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A. L. Chakraborty and A. Roy
experimental relative transmission in a least squares sense and the mole fraction
and pressure are extracted. Note that the relative transmission is independent of the
laser intensity. This is important because it is impossible to ensure that the received
light intensity remains constant with time. This simple fact makes TDLS with direct
detection an inherently absolute measurement technique. The measurements do not
need to be calibrated against other gold standards (assuming of course that the path
length and the spectroscopic parameters are correctly known). This aspect, although
often under-appreciated, is extremely important from a practical point of view. TDLS
with direct detection is therefore the preferred method whenever it can be used
because the experimental arrangement is simple and signal extraction and processing
is straightforward.
While direct detection is simple and an absolute measurement technique, it suffers from poor signal-to-noise ratio (SNR) and cannot be used for high-sensitivity
applications. The photo-detector output has a large sloping baseline (as shown in
Fig. 6b) that contains no information about the mole fraction. The informationbearing absorption-dependent signal, which is weak for low mole fractions, appears
on this large dc background and is very hard to measure because it is impossible to
selectively amplify that component. Additionally, direct detection being a dc measurement technique, must compete with the 1/f noise that is dominant at low operating
frequencies. The low SNR is especially problematic for weakly absorbing species,
low mole fractions and short path lengths. Although multi-pass gas cells can be used
to increase the effective path length by as much as 30 times, they are costly and not
easy to align. The other option is to interrogate the strong fundamental mid-infrared
gas lines using a mid-infrared laser. An additional practical issue with direct detection is that the recovery of the absolute absorption line shape by normalization is
susceptible to baseline fitting errors that cause artificial broadening or narrowing of
the line shape leading to significant errors in gas parameter extraction.
4 Wavelength Modulation Spectroscopy
The final option for increasing SNR is to use switch to modulation spectroscopy,
which is broadly classified as frequency modulation spectroscopy (FMS) [45] and
wavelength modulation spectroscopy [4, 46, 47]. The underlying principle of both
techniques is to translate the detection to a higher frequency at which the 1/f noise
reduces drastically. This makes it possible to use narrow-band detection techniques
to reduce the noise power and to thereby improve the SNR. FMS and WMS differ
in their operating regimes, mathematical formalism and applications. FMS systems
are not as widely used for trace gas sensing because they require high-speed and
expensive electro-optic modulators and wide-band photo-detectors.
A. L. Chakraborty and A. Roy
experimental relative transmission in a least squares sense and the mole fraction
and pressure are extracted. Note that the relative transmission is independent of the
laser intensity. This is important because it is impossible to ensure that the received
light intensity remains constant with time. This simple fact makes TDLS with direct
detection an inherently absolute measurement technique. The measurements do not
need to be calibrated against other gold standards (assuming of course that the path
length and the spectroscopic parameters are correctly known). This aspect, although
often under-appreciated, is extremely important from a practical point of view. TDLS
with direct detection is therefore the preferred method whenever it can be used
because the experimental arrangement is simple and signal extraction and processing
is straightforward.
While direct detection is simple and an absolute measurement technique, it suffers from poor signal-to-noise ratio (SNR) and cannot be used for high-sensitivity
applications. The photo-detector output has a large sloping baseline (as shown in
Fig. 6b) that contains no information about the mole fraction. The informationbearing absorption-dependent signal, which is weak for low mole fractions, appears
on this large dc background and is very hard to measure because it is impossible to
selectively amplify that component. Additionally, direct detection being a dc measurement technique, must compete with the 1/f noise that is dominant at low operating
frequencies. The low SNR is especially problematic for weakly absorbing species,
low mole fractions and short path lengths. Although multi-pass gas cells can be used
to increase the effective path length by as much as 30 times, they are costly and not
easy to align. The other option is to interrogate the strong fundamental mid-infrared
gas lines using a mid-infrared laser. An additional practical issue with direct detection is that the recovery of the absolute absorption line shape by normalization is
susceptible to baseline fitting errors that cause artificial broadening or narrowing of
the line shape leading to significant errors in gas parameter extraction.
4 Wavelength Modulation Spectroscopy
The final option for increasing SNR is to use switch to modulation spectroscopy,
which is broadly classified as frequency modulation spectroscopy (FMS) [45] and
wavelength modulation spectroscopy [4, 46, 47]. The underlying principle of both
techniques is to translate the detection to a higher frequency at which the 1/f noise
reduces drastically. This makes it possible to use narrow-band detection techniques
to reduce the noise power and to thereby improve the SNR. FMS and WMS differ
in their operating regimes, mathematical formalism and applications. FMS systems
are not as widely used for trace gas sensing because they require high-speed and
expensive electro-optic modulators and wide-band photo-detectors.