1. 69th. International Symposium on Molecular Spectroscopy
June 17, 2014, The University of Illinois at Urbana-Champaign
TJ13
MID-IR SUB-DOPPLER RESOLUTION SPECTROMETER USING AN ENHANCED-CAVITY ABSORPTION CELL COUPLED WITH A WIDE BEAM
M. ABE, K. IWAKUNI, S. OKUBO* and H. SASADA
Department of Physics, Faculty of Science and Technology,
Keio University, Yokohama, Japan.
*Current address: National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST)

2. Outline Motivation Design of an enhanced-cavity absorption cell (ECAC)
Spectroscopy of 12CH4 with a new ECAC
Spectroscopy of 12CH3D and absolute frequency determination of the A1-A2 components
Summary

3. Transit-time broadening
Motivation
Our spectrometer
enhanced-cavity
absorption cell
(ECAC)
pump
Nonlinear crystal
signal
idler
the n3 band of 12CH4
Linewidth of Lamb dip
Q(10)
Pressure broadening
Transit-time broadening
Source width
Natural linewidth ≒
Power broadening

4. Transit-time broadening
Motivation
Our spectrometer
enhanced-cavity
absorption cell
(ECAC)
pump
Nonlinear crystal
signal
idler
the n3 band of 12CH4
Linewidth of Lamb dip
Q(10)
Pressure broadening
Transit-time broadening
Source width
Natural linewidth ≒
Power broadening
light molecules: The transit-time broadening is dominant.
⇒introduce a novel ECAC coupled with a wide beam
⇒The linewidth of the Lamb dip reduces by decreasing power broadening.

5. Transit time in a Gaussian beam
Intensity distribution of a Gaussian beam
Transverse Beam Size v
2w0 I

6. Transit time in a Gaussian beam
Intensity distribution of a Gaussian beam
Transverse Beam Size v
2w0 I
Curved Wavefront

7. Transit time in a Gaussian beam
Intensity distribution of a Gaussian beam
Transverse Beam Size v
2w0 I
Curved Wavefront v k
Doppler effect

8. Transit time in a Gaussian beam
Intensity distribution of a Gaussian beam
Transverse Beam Size v
2w0 I
Curved Wavefront v k
The total broadening due to these effects is constant for any longitudinal position.
Doppler effect
Transit-time broadening:

9. Comparison of ECACs 3.8 mm 1.4 mm 24 cm 38 cm 2 m 7 m
ミラー損失を0.3%と同じにしているため

10. Experimental setup Feedback Signal Intensity Triangle Frequency
Lamb dip
Intensity
Triangle
Frequency
1.06 mm
Sample gas
InSb
detector
Nd:YAG Laser
Linewidth: a few kHz
PZT
3.4 mm
Waveguide Periodically poled lithium niobate (PPLN)
enhanced-cavity
absorption cell
(ECAC) FA
12 T=LandC band
EOM
Extended cavity laser diode（ECLD）
Linewidth< 500 kHz
1.5 mm
Feedback Signal 10 10

11. Transmission spectrum of ECAC
FSR: 400 MHz
TEM00 ＋freqency modulation
520 kHz
sideband
sideband
finesse = 770

12. Observed spectrum
the Q(12) transition of 12CH4 in the n3 band

13. Mid-IR frequency control
Er-OFC
frep = 67 MHz n
PZT
pump (1.06 mm)
Nd:YAG
idler (3.4 mm)
PPLN
ECAC FA
signal (1.5 mm)
EOM
Feedback Signal
ECLD

14. Mid-IR frequency control
Er-OFC
frep = 67 MHz n
PZT
pump (1.06 mm)
Nd:YAG
idler (3.4 mm)
PPLN
ECAC FA
signal (1.5 mm)
EOM
Feedback Signal
ECLD

15. Mid-IR frequency control
Synthesizer
dsignal
Er-OFC
frep = 67 MHz n
PZT
pump (1.06 mm)
Nd:YAG
idler (3.4 mm)
PPLN
ECAC FA
signal (1.5 mm)
EOM
Feedback Signal
ECLD

16. A1-A2 splittings of 12CH3D The transition frequency list the n1 band
Pressure: 0.1 – 0.4 Pa
linewidth ~ 300 kHz
Q(J = 3, K = 3)
Q(J = 4, K = 3)
Q(J = 5, K = 3)
D = 0.29 MHz
D = 0.92 MHz
D = 3.84 MHz
The transition frequency list
Trans.
Comp.
n (MHz)
the n1 band
R (3, 3) A1
(0.09)
Q (3, 3) A2
(0.06)
(0.14)
(0.05)
the n4 band
Q (4, 3)
(0.10)
pP (4, 3)
( - )
(0.12)
( - )
Q (5, 3)
( - )
pP (3, 3)
( - )
( - )
( - )

17. Summary
We have introduced a novel enhanced-cavity absorption cell to reduce the transit-time broadening. It enables us to decrease the input power and the sample pressure and eventually to observe narrow Lamb dips.
We have determined the absolute frequencies of six A1-A2 pairs of 12CH3D using the optical frequency comb with a relative uncertainty of 10–9.

18. Thank you for your attention.
Summary
We have introduced a novel enhanced-cavity absorption cell to reduce the transit-time broadening. It enables us to decrease the input power and the sample pressure and eventually to observe narrow Lamb dips.
We have determined the absolute frequencies of six A1-A2 pairs of 12CH3D using the optical frequency comb with a relative uncertainty of 10–9.
Thank you for your attention.

19. Appendix

20. CH3D A1-A2 splitting E K-l=2 K-l=3 K-l=4 Vibrational mode CH4 CH3D J=5
Vibrational excited state Td
C3v
J=4
n1 (DK=0)
non-degenerate
J=3
J=2 n3
Q(J=3,K=3)
n4 (DK=±1)
Doubly degenerate
Triply degenerate
J=5
Vibrational ground state
J=4
J=3 E
J=2 A1
K=2
K=3
K=4 A2

21. Tunable range R(8) P(12) CH4 ECLD (signal) Fiber Amp. (signal) PPLN
DFG (idler) = Nd:YAG laser (pump) − tunable 1.5 mm ECLD (signal)
mid-IR Freq.
/ THz 85 90 95
R(8)
P(12)
Phys. Rev. 48, 864 (1935)
CH4
ECLD (signal)
ECLD tunable range~30THz
Fiber Amp. (signal)
C-Band
L-Band
PPLN
Wavenumber
/ cm-1
2900
3000
3100

22. previous enhanced-cavity absorption cell
bellows
PZT
Mirrors are
optical windows.
mirror separation (FSR)
reflectivity
(transmittance)
finesse (FWHM)
23.6 cm (636 MHz)
99.0% (0.7%)
300 (2.1 MHz)
The optical cavity is also used as an absorption cell. This picture shows an enhanced cavity absorption cell. A pair of mirrors are also optical windows to seal the sample gas. The mirror separation is 23.6 cm corresponding to the free spectral range of 636 MHz. The mirrors have the reflectivity of 99.0% and the transmittance of 0,7 %. The absorption length effectively increases 200 times. Because the field strength is enhanced 17 times at the anti nodes, the sub-Doppler spectral lines are observed easily.
effective absorption length
sensitivity
optical field strength
at antinodes
×198
×139
×17
From Sasada presentation
in International Symposium on Molecular Spectroscopy 67th Meeting

23. DFG frequency measurement using a frequency comb
Er fiber comb
rep. freq.: 65 MHz ~ 1.57 mm
A nonlinear fiber broadens comb spectrum.
1.0 ~ 2.0 mm
We measure the DFG frequency using a frequency comb. This picture shows an Er fiber comb made by Kana Iwakuni for this measurement. The repetition rate is 65 MHz, and the comb spectrum is spread between and 1.57 micrometer. It is broadened further using a nonlinear fiber. Then the comb spectrum covers 1 to 2 micrometer.
From Sasada presentation
in International Symposium on Molecular Spectroscopy 67th Meeting