Lasers, gas

These lasers are also called—incorrectly— excimer lasers. It will be clear that they could be called exciplex lasers. The active material is a gas mixture which contains a halogen (F2 or Cl2 in most cases) and a rare gas such as Kr, Ar or Xe. These cannot form any stable compounds in their ground states, but excited state species do exist and can fluoresce. These excited state species e.g. KrF) are formed through the recombination of ions, for instance 

One of the most common and familiar examples of the neutral atom gas laser is the He-Ne laser. Examples of ionic gas lasers are the Ar+ or Kr+ lasers. The particular oscillating transitions and operation mechanisms can be found 

In gases (atomic or ionic) the electronic energy levels of free atoms are narrow, since they are diluted systems and perturbation by the surroundings is very weak. An important fact derived from the discrete nature of the electronic levels in a gas is the high monochromaticity of the laser lines in this type of laser, compared to that of solid-medium based lasers. The high degree of coherence achievable with gas lasers is also a characteristic feature related to the narrow linewidth. 

Usually, mainly Doppler broadening determines the gain profile of a particular laser transition. Indeed, due to the different configurations achievable with gas lasers (namely, a large cavity length), the laser line can be narrower than the Doppler linewidth. Different experimental realizations of single-mode lasers are detailed elsewhere (Demtroder, 2(X)3). 

EXAMPLE 2.4 Obtain the separation between adjacent modes in a He Ne laser whose cavity length is 1 m. The emission wavelength is X = 632.8 nm. 

Different axial modes (optical standing waves) can be set up in a cavity of length L provided that the frequency v fulfills the condition 

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Chantry P J 1982 Negative ion formation in gas lasers Applied Atomic Collision Physics Vol 3, Gas Lasers ed FI S W Massey, E W MoDaniel, B Bederson and W L Nighan (New York Aoademio)... 

Smith K and Thomson R M 1978 Computer Modelling of Gas Lasers (New York Plenum)... 

EIOs), backward wave oscillators (BWOs) or magnetrons are available. Their spectral characteristics may be favourable however, they typically require highly stabilized high-voltage power supplies. Still higher frequencies may be obtained using far-infrared gas lasers pumped for example by a CO- laser [49]. 

For an air/glass interface, tan 0b = n, the refractive index of glass. In a gas laser, the light must be reflected back and forth between mirrors and through the gas container hundreds of times. Each time the beam passes through the cavity, it must pass through transparent windows at the ends of the gas container (Figure 18.10b), and it is clearly important that this transmission be as efficient as possible. 

By varying the types of gases inside the cavity, the wavelength of the laser emission can be varied (Table 18.3). These gas lasers are useful because the emitted light lies mostly in the ultraviolet... 

The reaction path shows how Xe and Clj react with electrons initially to form Xe cations. These react with Clj or Cl- to give electronically excited-state molecules XeCl, which emit light to return to ground-state XeCI. The latter are not stable and immediately dissociate to give xenon and chlorine. In such gas lasers, translational motion of the excited-state XeCl gives rise to some Doppler shifting in the laser light, so the emission line is not as sharp as it is in solid-state lasers. 

Molecular Interaction. The examples of gas lasers described above involve the formation of chemical compounds in their excited states, produced by reaction between positive and negative ions. However, molecules can also interact in a formally nonbonding sense to give complexes of very short lifetimes, as when atoms or molecules collide with each other. If these sticky collisions take place with one of the molecules in an electronically excited state and the other in its ground state, then an excited-state complex (an exciplex) is formed, in which energy can be transferred from the excited-state molecule to the ground-state molecule. The process is illustrated in Figure 18.12. 

If the flash lamp is pulsed very rapidly, the emergent beam appears at a rate governed by the lifetime of the inverted population. The resulting laser beam becomes almost continuous because the pulses follow each other so rapidly. However, such a solid-state laser should not be pulsed too rapidly because, if it is, the rod heats to an unacceptable extent, causing distortion and even fracture. Generally, solid-state lasers are not used in continuous mode because of this heating aspect. Liquid or gas lasers do not suffer from this problem. 

Brightness. This is defined as the power emitted per unit area of the output mirror per unit solid angle and is extremely high compared with that of a conventional source. The reason for this is that, although the power may be only modest, as in, for example, a 0.5 mW helium-neon gas laser, the solid angle over which it is distributed is very small. 

In some gas lasers it is preferable to use a mixture of the lasing gas M and a second gas N, where N serves only to be excited to N by collisions with electrons and to transfer this energy to M by further collisions ... 

The helium-neon laser is a CW gas laser which is simple and reliable fo operafe and, if fhe laser is of relatively low power, quife inexpensive. 

The CO2 laser is a near-infrared gas laser capable of very high power and with an efficiency of about 20 per cent. CO2 has three normal modes of vibration Vj, the symmetric stretch, V2, the bending vibration, and V3, the antisymmetric stretch, with symmetry species (t+, ti , and (7+, and fundamental vibration wavenumbers of 1354, 673, and 2396 cm, respectively. Figure 9.16 shows some of the vibrational levels, the numbering of which is explained in footnote 4 of Chapter 4 (page 93), which are involved in the laser action. This occurs principally in the 3q22 transition, at about 10.6 pm, but may also be induced in the 3oli transition, at about 9.6 pm. 

Unstable monohaUdes of xenon ([16757-14-5], XeF [55130-03-5], XeCl [55130-04-6], XeBr and [55130-05-7], Xel), have been produced in the gas phase by electron bombardment methods (43,44) and in soHd matrices by gamma and ultraviolet inradiation methods (45,46). Although short-Hved in the gas phase, these haUdes are of considerable importance as light-emitting species in gas lasers (qv). 

Only some of the configurations that commonly occur in low power gas lasers are described herein. The modes are denoted by the nomenclature TEM where the term TEM stands for transverse electromagnetic, and where m and n are small integers. Eigure 3 shows some TEM modes that are... 

The most familiar gas laser is the helium—neon laser (23,24). Sales of commercial helium—neon lasers exceed 400,000 units per year. The helium—neon laser is a compact package that produces a continuous beam of orange-red light. The inside diameter of the tube is commonly around 1.5 mm. The output of helium—neon lasers available commercially ranges from a fraction of a milliwatt to more than 35 mW. They have many appHcations in the areas of alignment, supermarket scanning, educational demonstrations, and holography. 

Krypton lasers are also ionized gas lasers and are very similar in general characteristics to argon lasers (27). Krypton lasers having total multiline output up to 16 W are available commercially. The strongest line at 0.6471 p.m is notable because it is in the red portion of the spectmm, and thus makes the krypton laser useful for appHcations such as display and entertainment. 

The helium—ca dmium laser, which has emission at 0.442 and 0.325 p.m, is a somewhat different type of ionized gas laser (28). It operates using the ionized states of cadmium, produced by heating ca dmium in a furnace. The output of continuous, commercially available helium—ca dmium lasers ranges up to 150 mW. 

Whereas the gas lasers described use energy levels characteristic of individual atoms or ions, laser operation can also employ molecular energy levels. Molecular levels may correspond to vibrations and rotations, in contrast to the electronic energy levels of atomic and ionic species. The energies associated with vibrations and rotations tend to be lower than those of electronic transitions thus the output wavelengths of the molecular lasers tend to He farther into the infrared. 

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