+ +

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).

Presentation of the EX 500 wavelight excimer laser

EX 500 with his surgical bed pivoting Wavelight excimer laser: the simplified path of the beam from the cavity is represented below: it is issued to about 25 m from the eye.

 

 

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

 

Pages are devoted to the principles of the laser show and the technology excimer.

 

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.

mixed gas Argon fluorite for EX 500 excimer laser

The "body" of the excimer laser contains a supply of gas containing Fluorine-Argon mixture. The gas lines connect the mixture of Argon fluoride in the laser cavity where it is injected.

 

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:

excimer laser cavity

The laser cavity where the gas mixture are injected overlooks the nitrogen generator (which extracts nitrogen from the air for injecting it along the optical path in order to drive out the oxygen molecules), and particles suction system, also designed to guarantee the quality of the laser until its final issue.

 

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.

output of the laser cavity

The route connecting the cavity and the optical path is represented here.

 

 Arms of the laser and optical path

 

Arms of the excimer laser

The excimer laser arm contains the optical path of the laser: it is waterproof to avoid contamination by certain gases (oxygen) and particles. We see a part of the system controlling scanning laser (scanner), and video acquisition systems at the level of the ocular of the microscope, and the head-up display system - which allows the surgeon to see superimposed the name of the patient, the operated side, and the percentage granted treatment.

 

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.

End of the arm of the laser and visualization systems

Next to the end of the laser arm, are the elements for the illumination of the operating field, and cameras for imaging of the eye operated in real time.

 

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).

 

 

Leave a comment

You can ask questions or comment on this content: for this, use the "comments" form located below. The questions and comments of a general interest will be processed and published, and the information provided on the relevant pages should be clarified or supplemented.

Your e-mail address will not be published. Required fields are indicated with *