Environnement AS32M, Technical Manual, Page: 27 / 186

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AS32M
Environnement
S.A
DECEMBER 2013
2–3
2 PRINCIPLE OF OPERATION
Refer to Figure 2–1.
2.1 THEORETICAL BASIS: OPTICAL CAVITY
Molecules of NO
2
absorb photons at a wavelength of 450 nm with an absorption coefficient of
4.5x10
-19
cm
-1
. In order to measure low concentrations (a few ppb) of NO
2
, it is necessary to
increase the time that the photons interact with the NO
2
molecules. This is achieved by closing the
optical cavity at both ends with a high reflectivity mirror (R > 99,99 %). The photons emitted by the
LED enter the optical cavity through the first mirror and are trapped between both mirrors, which
transmit less than 0.01 % of the photons. Then, the photons trapped inside the cavity are reflected
back and forth between the two mirrors, with a small fraction transmitting through each mirror with
each pass. This increases their path from 26 cm (optical cavity length) to approximately 1 km.
2.2 MEASUREMENT PRINCIPLE
The measurement principle of the AS32M is based on the cavity-attenuated phase-shift spectroscopy
(CAPS) technique. Because of the long effective optical path covered by the photons in the optical
cavity, the shape of the light intensity received by the detector located behind the cavity is different
from the signal shape emitted by the LED. In the AS32M, the shape of the luminous signal emitted by
the LED is square, and the shape of the signal received by the detector is a succession of increasing
and decreasing exponential curves. This distortion is measured as a phase shift
θ , comparable to the
measurement of the decay time

in the optical cavity. This phase shift depends on the following:

Speed of light
c

Coefficient of mirror reflectivity
R

Modulation frequency
f
of the LED

Length of optical cavity
L

Concentration of absorbent species
N

Absorption cross section

of absorbent species
   
  

.
. . 2
1
cot
,
. 2
. .
cot cot
0
0
L f
R c
f
N c




 


 
Figure 2–1 – Diagram of the optical cavity