Anatomy of an excimer laser
The EX 500 of the company Wavelight excimer laser will serve as an example: the "bowels" of this machine have been photographed during his installation at the Institute Laser Vision (Rothschild Foundation) in 2012. This laser is among the instrument currently the newest and most advanced technologically to corneal refractive surgery. It uses a technology of scan by spots (flying spots).
The excimer laser block consists of an Assembly of various components:
-the laser cavity, fueled by the Argon Fluorine gas mixture, and subjected to electric shocks
-the optical path outside the cavity, along which the laser beam is model, and debris sucked
-the licensing system that allows scanning of the cornea with the laser spots according to the issued ablation profile
-various opto-electronic modules: visualization for the control screen and the eye tracker cameras
Container that contains the mixture of Argon-fluoride
The gas mixture, Argon - fluorite, which is the responsive environment (formation of dimers excited under the effect of electrical discharges) is contained in a container that is plugged into the laser cavity.
To ensure the purity of the optical beam path, the WaveLight® EX500 has an auto-generateur of integrated nitrogen that can protect all of the optical route taken by the radiation, excimer laser (193 nm) since the cavity until the mirrors of exit, located about 25 cm from the eye of the patient. This increases the service life of the optics and the laser head, avoiding the creation of ozone. This also allows a better homogeneity of the laser beam and participates in specific refractive outcomes.
Cavity, nitrogen generator and filter
The laser cavity is the fundamental element of the excimer laser:
Once the laser beam generated output (rectangular perimeter and area of a few square centimetres), must give it a circular edge and allow its focus on the cornea: the spot diameter is 1 mm.
Arms of the laser and optical path
At the end of the arm of the excimer laser, the optical acquisition modules are arranged: infrared camera of the eye tracker (system allowing to acquire images of the pupil and the corneal reflection, which allows to calculate the position of the eye of 1000 times per second!). Before the photoablation, a boom with the infra-red illumination system down to the patient's head.
The Eye-tracker of the WaveLight® EX500 analysis of eye movements in 6 degree of freedom, and has a frequency of 1050 Hz with a response time of only 2.0 milliseconds. With morphological data collected by the instrument Topolyzer (images of the iris), the laser is able to track the movements of the eye using both the blade and the recognition of the iris of the patient.
The integrated pachymetre allows to measure the thickness of the cornea in real time: before and after the uprising of the flap, and during the broadcast of the firing laser (photoablation of the cornea). This is accomplished by a product module non-dynamic contact integrated laser, which allows the measurement of the thickness of the stromal bed before, during and after the laser procedure.
Characteristics of the laser (Wavelight EX500) shots
The frequency of firing of the laser EX500 is 500 Hz (500 shots per second). The laser fluence is 200 mJ/cm2, and each spot produces an ablation of a depth of 0.65 microns.
The correction of a diopter of myopia for an optical area of 6.5 mm is made in only 1.4 seconds, and induced a depth of ablation of 15.5 micron. Accelerated processing time reduces the dehydration of the stroma and the time of fixation of the patient, and will produce patient results more predictable and reproducible.
See: issued by the excimer laser ablation profiles and allowing the correction of myopia, to the hyperopia, and astigmatismthe pages devoted to the use of the excimer laser in LASIK and surface technology)PKR).