Introduction
1
0
1
-
9
2
-
4
A
.
v
e
r
9
7
0
2
0
-
0
0
0
9
4
–
5
Due to their inherent structure, the molecules in the sample gas each have
characteristic natural frequencies (or resonances). When the output of the
laser is tuned to one of those natural frequencies, the molecules with that
particular resonance will absorb energy from the incident beam. That is, as the
beam of incident intensity,
I
0
(
λ
)
,
passes through the sample, attenuation occurs
via absorption by the trace gas with absorption cross section
σ
(
λ
). According
to the Beer-Lambert absorption law, the intensity remaining,
I
(
λ
), as measured
by the detector at the end of the beam path of length
l
(cell length
×
number
of passes), is given by
)
1
(
,
where
N
represents the species concentration. Thus, the ratio of the absorption
measured when the laser is tuned on-resonance versus off-resonance is
directly proportional to the number of molecules of that particular species in
the beam path, or
)
2
(
.
Figure 1–2 shows typical raw data from a laser absorption spectrometer scan
including the incident laser intensity,
I
0
(
λ
)
,
and the transmitted intensity,
I
(
λ
),
for a clean system and one with contaminated mirrors (shown to illustrate the
systems relative insensitivity to mirror contamination). The positive slope of
the raw data results from ramping the current to tune the laser, which not only
increases the wavelength with current, but also causes the corresponding
output power to increase. By normalizing the signal by the incident intensity,
any laser output fluctuations are cancelled, and a typical, yet more
pronounced, absorption profile results, as shown in Figure 1–3. Note that
contamination of the mirrors results solely in lower overall signal. However, by
tuning the laser off-resonance as well as on-resonance and normalizing the
L
D
DETECTOR
LASER
R
O
R
R
I
M
R
O
R
R
I
M
I
0
(
λ
)
I
(
λ
)
TRACE GAS ABSORPTION
α
(
λ
)
GAS IN
GAS OUT
Figure 1–1
Schematic of a typical laser diode
absorption spectrometer.
I
λ
( )
I
0
λ
( )
exp
σ λ
( )
lN
–
[
]
=
N
1
–
σ λ
( )
l
--------------
I
λ
( )
I
0
λ
( )
-------------
ln
=
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