288
A. Maity et al.
per trillion (ppt) are extremely difficult. Recently, an optical cavity-enhanced technique such as cavity ring-down spectroscopy (CRDS) has received a lot of research
interests in scientific community because of its nature of ultra-sensitive absorption
measurements in the time domain. CRDS is a direct quantitative absorption measurement with high-temporal and spatial resolutions. The leaking light intensity from the
cavity which decays exponentially is measured in the time domain. This decay is
called the “ring-down” and cavity rings down with 1/e time constant. O’Keefe and
Deacon first introduced the concept of cavity ring-down spectroscopy (CRDS) in
1988, while measuring the reflectivity of the mirrors. They have shown that the
detection of highly forbidden transition of molecular oxygen is possible by this
optical method [1].
However, the group of van Zee et al. in 1999 demonstrated the CRDS technique
with pulsed laser and they have shown that with single mode excitation, a relative
precision of 0.03% for the ring-down measurement is feasible [2]. But the concept
of continuous wave (cw) CRDS technique was first introduced by Romanini and his
group [3]. This methodology is suitable for high-resolution molecular spectroscopy
in the gas-phase species. Later on, several research groups have taken the steps
forward towards its technological advancements as well as numerous applications in
various fields as described later on. The technique assists to measure weak absorption
of molecular species along with molecular concentrations in ultra-low levels.
The technique has several advantages compared to the traditional absorption spectroscopic methods. The ring-down signal is monitored in the domain of time, thus
providing the signal which is insensitive to fluctuations of the intensity of the optical
light source. As the light travels inside the optical cavity many times, the effective
optical path length is enormous, usually several kilometers, resulting high-sensitivity
for any weak absorption of molecular species. The typical sensitivity is in the order
of 10
–8 –10
–10 cm
−1 . For the absorption measurement, there is no need for secondary
calibration if the molecular absorption coefficient is known at the probed wavelength.
Moreover, the CRDS systems are usually compact and robust, giving its potential
for field deployment measurements.
In this chapter, at first, we have described the working principle of CRDS, its sensitivity and the typical detection limit. We have explained how the modes are established in the optical cavity. We have then briefly described the operating principle of
continuous wave (cw) CRDS technique. Moreover, few important variants of CRDS
technique are also described in brief. Finally, the applications of CRDS technique
in some important fields like high-resolution fundamental molecular spectroscopy,
atmospheric gas sensing and biomedical diagnostics are also presented.
2 A Brief Overview of Cavity Ring-Down Spectroscopy
The working principle of a basic CRDS instrument comprised of two-mirrors in an
optical cavity is shown in Fig. 1.
A. Maity et al.
per trillion (ppt) are extremely difficult. Recently, an optical cavity-enhanced technique such as cavity ring-down spectroscopy (CRDS) has received a lot of research
interests in scientific community because of its nature of ultra-sensitive absorption
measurements in the time domain. CRDS is a direct quantitative absorption measurement with high-temporal and spatial resolutions. The leaking light intensity from the
cavity which decays exponentially is measured in the time domain. This decay is
called the “ring-down” and cavity rings down with 1/e time constant. O’Keefe and
Deacon first introduced the concept of cavity ring-down spectroscopy (CRDS) in
1988, while measuring the reflectivity of the mirrors. They have shown that the
detection of highly forbidden transition of molecular oxygen is possible by this
optical method [1].
However, the group of van Zee et al. in 1999 demonstrated the CRDS technique
with pulsed laser and they have shown that with single mode excitation, a relative
precision of 0.03% for the ring-down measurement is feasible [2]. But the concept
of continuous wave (cw) CRDS technique was first introduced by Romanini and his
group [3]. This methodology is suitable for high-resolution molecular spectroscopy
in the gas-phase species. Later on, several research groups have taken the steps
forward towards its technological advancements as well as numerous applications in
various fields as described later on. The technique assists to measure weak absorption
of molecular species along with molecular concentrations in ultra-low levels.
The technique has several advantages compared to the traditional absorption spectroscopic methods. The ring-down signal is monitored in the domain of time, thus
providing the signal which is insensitive to fluctuations of the intensity of the optical
light source. As the light travels inside the optical cavity many times, the effective
optical path length is enormous, usually several kilometers, resulting high-sensitivity
for any weak absorption of molecular species. The typical sensitivity is in the order
of 10
–8 –10
–10 cm
−1 . For the absorption measurement, there is no need for secondary
calibration if the molecular absorption coefficient is known at the probed wavelength.
Moreover, the CRDS systems are usually compact and robust, giving its potential
for field deployment measurements.
In this chapter, at first, we have described the working principle of CRDS, its sensitivity and the typical detection limit. We have explained how the modes are established in the optical cavity. We have then briefly described the operating principle of
continuous wave (cw) CRDS technique. Moreover, few important variants of CRDS
technique are also described in brief. Finally, the applications of CRDS technique
in some important fields like high-resolution fundamental molecular spectroscopy,
atmospheric gas sensing and biomedical diagnostics are also presented.
2 A Brief Overview of Cavity Ring-Down Spectroscopy
The working principle of a basic CRDS instrument comprised of two-mirrors in an
optical cavity is shown in Fig. 1.