Tunable Mid-IR Frequency Comb for Molecular Spectroscopy
Todd Johnson and Scott DiddamsNational Institute of Standards and Technology
Boulder, [email protected]
bandwidth of spectrum
BW=1/τ(range of spectral coverage)
repetition rate
frep=1/T(separation between modes)
pulse periodT τ
Frequency comb
frequency
Frequency comb modes
frequency comb laser source
Frequency combs for gas detection
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
frequency
Comb modes Transmitted comb modesMolecular absorption profile
detectionmolecularsample
frequency comb laser source
Thorpe, Ye, et al, Science 311, 1595 (2006)Diddams, Hollberg, Mbele, Nature 445, 627 (2007)Gohle, Hänsch, et al, Phys. Rev. Lett. 99, 263902 (2007)Coddington, Swann, Newbury, Phys. Rev. Lett. 100, 013902 (2008)Bernhardt, Picque, et al, Nat. Phot. 4, 55 (2010)
Yb femtosecond laser1 and amplifier
microstructured fiber
PPLN
Use difference frequency generation
• 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)
Mid-infrared frequency comb
Yb femtosecond laser1 and amplifier
microstructured fiber
PPLN
Use difference frequency generation
• 100 MHz repetition rate• 125 fs pulse duration• ~2.5 W average power
increasing input power1X. Zhou, et al, Opt. Exp. 16, 7055 (2008)2Knox, (2001); Molenauer, (1986)
Mid-infrared frequency comb
• Nonlinear broadening• Raman shifted soliton2
• 10-20mW average power
Mid-infrared frequency comb
Yb femtosecond laser1 and amplifier
microstructured fiber
PPLN
• 100 MHz repetition rate• 125 fs pulse duration• ~2.5 W average power
• Up to 40mW of MIR light• DFG results in comb with zero offset frequency
gascell
Use difference frequency generation
3100 3200 3300 3400 3500 3600wavelength[nm]
1.0
0.8
0.6
0.4
0.2
0.0
(re
lativ
e p
ow
er)
methane absorption, 40 torr, 11.7cm
1X. Zhou, et al, Opt. Exp. 16, 7055 (2008)2Knox, (2001); Molenauer, (1986)
• Nonlinear broadening• Raman shifted soliton2
• 10-20mW average power
Virtually Imaged Phased Array (VIPA)
• 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
increasing frequency axis
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)
VIPA freespectral range
grating dispersion
VIPAdispersion
(CCD image plane)
CCD
VIPA grating
Direct 2-D imaging at 3µm would require a mid-IR cameraand development of VIPA with IR substrates and coatings
Upconversion of mid-IR comb
• 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
Yb fiber femtosecond
laser PPLN #1 PPLN #2
500mWNd:YAG
1064nm CW
single mode fiber
VIPA 2-Dspectrometer
E. J. Heilweil, Opt. Lett. 14, 551 (1989)K. J. Kubarych, et al, Opt. Lett. 30, 1228 (2005)
gascell
1 and 1.5 μm Mid-IR 805nm
VIPA image of upconverted mid-IR comb
1064nm CW YAG
VIPA 2-Dspectrometer
Yb fiberlaser
10 torrmethanein cell,
11.7cm path
subtractedsignal
10 GHzTi:Saphcomb
calibration
vacuumin cell
809nm wavelength 804nm(3374nm MIR wavelength 3288nm)
~1.4GHz resolution
gas cell
Reconstructed MIR methane spectrum
1 torr methane, 11.7cm path length2 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
blue – measurementred - HITRAN
gascell
multipassgas cell
Avenues to increased sensitivity
• 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
Yb fiber femtosecond
laser PPLN #1 PPLN #2
single mode fiber
VIPA 2-Dspectrometer
500mWNd:YAG
1064nm CW
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 temperaturefluctuations in VIPA imaging
Avenues to increased sensitivity
Sensitivity1 torr methaneL=11.7cm path lengthT=30 second pump cycle (2 s exposures)SNR=70 (~1.5% intensity noise)M=1500 (spectral elements, span/resolution)
10 mtorr methaneL=210m path lengthT=30 second pump cycle (5 s exposures)SNR=20 (~5% intensity noise)M=2500 (spectral elements, span/resolution)
Sensitivity1 torr methaneL=11.7cm path lengthT=30 second pump cycle (2 s exposures)SNR=70 (~1.5% intensity noise)M=1500 (spectral elements, span/resolution)
1/2141/2
Hzcm101.7~M
T
SNRL
1NEA
10 mtorr methaneL=210m path lengthT=30 second pump cycle (5 s exposures)SNR=20 (~5% intensity noise)M=2500 (spectral elements, span/resolution)
1/2171/2
Hzcm102.6~M
T
SNRL
1NEA
Conclusions and Acknowledgements
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 TechnologyDepartment of Homeland SecurityNational Research Council
• Mid-IR comb spanning ~5 THz, tunable from 2.7-4.7μm, up to 40mW
• VIPA imaging of upconverted signal can be reconstructed into gas absorption spectrum
VIPA with absolute frequency calibration
Yb fiber femtosecond
laser PPLN PPLN
500mWNd:YAG
1064nm CW
single mode fiber
VIPA 2-Dspectrometer
#1 Lock Yb repetition rate (already done)#2 Stabilize Nd:YAG to known frequency#3 Add ~805nm stabilized CW light to VIPA
#2
#3
#1
#4 For resolved MIR comb modes, add filter cavity after second PPLN
#4