Humanity’s familiarity with both continuous and pulsed sources of light dates back to time out of mind. Like the natural and artificial sources of light, lasers also operate both in the continuous and pulsed modes. The first laser, a solid-state (ruby) laser, that Maiman operated in the spring of 1960 at Hughes Research Laboratory, USA, was a pulsed laser. The very next laser, a helium-neon laser, operated shortly afterward by Ali Javan and his coworkers at the Massachusetts Institute of Technology, USA, was a continuously working (cw) laser. Thecw lasers under a steady condition of pumping yield a steady output in time. Maintaining population inversion over such a steady state condition at a higher rate of pumping is challenging, and, consequently, power emitted by thecw lasers does not normally exceed the kilowatt range. A pulsed laser, on the other hand, does not need to operate under steady state conditions, and consequently creation and sustenance of population inversion, over a brief period, is straightforward. In fact, the shorter the duration of the laser pulse is, the easier it is to meet the condition of population inversion. Today, we have lasers that are capable of producing pulses of femtosecond duration, a time practically of the order of the period of optical cycles. Thecw and pulsed lasers complement each other in a remarkable manner as far as the applications of the lasers are concerned. This chapter aims to follow a largely nonmathematical approach to impart to the mind of the readers a comprehensive knowledge on the working of bothcw and pulsed lasers.

1. 1.

   Nonradiative relaxation is the transition from a higher to lower energy level not involving emission of any light, meaning it is a radiation-less transition. Such transitions are very common in case of solids where the excess energy is released as phonons associated with vibration of the lattice. Liquid too presents conditions conducive for relaxation of excess energy in the nonradiative manner. In the case of gas, wherein the atoms or molecules are barely in contact, there is hardly any possibility to dissipate the excess energy nonradiatively.

2. 2.

   The number of pulses that a pulsed laser gives out in every second is called its repetition rate or pulse repetition frequency (prf) and is expressed as Hz. Today we have lasers with prf ranging from several Hz to the range of MHz.

3. 3.

   The phenomenon of gain clamping, which has a strong bearing on the operation of a laser and known to manifest into a host of interesting effects, will be looked into greater detail in the next chapter.

4. 4.

   Cavity decay rate is essentially the rate at which photons escape from an optical cavity. A cavity with higherQ will decay at a lower rate as it can retain photons for a longer time and vice versa.

5. 5.

   Thermal lensing effect usually occurs in the operation of high-power solid-state lasers. The fraction of the pump energy that stays back in the system, as we know, is often dissipated as heat into the lasing medium. In the case of a solid-state laser, the laser rod, as a result, stays warmer at the center compared to the periphery. This inhomogeneous cooling of the rod establishes a refractive index gradient laterally across it that, in turn, significantly modifies the propagation of the intracavity Gaussian beam degrading greatly the performance of the laser.

6. 6.

   A switch is a device that can make or break the connection of an electrical circuit. A high voltage high current switch, usually a spark gap or a thyratron, that can hold a voltage of several kV to tens of kV and capable of withstanding pulse current in the range of kiloampere is a key component in the operation of a pulsed laser.

7. 7.

   In the case of a glass substrate of refractive index 1.5, 50% of the S-polarized light is reflected at the Brewster angle of reflection. The fraction of reflection monotonically increases with increasing refractive index of the Brewster plate.

8. 8.

   Wall-plug efficiency in the present context means the efficiency with which the laser converts the total input electrical power eventually into the coherent optical output and will be dealt in greater detail in the following sections.

9. 9.

   A ballast resistance, in the present context, is an ohmic load inserted into an electrical loop to ensure a stable operation of a gaseous discharge by limiting the value of the current flowing through the circuit.

10. 10.

    A collision between two or more objects is said to be inelastic if only momentum and not energy is conserved after the collision. In an elastic collision, on the other hand, both momentum and energy are conserved. It is of interest to note that majority of collisions that we witness in our everyday life are inelastic in nature.

11. 11.

    Breakdown voltage of an electrical gap between two conductors is defined as the minimum voltage required to be applied across it to break the gap into conduction.

Authors and Affiliations

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

   Dhruba J. Biswas

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