LASER stands for Light Amplification by Stimulated Emission of Radiation. It is a high energy beam of light which is used to treat the eyes in various ways. Laser technology utilized in eye surgery has been around for quite some time now, and has become such a low risk and relatively easy-to-execute procedure.
Here in the article we will have a deeper look on relatively recent lasers: Femto and Excimer lasers.
HOW LASER WORKS WITH TISSUE
Laser amplifies light and focuses it into a narrow unidirectional beam, allowing its energy to be delivered to a small, very precise target.
Laser and Cornea Interaction
The important interactions are absorption and transmission. Transmission is maximal at wavelengths between 400 and 1600nm; this is the case of argon and YAG lasers, which pass through the cornea without significant interactions. Absorption becomes predominant at wavelengths below 350 nm. This is the principal effect used for photo ablative corneal surgery.
Absorption in cornea
Absorption itself can be broken down into three distinct effects: photothermal, photo disruptive, and photochemical. The photothermal effect is linked to the molecular vibrations induced by photonic energy, and results in a temperature increase. The photo disruptive effect follows ionization. It emerges only at very high wavelengths of the micron order (infrared). This is the mechanism of action of the YAG and femtosecond lasers. The photochemical effect usually occurs at short wavelengths.
There are two principal types of photochemical effect: photo radiation and photoablation. The property used for refractive surgery is photoablation, which is obtained with ultraviolet radiation associated with very high energies.
Photoablation is linear absorption of light energy, which leads to the process known as photoablation that is produced by excimer lasers.
BASIC THEORY OF LASER TYPE
Femtosecond Laser
It have increasing popularity and expanding applications in ophthalmology refractive surgery in particular.
Femtosecond is one millionth of a billionth of a second.
Femtosecond Laser Interaction With Tissue: Photo disruption
Femtosecond lasers produce a different tissue interaction, however, known as photo disruption. The application of many photons of laser energy at the same place and time leads to a nonlinear absorption of femtosecond laser energy. Due to the multiphoton effect, as well as the electron avalanche phenomenon, energy absorption by tissue eventually exceeds the threshold for optical breakdown. This process of photo disruption creates plasma. It also produces an acoustic shockwave, some thermal energy, and then a cavitation bubble, which expands at supersonic speed, slows down, and then implodes. A gas bubble subsequently forms that is composed of carbon dioxide, water, nitrogen, and other elements
The excimer laser
It is based on the combination of two gases: a noble gas and halogen.
The specific wavelength of an excimer laser depends on the composition of the gases used in the laser system. The argon-fluorine excimer lasers emit energy at a wavelength of 193 nm.
In the early 1980s, argon fluoride (ArF, 193 nm) excimer laser was being explored as a means of reshaping the surface of the cornea to correct vision defects.
Excimer Laser Interaction With Tissue: Photoablation
The photoablation effect is based on the delivery of sufficient energy into the tissue to ablate it in a short time before any heat is transferred to the surrounding tissue.
Photoablation is typified by excimer laser, which generate intense ultraviolet energy beams. At 193 μm, the ultraviolet argon fluoride excimer laser energy breaks molecular bonds, disintegrating tissue into molecular fragments that are ejected from target sites at high velocities.
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