Cavity ring down spectroscopy
Encyclopedia
Cavity ring-down spectroscopy (CRDS) is a highly sensitive optical spectroscopic
technique that enables measurement of absolute optical extinction by samples that scatter
and absorb
light. It has been widely used to study gaseous samples which absorb light at specific wavelength
s, and in turn to determine mole fractions down to the parts per trillion level. The technique is also known as cavity ring-down laser absorption spectroscopy (CRLAS).
A typical CRDS setup consists of a laser
that is used to illuminate a high-finesse optical cavity
, which in its simplest form consists of two highly reflective mirror
s. When the laser is in resonance
with a cavity mode
, intensity
builds up in the cavity due to constructive interference. The laser is then turned off in order to allow the measurement of the exponentially decaying light intensity leaking from the cavity. During this decay, light is reflected back and forth thousands of times between the mirrors giving an effective path length for the extinction on the order of a few kilometers.
If something that absorbs light is placed in the cavity, the amount of light decreases faster—it makes fewer bounces before it is all gone. A CRDS setup measures how long it takes for the light to decay to 1/e of its initial intensity, and this "ringdown time" can be used to calculate the concentration of the absorbing substance in the gas mixture in the cavity.
. In CRDS, a laser pulse is trapped in a highly reflective (typically R > 99.9%) detection cavity
. The intensity of the trapped pulse will decrease by a fixed percentage during each round trip within the cell due to both absorption and scattering by the medium within the cell and reflectivity losses. The intensity of light within the cavity is then determined as an exponential function
of time.
The principle of operation is based on the measurement of a decay rate rather than an absolute absorbance. This is one reason for the increased sensitivity over traditional absorption spectroscopy. The decay constant, τ, is called the ring-down time and is dependent on the loss mechanism(s) within the cavity. For an empty cavity, the decay constant is dependent on mirror loss and various optical phenomena like scattering and refraction:
where n is the index of refraction within the cavity, c is the speed of light
in vacuum, l is the cavity length, R is the mirror reflectivity, and X takes into account other miscellaneous optical losses. This equation uses the approximation ln(1+x) ≈ x for x close to zero, which is the case under cavity ring-down conditions. Often, the miscellaneous losses are factored into an effective mirror loss for simplicity. An absorbing species in the cavity will increase losses according to the Beer-Lambert law
. Assuming the sample fills the entire cavity,
where α is the absorption coefficient for a specific analyte concentration. The decadic absorbance, A, due to the analyte can be determined from both ring-down times.
Alternately, the molar absorptivity
, ε, and analyte concentration, C, can be determined from the ratio of both ring-down times.
First, it isn't affected by fluctuations in the laser intensity. In most absorption measurements, the light source must be assumed to remain steady between blank (no analyte
), standard (known amount of analyte), and sample (unknown amount of analyte). Any drift (change in the light source) between measurements will introduce errors. In CRDS, the ringdown time does not depend on the intensity of the laser, so fluctuations of this type aren't a problem.
Second, it is very sensitive due to its long pathlength. In absorption measurements, the smallest amount that can be detected is proportional to the length that the light travels through a sample. Since the light reflects many times between the mirrors, it ends up traveling long distances. For example, a laser pulse making 500 round trips through a 1 meter cavity will effectively have traveled through 1 kilometer of sample.
Thus the advantages include:
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
technique that enables measurement of absolute optical extinction by samples that scatter
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
and absorb
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
light. It has been widely used to study gaseous samples which absorb light at specific wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s, and in turn to determine mole fractions down to the parts per trillion level. The technique is also known as cavity ring-down laser absorption spectroscopy (CRLAS).
A typical CRDS setup consists of a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
that is used to illuminate a high-finesse optical cavity
Optical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
, which in its simplest form consists of two highly reflective mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
s. When the laser is in resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
with a cavity mode
Normal mode
A normal mode of an oscillating system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation. The frequencies of the normal modes of a system are known as its natural frequencies or resonant frequencies...
, intensity
Intensity (physics)
In physics, intensity is a measure of the energy flux, averaged over the period of the wave. The word "intensity" here is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech...
builds up in the cavity due to constructive interference. The laser is then turned off in order to allow the measurement of the exponentially decaying light intensity leaking from the cavity. During this decay, light is reflected back and forth thousands of times between the mirrors giving an effective path length for the extinction on the order of a few kilometers.
If something that absorbs light is placed in the cavity, the amount of light decreases faster—it makes fewer bounces before it is all gone. A CRDS setup measures how long it takes for the light to decay to 1/e of its initial intensity, and this "ringdown time" can be used to calculate the concentration of the absorbing substance in the gas mixture in the cavity.
Detailed description
Cavity ring down spectroscopy is a form of laser absorption spectrometryLaser absorption spectrometry
Laser absorption spectrometry refers to techniques that use lasers to assess the concentration or amount of a species in gas phase by absorption spectrometry ....
. In CRDS, a laser pulse is trapped in a highly reflective (typically R > 99.9%) detection cavity
Optical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
. The intensity of the trapped pulse will decrease by a fixed percentage during each round trip within the cell due to both absorption and scattering by the medium within the cell and reflectivity losses. The intensity of light within the cavity is then determined as an exponential function
Exponential function
In mathematics, the exponential function is the function ex, where e is the number such that the function ex is its own derivative. The exponential function is used to model a relationship in which a constant change in the independent variable gives the same proportional change In mathematics,...
of time.
The principle of operation is based on the measurement of a decay rate rather than an absolute absorbance. This is one reason for the increased sensitivity over traditional absorption spectroscopy. The decay constant, τ, is called the ring-down time and is dependent on the loss mechanism(s) within the cavity. For an empty cavity, the decay constant is dependent on mirror loss and various optical phenomena like scattering and refraction:
where n is the index of refraction within the cavity, c is the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
in vacuum, l is the cavity length, R is the mirror reflectivity, and X takes into account other miscellaneous optical losses. This equation uses the approximation ln(1+x) ≈ x for x close to zero, which is the case under cavity ring-down conditions. Often, the miscellaneous losses are factored into an effective mirror loss for simplicity. An absorbing species in the cavity will increase losses according to the Beer-Lambert law
Beer-Lambert law
In optics, the Beer–Lambert law, also known as Beer's law or the Lambert–Beer law or the Beer–Lambert–Bouguer law relates the absorption of light to the properties of the material through which the light is travelling.-Equations:The law states that there is a logarithmic dependence between the...
. Assuming the sample fills the entire cavity,
where α is the absorption coefficient for a specific analyte concentration. The decadic absorbance, A, due to the analyte can be determined from both ring-down times.
Alternately, the molar absorptivity
Molar absorptivity
The molar absorption coefficient, molar extinction coefficient, or molar absorptivity, is a measurement of how strongly a chemical species absorbs light at a given wavelength...
, ε, and analyte concentration, C, can be determined from the ratio of both ring-down times.
Advantages of CRDS
There are two main advantages to CRDS over other absorption methods:First, it isn't affected by fluctuations in the laser intensity. In most absorption measurements, the light source must be assumed to remain steady between blank (no analyte
Analyte
An analyte, or component , is a substance or chemical constituent that is of interest in an analytical procedure. Grammatically, it is important to note that experiments always seek to measure properties of analytes—and that analytes themselves can never be measured. For instance, one cannot...
), standard (known amount of analyte), and sample (unknown amount of analyte). Any drift (change in the light source) between measurements will introduce errors. In CRDS, the ringdown time does not depend on the intensity of the laser, so fluctuations of this type aren't a problem.
Second, it is very sensitive due to its long pathlength. In absorption measurements, the smallest amount that can be detected is proportional to the length that the light travels through a sample. Since the light reflects many times between the mirrors, it ends up traveling long distances. For example, a laser pulse making 500 round trips through a 1 meter cavity will effectively have traveled through 1 kilometer of sample.
Thus the advantages include:
- High sensitivity due to the multipass nature (i.e. long pathlength) of the detection cell.
- Immunity to shot variations in laser intensity due to the measurement of a rate constant.
- Wide range of use for a given set of mirrors; typically ±5% of the center wavelength.
- High throughput, individual ring down events occur on the millisecond time scale.
- No need for a fluorophore, which makes it more attractive than LIF or REMPIREMPIResonance Enhanced Multi-Photon Ionization is a technique applied to the spectroscopy of atoms and small molecules. In practice, a tunable laser can be used to access an excited intermediate state. The selection rules associated with a two-photon or other multi-photon photoabsorption are different...
for some (e.g. rapidly predissociating) systems.
Disadvantages of CRDS
- Spectra cannot be acquired quickly due to the monochromatic laser source which is used. Having said this, some groups are now beginning to develop the use of broadband LEDLEdLEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....
or supercontinuumSupercontinuumIn optics, a supercontinuum is formed when a collection of nonlinear processes act together upon a pump beam in order to cause severe spectral broadening of the original pump beam. The result is a smooth spectral continuum...
sources for CRDS, the light of which can then be dispersed by a gratingDiffraction gratingIn optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
onto a CCDCharge-coupled deviceA charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
, or Fourier transformFourier transformIn mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...
ed spectrometer. Perhaps more importantly, the development of ICOS based techniques have now been demonstrated over the range from the near UV to the mid-infrared. - Analytes are limited both by the availability of tunable laser light at the appropriate wavelength and also the availability of high reflectance mirrors at those wavelengths.
- Expense: the requirement for laser systems and high reflectivity mirrors often makes CRDS orders of magnitude more expensive than some alternative spectroscopic techniques.
See also
- Absorption spectroscopyAbsorption spectroscopyAbsorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...
- Laser absorption spectrometryLaser absorption spectrometryLaser absorption spectrometry refers to techniques that use lasers to assess the concentration or amount of a species in gas phase by absorption spectrometry ....
- Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular SpectroscopyNoise-Immune Cavity-Enhanced Optical-Heterodyne Molecular SpectroscopyNoise-immune cavity-enhanced optical-heterodyne molecular spectroscopy is an ultra-sensitive laser-based absorption technique that utilizes laser light to assess the concentration or the amount of a species in gas phase by absorption spectrometry .- Principles :The NICE-OHMS technique combines...
(NICE-OHMS) - Tunable Diode Laser Absorption Spectroscopy (TDLAS)TDLASTunable diode laser absorption spectroscopy is a technique for measuring the concentration of certain species such as methane, water vapor and many more, in a gaseous mixture using tunable diode lasers and laser absorption spectrometry...