Excimer laser

Excimer laser eye surgery

(To excimers) excimer lasers are prominent in the landscape and lasers in refractive surgery: these are lasers of choice to emit powerful radiation in of short wavelengths as the ultraviolet. The photons in the ultraviolet range are more energetic than photons infra-red or visible spectrum and in fact, they can produce special effects such as the photoablation of the cornea in surgery for myopia and other ocular defects.

The excimer laser has revolutionized the refractive surgery, because he brought to this discipline a major gain in terms of precision and safety. () Excimer laser corneal refractive surgery techniquesLASIK and PKR) dominate with supremacy the landscape of surgical correction of Visual defects such as myopia, hyperopia or theastigmatism. The use of the excimer laser is thus a must for a refractive surgeon. Femto LASIK is based on the successive use of a femtosecond laser and an excimer laser.

The generalization or trivialization of this technology should not overshadow the real technological prowess and engineering allowing it, when he press the pedal of shooting, to focus with precision on the cornea an energy ultraviolet radiation, including dosage and precise distribution of this Act the most accurate surgery, all disciplines.

Today, the last generation excimer lasers can deliver the optical correction in a reduced thanks to a high frequency of shooting time (ex: 500 Hz or 500 shots per second). The following video shows the sequence of shots issued to correct myopia 10 dioptres and a myopic astigmatism of 5 diopters, on an optical area of 6.5 mm. At this rehearsal (500 Hz), a diopter is corrected in less than a second and a half. (193 nm) ultraviolet laser photons shooting is done on a disc of plastic: each impact emissions of photons in the 'blue '.

This video shows a sequence of shots excimers for correction of myopia and astigmatism... view from the camera of an iPhone positioned under the laser, in the line of fire.

In addition to medical applications, lasers excimers participate in many fields of application which have in common the need for extreme precision as the engraving of electronic circuits, the manufacturing of semiconductors, as well as biological and chemical research. The highlight of the excimer technology lies in the fact that it allows the issuance of a laser radiation in the ultraviolet (UV), where photons are more energy; the energy carried by these photons is greater than the energy of cohesion of the atoms within the molecules. A UV photon can act in a targeted way by breaking an inter atomic bond. In the case of corneal refractive surgery, this interaction will remove a very thin layer of corneal tissue with each impact.

The use of the wavelengths between 350 nm and 230nm is banned from medical applications. Indeed, lower than 230nm wavelengths, the intracellular penetration of ultraviolet radiation is limited in depth.  Beyond 350 nm, the energy of the incident photons is lower than the minimum threshold to disrupt the cell structures. It is between 230 nm and 350 nm risk changing the intracellular structures like the DNA of the nucleus is not negligible. In general, radiation whose wavelength is less than 230 nm or greater than 3 microns do not penetrate beyond a micron depth corneal.

 

Definition of the word "excimer".

The word excimer is a contraction of the term "Dimer" excited The excitation media of excimer lasers is gaseous and gas (Helium, Argon, Krypton etc.) or halides (Fluorine, bromine, etc.). The noble gases are the last column on the right in the table of periodic classification of the elements, and the penultimate halides.

These are naturally reluctant to assemble, except under certain physical conditions that must be met in the cavity laser (high pressure, electric shocks): they can then form a excited Dimer. Strictly speaking, is a Dimer of two identical atoms, and we should rather use the term "exiplexe" to characterize the Assembly of two different monomers.

 

History of the excimer laser

The first laser emitted in the utraviolet radiation was generated by a Soviet team led by NG Basov in 1970. An intense search if followed, and many laboratories realized their own in the mid-1970s excimer laser. The first commercial development was accomplished by the German company Lambda Physik in 1977, and the first applications of excimers lasers were performed in the industrial field (engraving of electronic circuits) and research.

The in-depth study and understanding of the process of photoablation (called in the publication principles of Leigh and Srinivasan ' ") Ablative Photodecomposition ('') was a milestone in the development of the excimer technology. It is said that the idea of 'work' a biological tissue was born in the mind of the American scientist employee of IBM of Indian origin Rangaswamy Srinivasan in November 1981, the day of the Thanksgiving Feast, and this at a time where he sipped family the traditional Turkey.  The next day, he submitted a piece of Turkey meat rest to the action of a 193 nm excimer laser in his lab, and was pleasantly surprised to verify that accomplished by laser radiation tissue ablation was accompanied byno visible trace of burning or specific injury. His team continued experiments on various biological tissue and found that this "own ablation" occurred consistently.

A first patent for medical applications of excimer radiation was deposited and recorded in 1988 (US patent: 4.784.135: the authors of the initial - Srinivasan, Blum and Wynne - patent have been awarded the National Medal of technological Innovation by president Barack Obama in 2012).  Many pictures attesting to the accuracy and the absence of collateral damage from the photoablatif effect of the radiation at 193 nm were produced by Srinivasan, whose sculpted one of a human hair to a depth of 50 microns.  These discoveries were amplified for the general public by the IBM company, who during an official announcement put forward applications of laser excimer for burning electronic circuits and surgery.

One of the first ophthalmologists to work with Srinivasan for implementation in eye surgery was Dr. Stephen Trokel, Associate Professor of ophthalmology at the "Columbia Presbyterian Medical Center" of New York University. A seminal paper titled "Excimer laser surgery of the cornea," (excimer laser corneal surgery) was published in 1983 by the American Journal of Ophthalmology (Trokel SL, Srinivasan B, Braren B. Excimer laser surgery of the cornea.) Am J invest, 1983; 96: 710).

Stephen Trokel

Photo taken with Stephen Trokel during the Congress of the American Academy of Ophthalmology, November 2013

The ablative precision allowed by the 193 nm radiation associated with the lack of collateral or mutagenic effect enabled the further development of a range of eye operations corrective aiming (PKR and LASIK): this application certainly is now the most popular lasers excimers, and one of the leading causes of their commercial success.

Various companies compete for refractive surgery market; In addition to VISX (acquired by the American firm Abbott) and which was the first company to manufacture in large series refractive aiming lasers in the United States, German companies Wavelight, Technolas, Schwind, Zeiss and Nidek Japanese are the main players of the market of the vision correction.

 

Excimer radiation

Excimer radiation used in refractive surgery (193 nm) is obtained from a mixture Argon and Fluorine (Ar - F).

rare gas containers

Containers containing gases that fuel the cavity of an excimer laser. Helium (He) and Neon (not) are buffer gases: Argon (Ar) and Fluorine (F2) are the parties likely to form the excited Dimer. Corrosive gas, and it is important that the distribution of the gas system is tight.

 

Rare gases such as Argon are inert elements, whose electronic levels are completely saturated. in fact, they are not the natural state of stable molecules with other elements and tend to repel. The excitement by an intense electric shock provides one of the elements (example: Argon Atom) an electron, and this charge allows him to draw and constitute a "excited Dimer" ephemeral way with a second element (ex 2 Ar + F2 → 2 ArF *). This reaction, to get the probability of encounter between these elements is high enough, which implies a high density and a high pressure in the cavity of the laser. During the separation of the Dimer, the breaking of the binding energy is produced in the form of a photon whose wavelength is located in the ultraviolet)193 nm).

The genesis of the laser radiation is special since each created Dimer molecule is excited and corresponds to a population inversion, without fundamental State. It is relatively easy to create the excited Dimer. However, this reversal must be much greater than for a 'classic' gas laser to compensate for the loss of radiation within the cavity. Indeed, excited Dimer have an ephemeral life of the order of some nanoseconds, and can emit energy in the form of heat or light; It is issued when the 'natural' dissociation and corresponds to a show spontaneous, non-contributory to the laser radiation, which requires that the dissociation are caused by an incident photon. Heat output to the level of some lasers cavity is important, and some involve a third element gas (Helium) in order to dispel some of this heat.

For example, to increase the density in excited Dimer, it is necessary to provide an energy very important contribution to the gaseous environment, what is feasible on the very short durations  because of the intensity and tension required.

Pages are devoted to the basic principles of the laser show and to theamplification of laser light. The gain of the cavities of the excimer lasers is lower than for other types of lasers which media consists of ions; We therefore have to produce intense electric shocks to get the laser radiation, which is emitted naturally in the form of pulse duration of a few nanoseconds. Electric shocks are transmitted by electrodes located on both of the cavity where the Ar and F2 gases are introduced.

parameters of excitation of excimer laser

The gas mixture is introduced into the cavity under certain pressure and an electrical excitation (23 kV voltage, frequency 10 Hz) aims to get an elergie of 135 mJ cavity output.

 

The cavity of an excimer laser is equipped with mirrors that allow amplification of the laser beam: the gas travels quickly between the electrodes and the stimulated emission produces ultraviolet laser radiation.

 

schema cavie laser

In a cavity excimer laser, gas containing the exciplexe (Argon fluorite) mixture is injected between the electrodes, which shall submit it to intense electric shocks (excitation). The produced radiation is amplified by a few go returns within the cavity before be directed outside the cavity. Some devices like the pre ionisers, insulation and cooling systems / movement of the cavity are not shown.

 

Beam excimer cavity output

Out of the cavity, the collected laser beam section is generally a surface of some cm2.

This image shows the outlet of the beam of the cavity, as well as its footprint gathered on a photosensitive film:

the excimer laser cavity output

Left: output port of the cavity (laser Nidek EC5000), after withdrawal of a mirror. On the right, there are three fingerprints collected on a polaroid film: vertical beam diameter is 30 mm.

 

In normal operation, a mirror to return the beam coming out of the cavity to the optical path within the instrument.

the laser output mirror

The mirror cavity output allows to direct the beam excimer to the optical path of the instrument.

As shown in the following video, we can measure the energy of the laser cavity output. The frequency of pulses is 10 Hz (10 pulses per second).

 

If you take away the energy detector, the beam spreads freely... until ' at the next material obstacle with which photons will interact (here, the door of the operating room). At 193 nm ultraviolet photons are very energetic and cause a photoablation. On the video, blue radiation correspond to a secondary show less energetic photons at laser/material interaction.

 

Licensing system

In the case of applications in corneal refractive surgery, radiation generated by the cavity must then be shaped and delivered precisely to the desired location. This role is dedicated to the licensing system, which must ensure that the final show of the impacts of the laser on the cornea, which is the spatial summation to the profile of ablation.

This imposes a the addition of various technological elements (see also the page dedicated to the)Anatomy of an excimer laser). "" Excimer lasers used in eye surgery associate a system that allows not only to generate but also of "shape" then deliver the radiation on the cornea of the eye surgery.

excimer laser: the laser beam path

Schematic representation of the path of the beam excimer output (Nidek laser). The system issue, located in the horizontal arm of the laser module the shape and direction of the beam on the cornea.

Excimer lasers used in eye surgery also receive modules allowing the pursuit of eye movements during operation (eye tracker), the integration of data specific to the operated eye (corneal topography, wave front, etc.). Part of the electronic components dedicated to these missions is shown here:

electronic excimer lasers

The excimer laser are equipped with many modules electroniqes to ensure the issuance of a beam of power set, a precise and controlled way to the surface of the operated eye.

Before each use, the laser must be "calibrated": the intensity of electric discharge intended to excite the gas environment is modulated in order to ensure a homogeneous and meets the specifications of the laser fluence according to the physical conditions in the cavity (quality of the gas mixture) and in the operating room (temperature, humidity, etc.). The calibration allows to compensate for a reduction in the power output by increasing the voltage delivered, and vice versa.

The following video shows an example of calibration with the Bausch & Lomb - Technolas Z100 laser. An orange plastic calibration plate is covered with a thin film of aluminium. This laser delivers the spots with diameters between 1 and 2 mm. When the fluence is optimal, there are 65 + / pulses to achieve two complete holes in the metal layer, naked putting the plastic layer. If the number of pulses required is more high, it must increase the intensity of electrical discharges in the cavity to increase the power emitted out of the cavity. If the number of pulses is less than 63, it should be instead of reducing tension in the cavity.

 

Characteristics of excimer cavities

The intensity of the tensions at play (on the order of some tens of kilo-Volt kV) imposes special constraints on the cavity and has some consequences that are a particular challenge of conducting an excimer laser cavity.

To avoid creating an electric arc between the electrodes, a cavity pre-ionisation is needed by an electric current writ through secondary electrodes. The duration of the discharge of the main electrode does not generally exceed 50 nanoseconds, but its intensity requires the presence of a high-performance insulating material in contact with the cavity.

To ensure a good quality radiation, must avoid the calorimetric fluctuations, and design a powerful cooling system.

The elements of the cavity subjected to these extreme conditions can degrade more quickly than for other types of laser and train the creation of dust, which must constantly be evacuated under penalty to alter the quality of the emitted beam.

So, the present gas mixture in the cavity next to the electrodes (either on a distance of a few centimetres) must be constantly renewed to dissipate heat and ensure a sufficient density in reactive elements to ensure a sufficient stimulated emission. This renewal must be made between each pulse, and this requires a quick movement of the order of several meters per second, faster than the firing frequency (between 100 and 500 Herz for excimers to surgical aiming lasers). This rapid movement requires the implementation of constraints of the fluid dynamics. Excimers eye lasers, however, are among those for which these constraints are relatively muted, as the sculpture of the cornea does not require high energy necessary for certain ceramic materials for example.

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