B.  Laser pumping sources

Laser pumping sources are the means by which energy is transferred into the laser gain medium to produce the required population inversion DN₂₁. These pumping sources generally consist of either electrons flowing within the medium or light being absorbed by the medium.

Electron pumping—Electron pumping is used primarily in gaseous or semiconductor gain media. In gases, many electrons are produced when a few initial electrons within the gain medium are accelerated by an electric field within the medium and these many electrons then collide with neutral atoms, exciting those atoms to higher-lying energy levels and even ionizing some of the atoms (removing an electron). The freed electrons are also accelerated, producing an avalanche of electrons and therefore an electrical current within the medium. The electrons lose their energy by transferring it to the atoms during the collision process. Some of the lasers operate on a pulsed basis, applying a large amount of current for a short period of time. Others operate on a continuous (cw) basis, using a much smaller but continuous current.

In semiconductors, the electrons flow through the semiconducting material by applying a voltage across the pn junction with the positive voltage on the side of the p-type material. This leads to recombination radiation when the electrons combine with the holes in the junction. The heat loading of the semiconductor limits the current.

Optical pumping—Optical pumping of lasers generally applies to the pumping of liquid (dye) lasers and to dielectric solid-state lasers and is provided by either flashlamps or other lasers.

The most common types of flashlamps used for pumping lasers are narrow, cylindrical quartz tubes with metal electrodes mounted on the ends, filled with a gaseous species such as xenon that serves as the radiating material within the lamp. A voltage is applied across the electrodes of the flashlamp and current flows through the gas, populating excited levels of the atoms within the gas that radiate and produce intense light emission. The process is similar to that of electron excitation of lasers described above except that a population inversion is not produced and the radiating material of the lamp radiates via spontaneous emission, rather than by stimulated emission as in the case of a laser gain medium. The pumping wavelength of the flashlamp is determined by the gaseous medium inserted within the flashlamp tube. Xenon is the most common species because of both its radiating efficiency and its emission of a broad spectrum of wavelengths from which to choose in matching the lamp emission to the pumping absorption bands of the laser.

Examples of flashlamp configurations for pumping lasers are shown tin Figure 5-9. Figure 5-9a shows the flashlamp in the form of a helix wrapped around the laser rod. Figures 5-9b and 5-9c show the flashlamp inserted into an elliptically-shaped or circularly-shaped elongated laser cavity. In Figure 5-9b the flashlamp is located at one focus of the ellipse and the laser rod to be pumped at the other focus of the ellipse. Figure 5-9d shows two flashlamps used in a double elliptical cavity, one of the most favorable arrangements, with the laser rod in the center. Sometimes the laser mirrors are coated onto the ends of the laser rod and sometimes they are mounted externally to the rod, along the longitudinal axis of the cavity.

Figure 5-9  Flashlamp configurations for pumping lasers

Laser pumping is used in cases in which the pumping energy must be concentrated into a relatively small volume or for a very short time, or if the pumping wavelength must be provided over a fairly narrow-wavelength bandwidth. Pumping lasers include the argon ion or doubled Nd:YAG cw lasers for pumping titanium-sapphire lasers, excimer lasers for pumping dye lasers, and gallium arsenide semiconductor lasers for pumping Nd:YAG lasers. In most cases the laser is focused to a relatively small gain region, a line focus for dye lasers and a spot focus for the other lasers. Two examples of diode pumping of Nd:YAG lasers are shown in Figure 5-10.

Figure 5-10  Examples of laser diode pumping of Nd:YAG lasers