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 ?