1. Background
In essence, the principle of Cavity Ring-Down Spectroscopy or CRDS is based on absorption spectroscopy. Absorption spectroscopy provides the most general spectroscopic method for detection of important trace species. In the gas phase, the spectrum of a species consists of many sharp rotational lines that provide high selectivity (a molecular 'fingerprint.') The strength of a line and the number of molecules in the lower state of the transition determine the sensitivity of this method. Water vapor is particularly favorable because its spectrum extends into the near infrared where several widely spaced rotational lines with strong absorption can be found.

In a conventional absorption measurement, the intensity of light transmitted through a sample is compared to the transmission without the sample. The smallest number density that can be determined by this method is given by:



where is the smallest fractional change in light intensity that can be detected. It is clear from this formula that to maximize sensitivity, one wants to select a molecular transition with a large , as well as have the longest possible pathlength L, and the smallest possible . The pathlength L in the conventional absorption method is limited by the physical size of the apparatus. One can increase the effective pathlengths from 10 to 100 times the physical lengths using "multiple pass" cells, such as White [1] or Herriott [2] cells. Therefore, the sensitivity and accuracy is often limited by the amplitude noise of light sources and the detection system.

2. The Power of CRDS
CRDS, on the other hand, is not affected by the laser noise because it only measures "time". CRDS is an emerging technique that has been demonstrated to dramatically increase the sensitivity of absorption spectroscopy [3,4]. It utilizes optical excitation of a stable optical resonator formed from two ultra-high reflectivity mirrors separated by a distance d, the ring-down cavity (RDC). In a cavity ring-down measurement, a fraction of radiation from a narrow bandwidth laser is coupled into the cavity and then abruptly turned off. Light inside the cavity is reflected by the high reflective mirrors many times, leaking out a tiny amount upon each reflection. Light leaking out of the cavity is the ring-down signal. This signal has an envelope that is simply a first order exponential decay.



This decay arises from losses in the mirror coatings and absorption by the gas sample contained between the mirrors. When light is tuned away from the molecular resonance or the cell is empty, the ring-down time, , is determined by

When there are gas molecules present inside the cell, the ring-down time, , is then determined also by absorption by the molecule,

In the above equations, is the ring-down time of the empty cavity, is the ring-down time at frequency ; d is the physical separation of the two cavity mirrors; R is the reflectivity of the mirrors (assumed to be the same for both mirrors); is the absorption cross section of the particular molecules that absorb light at frequency ; N is their number density, which is proportional to the absolute concentration. CRDS measurement involves initial measurement of ring-down time , often measured at a frequency away from molecular absorption, which is independent of frequency in the high reflective region of the mirrors, and the ring-down time at the peak frequency of molecular absorption. Concentration N can be calculated from the following equation:

Dr. Daniele Romanini and Prof. Lehmann [4] have shown that the Ring-Down Cavity Cell (RDC) can be viewed as having an effective pathlength L=d/(1-R), where R is the reflectivity of the mirrors. Since mirrors are commercially available today with reflectivity on the order of 99.999% [5], the effective optical pathlength of the cell is increased by times over the physical dimensions of the device. Thus an absorption pathlength of 10,000 ­10,0000 meters is achieved in the cell on the order of one meter physical length. This can be compared with traditional "multiple pass" cells, where effective pathlengths 10-100 meters are the practical limit.

3. Recent Technical Developments
Since its introduction [3] in1988, CRDS has been applied to detect molecular species in samples from many different scientific research fields, ranging from such hostile environments as combustion and flames [6], in high magnetic fields [7], to discharges [8-10] and molecular beam expansions [11-14]. Its applications also include assignment of diffuse interstellar bands [15] and spectroscopic characterization of biological relevant systems [16].

In early work using CRDS, the excitation source was a nanosecond pulse length dye laser [3]. This provides a convenient light source in the laboratory, but is not practical for industrial applications. After careful review of the basic physics of CRDS, Prof. Kevin Lehmann of Princeton University discovered that Continuous Wave Semiconductor Diode Lasers (CW) laser sources can be used in CRDS, contrary to the wide belief that only pulsed sources can be used.

As a result of this work, Princeton University was awarded a patent that covers the use of any Continuous Wave laser excitation in CRDS. [17] CW, single-mode semiconductor diode lasers are attractive for CRDS excitation, thanks to their small size, low power requirements, long lifetime, monolithic construction, and modest cost. These lasers also offer RDC designers a well-developed technology that has been advanced by widespread industrial applications from bar codes to telecommunications.

Diode lasers in the visible to near infrared spectral region are particularly advanced thanks to their importance in consumer markets and telecommunications. These lasers can operate at ambient temperature and detectors for this spectral range do not require cooling. These features are very convenient for making instruments that are easy to operate and with low energy consumption.

For moisture detection, the effort has relied upon a 1.39 diode laser that can be tuned on and off of resonance with a strong absorption line in the first overtone band of water. Based upon the known cross-section of this absorption line [18] and the observed value for the noise in the determination of the ring-down time, the instrument should be able to detect water vapor at a few parts per trillion (ppt) levels in nitrogen or other inert gases. In addition, it will be directly calibrated since the method will yield the absorption coefficient of the gas. This absorption coefficient is related to the number density by a known factor, the cross-section of the line.

Figure 1 shows a schematic of a typical experimental set-up. The major components are: a single mode diode laser, a Ring-Down Cavity (RDC) with a pair of high reflective mirrors, a photo detector to detect the transmitted light through the RDC. Other components are mode-matching optics to condition the laser beam shape for optimum coupling, an acousto-optic modulator (AOM) to quickly deflect the laser out of the cavity when sufficient light builds up inside the RDC, an optical isolator to prevent light feedback to the diode laser, a computer with data acquisition for controlling the system, taking data and analysis.

4. Progress to Date
Over the past several years, Professor Lehmann, in collaboration with MEECO, Inc., has been developing CRDS for trace-level moisture measurement. This work provided the first demonstration that CRDS could be used with highly attractive continuous wave, semiconductor diode lasers.

Preliminary data shows actual detection of 80 ppb of moisture with a signal-to-noise ratio on the order of 1000. This means CRDS could easily detect less than a 100 ppt moisture level, based on a SNR of 1. Figure 1 shows a schematic of a typical experimental set-up, and Figure 2 shows the observed time-dependent RDC transmission. Figure 3 shows a water spectrum by CRDS, in terms of vs. wavelength of the diode laser.

The effort has been focused on using a 1.39 diode laser manufactured by Sensors Unlimited, Inc. of Princeton, New Jersey. This diode can be tuned on and off of resonance with a strong absorption line in the first overtone band of water. Based upon the known cross section of this absorption line [18] and the observed value for the noise in the determination of the ring-down time, the instrument should be able to detect water vapor at a few ppt levels in nitrogen or other inert gases. In addition, it will be directly calibrated since the method will yield the absorption coefficient of the gas. This absorption coefficient is related to the number density by a known factor, the cross-section of the line.