1. CLEO/QELS
May 9, 2012
In-Line Reference Cell for Real-Time Calibration of
Laser Absorption Spectrometers
Clinton J. Smith1, Amir Khan2, Mark A. Zondlo2, and Gerard Wysocki1
1. Dept. of Electrical Engineering, Princeton University, Princeton, NJ 08544
2. Dept. of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544
pulse.princeton.edu

2. Project Goal & Outline
The project goal:
• Develop and implement a technique for real-time calibration of
portable trace-gas sensors
 Use modeling of wavelength modulation spectroscopic spectra
 Differentiate between sample and reference based on physical
http://www.coas.oregonstate.edu/research/po/satellite.gif
parameters of the gas
Outline
• Key challenges to long-term sensor measurement stability
 Immunity to noise and long term drift
• Conventional calibration solutions are not compatible with the need
for low-power, compact sensors
• Overview of the permanent in-line reference cell implementation
• Simulations of the technique
• Experimental results
• Conclusions and Future directions
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3. Measurement Noise & Drift Reduce Sensitivity
Measurement drift can be induced
by many factors:
• Electronics instability
• Environmental dependence Allan Deviation
• Opto-mechanical instability
 Shear, torque, compressive,
stress
 Beam steering
• Laser & Detector Drift
 Optical power fluctuation
• Fabry-Perot Fringing
Averaging Time (sec)
Recurring Calibration Required
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4. Traditional Calibration Vs. In-Line Reference Cell
Multi-Pass Gas Cell
I0
• Split off beam IDet,1 • Use single cell Ambient
• Use separate Detector • Cycle between Ref. Gas
Inlet Outlet
reference cell reference and
Ref. Cell IDet,2 I0 IDet
sample gases
nRef, LRef , PRef , σRef Detector Detector
Separate reference cell signal and ambient signal
using gas parameters and WMS
• Permanently insert a low-pressure reference cell in the beam path
 Contains the same gas as sampled
• Reference beam experiences the same fringes as the ambient/sampling beam
• No complex gas handling required
• Only single detector is needed
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5. Simultaneous Detection of Sample and Reference
IDet Detector 2f ambient 6f reference
2f
I0
IDet Detector
• Simultaneous 2f & 6f demodulation
IDet Detector
• Selectively suppress ambient sample or low pressure reference signal
• Real-time, in-line calibration is possible 6f
I0
IDet Detector
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6. Key WMS Parameters
Laser frequency change
Amplitude = β
Key characteristics of Wavelength Absorption Line Shape
Modulation Spectroscopy (WMS) νHWHM
Laser Detector
• Modulate laser wavelength at a
high frequency (f) and with
modulation depth/amplitude (β) 1f
 Avoids 1/f noise
• Demodulate and filter at
multiples of f
 Low-noise, “derivative-like”
spectral envelopes 2f
• WMS Signal is proportional to
laser power and depends on
3 key parameters:
 Line-width, νHWHM (cm-1)  3f
 Modulation depth, β (cm-1) m
 Harmonic (e.g., 1f, 2f, 3f, …)  HWHM
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7. Optimizing 2f WMS Detection
S amb
S ref
Ambient Pressure
• 1% CO2 Absorbance in the reference cell
• Pressure affects the νHWHM of the target line
• Simulate 2f spectrum for different reference cell pressures and β
 Select reference cell pressure that minimizes “crosstalk”
 Samb/Sref  max.
• Both higher and lower pressures in reference cell can provide better signal
contrast
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8. Experimental Validation of Samb/Sref vs. Pressure for 2f
• Experimentally varied the modulation index for pressures from 50 to 1000 Torr
• Line-center value used for comparison
• Good agreement between experiment and simulation
• Reference cell signal into ambient signal cross-talk of ~13%
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9. Optimizing 6f WMS Detection
S ref
Ambient Pressure S amb
• 1% CO2 Absorbance in the reference cell
• Simulate 6f spectrum for different reference cell pressures and β
 Select reference cell pressure that minimizes “crosstalk”
 Higher Sref/Samb ratio at 6f can be achieved than Samb/Sref at 2f (16 vs. 10)
 Optimum pressure: 100-150 Torr
• Only low pressure and low modulation depth give higher ratio
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10. Experimental Validation of Sref/Samb vs. Pressure for 6f
• Experimentally varied the modulation index for pressures from 50 to 1000 Torr
• Line-center value used for comparison
• Good agreement between experiment and simulation
• Ambient signal into reference cell signal cross-talk of ~4%
 Ambient signal is at or below the observed noise floor
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11. Conclusion and Future Work
• A novel in-line calibration technique has been presented
 Single detector is used
 Sample and reference signals experience the same parasitic optical fringes
• Reference cell contains the same gas as the sampled gas
• Physical properties of the gas in conjunction with WMS are used to distinguish
the reference from the sample
Future Improvements and Potential Applications
• Further investigate the degree to which crosstalk reduces precision
• Perform long-term measurements and drift analyses
• Further enhancement of the signal and reference contrast
 Investigate full spectral fitting
• Potential Applications
 Use with gases that do not require ultra-high precision (e.g. ambient NH3 requires
~5% precision)
 Use with gases with low variability (e.g., 4% cross-talk with a sample of 20%
variability yields 0.8% precision)
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12. Acknowledgements
This work was sponsored in part by:
The National Science Foundation’s MIRTHE Engineering Research Center
An NSF MRI award #0723190 for the openPHOTONS systems
An innovation award from The Keller Center for Innovation in Engineering
Education
National Science Foundation Grant No. 0903661 “Nanotechnology for Clean
Energy IGERT”
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13. Vary Linewidth, Mod. Index, & Detection Order
Optimal contrast: N=2 (mod = 1 * LWamb) Simulation
• Ambient 0.018
• Samb>>Sref N=2 (mod = 5 * LWamb)
• Reference 0.016
• Sref>>Samb 0.014
Linecenter
Simulation 0.012
Parameters:
ambient
• 1% CO2 0.01
Absorbance 0.008
• Variables N=6 (mod = 5 * LWamb)
• Pressure 0.006
• Modulation 0.004
Depth
• Harmonic 0.002
N=6 (mod = 1 * LWamb)
0
0 200 400 600 800 1000 1200 1400 1600
Reference Cell Pressure (Torr)
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