FAQ - Gas Lasers

In He-ne laser, neon atoms are responsible for laser emission. It uses a mixture of helium and neon gases up to a pressure of few torr, with the mixture containing almost ten times helium than neon. Electrical discharge raises the helium and neon atoms to the excited levels. The more abundant helium atoms transfer their excitation energy to neon atoms by collision, thus creating a population inversion condition for neon atoms, resulting in laser emission at 632.8nm.

In CO₂ laser, carbon dioxide molecule is the active laser medium. It uses a mixture of carbon dioxide, nitrogen and helium, usually in the ratio of 9, 90, 1 percentage respectively. During the electrical discharge, the upper laser levels of carbon dioxide molecules are populated by collision with the abundant excited nitrogen molecules. Helium helps in not only in depopulating lower laser level but also in removing the heat from the system. The population inversion thus generated is responsible for CO₂ laser emission at 10.6 micron.

Metal vapour laser works at high temperatures (above 1000^oC) and employs ceramic tubes containing metal pellets of gold or copper, as the case may be, and filled with neon gas. The electrical discharge through neon gas heats the metal pellets generating corresponding metal vapours at low pressure. Metal vapour laser is basically a pulsed laser and as it works at a high repetition rate (few kHz) it appears to be giving continuous output. Copper vapour laser gives output at 511 and 578nm and gold at 628nm.

Excimer lasers give pulsed output in the UV region of nanoseconds duration. Excimer lasers employ noble gas molecules, which form compounds that have no stable ground state, but have excited states that are bound temporarily. i.e. they combine for a short period as transient short lived molecule. The diatomic molecule, formed by the union of two atoms, is called an excimer, for example Xenon fluoride (XeF). When a gas mixture of xenon and fluorine is excited in a pulsed discharge device, excited state xenon fluoride (XeF*) is formed. But this metastable excited state exists only for a short period and dissociates as per the reaction
where the photon hn corresponds to a wavelength of 351nm.
It may be noted that as soon as the photon is emitted, XeF* molecule breaks up to form Xe and F.
Other important excimer lasers are KrF (krypton fluoride-249nm), CaF2 (calcium fluoride-193nm), ArF (argon fluoride -191nm), XeBr (xenon bromide- 282), and XeCl (xenon chloride-308nm). High-energy electron beams are also used to excite excimer lasers.

Argon ion lasers require high current density discharge for the generation of ionized argon as the laser medium. In this case, the energy levels responsible for laser action belong to singly ionized argon gas, the lower level being the ground state of argon ion. Since the upper energy level is about 20 ev above the ionized ground state, considerable amount of energy has to be supplied to raise the neutral argon atom to this level. Though there are a number of output wavelengths, most important are at 488 and 514nm.

He-Cd laser is a very unique laser in the sense that it is basically an ion laser as the energy levels of ionized gaseous cadmium is used for laser operation. Like metal vapour laser, cadmium metal contained in a reservoir near the anode is heated to give the optimum vapour pressure. The electric discharge generates ionized cadmium gas. Interestingly it has few similarities with He-Ne laser as well. During electric discharge, excited helium atoms produced by collision with electrons, collide with cadmium atoms in the ground state to produce excited levels of cadmium ion. He-Cd laser generates output at 441 and 325 nm.

GDL works on the principle that rapid heating and cooling can produce population inversion in molecular systems. When high-pressure hot gas contained in a chamber is rapidly expanded through a bank of convergent -divergent supersonic nozzles to a high mach number, gas is rapidly cooled due to supersonic expansion. Population inversion is created between two molecular energy levels by the differential vibrational relaxation processes. Due to the quickness of expansion, the upper laser level cannot follow the rapid change in temperature and pressure and its population is frozen for a long time downstream of the nozzle. At the same time, the lower level relaxes in a time shorter than expansion time, especially with the addition of a catalyst like water, the lower level population decreases rapidly with in the nozzle and continues to do so till it almost becomes nil downstream of the nozzle. Thus population inversion of a thermally pumped system is produced gasdynamically by rapid expansion through the supersonic nozzle. The laser cavity at the downstream of the nozzle extracts the laser output, perpendicular to the direction of the flow of the gas. In the combustion driven gas dynamic laser (CDGDL) employing CO₂ laser, benzene and nitrous oxide are burnt in a combustion chamber with nitrogen added to the mixture. This generates CO₂, N₂ and water vapour in the required ratio to produce laser output at 10.6μ.

Chemical oxygen iodine laser (COIL) made its debut in 1978. In this laser system, a reaction generated between chlorine and hydrogen peroxide excites oxygen atoms, known as singlet oxygen. These singlet oxygen species transfer their energy to iodine atoms, which are the lasing species. This transfer of energy causes the iodine atoms to become excited, creating a laser with a wavelength of about 1.3 microns. This is the shortest wavelength as compared to other chemical lasers e.g. hydrogen fluoride or deuterium fluoride. This smaller wavelength means that smaller optics can be used to develop a useful system.

In Q-switching, increase of stored energy and the amplifier gain is limited to only for the period of the lifetime of the upper lasing level. The pumping carried out above this period will result in losing the stored energy by fluorescence. Therefore, more energy can be stored in the active medium if the fluorescent lifetime is greater. In solid-state lasers, the fluorescent lifetime is of the order of few hundred microseconds, which is sufficiently high for Q-switching. But in certain lasers, the upper laser lifetime is too small to build up a large stored energy and as such cannot be Q-switched. Ion lasers fall in to this category.

It is a four level atom laser with a mixture of helium and neon. Though it lases at a number of wavelengths, its most popular output is at 633nm (red). Other available outputs are at 543nm (green), 594nm (yellow), 612nm (orange) and 1523nm (infra-red). Though neon is the lasing gas, it is the minor constituent (15% of the total mixture), helium taking the bigger share. Electrical discharge excites the helium atoms to the higher energy states, which are populated by electronic collisions.

To extract a light beam from the resonator, it is necessary that one of the two resonator mirrors, usually called the output coupler, has a reflectivity of 99% so that 1% of the photons incident on it travel out of the resonator to produce an external laser beam. The other mirror, called the high reflector, should be as reflective as possible. The diameter, bandwidth, and polarization of the HeNe laser beam are determined by the properties of the resonator mirrors and other optical components that lie along the axis of the optical resonator.

Carbon dioxide molecule is a tri-atomic molecule consisting of two oxygen atoms covalently bonded to a central carbon atom. It has three fundamental modes of vibration, namely, symmetric, bending and asymmetric stretching modes, which are shown in figure below. In the symmetric mode, carbon atom is in the center and the two oxygen atoms oscillate symmetrically along the axis of the molecule in unison, either away from or towards each other. In bending stretch mode, the oscillation of the molecules is in perpendicular direction to the axis. In the asymmetric mode, though the molecules oscillate along the axis, only one of the oxygen atoms comes close to the central carbon atom at a time and as this atom moves away from the center, the other atom comes towards the carbon atom and they alternate the movements.

CO₂ laser uses CO₂, N2, He and sometimes some hydrogen (H2) and or water vapor mixtures. Nitrogen has only one vibrational mode and its energy level is very near to the CO₂ (001) level. Collision between CO₂ molecule in the ground state (000) and N2 molecule results in the transfer of energy to CO₂ molecule. Consequently, CO₂ molecule will be at (001) state. The presence of helium gas helps in accelerating the de-excitation of (010) to (000) level, thus increasing the efficiency of the system. The role of hydrogen or water vapor (2-5 %) is to help (particularly in sealed-tube lasers) to reoxidize carbon monoxide (formed in the discharge) to carbon dioxide.

Total reflecting mirror is a highly polished solid molybdnium or silicon with high reflectivity coatings or gold-coated copper. Germanium is another choice, but this has to be cooled, especially for high output. For high power applications, gold mirrors and zinc selenide windows and lenses are preferred. Recently diamond windows and even lenses are also being used. Diamond windows are extremely expensive, but their high thermal conductivity and hardness make them useful in high-power applications. Output coupler is normally Zinc Selenide (ZnSe) with reflectivity typically around 5-15%.

A shorter pulse is possible by using an internal electro-optic modulator, such as a Q-switch (or cavity dumper). It is also very easy to actively Q-switch a CO₂ laser by means of a rotating mirror or an electro-optic switch, giving rise to Q-switched peak powers up to gigawatts (GW) of peak power. Previously, Q-switching was limited to military and very high-value applications, because the limited availability of cadmium-telluride (CdTe) modulator crystals. Even then, the modulators had a short lifetime because of poor damage threshold properties of these crystals. However, recent innovations have eliminated these drawbacks. Now, a modulator with advanced growth techniques suitable for CdTe crystals produces devices with very high optical damage threshold.
Properties of some resonantly absorbing molecules have also been experimentally investigated by making use of Q-switching techniques. SF6 has been used to passively Q-switch CO₂ lasers.

The wavelength of a YAG laser (1.064 microns) is exactly ten times smaller than the CO₂ wavelength (10.64 microns) and therefore, has a resulting spot size that is 10 times smaller than a CO₂ (in the same set-up). The power density is almost 100 times more in case of YAG laser for the same output power of both the lasers.
In materials processing, the shorter wavelength of the Nd:YAG couples better to metal while the longer CO₂ wavelength is more suitable for cutting plastics, ceramics and other organic materials.

10.6 μm are totally invisible to the human eye and conventional solid-state sensors cannot work. Therefore, thermal approaches are generally used to measure beam power or determine beam profile. These include low cost CO₂ viewing plates, thermo-couple based power meters.

10 MW produced by Russia based on Gas Dynamic principle.

The typical sealed CO₂ tube has an operating voltage of between 3 and 12 kV at 2 to 15 mA DC.

The electrical to optical efficiency of a typical sealed CO₂ laser is around 10 - 20 percent as compared to much less than 1 percent for a HeNe laser.

The best efficiency is only achieved when tube diameter and gas pressure are optimal. For example, the optimum gas pressure for a sealed CW CO₂ laser using an 8-10 mm inside diameter glass tube is about 14 -15 Torr.
Optimum gas pressure varies, as 1/D. Where D is the diameter of the tube.

CO₂ lasers can be operated using radio frequency (including microwave) excitation instead of a direct electrical discharge but this results in more complex resonator/electrode configurations, more complex driving electronics and additional safety issues.
One of the reasons RF is promoted for high-power fast-flow CO₂ lasers are that you don't have internal electrodes that tend to sputter and contaminate the resonator optics. In case of DC excitation, there is voltage drop at the cathode, which results in heat dissipation and doesn't contribute to the discharge.
However, a good design of cathode can reduce sputtering contamination a lot. Moreover, the efficiency in DC excited lasers can be achieved greater than 20%, which makes them cheaper as compared to RF excited lasers. For the same output power requirements.

Population inversion takes place between 4p and 4s level. 4s level has short life time and decays to the ion ground state. Argon ion recaptures and electron and moves to argon atom ground level.

Argon ion lasers can generate more than 30 discrete laser lines (wavelengths) ranging from the UV (275.4nm) to near infrared (752 nm) with the majority of the power being developed at the 488nm and 514.5nm lines. However, unlike HeNe lasers, the energy level transitions that contribute to laser action come from ions of argon atoms that have had 1 or 2 electrons stripped from their outer shells. Spectral lines at wavelengths less than 400 nm come from atoms that have had 2 electrons removed. Longer wavelengths come from singly ionized atoms. There are many possible transitions in the UV, visible, and IR portions of the spectrum. With suitable optics coherent light from a single spectral line or many lines may be produced simultaneously.

The comparative strength of some of the important argon ion laser lines are:

Argon ion lasers are excited by electric discharge through the gas, after an initial high voltage pulse that ionises the gas. Electrons traveling through the gas collide with atoms and transfer energy through the collision. Since these atoms require large amounts of energy to reach ionisation, many collisions must take place in a short time, which means that high current density is required these types of lasers. Once the gas ions are sufficiently excited, lasing may occur on several different transitions. The ground state of the ion is about 16eV above the neutral atom ground state, so a total of 36eV is required to excite an argon atom from its neutral atom ground state to the upper lasing level. This is a lot of energy considering that electrons can only provide between 2eV and 4eV per collision. Thus, many collisions are required to raise a neutral atom from its ground state to the ion ground state, and then to the upper lasing level. The fact that many collisions are required implies large currents.

The heart of any argon laser is the plasma tube, and the key component of the plasma tube is the bore. The design of the plasma tube must be such that it can sustain extremely high temperatures without damage while maintaining an excellent vacuum seal. Further, in addition to the heat, the tube material must also be able to withstand the intense UV radiation emitted by ions dropping from the lower laser level to the ground state. Since plasma temperature is in the range of 1500 - 2000^o C, there are only few materials that can go into argon plasma tube and survive are: BeO, kovar, tungsten, aluminum nitride, pyrolytic graphite and molybdenum. The material of choice for the bore of an argon ion laser plasma tube is usually BeO since it has a low vapor pressure and can be produced with a high chemical purity. When properly sealed, a plasma tube utilizing a BeO bore will allow the argon gas pressure within the tube to remain at its approximate 1 torr level for many years, thus assuring many hours of reliable laser operation. In addition, BeO is also an excellent thermal conductor. As such, the large amount of heat, generated by the plasma discharge within the bore, is readily conducted to the exterior of the BeO bore where it is then removed by means of forced air cooling (low argon lasers) or flowing water in a water jacket (high power argon lasers). Beryllium oxide is also preferred as it conducts heat 5 times faster then most metals.

In some of the designs, magnetic field is also applied coaxially to the laser tube to further concentrate the current at the center of the laser tube, resulting in higher current density and fewer collisions with the tube walls. Reducing the collisions with the wall of the tube also helps in reducing the tube temperature.

As Argon ion lasers simultaneously run on several lines unless there is a dispersive element (prism or grating) in the cavity. With an intra-cavity prism, different lines can be selected. Most of these lasers have a hemispherical cavity, with a flat high-reflector mirror and a long-radius output coupler. The mirrors are designed for specific wavelengths. An intra-cavity prism is used for the selection of the various lines, with the prism shaped so that the beams strikes it at or near Brewster's angle on both surfaces. The prism and the high reflector are usually mounted together in a single unit.

For applications like holography, one requires a single transverse mode, single line and single frequency i.e. single longitudinal mode argon ion lasers. TEM₀₀ mode operation can be realised by inserting a variable aperture in the resonant cavity. Single line operation is achieved by placing an intracacity prism. However, the number of longitudinal oscillating modes in any laser is approximately equal to the laser line-width divided by the mode spacing. In order to prevent more than one mode to lase and thus to ensure single frequency operation, we need to add an etalon into the cavity.

A quality factor, M² is defined to describe the deviation of the laser beam from a theoretical Gaussian. For a theoretical Gaussian, M²=1. However, for practical laser systems, the value of M²>1. Helium neon lasers typically have an M² factor that is less than 1.1. For Argon ion lasers, the M² factor is typically between 1.1 and 1.3. Collimated TEM₀₀ diode laser beams usually have an M² ranging from 1.1 to 1.7. For high-energy multimode lasers, the M² factor can be as high as 3 or 4.

Pulsed Argon on lasers can be realized in the following manner:

• Power on Demand power supplies are used for pulsed medical ion laser systems, these power supplies consist of a large capacitor bank charged by a switching supply to enable multi watt lasers to run off common single phase power supplies.
• An intra-cavity acousto-optic device can mode lock a laser source. The acousto-optic modulator device suitable for a particular wavelength has been used to mode lock the ultraviolet lines at 3511, 3638 and 5145Å from an argon ion laser. Pulses of 0.2 nsec and 0.17 nsec for UV and visible wavelengths have been produced.

Krypton ion laser and argon ion lasers are similar in construction and performance, with the argon system producing higher powers for longer lifetimes. Krypton-ion lasers are almost identical in construction and reliability to argon lasers. Krypton lasers emit at several wavelengths : in the visible range it emits at 406.7 nm, 413.1 nm, 415,4 nm, 468.0 nm, 476.2 nm, 482.5 nm, 520.8 nm, 530.9 nm, 568.2 nm, 647.1 nm, 676.4 nm. The argon laser has its strongest output at 514 nm (green) and 488 nm (blue). The krypton laser is known for its red (647 nm) and yellow (568 nm) output.

Krypton ion and argon ion lasers are very similar - they are both rare gas ion lasers, their basic principles of operation are similar, and the same basic hardware configuration and power supplies can usually be used. Differences are primarily in gas fill of the plasma tube and the mirrors/prisms for selecting the output wavelength.