Gas Lasers
Study of characterstics of Gas Lasers
In gas lasers, the active medium is in the gaseous state. Since the laser media is a gas, it is kept in a plasma tube, with proper electrodes for electrical discharge to produce ionization, enclosed with dielectric mirrors. One may think that gas laser is a simple device, as there is no basic preparation required for the lasing medium, as in the case of a solid sate laser. But in practice, it is a complex device, as it needs optimization of gas mixture, gas discharge parameters, mirror and container configuration etc. The same have to be properly designed to create suitable conditions for population inversion. Further, gas discharge produces heat and it has to be removed to avoid detrimental effect on gas discharge and the optical components.

Gas lasers may be grouped as, atom lasers, molecular lasers, ion lasers, etc. In the atom lasers, the lasing medium contains atoms, which are electrically neutral. He-Ne laser is an excellent example of this group. Molecular lasers have molecules as the lasing medium, as in the case of carbon dioxide, carbon monoxide and nitrogen lasers. Important ion lasers, such as argon and krypton lasers have ionized gases as their active laser medium. Interestingly, helium-cadmium laser has metal ions as the active laser medium. Some of the important lasers will be discussed in the coming paragraphs. Interested readers may look up the references, given at end of this section for detailed study.
He-Ne laser
Ali Javan invented helium-neon laser in 1961 at Bell Telephone Laboratories, USA. 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), 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. A characteristic of Helium is that its first states to be excited, 21S1 and 21S0 are metastable (lifetime ~ seconds), i.e. optical transitions to the ground state 11S0 are not allowed, because this would violate the selection rules for optical transitions. As a result of gas discharge, these states are populated by electron collisions. The upper energy level of helium atoms nearly matches with the upper energy levels of the neon atoms (within the thermal energy kT). Besides electron collision, atomic collision also plays very important part. The excited helium atoms reach ground state after loosing their energy after exciting the ground state neon atoms to higher energy levels. These two processes create population inversion in the neon system. The simple energy level diagram of a Helium-Neon laser is given below with only some of the major transitions indicated. Since it has two lower laser levels, it is capable of lasing at a large number of wavelengths. As can be seen, the generation of laser output at 632nm is between 3s and 2p energy levels.

The lifetime of s-states is about an order longer than that of p states; population inversion exists between 3s and 2p levels. Further, 2p level is almost emptied into 1s level due to fast radiative (spontaneous) transitions. Neon atoms reach the ground state via collisions with tube wall, as optical transition is not allowed. Therefore to increase losses, one has to choose a laser tube with a capillary as small as possible. But this effectively reduces the available amount of lasing atoms. This contradiction is the main reason for the low output of He-Ne laser. Another reason for the low output is the low gain of He-Ne laser. Consequently, maximum output He-Ne laser is 100mw. Another interesting aspect is that since the discharge has a negative resistance, a ballast resistance is to be used in series with the laser to make the overall impedance positive. But with its high spatial and temporal coherence as well as low divergence, it is an excellent tool for holographic work. It is a rugged, compact and comparatively less expensive laser with long life (more than 10,000hours).

HeNe laser system consists of Power supply for ionizing the HeNe gas and sustaining the operation by supplying a regulated current, Ballast resistor placed in series with the HeNe laser anode to have a stable electrical discharge and the main HeNe laser tube containing the helium and neon gas mixture that generates light at the desired wavelength. The following page contains more detailed information concerning how a HeNe laser. The schematic diagram of He-Ne laser head is shown below:
Some of the typical parameters of Helium Neon Lasers are listed below
Parameter Value
Wavelengths 6328 Å, 1.15 μm, 3.39 μm
Small signal gain 'g0' (1.3 - 2.1)%
Saturation Intensity, Isat 29.2 W/cm2
Beam divergence 0.5 - 1.5 mrad
Beam diameter 0.5 - 1.25 mm
Efficiency 0.01 to 0.1 %
Output Power 0.5 - 75 mW

Some of the important applications of He-Ne lasers include
Carbon Dioxide Lasers
C.K.N.Patel invented this molecular laser in 1964, at Bell Telephone Labs, USA. The active medium responsible for lasing is the carbon dioxide molecules, giving output at 10.6 μm and 9.6 μm. The very first CO2 laser produced only a few milliwatt output. CO2 lasers typically emit at a wavelength of 10.6 μm, but there are other lines in the region of 9-11 μm (particularly at 9.6 μm). In most cases, average powers are between some tens of watts and many kilowatts. The power conversion efficiency can be between 10 - 20 %. It is higher than for most flash lamp pumped solid-state lasers, but much lower than for diode pumped solid-state lasers. With the technological advancements, the present class of CO2 lasers produces CW output of megawatts.

The salient features of fundamentals of lasing action in CO2 laser are as follows. 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.

The three vibrational states of CO2 are referred to by the number of vibrational quanta n1, n2 and n3 related to the symmetric, bend and asymmetric stretch modes respectively. It may be noted that it is possible for a molecule to execute all the three modes at a time or a molecule excited to a level can have more than one quantum of energy in any one of the modes or in all of the three modes. For example, (001) means that the CO2 molecule has a single vibrational quantum in asymmetric stretch mode.

There are many vibrational energy exchange processes that take place between different molecules in the mixture of gases. Transitions between vibrational energy levels results in emission in the infrared, where as transitions between rotational states emit photons in the microwave region. Gas mixture in the CO2 laser consists of helium, nitrogen and carbon dioxide gases. A total pressure of 6-20 torr is made up of 10 to 20% N2 and 10 to 15% CO2 and rest being helium. Important vibrational relaxation processes that occur in CO2 and N2 system are shown in the figure given here. The laser transition is between upper laser level (001) and the lower laser level (100), emitting out put at 10.6μm. Lot of transitions takes place for generating 10.6μm laser emission. By electron impact nitrogen (N2) excitation takes place to vibration level, n = 1. Transfer of energy between nitrogen and the nearly resonant first asymmetric stretch level (001) of carbon dioxide molecules takes place, since the restoring force constant of N2 and that of CO2 molecules are almost identical and results in populating the upper laser level. It may be noted that the population is shared between n1 and n2 modes and the vibrational energy in the n2 manifolds is converted in to translational energy by collisions with helium. (100) state and (020) state have only even spin members. Further, members of the same vibrational states are in thermal equilibrium and the available energy is redistributed between them, as determined by Boltzmann statistics. Two important functions are carried out by helium, namely, it maintains plasma discharge and also helps in depopulating the lower laser level. Collision between carbon dioxide and helium atoms results in the transfer of energy to the helium atom.

After the completion of laser transition from (001) state to (100) state, CO2 molecules still have lot of energy, which they have to loose before getting excited back to the higher level. This takes place in two steps. i.e. first from (100) to (010) and then from (010) to (000) level, which is the ground state.

It would not be out of place to discuss the role of nitrogen and helium in the lasing of CO2 laser. Nitrogen has only one vibrational mode as it is a diatomic molecule and it has only one vibrational quantum number. With reference to the figure above, it can be seen that nitrogen level (n = 1) is very near to the CO2 (001) level. Collision between CO2 molecule in the ground state (000) and N2 molecule (n = 1) results in the transfer of energy to CO2 molecule. Consequently, CO2 molecule will be at (001) state and N2 will be at (n = 0) state. In an electric discharge laser, nitrogen is used to transform the available energy to excite CO2 molecules to the upper lasing level.

De-excitation of CO2 from (100) level to (010) level is very efficient. But the relaxation from (010) level to (000) state is very inefficient in pure CO2. This is due to the fact that the de-excitation is by collision and the rate depends on the nature of the particles involved. At a pressure of say 1 torr, each CO2 molecule under goes about 100 collisions per second, where as in the presence of helium gas, the rate increases to about 4,000, due to which the de-excitation is accelerated greatly, thus increasing the efficiency of the system. Water vapour or helium is used as a catalyst to improve the efficiency of the CO2 laser. Though the collision rate in the presence of water vapor is about 100,000 per second at a pressure of 1 torr, it helps only in depopulating the lower laser level, where as helium helps in de-excitation of the lower laser level as well as in maintaining the excitation to upper level.

At this juncture, we would like to draw the attention of the reader to certain specific factors related to the design of CO2 lasers, namely methods of excitation and cooling of laser gas mixtures. Electrical discharge (radio frequency and direct current), gas dynamic (thermal) and electron beam (high energy) excitation techniques have been employed to generate high power output from CO2 lasers. Like other lasers, CO2 laser is also not a very efficient device. It has efficiency about 20%, the rest of the energy going as heat and as such cooling is of great importance in the design and development of CO2 lasers. Unused energy increases the temperature of the system and the lower level population cannot be emptied fast enough to ground state. This naturally affects the population inversion and laser action ceases altogether due to temperature rise.

CO2 laser uses CO2, N2, He and sometimes some hydrogen (H2) and or water vapor mixtures. 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. It may be mentioned that not only the total pressure of laser gas, but also the proportion at which they are mixed are also very important to produce maximum output from the system. For example, to start with one may begin with 10-20 % CO2, 10-20% N2 and the rest 60 - 80 % being He. Then the proportion of these gases may be optimized to get maximum output. Here one cannot forget the importance of current, higher the current, higher is the laser output. Such a laser is electrically pumped via a gas discharge, which can be operated with DC current, with AC current (e.g. 20-50 kHz) or in the radio frequency (RF) domain.

The gas mixture in a carbon dioxide laser is subjected to an electric discharge causing the low-pressure gas to form plasma. In the plasma, the molecules take up various excited states as expected from the Boltzmann distribution. Some will be in the upper state (001), which represents an asymmetric oscillation mode. This molecule may lose its energy by collision with the walls of the cavity or by spontaneous emission. Through spontaneous emission the state falls to the symmetric oscillation mode (100) and a photon of light of wavelength 10.6 μm is emitted traveling in any direction. One of these photons, by chance, will be traveling down the optics axis of the cavity and will start oscillating between the resonator mirrors.

CO2 laser can work in the pulsed as well as in the CW mode. Not only DC excitation generate CW output, but high frequency (HF) and radio frequency (RF) can also generate CW output, since the life times of the excited molecules are long when compared with HF and RF frequency. Maximum pulse frequency of the system is limited by the speed of response of the discharge.

Basically, CO2 laser system arrangement is very similar to other gas lasers, i.e. a gas filled tube with a pair of mirrors at the ends and the gas excited with DC or RF electrical discharge. Total reflecting mirror is a highly polished solid molybdnium or silicon with high reflectivity coatings or gold-coated copper. Output coupler is normally Zinc Selenide (ZnSe) as it has very low transmission losses at CO2 laser wavelength. 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.

The most basic form of a CO2 laser consists of a gas discharge (with a mix close to that specified above) with a total reflector at one end, and an output coupler (usually a semi-reflective coated zinc selenide mirror) at the output end. The reflectivity of the output coupler is typically around 5-15%. The laser output may also be edge-coupled in higher power systems to reduce optical heating problems.
CO2 lasers have been operated in the following modes
The maximum attainable power output depends upon the efficiency of excitation and the cooling of the Laser gas. Thus the practical designs of the Laser vary according to the form of selective excitation, the cooling of the gas and with it the attainable power output. The development of a cost effective power Semiconductor for the high frequency region made it possible to realise high frequency excited discharging. They also have the advantage against parallel flow discharge as the power can be connected to the Laser gas almost without losses. The way the waste heat is rejected (or gas cooling method) has a large influence on the laser system design. In principle, it can be performed by two possible methods. The first method is based on the conventional process of natural diffusion of the heated gas to the tube wall, which is the operating principle of the sealed, and slow axial flow lasers. The second method is based on the gas forced convection, which is the operating principle of the fast flow lasers.
Sealed off Laser
A sealed off laser is a laser in which the ends of the gain tube are closed off and the gas is not allowed to flow into or out of the tube. The scheme of the Laser systems as a whole, at first seems rather simple; but the various details adopted for system life are quite complex. Contamination of the laser gas due to sputtering of the electrodes and production of heat are the major problems encountered in the working of the system. The electrodes must be made of a precious metal so that the continual material erosion does not react with the gases and especially with its disassociation products. In this case, heat is removed by helium removing the excess energy and moving to the walls of the gain tube where it gives up its energy to the wall through collisions. Another issue that arises in this case is that carbon monoxide is formed, which will cause laser action to cease. The addition of water vapour takes care that the formed CO gas will react with it and generates CO2 and finally re-mixes again with the CO2 gas. These types of lasers are capable of hundreds of watts of output power. Optimised systems achieve power outputs of up to 60 - 70 Watt per meter of discharged length. The cooling of gases takes place through diffusion- that means that the gas molecules diffuse from the center of the discharge to the wall, where they are cooled by collisions.
Slow Flow Laser
Contamination and thermal aspects can be overcome to a great extent by flowing the laser gases, where the electrodes are outside the discharge tube. These are the systems required for initial understanding of CO2 Laser development and are still very common keeping in view the complications associated with the sealed off lasers. For experimentation, this system is ideal with regard to Laser behaviour and basic capabilities. With an optimised system having an excitation length of 1 meter, Laser power of up to a maximum of 100 Watt can be achieved. The excitation takes place over a longitudinal direct current discharge, The excitation of the initial Laser level is achieved firstly by electron collision and secondly due to collisions with excited N2 molecules. The important features are: The primary factor that limits output power of these lasers is their inability to efficiently remove waste heat from the gas. Cooling is principally achieved by helium (He) collisions with tube walls. Air-cooling of CO2 laser tubes is possible, but this results in an elevated wall temperature and greatly reduces laser efficiency. Smaller CO2 lasers and those used in research often employ water-cooling. Industrial CO2 lasers usually use re-circulating oil and oil-to-water heat exchangers for better system stability and reduced maintenance. An increase of tube current beyond the recommended operating value results in more heat than can be effectively removed from the system in this manner. Increases in tube diameter also decrease cooling efficiency by increasing the path length necessary for (He) atoms to reach the walls from the center of the tube. Thus, the only effective method of increasing output power of this type of CO2 laser is to extend the active length. For best results, this must also be accompanied by an increase in gas flow rate. In larger systems the gas is re-circulated with a few percent being replaced on each cycle.

Slow axial flow lasers can generate few hundreds of watts of output, typically about 50 to 70 watts per meter length of the tube, independent of the tube diameter. To generate hundreds of watts, few meters long tube will be required, which is not very practical. In order to overcome this problem, folded tube technique has been employed. But the folding mirrors increase the complexity of the system. The optical quality of the laser beam is also affected adversely. Further, alignment of the mirrors and cleaning of the same poses practical difficulty in maintaining the system. The laser gases are cooled by direct cooling of the gases by conduction with the walls, where the cavity walls are cooled by water jackets Typically slow flow CO2 lasers generate a maximum of 400watts CW output with a gas flow of 20 l/minute and a coolant flow of 7 l/minute using a water jacket.
Fast Flow Laser
To generate few kilowatt output, one has to go for fast axial flow CO2 laser systems. With this system, it is possible to obtain 4-5 KW outputs, which depends on the rate of mass of flow rather than on the length of the tube, unlike in the case of slow flow system. Typical rate of gas flow is about 400 l/second and gas is recalculated after cooling through a heat exchanger. Fast axial flow lasers generally operate at less than 150mbar. Another aspect is that the modes can be controlled with the diameter of the laser tube as it acts as an optical aperture. The hot Laser gas is sucked off from the discharge chamber with the help of a Roots vacuum pump. In the process it passes through the heat exchanger. The laser gas mixture is cooled when it passes through the heat exchangers kept out side the laser cavity. Fast axial flow CO2 laser systems are complex due to size and power requirements of re-circulation pumps and power supplies. The power output is limited to 5KW. Increasing the diameter of the laser tube increases the beam divergence, especially in the higher modes and the reduction generates thermal lensing and consequently increases beam divergence, distorting the beam. For Laser powers higher than 5 kW, however, the transversal excitation system of is more advantageous, because here, due to the far lower electrode distance, the necessary ignition and drop voltage is comparatively low. Lasers of this type are capable of producing several kilowatts of power with fast flow geometry.
Transverse Excited Atmosphere (TEA)
As stated above, the pressure in the CO2 laser tube is of the order of millibars and output is limited to 5 kilowatts. By operating the laser at a pressure of about one atmosphere as well as by passing a pulsed current transversely through the laser gas, it is possible to produce pulsed energy output of hundred of joules with pulse width ranging from microsecond to nanosecond region and pulse repetition rate ranging from single shot to 300 Hz. This type of laser is referred to as TEA (transversely excited atmospheric) laser. TEA lasers are operated in pulsed mode only, as the gas discharge would not be stable at high pressures, and are suitable for average powers of tens of kilowatts

Unlike sealed off and slow flow lasers, where problem of heat disposal is solved through convection and diffusion, the development of a pulsed CO2 Laser forms the basis of TEA laser to solve the problem of heat disposal of the Laser gas, mainly through the utilisation of the specific heat of the Laser gas. This eliminates the continuous operation, because through the increased number of collisions, inversion is destroyed. However, if the excitation pulse is faster than the destruction of inversion, then a Laser pulse can be generated whose time constant depends on the mechanisms time constant. The gas flow is low and the gas pressure is high. The excitation voltage is around tens thousand volts. The laser beam energy distribution is uniform over a relatively large area. As the voltage required for a longitudinal discharge would be too high, transverse excitation is done with a series of electrodes along the tube. Its peak power is very high up to 1012 W as its pulse duration is very small. However it is very difficult to focus its laser beam to a small spot due to multimode operation. Lasers of this type are used in big research plants under extremely high peak power to examine the nature of matter. A CO2 Laser system built in the scientific laboratory in Los Alamos has a terawatt of power with a nanosecond pulse width.
Wave guide CO2 Laser
Wave-guide CO2 laser is a very compact laser. Basically it has two electrodes separated by an insulator. Since the electrode separation is only a few millimeters, it acts as a bore of same dimension and the beam propagation is in the 'wave guide mode'. Normally sealed laser tube has a reservoir. High-pressure operation due to small bore allows rapid heat removal, leading to high gain and consequent high output. The beam diameter and divergence of the beam are 1 - 2 mm and 3 - 5 mrad respectively.
Gas Dynamic CO2 Laser
There are gas dynamic CO2 lasers for multi-megawatt powers for military applications, where the energy is not provided by a gas discharge but by a combustion process just like rocket engine. These types of lasers are a class in itself and will be discussed separately
Q - Switched CO2 Lasers
Fast pulsing has proven to be more difficult. The simplest way to pulse the laser is to switch the RF power source to excite the CO2 plasma. Due to the high life time of the Laser output level (001) of the order of msec, a switching technique is possible for the production of higher peak power. With the correct duty cycle, one can produce pulses in the range of 100 microseconds to 10 ms.

A shorter pulse is also 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.
Applications
There are numerous applications for CO2 lasers. The ranges of power that can be produced by the CO2 laser, the relatively high efficiency, and low cost of the CO2 laser make it a very strong candidate for the materials industry as well as the medical profession. For a detailed understanding of CO2 laser, the reader is referred to references.
References
Argon ion laser
W. Bridges invented Argon ion laser in 1964 at Hughes Aircraft Labs. Argon gas, in an ionized state (Ar+), is the lasing medium. It is an important member of the noble gas lasers and can generate large number of discrete laser lines (wavelengths) ranging from the UV (275.4 nm) 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. An adjustable intra-cavity prism can even be included to permit the desired wavelength to be selected via a thumbscrew adjustment. The energy level diagram of Argon ion laser is given here.

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.

To generate ionised argon gas, high current (tens of amperes) low voltage (few hundred volts) power supply is employed. Water-cooling is essential for most Argon ion lasers, but air cooled low power argon lasers are also available. Argon-ion lasers can emit tens of watts at some lines in the green and blue, and up to 100W emitting on all lines when operating in the CW mode. In the pulse mode, the output power can easily reach several kilowatts.

The important components of Argon ion laser include power supply, plasma tube and resonator assembly.

Argon ion lasers, like the HeNe 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. In addition, lifetimes of the atomic and ionic states involved in the excitation process are short, so the atoms tend to drop in energy very quickly after each collision. So to keep the excited state population high, collisions must occur at a rapid rate, which again requires high current densities. For these reasons, ion lasers operate at very high current densities. To achieve high current densities one can either increase the magnitude of the current or reduce the tube diameter or both. Current amplitudes in ion lasers are very large, typically between 10 and 70A depending on the laser. Further laser tubes are constructed so as to have the smallest bore diameters possible without introducing diffraction losses. 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.

So the power supply must not only supply an initial triggering pulse (6KV to 8KV) to initiate the plasma discharge, but it must also maintain the plasma discharge. For small to medium size argon ion lasers, the power supply is rating is up to 10 - 15 amps of DC current at up to 140VDC. For higher power lasers, the current requirement may be as high as 50 - 70 amps of DC current at up to 600VDC. Major advances in semiconductor technology, coupled with significant enhancements of electronic components in general, have lead to many significant improvements in power supplies for all types of ion lasers. Modern argon laser power supplies are much smaller, more reliable, more efficient, and provide overall better laser performance. Today, state-of-the-art ion laser power supplies operate at very high efficiencies (>93%).

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 - 2000o C, there are only few materials that can go into an argon plasma tube and survive the 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. However, one has to be careful while handling BeO, as its powder is extremely dangerous for lungs. The typical ion tube has a thick helix of tungsten-based cathode. This cathode is made of specially processed tungsten and coated to promote the formation of a sufficiently dense electron cloud so that high current can be passed through the tube with minimal additional heating at the cathode itself. The hot cathode results in thermionic emission of electrons from its surface to free space.

Bore diameters usually range from 0.55 to 0.75 mm for small lasers and up to 2 mm for larger ones. Further longer tubes require larger bores.

For producing laser energy, the bore must function as part of an optically resonant cavity. To accomplish this, mirrors are placed at each end of the bore facing perpendicular to the length of the bore. One of these mirrors is a highly reflective mirror while the other is partially reflective. Usually the mirrors are permanently bonded, in a vacuum tight manner.

As Argon ion lasers will simultaneously run on several lines unless there is a dispersive element (prism or grating) in the cavity. With an intra-cavity prism, approximately 30 different lines can be selected, depending on the laser power and the type of mirrors used.

Most of these lasers have a hemispherical cavity, with a flat high-reflector mirror and a long-radius output coupler. A typical 5 W Argon ion laser is about 1m long, while higher power lasers are much longer. The mirrors are designed for specific wavelengths; for the ultraviolet range special mirrors are required. 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. The optical cavities of Argon ion lasers often contain an adjustable aperture located between the output coupler and the end of the laser tube, which serves to depress unwanted transverse modes. When the aperture is set for the proper diameter, only the TEM00 mode will lase.

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. The gain bandwidth of an argon laser is typically of the order of 10Ghz, which means that many modes will lase simulaneously, unless prevented from doing so. 10Ghz corresponds to a length of a little more than an inch (3cm), which means that the coherence length is of this order implying that the maximum recordable depth of a hologram will be only an inch or so. 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. Two types of etalons are common in Argon ion lasers. In one, the etalon is an approximately 1cm thick solid piece of fused quartz with coated surfaces to achieve the correct reflectivity. In the second type, an air-spaced etalon is used, with two quartz windows mounted on the ends of a hollow cylinder. The oscillating frequency of the (single) mode is adjusted by slightly changing the cavity length of the etalon either by tilting the etalon slightly, or by changing the temperature of the etalon and thereby its thickness.

The efficiency of the ion lasers is very poor. The problem is that the lower laser level is about 16 eV from the ground state, resulting in a loss of large percentage of the input energy, before the ions relax to the ground state. The end result is a poor efficiency in these lasers, with typical efficiencies the order of 0.05%. Typically, a 5 W argon ion laser will also generate about 10kW of heat. In fact, the limitation on the output power of these lasers is determined by the ability to remove the waste heat from the laser and maintain the tube at a low enough temperature so as not to damage the tube. That is why; high power ion lasers are always water-cooled.

Pulsed Argon ion lasers can be realized in the following manner:
Applications
Argon ion lasers are used in a wide variety of applications. These are being used extensively in scientific, research, educational, medical and commercial applications. The applications include:
Summary of some important properties of Argon Ion Lasers

Property Value
Strongest Wavelengths 514.5 and 488 nm
Power Range Few miliwatts to about 100 W on all the lines
Electrical efficiency 0.05 to 0.1 %
Small signal gain 0.005 cm-1
Saturation Intensity 16.3 W/cm2
Beam diameter 1 - 2 mm
Beam divergence 0.5 mrad
Typical operating current 50 A
Magnetic Field 600 - 1200 G
Operating Life 5000 - 10000 hrs
Pressure inside plasma tube 0.1 - 1.0 torr

Krypton laser
Krypton laser is another important member of the Ion lasers category. Argon lasers are though by far the most important members of this group, but krypton ion lasers are also in wide use. Ion lasers are gas lasers in which the stimulated emission process occurs between two energy states of an ion. The excitation mechanism of such a laser first must supply the necessary energy to remove an electron from the lasing atom to produce the ion, and then must supply additional energy to raise the ion to the appropriate excited state. These large input energy requirements result in low efficiencies for essentially all ion lasers. The energy levels of krypton ion lasers are similar to those of argon, and they also may be operated either on one line at a time or on several lines at once. A unique feature of krypton lasers is that, with the proper mirrors, they will lase on four lines that are red, yellow, green, and blue in color. This produces an output beam that is white in appearance and is uniquely suited to laser light shows. Krypton ion laser employs ionised krypton gas (Kr+) and lases at a number of wavelengths (more than 10), most important being in the visible region of electromagnetic spectra.

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. The two gases can be combined to produce a laser with quasi-white all-lines output. Under some conditions krypton lasers can produce wavelengths over the full visible spectrum with lines in the red, yellow, green and blue. The 647.1 nm and 676.4 nm red lines, however, are the strongest and result in the best performance. The relative power of various lines is shown in this table.

Wavelength Relative Power
406.7 nm .036
413.1 nm .07
415.4 nm .02
468.0 nm .02
476.2 nm .016
482.5 nm .016
520.8 nm .028
530.9 nm .06
568.2 nm .044
647.1 nm .014
676.4 nm .048

Efficiencies of Krypton ion lasers is also low because of the large amount of input energy required to ionize atoms and excite them to the proper state. The most general expression for the output power of an ion laser indicates that the power increases approximately with the square of the current density. Implying that smaller bores produce higher powers for the same current values. Thus, ion lasers are constructed to have the smallest bore diameters possible without introducing excessive diffraction loss or erosion.

Krypton lasers, like argon ion lasers, may be operated at gas pressures from 250 to 500 millitorr, but the best operation is usually in the range of 300 to 350 millitorr. Specific systems are designed to operate best at a specific gas pressure, and the laser power supply is matched to tube characteristics at that pressure. If gas pressure is too low or too high, electrical characteristics of the tube may become incompatible with the power supply, resulting in failure of the tube to ignite or in unstable operation. The only two materials that are practical for such tubes are graphite and Be0. The discharge tubes typically have an inner diameter of 2 to 5 mm., and a length of 350 to 450 mm. Passing a high-current discharge through a tube, in general, excites ion lasers. The discharge is concentrated in a small-diameter bore at the center of the tube, which is where laser action occurs. An initial spike of a few thousand volts breaks down the gas, then voltage drops to 90-400 V while the current jumps to 10-70 A in the sustained discharge. An external magnet producing a magnetic field parallel to the bore axis can help confine the discharge to the bore. Krypton generates only 10%-30% as much power as argon used in the same tube. The strongest line of singly ionized krypton (Kr+) is at 647.1 nm (red), but other lines in violet, yellow, green and red (416 nm, 530.9 nm, 568.2 nm, 676.4 nm, 752.5 nm, 799.3 nm) can produce up to one half or slightly less as much power, The violet lines are important for dye laser pumping; the red and yellow lines are important for displays. Doubly ionized krypton (Kr2+) has three near-UV lines. Laboratory lasers have also been shown operating on Kr3+ lines, which have generated CW powers at 242 to 266 nm and wavelengths as short as 219 nm.

The output power of Krypton ion laser also depends on the magnetic field strength. If there is no field, current density in the center of the bore is reduced and more energy is being lost through collisions with the walls of the tube. An increase of magnetic field strength increases current density and output power. The optimum value of the magnetic field is in a range of 600 to 1,200 gausses

When a gas is subjected to a strong magnetic field, each of its energy levels splits into several closely spaced levels due to Zeeman effect and is proportional to the strength of the magnetic field. The magnetic field in ion lasers results in a broadening of the laser gain curve that is proportional to the applied magnetic field strength. At field strengths below about one kilogauss, this produces a broadened output spectrum with more cavity modes and higher output power. Above about one kilogauss, the gain curve becomes so broadened and flat that its edges fall below the lasing threshold and the output power and spectral lines will both begin to drop. Each laser line has an optimum magnetic field strength.

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. Sealed plasma tube with internal or external mirrors and high current regulated power supply. Combined Ar/Kr produces lines in red, green, and blue, and is therefore considered a 'white light laser'.

Normally, optics is selected to support the mission of the laser. For example, surgery wants only the blue lines; ophthalmology needs green, red, and yellow; Raman Spectroscopy needs 647 and 676 nm; laser shows use argon for blue, green, and violet, and krypton for red and yellow. Mixed gas lasers use optics selected for 55-60 % red, 20% green, and 20 - 25% blue and violet. To suppress a line, one of the optics is made more then 15% transmission for that line.

Since the krypton and argon lasing mediums have substantial gain at several spectral lines, a given laser can be set up to output on a single line or more than one at the same time depending on how the optics are designed and adjusted. The tube current also affects this to some extent as increasing the current will bring in progressively more lower gain lines.

A laser set up for multi-line operation will usually result in highest total output power but there are many applications like holography where a monochromatic beam is required. Single line operation is achieved by replacing the multi-line rear mirror with an intracavity prism assembly as mentioned in the case of Argon ion lasers. This assembly consists of an internal prism aligned to properly deflect the intra-cavity optical path to the High Reflector. Because of the dispersive properties of the prism, only one wavelength at a time will be properly aligned and produce lasing. The wavelength selector thus allows easy tunability and selection of any of the individual lasing wavelengths. As mentioned earlier using special optics sets coated to transmit more then 15% for the lines to be suppressed can also be used to implement single line operation, thus stopping them from oscillating.

Multi-line operation requires a set of mirrors with reflectivities designed to achieve laser operation for all the desired spectral lines. Any intracavity prisms are removed.

Beam quality is very high and one can achieve TEM00 mode Gaussian beam, for all lines using suitable optics.
White Light Lasers
The term 'white light laser' typically refers to one that is capable of producing a set of wavelengths which if mixed in the proper proportion can 'simulate' the effect of a white light source in full color displays and laser shows and also for some spectroscopy applications. However, they generally don't produce a broad spectrum like an incandescent light bulb.

Though under some conditions krypton lasers as such can produce wavelengths over the full visible spectrum with lines in the red, yellow, green and blue. However, the most common white light lasers are large frame ion types with a mixture argon and krypton for the gas fill.

White light lasers are now even available in air-cooled format. All use a mix of argon and krypton. Many are made for a roughly 60:20:20 ratio of red, green, and blue lines for proper white balance.
Applications
The applications of Krypton ion lasers are almost the same as those of Argon ion lasers. These include areas such as very high performance printing, copying, typesetting, photo-plotting, image generation, forensic medicine, general and ophthalmic surgery, laser shows for entertainment, holography, electro-optics research, optical 'pumping' source for other lasers, spectroscopy etc. Krypton lasers are also used in medicine for example for coagulation of retina.
Nitrogen Lasers
Nitrogen laser was developed as one of the first ultraviolet lasers in 1963. These lasers are convenient and economical sources of short, nanosecond, ultraviolet (337.1 nm) pulses. The gain medium is nitrogen molecules in the gas phase. The nitrogen laser is a 3-level laser: the upper laser level is directly pumped, imposing no speed limits on the pump. These lasers are based on a fast electrical discharge through N2 gas. Traditional designs require vacuum pumps and flowing gas. Smaller sealed tubes, a more recent variant, are much more convenient to handle. These lasers are based on pure nitrogen, nitrogen-helium mixture, and sometimes even simply air. Emission typically occurs at 337.1 nm. The high gain leads this to relatively efficient super-radiant pulsed molecular laser. Superluminescent emission implies that laser can works even without a laser resonator. As a superluminescent source, it has a very low temporal coherence. It contains a laser gain medium, which is excited in order to emit and then amplify luminescent light. Since it has a large emission bandwidth of about 0.1nm compared to most other lasers: that is why its temporal coherence is very low. Nitrogen lasers are relatively easy to build and operate, and have been made by many hobbyists without refined laboratory equipment.

Although nitrogen lasers have been operated over a range of partial pressures from a few Torr to more than 1 atmosphere, it is common to divide them into two categories: low pressure and atmospheric pressure. The upper-state lifetime of nitrogen is inversely related to pressure; it is approximately 40 nsec at pressures of a few Torr, decreasing to around 2 nsec at 1 atmosphere. However, the lower laser level has much longer lifetime thus causing the laser as self-terminating. This effect seriously limits both the duration of the pulse and the efficiency of the laser. There are two versions a nitrogen laser, which may be constructed. The first approach is a low-pressure design - it is more 'traditional' and requires a vacuum pump. It produces a pulse of 5 to 10nS in duration, which has a cross-section of about 10mm by 2mm. The power output and beam shape are quite suitable for pumping a dye laser. The second approach is a TEA version of the nitrogen laser. TEA lasers (for Transverse Electrical-discharge at Atmospheric pressure) do not require a vacuum system at all as they operate at atmospheric or even greater pressures. Either laser, if built efficiently, can even operate using regular air as the lasing medium. In the case of the TEA laser using air, no gas housing is required. Although the idea of a TEA laser is tempting, since no vacuum pump required, construction of this type of laser is bit difficult than the type employing a vacuum pump. Ultra-fast discharges are required for the TEA laser (10 times faster than a 'normal' low-pressure nitrogen laser).

Nitrogen lasers are capable of generating peak power output of few megawatts, with a pulse width of 10 nsec, at rep rate 1000 Hz. at 337.1 nm in the UV region, with average energy of hundreds of milliwatts. Since the life times of it's upper and lower laser levels are about 40 n sec and 10 microseconds respectively, CW operation is not possible. The energy level diagram of the Nitrogen laser is shown below.

Energy level diagram of Nitrogen laser A fast high-voltage discharge populates the upper laser level, an excited electronic state with 40-ns lifetime, which emits at 337.1 nm when it drops to the lower laser level as shown in the figure. The transition is a vibronic one, in which both electronic and vibrational energy levels change, making it broadband. The lower level has a 10- microsecond decay time, much longer than the upper level, and drops to a metastable state with a lifetime of the order of seconds. This implies that laser action is possible only if a fast electric discharge populates the upper N2 level very efficiently. Pumping is normally provided by direct electron impact; the electrons must have sufficient energy, or they will fail to excite the upper laser level. Typically reported optimum values are in the range of 80 to 100 eV per Torr-cm pressure of nitrogen gas.

A fast strong electrical pulse does this where electron collisions cause the preferential population of the upper energy band first. After about 20nSec, the population of molecules at the upper laser level starts decaying to the lower laser level where it will stay because of much longer life time thereby quickly ceasing the laser action after the electrical pulse. Nitrogen lasers are hence self terminating. The pulse length of the low-pressure Nitrogen laser, then, is limited by the lifetime of the upper laser level i.e. 20nS. After this time, population inversion is no longer possible since half of the molecules in the upper energy state have decayed to the lower state

The lifetime of the upper-lasing level is dependent on the pressure in the laser tube. As pressure rises, the lifetime shortens according to:
         t = 36/( 1+12.8*p(bar)) ns

                        Or

         t = 36/(1+p(torr)/58) ns
There is 40 ns upper limit of laser lifetime at low pressures and the lifetime becomes shorter as the pressure increases. For TEA nitrogen laser where the pressure is 760 torr (one atmosphere) the lifetime is about 1 - 3 ns.

These features result in making N2 lasers very high gains during its short pulses; small signal gain is typically 1-2 cm-1. This avoids the need for a high-quality resonator. The laser gain is extraordinarily high, so much so that a single pass of light down the laser tube amplifies radiation enough to produce a powerful output beam. No mirrors are required for this laser - this is why, it is called a super radiant laser. The output beam simply passes through a thin glass microscope slide to exit the laser tube. This also eliminates mirror alignment problems mostly encountered with most other types of lasers. Though all nitrogen lasers can operate in super-radiant mode without cavity mirrors, but output can be enhanced significantly typically more than double, with a simple cavity with 100 percent reflective rear mirror. Further the use of rear mirror ensures that the output is emitted from the opposite end. Use of a high-reflectance rear mirror not only increases the peak power output but also decreases beam divergence, and improves beam homogeneity. Cavities are normally are 15 to 50 cm long.

The greatest challenge is the need to deliver a pulse of 15 to 40 kilovolts (kV) with rise time less than 10 ns. Previous designs involved the use of spark gaps, however, in recent designs, high efficiency thyratron switch provides the high energy required for pumping.
Some other important features of nitrogen lasers are
The important properties can be summarized as follows
Important applications include
References
Excimer Lasers
The name excimer refers to the electronically excited species such as monomers, dimers and other com¬plexes, which exist in the electronically excited state only. Excimers are characterized by short radiative lifetimes of the order of nanoseconds and large cross sections for stimulated emission, which enables an efficient laser operation. N. G. Basov et al. obtained the first experimental evidence of excimer lasing by exciting liquid xenon in 1971. However, the excimer lasers, which we know today like rare gas halides were first, demonstrated in Xenon bromide (XeBr) by Searl and Hart in 1975. The term excimer stands for 'excited dimer' where a dimer refers to a molecule of two identical or similar parts. In the case of excimer lasers as we know which consists of noble gas halides, both the molecules are different. In this case exciplex should be used for 'excited complex'. Most "excimer" lasers are of the noble gas halide type, for which the term excimer is strictly speaking a misnomer. The correct name for these lasers is exciplex laser. However, we all know these lasers as excimer lasers. The excimer laser is really an exotic laser in the sense that the lasing molecule exists only in the excited state and separates in to the original atoms in the ground state. Excimer laser molecule contains a noble gas atom (Argon, Krypton, Xenon etc) and a halogen atom (Fluorine, Chlorine, Bromine, Iodine etc). Excimer lasers are based on electronic transitions and emit mainly in the UV spectral range. These lasers are gas lasers that emit pulses of light with duration of about 10 ns in the ultraviolet (UV) spectral range. They are the most powerful lasers in the UV. While a lot of different excimer laser transitions have been used to generate light pulses at various wavelengths between 126nm and about 660nm, the most commonly used excimer lasers are krypton fluoride (KrF, 248 nm), argon fluoride (ArF, 193 nm), xenon chloride (XeCl, 308 nm) and xenon fluoride (XeF, 351 nm). The relative power of various lasers are given below:

Wavelength Active Gas Relative Power
248nm Krypton Fluoride 100
193nm Argon Fluoride 60
308nm Xenon Chloride 50
351nm Xenon Fluoride 45

Laser action in an excimer molecule occurs because it has a bound excited state, but a repulsive ground state. This is because inert gases such as xenon and krypton do not usually form chemical compound. However, when in an excited state, they can form temporarily-bound molecules with themselves or with halogens such as fluorine and chlorine. This bound state is the upper laser level in the case of excimer laser. The excited compound can give up its excess energy by undergoing spontonaneous or stimulated emission, resulting in a strongly repulsive ground state molecule which very quickly dissociates back into two unbound atoms. Since the excimer molecule returns to the unexcited ground state and separates into atoms, the population inversion condition is achieved the moment excited state is created, since the population of ground level is nil. The lifetime of the excited state is of the order of 10 n sec.

The excimer laser contains about 90-95 % helium or neon, less than 0.2% of halogen, the rest being the corresponding noble gas. The entire laser unit consists of discharge chamber (gas tube), an optical resonator, high voltage system, and the system serving for pumping and mixing of gases. The electrical high voltage discharge is transverse with respect to the length direction of the gas tube. Therefore the output beam of an excimer laser normally has a rectangular cross-section. To break the halogen and noble gas molecules to form the excited state, it is necessary to pass short duration high power electric pulses, sometimes of the order of megawatts / cm3. As the gain of the laser medium is high, it is sufficient to use a fully reflecting rear mirror and an ordinary window as the output coupler. The wavelength output of an excimer laser can be changed simply by changing the gas mixture. However, the laser mirrors may have to be replaced to obtain maximum output. Energy level diagram of Excimer laser The energy level diagram of excimer laser is shown here. The general principle of the excimer laser transitions is shown in the adjoining Fig. For example, in case of KrF, the upper laser level is an ionically bound charge transfer state of the rare gas positive ion and the halogen negative ion. For example for Krypton fluoride lasers, it is 2P rare gas positive ion (Kr+) and the 1S halogen negative ion (F-). The upper laser level is populated by a three-body collision involving Kr+, F-, and a third collision partner (called buffer gas, for example Ne or He). While we can see that there is a minimum in the potential energy curve in the upper state it is still rather unstable. The excited laser molecules decays after several nanoseconds via emission of a photon into individual atoms like Kr and F. These form the ground state, which is covalently bonded and consists of separate Kr and F atoms for large internuclear separations.

Some of the important characteristics of excimer lasers include:
References
Helium-Cadmium Lasers
The helium-cadmium laser was discovered by William Silvast in 1966 at the University of Utah (Salt Lake City). The transitions in Helium-Cadmium laser are between energy levels of singly ionized Cadmium atoms, and there are about twelve lines. These wavelengths are in the shorter wavelength region, violet and Ultra-Violet (UV). The most prominent wavelengths are 441.6 nm and 325 nm. The helium buffer gas plays basically the same role for He-Cd as it does in the helium-neon laser. Excited helium in the discharge couples extremely well to the upper laser level of the cadmium ion. Helium-cadmium lasers typically have less than 10 Torr of helium constrained in a bore of the order of 1 mm in diameter, with vaporized cadmium at about 0.1% of the helium concentration.

The main mechanism identified for the laser transitions is a process known as Penning ionization, in which highly excited helium atoms transfer their energy to cadmium in a way similar to the operation of He-Ne laser. However, in this case cadmium ions are produced in the process, instead of neutral atoms as in the case of neon, owing to much lower ionization potential of cadmium than of neon. That is why Helium-Cadmium lasers can be categorized either in Metal vapor Lasers since cadmium is a metal and the lasing action in Helium Cadmium laser occurs between energy levels of cadmium ions OR a Gas Laser since the properties of Helium-Cadmium laser are similar to those of Helium Neon Laser, which is a neutral atom gas laser. Excitation energy for the helium-cadmium laser is provided by a direct-current discharge passing through the laser tube. Energy level diagram of Helium-Cadmium laser Typical discharges are around 700 - 2000 volts, with current densities in the small-diameter bore (2 -mm) of the order of 3 - 5 amperes per square centimeter of cross section. Helium atoms in the laser gas absorb energy from the discharge and then transfer that energy to cadmium ions. The energy levels of cadmium and helium involved in the principal He-Cd lines are shown in the adjoining figure. The most prominent transitions, which can be easily, produced are 441.6-nm (blue transition) and the 325-nm (ultraviolet transition).

The most important energy transfer mechanism for the narrow-bore tubes mainly used for blue and ultraviolet lasers is Penning ionization. Penning ionization is a form of chemi - ionization, an ionization process involving reactions between neutral atoms and/or molecules. The process is named after the Dutch physicist Frans Michel Penning, who first reported it in 1927. Chemi-ionization is the formation of an ion through the reaction of a gas phase atom or molecule with an atom or molecule in an excited state and should not be confused with chemical ionization.

In Penning ionization, energy from an excited helium atom ionizes a cadmium atom:
He* + Cd → He + Cd+ + e-
The energy levels of cadmium and helium involved in the principal He-Cd lines are shown in the above figure. The 441.6-nm blue transition and the 325-nm ultraviolet transition, which are closely related, are the easiest to produce and the most widely available. Considerably more energy is needed to raise Cd+ ions to the upper laser levels of the red and green transitions which themselves are also closely related. In practice, different discharge conditions are necessary. The blue and ultraviolet lines can be generated in the positive column of the discharge, the region from the middle of a discharge to the anode where the electric field is not steeply graded and the electrons are not accelerated rapidly. The higher energy needed to produce the red and green lines is available only in the region near the cathode, where there is a larger change in electric field and a stronger electron acceleration. This difference means that special tube designs are needed to generate the red and green lines.

Because helium's lowest excited energy level is well above the ground state of Cd+, Penning ionization produces excited cadmium ions. The metastable helium excited states transfer energy to the levels of Cd+, which have lifetimes of the order of 100 nanoseconds (ns). The lower levels of Cd+ also receive some energy but are quickly depopulated because of their very low lifetime of the order of 1ns. The result is a population inversion between different states of cadmium, allowing laser action on either the 442- or 325-nm lines. The prism combination can be used to select the wavelength required for a particular application.

Cadmium, a metal, is solid at room temperature and for lasing it needs to be sufficiently heated to have required partial pressure of cadmium vapors in the discharge tube. Normally a uniform vapor is obtained within a chamber placing the vapor source either liquid or solid, and heating the entire chamber uniformly. In case of cadmium, it is not practical to heat the complete chamber to approximately 260oC because of the other components like mirrors, windows, electrodes etc. present in the chamber. Thus the challenge in making HeCd lasers operate continuous-wave (CW) is dealing with cataphoresis. This is the term given to the migration of the positively charged metal ions toward the cathode where they may condense, depleting the supply of vapor and contaminating tube components and optical surfaces. A critical breakthrough has been achieved to put cataphoresis to good use in distributing vaporized cadmium evenly throughout the discharge, which has made possible CW operation of the UV line. This is achieved by using cataphoresis to control the cadmium vapor distribution. In this process the cadmium metal is heated and vaporized at the anode (which is at positive potential) end of the discharge and is transported towards the cathode end of the discharge by the electric field acting upon the cadmium ions that are produced by the discharge current. The practical problem in Helium-Cadmium laser is to maintain homogeneous distribution of the metal vapor inside the electrical discharge tube. Once the vapors go out of the bore, it may deposit on cold surfaces. Thus it is important that its vapor remains at proper areas. In order to prevent coating of the windows with Cadmium, cold traps are put before the laser windows. The cadmium atoms then condense in a pocket near the cathode region. The laser species are mainly flowing down the capillary at a very slow rate: typically one to two gms per 5000 hours. The discharge current heats the gain region sufficiently so that cadmium does not condense in that region while it is being transported through the capillary.

The construction of helium cadmium (HeCd) lasers is far more complex than that of other helium based lasers. The laser tube contains a reservoir for cadmium and a heater to vaporize the metal. As a result, a heated filament cathode is often used in placed of the cylindrical tube that comprises a HeNe laser. Additionally, the laser itself needs to sustain a higher level of internal pressurization allowing the vaporized cadmium to remain in the tube. The lifetime of a specific helium cadmium laser is dictated by the amount of cadmium in the reservoir. Once the cadmium supply is exhausted, the tube must be replaced.

He-Cd laser tubes are more complex than those used for He-Ne lasers. In addition to often using a heated filament/cathode, they also include a reservoir for the cadmium metal and a heater to control its vapor pressure, a mechanism to add helium as needed to maintain correct pressure, possibly an overall heater and thermal insulation to control tube temperature, and various sensors inside the envelope to monitor these parameters for use by several feedback loops in the power supply. The power supplies are also correspondingly more complex with multiple feedback loops and power sequencing logic.
Some common features of He - Cd lasers are
Applications
Important properties of He-Cd laser can be summarized as follows
References
Metal vapour lasers
In this system, vapour of metal atoms is the active lasing medium. Copper vapour laser 510.6 nm (green), 578.2 nm (yellow), Gold vapour laser 627.8 nm (red), Helium-selenium (HeSe) metal-vapour laser up to 24 wavelengths between red and UV, Helium-silver (HeAg) metal-vapour laser 224.3 nm, Strontium Vapour Laser 430.5 nm, Neon-copper (NeCu) metal-vapour laser 248.6 nm belong to this group of neutral metal atom vapour lasers. Most popular among them are Copper vapour laser (CVL) and Gold vapour laser (GVL) and are being discussed here.
Copper Vapor lasers
Copper vapour laser (CVL), invented in 1965 by scientist William Walter, is a three-level laser and uses copper vapours as the lasing medium. It produces green laser light at 510.6 nm and yellow laser light at 578.2 nm. It consists of a sealed zirconia tube filled with neon gas at a pressure of 25-40 torr. A solid block of pure copper metal is kept in the middle of the tube. Copper is heated above 10830C (laser operating temperature is about 16500C) to generate copper vapour, which is the active laser medium. High voltage is applied between the two electrodes at the end of the zirconia tube. As a result, the temperature rises inside the tube cavity to about 1400 - 1700 0C, until the Copper evaporates, and the vapour pressure of the Copper is about 0.1 torr. During the laser operation only a small fraction of the copper atoms are ionized, and they move toward the ends of the tube because of electrical attraction. There, the vapour cools down, and transforms to solid metal, which reduces the quantity of copper in the tube. To continue the operation of the laser, it is necessary to replenish the copper metal in the tube after few hundred hours of operation. Electrons, accelerated by the high voltage applied to the electrodes, collide with the copper vapour molecules, exciting them into one of the available high laser energy levels. The lasers are self-heated such that most of the energy provided by the discharge current provides heat to bring the plasma tube to the necessary temperature. Energy level diagram of Copper Vapor laser Excitation occurs by electrons colliding with neutral copper atoms to excite them to the relevant laser-related energy levels. Inelastic collisions of electrons with copper vapour atoms causes excitation of the copper atoms, so the inversion population occur and laser oscillation due to electron transition from upper level (P1/2, P3/2) to the lower meta-stable level (D5/2, D3/2) take place at the 578.2- and 510.6nm, respectively. Two principal outputs having wavelengths of 510.6 nm (green line 2P3/2 - 2D5/2) and 578.2 (yellow line 2P1/2 - 2D3/2) are obtained. The lower laser levels (2D) are metastable leading to self-termination of the laser action. Upper and lower laser levels are shown in the energy level diagram.

The Copper Vapour Lasers (CVL) can be realized by two different ways. First one is the development of lasers using copper as such and the second one involves the vapours of copper-bearing compounds, mainly the copper halides, CuCl, CuBr, CuI. In case of copper based CVL lasers, Copper must be heated to 1400 to 1700oC in order to achieve a suitable vapour pressure. However, in case of CVL utilizing copper based compounds, it is possible to achieve a sufficient copper concentration for lasing in the 300 to 600oC range depending upon the type of compound. Typically Halide based lasers such as copper bromide (CuBr), copper chloride (CuCl) and copper iodide (CuI) lasers are necessary to be heated to 400, 500, 600 oC, respectively. The use of copper halide instead of pure copper as the active medium not only reduces the requirement for the heating but also is much easier to handle in terms of materials involved for the resonator and cooling requirements for electrodes and windows or mirrors. The benefit of the copper halide however has the disadvantage, of a more complex power supply. To dissociate the copper from its halogen atoms, a double pulse power supply is needed that is capable of firing two HV pulses within the recombination time of the copper halide molecule. Typically two energizing pulses in quick succession are required, the first to dissociate vapour molecules, and the second to cause the dissociated ions to lase. The first pulse provides copper atoms by dissociating the halide. The second pulse is delayed until an adequate copper atom concentration has built up. The second pulse is a fast discharge pulse that pumps the copper atoms to the upper laser levels by electron collisions. For better performance, the laser requires fast excitation pulses of rise time of the order of 100ns or less. The time gap between dissociation pulse and excitation pulse should be dependent on how fast the chloride and copper atoms recombine to copper halide. So HV pulse power supplies operating in the kHz range require fast switching devices such as hydrogen thyratrons.
Some of salient features of Copper Vapour Lasers
Typical Parameters of Copper Vapour Lasers
Applications
Gold vapour laser (GVL)
The principle of operation and structure of the GVL is quite similar to CVL. Even the same laser tube and power supply can be used for both lasers. The only change is that instead of solid copper metal, gold wire is employed to produce gold vapour and it lases at 627.8 nm in the red region. The laser head can withstand a temperature of about 1700oC. The discharge circuit, like CVL, makes use of the thyratron.

Typical efficiency of GVL (627.8 nm) is 0.2 % and the frequency doubled GVL (312.2 nm) is 0.02%.

As compared to Copper Vapour Lasers, which can emit more than 200 W of average power, Gold vapour can produce few tens of Watts only.

Gold vapour lasers find their main applications in dermatological and experimental cancer treatment of photodynamic therapy
References


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Updated: 6 April, 2015
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