View
29
Download
4
Category
Preview:
DESCRIPTION
CEAS - ICOS - absorption of IR 940 nm by water vapor
Citation preview
CAVITY ENHANCED ABSORPTION SPECTROSCOPY: NEAR INFRARED
ABSORPTION BY WATER VAPOR USING A LED (LIGHT EMITTING DIODE)
Maheshwar Ghimire
A Project for Master’s Level of Science, Department of
Physics
Date: July 30, 2012
•Why water vapor?
•Why absorption in infrared region?
•Why cavity enhanced technologies ?
•Why ICOS ?
•Why Broadband LED source ?
Outline
Beers Law
Where C = molecular concentration = Absorption cross section
Absorption Depends Upon Absorption Cross section The Path Length
Cavity Enhanced Absorption Spectroscopy (CEAS)
Light Source
Dectetor
Highly reflective cavity Mirrors
Type• CRDS (Cavity ring down spectroscopy) • ICOS (Integrated cavity output
spectroscopy)
Cavity Ring Down Spectroscopy
In CRDS, a beam of light is injected into the cavity and the rate of decay of light which is transmitted is measured
On Axis TEM(3,0) On Axis TEM(1,1)
On Axis TEM(1,0) Cylindrical mode
Experiment on CRDS
Cavity condition and relation for concentration
For Spherical Mirrors, the stable cavity condition is
Where L is the length of the cavity and and are the radii of
curvature of the spherical mirrors.
Output response [*]
Where is the Single pass absorption loss and is the cavity gain factor.
[*].Barbara A. Paldus and Alexander A. Kachanov, An historical overview of cavity enhanced methods , Can. J. Physics 83: 975-999 (2005)
Experimental Set Up
A. Combi Controller (ITC 502)
B. LED Mount(TCLDM 9)
C. Iris DiaphragmsD. Gas ChamberE. Cavity MirrorF. Converging LensG. Optical ChopperH. IR Sensor
(PDA 155)I. Lock in Amplifier
(SR 510)J. Chopper Controller
(SR 540) K. (Highly Pure N2 tanksL. Throttling valve M. Humidity Sensor
(HoneyWell HIH-4000-001)N. Collimating system
N
Broadband LED Source (840-1020 nm)
9615 10115 10615 11115 11615 121150
5
10
15
20
25
30
35
40
45
Water Absorption Cross-section
Wavenumber cm-1
Cro
ss-s
ecti
on
(m
2)/m
ole
cule
*10
E-2
6
9615 10115 10615 11115 11615 121150
0.001
0.002
0.003
0.004
0.005
0.006
0.007
LED Intensity Input /Output
InputOutput
Wavenumber cm-1Probability
Water vapor cross section is taken from the HITRAN database and LED Input intensity distribution over frequency is considered to be Gaussian. The expected output is at 25% RH at 1 atm pressure and 100 m path length
Primary Experimental Result: Accuracy of measurement
Cavity is aligned
Output signal with free air medium : 195 μV
Output signal fluctuation : 0.5 μV
From ICOS calculation, absorption of 1.5 μV of
intensity with total output of 195 μV signal is
equivalent to Relative Humidity of 0.66%.
Hence, Broadband LED source could be used to
study water vapor concentration with sufficient
accuracy.
Expectation:
Signal level Increases slightly while passing Pure
Nitrogen!
Signal level decreases by different amount with
different concentration of the water vapor as
shown in graph!
Ready To Take the Data !
Problem arises! Signal level goes down significantly
while Passing Nitrogen
What can be the cause?
Gas is cool enough to make surrounding water
vapor condensed on the mirror?
Gas is heated up by passing through a long
copper tube immersed in flowing hot water!
Some effect of Trublance of the Supplied
Nitrogen?
I have used metallic vacuum chamber so that
we can trap the Nitrogen !
Presence of Hydrocarbons in the commercially available Nitrogen?
• Used Chromatographic Charcoal filter and water vapor filter!
NO!
Problem Solved ?
What is the structure of output intensity of the LED?
Measured Manufacture’s Information sheet!
What is the Problem:
Mirrors ?All the above experiments shows that The
problem can be because of reflectivity of the
mirrors!
We have tested the mirrors for change of
reflectivity in Humid environment!
Manufacturing company also mentioned that
some of the coating can be effected by Humid
environment depending upon the process of
coatings! http://www.precisionphotonics.com/faq.asp
Source Detector
Conclusions
We can form sensitive Cavity by using 940 nm
LED.
Mirrors ( With dielectric coating by e-beam
coating ) are very sensitive to moisture and they
can be used to detect presence of water vapor
(further study required).
For the water vapor absorption cavity enhance
spectroscopy , use of mirrors with e-beam coating
are not good.
What Next ?
Calculation of output pattern by Using exact output pattern of LED instead of assuming Gaussian like output.
Single mode 940 nm Infrared Laser Diode are available in Thor Labs ! We can test our system with these to further conform that the mirrors are moisture sensitive.
Replace the mirror I have used by the mirrors coated by Ion Beam Sputtering (IBS).
If collimation of the LED beam can be Improved, narrow beam of the Infrared light can be used so that we can lower the no of cavity mode and can improve the sensitivity of the system.
Acknowledgement
Colleagues
Tyler Reese
Melanie Brady
Rajendra Timilsina
Ek Raj Adhikari
Advisor Prof Christopher Winstead
Questions ?
Recommended