Transcript Femtosecond Laser Frequency Comb

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
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;
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.
Highly collimated atomic beam
Spectroscopy with a single
comb component
High-denslty narrow divergence atomic beam
1015/cm3 densities
<3 mrad divergence corresponding to 2.3(1)MHz Doppler width
Advantages
Disadvantages
Full knowledge and control of the optical frequencies;
Tunability
Each component linewidth less than a Hz1
Low intensity
Presence of many spectral components
Also:
Optical frequency measurements
- 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;
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
- Reference to NIST atomic fountain5
- Measured optical frequencies with a CW laser6,7
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.
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.
F-F’
Previous4 (kHz)
This work (kHz)
Difference (kHz)
F3-F3 335120562759.7(4.9)
335120562753.7(85.0)
-6.0 ( 0.1 sigma)
F3-F4 335121730483.2(5.3)
335121730500.8(16.4)
17.6 (1 sigma)
F4-F3
335111370130.2(4.6)
335111370146.3(10.5)
16.1 (1.4 sigma)
F4-F4 335112537853.9(4.0)
335112537861.7(28.0)
7.8 ( 0.3 sigma)
REFERENCES
Optical frequencies of the D2 line components
F-F’
Previous5 (kHz)
This work (kHz)
1Bartels
Difference (kHz)
F3-F2 351730549621.5(5.5) 351730549616.3(9.7)
-5.2 (0.5 sigma)
F3-F3 351730700845.9(5.5) 351730700766.1(98.5)
-79.8(0.8 sigma)
F3-F4 351730902133.2(5.6) 351730902116.9(34.2)
-16.3 (0.5 sigma)
F4-F3 351721508210.5(5.5) 351721508195.1(21.7)
-15.4 (0.7 sigma)
F4-F4 351721709496.9(5.5) 351721709471.6(167.8)
-25.3( 0.2 sigma)
F4-F5 351721960585.7(5.5) 351721960563.5(4.5)
-22.2( 3 sigma)
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., Metrologia 39 (4) 321, 2002
6Gerginov, et al., in preparation
7Gerginov, et al., PRA 70, 042505, 2004
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.