CAVITY RING DOWN SPECTROSCOPY

AYSENUR BICER

Outline

• What is CRD spectroscopy • A simple CRDS experiment• Pulsed laser CRDS versus CW-CRDS• CW- CRDS experimental schema• Experimental results • Knife edge method

What is CRD spectroscopy• CRDS is a sensitive absorption technique in which the rate of

absorption in an optical cavity is measured• It has significantly high sensitivity

1. The effective absorption path length is very long2. The sensitivity is independent of intensity fluctuations of the light

source• Small fractional absorptions sub- ppm levels CO2 400 ppm

(open air)A Simple CRDS Experiment

A laser pulse coupled into an optical cavity The decay time is determined by measuring the time dependence of the light

leaking out of the cavityBy measuring the decay time the rate of absorption is determined directly

providing the losses on an absolute scale

• After one pass-through the cavity the intensity of the first optical pulse (Beer-Lambert’s law)

• The intensity of the second pulse

• After n complete round trip the pulse intensity behind the cavity will be

)exp(20 LTII laser

)2exp(201 LRII

)2exp(2 LnRII non

Pulsed laser CRD spectroscopy

• Pulsed lasers promise Fourier transform limited line widths of the order of 100 MHz, in practice it is difficult to archive

• The length of the cavity, L, and the radius of the mirrors curvature of the mirrors should be chosen such that cavity is optically stable

• They are rather bulky, require massive amounts of electricity to run, and cost several hundred thousand dollars

• The pulsed lasers have the advantage of broad wavelength coverage

Continuous Wave CRD spectroscopy

• The main advantage of using CW laser radiation sources in any spectroscopic system is the increased resolution in the frequency domain

• (Trigger event ) In order to observe a ring down transits CW have to be switched of

• The bandwidth of these lasers is very small so can be only scanned over small wavelength regions

• each mode can have various allowed longitudinal modes associated with it

• The frequency spacing between two successive transverse modes is usually much smaller than the spacing between two successive longitudinal modes and depends on the characteristics of the cavity (length, mirror radii)

CRD spectroscopy Using Continuous Wave Laser

• Because of narrow line width of the laser and high finesse of the cavity, spectral overlap between the laser frequency and the frequency of the cavity modes are no longer obvious • 1605.74nm- 1602.31nm infrared light region to solution He- Ne laser can be used The helium-neon laser (He-Ne) was the first gas laser. The most

widely used laser wavelength is the red wavelength (632.8 nm) with a CW power output ranging from 1mW to 100mW and laser lengths varying from 10 to 100 cm.

DFB diode laser

He-Ne laser

AOM

Photodiode

Diode laser controller

Wavemeter or OSA

Scope

DFB diode laser

1.6~1.61µ m

He-Ne laser

PD preamp

AOM

AOM Driver

PZT driver

L

First step

• The DFB laser has a stable wavelength that is set during manufacturing by the pitch of the grating, and can only be tuned slightly with temperature.

• It has elliptical beam shape• The beam pass through wave plates

Second step• AOM uses the acousto-optic effect to diffract and shift the frequency of light using

sound waves so we can use it in CRD spectroscopy for frequency control• The laser light that passes through AOM will be diffracted into multiple orders • The first order diffracted beam is directed through the optical cavity• Frequency of radiation from CW laser is coincident with cavity mode, power is

likely build up within the optical cavity• Trigger pulse is sent to AOM to switch it off• The first order beam is quickly extinguish 150ns/mm

Third step

• The ring down signal registered by photodiode to oscilloscope.

1600 1601 1602 1603

6.5

7.0

7.5

8.0

8.5

Experimental Data

12CO2 98.42% of 400ppm

H2O 1.5%

13CO2

1.11% of 400ppm

at atmospheric pressure and room temperature

Wavelength (nm)

CR

D D

ecay

Tim

e (

s)

0.5

0.4

0.3

0.2

0.1

0.0

Calculated H

itran Absorbance

C0Temperature (Celsius) 13 - 30.30 increasing by 0.10Wavelength between 1600.566nm – 1602.534nm

C0 C0

1590 1595 1600 1605 1610 1615 1620

0.0

0.1

0.2

0.3

0.4

0.5

12CO2 98.42%

13CO2

1.11%

at atmospheric pressure and room temperature

Wavelength (nm)

12C

O2 ~

400ppm

abso

rban

ce

0.000

0.001

0.002

0.003

0.004

0.005

0.006

13C

O2 ~

4ppm

abso

rbance

1600 1602 1604 1606 1608 1610

0.0

0.1

0.2

0.3

0.4

0.5

12CO2 98.42%

13CO2

1.11%

at atmospheric pressure and room temperature

Wavelength (nm)

12C

O2 ~

400pp

m a

bsor

banc

e

0.0

0.1

0.2

0.3

0.4

0.513C

O2 ~

4ppm

absorbance

1605.0 1605.2 1605.4 1605.6 1605.8 1606.0

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

12CO2 98.42%

13CO2

1.11%

at atmospheric pressure and room temperature

Wavelength (nm)

12C

O2 ~

400pp

m a

bsor

banc

e

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.1413C

O2 ~

4.4 ppm absorbance

1604 1605 1606 16076

7

8

9

10

Wavelength (nm)

Dec

ay T

ime

(s)

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

8/13/2012 Measured with CRD, Yokogawa OSA

(0.1nm accuracy) Calculated from Hitran(400ppm CO

2 atmosphere

room temperature)

Absorbance(ln(I/I0 ))

1604 1605 1606 16073

4

5

6

7

8

9

8/14/2012 Measured with CRD, Yokogawa OSA

(0.1nm accuracy) Calculated from Hitran(400ppm CO

2 atmosphere

room temperature)

Wavelength (nm)

Dec

ay T

ime

(s)

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

Absorbance(ln(I/I0 ))

1604.5 1605.0 1605.5 1606.0 1606.5 1607.07

8

9

10

Experimental Data(wavelength measured with Yokogawa, +/- 0.2nm)

12CO2 98.42% of 400ppm

H2O 1.5%

13CO2

1.11% of 400ppm

at atmospheric pressure and room temperature

Wavelength (nm)

CR

D D

eca

y Tim

e (

s)

0.3

0.2

0.1

0.0

Calcu

late

d H

itran A

bsorb

ance

L=60 cm

R=∞ R=200cm

]1[)(2

z

zzzR R

W0= 0.683 mmW1= 0.816 mm zR = 916.5 mm

Knife edge method

84.0

),(

),(

16.0

),(

),(

)(

2

2

)(22

22

yxdxIdy

yxdxIdy

yxdxIdy

yxdxIdy

exI

w

w

w

yx

• First order diffracted beam 10.42mW 10.42×0.84=8.752mW

9.9775mm 10.42×0.16=10.7950mW 10.7950mm10.7950-9.9775=0.817mm

• First order diffracted beam 10.54mW

10.54×0.84=8.8636mW 16.690mm 10.54×0.16=1.6864mW 17.350mm 17.350-16.690=0.660mm

References

• Berden, G., Engeln, R. (2009). Cavity ring-down spectroccopy: Techniques and applications. A John WILEY and Sons, Inc., Publication.

• http://massey.dur.ac.uk/resources/grad_skills/KnifeEdge.pdf