gas and vacuum cells (original) (raw)

Definition: a cell filled with some gas, normally used in laser absorption spectroscopy

Alternative terms: vapor cells, absorption cells, spectroscopic cells

Category: photonic devices

• gas and vacuum cells
  • multipass gas cells

Related: multipass gas cellslaser spectroscopylaser absorption spectroscopy

Page views in 12 months: 1005

DOI: 10.61835/dw3 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

Content quality and neutrality are maintained according to our editorial policy.

📦 For purchasing gas and vacuum cells, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.

Contents

What are Gas and Vacuum Cells?

In photonics, one sometimes needs to send light through some volume of gas or through empty space (vacuum); for that, gas and vacuum cells are used. In any case, one has some kind of container. Often, that container has optical windows for injecting and extracting light — typically, in the form of a light beam, or in particular a laser beam.

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.

Gas cells can be considered as a specific type of flow cells; there are flow cells for gases, but also for liquids.

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 appropriate 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 by 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 leak.

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 differ 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.

Gas Pressure

In some cases, one uses a rather low gas pressure, e.g. to minimize pressure broadening of the spectral lines. In other cases, a high gas pressure is desired, e.g. 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 to achieve a sufficient 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; 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.

Purposes of Vacuum Cells

Vacuum cells can be used for optical trapping, e.g. of atoms, ions or molecules, using light forces. A possible application is in optical metrology for optical clocks of extremely high precision. Strictly speaking, such a cell is then not completely empty, but one ideally wants it to be empty apart from some quite limited number of atoms or ions — sometimes only a single one.

Similarly, there are applications where one has a gas jet going through a vacuum cell — for example, for high harmonic generation. The gas jet continuously provides a fresh supply of gas (which may be degraded by the intense optical fields applied). One will not have a very high vacuum, but the intention is clearly to have nothing else in the cell than the gas jet. Wakefield acceleration is another application of somewhat similar kind.

In some cases, experiments with intense laser beams need to be carried out in a vacuum cell to avoid nonlinear optical effects or other disturbing effects in air.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a gas cell in photonics?

A gas cell is a container, usually with transparent optical windows, designed to hold a specific gas. It allows light, typically from a laser beam, to pass through the gas for applications like laser spectroscopy.

What is the difference between an open and a sealed gas cell?

An open gas cell allows a gas to flow through it, which is useful for real-time analysis like air pollution monitoring. A sealed gas cell is permanently filled with a specific gas and is often used as a stable reference for optical frequency standards.

What is a vapor cell?

A vapor cell is a specific type of gas cell containing a substance, often an alkali metal like rubidium or cesium, which is heated to create a gas (vapor) with sufficient pressure for spectroscopic analysis.

How can the sensitivity of spectroscopic measurements with a gas cell be increased?

Sensitivity can be increased by realizing a long path length for the light within the gas. This is often achieved with compact multipass gas cells or by using a gas-filled hollow-core fiber.

What is a multipass gas cell?

A multipass gas cell uses internal mirrors to fold a light beam's path, causing it to travel through the gas multiple times. This creates a long effective path length in a physically compact device, enhancing measurement sensitivity.

For what purposes are vacuum cells used?

Vacuum cells provide an empty environment for experiments such as the optical trapping of atoms or ions for optical clocks, for high harmonic generation in a gas jet, or to prevent air from disturbing experiments with intense laser beams.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 12 suppliers for gas and vacuum cells. Among them:

⚙ hardware

Knight Optical can provide custom gas cells to our customers, either as separate components — the cylinder and the transmission window — or as a complete gas cell. We are also able to supply filled gas cells with more common gases such as CH4, NH3, C2H2, and more.

⚙ hardware

Geola Digital specializes in designing and manufacturing vacuum cells for pulsed laser applications. These cells can be produced with a central pinhole (VSF) or without one (VC). For instance, the VSF or VC models are used to prevent air breakdown when employing a Kepler-type refocusing telescope.

⚙ hardware

PowerLink brings together the unique optical performances of GLO hollow-Core fibers and the gas–light interactions.

The PowerLink product represents a standard tool, that functionalizes and gives access to our gas-fillable hollow-core photonic crystal fiber technology’s potential and high performance in the simplest way, with strong gas-light interaction. Thanks to it, we provide a gas-fillable HCPCF-based platform with a certain degree of freedom to the optics community and their applications (non-linear optics, gas spectroscopy…).

Bibliography

(Suggest additional literature!)

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.