field or two that I’m only barely using in my current job. I decided to put that deadly combination to good use into this otherwise culturally- and sociologically-minded blog that I’m proud to be a part of. Therefore, future fans, I give you my new weekly feature. I hope it will expand your understanding of the world you live in as much as it satisfies my rampant tendency towards intellectual self-indulgence.

This week’s topic: LASERS.

As the world of modern technology and digital data races forward, the need for larger storage media is constantly growing to meet the demands of an innovation-hungry population. Recently, as inspired by my Blu-ray tangent from the podcast, I’ve been following news about the development of the HVD (Holographic Versatile Disc). While this technology is still in the development phase and currently prohibitively expensive (and will likely stay that way until about 2020), gigabyte-touting HD enthusiasts have been heralding the end of blu-ray since the HVD was conceived in the mid-2000s. But it’s important for me to reflect on the fact that innovations like this are possible because of the single most important tool in medicine, industrial and commercial manufacturing, and analytical scientific research: the laser.

In today’s modern word, lasers are everywhere from consumer electronics to information technology, from science and medicine to industry and entertainment. They also play an important role in law enforcement  in the military. They first emerged on the scene with the supermarket barcode scanner, which became common in 1974. The CD player and the laser printer weren’t far behind. Lasers are ubiquitous, essential, and responsible in some way or another for nearly all of the discovery and innovation that has taken place over the last fifty years.

What makes them special:

from the UC Davis optics club websiteLASER stands for Light Amplification by Stimulated Emission of Radiation. As I will no doubt return to in the future, please don’t be alarmed by the word “radiation.” In most cases, radiation simply describes electromagnetic waves propagating through space, i.e. how visible light travels from a bulb in your house to your eye. It radiates outward. In most cases when we discuss lasers, we’re still talking about visible light -- the colored beam that you’re told not to shine in your friends’ eyes. It is possible to produce other light-emitting laser devices (for UV light, microwaves, X-rays) using the same technology, and in 1959 when Gordon Gould first published the term LASER, he hoped that the –aser suffix would catch on (uvaser, maser, xaser).

It didn’t.

What makes a laser unique and exciting is that it produces a beam that is narrow, coherent, colinear, monochromatic, and low-divergence. Here’s what that means:

Light, which has a dual nature of being both a particle and a wave, illuminates a room by spreading out. All the different colors, phases, and directions of these light waves together make what we see as white light. Laser light, however, produces a beam of just a single, narrow-band wavelength, meaning it is only one color. And you’ll notice that this beam stays essentially straight over nontrivial distances – it doesn’t disperse. Coherence and colinearity are slightly more complicated features of laser light that have to do with diffraction and other properties that describe how a wave travels in both space and time, but a simple way of describing it would be to say that all the waves are “in-step” with each other. They have the same “size” and “shape” and they travel in the same direction along the same line. That makes it a lot easier to predict and measure how they interact with each other, with other waves, or with other matter.

How they work…sort of

This is not a phenomenon that is particularly easy to explain, especially without a background in quantum mechanics, a few well-designed figures, and hours of classroom time. What’s important to remember, however, is that it takes energy to produce energy. A laser has to be plugged into something, and the input is always larger than the output, making portability somewhat of a challenge.

A controlled material inside of a tube tube absorbs energy that excites the electrons in the material into a higher, slightly unstable energy state. When these electrons “fall” back down to their lower energy state, they emit that energy back out in the form of a light particle (a photon). This emission can be spontaneous, or stimulated by some other light particle passing by. In the latter case, the emitted photon comes out traveling in the same direction as the one that caught its attention, and with the same characteristics, which is how we get the focused beam. When there are more excited electrons than there are stable ones, you get something called population inversion, and we get more stimulated emission than energy-absorption, so the light coming out is amplified.

If you giggled at all during that last paragraph, don’t be too ashamed. It happens to the best of us.

Laser applications and specifications:

There are two main types of laser output: continuous wave or pulsed operation. A continuous wave laser stays on, a pulsed laser’s output alternates between on and off periods. They’re each used for different applications, but it’s worthwhile to note that a laser can achieve much higher peak power output in pulsed operation. You can think about that this way – if you apply an amount of energy to something over a longer time, that energy can disperse into other things like the air or surrounding materials. But if it’s applied for a very short time, it can be very focused at its maximum intensity. Lasers that produce a continuous beam are compared by their average power. Lasers that produce pulses can be characterized by the peak power of each pulse, which is usually significantly greater than the average.

You can find lasers in medicine (LASIK eye surgery, drying and sealing dental fillings), industry (cutting, welding, heat treatment, non-contact measurement), law enforcement (fingerprint detection in forensic identification), cosmetic treatments (acne, cellulite reduction, hair removal), scientific research (spectroscopy, scattering experiments, laser cooling, single atom capture) and of course, in the military (marking targets, guiding munitions, missile defense, and the offspring of the RADAR (which uses radio waves), the LIDAR).

Here is a list of the average power required to operate some devices you might be familiar with:

  • 1 milliwatt (one one-thousandth of a Watt): laser pointersIndustrial laser cutting at its finest
  • 5–10 mW – CD and DVD players
  • 200-300 mW– Consumer DVD-R burner
  • 1 Watt – green laser used in HVD development
  • 1–20 W – commercial solid-state lasers
  • 30–100 W – Surgical lasers
  • 100–3000 W– Industrial cutting lasers
  • 1.3 petawatt (1.3×1015 , 1,300,000,000,000,000, or 1.3 quadrillion Watts) – 1998’s most powerful pulsed laser, located in the Lawrence Livermore Laboratory

Okay, so when can we make laser guns?

Yes, I realize that some barcode scanners look a lot like Star Trek phazors, but I have to break the  unfortunate news to you: actual laser weapons are impractical, and are only beginning to enter the scene as anything other than a way to help aim. The theory is to hit a target with a high-output pulsed laser, causing rapid heating, evaporation and expansion of a surface. But the power needed to project a high-powered laser beam is not particularly conducive to the ease of mobility and light-weight requirements that an infantry might require. Certainly, there is the potential for mounted laser weaponry on vehicles, a concept that is being experimentally implemented in some areas of the military. And using lasers for missile detection and defense has been a long-standing practice.

But as far as laser guns go? Don’t get your hopes up too high just now. Sure, lasers can potentially be used as incapacitating weapons – you can put someone’s eyes out. Lasers of even a fraction of a watt in power can produce immediate and permanent vision loss under the right conditions, which is why your teachers warned you not to play around with the laser pointer. Fortunately, however, weapons designed to intentionally cause blindness have been banned by international humanitarian law. barcode scanner phasor

Well that’s it for this week’s topic. I’ll be doing my best to keep up with interesting news, tech development, controversy, or other charge-shot writers’ articles to draw ideas for future posts, but I would love to take any and all requests. If you have any feedback, questions, or topic suggestions that you would like to see featured, feel free to sound off in the comments section or email me at science@charge-shot.com. Until next time, appreciate your gadgets and respect the science!