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In the last 20 years, green lasers have shrunk from table-size lab equipment to pocket-portable presentation tools (not to mention cat toys).
When green laser pointers first hit the market in the 1990s, they would set you back about $400.
The trick is to convert two photons of long-wavelength, low-energy infrared light into one photon of short-wavelength, high-energy green light in a process called frequency doubling.
The standard green laser pointer also includes a shield to keep any of the infrared light from escaping. Hagley noticed the problem when he bought three $15 laser pointers last December as Christmas presents for his in-laws.
The total power isn’t that much, about a thousandth of the output of a typical flashlight, Hagley noted. So before you let your cat chase a laser pointer beam across the floor, the authors suggest a do-it-yourself test to see how much infrared light your laser puts out. Place a piece of paper with a hole in it between the laser and the CD, and aim the laser through the hole. But in the pointer that Clark and his colleagues examined, the shield was entirely missing.

Each pointer claimed to emit 10 milliwatts of power, but one of them glowed with a much dimmer green beam. The danger is that laser light is a focused beam of a single wavelength of light, meaning 20 milliwats is enough to burn a hole in your retina before you blink. Most digital cameras or camera phones are sensitive only to visible light, but webcams can take images of light well into the infrared portion of the spectrum (or can be easily modified to do so). The light reflects off the CD and onto the paper, where it can be photographed by either the digital camera or the webcam. The top image shows the visible diffraction pattern; the bottom shows extra light in the infrared. A new study from the National Institute of Standards and Technology reports that some cheap laser pointers can emit more than 10 times as much invisible infrared light as bright green light, making them more likely to blind kids and pets. The average pointer makes its bright beam of light in three steps, each of which was a highlight in laser development when it first came out.
That light gets funneled into a crystal of a material called neodymium-doped yttrium orthovanadate, which is common to lab lasers.
Not only was the dim pointer missing its infrared shield, it also turned out to emit 20 milliwatts of invisible infrared light during normal use.

The authors suggest cutting a few notches in two paper cups, one to stabilize the laser and the other to hold a CD vertically. So, add us to your ad blocker’s whitelist or pay $1 per week for an ad-free version of WIRED.
The crystal’s electrons respond by getting excited and emitting infrared light at 1064 nanometers, which goes through a second crystal made of potassium titanyl phosphate.
The extra infrared is probably due to a misalignment between the diode laser and the crystals, making the conversion from infrared to green light less efficient. The CD acts as a diffraction grating, which spreads the laser light out across all its wavelengths.
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That crystal combines two infrared photons into one photon with half the wavelength and double the energy, the familiar 532-nanometer green light.

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