Scientists at the University of Konstanz in Germany and the National Institute of Standards and Technology (NIST) have demonstrated an ultrafast laser that offers a record combination of high speed, short pulses and high average power. [about 100 times faster and more powerful] The same NIST group also has shown that this type of laser, when used as a frequency comb—an ultraprecise technique for measuring different colors of light—could boost the sensitivity of astronomical tools searching for other Earthlike planets as much as 100 fold.
Among its applications, the new laser can be used in searches for planets orbiting distant stars. Other possible applications of the new laser include remote sensing of gases for medical or atmospheric studies, and on-the-fly precision control of high-speed optical communications to provide greater versatility in data and time transmissions.
Astronomers look for slight variations in the colors of starlight over time as clues to the presence of a planet orbiting the star. The variations are due to the small wobbles induced in the star’s motion as the orbiting planet tugs it back and forth, producing minute shifts in the apparent color (frequency) of the starlight. Currently, astronomers’ instruments are calibrated with frequency standards that are limited in spectral coverage and stability. Frequency combs could be more accurate calibration tools, helping to pinpoint even smaller variations in starlight caused by tiny Earthlike planets. Such small planets would cause color shifts equivalent to a star wobble of just a few centimeters per second. Current instruments can detect, at best, a wobble of about 1 meter per second.
Standard frequency combs have “teeth” that are too finely spaced for astronomical instruments to read. The faster laser is one approach to solving this problem. In a separate paper,** the NIST group and astronomer Steve Osterman at the University of Colorado at Boulder describe how, by bouncing the light between sets of mirrors a particular distance apart, they can eliminate periodic blocks of teeth to create a gap-toothed comb. This leaves only every 10th or 20th tooth, making an ideal ruler for astronomy.
Both approaches have advantages for astronomical planet finding and related applications. The dime-sized laser is very simple in construction and produces powerful and extremely well-defined comb teeth. On the other hand, the filtering approach can cover a broader range of wavelengths. Four or five filtering cavities in parallel would provide a high-precision comb of about 25,000 evenly spaced teeth that spans the visible to near-infrared wavelengths (400 to 1100 nanometers), NIST physicist Scott Diddams says.
Osterman says he is pursuing the possibility of testing such a frequency comb at a ground-based telescope or launching a comb on a satellite or other space mission.
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