10-GHz Self-Referenced Download full-text
Optical Frequency Comb
Albrecht Bartels,1* Dirk Heinecke,1,2Scott A. Diddams2*
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
forward mannerand are thus not accessible for indi-
vidual use. Combs with large mode spacings (i.e.,
both in terms of bandwidth and average power,
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).
Ourfrequency combisbasedon aTi: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
mode-locked femtosecond laser emits an
evenly spaced grid of frequencies that can
be phase-coherently linked to a primary
average output power is relatively high; however,
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-
as f0is measured with a nonlinear f-2f interferom-
eter after spectral broadening of the laser output to
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 f0and fRto frequency refer-
encesthatarecalibrated by a Cs atomic clock or a
more readily accessible representation of the sec-
the global positioning system.
of l/Dl = 6 × 104is 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 modesareavailable asprecise 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
andcanprovide afreelyprogrammablearrayof pre-
cisely defined light sources with an inherently high
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
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,
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
Materials and Methods
References and Notes
14 July 2009; accepted 10 September 2009
1Center for Applied Photonics, University of Konstanz, Univer-
sitätsstraße 10, 78457 Konstanz, Germany.2National Insti-
tute of Standards and Technology (NIST), 325 Broadway Mail
Stop 847, Boulder, CO 80305, USA.
*To whom correspondence should be addressed. E-mail:
firstname.lastname@example.org (A.B.); scott.diddams@
Fig. 1. (A) Illustration of the 10-GHz laser cavity. A 0.02-€ coin is shown for size comparison. The
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.
VOL 326 30 OCTOBER 2009
on November 5, 2009