Built on Facts

As you might expect for a guy who does experimental optical physics, I get to spend a tremendous amount of time in labs with some fairly snazzy lasers. Most of them are fairly specialized pieces of equipment that aren't really designed simply to dump huge amounts of power in industrial applications. As far as danger goes, they're not going to come to life and murder you in your sleep. But still, we have open beamlines of infrared lasers with average powers on the order of 1-4 watts. Unfocused they usually won't do much to exposed skin other than make you uncomfortable (for instance, like a Christmas light pressed against the skin for the 4 watt laser). Focused they'd have no trouble poking a very clean little pinhole in you. Unfortunately your eye is a focusing lens, and direct eye exposure would be instant permanent damage. And the infrared beam is invisible to the naked eye, so that's an extra challenge.

Of course we have pretty robust safety procedures and I've never heard of any injuries in our AMO group. We also have a wide assortment of surprisingly expensive safety glasses that are designed to filter out the wavelength of particular lasers. For lasers with a continuous beam this is not so hard since all the light is emitted at pretty much exactly one frequency. For lasers that generate ultrashort pulses the light spans a wider portion of the spectrum and the glasses have to remove more bandwidth.

This particular laser is a 15 watt Nd:YLF laser we use to pump an ultrafast amplifier. While it's possible in theory to pop the lid and get a good picture of its innards, it's not a great idea to shut down and open up expensive equipment for no research-related reason. So here's a picture of some green light diffusely reflecting from the gap between the pump laser and the amplifier:

And here's the same thing with some safety glasses held in front of the camera:

Though the images are not great, you can see that the green light is totally blocked. You can still see most of the world pretty well because the glasses still let in light with longer wavelengths than green. (For the curious, these block everything from about 540nm to around 160nm. This protects against frequency doubled Nd lasers and excimer lasers, though our lab only uses the former.)

So what's this have to do with the now-infamous vuvuzela? If you've been living under a rock lately, you may have missed watching or at least seeing clips of the World Cup being held in South Africa. Apparently it's tradition to bring this instrument to the games. It sounds something like an oversized kazoo and in the aggregate makes the stadium sound like the world's biggest beehive. But in analogy to the laser, it just so happens that the sound produced by the vuvuzela is generated in a relatively narrow frequency range - mostly at 233 Hz, 466 Hz, 932 Hz, and 1864 Hz. Block those frequencies electronically and you remove that buzzing while preserving most of the rest of the noise of the games. Various news organizations have reported that some people are doing just that. Here's the web site of the guy most of the news articles mention. It's in German, but if you scroll to the bottom the before/after sound samples are easy to find. It's not quite perfect. The vuvuzela is not quite a perfectly narrowband instrument and some of those removed frequencies contain significant parts of the spectrum of the human voice which leaves vocal audio sounding a tiny bit tinny. But it's really a remarkable improvement nonetheless.

Maybe not quite as important to health as a good pair of laser safety glasses, but certainly an application of band-stop filtering that's likely to be useful to more people.