Tunable Mid-IR Frequency Comb for Molecular Spectroscopy

Todd Johnson and Scott DiddamsNational Institute of Standards and Technology

Boulder, COtoddj@nist.gov

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