A mode-locked femtosecond laser emits an evenly spaced grid of frequencies that can be phase-coherently linked to a primary frequency standard, that is, to a cesium atomic clock. Such frequency combs can cover the entire visible and near-infrared spectral regions and have become invaluable as precise frequency rulers for modern optical frequency metrology (1). However, the teeth of the combs have been too densely spaced (0.1 to 1 GHz) to be spectrally resolved in a straightforward manner and are thus not accessible for individual use. Combs with large mode spacings (i.e., greater than 10 GHz) have required compromises both in terms of bandwidth and average power, resulting in pulses that are too weak and too long in duration for efficient nonlinear spectral broadening. By taking advantage of a combination of laser and fiber optic technology, we overcame these limitations of power and bandwidth to directly make a 10-GHz frequency comb. The result is more than 50,000 modes spanning a wavelength range from 470 to 1130 nm that can be directly resolved with a diffraction grating, a result that should accelerate progress in diverse applications including precision spectroscopy with individual comb teeth (2); calibration of high-resolution astronomical spectrographs (3, 4); and synthesis of optical, terahertz, and microwave waveforms via line-by-line pulse shaping (5).Our frequency comb is based on a Ti:sapphire laser with a 30-mm-long ring cavity ( Fig. 1) (6, 7). The roundtrip period is only 100 ps, resulting in a repetition rate and thus a frequency comb spacing of f R = 10 GHz. For a femtosecond laser, the 1.2-W average output power is relatively high; however, this translates to a pulse energy of merely 120 pJ at the output and only~6 nJ circulating inside the cavity. At such low pulse energies, care must be taken to maintain a high peak intensity in the Ti:sapphire crystal in order to support stable pulsed operation via Kerr-lens-mode-locking. We account for this requirement by using tight focusing into the gain crystal and appropriately balancing the intracavity dispersion to support pulses with a duration below 40 fs. The direct output spectrum of the laser (Fig. 1) shows that, for the~1200 modes within the full width at half maximum, 0.5 mW per individual 10 GHz mode is exceeded, an impressive combination of power and bandwidth among existing frequency comb sources.Absolute frequency stabilization of the comb requires measurement and control of both the repetition rate (f R ) and the comb's offset frequency ( f 0 ). f R is easily measured with a fast photodiode, whereas f 0 is measured with a nonlinear f-2f interferometer after spectral broadening of the laser output to more than an octave (1). In our case, spectral broadening is achieved in a microstructured fiber with a 1.5-mm core and negative group velocity dispersion at the wavelength of the laser (7). A key feature of the fiber is its sealed input, which allows us to achieve coupling efficiency of 50%, yielding more than 500 mW of average power at its o...

We report a mode-locked Ti:sapphire femtosecond laser emitting 42 fs pulses at a 10 GHz repetition rate. When operated with a spectrally integrated average power greater than 1 W, the associated femtosecond laser frequency comb contains approximately 500 modes, each with power exceeding 1 mW. Spectral broadening in nonlinear microstructured fiber yields comb elements with individual powers greater than 1 nW over approximately 250 nm of spectral bandwidth. The modes of the emitted comb are resolved and imaged with a simple grating spectrometer and digital camera. Combined with absorption spectroscopy of rubidium vapor, this approach permits identification of the mode index and measurement of the carrier envelope offset frequency of the comb.

The high power per mode of a recently developed 10 GHz femtosecond Ti:sapphire frequency comb permits nonlinear Doppler-free saturation spectroscopy in 87 Rb with a single mode of the comb. We use this access to the natural linewidth of the rubidium D 2 line to effectively stabilize the optical frequencies of the comb with an instability of 7 ϫ 10 −12 in 1 s of averaging. The repetition rate is stabilized to a microwave reference leading to a stabilized and atomically referenced comb. The frequency stability of the 10 GHz comb is characterized using optical heterodyne with an independent self-referenced 1 GHz comb. In addition, we present alternative stabilization approaches for high repetition rate frequency combs and evaluate their expected stabilities.