Frequency combs help untangle astrophysical spectra

By resolving light into its component frequencies, spectrographs can reveal, for example, the Doppler shift in light that travels to Earth from other solar systems with orbiting exoplanets or from the far reaches of the universe. But before it can be put to work, a spectrograph must be calibrated, and that process inevitably introduces measurement uncertainty. To reduce those uncertainties, Tilo Steinmetz and colleagues developed a calibration procedure that can dramatically improve spectroscopic precision, and they demonstrated their technique by obtaining the most accurate spectrum of the Sun’s photosphere to date. The key ingredient is a laser frequency comb, a series of equally spaced, precisely known frequency spikes that, as the white stripes in the figure show, can serve as a template with which astrophysical spectra can be compared (see Physics Today, June 2000, page 19 , and December 2005, page 19 ). Because the comb is reproducible, the calibration can be replicated from run to run. Moreover, frequency combs may help isolate systematic spectrograph uncertainties that, one hopes, can be understood and minimized. A plausible goal, according to Steinmetz and company, is to measure redshifts of objects whose speed along the line of sight is changing by 1 cm/s over a year’s time. That would allow for a direct measurement of cosmic acceleration. Closer to home, higher-precision spectroscopy would enable astrophysicists to identify Earth-like exoplanets by measuring the characteristic Doppler shifts experienced by a star as it is gravitationally tugged by an orbiting planet. (T. Steinmetz et al., Science 321 , 1335, 2008 http://dx.doi.org/10.1126/science.1161030 .)