1. Two-Photon Absorption Spectroscopy of Rubidium
with a Dual-Comb Technique
Akiko Nishiyamaa,b,c, Satoru Yoshidaa,b, Takuya Harikia, Yoshiaki Nakajimaa,b, Kaoru Minoshimaa,b
a. University of Electro-Communications (UEC),
b. JST, ERATO MINOSHIMA Intelligent Optical Synthesizer (IOS) Project 　　　　　　　　　　　　　　　　　　　　　　　
c. Research Fellow of the Japan Society for the Promotion of Science (JSPS)
This work is supported by JST, ERATO MINOSHIMA Intelligent Optical Synthesizer (IOS) project

2. Dual-comb spectroscopy
Dual-comb is
・Direct comb spectroscopy
・Fourier transform spectroscopy
※I. Codington, et.al., Phys. Rev. Lett. 100, (2008)
Comb
frep
Δfrep
Interferogram
RF spectrum
Optical frequency
frep+Δfrep
Broadband, high precision, high resolution

3. Doppler broadening > 100 MHz
Resolution of dual-comb spectroscopy
Capability : high-resolution of repetition rate (frep) ～ MHz
or comb-mode linewidth ～ 10 kHz
Doppler broadening > 100 MHz
Optical frequency
frep
… Actual resolution is limited by Doppler broadenings

4. A. Nishiyama, et.al., Opt. Express 24, 25894 (2016)
Motivation
To apply the dual-comb spectroscopy to
Doppler-free high-resolution spectroscopy
Previous work ～ Double-resonance spectroscopy Rb
A. Nishiyama, et.al., Opt. Express 24, (2016)
4D5/2
4D3/2
46 MHz
36 MHz
F = 2 3
F =
5S1/2
5P3/2
4D5/2
Narrow cw
Dual-comb
Doppler-free high resolution, but low sensitivity
→ Doppler-free high resolution
→ High sensitivity using the fluorescence detection
This work ～ Two-photon absorption spectroscopy

5. Principle of the two-photon absorption spectroscopy using the dual-comb
A. Hipke, et al., Phys. Rev. A 90, (R) (2014)
nΔfrep = f0
Fluorescence
nfrep
n(frep+Δfrep)
Energy
Transition frequency v0
= f0×frep/Δfrep
Comb １
Comb 2
・Contribution of many comb mode pairs
→Two-photon transitions (nonlinear processes) are observable
・Fluorescence detection
→Background free and high-sensitive measurement are realized

6. PPLN：periodically poled lithium niobate
Experimental setup
2nd harmonic output
Dual-comb system
Power: mW
Δλ: 5 nm
λc: ～778 nm
Power: mW
λc: ～1.55 μm
frep scan
RF stabilized
comb
lock
PPLN
Fiber amplifier
lock
cw laser
lock
High-speed
controllable comb
PPLN
Fiber amplifier
Comb : Er-doped mode-locked fiber laser
frep ≈ 57 MHz, Δfrep = 100 Hz
PPLN：periodically poled lithium niobate

7. Experimental setup <Rb two-photon transitions> Dual-comb system
410 nm
389 nm
(778 nm ×2)
<Rb two-photon transitions>
Dual-comb system
frep scan
RF stabilized
comb
lock
PPLN
Fiber amplifier
lock
cw laser
lock
Digitizer
(14 bit, 256 MB)
87,85Rb
High-speed
controllable comb
PPLN
Fiber amplifier
Filter
Computer
PMT
average
FFT
Low Pass
1.9MHz

8. Doppler-limited two-photon spectroscopy
<Observed interferogram>
Fourier-transform and
conversion to optical frequency axis
＜Observed spectra＞
5D3/2
5D5/2
Averaging：20 s, cell temp: 100 oC
Zoom
・Averaging of the time domain signal
→Detector noise is reduced
・Background free measurement is achieved

9. Doppler-fee two-photon spectroscopy
A. Hipke, et al., CLEO conference paper (2013) CTh5C.8
Counter-propagating pulses cancel the Doppler-shift Rb u
fcomb(1-u/c)
fcomb(1+u/c)
Doppler-free signal
Doppler background
・・・caused by mode pairs in a pulse .
Optical frequency

10. Setup for Doppler-free measurement
To eliminate the Doppler background, pulses are separated in two color parts (>778 nm, <778 nm)
PMT
λ/4
comb 1
comb 2
λ/2
Diffraction Grating DG
Delay

11. <Reduction of background>
Observed Doppler-free spectra
5S1/2 - 5D5/2 transition
<Reduction of background>
87Rb
F”= 1
87Rb
F”= 2
85Rb
F”= 3
85Rb
F”= 2
Zoom
1 mm delay
5 mm
10 mm
The delay of the pulses reduce the Doppler background

12. Hyperfine spectra of Rb (5S1/2-5D5/2)
＜Measurement conditions＞
87Rb, F”= 2
F’ =
Sampling：scan step of frep = 0.03Hz
→scan step at 389 nm = 400 kHz
Laser power: 8 mW
Averaging： 40 sec/point
Total measurement time: 100 min
Cell temperature： 40 oC
F’ =
85Rb, F”= 3
F’ =
85Rb, F”= 2
F’ =
87Rb, F”= 1
FWHM
～5 MHz .
50 MHz
Fully-resolved hyperfine spectra are observed

13. ※CIPM recommended values (2005)
Determination of absolute frequencies
※CIPM recommended values (2005)
5S1/2-5D5/2
Transition
This work
(kHz)
Width
(MHz)
Recommended
value※ (kHz)
Diff
87Rb F”= 2
F’ = 4
4.70 15
F’ = 3
3.87
-39
F’ = 2
F’ = 1
4.07
1.98
-49
-82
85Rb F”= 3
F’ = 5
5.06 98
5.36
153
4.89 65
4.64
226
85Rb F”= 2　
4.88
-22
5.03
4.81
-33
3.62
472
87Rb F”= 1
4.26 59
4.18 21
4.44 40
Deviations from the recommended values
< 100 kHz

14. Summary We demonstrated Doppler-free two-photon dual-comb spectroscopy
High sensitivity using the fluorescence measurement
・Dual-comb averaging scheme reduces detector noise
・Delayed pulse scheme realized Doppler background free
Measurement of the hyperfine spectra of Rb
　Resolution：Linewidth ～ 5 MHz
　Precision ：Difference from CIPM < 100 kHz
Doppler-free two-photon dual-comb spectroscopy is applicable to various gaseous species