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 100 016 kHz 100 MHz 989.25 MHz Repetition rate of LO comb 100 017 kHz 989.15 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 ~1×104 ~1.×105 ~1×103 Acquisition time tacq ~ 400 μs 467 μs ~ 10 μs Signal to noise ratio SNR 35 dB 20 dB ~20 dB ΔT= 1 f rep1 − 1 f rep2 = Δ f r f rep1 f rep2 t acq =4 Δν BW f rep1 −2

Compare to other groups   sign 2008 Boulder group 2014 Garching group Our lab Repetition rate of signal comb 100 016 kHz 100 MHz 989.25 MHz Repetition rate of LO comb 100 017 kHz 989.15 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 ~1×104 ~1.×105 ~1×103 Acquisition time tacq ~ 400 μs 467 μs ~ 10 μs Signal to noise ratio SNR 35 dB 20 dB ~20 dB ΔT= 1 f rep1 − 1 f rep2 = Δ f r f rep1 f rep2 t acq =4 Δν BW f rep1 −2

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 laser @ 822 nm Repetition rate 988.225425 MHz 1.23MHz 1.2 MHz 1.24 MHz 852 nm + 794 nm 822 nm + 822 nm

Future work High resolution(<100 kHz) CF4 stimulated Raman spectrum by dual comb system 1.Why using Raman spectrum? Nondestructive measurement Avoid to build an IR frequency comb 2.Why do we choose CF4? High symmetry Strong Raman signal at 909, 435, 631 cm-1

RF domain

Thank you for your listening

70 MHz

Not perfect Doppler free two photon transition The TPT is very easy to reduce the first order Doppler effect by using a Doppler-free scheme. This scheme is very simple that one just need a mirror to reflect the laser. As we know the Doppler effect is due to the thermal velocities of the atom in vapor. The frequency shift of this atom whose velocity is V and toward the input beam is equal to .And to the reflected beam. So that the Velocity effect is canceled while the laser frequency is on resonance. So that all the atoms could absorb two photons on resonance frequency. It will contribute to a very sharp Loretzain lineshape. A Gaussian curve of small intensity and broad width corresponding to the absorption of two photons from the same direction beams. In the case of S-S TPT, we can use a circular polarized light to eliminate this Doppler background.

(𝐧 𝒇 𝟏 + 𝜹 𝟏 − 𝒇 𝐜𝐰𝟏 ) n 𝒇 𝟏 − 𝒇 𝟐 +( 𝜹 𝟏 −𝜹 𝟐 ) (n 𝒇 𝟐 + 𝜹 𝟐 − 𝒇 𝐜𝐰𝟏 ) (𝐦 𝒇 𝟏 + 𝜹 𝟏 − 𝒇 𝐜𝐰𝟐 ) m 𝒇 𝟏 − 𝒇 𝟐 +( 𝜹 𝟏 −𝜹 𝟐 ) (m 𝒇 𝟐 + 𝜹 𝟐 − 𝒇 𝐜𝐰𝟐 ) (3n-2m) 𝒇 𝟏 − 𝒇 𝟐 +( 𝜹 𝟏 −𝜹 𝟐 ) 10(n-m) 𝒇 𝟏 − 𝒇 𝟐

Offset frequency tuning range:

Rayleigh and Raman Scattering

Experiment setup

Set up experiment 794 nm stimulated Raman 852 nm stimulated Raman

RF domain