1. Dual-Comb Spectroscopy of C2H2, CH4 and H2O over 1.0 – 1.7 μm
69th International Symposium on Molecular Spectroscopy,
17 June 2014, Champaign-Urbana, the University of Illinois
TJ03
Dual-Comb Spectroscopy of C2H2, CH4 and H2O over 1.0 – 1.7 μm
Keio University a, NMIJ, AIST b,
ERATO Minoshima Intelligent Optical Synthesizer Project, JST c
Kana Iwakunia,b,c, Sho Okubob,c, Hajime Inabab,c, Kazumoto Hosakab,c,
Atsushi Onaeb,c, Hiroyuki Sasadaa,c, Feng-Lei Hongb,c

2. Outline Optical frequency comb as a light source for spectroscopy
Principle of dual-comb spectroscopy
Observed spectra and absolute frequency measurements

3. Optical frequency comb
𝑓 rep = 1 𝑇 rep
: Repetition rate
Fourier transformation
Trep
2 μm
1 μm
n th mode frequency
νn = fceo + n frep

4. - Direct frequency comb spectroscopy -
Application of combs
- Direct frequency comb spectroscopy - ν ν 工夫
A few tens - MHz mode spacing
Over 1 octave
Accurate mode frequency
A number of comb modes
High resolution
Broad bandwidth
Absolute frequency measurement
Rapid data acquisition

5. Principle of dual-comb spectroscopy
I. Coddington et. al , PRL, 100, (2008)
Typical values in our experiment
frep = 50 MHz
1/frep = Trep = 20 ns
Δfrep = 10 Hz
1/Δfrep = 100 ms
ΔTrep= 4 fs LO
Detector

6. Principle of dual-comb spectroscopy - frequency domain -
frep,S
Δfrep
2Δfrep
frep,L
Signal comb ν
LO comb ν
Δfrep = frep,S － frep,L
Δfrep RF f
Δfrep
2Δfrep

7. To observe broad spectrum…
One-to-one correspondence between signal and RF comb modes
frep/2
Observable
spectral bandwidth
< 𝑓 rep 2 2Δ 𝑓 rep ν ν
Aliasing RF f
Δfrep
frep/2

8. To observe broad spectrum…
One-to-one correspondence between signal and RF comb modes
frep/2
Observable
spectral bandwidth
< 𝑓 rep 2 2Δ 𝑓 rep ν ν
Observable spectral bandwidth
Aliasing RF f
Δfrep
frep/2

9. To observe broad spectrum…
One-to-one correspondence between signal and RF comb modes
frep/2
Observable
spectral bandwidth
< 𝑓 rep 2 2Δ 𝑓 rep ν ν
Δfrep must be small.
Observable spectral bandwidth

10. To observe broad spectrum…
One-to-one correspondence between signal and RF comb modes
frep/2
Observable
spectral bandwidth
< 𝑓 rep 2 2Δ 𝑓 rep ν ν
Observable spectral bandwidth
Separate comb modes
Relative linewidth RF
The relative linewidth
of the two combs
< Δfrep f
Δfrep

11. To observe broad spectrum…
One-to-one correspondence between signal and RF comb modes
frep/2
Observable
spectral bandwidth
< 𝑓 rep 2 2Δ 𝑓 rep ν ν
Observable spectral bandwidth
Separate comb modes
Relative linewidth RF
The relative linewidth
of the two combs
< Δfrep f
Δfrep
The relative linewidth must be narrow.

12. Fiber-based optical frequency comb
Y. Nakajima et. al, Opt. Express, 18, 1667 (2010)
K. Iwakuni et. al, Opt. Express, 20, (2012)
LD 1480 nm
EOM
λ/4
λ/2
λ/2
λ/4
PZT
frep = 48 MHz
output
Average power:
a few mW
EDF
Peltier element
Delay line
0.5 MHz

13. Measurement of the relative linewidth
RBW: 1Hz
VBW: 1Hz
Less than 1 Hz
Signal / arb. unit (linear scale)
5 Hz
Relative frequency / Hz

14. Experimental setup Signal comb fceo frep,S = 48.000 007 MHz
12C2H2, 20 Torr
Digitizer
Signal comb
・・・
White cell
fceo
frep,S = MHz
LPF
Reference signals
Δfrep ≈ 7 Hz
Bandwidth
100 MHz
Spectrum μm RF
30 MHz CW
1535 nm
detector
LO comb
λ/2
PBS
PBS
・・・
fceo
frep,LO = MHz

15. Observed interferogram
Free Induction Decay (FID)
12C2H2, 20 Torr, White cell (15cm, 13 round trips ) 2 4 6 8 10
Time / ns
- 40
- 20 20 40
1 / 2B
Time / ns
Δfrep = 9 Hz
Minimum acquisition time to
observe whole spectrum: 110 ms
Average: 50,000 times
Total measurement time: 90 min
0.36
0.37
0.38
0.39
0.4
Time / ns

16. Observed spectrum 12C2H2, 20 Torr, White cell (15cm, 13 round trips )
CH4, 20 Torr, Cell (50cm)
Wavenumber (cm-1)
6, , ,000
15 cm,
13 round trips
CH4, 20 Torr
White cell
50 cm
12C2H2, 20 Torr LO
Detector
Frequency (THz)
Δfrep = 7 Hz
Minimum acquisition time to
observe whole spectrum : 140 ms
Average: 400, 000 times
Total measurement time: 16 h

17. Observed spectrum 12C2H2, 20 Torr, White cell (15cm, 13 round trips )
CH4, 20 Torr, Cell (50cm)
Wavenumber (cm-1)
6, , ,000 10 1
Transmitted power (arb. unit)
0.1
0.01
Δfrep = 7 Hz
Minimum acquisition time to
observe whole spectrum : 140 ms
Average: 400, 000 times
Total measurement time: 16 h
Frequency (THz)

18. Observed spectrum 12C2H2, 20 Torr, White cell (15cm, 13 round trips )
CH4, 20 Torr, Cell (50cm)
Wavenumber (cm-1)
6, , ,000 10 40 1
Transmitted power (arb. unit)
1.67 μm
CH4, 2ν3 30
0.1
Transmitted power (arb. unit) 20 10
0.01
Frequency (THz)

19. Observed spectrum 12C2H2, 20 Torr, White cell (15cm, 13 round trips )
CH4, 20 Torr, Cell (50cm)
Wavenumber (cm-1)
6, , ,000 10 30 1
Transmitted power (arb. unit)
1.53 μm
12C2H2, ν1＋ν3 20
0.1
Transmitted power (arb. unit) 10
0.01
Frequency (THz)

20. Observed spectrum 12C2H2, 20 Torr, White cell (15cm, 13 round trips )
CH4, 20 Torr, Cell (50cm)
Wavenumber (cm-1)
6, , ,000 10 10 1
Transmitted power (arb. unit)
1.46 μm
H2O, 2ν2+ν3 8 6
0.1
Transmitted power (arb. unit) 4 2
0.01
Frequency (THz)

21. Observed spectrum 12C2H2, 20 Torr, White cell (15cm, 13 round trips )
CH4, 20 Torr, Cell (50cm)
Wavenumber (cm-1)
6, , ,000 10 3 2 1 4 5 　
Frequency (THz)
Transmitted power (arb. unit)
12C2H2, 2ν1+ν3
1.03 μm 1
Transmitted power (arb. unit)
0.1
0.01

22. Observed spectrum 12C2H2, 20 Torr, White cell (15cm, 13 round trips )
CH4, 20 Torr, Cell (50cm)
Wavenumber (cm-1)
6, , ,000 10
Transmitted power (arb. unit) 1
0.1
0.01
1.67 μm
CH4
1.53 μm
12C2H2
1.46 μm
H2O
1.03 μm
12C2H2
Transmitted power (arb. unit)

23. S/N ≈ 38 (without fringes)
Sensitivity
12C2H2, 20 Torr, White cell (15cm, 13 round trips )
ν1+ν3 vibration band
10 times avg.
(Measurement time: 1 s)
100 times avg.
(Measurement time: 10 s)
1000 times avg.
(Measurement time: 100 s)
S/N ≈ 5
S/N ≈ 11
S/N ≈ 38 (without fringes)
S/N ≈ 15 (with fringes)
Δfrep = 9 Hz
Minimum acquisition time to
observe whole spectrum: 110 ms
Limited by background fringes

24. Absolute frequency measurement
12C2H2, 20 Torr, White cell (15cm, 13 round trips )
Δfrep = 9 Hz
Minimum acquisition time to
observe whole spectrum: 110 ms
Average: 50,000 times
Total measurement time: 90 min
Wavenumber / cm-1
ν1＋ν3
Transmitted signal
Transmitted signal
P(23)
Phase / rad
Frequency / THz
The discrepancy comes from
the pressure shift and
the residual fringes.
Frequency / THz
Determined frequency from the fitting
( ± 2.6 ) MHz
Previous work (sub-Doppler resolution)
A. Madej et. al, JOSA B, 23, 2200 (2006)
( ± ) MHz

25. Summary Acknowledgments
We developed the dual-comb spectrometer using two combs with
narrow relative linewidth, and simultaneously observed absorption
spectrum of 12C2H2, CH4 and H2O over μm in 140 ms.
The sensitivity is currently limited by the residual fringes.
Acknowledgments
We are grateful to Dr. K. M. T. Yamada for his helps to this research.
This research is financially supported by Grand-in-Aid for Scientific Research (A)
of the Ministry of Education, Culture, Sports, Science and Technology, Japan.

26. Scale the horizontal axis
fRF
νopt
𝑓 rep,S 2
𝑓 0
𝜈 CW + 𝑓 beat,S
Measure the 𝜈 CW with two combs
𝑓 beat,S
Signal
𝑓 0 𝜈
Calculate 𝑓 0 LO 𝜈
𝑓 beat,L
𝜈 CW
𝜈 opt =± 𝑓 rep,S Δ 𝑓 rep 𝑓 RF − 𝑓 0 + 𝜈 CW + 𝑓 beat,S

27. Observed five interferograms
Lab. time / ms
Effective time / ns
effective time / ns
lab. time / ms
Δfrep = 95 Hz
1 shot measurement time: 53 ms
Average: 30 times
Total measurement time: 1.6 s

28. Separation of comb modes 13C2H2 ν1+ ν3 R (12) R (13) R (12) frep,S
500 MHz
Absolute frequency / THz

29. Measurement of the relative linewidth
EOM, PZT, Peltier
RBW: 1Hz
VBW: 1Hz
Signal comb
fbeat
Less than 1 Hz
・・・
fceo
5 Hz
CW laser #1
Function generator 30 MHz
Pump LD
Pump LD
LO comb
PZT
Temp.
CW laser #2
・・・
fceo
EOM, PZT, Peltier

30. In-loop beat signal RBW : 30 kHz VBW : 10 kHz fceo fbeat 700 kHz 4 MHz
CWの周波数は既知

31. Application of combs
Frequency counter
fbeat
Sample gas ν
laser 工夫