Modes: Continuous Wave, Pulsed, and Q Switching

The light generated with a laser, in general, can be delivered in 2 ways: as a constant flow of energy (continuous-wave [CW] laser or as multiple discrete pulses (pulsed laser). The 2 types of lasers are fundamentally different in design, light delivery, and operation.

A CW laser is generated by continuously pumping energy into the active medium to achieve an equilibrium between the number of atoms raised to the excited state and the number of photons emitted. At such an equilibrium, continuous laser output results. The duration of a CW laser pulse is approximately 0.25 s. With this duration and with relatively constant power delivery to tissues, significant thermal damage occurs. To minimize their destructive effects, CW lasers have been modified to emit beams in a pulsatile fashion by adding electronically controlled, mechanically gated, timed shutters to interrupt the output beam at preset intervals. This system is not a true pulsed laser per se because the laser beam is physically chopped off to produce the pulse effect.

Pulsed lasers, in contrast, deliver high-energy beams in very short pulses in the range of milliseconds without the use of a shutter. Emissions are produced when the pump is modulated to create discrete laser pulses, which usually are broad and randomly shaped.

Both types of lasers can be further modified to produce even shorter pulses, usually in the range of 10-250 ns, by using a method referred to as Q switching. With this technique, a large population inversion builds before emission is stimulated. This population inversion is accomplished by using a mechanical opaque shutter or by inserting a high-speed, electrically sensitive, polarizable optical shutter known as a Pockel cell between the 2 mirrors of the laser.

Because this shutter effectively blocks the photons' path between the 2 mirrors and prevents resonation, stimulated emission does not occur. When the population inversion reaches its maximal level, the opaque mechanical shutter opens. When Pockel cells are used, an electrical pulse is applied to the cell that changes it from opaque to transparent, allowing the photons to reflect back and forth and subsequently generate an intense beam.

Since their initial use to coagulate ophthalmic lesions in 1963, laser-equipped devices are now used in almost every specialty of medicine. They have revolutionized the science of medicine and have become valuable and indispensable medical tools. With the rapid pace of technologic advances, other novel applications are likely to be discovered.

Дата добавления: 2015-07-25