LASER SYSTEM HARDWARE

The essential components of a laser system include a laser medium containing the atoms whose electrons create the laser light, resonating mirrors to boost laser efficiency, and an energy source to excite or pump the atoms of the laser medium into producing laser light ( Fig. 67-3 ).

The different types of lasers applied in clinical practice use a variety of laser media and energy pumps. Some clinical lasers use a gaseous medium such as carbon dioxide (CO₂ ), argon, krypton, or helium-neon and are pumped by electric discharge through the gas. Gas lasers may produce a continuous or an intermittently pulsed beam output. Other lasers use solid rods of laser-passive material containing small quantities of ionic impurities, known as dopants, which are the actual laser materials. Dopants commonly used for their laser potential include chromium (as in the ruby laser), neodymium (Nd), and holmium (Ho). A synthetic gem crystal known as yttrium-aluminumgarnet (YAG) is commonly used as a passive host matrix, but even glass has been used. Solid lasers usually are pumped by high-energy photons from a xenon flash lamp and therefore produce a pulsed beam. Lasers are also made from dyes in liquid media and semiconductors, but these technologies have yet to appear to a significant extent in surgical practice. Some medically relevant laser media and their respective output wavelengths are listed in Table 67-2 .

Because lasers are not very efficient at converting electricity into light, they require a large power supply. For example, a laser with a 10-W output requires in excess of 1000 W of alternating current from a wall socket; consequently, some laser systems may require special wiring for the high current load. This electrical energy is converted

Figure 67-3 Generic laser hardware. A laser system consists of several components, regardless of whether the laser is a solid-, liquid-, or gas-based device. The central component is the laser medium itself, which may, for example, be a solid crystal of yttrium-aluminum-garnet (YAG) with a small concentration of neodymium as dopant or may be a tube containing carbon dioxide (CO₂ ). The energy pump provides the means of obtaining a population inversion of orbital electrons; it may consist of a xenon flash lamp or an electric spark generator. A pair of axial mirrors permits repeated passes of collimated photons through the medium, allowing maximum amplification by stimulated emission. The mirror on the right is not 100% reflective, allowing the beam to escape eventually. The optional Q switch increases the efficiency of pulsed lasers by allowing a small delay to increase the pumping.

Frequency doublers convert laser light to a different wavelength, enhancing therapeutic flexibility. A beam of laser light passed through a crystal of potassium-titanium phosphate (KTP) emerges with a mixture of light of the original wavelength and light of a wavelength that is one half of the original (i.e., double the original frequency). In medical lasers, KTP is most often used with Nd:YAG. Truly "tunable" or frequency-adjustable lasers exist, but they are still relatively low-powered devices. A light guide directs the laser beam to the surgical site, as illustrated in Figure 67-4 . Fiberoptic bundles provide a convenient, flexible conduit for visible and near-infrared wavelengths. Wavelengths outside this range, such as the long infrared from a CO₂ laser, require an articulated arm containing front-surface mirrors at each joint or newly developed exotic materials in a fiberoptic bundle. After the laser beam is delivered in proximity to the surgical site, it is focused to the site by the lens of an operating microscope or the shape of the beam is intentionally altered by passing it through a contact probe directly on the tissue to be ablated.

An operating microscope accurately aims a laser by directing a low-powered (about 1-mW) visible beam (usually from a low-powered helium-neon gas laser) through

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For surgical procedures that do not require a "no-touch" technique, there are special heat-resistant (sapphire), direct-contact probes^[5] ( Fig. 67-4 ) that are interchangeable and whose shapes can be designed as needed for sharp cutting or diffuse coagulation. However, these probes require active cooling in the form of a compressed gas or liquid jet, a feature that has contributed significantly to laser-related morbidity and mortality.^[6] The mechanism of action of the contact probe is probably a combination of thermal conversion of most of the laser energy within the sapphire (heating the surface to > 800°C) and transmission

Figure 67-4 Light guides. A, Schematic representation of a carbon dioxide laser guide such as may be found in an operating microscope or a hand-held wand. The guide consists of rigid hollow tubes with hinged, aligned mirrors, which reflect the beam from its source through the focusing lens. B, Schematic of a flexible fiberoptic guide with a sapphire contact scalpel and a coaxial cooling system.