In laser spectroscopy, one often needs to measure the absorption coefficient of light in a gas, or some other effect resulting from the interaction of the gas with light – for example, frequency-dependent phase changes. Typically, small changes to the light beam caused by the passage through the gas are measured as a function of the optical frequency of the laser beam, and the results are presented in the form of a spectrum – for example, an absorption spectrum. The obtained peaks in such a spectrum can be used to identify certain chemical species and to measure their concentration. For such measurements, one often uses wavelength-tunable single-frequency lasers or other narrow-linewidth lasers. Another method is photoacoustic spectroscopy, where one measures acoustic signals induced by absorption-related heating of the gas, in this case using laser pulses.
The obtained spectroscopic data may be used in environmental monitoring or for medical diagnosis, for example.
Open and Sealed Gas and Vapor Cells
The gas to which the spectroscopic method is applied is often filled into some kind of gas cell. It frequently has a cylindrical shape and is made of some transparent material such as borosilicate, pyrex or fused quartz.
For analytical applications, one needs to fill the cell with the gas of interest during operation. Obviously, the gas cell needs to have one or two appropriate openings for that. In some cases, a continuous flow of gas through the cell is used during operation – e.g., in the context of air pollution monitoring –, using a gas inlet and outlet.
Another application of spectroscopic gas cells is to obtain optical frequency standards. Here, a single-frequency laser is locked to a certain absorption feature of a gas, using some automatic feedback system. In other cases, a gas cell is used only temporarily for optical frequency calibration procedures. Such reference gas cells can be filled with the wanted type and amount of gas during production and then sealed. They need to have appropriate optical windows (with high transmissivity over the whole relevant spectral region) for the light to enter and leave the cell.
Reference gas cells are commercially available with many different gases, including both atomic and (often diatomic) molecular gases. Typical examples are iodine (I2), hydrogen (H2), helium, carbon monoxide (CO) and acetylene (C2H2). A wide range of standard spectral lines can thus be used. In some cases, alkali metals such as sodium (Na), potassium (K), rubidium (Rb) or cesium (Cs) are used, which develop a sufficiently high vapor pressure at least when electrically heated to some appropriate temperature. Such cells may be called vapor cells.
Obviously, a sealed gas cell should be reliably leak-free. Therefore, helium leak testing is often applied. Any helium in the cell after exposure to some helium atmosphere over some time can easily be detected with means of spectroscopy. As helium has a particularly high diffusion coefficient, the absence of a leak under helium testing is a good sign for the absence of any significant legal.
The Sensitivity Issue
In many cases, a very high sensitivity of laser spectroscopy is desired. That depends on different factors:
- A high sensitivity is easier to achieve when addressing spectroscopic features of the gases with high absorption cross sections. Such transitions are often found in the mid infrared spectral region. There are various types of suitable mid-infrared laser sources, e.g. based on optical parametric oscillators, but unfortunately these are substantially more difficult to make (with good performance) than e.g. near-infrared lasers.
- A high gas pressure, corresponding to an accordingly increased particle density, could be helpful in terms of sensitivity, but is often not practical to realize and may also cause pressure broadening of spectral lines.
- Another approach is to realize a very long path length of the light in the gas. In that case, even weak overtone transitions may be sufficient for obtaining the required sensitivity.
- Using higher optical powers can also help, but may substantially increase the cost of the required laser system and also the electricity consumption, which is particularly relevant for portable spectrometers.
- Different techniques of laser spectroscopy also differs substantially in terms of their sensitivity to noise sources, in particular to laser noise.
Because those performance aspects which can be addressed with the laser sources are often associated with high cost, it is desirable to optimize the performance on the side of the gas cells, where substantial advances can often be achieved at moderate cost.
In some cases, one uses a rather low gas pressure, e.g. in order to minimize pressure broadening of the spectral lines. In other cases, a high gas pressure is wanted, e.g. in order to achieve a higher sensitivity. Obviously, the glass housing must be stable enough to withstand the pressure difference to the ambient atmosphere.
In some cases, a buffer gas is used to carry the actual substance of interest. One may then have a total pressure equal to the ambient pressure while the partial pressure of the substance of interest is only a fraction of that.
Purity of the Gas
The used gas should usually exhibit high purity. It should not be contaminated e.g. by chemical species from the glass which may diffuse into the gas. Therefore, reference spectroscopic cells may be baked under vacuum for some time before filling them with the used gas.
Normally, the fill gas contains a natural mixture of isotopes, which can somewhat differ in terms of the transition frequencies, but in some cases purified isotopes are also available.
For spectroscopic gas cells with continuously exchanged gas, one should avoid depositions of unwanted substances such as dust. Therefore, one may have to filter the gas before sending it into the cell.
Heated Gas Cells
Some gas cells need to be heated during operation – in most cases either in order to achieve a sufficiently vapor pressure, e.g. when an alkaline metal is used, or for avoiding condensation of some species. The maximum allowed cell temperature depends very much on the used materials; for some cells, it is only 200 °C, while others tolerate 800 °C and more.
Realizing a Long Path Length
Multipass Gas Cells
In principle, one could use a correspondingly long single-pass gas cell; in order to avoid excessive beam divergence, one would need to use a laser beam with relatively large beam radius. This approach, however, is often not practical, mostly since the gas cell would become too bulky to be integrated into a compact device. Therefore, one often uses various types of multipass gas cells, where a long path length is realized by multiple passes (i.e., a folded path, usually using mirrors) through a moderately long cell.
Gas Cells with a Hollow-core Fiber
Another possible approach is to use a hollow-core fiber which can be filled with the gas to be analyzed. Even if the fiber is e.g. several meters long, a compact setup can be achieved simply by winding the fiber on a coil. (A limit is set by bend losses, which steeply rise for bending beyond a certain critical bend radius.) An important advantage is that such a gas cell itself does not need any alignment. However, the input beam needs to be carefully aligned for efficiently launching the light into the fiber – which is often single-mode.
In practice, the usable fiber length may be limited not by how much fiber can be wound up, but by the time required to replace the gas, since the tiny core diameter does not allow for a high velocity of gas flow.
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