1. CAVITY RING DOWN SPECTROSCOPY
AYSENUR BICER

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

3. 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
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 CO 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

4. 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

5. Continuous Wave CRD spectroscopy Pulsed laser 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)
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

6. 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
nm nm
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.

7. AOM
He-Ne laser
Photodiode
DFB diode laser

8. Diode laser controller
1.6~1.61µ m
DFB diode laser
Diode laser controller
He-Ne laser
AOM
AOM Driver PC
DAQ
Wavemeter or OSA
Scope PD
preamp L
PZT driver

9. 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

10. 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

11. Third step
The ring down signal registered by photodiode to oscilloscope.

15. Temperature (Celsius) 13 - 30.30 increasing by 0.10
Wavelength between nm – nm

22. R=∞
R=200cm
L=60 cm
W0= mm
W1= mm
zR = mm

23. Knife edge method

24. First order diffracted beam 10.54mW
First order diffracted beam 10.42mW 10.42×0.84=8.752mW
9.9775mm
10.42×0.16= mW mm
=0.817mm

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