Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 79

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.