CAVITY RING DOWN
SPECTROSCOPY
AYSENUR BICER
Outline
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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.
2.
The effective absorption path length is very long
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 cavity
By 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)
I 0  I laser T
2
exp(   L )
• The intensity of the second pulse
I1  I 0 R
2
exp(  2 L )
• After n complete round trip the pulse intensity
behind the cavity will be
In  IoR
2n
exp(  2 n  L )
Pulsed laser CRD spectroscopy
Continuous Wave CRD spectroscopy
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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
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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.
AOM
He-Ne laser
Photodiode
DFB diode laser
1.6~1.61µ m
He-Ne laser
AOM
DFB
diode
laser
Diode
laser
controller
AOM Driver
Wavemeter
or OSA
PD
L
PZT
driver
preamp
Scope
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
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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.
Temperature (Celsius) 130 C - 30.30 0 C increasing by 0.10 0 C
Wavelength between 1600.566nm – 1602.534nm
0.0
0.1
8.0
0.2
7.5
0.3
7.0
Experimental Data
12
CO2 98.42% of 400ppm
0.4
H2O 1.5%
13
6.5
CO2 1.11% of 400ppm
at atmospheric pressure
and room temperature
1600
1601
1602
Wavelength (nm)
0.5
1603
Calculated Hitran Absorbance
CRD Decay Time (s)
8.5
0.5
CO2 98.42%
13
CO2 1.11%
at atmospheric pressure
and room temperature
0.006
0.005
0.3
0.003
0.2
0.002
0.1
12
0.001
0.0
1590
0.000
1595
1600
1605
1610
Wavelength (nm)
1615
1620
13
0.004
CO2 ~4ppm absorbance
CO2 ~400ppm absorbance
0.4
12
0.5
0.5
CO2 98.42%
13
CO2 1.11%
0.4
at atmospheric pressure
and room temperature
13
0.3
0.2
0.2
0.1
0.1
0.0
0.0
12
0.3
1600
1602
1604
1606
Wavelength (nm)
1608
1610
CO2 ~4ppm absorbance
CO2 ~400ppm absorbance
0.4
12
0.14
13
CO2 1.11%
0.14
0.12
at atmospheric pressure
and room temperature
0.10
13
12
CO2 98.42%
0.10
0.08
0.08
0.06
0.06
0.04
0.04
0.02
0.02
0.00
0.00
1605.0
1605.2
1605.4
1605.6
Wavelength (nm)
1605.8
1606.0
CO2 ~4.4 ppm absorbance
CO2 ~400ppm absorbance
0.12
12
10
0.0
-0.1
-0.2
8
7
-0.3
8/13/2012
Measured with CRD, Yokogawa OSA
(0.1nm accuracy)
Calculated from Hitran(400ppm CO2 atmosphere
-0.4
-0.5
room temperature)
6
1604
1605
1606
Wavelength (nm)
-0.6
1607
Absorbance(ln(I/I0))
Decay Time (s)
9
Decay Time (s)
0.0
8
-0.1
7
-0.2
6
-0.3
5
4
3
1604
8/14/2012
Measured with CRD, Yokogawa OSA
(0.1nm accuracy)
Calculated from Hitran(400ppm CO2 atmosphere
room temperature)
1605
1606
Wavelength (nm)
-0.4
-0.5
-0.6
1607
Absorbance(ln(I/I0))
9
10
9
0.1
Experimental Data(wavelength
measured with Yokogawa, +/- 0.2nm)
12
CO2 98.42% of 400ppm
8
H2O 1.5%
13
0.2
CO2 1.11% of 400ppm
at atmospheric pressure
and room temperature
7
1604.5
1605.0
1605.5
1606.0
Wavelength (nm)
1606.5
0.3
1607.0
Calculated Hitran Absorbance
CRD Decay Time (s)
0.0
R=∞
R=200cm
L=60 cm
W0= 0.683 mm
W1= 0.816 mm
zR = 916.5 mm
2
 zR 
R ( z )  z [1  
 ]
 z 
Knife edge method
2 ( x
I (x)  e
2
w
 y
2
)
2
w

2
 dy  dxI ( x , y )




 0 . 16
 dy  dxI ( x , y )


w

2
 dy  dxI ( x , y )




 dy  dxI ( x , y )


 0 . 84
• 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
• First order diffracted
beam 10.42mW
10.42×0.84=8.752mW
9.9775mm
10.42×0.16=10.7950mW
10.7950mm
10.7950-9.9775=0.817mm
References
• Berden, G., Engeln, R. (2009). Cavity ringdown spectroccopy: Techniques and
applications. A John WILEY and Sons, Inc.,
Publication.
• http://massey.dur.ac.uk/resources/grad_skills/
KnifeEdge.pdf