Center for Applied Photonics, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany.
The femtosecond laser-based frequency comb has played a key role in high-precision optical frequency metrology for a decade. Although often referred to as a precise optical frequency ruler, its tick marks are in fact too densely spaced for direct observation and individual use, limiting important applications in spectroscopy, astronomy, and ultrafast electromagnetic waveform control. We report on a femtosecond laser frequency comb with a 10-gigahertz repetition rate that creates a stabilized output spectrum with coverage from 470 to 1130 nanometers. The individual modes can be directly resolved with a grating spectrometer and are visible by eye.