Spectroscopic Applications of Quantum Cascade Lasers

Transcription

1 Spectroscopic Applications of Quantum Cascade Lasers F.K. Tittel, A. Kosterev, and R.F. Curl Rice University Houston, USA OUTLINE PQE 2000 Snowbird, UT Motivation and Background Design of QC-DFB Laser Spectrometers Real World Applications Outlook and Summary

2 Wide Range of Gas Sensor Applications Urban and Industrial Emission Measurements Industrial Plants Combustion Sources Automobile, Trucks and Aircraft Rural Emission Measurements Agriculture Environmental Monitoring Atmospheric Chemistry Volcanic Emissions Spacecraft and Planetary Surface Monitoring Crew Health Maintenance & Life Support Chemical Analysis and Industrial Process Control Semiconductor Industry Medical Applications Law enforcement Applications

3 Vulcano, Italy

4 Stromboli, Italy

5 International Space Station

6 Mars NASA Pathfinder Climate Monitoring

7 This image cannot currently be displayed. Air Composition Main Components Trace Components Nitrogen 78% Oxygen 21% Water 0.8% CO % Methane CO N 2 O O ppm 0.4 ppm 0.3 ppm 0.03 ppm

8 Existing Techniques for Trace Gas Detection

9 Absorption Spectroscopy

10 Spectral Coverage by Diode/QC Lasers

11 Key Characteristics of Quantum Cascade Lasers Laser wavelengths cover entire range from 3.4 to 17µm determined by layer thickness of same material Intrinsically high power lasers (determined by number of stages) CW: 80 K, ~100 mw single frequency Pulsed: 0.5W peak at room temperature, ~15 mw 300 K High Spectral purity (single mode) Wavelength tuning by current or temperature scanning High reliability: low failure rate, long lifetime, robust operation and extremely reproducible emission wavelengths

12 Trace Gas Detection with a Multipass Cell

13 Motivation for CH 4 Detection Contributor to global warming Important in tropospheric and stratospheric chemistry Emitted by microorganisms Can leak from gas pipelines

14 CH 4 and N 2 O Absorption Spectra QC laser frequency Line intensity, cm (1) (2) CH 4 Ν 2 Ο Frequency, cm -1

15 Absorption Spectrum of Room Air 10 HDO Absorption, % 8 CH4 6 4 CH4 N2O N2O 2 18 H2 O Frequency, cm

16 13 CH 4 Absorption Line at cm -1 on the Shoulder of H 2 O line Absorbance Frequency, cm -1

17 Isotopic Composition of CH 4 Pressure: 16.9 Torr Pathlength: 43 cm

18 Continuous Detection of Ambient CH 4 for a 7 Day Period

19 Laser-Based CH 4 Detection Reported to Date First overtone band: 2ν 3 line intensity: ~ cm source: diode laser at 1.66 µm (Uehara and Tai, 1992) sensitivity: 600 ppb m Hz-1/2 Fundamental band: ν 3 (CH asymmetric stretch) line intensity: ~ cm source: diode laser at 3.2 µm DFG (Rice, 1995) sensitivity: 12 ppb m Hz-1/2; 4 ppb m Hz-1/2 (Rice, 1999) band: ν 4 (CH bend) line intensity: ~ cm source: lead-salt diode laser at 7.8 µm (Webster et al, 1994) sensitivity: 14 ppb m Hz-1/2 ; 5 ppb.m.hz -1/2 (Rice,1999)

20 Measured Emission from Rice Paddies of Different Countries Country Total Area of Rice Paddies (10 10 m 2 ) Percent of World Rice Area Total Rice Grain Yield (000 t) Percent of World Rice Grain Annual CH 4 Emissions (Tg) Percent of Average CH 4 World Emission China , India , Japan , Thailand , Philippines , USA , Total , World Total ,

21 Monitoring Methane in Rice - Based Agroecosystem

22 Ethanol Absorption Spectrum at cm-1 Absorbance Frequency, cm

23 Nitric Oxide: Various Human Functions

24 Cavity Enhanced QC Laser Spectroscopy

25 Cavity Enhanced Spectroscopy of CO 2, H 2 O 0,2 and NO at 5.2 µm 0,0 Absorption -0,2-0,4-0,6 NO HO 2-0,8-1,0 HO 2 CO (4%, breath) ,5 1920,0 1920,5 1921,0 Frequency, cm -1

26 Summary QC-DFB Laser Based Trace Gas Sensors Compact, tunable, robust High sensitivity (<10-4 ) and selectivity (<50 MHz) Fast data acquisition and analysis Detected trace gases: CH 4, N 2 O, H 2 O, NO, CO 2 at 5.2 and 8 µm Isotopic Compositions Current Applications in Trace Gas Detection CH 4 : NOAA, NASA-JPL, and gas industry Future Directions More efficient suppression of optical interference fringes Pulsed quasi room temperature operations Detection of complex molecules Cavity enhanced spectroscopy Medical Diagnostics: NO, CO, CO 2 and NH 3

27 Strategy for NH 3 Concentration Measurements cm -1 ~1177 cm -1 NH 3 H 2 O (B.A. Paldus et al.) N 2 O CO Frequency, cm -1

28 Worldwide Megadirty Megacities

29 CRLAS Principle QC-DFB Laser Ringdown Cavity Detector Computer Digital Storage Oscilloscope I(t) = I 0 exp [- t (1 - R) +A ] A = tr/2(1/τ-1/ τ o ) t r /2

30 Cavity Enhanced QC-Laser Spectroscopy Function Generator Trigger Laser diode driver QC-DFB laser in dewar Collimating optics Data Acquisition Card PC laptop Gas cell with high-q optical cavity Signal Preamplifier R=99,995% InSb detector α = ( 1 R) ( 1 R) + 4RT 1 L 2 4R Tn n 2

31 Cavity Enhanced Absorption Observed absorption : A obs = I empty I I empty sample = T αl + αl Absorption gain : Aobs 1 1 g = = ; g if αl << T αl T + αl T Observed absorption T=5*10-5 T=3*10-5 Absorption gain T=5*10-5 T=3* αl αl

32 Detection of CH4 Near 1241 cm-1 4 P+(10) Absorption, % 6.5 mtorr, pure CH4 P+(10) 3 HO P+(11) * ** Absorption, % 16 ppm of CH4 in 50 Torr of air Wavenumber, cm

33 Estimate of Detection Sensitivity 0.06 Absorption, % % 0.011% Simulated false lines are always 0.01% The detection limit for absorption is 10-4

34 Summary and Future Outlook A cw QC-DFB laser based gas sensor at 8 µm was designed and tested for methane and nitrous oxide detection A detection limit of 10-4 for absorption was obtained Isotopic composition measurements were demonstrated Future development More efficient suppression of optical interference fringes Detection of more complex organic molecules Room-temperature operation

35 Cavity Enhanced QC Laser Spectroscopy

36 HITRAN Simulated NO Absorption Spectrum NO H 2 O CO 2 Q-branch (suitable for LMR spectroscopy ) ~1876 cm -1 ~1920 cm Frequency, cm -1