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

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)

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, 263902 (2007) Coddington, Swann, Newbury, Phys. Rev. Lett. 100, 013902 (2008) Bernhardt, Picque, et al, Nat. Phot. 4, 55 (2010)

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)

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

Mid-infrared frequency comb methane absorption, 40 torr, 11.7cm Use difference frequency generation 3100 3200 3300 3400 3500 3600 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)

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)

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)

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 804nm (3374nm MIR wavelength 3288nm) ~1.4GHz resolution 10 torr methane in cell, 11.7cm path 10 GHz Ti:Saph comb calibration

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

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

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

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)

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)

Conclusions and Acknowledgements 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 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

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