1. Fiber-laser-based NICE-OHMS
for Trace Species Detection
Aleksandra Foltynowicz
Weiguang Ma
Ove Axner
Laser Physics Group
Department of Physics
Umeå University
Umeå, Sweden
International Symposium on Molecular Spectroscopy
OSU, Columbus, Ohio
June 23, 2009

2. Principles of NICE-OHMS
Noise-Immune
Cavity-Enhanced ?
resonant cavity
increased interaction length
enhanced power
Optical Heterodyne
Molecular Spectrometry
frequency modulation for noise reduction
high sensitivity
detectability – cm-1
close to shot-noise-limited performance
weak molecular overtone transitions
trace gas detection 2

3. Principles of NICE-OHMS Frequency Modulation
electro-optic modulator
FM triplet
RF signal
(MHz)
modulation index
no analyte – constant intensity
Laser
EOM
Absorber PD
double balanced
mixer
Phase
shifter
with analyte – signal at vm
low pass
filter LP
FM signal
in-phase
dispersion
out-of-phase
absorption 3

4. Principles of NICE-OHMS Cavity Enhancement – Fabry-Perot Resonator
EOM
PBS PD
Laser
frequency
control
servo Ph
DBM LP
Cavity
finesse
Laser
effective length
intracavity power
free spectral range
Pound-Drever-Hall
laser stabilization technique
cavity mode width 4

5. Principles of NICE-OHMS
Noise-Immune
Cavity-Enhanced
Optical Heterodyne
Molecular Spectroscopy
alternative name:
Cavity-Enhanced Frequency Modulation Spectroscopy
absorption
dispersion
Noise
Immunity 5

6. Experimental Setup EDFL 1531 nm Laser frequency control fm-NICE-OHMS
fiber EOM
polar.
lens
PBS
Cavity
with Absorber
Laser
frequency
control
Phase 20
MHz
DBM
fm-NICE-OHMS
signal
Gain LP
Phase
Scan
DBM
wm-NICE-OHMS
signal
Lock-in OI OI
360 MHz
FSR
control
Phase BP
380
MHz
DBM
PD1
PD2
fiber
polarizer
EDFL
1531 nm
PD – photodetector
PBS – polarizing beamsplitter
OI – optical isolator
DBM – double balanced mixer LP – low pass filter
BP – bandpass filter
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).

7. Experimental Setup Fiber Laser
DFB-laser pumped erbium doped fiber laser
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) – nm, detection of C2H2, NH3, N2O, CH2O, CO2, CH4
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 7

8. Experimental Setup Fiber-coupled EOM, Cavity
LiNbO3 phase modulator
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
Cavity
Zerodur spacer
Two ring-shaped piezo actuators
Finesse 4800, 5700
Length 40 cm

9. NICE-OHMS Detection Modes
Doppler-broadened
sub-Doppler
fm-NICE-OHMS
wm-NICE-OHMS
absorption
CO2
absorption
CO2
absorption
C2H2 fm
C2H2
dispersion
CO2
dispersion
C2H2 wm
C2H2
dispersion
CO2 9

10. Doppler-broadened Signals Lineshapes
absorption
dispersion
Standard FM nomenclature and Fourier-series-based WM theory
Detection at arbitrary phase
fm-NICE-OHMS
1000 ppm of C2H2
at 130 mTorr of N2
wm-NICE-OHMS
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).

11. Doppler-broadened Signals Signal Strength
Independent of FM detection phase
Influence of optical saturation
absorption signal reduced
dispersion signal unaffected
<<1
Linear with pressure/concentration for small absorption
1000 ppm of C2H2
at 10 mTorr of N2
Saturation power
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).

12. Sub-Doppler Signals Lineshapes
On-resonance dispersion signal
Lorentzian up to high degrees of saturation 70
8.4
3.7
0.4
0.19
0.02
fm-NICE-OHMS
10 µTorr of C2H2
4.1 W
0.49 W
wm-NICE-OHMS
A. Foltynowicz, W. Ma, and O. Axner, Opt. Express 16 (2008). 12

13. Sub-Doppler Signals Signal Strength
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
Axner et al.
Ma, Hall et al.
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). 13

14. Sensitivity Doppler-broadened sub-Doppler cavity parameters
minimum detectable absorption
8  cm-1 (0.7 s)
minimum detectable sub-Doppler phase shift 5.7  cm-1 Hz-1/2
finesse
4800
FSR
380 MHz
cavity length
40 cm
effective length
1.2 km
intracavity power
< 4.5 W
transitions parameters
gas
wavelength
[nm]
transition strength
[cm-1/(molec cm-2)]
C2H2
1.2  10-20
CO2
8.4  10-26
fm-NICE-OHMS signal
6  10-9 cm-1 of pure CO2
Allan variance 25 ppm of C2H2 in 20 mTorr of N2
Detection limit for C2H2
3.5 nTorr
46 ppt
mTorr
40 ppb
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).

15. Summary 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  cm-1 (46 ppt of C2H2 )
sub-Doppler optical phase shift 5.7  cm-1 Hz-1/2 (0.8 nTorr/110-12 atm of C2H2)
Dynamic range

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