Tuesday, May 13, 2008

New Laser Technology Aiding the Search for Exo-Planets

8108_web_2 A laser-generated optical comb might sound like something Flash Gordon would use to straighten his hair. It's actually a super precise measuring device, able to find the frequency of radiation more accurately than any other method. That might sound less exciting than a new kind of paint-dryer, but has applications in little things like the measurement of time, probing the fundamental constants of the universe, and finding other planets. You know, small stuff.

Every light source, radio signal or cosmic ray we encounter has a frequency - find the frequency and you have information about the radiation. Electronics can accurately measure these up to about a hundred gigahertz, (100,000,000,000 cycles per second!) which sounds like a lot - but some critical sources go up into the hundreds of Terahertz, a thousand times higher and more. There's a quick trick to simplify the problem: instead of measuring the absolute frequency of a source you can measure the frequency difference with a known reference - combining the two creates "beats", a musically-monickered phenomenon with a frequency equal to the difference of the two component parts. So if we had a source of known frequencies close to what we want to measure, this "beat frequency" can be small enough to be measured by regular electronics.

This is where the optical frequency comb comes in. By setting up a laser cavity very carefully, mode-locking conditions mean that only certain regular frequencies can circulate - the fundamental, and the harmonics which are twice that, three times that, and onwards up. This is where the idea of the comb as a "ruler" comes from - a series of regular markings in frequency, just like your ruler has little ink markings at known intervals of distance. It's also where the "comb" comes from - a plot of frequencies output is a series of spikes separated by narrow gaps, looking just like the classic Fonz-favored hair-straightener. Combine your comb with a photodiode and the right electronic gadgetry and you can measure high frequencies with unprecedented precision.

This is nothing particularly new - optical combs have been a very active field since Halls and Hänsch won a little thing called the Nobel Prize for work on the technique in 2005. More recently, researchers at the University of Konstanz and the National Institute of Science and Technology have developed an ultra-small, ultra-accurate comb which could even be used in space. This would help satellite telescopes in the search for exoplanets.

If you tell somebody a planet is a hard thing to spot, they might look at you funny - unless they're an astronomer. Planets (small, rock, don't emit light) tend to hang out near stars (gigantic, fusion reactors, emit light pretty much by definition) and can be hard to make out, especially since the only light they emit is reflected from the parent star (the gigantic mega-bright source of the exact same radiation). We do have one chance - as the planet orbits, the frequencies reflected get shifted up and down around the original value by the Doppler shift (just as ambulance sirens get higher then lower as they shoot past you in the street). The problem so far has been that these frequency shifts are orders of magnitude below what we can resolve - until the new super-miniature comb, built by Albrecht Bartels at the Konstanz Center for Applied Photonics, gets in on the action.

So, when you think combs are just for picking the imperfections out of your coiffure (or making truly terrible noise with a sheet of wax paper), remember that high-tech hardware is combing the universe for alien worlds.

Posted by Luke McKinney.

Optical comb smaller than a real comb

2005 Nobel Prize

Original here

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