Aleksandra FoltynowiczWeiguang MaOve Axner

International Symposium on Molecular SpectroscopyOSU, Columbus, Ohio

June 23, 2009

Laser Physics Group Department of PhysicsUmeå UniversityUmeå, Sweden

Fiber-laser-based NICE-OHMSfor Trace Species Detection

Noise-Immune

Optical Heterodyne Molecular Spectrometry

Cavity-Enhanced

• resonant cavity• increased interaction length• enhanced power

• high sensitivity • detectability 10-10 – 10-13 cm-1

• close to shot-noise-limited performance

• weak molecular overtone transitions

• trace gas detection

• frequency modulation for noise reduction

?

Principles of NICE-OHMS

c mi t tfm t E e

2 sin 20

1 ˆ2

E ε

Principles of NICE-OHMS Frequency Modulation

RF signal (MHz)

c m c mci t i ti t

fm t E J e J e J e2 220 1 0 1

1 ˆ2

E ε

electro-optic modulator

modulation index

ci tin t E e 2

01 ˆ2

E ε

m

FM triplet

Laser EOM

m

Absorber PD

m

c

BGI c E I20 0 0

12

A m mI t I t t0 1 0 1 1 12 sin 2 cos 22

• with analyte – signal at vm

Phaseshifter

LP

FM signal

low passfilter

out-of-phaseabsorption

in-phasedispersion

II

6

min

10

• no analyte – constant intensitydouble balanced

mixer

FSR2 c

cFSR

nL

2

cFSR

F2

• free spectral range

• cavity mode width

Principles of NICE-OHMS Cavity Enhancement – Fabry-Perot Resonator

.R0 9995

L0.5m MHzFSR 300

kHzc 25

F 6300

kmeffL 2

c incP P2000

c incF

P P

• intracavity power

effF

L L

2

• effective length

RF

R

1

• finesse

1 R T

incP

rP

tP

L

cP

CavityEOM

PBS

PD

4/

Laserfrequency

controlservo

Ph

DBM

pdh

LP

7

min

10II

Laser

Pound-Drever-Halllaser stabilization technique

Principles of NICE-OHMS

Cavity-Enhanced Frequency Modulation Spectroscopy

alternative name:

NoiseImmunit

y

m FSR

Noise-Immune

Optical Heterodyne Molecular Spectroscopy

Cavity-Enhanced13

min

5 10II

absorption dispersion

EDFL1531 nm

PBSlens / 4/ 2

fiberpolarizer

PD2

fiber EOMCavity

with Absorber

OI

polar.

PD1

OI

Laserfrequency

control

Phase

20MHz

DBM wm-NICE-OHMSsignal

Lock-in

PD – photodetectorPBS – polarizing beamsplitterOI – optical isolator

DBM – double balanced mixer LP – low pass filterBP – bandpass filter

360 MHz

FSRcontrol

Phase

BP

380MHz

DBM

DBM

Experimental Setup

fm-NICE-OHMSsignal

Gain

LP

Phase

Scan

DBM

F. M. Schmidt, A. Foltynowicz, W. Ma, and O. Axner, J. Opt. Soc. Am. B 24 (2007).F. M. Schmidt, A. Foltynowicz, W. Ma, T.Lock, and O. Axner, Opt. Express 15 (2007).

Extremely narrow free-running linewidth (1 kHz/120 µs)

Fast tuning (PZT stretching the fiber) with bandwidth up to 100 kHz

Low bandwidth and simple transfer function of the locking servo

Gaussian beam – easy mode-matching to the cavity

Compact setup – short free-space optical path

Working range (our laser) 1530.8 – 1531.8 nm, detection of C2H2, NH3, N2O, CH2O, CO2, CH4

DFB-laser pumped erbium doped fiber laser

Limited fast (PZT) tuning range (ca 3 GHz)

Limited total (temperature) tuning range (1 nm)

Presently available in three wavelength ranges (1030–1121, 1525–1585 and 1710–2100 nm)

PZT resonances at kHz frequencies, limiting the bandwidth of locking servo

Experimental Setup Fiber Laser

Wide working frequency range (30 kHz – 10 GHz) – one modulator sufficient to create sidebands at both frequencies needed in the experiment

Low half-wave voltage (ca 6 V) – low RF input power, less electronic pick-up

Smaller and less expensive than free-space EOMs

No optical alignment needed

LiNbO3 phase modulator

Experimental Setup Fiber-coupled EOM, Cavity

Cavity• Zerodur spacer

• Two ring-shaped piezo actuators

• Finesse 4800, 5700

• Length 40 cm

dispersion

C2H2

absorption

C2H2

NICE-OHMS Detection Modes

Doppler-broadened

fm-NICE-OHMS wm-NICE-OHMS

sub-Doppler

dispersion

CO2

dispersion

CO2

absorption

CO2

absorption

CO2

wm

C2H2

fm

C2H2

Doppler-broadened SignalsLineshapes

fm-NICE-OHMS

absorption dispersion

wm-NICE-OHMS

• Standard FM nomenclature and Fourier-series-based WM theory

• Detection at arbitrary phase

F. M. Schmidt, A. Foltynowicz, W. Ma, and O. Axner, J. Opt. Soc. Am. B 24 (2007).A. Foltynowicz, W. Ma, F. M. Schmidt, and O. Axner, J. Opt. Soc. Am. B 26 (2009).

1000 ppm of C2H2 at 130 mTorr of N2

Doppler-broadened SignalsSignal Strength

0 1 0 002

( ) ( )fm no crel

FS J J P c L

• Independent of FM detection phase

• Linear with pressure/concentrationfor small absorption

<<1

F. M. Schmidt, A. Foltynowicz, W. Ma, and O. Axner, J. Opt. Soc. Am. B 24 (2007).W. Ma, A. Foltynowicz, and O. Axner, J. Opt. Soc. Am. B 25 (2008).

A. Foltynowicz, W. Ma, F. M. Schmidt, and O. Axner, J. Opt. Soc. Am. B 25 (2008).

• Saturation power

1000 ppm of C2H2 at 10 mTorr of N2

Influence of optical saturation

• absorption signal reduced• dispersion signal unaffected

Sub-Doppler SignalsLineshapes

A. Foltynowicz, W. Ma, and O. Axner, Opt. Express 16 (2008).

4.1 W

0.49 W

• On-resonance dispersion signal

• Lorentzian up to high degrees of saturation

fm-NICE-OHMS

wm-NICE-OHMS

708.4

3.70.4

0.190.02

708.4

3.70.4

0.190.02

10 µTorr of C2H2

Sub-Doppler SignalsSignal Strength

O. Axner, W. Ma, and A. Foltynowicz, J. Opt. Soc. Am. B 25 (2008).A. Foltynowicz, W. Ma, and O. Axner, Opt. Express 16 (2008).

Axner et al.

Ma, Hall et al.

Sub-Doppler optical phase shift

• high degrees of saturation

• revised expression

• not related to attenuation by the Kramers-Kronig relations

Signal strength

• pressure dependence

• concentration dependence

finesse 4800

FSR380 MHz

cavity length 40 cm

effective length

1.2 km

intracavity power

< 4.5 W

gaswavelengt

h[nm]

transition strength

[cm-1/(molec cm-

2)]

C2H

2

1531.59 1.2 10-20

CO2 1531.19 8.4 10-26

Sensitivity

Allan variance25 ppm of C2H2 in 20 mTorr of N2

fm-NICE-OHMS signal6 10-9 cm-1 of pure CO2

cavity parameters

transitions parameters

minimum detectable absorption

8 10-11 cm-1 (0.7 s)

3.5 nTorr46 ppt

F. M. Schmidt, A. Foltynowicz, W. Ma, T. Lock and O. Axner, Opt. Express 15 (2007).A. Foltynowicz, W. Ma, and O. Axner, Opt. Express 16 (2008).

Doppler-broadened sub-Doppler

minimum detectable sub-Doppler phase

shift 5.7 10-11 cm-1 Hz-1/2

0.8 nTorr @ 20 mTorr40 ppb

Detection limit for C2H2

First realization of fiber-laser-based NICE-OHMS

• easily stabilized laser

• fiber-coupled components

• compact setup

• a step towards practical trace species detection

Theoretical description of signal shape and strength

• Doppler-broadened NICE-OHMS

• sub-Doppler NICE-OHMS

• influence of optical saturation

High sensitivity

• Doppler-broadened absorption 8 10-11 cm-1 (46 ppt of C2H2 )

• sub-Doppler optical phase shift 5.7 10-11 cm-1 Hz-1/2 (0.8 nTorr/110-12 atm of C2H2)

Dynamic range 105 - 106

Summary

F. M. Schmidt, A. Foltynowicz, W. Ma, and O. Axner: Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry for Doppler-broadened detection of C2H2 in the parts per trillion range, J. Opt. Soc. Am. B 24, 1392-1405 (2007)

F. M. Schmidt, A. Foltynowicz, W. Ma, T. Lock, and O. Axner: Doppler-broadened fiber-laser-based NICE-OHMS - Improved detectability, Opt. Express 15, 10822-10831 (2007)

W. Ma, A. Foltynowicz, and O. Axner: Theoretical description of Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectroscopy under optically saturated conditions, Opt. Soc. Am. B 25, 1144-1155 (2008)

A. Foltynowicz, W. Ma, F. M. Schmidt, and O. Axner: Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signals from optically saturated transitions under low pressure conditions, J. Opt. Soc. Am. B 25, 1156-1165 (2008)

O. Axner, W. Ma, and A. Foltynowicz: Sub-Doppler dispersion and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy revised, J. Opt. Soc. Am. B 25, 1166-1177 (2008)

A. Foltynowicz, F. M. Schmidt, W. Ma, and O. Axner: Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy: Current status and future potential, Appl. Phys. B 92, 313-326 (2008)

A. Foltynowicz, W. Ma, and O. Axner: Characterization of fiber-laser-based sub-Doppler NICE-OHMS for trace gas detection, Opt. Express 16, 14689-14702 (2008)

A. Foltynowicz, W. Ma, F. M. Schmidt, and O. Axner: Wavelength modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signal line shapes in the Doppler limit, to be published in J. Opt. Soc. Am. B (2009)

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