10-GHz Self-Referenced Optical Frequency Comb Albrecht Bartels, 1 * Dirk Heinecke, 1,2 Scott A. Diddams 2 * 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 straight- forward manner and are thus not accessible for indi- vidual 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 limita- tions 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); cali- bration of high-resolution astronomical spectrographs ( 3, 4); and synthesis of optical, terahertz, and micro- wave 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 cav- ity. 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 combi- nation of power and bandwidth among existing frequency comb sources. Absolute frequency stabilization of the comb requires measurement and control of both the rep- etition rate ( f R ) and the comb’ s offset frequency ( f 0 ). f R is easily measured with a fast photodiode, where- as f 0 is measured with a nonlinear f-2f interferom- eter after spectral broadening of the laser output to more than an octave ( 1). In our case, spectral broad- ening 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 output. We achieve spectral coverage from about 470 to 1130 nm (Fig. 1). Common servo techniques are used to phase-lock f 0 and f R to frequency refer- ences that are calibrated by a Cs atomic clock or a more readily accessible representation of the sec- ond, for example, a quartz oscillator disciplined by the global positioning system. Because of the large mode spacing of the comb, a simple grating spectrometer with a resolving power of l/ Dl = 6 × 10 4 is sufficient to resolve and spa- tially separate the individual comb elements. Real- color images of the resolved modes were acquired at wavelengths of 490 nm, 540 nm, 583 nm, and 632 nm, through a microscope with a digital cam- era (Fig. 1). Although the modes at the longest three wavelengths are clearly resolved, we are ap- proaching the resolution limit at 490 nm. Once re- solved, the modes are available as precise frequency markers, for example, in astronomic spectrograph calibration. This application should specifically ben- efit from the wide spectral coverage of our source extending over ~350 THz at a power level exceed- ing 1 nW per mode, a performance currently un- achievable with existing mode-filtering approaches. Selection of individual modes via simple spatial fil- ters or even manipulation in amplitude and phase by use of spatial light modulators is straightforward and can provide a freely programmable array of pre- cisely defined light sources with an inherently high degree of mutual coherence. Such a device would be highly valuable for spectroscopy or Fourier synthe- sis of arbitrary waveforms via linear superposition and nonlinear mixing and frequency conversion. References and Notes 1. Th. Udem, R. Holzwarth, T. W. Hänsch, Nature 416, 233 (2002). 2. M. C. Stowe et al., in Advances in Atomic, Molecular and Optical Physics, E. Arimondo, P. Berman, Eds. (Elsevier, London, 2007), vol. 55. 3. C.-H. Li et al., Nature 452, 610 (2008). 4. T. Steinmetz et al., Science 321, 1335 (2008). 5. Z. Jiang, C. B. Huang, D. E. Leaird, A. M. Weiner, Nat. Photonics 1, 463 (2007). 6. A. Bartels, D. Heinecke, S. A. Diddams, Opt. Lett. 33, 1905 (2008). 7. Materials and methods are available as supporting material on Science Online. 8. We thank T. Fortier and C. Oates for thoughtful comments on this manuscript. A.B. is chief executive officer and partial owner of Gigaoptics GmbH. NIST and Gigaoptics hold patents (6,618,423 and 6,850,543) relating to some of the technologies used in the present submission. This work is supported by the Center for Applied Photonics at the University of Konstanz and NIST. Supporting Online Material www.sciencemag.org/cgi/content/full/326/5953/681/DC1 Materials and Methods SOM Text Fig. S1 References and Notes 14 July 2009; accepted 10 September 2009 10.1126/science.1179112 BREVIA 1 Center for Applied Photonics, University of Konstanz, Univer- sitätsstraße 10, 78457 Konstanz, Germany. 2 National Insti- tute of Standards and Technology (NIST), 325 Broadway Mail Stop 847, Boulder, CO 80305, USA. *To whom correspondence should be addressed. E-mail: albrecht.bartels@uni-konstanz.de (A.B.); scott.diddams@ nist.gov (S.A.D.) Fig. 1. (A) Illustration of the 10-GHz laser cavity. A 0.02-€ coin is shown for size comparison. The nonlinear fiber and diffraction grating dispersing the white light are also illustrated. (B) Real-color image of the spectrally dispersed visible part of the continuum and a magnified view of the individually resolved frequency comb modes at wavelengths of 490 nm, 540 nm, 583 nm, and 632 nm. (C) Low-resolution measurements of the direct laser output spectrum (gray line) and quasi-continuum output after broad- ening in nonlinear fiber (yellow line) on a power-per-mode scale. www.sciencemag.org SCIENCE VOL 326 30 OCTOBER 2009 681 on November 5, 2009 www.sciencemag.org Downloaded from