Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 78

From the point of view of the industrial hygienist or safety specialist, it is not really essential to understand how a laser works in any great detail. Determining the most appropriate hazard control measures really does not depend upon such knowledge. However, it is always useful to be somewhat conversant in laser jargon if one is appointed a LSO. In this section, we shall try to succinctly summarize how lasers operate.

All lasers have at least three fundamental aspects:

1 an active medium (a solid, liquid, or gas) that defines the possible emission wavelengths;

2 an energy source (e.g. electric current, pump lamp, or chemical reaction); and

3 a resonant cavity with output coupler (generally two mirrors with one partially transmitting).

Most practical laser systems outside of the research laboratory also have a beam delivery system with lenses and mirrors to direct the beam, or a flexible delivery system, such as an optical fiber or articulated arm with mirrors to direct the beam. In most surgical and material processing applications, a focusing lens concentrates the beam on to the target tissue or the material to be welded, etched, or cut.

Figure 5 illustrates the three basic elements of a laser. Identical atoms or molecules are brought to an excited state by energy delivered by the energy source (e.g. a pump lamp). When the atoms or molecules are in an excited state, a photon (“particle” of light energy) can stimulate an excited atom or molecule to emit a second photon of the same energy (wavelength) traveling in phase (coherent) and in the same direction as the stimulating photon. Thus, light amplification by a factor of two has taken place. This same process repeated in a cascade causes a light beam to develop that travels along one axis (generally reflecting back and forth between two mirrors that form a resonant cavity).

This has been a very brief description of the process of laser action: Light Amplification occurring by the Stimulated Emission of Radiation. In actual practice, this amplification process usually requires a long path length for the light to travel, and it is not practical to build a laser as a very long tube. Instead, the long path length for the generation of the laser beam is created by forcing the light to travel between mirrors. These mirrors are placed at both ends of a short cylinder and send the photons bouncing back and forth within the energized medium. The space formed by the optical medium bounded by the two mirrors is a special optical space called a resonant cavity (Figure 5). One of the mirrors is only partially silvered to allow some of the light to leave the cavity and to escape as a collimated beam. The laser is designed so that enough light reflects back into the cavity to maintain the laser action. In practice, the mirrors usually have a certain curvature to better control the reflections of light within the cavity and produce a stable output. This has the effect of altering the distribution of light within the laser output beam. As we shall learn later, the beam profile is important in medical applications. Sometimes, the optical gain within one pass through the cavity is so great, that the output mirror is not needed, but in this instance, the laser may only operate in a “superradiance” condition. Although the two parallel mirrors shown in Figure 1 are generally curved in larger cavities they may be nearly parallel at a semiconductor junction for a diode laser. In any case, the source must have gain for the device to be termed a “laser,” and the basic principle holds for all lasers. Light‐emitting diodes (LEDs) do not have gain and cannot be considered lasers as a few publications seem to suggest.

FIGURE 5 A laser with an energy source, active medium, and resonant cavity.

All laser action originates in an active medium bounded by the two mirrors. Both mirrors reflect photons but the output mirror is semi‐transparent to allow laser light to leave the cavity. An energy source is required to excite the active medium and initiate laser action, e.g. light from a flash lamp or an electric discharge, or a semiconductor diode. Other components that may be within the cavity include apertures to shape the beam, and shutters to control laser action. The special properties of the emitted light beam, produced by stimulated emission during this multiple passage of light between the mirrors of the resonant cavity, arise because the characteristics of each stimulating photon are maintained in the cascade of emitted photons. The laser light is highly monochromatic, coherent, directional, and extremely bright.