Laser frequency combs can help telescopes achieve unprecedented accuracy in spectral measurements, and may someday aid detection of Earth-sized exoplanets. A team from three German institutions attached a laser frequency comb to the Kiepenheuer Institute for Solar Physics’ solar telescope in Tenerife in the Canary Islands. Light from both the comb and the sun was coupled to a single optical fiber connected to a spectrograph. “This yields nearly perfect spatial mode matching between the two sources,” the researchers wrote in a study published in New Journal of Physics (doi: 10.1088/1367-2630/17/2/023048 [open access]). “In combination with the absolute calibration provided by the frequency comb, this method enables extremely robust and accurate spectroscopic measurements.” Laser frequency combs are created by a laser that emits continuous pulses of light containing millions of different colors, often spanning almost the entire visible spectrum. They are used for precisely measuring the frequency of light from a variety of sources. When the light is separated, its individual frequencies form a comb-like graph with finely spaced lines like the teeth of a comb. “An important aspect of our work is that we use a single-mode fiber, which takes advantage of the wave nature of light to enable a very clean and stable beam at its output,” said Rafael Probst, a doctoral candidate at the Max Planck Institute of Quantum Optics. “This type of fiber is quite common in telecom and laser applications, but its applications in astronomy are still largely unexplored. The (laser frequency comb) at the solar telescope on Tenerife is the first installation for astronomical use based on single-mode fibers.” The researchers said their technique improves calibration by about a factor of 100 over a temporally separated fiber transmission. They envision using the new technique to not only study the sun, but also distant stars and exoplanets. When a planet orbits a star, the star does not stay completely stationary, but instead moves in a very small circle or ellipse. This movement can be detected through the Doppler shift in the light from the star. The laser frequency comb could allow astronomers to more accurately measure these Doppler shifts, thereby increasing the chance of spotting Earth-sized, habitable planets. Current calibration techniques can detect changes in speed of roughly 1 m/s over large periods of time. Laser frequency combs could enable measurements with an accuracy of 1 cm/s. The comb technique could also allow improved spectral analysis of distant stars, the researcher said, as well as advance research in other areas of astrophysics, such as obtaining detailed observations of the sun and measuring the acceleration of the universe by observing distant quasars. For more information, visit www.mpq.mpg.de.