1. Tunable Mid-IR Frequency Comb for Molecular Spectroscopy
Todd Johnson and Scott Diddams
National Institute of Standards and Technology
Boulder, CO

2. Frequency comb T τ frep=1/T BW=1/τ pulse period repetition rate
laser source
pulse period T τ
frequency
Frequency comb modes
repetition rate
frep=1/T
(separation between modes)
bandwidth of spectrum
BW=1/τ
(range of spectral coverage)

3. Frequency combs for gas detection
Comb modes
Transmitted comb modes
Molecular absorption profile
detection
molecular
sample
frequency comb
laser source
Frequency comb can provide the advantages:
Broad spectral coverage with high brightness
High frequency precision on each comb tooth
Strong molecular absorptions in mid-infrared (3-5 microns)
Requires frequency comb source in mid-IR
Requires broad detection over large comb range
Thorpe, Ye, et al, Science 311, 1595 (2006)
Diddams, Hollberg, Mbele, Nature 445, 627 (2007)
Gohle, Hänsch, et al, Phys. Rev. Lett. 99, (2007)
Coddington, Swann, Newbury, Phys. Rev. Lett. 100, (2008)
Bernhardt, Picque, et al, Nat. Phot. 4, 55 (2010)

4. Mid-infrared frequency comb
Use difference frequency generation
Yb femtosecond
laser1 and amplifier
microstructured fiber
PPLN
100 MHz repetition rate
125 fs pulse duration
~2.5 W average power
Nonlinear broadening
Raman shifted soliton2
10-20mW average power
1X. Zhou, et al, Opt. Exp. 16, 7055 (2008)
2Knox, (2001); Molenauer, (1986)

5. Mid-infrared frequency comb
Use difference frequency generation
Yb femtosecond
laser1 and amplifier
microstructured fiber
PPLN
100 MHz repetition rate
125 fs pulse duration
~2.5 W average power
Nonlinear broadening
Raman shifted soliton2
10-20mW average power
1X. Zhou, et al, Opt. Exp. 16, 7055 (2008)
2Knox, (2001); Molenauer, (1986)
increasing input power

6. Mid-infrared frequency comb
methane absorption, 40 torr, 11.7cm
Use difference frequency generation
wavelength[nm]
1.0
0.8
0.6
0.4
0.2
0.0
(relative power)
Yb femtosecond
laser1 and amplifier
microstructured fiber
PPLN
gas
cell
Up to 40mW of MIR light
DFG results in comb with
zero offset frequency
100 MHz repetition rate
125 fs pulse duration
~2.5 W average power
Nonlinear broadening
Raman shifted soliton2
10-20mW average power
1X. Zhou, et al, Opt. Exp. 16, 7055 (2008)
2Knox, (2001); Molenauer, (1986)

7. Virtually Imaged Phased Array (VIPA)
grating dispersion
VIPA
dispersion
(CCD image plane)
CCD
VIPA
grating
VIPA free
spectral range
increasing frequency axis
VIPA has large vertical dispersion
Grating has smaller horizontal dispersion
2-D spectrum imaged onto a camera
CCD camera image can be reconstructed into frequency scale
Direct 2-D imaging at 3µm would require a mid-IR camera
and development of VIPA with IR substrates and coatings
M. Shirasaki, Opt. Lett. 21, 366 (1996)
S. Xiao, A. Weiner, Opt. Exp. 12, 2895 (2004)
S. A. Diddams,et al, Nature 445, 627 (2007)

8. Upconversion of mid-IR comb
1 and 1.5 μm
Mid-IR
805nm
Yb fiber
femtosecond
laser
PPLN #1
PPLN #2
500mW
Nd:YAG
1064nm CW
single mode
fiber
VIPA 2-D
spectrometer
gas
cell
Sum frequency generation in second PPLN
100nW of ~805nm upconverted light
Single mode fiber transfer to VIPA spectrometer
Silicon CCD for 2-D VIPA imaging
E. J. Heilweil, Opt. Lett. 14, 551 (1989)
K. J. Kubarych, et al, Opt. Lett. 30, 1228 (2005)

9. VIPA image of upconverted mid-IR comb
1064nm
CW YAG
VIPA 2-D
spectrometer
Yb fiber
laser
gas cell
vacuum
in cell
subtracted
signal
809nm wavelength nm
(3374nm MIR wavelength nm)
~1.4GHz resolution
10 torr
methane
in cell,
11.7cm path
10 GHz
Ti:Saph
comb
calibration

10. Reconstructed MIR methane spectrum
blue – measurement
red - HITRAN
1 torr methane, 11.7cm path length
2 second camera exposures
~30 second pump cycle
Compare to HITRAN database and
- account for pressure, Doppler broadening
- account for optical blur profile (resolution)
- adjust global offset of frequency scale

11. Avenues to increased sensitivity
Yb fiber
femtosecond
laser
PPLN #1
PPLN #2
single mode
fiber
VIPA 2-D
spectrometer
multipass
gas cell
gas
cell
500mW
Nd:YAG
1064nm CW
Increase absorption path length (higher losses)
Shorter PPLN #2 for wider conversion bandwidth (lower efficiency)
Temperature stabilization of VIPA spectrometer
Increase 1064nm power for upconversion

12. Avenues to increased sensitivity
1Torr methane
11.7cm single pass cell
3.4mm length upconversion PPLN
10mTorr methane
210m multipass cell
(238 passes lead to 98.5% MIR loss)
2mm length upconversion PPLN
(trade higher bandwidth for lower efficiency)
Oscillations likely due to temperature
fluctuations in VIPA imaging

13. Sensitivity 1 torr methane L=11.7cm path length
T=30 second pump cycle (2 s exposures)
SNR=70 (~1.5% intensity noise)
M=1500 (spectral elements, span/resolution)
10 mtorr methane
L=210m path length
T=30 second pump cycle (5 s exposures)
SNR=20 (~5% intensity noise)
M=2500 (spectral elements, span/resolution)

14. Sensitivity 1 torr methane L=11.7cm path length
T=30 second pump cycle (2 s exposures)
SNR=70 (~1.5% intensity noise)
M=1500 (spectral elements, span/resolution)
10 mtorr methane
L=210m path length
T=30 second pump cycle (5 s exposures)
SNR=20 (~5% intensity noise)
M=2500 (spectral elements, span/resolution)

15. Conclusions and Acknowledgements
Mid-IR comb spanning ~5 THz,
tunable from μm, up to 40mW
VIPA imaging of upconverted signal can be
reconstructed into gas absorption spectrum
Esther Baumann, Nate Newbury, Lora Nugent-Glandorf, Alex Zolot (NIST)
Dirk Richter (NCAR)
Masaaki Hirano (Sumitomo Electric Industries)
Yohei Kobayashi (ISSP, University of Tokyo)
Ingmar Hartl (IMRA)
National Institute of Standards and Technology
Department of Homeland Security
National Research Council

17. VIPA with absolute frequency calibration#4
Yb fiber
femtosecond
laser
PPLN
500mW
Nd:YAG
1064nm CW
single mode
fiber
VIPA 2-D
spectrometer #1 #2
#1 Lock Yb repetition rate (already done)
#2 Stabilize Nd:YAG to known frequency
#3 Add ~805nm stabilized CW light to VIPA
#4 For resolved MIR comb modes,
add filter cavity after second PPLN #3