Results

1. Advantages Full knowledge and control of the optical frequencies; Tunability Each component linewidth less than a Hz1 Disadvantages Low intensity Presence of many spectral components Direct spectroscopy of cesium with a femtosecond laser frequency comb V. Gerginov1, S. Diddams2, A. Bartels2, C. Tanner1, L. Hollberg2 1University of Notre Dame, Notre Dame, IN 2Time and Frequency Division, NIST, Boulder, CO Motivation In metrology, Femtosecond Laser Frequency Combs (FLFC) provide the link between CW lasers which do the spectroscopy, and the microwave standards which provide the frequency calibration. FLFC are also used for studying ultrafast phenomena3 and doing multi-component spectroscopy4. In this work, we show that they can also be used for single-photon linear spectroscopy and to create a simple optical clock. Femtosecond Laser Frequency Comb: Solid-state laser pumped Ti:Sapphire modelocked laser. Time domain: Output consists of femtosecond pulses; Pulses repetition rate 1 GHz1; Frequency domain: 1 GHz spaced discrete frequencies; Less than a Hz linewidth2 per spectral component; Highly collimated atomic beam High-denslty narrow divergence atomic beam 1015/cm3 densities <3 mrad divergence corresponding to 2.3(1)MHz Doppler width Spectroscopy with a single comb component Also: - Cs atomic lines within the FLFC spectrum: electric-dipole allowed 6s 2S1/2 - 6p 2P1/2,3/2 transitions in the near infrared. - 14 nW @ 895 nm and 1.5nW @ 852 nm per component; - Reference to NIST atomic fountain5 - Measured optical frequencies with a CW laser6,7 Optical frequency measurements 10% of the filtered FLFC output is sent to the atomic beam. The comb spectrum is referenced to the hydrogen maser at NIST. A single comb component of the laser output excites the atomic transitions when the component frequency is close to an optical transition, fc. The repetition rate of the laser is scanned with a computer, and the fluorescence is detected with a photodetector. The interference filter (IF) is used to limit the spectral width around the wavelength of interest. The corner cube is used only to make the laser-atomic beam angle equal to 900. An acousto-optic modulator is used to stabilize the Cesium optical clock If the femtosecond laser component used to probe the atomic transition is locked to this transition, the repetition rate of the comb becomes frep=(fopt±fceo)/N, where N~300000 and fceo is the carrier-envelope offset frequency. To lock the FLFC component to the atomic transition, the repetition rate is modulated at 27Hz with 15Hz modulation depth, and a lock-in detection is used. The fractional frequency uncertainty is 1x10-10/s which is nonetheless competitive with other simple laboratory atomic references. The main limitation is the width of the atomic resonance of 8 MHz. Typical data for F=4-F'=4 transition of D1 line taken in ~6 hours. The previous optical frequency measurements6 of this line is represented by the shaded area. The Doppler shift due to laser-atomic beam misalignment is compensated on the order of a single-measurement error bar or ~40 kHz. CONCLUSIONS 1. A high-resolution atomic beam spectroscopy using a single femtosecond laser spectral component is performed, resulting in optical frequency measurements with precision approaching that of the CW laser experiments. Such spectroscopy can be performed in any part of the optical spectrum of the comb by filtering out the desired wavelength with a commercial interference filter. 2. Using a single femtosecond laser spectral component, a simple optical clock is realized. This creates a grid of absolute optical frequencies in addition to the divided-down microwave signal. The present accuracy is limited to 40 kHz (10-10 level) due to the 8 MHz width of the optical resonance. Using narrower transitions and higher laser output, even better accuracies can be achieved with extremely simple experimental setup. Results The optical frequencies of the D1 and D2 components were measured using a single FLFC component. Typical spectra are shown in the Figure below. The spectra repeat every 3 kHz change of the repetition rate. The constant background is due to the multiple comb components which are not resonant with the atomic transitions but contribute to the scattered light. D1 line - 14 nW per component, D2 line - 1.5 nW per component. No systematic corrections are included. Optical frequencies of the D1 line components. REFERENCES 1Bartels et al., Opt. Lett.27(20) 1839, 2002 2Bartels et al., Opt. Lett.29(10) 1081,2004 3Diels and Rudolph, "Ultrashort Laser Pulse Phenomena", Academic Press 1996. 4Shaden et al., Opt. Commun.125(1-3) 70,1996; Marian et al., Science, 2004. 5Jefferts et al., Metrologia39 (4) 321, 2002 6Gerginov, et al.,in preparation 7Gerginov, et al., PRA 70, 042505, 2004 Optical frequencies of the D2 line components