Abstract
In this chapter, the most important types of lasers involving low density active media are considered, namely gas, chemical and free electron lasers. Some considerations on X-ray lasers involving highly ionized plasmas will also be presented. The main emphasis, again, is to stress the physical behavior of the laser and to relate this behavior to the general concepts developed in the previous chapters. Some engineering details are also presented with the main intention again of providing for a better physical insight into the behavior of the particular laser. To complete the picture, some data relating to laser performances (e.g., oscillation wavelength(s), output power or energy, wavelength tunability, etc.) are also included to help provide some indication of the laser’s field of application. For each laser, after some introductory comments, the following items are generally covered: (1) Relevant energy levels; (2) excitation mechanisms; (3) characteristics of the laser transition(s); (4) engineering details relating to the laser structure(s); (5) characteristics of the output beam; (6) applications.
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Notes
- 1.
With reference to the discussion of Sect. 6.4.4., we recall that direct electron ion recombination cannot occur in the discharge volume since the process cannot satisfy, simultaneously, both energy and momentum conservation. Electron-ion recombination can therefore only occur in the presence of a third partner e.g., at the tube walls or at the cathode surface.
- 2.
According to e.g., (2.6.2), the rate of excitation, \({k}_{{A}^{{_\ast}}B}\), of the general process Eq. (10.2.4) can be defined via the relation \({(\mathit{dN}/\mathit{dt})}_{{\mathit{AB}}^{+}} = {k}_{{A}^{{_\ast}}B}\,{N}_{{A}^{{_\ast}}}{N}_{B}\), where \({(\mathit{dN}/\mathit{dt})}_{{\mathit{B}}^{+}}\) is the number of species B + , which are produced per unit volume per unit time, and \({N}_{{A}^{{_\ast}}}\) and N B are the concentrations of the colliding partners.
- 3.
The superscript (which we will denote by l) on the bending quantum number arises from the fact that the bending vibration is, in this case, doubly degenerate. In fact, it can occur both in the plane of Fig. 10.10 and in the orthogonal plane. A bending vibration therefore consists of a suitable combination of these two vibrations and the superscript l characterizes this combination; more precisely, lℏ gives the angular momentum of this vibration about the axis of the CO2 molecule. For example, in the 0200 state (l = 0), the two degenerate vibrations combine in such a way to give an angular momentum lℏ = 0.
- 4.
Relaxation processes in which vibrational energy is given up as vibrational energy of another like or unlike molecule are usually referred to as VV relaxations.
- 5.
For symmetry reasons, only levels with odd values of J are occupied in a CO2 molecule.
- 6.
Anharmonic pumping arises from a collision of the type \(\mathrm{CO}(\upsilon = n) + \mathrm{CO}(\upsilon = m) \rightarrow \mathrm{CO}(\upsilon = n + 1) + \mathrm{CO}(\upsilon = m - 1)\) with n > m. Because of anharmonicity (a phenomenon shown by all molecular oscillators), the separation between vibrational levels becomes smaller for levels higher up in the vibrational ladder (see also Fig. 3.1). This means that, in a collision process of the type indicated above, with n > m, the total vibrational energy of the two CO molecules after collision is somewhat smaller than before collision. The collision process therefore has a greater probability of proceeding in this direction rather than the reverse direction. This means that the hottest CO molecule [CO(υ = n)] can climb up the vibrational ladder and this leads to a non-Boltzmann distribution of the population among the vibrational levels.
- 7.
Under different operating conditions laser action can also take place, in the near infrared \((0.74 - 1.2\,\mu \mathrm{m})\), in the first positive system involving the B 3 Π g → A 3 Σ u + transition.
- 8.
Strictly speaking these should not be referred to as excimers since they involve binding between unlike atoms. In fact, the word exciplex, a contraction from excited complex, has been suggested as perhaps being more appropriate for this case. However, the word excimer is now widely used in this context, and we will follow this usage.
- 9.
According to this definition, the gas-dynamic CO2 laser, briefly considered in Sect. 6.1., should not be regarded as a chemical laser even though the upper state population arises ultimately from a combustion reaction.
- 10.
For example, a mixture of H2, F2, and other substances (16% of H2 and F2 in a gas mixture at atmospheric pressure) has a heat of reaction equal to 2,000 J/liter, of which 1,000 J is left as vibrational energy of HF (a large value in terms of available laser energy).
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Svelto, O. (2010). Gas, Chemical, Free Electron, and X-Ray Lasers. In: Principles of Lasers. Springer, Boston, MA. https://doi.org/10.1007/978-1-4419-1302-9_10
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