Ultrasensitive, high accuracy measurements of
trace gas species
Figure from HDR Architecture, Inc.
D. A. Long, A. J. Fleisher, J. T. Hodges, and D. F. Plusquellic
ISMS
24 June 2015
Outline
• A series of new spectroscopic techniques
• Each utilizes electro-optic modulators to enable high speed scanning
• They use optical cavities to achieve high absorption sensitivity
• They offer a range of complexity, speed, and sensitivity
Cavity ring-down spectroscopy (CRDS)
recirculating field
Single-mode excitation
with
lockedmirror
cavity
low-loss
dielectric
detector
incident laser beam
ring-down signal
Advantages:
Empty-cavity
•High effective pathlength and sensitivity
•Insensitive to laser intensity noise
Absorbing medium
•Small sample volume
t
Measurements with frequency-stabilized cavity
ring-down spectroscopy (FS-CRDS)
SNR=1,800,000:1
Hyperfine structure
Measured
CO2, CO, H2O,
O2, CH4
O2 A-band line list for HITRAN
Line positions
linked to OFC
Very powerful. But very slow.
The problem
• To record a spectrum you need to tune the laser frequency
• This generally requires thermal or mechanical tuning
• This is usually non-linear and very slow
• Things are even more difficult with cavity-enhanced spectroscopy (discrete frequencies)
frequency
One solution
• Stabilize the cavity’s length (actively or passively)
• This ensures we know exactly where the cavity mode are located
• Use an electro-optic modulator (EOM) to program the laser frequency
• We can do this with microwave-level accuracy
• These waveguide based devices are amazing:
• Very high bandwidth (up to 300 GHz!)
• Ultra-low insertion loss
• Very low Vπ (i.e. required microwave power)
• Very high efficiencies
• Fiber coupled
Frequency-agile, rapid scanning (FARS) spectroscopy
Method:
• Use waveguide electro-optic phase-modulator (EOM) to generate tunable sidebands
• Drive PM with a rapidly-switchable microwave (MW) source
• Fix carrier and use ring-down cavity to filter out all but one selected side band
MW source
side-band spectrum
ring-down cavity
cw laser
Detector
gas analyte
EOM
Advantages:
• Overcomes slow mechanical and thermal scanning
• Links optical detuning axis link to radio-frequency (RF) standards
• Wide frequency tuning range (> 130 GHz = 4.3 cm-1)
G.-W. Truong et al., Nature Photonics, (2013)
FARS operating principle
wf
cavity
resonances
frequency
scanning
wC+d
wC+d+wf
wC+d+2wf
G.-W. Truong et al., Nature Photonics, (2013)
Very high acquisition rates: scanning
No dead time due to tuning
Rates as high as 8 kHz limited
by cavity response time
D. A. Long et al., Appl. Phys. B, (2013)
Lower finesse = faster rates
Finesse = 20,000
RD acq. rate = 8 kHz
st/t = 0.008 %
NEA = 1.7×10-12 cm-1 Hz-1/2
t = 30.85 ns  1.2 ns
Finesse = 60
RD acq. rate = 5 MHz
st/t = 4 %
NEA = 1.9×10-8 cm-1 Hz-1/2
D. A. Long et al., Appl. Phys. B, (2013)
K. O. Douglass, JOSA B, (2013)
Extremely precise frequency axis
Due to the quality of our
frequency axis we can record
the individual cavity
resonances
~130 Hz relative laser linewidth
Uncertainty of the fitted
resonance frequency ~1 Hz
The width of the resonances
provides a complimentary
measure of the absorption
Uncertainty of the fitted width
of the resonances ~0.04%
D. A. Long et al., Appl. Phys. B, (2013)
High accuracy of frequency axis-linked to OFC
• Absolute accuracy of 4 kHz
• Two measurements agree to within 4 Hz
cavity dispersion
gDD  40 fs2
Spectra of entire absorption bands
2 THz wide spectra
recorded in 45 minutes
425 ppm of CO2 in air
ECDL grating moved
every 12 GHz
Each point is the
average of 100 RDs
D. A. Long, et al., JQSRT, (2015)
Differential FARS-CRDS
st/t = 0.023%
NEA = 4x10-12 cm-1 Hz-1/2
st/t = 0.014%
NEA = 9x10-12 cm-1 Hz-1/2
Differential approach
increases optimal
averaging time by
more than three orders
of magnitude
See Zach Reed’s talk for
more information (WF03)
Dq = 1
30 s
What if I want higher sensitivity?
• To reduce 1/f noise, we want to make the measurement away from DC
• Also need to rapidly compare ring-down time constants at different wavelengths
Improving the sensitivity:
Heterodyne measurements
• To do this we adapted the approach of Ye and Hall
J. Ye and J. Hall., Phys. Rev. A, (2000)
Heterodyne measurements:
Our approach
• Replace their two AOMs with a single EOM
• This enables rapid scanning
• Our approach also allows for a continuous lock (greatly reduces complexity)
D. A. Long, et al., Appl. Phys. B, (2014)
Heterodyne measurements:
Reaching the quantum-noise limit
• Utilized both a traditional InGaAs detector and an APD
• Able to reach the quantum-noise limit with the APD
• The traditional InGaAs allows for an NEA of 6E-14 cm-1 Hz-1/2
APD
PIN
D. A. Long, et al., Appl. Phys. B, (2014)
What if I don’t want to scan?
• Then multiplex!
Mode-locked femtosecond OFCs
Advantages:
• Wide bandwidth (octave-spanning)
• Can be self-referenced (absolute freq. axis)
Disadvantages:
• Essentially fixed repetition rate
• Low power per tooth (nW to µW)
• Large and expensive
Image from Menlo Systems
An alternate approach: electro-optic modulators
Ideal for targeted
measurements of
selected species
Dual-drive MZM allows for
power-leveling of the comb
T. Sakamoto, et al., Electron. Lett., (2007)
Variable pitch
Pitch can be changed in <100 µs
D. A. Long, et al., Opt. Lett., (2014)
Multiheterodyne spectroscopy
Common mode
(no need for
phase locking)
D. A. Long, et al., Opt. Lett., (2014)
Referencing
D. A. Long, et al., Opt. Lett., (2014)
Multiheterodyne spectra
Acquired in 30 s
(average of 10,000 spectra)
D. A. Long, et al., Opt. Lett., (2014)
Dense OFC generation
EOMLO
MZMLO
FS
FS
EOMprobe
MZMprobe
Replicate our original DD-MZM OFC
generator using a second
modulator at high frequency (≈1020 GHz).
Increase optical bandwidth to >150
GHz.
Still acquire the down-converted RF
spectrum in 2 ms.
Frequency (a.u.)
Down-converted RF comb
>280 optical modes
Where are we headed?
• The mid-infrared.
Mid-infrared CRDS
MDA < 6×10−12 cm−1 at 4 s
Quantum-noise-limited measurements
high noise PD
low noise PD
Detector-Noise-Limited Fit Residuals
Quantum-Noise-Limited Fit Residuals
More on this in A. J. Fleisher’s talk (WF04)
Mid-infrared sensitivities
NIST value
Conclusions
• Presented several new techniques for rapid, ultrasensitive detection
– Frequency-agile, rapid scanning (FARS) spectroscopy
• Use an EOM to step scan the laser frequency
• Scanning rates limited only by the cavity response time
– Heterodyne-detected cavity ring-down spectroscopy (HD-CRDS)
• Make the measurement well above DC
• Leads to quantum-noise-limited sensitivity
• Does not compromise scanning rate
– Multiheterodyne spectroscopy with EOM-generated optical
frequency combs
• Allows for multiplexed detection of several trace gases
• Far lower costs and complexity than with femtosecond lasers
• Inherently common-mode
Acknowledgements
• Joseph Hodges, David Plusquellic, Adam Fleisher, Gar-Wing Truong,
Szymon Wojtewicz, Katarzyna Bielska, Hong Lin, Qingnan Liu,
Zachary Reed, Kevin Douglass, Stephen Maxwell, Roger van Zee
– NIST
•
A NIST Innovations in Measurement Science (IMS) award
•
The NIST Greenhouse Gas Measurements and Climate Research Program
Always looking for good post-docs
Email: David.Long@nist.gov