Searches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision of 60 cm s −1 (ref 1), which is sufficient to find a 5-Earth-mass planet in a Mercury-like orbit around a Sun-like star. To find a 1-Earth-mass planet in an Earthlike orbit, a precision of ∼ 5 cm s −1 is necessary. The combination of a laser frequency comb with a Fabry-Pérot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration 2−4 , with recent encouraging results 5 . Here we report the fabrication of such a filtered laser comb with up to 40-GHz (∼ 1-Å) line spacing, generated from a 1-GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-line-spacing comb, or 'astro-comb', is well matched to the resolving power of high-resolution astrophysical spectrographs. The astro-comb should allow a precision as high as 1 cm s −1 in astronomical radial velocity measurements.The accuracy and long-term stability of state-of-theart astrophysical spectrographs are currently limited by the wavelength-calibration source 6,7 , typically either thorium-argon lamps or iodine absorption cells 8 . In addition, existing calibration sources are limited in the red-tonear-IR spectral bands most useful for exoplanet searches around M stars 9 and dark matter studies in globular clusters 10 . Iodine cells have very few spectral lines in the red and near-IR spectral bands, while thorium-argon lamps have limited lines and unstable bright features that saturate spectrograph detectors. Recently, laser frequency combs 11 have been suggested as potentially superior wavelength calibrators 2,3 because of their good longterm stability and reproducibility, and because they have useful lines in the red-to-near-IR range. The absolute optical frequencies of the comb lines are determined by f = f ceo + m × f rep , where f rep is the repetition rate, f ceo is the carrier-envelope offset frequency and m is an integer. Both f rep and f ceo can be synchronized with radio-frequency oscillators referenced to atomic clocks. For example, using the generally available Global Positioning System (GPS), the frequencies of comb lines have long-term fractional stability and accuracy of better than 10 −12 . For the calibration of an astrophysical spectrograph, fractional stability and accuracy of 3 × 10 −11 are sufficient to measure a velocity variation of 1 cm s −1 in astronomical objects. In addition, using GPS as the absolute reference allows the comparison of measurements at different observatories.For existing laser combs, f rep is usually < 1 GHz (ref. 12), which would require a spectrograph with a resolving power of R = λ/δλ 10 5 to resolve individual comb lines (here δλ is the smallest difference in wavelengths that can be resolved at wavelength λ). In practice, astrophysical spectrographs tend to have a resolving power of R ∼ 10 4 − 10 5 owing to physical limitations on the in...
We report a new absolute frequency measurement of the Cs 6s-8s two-photon transition measured using frequency comb spectroscopy. The fractional frequency uncertainty is 5x10(-11), a factor of 6 better than previous results. The comb is derived from a stabilized picosecond laser and referenced to an octave-spanning femtosecond frequency comb. The relative merits of picosecond-based frequency combs are discussed, and it is shown that the AC Stark shift of the transition is determined by the average rather than the much larger peak intensity.
One of the dominant systematic effects that shift resonance lines in high-precision measurements of twophoton transitions is the dynamic ͑ac͒ Stark shift. For suitable laser frequencies, the ac Stark shift acquires an imaginary part which corresponds to the rate of resonant one-photon ionization of electrons into a continuum state. At the current level of spectroscopic accuracy, the underlying time-dependent quantum dynamics governing the atomic two-photon excitation process must be well understood, and related considerations are the subject of the present paper. In order to illustrate the basic mechanisms in the transient regime, we investigate an analytically solvable model scenario for the population dynamics in the density matrix formalism and describe in detail how to generalize the corresponding equations of motion for individual experimental use. We also calculate the dynamic Stark shift for two-photon S-S and S-D transitions in bound two-body Coulomb systems and the corresponding two-photon transition matrix elements. In particular, we investigate transitions for which the 1S ground state or alternatively the metastable 2S state acts as the lower-energy state, and for which states with n ഛ 20 represent the upper states. Relativistic and radiative corrections to the excitation dynamics, and the corresponding limitations to the accuracy of the measurements, are briefly discussed. Our considerations suggest the general feasibility of a detection mechanism, offering high quantum efficiency, based on two-step three-photon resonant ionization spectroscopy, for large classes of experimentally relevant two-photon transitions in two-body Coulomb systems.
A nearly two-octave wide coherent mid-infrared supercontinuum is demonstrated in a dispersion-engineered step-index indium fluoride fiber pumped near 2 µm. The pump source is an all-fiber femtosecond laser with 100 fs pulse width, 570 mW average power and 50 MHz repetition rate. The supercontinuum spectrum spans from 1.25 µm to 4.6 µm. Numerical modelling of the supercontinuum spectra show good agreement with the measurements. The coherence of the supercontinuum is calculated using a numerical model and shows a high degree of coherence across the generated bandwidth allowing it to be used for frequency comb applications.
Generation of low-timing-jitter 150 fs pulse trains at 1560 nm with 2 GHz repetition rate is demonstrated by locking a 200 MHz fundamental polarization additive-pulse mode-locked erbium fiber laser to high-finesse external Fabry-Perot cavities. The timing jitter and relative intensity noise of the repetition-rate multiplied pulse train are investigated.
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