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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.

      3.1 Q‐switching

The term “Q‐switching,” or “Q‐spoiling,” is derived from an electrical engineering term used to describe the resonant quality of an electronic circuit. Some of the first laser designers were electrical engineers and they used electronic circuit terminology to describe the operation of the laser's resonant cavity, and referred to laser technology as “quantum electronics” (14). The “Q” term was originally used to describe the resonant quality of a circuit and came to be used by these engineers to describe the analogous change in resonant quality of a laser cavity which forces short pulses. These changes involve either one mirror's position (active Q‐switching) or an interruption of light traveling throughout the laser cavity by an electro‐optic shutter (active Q‐switching) or by a saturable dye (passive Q‐switching). All of these techniques block the passage of light in the laser cavity in a controllable manner.

      A number of engineering techniques have been developed for Q‐switching. They all, in common, interrupt the light beam in a controllable manner so that laser action is delayed until maximal population inversion has been achieved in the active medium. A dye‐cell Q‐switch will remain opaque to transmitted light until the light concentration builds up to a certain threshold level. The bleaching of the dye then corresponds to the mechanical movement of a shutter or mirror as it opens the optical path to the passage of light.

      The most commonly used active Q‐switches are electro‐optic shutters known as Kerr Cells or the more typical Pockel Cells which change polarization very rapidly with an applied high‐voltage pulse. With an adjacent fixed polarizer, an electric pulse will suddenly change the polarization of the Kerr Cell. This aligns the Kerr Cell to the adjacent fixed polarizer to permit light transmission and laser action. The most commonly used passive switch is a saturable dye, which may be either in a solution in an optical cell or dispersed throughout a plastic film. In this method, the dye cell or film is placed between the laser medium and one mirror. When the dye is exposed to a very intense beam of light above a certain threshold irradiance (Watts/cm²), the absorbing dye bleaches and suddenly becomes nearly transparent. This bleaching abruptly makes the two mirrors of the resonant cavity available to the free passage of light and the beam can reflect back and forth. A very short, giant pulse results.

      3.2 Mode‐locking

      Unfortunately, the term “mode” is confusing as it has many different meanings in optics and laser technology. It may refer to the time distribution or “temporal mode” of the output. That is, a pulsed or CW laser can be described as being a “pulsed mode” or “CW mode” laser. Similarly, a burst of laser pulses can be described as a “burst mode,” or “normal mode.” It may also refer to the spatial distribution of light within the laser beam. The term “mode‐locking” describes a method that results in a train (i.e. group) of evenly‐spaced pulses, where each individual pulse has an extremely short duration measured in picoseconds (ps); such pulses are sometimes referred to as “ultra‐short pulses.” Characteristically mode‐locking produces light pulses of higher power than does Q‐switching because the pulses are very much shorter.

      3.3 Beam Profiles

      Transverse modes describe the energy distribution across the beam profile and determine the spatial distribution of the laser light in the beam and the nature of the laser focus. This description of the radiant power or energy in the beam's cross‐section is one way to know if the laser will focus on a clean circular pattern or form several patches of light distributed over a larger area. A “single mode,” or “fundamental mode” (noted as TEM₀₀), has a Gaussian (normal) distribution of power in the beam profile. This is desirable in many circumstances, since it can be focused to the smallest possible spot of any transverse mode. On the other hand, the fundamental mode can represent only a small fraction of the laser's output and may not be desirable where extremely high energy levels are required for a given task.

      Most laser designers attempt to achieve a single transverse mode of operation when possible, since the beam of a single‐mode laser can be focused to the smallest theoretical spot for that wavelength. However, single‐mode operation is not always possible and the beam profile may be irregular in shape. This irregular profile is referred to as “multimode.”

      The term “mode‐locking” describes a method that fixes the way the photons bounce back and forth in the resonant cavity. Technically, mode‐locking controls the number of longitudinal modes in a cavity. The net result is that a train (i.e. group) of evenly‐spaced pulses is produced, where each individual pulse has an extremely short duration; such pulses are sometimes referred to as “ultra‐short pulses.” One can visualize longitudinal modes by imagining several groups of photons separated in space along the longitudinal or long axis of the cavity. These groups of photons are racing separately at the speed of light (c) between the two mirrors which are separated by the cavity length (L). The transit time between mirrors is c/L seconds and the time for a complete round trip to the starting point is 2 c/L seconds. If only one bundle of photons move back and forth between mirrors, a short pulse of light will leave the cavity through the partially silvered mirror every 2 c/L seconds. For example, since light travels about 30 cm (1 foot) in one nanosecond, a one‐foot long cavity would have ultra‐short pulses being emitted every 2 ns. This allows control of the light in the laser so the light energy bunches into a very concentrated short packet, delivered in a time determined by the length of the laser cavity.

4 HAZARD EVALUATION MEASUREMENTS AND CLASSIFICATION

Current laser safety standards throughout the world follow the practice of categorizing all laser products into several hazard classes, and control