Results

1
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 Freque
ncy 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 TiSapphire modelocked
laser. Time domain Output consists of fe
mtosecond pulses Pulses repetition rate 1 G
Hz1 Frequency domain 1 GHz spaced discre
te frequencies Less than a Hz linewidth2 pe
r spectral component
Highly collimated atomic beam High-denslty narrow
divergence atomic beam 1015/cm3 densities ad 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 _at_ 895 nm and 1.5nW _at_ 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 com
ponent used to probe the atomic transition is
locked to this transition, the repetition rate of
the comb becomes frep(foptfceo)/N, where
N300000 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 F4-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 spec
troscopy 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 femtoseco
nd 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) 183
9, 2002 2Bartels et al., Opt. Lett. 29(10) 1081,2
004 3Diels and Rudolph, "Ultrashort Laser Pulse P
henomena", Academic Press 1996.
4Shaden et al., Opt. Commun.125(1-3) 70,1996
Marian et al., Science, 2004. 5Jefferts et al., M
etrologia 39 (4) 321, 2002 6Gerginov, et al., in
preparation 7Gerginov, et al., PRA 70, 042505, 20
04
Optical frequencies of the D2 line components