Incoherent Broadband Cavity Enhanced Spectroscopy using LEDs

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Incoherent Broadband Cavity Enhanced Spectroscopy
using LEDs

• by David Lackner

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Outline

• Absorption spectroscopy of gases
• Cavity Enhanced Absorption (CEA)
• Experiment with LED
• Conclusion

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Absorption

• Beers Law
• With absorption cross-section s, pathlength d,
  number density n
• Absorption of gases is typically very small

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Absorption

• To overcome this
• Long pathlengths
• Multi-pass cells
• Now cavity enhanced absorption spectroscopy
• Idea trap light in cavity (Fabry-Perot
  interferometer) to increase effective
  pathlength

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CEA

• single pass absorption vs cavity enhanced
  absorption

d
Absorption 1, d 0.2m
Enhancement (1-R)-1
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CEA

• But, as sensitivity goes up, light throughput
  goes down rapidly

gt Strong sources
For empty cavity
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Light Sources
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Experimental Set Up
d
Cavity
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Experimental Set Up
Monochrometer
LED
PDA
d
Lens
Cavity
Lens
PC
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Experimental Set Up
O2
Ozone- generator
O3
LED
Cavity
Enhancement 550 free spectral range 1pm Length
d 19.8cm R 0.998
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LED Spectrum
mirror
LED Peak 654nm
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Measurements
Empty cavity Io
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Measurements
mirror
Empty cavity Io
With O3 I
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Measurements
single pass
mirror
multi pass
Empty cavity Io
With O3 I, I
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Concentration calculation
Measured n average
s O3
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Results
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Discussion

• Experiment successful
• Light was transmitted through the cavity
• Enhanced absorption was seen
• Improvements
• Higher curvature mirrors
• gt More light throughput gt smaller uncertainty
• Detector with less noise
• gt Less uncertainty better sensitivity

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Conclusion

• It is possible to use LEDs for IBBCEA
• Recent developments make LEDs up to 100 times
  brighter (especially in UV regime)
• Technique useable over wide range of wavelength
  from IR to UV
• Portable systems are possible (battery powered)
  to measure atmospheric gas concentrations with
  high spatial resolution

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Thanks

• To Dr. Andreas Ruth
• To Dr. Dean Venables, Michael Staak, Ed Gash
• To Dr. Mahbub Akhter _at_ nmrc
• For your Attention

Questions?