CLEO/QELS                                                                           May 9, 2012 In-Line Reference Cell for...
Project Goal & OutlineThe project goal:• Develop and implement a technique for real-time calibration of  portable trace-ga...
Measurement Noise & Drift Reduce SensitivityMeasurement drift can be inducedby many factors:• Electronics instability• Env...
Traditional Calibration Vs. In-Line Reference Cell                                Multi-Pass Gas Cell                     ...
Simultaneous Detection of Sample and Reference                           IDet Detector    2f ambient   6f reference       ...
Key WMS Parameters                                              Laser frequency change                                    ...
Optimizing 2f WMS Detection                                      S amb                                      S ref         ...
Experimental Validation of Samb/Sref vs. Pressure for 2f• Experimentally varied the modulation index for pressures from 50...
Optimizing 6f WMS Detection                                            S ref                   Ambient Pressure        S a...
Experimental Validation of Sref/Samb vs. Pressure for 6f• Experimentally varied the modulation index for pressures from 50...
Conclusion and Future Work• A novel in-line calibration technique has been presented       Single detector is used      ...
AcknowledgementsThis work was sponsored in part by:The National Science Foundation’s MIRTHE Engineering Research CenterAn ...
Vary Linewidth, Mod. Index, & Detection OrderOptimal contrast:                             N=2 (mod = 1 * LWamb)          ...
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Inline Reference Cell for Reatime Calibration of Laser Absorption Spectrometers

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Inline Reference Cell for Reatime Calibration of Laser Absorption Spectrometers

  1. 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 Wysocki11. Dept. of Electrical Engineering, Princeton University, Princeton, NJ 085442. Dept. of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544pulse.princeton.edu
  2. 2. Project Goal & OutlineThe 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 gasOutline• 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 2
  3. 3. Measurement Noise & Drift Reduce SensitivityMeasurement drift can be inducedby 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 3
  4. 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 4
  5. 5. Simultaneous Detection of Sample and Reference IDet Detector 2f ambient 6f reference 2fI0 IDet Detector • Simultaneous 2f & 6f demodulation IDet Detector • Selectively suppress ambient sample or low pressure reference signal • Real-time, in-line calibration is possible 6fI0 IDet Detector 5
  6. 6. Key WMS Parameters Laser frequency change Amplitude = βKey characteristics of Wavelength Absorption Line ShapeModulation 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 6
  7. 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 7
  8. 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% 8
  9. 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 9
  10. 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 10
  11. 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 sampleFuture 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) 11
  12. 12. AcknowledgementsThis work was sponsored in part by:The National Science Foundation’s MIRTHE Engineering Research CenterAn NSF MRI award #0723190 for the openPHOTONS systemsAn innovation award from The Keller Center for Innovation in EngineeringEducationNational Science Foundation Grant No. 0903661 “Nanotechnology for CleanEnergy IGERT” 12
  13. 13. Vary Linewidth, Mod. Index, & Detection OrderOptimal contrast: N=2 (mod = 1 * LWamb) Simulation• Ambient 0.018 • Samb>>Sref N=2 (mod = 5 * LWamb)• Reference 0.016 • Sref>>Samb 0.014 LinecenterSimulation 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) 13

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