Laser: an acronym for Light Amplification by Stimulated Emission of Radiation.
Photon: a packet of light energy, or quantum.
Excimer: a contraction of the two words Excited and Dimer. The dimer refers to the Fluoride-Argon molecules in the excited state. This dimer does not exist in nature in the unexcited state; in the unexcited state, the two atoms of Argon and Fluorine are not bound together. Fluorine gas is a halogen, and Argon gas is a rare or inert gas. Therefore, the excited dimer consists of atoms of Argon and Fluoride technologically bound together in a highly excited, temporary state to form a diatomic rare gas-halide. The decay of these unstable molecules (Argon-Fluoride) to a stable state results in the emission of a highly energetic photon of ultraviolet light. The emissions wavelength of the Argon-Fluoride Excited Dimer is 193 nanometers (193 nm). (One nanometer is one-billionth of a meter.) The excimer emissions are in a train of individual pulses, typically 10 nanoseconds (10 x 10-9 seconds) long, with a pulse repetition frequency of up to 50 Hertz (Hz), or 50 pulses per second. (Hertz is the international unit of frequency, equal to one cycle per second.) The typical pulse rate is 10 pulses per second, or 10 Hz coming out of the excimer laser.
Non-excimer ophthalmic lasers generally depend on one of two tissue/laser interactions:
1. Photocoagulation (typically achieved with Argon lasers):Photocoagulation occurs when laser light energy is absorbed by a target tissue and converted to heat. The thermal effects range from protein denaturation to vaporization and carbonization.
2. Photodisruption (typically achieved with Neodymium-YAG lasers): In photodisruption, a high-powered laser pulse causes optical breakdown and plasma formation with a resultant shockwave which mechanically disrupts the target tissue.
The excimer laser now offers a third tissue/laser interaction:
3. Photoablation: High powered, pulsed ultraviolet radiation removes tissue with an extremely high degree of precision (.25 microns per pulse, or 1/25000 of an inch per pulse). Initial studies of ultraviolet photoablation of organic polymers showed that such radiation could produce layer-by-layer removal of material and etch patterns with sub-micron precision. Moreover, ultraviolet photons are highly energetic and can directly break chemical bonds. Ultraviolet laser irradiation of biological tissue breaks these large molecules into diatomic and triatomic fragments and monomers which are ejected from the surface of the material being irradiated.
The Argon-Fluoride Excimer Laser is a revolutionary innovation and advanced treatment modality in an attempt to correct myopia, hyperopia and astigmatism, as well as superficial keratectomy to erase corneal scars and irregular corneal surfaces. When the Argon-Fluoride Excimer Laser is used in corneal reshaping to correct refractive errors, it breaks the carbon-to-carbon molecular bonds of the corneal tissue by the ultraviolet 193-nanometer wavelenghth of emission photochemical effect called photoablation. This photoablation effect is extremely superficial. Minimal thermal damage is created by the ultraviolet excimer laser, unlike traditional lasers in which the produced heat causes damaging effects to surrounding tissue. The pulsing excimer laser removes the tissue in microscopic layers, leaving virtually no underlying thermal trauma. The carbon-to-carbon bond holding most of the tissue together has an energy requirement of 3 electron volts. If an excimer laser photon is introduced, it can literally crack that bond. The photon-energy, or energy per photon, of the excimer photon is 6.4 electron volts, or 10-15 millijoules per photon. One laser pulse contains many photons. One excimer laser pulse contains 2.5 x 1016 photons. Therefore, the energy per pulse at the eye is equal to the 10-15 millijoules (single photon energy) times 2.5 x 1016 (number of photons in one pulse), which equals 25 millijoules (mj). (2.5 x 1016 = 25 billion million.) These excimer photons are like "photon scissors", breaking the carbon-to-carbon bonds of the corneal tissue. Hence, the excimer photon is incredibly energetic, having 3 times as much energy as the YAG laser photon and more than twice the energy as the Argon laser photon. The term that has been coined for the effect of the excimer laser on the tissue is photoablation. The key to the excimer laser is the short pulse duration (10 nanoseconds or 10 x 10-9 seconds) with high energy photons (energy per pulse is 25 millijoules at the eye) with the possibility of concentrating large numbers of these photons on tissue to crack the carbon-to-carbon bonding that holds tissue together.
For the first time, a no-touch system, or no-touch scalpel, with the ultimate resolution of a fraction of a micron, is available to surgeons. (One micron equals one one-thousandth of a millimeter.) So, without touching the eye, the excimer can change and sculpt the cornea (photon scissors) incredibly accurately with virtually no collateral damage conducted into the edges of the tissue affected. There is no significant mechanical effect to the surrounding tissues; and no crushing of tissue, as with a knife blade or scalpel.
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