The basics of molecular spectroscopy, an essential prerequisite to understand the working of molecular gas lasers, have been presented in the beginning of this chapter. The operation of CO₂ lasers, the most well-researched, versatile, and popular laser in this category, has been described in detail. The continuous and pulsed operation and their power scaling have been elaborated. The principle of working of a blazed grating and its use as a laser cavity tuner in the line tunable operation of CO₂ lasers form another endeavor of this chapter.

1. 1.

   The intrinsic rigidity of a solid inhibits such vibrating motion of the atoms constituting a molecule as in case of a gas or liquid.

2. 2.

   Reduced mass is a hypothetical concept that greatly simplifies the analysis of a two-body problem by reducing it to that of a single body. For example, the vibrating or rotating motion of two bodies will be recreated by a single body if its mass were the product of the individual masses of the two bodies divided by their sum.

3. 3.

   Named after its seventeenth-century discoverer, British physicist Robert Hooke (1635–1703), Hooke’s law states that the deformation of an object is linearly proportional to the deforming force as long as the magnitude of deformation is small.

4. 4.

   The role that moment of inertia plays in the rotational motion is akin to what mass plays in case of a linear motion. More specifically, moment of inertia governs the torque required to produce a desired angular acceleration of a rigid body about its axis of rotation just as mass determines the force essential to produce a desired acceleration.

5. 5.

   The rotation of a molecule sets in a centrifugal force that tends to pull the atoms apart. This, in turn, increases the moment of inertia of the molecule. With increasingJ, this effect becomes appreciable and is accounted for by adding a centrifugal distortion correction term to the expression of rotational energy level as,E_J = BJ(J + 1) − DJ²(J + 1)²,D is known as the centrifugal distortion constant.

6. 6.

   If two or more different physical states are located at the same energy level, then they are said to be degenerate. The number of such different states is called the degree of degeneracy.

7. 7.

   Laser ablation is a physical process wherein rapid deposition of optical energy on a material surface causes excessive localized heating of a thin layer and, in turn, its removal directly through vaporization.

8. 8.

   In a gaseous mixture, a vibrationally excited molecule can transfer its excitation to a molecule of another gaseous species through a collision. Such an energy transfer process is calledV-V relaxation. If the second molecule has a vibrational level closely matching that of the first, energy transfer process then occurs at an extremely fast rate and is called resonantV-V relaxation.

9. 9.

   Thermal population of an energy level is the population that intrinsically resides here following the Boltzmann distribution law at the ambient temperature of the medium.

10. 10.

    Pressure drop is a measure of the hindrance that a fluid experiences while flowing through a narrow and long pipe and is akin to the resistance that a conductor poses to the flow of electric current through it.

11. 11.

    Glow to arc transition time for a typical CO₂ laser gas mixture at atmospheric pressure is several hundreds of nanoseconds.

12. 12.

    Thyratron is a variant of the vacuum valve that offers very fast switching of high voltage gas discharge and capable of handling remarkably high average power making it ideally suited as a switch for high repetition operation of TE gas lasers.

13. 13.

    Although a grating can be both of reflection and transmission types, we would restrict ourselves to reflection gratings only as they normally find application in the operation of a tunable laser. A transmission grating is made up of multiple periodic slits. The light upon passing through these slits undergoes diffraction and, in turn, dispersion.

14. 14.

    As we know from the theory of diffraction, the grating elementd must be close to the wavelength of light to make the condition for its diffraction on the grooves favorable.

Authors and Affiliations

1. Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai, India

   Dhruba J. Biswas

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