Precision and Accuracy of Miniature Tunable Diode Laser Absorption Spectrometers M.B. Frish,* R.T. Wainner, M.C. Ladere

1. VG11-081 Precision and Accuracy of Miniature Tunable Diode Laser Absorption Spectrometers M.B. Frish,* R.T. Wainner, M.C. Laderer, K.R. Parameswaren, D.M. Sonnenfroh, and M.A. DruyPhysical Sciences Inc.20 New England Business CenterAndover, MA 01810-1077 SPIE Paper No.: 8032-8 Presented at:Next-Generation Spectroscopic Technologies IV SPIE Defense, Security and Sensing25 April, 2011Orlando, FL

2. TDLAS VG11-081 • Selective; generally insensitive to cross-species interference • Sensitive; sub-ppmdetection of many gas species • Fast; sub-second response time • Configurable; point, open-path, or standoff sensor • Non-contact; only the probe beam need interact with the analyte Tunable Diode Laser Absorption Spectroscopy (TDLAS) is an active optical method for detecting and quantifying one or more analyte gases mixed with other gases • Once considered a laboratory specialty, TDLAS is now accepted as rugged, reliable commercial industrial instrumentation • Reliability and accuracy => complexity and cost • High-volume emerging market opportunities (process control, environmental monitoring) demand reduced cost • Simplicity, autonomy, reliability, accuracy are key

3. TDLAS Examples VG11-081 • Standoff TDLAS • Remote Methane Leak Detector (RMLD™) • Fast, semi-quantitative alarm • >1100 units in use • Trace Gas Analyzers • Process measurement and control • Continuous operation in difficult environments • Quantitative; accuracy and precision required

4. Accuracy and Precision VG11-081 • Common Analyzer Requirements • Accuracy • Deviation of measured value relative to a standard • <1% of measured value • Precision • Measurement variance over a specified time interval • <1% of measured value or better • TDLAS Observations • With no analyte, Noise Equivalent Absorbance is typically 1-10 x 10-5 • Limited by temporal drift of coherent optical effects • Withanalyte present, accuracy and precision depend on quality of wavelength stability

5. Practical TDLAS Detection Limits for Some Gases VG11-081

6. Absorption Spectroscopy VG11-081 • Gas molecules absorb light at specific colors (“absorption lines”) • Beer-Lambert law: • I((t)) = It((t))·exp[-S(T) ·G(-c,T,P)·Ng(Pg,P,T)·ℓ] ≡ It((t))·exp[-(,T,P)] • where: • It((t)) = launched laser power of wavenumber  (W) • I((t)) = received laser power after propagation through the absorbing analyte (W) • t = time (s) •  = laser wavenumber (reciprocal of wavelength, cm-1) • c = wavenumber at absorption line center (cm-1) • l = optical pathlength through the analyte (cm) • S(T) = analyte spectral line strength (cm-1/molecule-cm-2) • T= temperature (K) • Ng= analyte number density (molecules/cm3) • Pg = analyte partial pressure (atm) • P = total pressure of gas sample (atm) • G( - c,T,P) = lineshapeparameter (1/cm-1) •  = Absorbance

7. TDLAS System Components A frequency agile (i.e. tunable) laser beam transits an analyte gas sample Laser wavelength scans repeatedly across absorption line unique to analyte gas Received signal processed to deduce analyte concentration Laser sources SWIR - Distributed Feedback (DFB) laser or SWIR - Vertical Cavity Surface Emitting Laser (VCSEL) MWIR - Interband Cascade Lasers LWIR – Quantum Cascade Lasers VG11-081

8. Example Industrial TDLAS System Architecture VG11-081 System Test Control Signals Demodulated WMS Signals Average Laser Power Signal Housekeeping Signals Linelock Error Signals Controller Analog and Digital Outputs Phase Comparison Signals Reference Receiver Electronics Reference Path Linelock Correction Signal Demodulated WMS Signals Average Laser Power Signal Housekeeping Signals Waveform Generators Transmitter Electronics Fiber Splitter Laser To additional Measurement Paths Measurement Path 1 • Sealed “reference cell” with independent measurement path, plus multiple modulations,provides wavelength stability and system health diagnostics • Needed for “alarm” sensors, when no analyte is normally present • Costly and complex • Eliminated in simpler but smarter WMS system Phase Comparison Signals Path 1 Receiver Electronics

9. Wavelength Modulation Spectroscopy (WMS) Laser wavelength scans repeatedly across absorption line: “modulation depth” δ ~ Γ (linewidth) causes amplitude modulation at fm; Absorption by target gas produces amplitude modulation at 2fm (F2) Lock-in detection at fm and 2fm provides narrow bandwidth measurement of F1 & F2 F1 represents received laser power F2 represents spectral absorbance VG11-081

10. Properly-tuned WMS Waveforms VG11-081 • Ng(t) = A(F2(t) – F2o)/F1(t) • A and F2o are constants determined by a two-point (zero and span) calibration. • F1, F2 and F2o are all proportional to wavelength-independent optical system transmittance • Ng is independent of transmittance changes • Detector signal • Absorbance = Detector signal (w/analyte) – Detector signal (evacuated) • 2f Demodulation @νo =νc • “Proper Tuning”

11. Noise and Drift: 1) Coherent Optical Effects VG11-081 • Lasers create interference patterns that alter optical throughput vs. wavelength • Etalons, speckle • Can appear similar to molecular absorption lines • Interference patterns add an offset to the spectral absorption signal • Amplitude depends only on optics • Independent of presence (or absence) of target gas • Equivalent absorbance ~ 1x10-5 • Precision is limited by temporal drift of offset due to environmental influences • Allan Deviation expresses precision vs. signal averaging period (=1/bandwidth) Ambient H2O, tuned off-line 101028

12. Improperly-tuned WMS Waveforms VG11-081 νo ≠νc νo F2 vs. νo≈ │lineshape 2nd derivative │ │F2│ νo • When νo ≠νc F2/F1 is non-linear • Diminishes accuracy of two-point calibration • Effect is significant when absorbance > 10-3

13. Examples (a) (b) (c) Time Noise and Drift: 2) Mistuning Effects VG11-081 • Fluctuations in laser current and temperature cause laser wavelength noise and drift • Wavelength noise creates noise in spectral absorption signals (F2, F1) • Translates to concentration measurement noise, decreasing precision • Noise amplitude is proportional to concentration • Wavelength drift (slowly varying deviation of laser wavelength νo from absorption line center) causes calibration error, • Decreases accuracy Concentration Concentration Concentration

14. Wavelength Stabilization VG11-081 • no linelocking; • sub-optimum modulation depth • Allan Deviations of a CO2 analyzer • Continuous wavelength stabilization (“linelocking”) using 3F feedback, • No reference cell • no linelocking; • optimum modulation depth; • with linelocking; • optimum modulation depth

15. Conclusions VG11-081 • Noise and drift limit the accuracy and precision of TDLAS gas analyzers • Drift is commonly attributed to coherent optical phenomena (“etalons” or speckle) that vary with environmental influences • This paper shows the noise and drift resulting from mistuning a simple TDLAS analyzer using Wavelength Modulation Spectroscopy • At high spectral absorbance, mistuning can cause significant error without etalons • Using techniques to avoid or mitigate the errors, we achieve precision suitable for discerning spectral absorbance changes smaller than 10-5 over periods of days or longer • Enables modest cost TDLAS sensors for tracking slowly changing low-concentration environmental gases such as carbon dioxide and methane.