Proceedings PaperMultiwavelength solid state laser for ophthalmic applications
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The four basic types of laser-tissue interactions are governed by tissue properties (scattering, absorption) and laser parameters (wavelength, energy/power, pulse width, repetition rate). Table 1. summarizes these interactions and the corresponding laser parameters. Lasers may be categorized by their lasing media which may be gas (such as excimer, CO2, argon, metal vapor, He-Ne), solid (such as Nd:YAG, Ho:YAG, Er:YAG, Nd:YLF, semiconductor diode laser, alexandrite, Ti:sapphire), or liquid (such as dye laser). Lasers may also be defined by their emitting spectra (wavelengths) which range from the ultraviolet (such as excimer and highorder harmonic of solid- state lasers) , to the visible (such as argon , metal vapor lasers) , to the infrared (such as c02, YAG lasers). Lasers may be further defined by their operation modes and/or pulse duration which are either continuous wave (CW) or pulsed (such as free running, AO and EO Q- switched). Most of the medical applications of lasers are governed by the tissue reactions to a laser at a specific laser wavelength and pulse duration. For tissues consisting of water the laser penetration depth is inversely proportional to the tissue absorption coefficient which has major peaks at 1.45, 1.93 and 2.94 microns. Examples of medical lasers using the thermal effects caused by a tissue's strong absorption are lasers with wavelengths at 1.3- 1.6 ,.m (Nd:YAG), 1.96-2.1 tm (Ho:YAG) and 2.94 jm (Er:YAG) which are referred to as i.ot" lasers. Alternatively, excimer lasers such as XeCl and ArF, with wavelengths at 308 nm and 193 nm, are referred to as "cold' lasers which may achieve direct bond breaking in tissues with minimal thermal damage to the surrounding tissue. The "cold" laser may also be provided by the high-order harmonic of a solid state laser (such as Nd:YAG and Ti:sapphire) or a dye laser, where frequency conversion techniques using nonlinear crystals have been commonly employed.