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A. Maity et al.
[14–17]. The group of Romanini and co-workers has utilized this technique [18]
and later on, another group, Mazzotti and co-workers applied this technique in the
mid-IR spectral region to record the ro-vibrational spectra of CO 2 molecules [19].
Consequently, the CRDS technique has transformed into a precision spectroscopy
when it is coupled with OFC strategy.
Moreover, in the context CRDS measurements, there is an important issue in the
optical cavity-based techniques which is known as optical feedback. This usually
disturbs the performance of the CRDS technique. But sometimes optical feedback
would be useful for the measurements of molecular species if it can be utilized under
some special conditions with suitable arrangement in the experimental setup [20].
In this method, a reference cavity is constructed where the optical feedback takes
place in resonance condition. Using this strategy, the central frequency of the laser
is stabilized at the resonance condition, generating narrow spectral line-width of the
laser. There is an important work by D. Romanini and his group who have shown the
optical feedback in a V-shaped optical cavity and utilized this method for methane
measurements in real-time [21, 22].
There is an important development in the field of optical cavity-based techniques
where frequency modulation spectroscopy is combined with the high-finesse cavity.
It is called (NICE-OHMS), which stands for noise-immune cavity-enhanced optical
heterodyne molecular spectroscopy where the laser-noise at the low-frequency side
is eliminated [23]. The frequency of modulation is used in such a manner that the FSR
of the cavity is matched with that one. The NICE-OHMS technique was employed
for the first time for studying the sub-Doppler spectroscopic detection of C 2 HD overtone transition [23]. It was also employed for the study of full Doppler or pressurebroadened spectra [24]. There is another important study for detection of C 2 H 2 in
ppt level utilizing NICE-OHMS technique with a fibre-based laser system [25].
Furthermore, CRDS technique coupled with new-generation quantum cascade
lasers (QCLs) technology has recently received enormous research interests in the
scientific community [26]. QCL provides to access molecular species in the mid-IR
region which is called “finger-print” region. In this region, the molecules have higher
absorption cross-sections compared to the overtone or combination bands that usually
occur in the near-IR region. Therefore, we have the opportunity to access most of the
fundamental vibrational bands of the molecular species or their isotopes. Thus, the
sensitivity of CRDS is highly enhanced when the technique is coupled with the newgeneration QCL technology as the optical source. Moreover, the QCL has several
other advantages such as wide tunability range (≥100 cm
−1 ), high optical power
(≥90 mW) and extremely narrow light width (~0.0001 cm
−1 ). Due to such unique
features, QCL coupled CRDS techniques are widely used for the high-resolution
spectroscopic applications and some commercial systems are also available for realtime gas sensing applications. In this context, to measure the NH 3 levels, a cw-DFBQCL was used at 8.5 μm and also optical feedback (OF) method was demonstrated
for high-resolution molecular spectroscopy [27].
A. Maity et al.
[14–17]. The group of Romanini and co-workers has utilized this technique [18]
and later on, another group, Mazzotti and co-workers applied this technique in the
mid-IR spectral region to record the ro-vibrational spectra of CO 2 molecules [19].
Consequently, the CRDS technique has transformed into a precision spectroscopy
when it is coupled with OFC strategy.
Moreover, in the context CRDS measurements, there is an important issue in the
optical cavity-based techniques which is known as optical feedback. This usually
disturbs the performance of the CRDS technique. But sometimes optical feedback
would be useful for the measurements of molecular species if it can be utilized under
some special conditions with suitable arrangement in the experimental setup [20].
In this method, a reference cavity is constructed where the optical feedback takes
place in resonance condition. Using this strategy, the central frequency of the laser
is stabilized at the resonance condition, generating narrow spectral line-width of the
laser. There is an important work by D. Romanini and his group who have shown the
optical feedback in a V-shaped optical cavity and utilized this method for methane
measurements in real-time [21, 22].
There is an important development in the field of optical cavity-based techniques
where frequency modulation spectroscopy is combined with the high-finesse cavity.
It is called (NICE-OHMS), which stands for noise-immune cavity-enhanced optical
heterodyne molecular spectroscopy where the laser-noise at the low-frequency side
is eliminated [23]. The frequency of modulation is used in such a manner that the FSR
of the cavity is matched with that one. The NICE-OHMS technique was employed
for the first time for studying the sub-Doppler spectroscopic detection of C 2 HD overtone transition [23]. It was also employed for the study of full Doppler or pressurebroadened spectra [24]. There is another important study for detection of C 2 H 2 in
ppt level utilizing NICE-OHMS technique with a fibre-based laser system [25].
Furthermore, CRDS technique coupled with new-generation quantum cascade
lasers (QCLs) technology has recently received enormous research interests in the
scientific community [26]. QCL provides to access molecular species in the mid-IR
region which is called “finger-print” region. In this region, the molecules have higher
absorption cross-sections compared to the overtone or combination bands that usually
occur in the near-IR region. Therefore, we have the opportunity to access most of the
fundamental vibrational bands of the molecular species or their isotopes. Thus, the
sensitivity of CRDS is highly enhanced when the technique is coupled with the newgeneration QCL technology as the optical source. Moreover, the QCL has several
other advantages such as wide tunability range (≥100 cm
−1 ), high optical power
(≥90 mW) and extremely narrow light width (~0.0001 cm
−1 ). Due to such unique
features, QCL coupled CRDS techniques are widely used for the high-resolution
spectroscopic applications and some commercial systems are also available for realtime gas sensing applications. In this context, to measure the NH 3 levels, a cw-DFBQCL was used at 8.5 μm and also optical feedback (OF) method was demonstrated
for high-resolution molecular spectroscopy [27].