Kana Iwakunia,b,c, Sho Okubob,c, Hajime Inabab,c, Kazumoto Hosakab,c, Atsushi Onaeb,c, Hiroyuki Sasadaa,c, Feng-Lei Hongb,c

Keio University a, NMIJ, AIST b, ERATO Minoshima Intelligent Optical Synthesizer Project, JST c

69th International Symposium on Molecular Spectroscopy, 17 June 2014, Champaign-Urbana, the University of Illinois

1

TJ03

Dual-Comb Spectroscopy of C2H2, CH4 and H2O over 1.0 – 1.7 μm

Outline

1. Optical frequency comb as a light source for spectroscopy

2. Principle of dual-comb spectroscopy3. Observed spectra and absolute frequency measurements

2

Optical frequency comb

Fourier transformation

n th mode frequency

νn = fceo + n frep

: Repetition rate

0

3

Trep

𝑓 rep=1𝑇 rep

1 μm2 μm

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

4

Application of combs - Direct frequency comb spectroscopy -

ν ν

frep = 50 MHz 1/frep = Trep = 20 ns Δfrep = 10 Hz 1/Δfrep = 100 ms ΔTrep= 4 fs

Principle of dual-comb spectroscopy

Detector

I. Coddington et. al , PRL, 100, 013902 (2008)

5

Typical values in our experiment

LO

Δfrep

6

RF

Principle of dual-comb spectroscopy- frequency domain -

Signal comb

LO comb

frep,S

frep,LΔfrep 2Δfrep

νν

Δfrep = frep,S － frep,L

f

2Δfrep

Δfrep

0

To observe broad spectrum…

frep/2

One-to-one correspondence between signal and RF comb modes

< Observable spectral bandwidth

7

νν

Δfrep

RFf

frep/20

Aliasing

To observe broad spectrum…

frep/2

One-to-one correspondence between signal and RF comb modes

< Observable spectral bandwidth

8

νν

Δfrep

RFf

frep/20

Aliasing

Observable spectral bandwidth

To observe broad spectrum…

frep/2

One-to-one correspondence between signal and RF comb modes

< Observable spectral bandwidth

Observable spectral bandwidth

νν

9

Δfrep must be small.

To observe broad spectrum…

frep/2

One-to-one correspondence between signal and RF comb modes

<

Separate comb modes

Δfrep

< Δfrep

Observable spectral bandwidth

The relative linewidth of the two combs

10

Observable spectral bandwidth

RF

Relative linewidth

νν

f

To observe broad spectrum…

frep/2

One-to-one correspondence between signal and RF comb modes

ΔfrepThe relative linewidth must be narrow.

11

Observable spectral bandwidth

RF

Relative linewidth

νν

f

<

< Δfrep

Observable spectral bandwidth

The relative linewidth of the two combs

Separate comb modes

Fiber-based optical frequency comb

EDF

frep = 48 MHz

EOM λ/2λ/2λ/4

LD 1480 nm

PZT

Peltier element

λ/4

Y. Nakajima et. al, Opt. Express, 18, 1667 (2010)K. Iwakuni et. al, Opt. Express, 20, 13769 (2012)

Average power: a few mW

0.5 MHz12

Delay line

output

13

RBW: 1HzVBW: 1Hz

Less than 1 Hz

5 Hz

Measurement of the relative linewidth

Sign

al /

arb.

uni

t (lin

ear s

cale

)

Relative frequency / Hz

Experimental setup

・・・

・・・

CW1535 nm

RF30 MHz

Signal comb

LO comb

fceo

fceo

detector

frep,LO = 48.000 000 MHz

frep,S = 48.000 007 MHz

Bandwidth 100 MHz

SpectrumBandwidth1.0-1.7 μm

White cell

Digitizer

LPF

Δfrep ≈ 7 Hz

λ/2

PBS PBS

14

Reference signals

12C2H2, 20 Torr

Observed interferogram

Δfrep = 9 Hz Minimum acquisition time to observe whole spectrum: 110 ms Average: 50,000 times Total measurement time: 90 min

Free Induction Decay (FID)12C2H2, 20 Torr, White cell (15cm, 13 round trips )

Time / ns0 20 40- 20- 40

Time / ns0 2 4 6 8 10

Time / ns0.36 0.37 0.38 0.39 0.4

1 / 2B

16

Observed spectrum

Δfrep = 7 Hz Minimum acquisition time to observe whole spectrum : 140 ms Average: 400, 000 times Total measurement time: 16 h

12C2H2, 20 Torr, White cell (15cm, 13 round trips ) CH4, 20 Torr, Cell (50cm)

6,000 8,000 10,000Wavenumber (cm-1)

180 200 220 240 260 280 300Frequency (THz)

CH4, 20 Torr

White cell50 cm12C2H2, 20 Torr

15 cm, 13 round trips

Detector LO

17

Observed spectrum

Δfrep = 7 Hz Minimum acquisition time to observe whole spectrum : 140 ms Average: 400, 000 times Total measurement time: 16 h

12C2H2, 20 Torr, White cell (15cm, 13 round trips ) CH4, 20 Torr, Cell (50cm)

6,000 8,000 10,000Wavenumber (cm-1)

180 200 220 240 260 280 300Frequency (THz)

Tran

smitt

ed p

ower

(arb

. uni

t)

10

1

0.1

0.01

180 200 220 240 260 280 300

18

Frequency (THz)176 178 180 182 184Tr

ansm

itted

pow

er (a

rb. u

nit)

40

30

20

10

Observed spectrum12C2H2, 20 Torr, White cell (15cm, 13 round trips ) CH4, 20 Torr, Cell (50cm)

0

CH4, 2ν31.67 μm

6,000 8,000 10,000Wavenumber (cm-1)

Tran

smitt

ed p

ower

(arb

. uni

t)

10

1

0.1

0.01

180 200 220 240 260 280 300

19

30

20

10

0

Frequency (THz) 194 196 198 200

Observed spectrum12C2H2, 20 Torr, White cell (15cm, 13 round trips ) CH4, 20 Torr, Cell (50cm)

12C2H2, ν1 ＋ ν3

Tran

smitt

ed p

ower

(arb

. uni

t)

1.53 μm

6,000 8,000 10,000Wavenumber (cm-1)

Tran

smitt

ed p

ower

(arb

. uni

t)

10

1

0.1

0.01

180 200 220 240 260 280 300

20

6

4

2

0

8

10

200 205 210 215 220 225 Frequency (THz)

Observed spectrum12C2H2, 20 Torr, White cell (15cm, 13 round trips ) CH4, 20 Torr, Cell (50cm)

Tran

smitt

ed p

ower

(arb

. uni

t)

1.46 μm H2O, 2ν2+ν3

6,000 8,000 10,000Wavenumber (cm-1)

Tran

smitt

ed p

ower

(arb

. uni

t)

10

1

0.1

0.01

180 200 220 240 260 280 300

21

Tran

smitt

ed p

ower

(arb

. uni

t)

10

1

0.1

0.01

Observed spectrum12C2H2, 20 Torr, White cell (15cm, 13 round trips ) CH4, 20 Torr, Cell (50cm)

3

2

1

4

5

286 287 288 289 　 290 291 292Frequency (THz)

0

Tran

smitt

ed p

ower

(arb

. uni

t)

12C2H2, 2ν1+ν31.03 μm

6,000 8,000 10,000Wavenumber (cm-1)

22Tran

smitt

ed p

ower

(arb

. uni

t)

Observed spectrum12C2H2, 20 Torr, White cell (15cm, 13 round trips ) CH4, 20 Torr, Cell (50cm)

12C2H2 1.46 μm H2O 12C2H21.53 μm 1.03 μmCH41.67 μm

Wavenumber (cm-1)6,000 8,000 10,000

Tran

smitt

ed p

ower

(arb

. uni

t)

10

1

0.1

0.01

Sensitivity

23

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)

ν1+ν3 vibration band12C2H2, 20 Torr, White cell (15cm, 13 round trips )

Δfrep = 9 Hz Minimum acquisition time to observe whole spectrum: 110 ms

Limited by background fringes

Absolute frequency measurement

24

( 194 742 519. 0 ± 2.6 ) MHz

P(23)

Frequency / THz

Tran

smitt

ed s

igna

l

( 194 742 536. 722 9 ± 0.0018 ) MHz

Determined frequency from the fitting

Previous work (sub-Doppler resolution)

The discrepancy comes from the pressure shift and the residual fringes.

A. Madej et. al, JOSA B, 23, 2200 (2006)

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 minν1 ＋ ν3

194 195 196 197 198 Frequency / THz

6500 6550 6600 Wavenumber / cm-1

Tran

smitt

ed s

igna

lPh

ase

/ rad

Summary

AcknowledgmentsWe 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.

25

We developed the dual-comb spectrometer using two combs with narrow relative linewidth, and simultaneously observed absorption spectrum of 12C2H2, CH4 and H2O over 1.0 - 1.7 μm in 140 ms.

The sensitivity is currently limited by the residual fringes.

Scale the horizontal axis𝑓 rep , S

2

fRF

νopt

Measure the with two combs

Calculate

𝜈CW + 𝑓 beat , S

𝑓 0

𝑓 beat ,S

𝜈CW

Signal

LO

𝑓 0 𝜈𝜈𝑓 beat , L

𝜈opt=±𝑓 rep , S

Δ 𝑓 rep( 𝑓 RF − 𝑓 0 )+𝜈CW+ 𝑓 beat , S

26

0 10 20 30 40 50

Lab. time / ms

0 25 50 75 100 125

Effective time / ns51.05 51.0525 51.055 51.0575

effective time / ns

lab. time / ms22.462 22.463 22.464 22.465

Observed five interferograms

Δfrep = 95 Hz 1 shot measurement time: 53 ms Average: 30 times Total measurement time: 1.6 s 27

Separation of comb modes

R (12) R (13)ν1+ ν3

192 194 196 198 200 202 Absolute frequency / THz

frep,SR (12)

500 MHz

13C2H2

28

・・・

・・・fceo

fceo

CW laser #1

Function generator 30 MHz

Pump LD

fbeat

29

Signal comb

LO comb

Pump LD

EOM, PZT, Peltier

EOM, PZT, Peltier

CW laser #2

RBW: 1HzVBW: 1Hz

Less than 1 Hz

5 Hz

PZTTemp.

Measurement of the relative linewidth

700 kHz

In-loop beat signal

4 MHz

RBW : 30 kHzVBW : 10 kHz

fceo fbeat

30

Application of combs

• Frequency counter

fbeat

laser

Sample gas

31

ν