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To take full advantage of the frequency accuracy and resolution of this comb, one must correctly and individually resolve each comb tooth in the final spectrum fr fr+Df Df 3Df 5Df Comb 1 Comb 2 Beat signal Dual-comb
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Tze-Wei Liu Y-C Hsu & Wang-Yau Cheng
Dual Comb Raman Spectroscopy on Cesium Hyperfine Transitions Toward
a Stimulated Raman Spectrum of CF4 Molecule Tze-Wei Liu Y-C Hsu
& Wang-Yau Cheng To take full advantage of the frequency
accuracy and resolution of this comb, one must correctly and
individually resolve each comb tooth in the final spectrum fr fr+Df
Df 3Df 5Df Comb 1 Comb 2 Beat signal Dual-comb Phase-Locking Two
Combs Together
Do not need a full octave Phase lock combs to two cw lasers For
high frequency accuracy, lock cw lasers to cavity Achieves sub-rad
optical coherence fr ~ 100 MHz Esignal 1535 nm 1550 nm Cavity
stabilized Lasers fr+Df ~ 100 MHz + 1 kHz ELO Comparison Scheme 1
by NIST Boulder group:
1.Have to lock two comb lasers 2.Well define all frequencies of
each mode Scheme 2 by Garching group: 1.Do not need to lock any
laser 2. The spectral frequency need to be calabrated Center
wavelength range
810 nm Spectrum bandwidth 35nm Pulse width 25fs Repetition rate ~1
GHz Cavity length 30 cm Operating power >300 mW Stability of
repetition rate 1 mHz (10s sampling time) Stability of comb mode 2
kHz (10s sampling time) The mode-lock lasers are in
free-running
(a) 3-hour measurement of the two rep. rates (b) the difference of
two rep. rates (Dfr) in which the drift of Dfr is almost
unobservable. Compare to other groups
sign 2008 Boulder group 2014 Garching group Our lab Repetition rate
of signal comb kHz 100 MHz MHz Repetition rate of LO comb kHz MHz
Repetition rate difference of two comb 1 kHz 350 Hz 100 kHz Period
difference of two comb plus train ~ 0.1 ps 0.035 ps Period of
interference signal 1 ms 2.86 ms 0.01 ms Maximum resolvable
bandwidth ~ 10 THz ~14.3 THz 5 THz Resolving bandwidth 1 THz 14.5
THz 2.5 THz Measured mode number N ~1104 ~1.105 ~1103 Acquisition
time tacq ~ 400 s 467 s ~ 10 s Signal to noise ratio SNR 35 dB 20
dB ~20 dB T= 1f rep1 1f rep2= f r f rep1f rep2 t acq =4 BW f rep12
Compare to other groups
sign 2008 Boulder group 2014 Garching group Our lab Repetition rate
of signal comb kHz 100 MHz MHz Repetition rate of LO comb kHz MHz
Repetition rate difference of two comb 1 kHz 350 Hz 100 kHz Period
difference of two comb plus train ~ 0.1 ps 0.035 ps Period of
interference signal 1 ms 2.86 ms 0.01 ms Maximum resolvable
bandwidth ~ 10 THz ~14.3 THz 5 THz Resolving bandwidth 1 THz 14.5
THz 2.5 THz Measured mode number N ~1104 ~1.105 ~1103 Acquisition
time tacq ~ 400 s 467 s ~ 10 s Signal to noise ratio SNR 35 dB 20
dB ~20 dB T= 1f rep1 1f rep2= f r f rep1f rep2 t acq =4 BW f rep12
The unique features of our dual-comb system
Narrow comb mode linewidth and high repetition rate Spatial light
modulator was used to vary the band pass ~ 800 nm wavelength Has
the possibility to decide the absolute frequency without the
reference laser How to chose a good frequency
reference of comb laser? Two underlying physics limit the precision
of AMO experiments
1.Power broadening One-photon saturation Good Signal-to-noise
ratio, bad linewidth 2.Light shifts transitions having intermediate
states like two-photon transition Bad Signal-to-noise ratio, narrow
linewidth Cs 62S1/2-82S1/2 two photon absorption Comb laser !!
8S1/2 794 nm 6P3/2 852 nm 6S1/2 How can two frequencies
of two lasers be perfectly coherent?Impassible! 8S1/2 Coherent
two-photon 1. Narrow linewidth 2. Good S/N 794 nm Comb laser !!
6P3/2 One-photon on resonance eliminate light shift 852 nm 6S1/2
I(f) f 822.5nm Mixed with direct and stepwise two-photon
transitions
preliminary results right circular polarization left circular
polarization right circular polarization Doppler free Direct
transition is required to follow M selection rule strictly The
linewidth is similar to what was obtained by direct two-photon
Doppler-free spectroscopy of a CW 822 nm Repetition rate MHz
1.23MHz 1.2 MHz 1.24 MHz 852 nm nm 822 nm nm Future work High
resolution(