Abstract

Mode-locked femtosecond lasers and related emerging miniaturized devices can produce optical frequency combs, which consist of a large number of precisely evenly spaced spectral lines. Almost two decades ago, such combs were introduced as tools for optical frequency metrology, motivated by the challenges of precision laser spectroscopy of atomic hydrogen as tests for fundamental physics laws. Laser frequency combs provide the long-missing clockwork for optical atomic clocks, which are now approaching relative frequency uncertainties of 10⁻¹⁸. Distant clocks can be compared via optical fiber links at the 10⁻¹⁹ level, opening new opportunities for relativistic geodesy and astronomical interferometry. As calibration tools for astronomical spectrographs, frequency combs are facilitating the search for exoplanets, and they may lead to direct evidence for the accelerating expansion of space in our universe. Laser combs are also becoming powerful instruments for broadband molecular spectroscopy. They can dramatically improve the resolution and recording speed of Fourier spectrometers, and they are creating intriguing new opportunities for highly multiplexed nonlinear spectroscopy and microscopy.

© 2015 IEEE