1. Results The optical frequencies of the D 1 and D 2 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. D 1 line - 14 nW per component, D 2 line - 1.5 nW per component. No systematic corrections are included. Direct spectroscopy of cesium with a femtosecond laser frequency comb V. Gerginov 1, S. Diddams 2, A. Bartels 2, C. Tanner 1, L. Hollberg 2 1 University of Notre Dame, Notre Dame, IN 2 Time 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 phenomena 3 and doing multi-component spectroscopy 4. In this work, we show that they can also be used for single-photon linear spectroscopy and to create a simple optical clock. REFERENCES 1 Bartels et al., Opt. Lett. 27(20) 1839, 2002 2 Bartels et al., Opt. Lett. 29(10) 1081,2004 3 Diels and Rudolph, "Ultrashort Laser Pulse Phenomena", Academic Press 1996. 4 Shaden et al., Opt. Commun.125(1-3) 70,1996; Marian et al., Science, 2004. 5 Jefferts et al., Metrologia 39 (4) 321, 2002 6 Gerginov, et al., in preparation 7 Gerginov, et al., PRA 70, 042505, 2004 Also: - Cs atomic lines within the FLFC spectrum: electric-dipole allowed 6s 2 S 1/2 - 6p 2 P 1/2,3/2 transitions in the near infrared. - 14 nW @ 895 nm and 1.5nW @ 852 nm per component; - Reference to NIST atomic fountain 5 - Measured optical frequencies with a CW laser 6,7 Femtosecond Laser Frequency Comb: Solid-state laser pumped Ti:Sapphire modelocked laser. Time domain: Output consists of femtosecond pulses; Pulses repetition rate 1 GHz 1 ; Frequency domain: 1 GHz spaced discrete frequencies; Less than a Hz linewidth 2 per spectral component; 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, f c. 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 90 0. An acousto-optic modulator is used to stabilize the F-F’Previous 4 (kHz)This work (kHz)Difference (kHz) F3-F3335120562759.7(4.9)335120562753.7(85.0)-6.0 ( 0.1 sigma) F3-F4335121730483.2(5.3)335121730500.8(16.4)17.6 (1 sigma) F4-F3335111370130.2(4.6)335111370146.3(10.5)16.1 (1.4 sigma) F4-F4335112537853.9(4.0)335112537861.7(28.0)7.8 ( 0.3 sigma) F-F’Previous 5 (kHz)This work (kHz)Difference (kHz) F3-F2351730549621.5(5.5)351730549616.3(9.7)-5.2 (0.5 sigma) F3-F3351730700845.9(5.5)351730700766.1(98.5)-79.8(0.8 sigma) F3-F4351730902133.2(5.6)351730902116.9(34.2)-16.3 (0.5 sigma) F4-F3351721508210.5(5.5)351721508195.1(21.7)-15.4 (0.7 sigma) F4-F4351721709496.9(5.5)351721709471.6(167.8)-25.3( 0.2 sigma) F4-F5351721960585.7(5.5)351721960563.5(4.5)-22.2( 3 sigma) Typical data for F=4-F'=4 transition of D 1 line taken in ~6 hours. The previous optical frequency measurements 6 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. Advantages Full knowledge and control of the optical frequencies; Tunability Each component linewidth less than a Hz 1 Disadvantages Low intensity Presence of many spectral components Spectroscopy with a single comb component Highly collimated atomic beam High-denslty narrow divergence atomic beam 10 15 /cm 3 densities <3 mrad divergence corresponding to 2.3(1)MHz Doppler width Optical frequencies of the D 1 line components. Optical frequencies of the D 2 line components 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 f rep =(f opt ±f ceo )/N, where N~300000 and f ceo 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. 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.