1. 7th International
DOAS Workshop
Brussels 2015
“High Precision DOAS” A new Long Path-DOAS system with reduced noise and new light source
Jan-Marcus Nasse, Denis Pöhler, Philipp Eger, Stefan Schmitt,
Udo Frieß and Ulrich Platt
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

2. Motivation Advantages of Long-Path DOAS:
analysis directly yields (path- averaged) concentrations
independent of solar radiation
own light source allows measurements in a very broad spectral range
Operation of LP-DOAS instruments is complex:
regular exchange of Xe-light bulbs
Optical adjustments
“Classical” configuration of the fibre bundle
Goals of instrument design:
improved usability and automation (  long term measurements)
better detection limits ( necessary for a number of research topics)
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

3. Origin of residual structures
1. Photon noise
 Linked to intensity of light source and transmission
2. Lamp structures
 unstable lamp spectrum
3. Insufficient mode mixing
inhomogeneous illumination of CCD
4. Stray light
 creates residual structures in particular in the UV
5. Imprecise literature cross sections
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

4. Laser-Driven Light Source (LDLS)
Standard light source for LP-DOAS:
Xenon arc lamps
short lifetimes
unstable
New light source: xenon lamp driven by infrared laser (LDLS)
Advantages:
Reduced abrasion of electrodes
 long life time
small and stable arc spot
 improved optical coupling
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

5. Comparison between xenon lamps and LEDs
Size of the arc spot:
“EQ-99” (experimental)
XBO-75 (new)
LED
0,014 mm2
0,125 mm2
1 mm2
Coupling into 800 μm optical fibre
Coupling into 200 μm optical fibre
Eger (2014)
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

6. New fibre configuration
Eger (2014)
residual
residual
“classical”
Eger (2014)
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

7. Origin of residual structures
Photon noise
 Linked to intensity of light source and transmission
Lamp structures
 Unstable light arc induces Xe structures
LDLS
Insufficient mode mixing
inhomogeneous illumination of CCD
Stray light
 creates residual structures in particular in the UV
Imprecise literature cross sections
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

8. Mode mixing New method:
Eger (2014)
from Platt and Stutz (2008) - adapted from Stutz and Platt (1997)
New method:
fibre roughened with 12 μm grit polishing sheet
 Like diffuser but less light loss
Eger (2014)
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

9. Mode mixing - Comparison
no mixing
vibration
diffuser
roughened
Eger (2014)
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

10. Origin of residual structures
Photon noise
 Linked to intensity of light source and transmission
Lamp structures
 Unstable light arc induces Xe structures
Insufficient mode mixing
inhomogeneous illumination of CCD
Stray light
 creates residual structures in particular in the UV
Imprecise literature cross sections
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

11. Stray light reduction I – “lens filter”
Exploiting the chromatic aberration of the lens coupling the into the fibre
Eger (2014)
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

12. Stray light reduction II -filters
Eger (2014)
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

13. LDLS with stray light reduction
lamp
lens
laser
moving
stage
filter
wheel
shutter
fibre
Eger (2014)
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

14. Origin of residual structures
Photon noise
 Linked to intensity of light source and transmission
Lamp structures
 Unstable light arc induces Xe structures
Insufficient mode mixing
inhomogeneous illumination of CCD
Stray light
 creates residual structures in particular in the UV
Imprecise literature cross sections
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

15. Combined reduction of residuals
chromatic aberration filter
band pass filter
LDLS
new fibre configuration
improved mode mixing
Eger (2014)
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

16. Comparison with conventional Xe-lamp
reversed
reversed
classical
classical
classical
classical
Eger (2014)
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

17. Origin of residual structures
Photon noise
 Linked to intensity of light source and transmission
Lamp structures
 Unstable light arc induces Xe structures
Insufficient mode mixing
inhomogeneous illumination of CCD
Stray light
 creates residual structures in particular in the UV
Imprecise literature cross sections
 Recording of own differential X-sections e.g. O2, O3
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

18. Improvement of the residuals by a factor of 4 to 5 x 10-5 RMS
Conclusions
Laser Driven Light Source (LDLS) offers important advantages in comparison with conventional Xenon lamps  small and stable arc spot; long lifetime
Optimal configuration: LDLS + “reversed” coupling into 200 μm fibre mode mixing by roughened fibres
Improvement of the residuals by a factor of 4 to 5 x RMS
Good temporal stability (in particular with LDLS)  adding up spectra is possible
Available differential cross sections are no longer sufficient  recording of own differential cross sections
Autonomous Long Path DOAS measurements are possible
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

19. Outlook: Long term measurements at Neumayer III/Antarctica
Investigation of chlorine, iodine and bromine chemistry
continuous measurements (full diurnal cycles and Polar night)
operation for at least one year
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015

20. Thank you for your attention!
7th International
DOAS Workshop
Brussels 2015
Thank you for your attention!
Institute of Environmental Physics / Heidelberg University / Jan-Marcus Nasse / 7th International DOAS Workshop 2015