Excimer laser for vision correction. What is an excimer laser

EXCIMER LASER

EXCIMER LASER

- gas laser, working on electronic transitions of excimer molecules (molecules that exist only in electronically excited states). Potential dependence the interaction energy of excimer atoms, located in the ground electronic state, from the internuclear distance is a monotonically decreasing function, which corresponds to the repulsion of nuclei. For an excited electron, which is the top level of a laser transition, this dependence has a minimum, which determines the possibility of the existence of the excimer itself (Fig.). The lifetime of an excited excimer molecule is limited

Dependence of the energy of an esimer molecule on distance R between its constituent atoms X and Y; The upper curve is for the upper laser level, the lower curve is for the lower laser level. The values ​​correspond to the center of the gain line of the active medium, its red and violet boundaries. time its radiation. decay. Since the lower state of laser transition in electron beam. is devastated as a result of the scattering of atoms of the excimer molecule, the characteristic of which (10 -13 - 10 -12 s) is significantly less than the radiation time. devastation top, state of laser transition containing excimer molecules, is active medium with enhancement at transitions between excited bound and main expansion terms of the excimer molecule.

The basis of the active medium of E. l. They are usually composed of diatomic excimer molecules - short-lived compounds of inert gas atoms with each other, with halogens or with oxygen. Emission length E. l. lies in the visible or near UV region of the spectrum. Gain linewidth of laser transition E. l. is anomalously large, which is associated with the expanding nature of the lower transition term. Characteristic values ​​of parameters of laser transitions for the most common electron beams. are presented in the table.

Excimer laser parameters

Optimal parameters of the active medium E. l. correspond to optimal conditions for the formation of excimer molecules. Most favorable conditions for the formation of dimers of inert gases correspond to the pressure range of 10-30 atm, when intensive formation of such molecules occurs in triple collisions involving excited atoms:

At such high pressures, the most effective. The method of introducing pump energy into the active medium of a laser involves passing a beam of fast electrons through the gas, which mostly lose energy. to ionize gas atoms. Conversion of atomic ions into molecular ions and subsequent dissociation of molecular ions accompanied by the formation of excited atoms of an inert gas, provide the possibility of eff. conversion of the energy of a beam of fast electrons into the energy of excimer molecules. Lasers on dimers of inert gases are characterized by ~1%. Basic The disadvantage of lasers of this type is the extremely high beat value. threshold energy input, which is associated with the short wavelength of the laser transition and, therefore, the width of the gain line. This imposes high demands on the characteristics of the electron beam used as a laser pumping source and limits the output energy of laser radiation to the level of fractions of a joule (per pulse) at a pulse repetition rate of no higher than several. Hz A further increase in the output characteristics of lasers based on noble gas dimers depends on the development of technology for electron accelerators with an electron beam pulse duration of the order of tens of nanoseconds and a beam energy of ~kJ.

E. l. have significantly higher output characteristics. on monohalides of inert gases RX*, where X is a halogen. Molecules of this type are effectively formed during pairwise collisions, for example or

These processes occur with sufficient intensity even at pressures on the order of atmospheric pressure, so the problem of introducing energy into the active medium of such lasers turns out to be technically much less complex than in the case of lasers based on inert gas dimers. Active medium E. l. on monohalides of inert gases consists of one or several. inert gases at a pressure of the order of atmospheric and a certain number (~10 -2 atm) of halogen-containing molecules. To excite the laser, either a beam of fast electrons or a pulsed electric beam is used. discharge. When using a beam of fast electrons, the output laser radiation reaches values ​​of ~ 10 3 J with an efficiency of several. percent and a pulse repetition rate well below 1 Hz. In case of using electric discharge, the output energy of laser radiation in a pulse does not exceed a fraction of a joule, which is due to the difficulty of forming a discharge that is uniform in volume, meaning a volume at atm. pressure for a time of ~ 10 ns. However, when using electric discharge, a high pulse repetition rate is achieved (up to several kHz), which opens up the possibility of a wide range of practical applications. use of lasers of this type. Naib. widespread among E. l. obtained on XeCl, which is due to the relative simplicity of implementing operation in the high pulse repetition rate mode. Cp. The output of this laser reaches a level of 1 kW.

Along with high energy. characteristics important attractive feature of E. l. is the extremely high value of the gain linewidth of the active transition (table). This opens up the possibility of creating high-power lasers in the UV and visible ranges with smooth wavelength tuning in a fairly wide range of the spectrum. This problem is solved using an injection laser excitation circuit, which includes a low-power generator of laser radiation with a wavelength tunable within the amplification line width of the active medium of the electron beam, and a broadband amplifier. This scheme makes it possible to obtain a laser with a line width of ~ 10 -3 HM, tunable in wavelength in a range of width ~ 10 HM and more.

E. l. are widely used due to their high energy. characteristics, short wavelength and the possibility of its smooth tuning in a fairly wide range. Powerful single-pulse electron beams excited by electron beams are used in installations for studying laser heating of targets for the purpose of carrying out thermonuclear reactions (for example, a KrF laser with HM, output energy per pulse up to 100 kJ, pulse duration ~ 1 ns). Lasers with a high pulse repetition rate, excited by a pulsed gas discharge, are used in technology. purposes in the processing of microelectronics products, in medicine, in experiments on laser isotope separation, in sensing the atmosphere in order to control its pollution, in photochemistry and in experiments. physics as an intense monochromatic source. UV or visible radiation.

Lit.: Excimer lasers, ed. C. Rhodes, trans. from English, M., 1981; EletskyA. V.. Smirnov B. M., Physical processes in gas lasers, M.. 1985. A. V. Eletsky.

Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-chief A. M. Prokhorov. 1988 .

See what "EXCIMER LASER" is in other dictionaries:

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EXCIMER LASER- (excimer laser) a laser used to remove very thin layers of tissue from the surface of the cornea of ​​the eye. This operation can be performed to change the curvature of the corneal surface, for example, in the process of treating myopia (photorefractive... ... Explanatory dictionary of medicine

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In this article we will look at the advantages of excimer lasers. Today, medicine has a wide range of all kinds of laser equipment for the treatment of complex diseases in hard-to-reach areas of the human body. help to achieve the effect of minimally invasiveness and painlessness, which has a huge advantage over those surgical interventions that are performed manually during abdominal operations, which are very traumatic, fraught with high blood loss, as well as long-term rehabilitation after them.

What is a laser?

A laser is a special quantum generator that emits a narrow beam of light. Laser devices open up incredible possibilities for transmitting energy over different distances at high speed. Ordinary light, which can be perceived by human vision, consists of small beams of light that spread in different directions. If these beams are concentrated using a lens or mirror, a large beam of light particles will be obtained, but even this cannot be compared with a laser beam, which consists of quantum particles, which can only be achieved by activating the atoms of the medium that underlies the laser radiation.

Varieties

With the help of colossal developments by scientists around the world, excimer lasers are today widely used in many areas of human activity and have the following varieties:

Origin

This type is ultraviolet, which is widely used in the field of eye surgery. Doctors use this device to perform laser vision correction.

The term "excimer" means "excited dimer" and characterizes the type of material that is used as its working fluid. For the first time in the USSR, such a device was presented in 1971 by scientists V. A. Danilichev, N. Basov and Yu. M. Popov in Moscow. The working fluid of such a laser was a xenon dimer, which was excited by a beam of electrons in order to produce radiation with a certain wavelength. After some time, noble gases with halogens began to be used for this, and this was done in 1975 in one of the US research laboratories by scientists J. Hart and S. Searles.

People often ask why excimer lasers are used for vision correction.

Its uniqueness

It was found that the excimer molecule produces by being in an excited "attractive" state as well as a "repulsive" state. This effect can be explained by the fact that xenon or krypton (noble gases) are highly inert and, as a rule, never form chemical compounds. An electrical discharge causes them to become excited, so that they can form molecules either with each other or with halogens, such as chlorine or fluorine. The appearance of molecules in an excited state creates, as a rule, a so-called population inversion, and such a molecule gives up its energy, which is stimulated or spontaneous emission. After this, the molecule returns to its ground state and disintegrates into atoms. The excimer laser device is unique.

The term “dimer” is usually used when identical atoms are connected to each other, but most modern excimer lasers use compounds of noble gases and halogens. Nevertheless, these compounds, which are used for all lasers of a similar design, are also called dimers. How does an excimer laser work? We will look at this now.

Operating principle of excimer laser

This laser is the main player in PRK and LASIK. Its working fluid is inert and halogen gas. When high voltage is introduced into the mixture of these gases, one halogen atom and one inert gas atom combine to form a diatomic molecule. It is in an extremely excited state and after a thousandth of a second it disintegrates into atoms, which leads to the appearance of a light wave in the UV range.

This principle of operation of the excimer laser has found wide application in medicine, since ultraviolet radiation affects organic tissues, for example, the cornea, in such a way that the bonds between molecules are separated, leading to the transfer of tissues from a solid to a gaseous state. This process is called "photoablation".

Wave range

All existing models of this type operate in the same wavelength range and differ solely in the width of the light beam, as well as in the composition of the working fluid. The excimer laser is the most commonly used laser for vision correction. But there are other areas of its use.

The first had a light beam diameter that was equal to the diameter of the surface on which evaporation occurred. The wide range of the beam and its heterogeneity caused the same heterogeneity in the upper layers of the cornea, as well as an increase in temperature on its surface. This process was accompanied by damage and burns. This situation was corrected by the creation of the excimer laser. The MNTK Eye Microsurgery has been using it for a very long time.

New generation lasers have undergone a long process of modernization, during which the diameter of the light beam was reduced, and a special rotational scanning system for delivering laser radiation to the eye was created. Let's look at how excimer lasers are used by doctors.

Application in medicine

In cross-section, such a laser beam looks like a spot moving in a circle, removing the upper layers of the cornea, and also giving it a different radius of curvature. In the ablation zone, the temperature does not rise because the effect is short-term. As a result of the operation, a smooth and clear surface of the cornea is observed. The excimer laser is indispensable in ophthalmology.

The surgeon performing the surgery determines in advance what portion of energy will be supplied to the cornea, as well as to what depth the excimer laser will be applied. From here, the specialist can plan the course of the process in advance and assume what result will be obtained as a result of the operation.

Laser vision correction

How does an excimer laser work in ophthalmology? The method that is popular today is based on the so-called computer repurposing of the cornea, which is the main optical lens of the human eye. The excimer laser that is used on it smoothes the surface of the cornea, removing the upper layers and, thus, eliminating all defects present on it. At the same time, normal conditions appear for the eye to receive the correct images, creating the correct refraction of light. People who have had this procedure see like everyone else who has initially good vision.

The procedure for repurposing the cornea does not cause high temperatures on its surface, which can be detrimental to living tissue. And, according to most people, the so-called burning of the upper layers of the cornea does not occur.

The most important advantage of excimer lasers is that their use for vision correction allows you to get an ideal result and correct almost all existing corneal anomalies. These devices are so precise that they allow “photochemical ablation” of the upper layers.

For example, if this process is carried out on the central zone of the cornea, then its shape becomes almost flat, and this helps correct myopia. If, during vision correction, the layers of the cornea in the peripheral zone are evaporated, then its shape becomes more rounded, and this, in turn, corrects farsightedness. Astigmatism is corrected through dosed removal of the upper layers of the cornea in its various parts. Modern excimer lasers, which are widely used in refractive microsurgery of the eye, guarantee high quality surfaces that undergo photoablation.

Features of use in medicine

Excimer lasers in the form they have today appeared quite recently, but they are already helping people all over the world get rid of vision problems such as myopia, farsightedness, and astigmatism. This solution to the problem, for the first time in many years of creating such equipment, meets all the requirements of painlessness, maximum safety and efficiency.

Eye diseases that can be treated by using

The field of ophthalmic surgery that deals with the elimination of these anomalies of the human eye is called refractive surgery, and such disorders are called ametropic and refractive errors.

According to experts, there are two types of refraction:

Ametropia, in turn, includes several subtypes:

• myopia (nearsightedness);
• astigmatism - the eye receives a distorted image when the cornea has irregular curvature, and the flow of light rays becomes unequal on different parts of its surface;
• hypermetropia (farsightedness).

There are two types of astigmatism - hypermetropic, which is close to farsightedness, myopic, similar to myopia, and mixed.

In order to correctly imagine the essence of refractive manipulations, it is necessary to have a minimal knowledge of the anatomy of the human eye. The optics system of the eye consists of three main elements - the cornea, the lens, which are the light-refracting parts, and the retina, which is the light-receiving part. In order for the resulting image to become clear and sharp, the retina is in the focus of the ball. However, if it is in front of the focus, which happens with farsightedness, or behind it, which happens with myopia, the resulting image becomes unclear and significantly blurred.

In humans, the optics of the eye can change throughout life, in particular from the moment of birth until the age of 16-20, it changes due to the growth and increase in the size of the eyeball, as well as under the influence of certain factors that can lead to the formation of certain anomalies . Thus, the patients of the eye refractive surgeon most often become adults.

Contraindications to excimer beam vision correction procedure

Vision correction with an excimer laser is not indicated for all people suffering from visual impairments. The use of this procedure is prohibited:

Possible complications after use

All existing excimer laser treatment methods today are highly safe and particularly effective. However, there are a number of complications that can occur after surgery using such techniques. These include:

1. Partial or incorrect growth of a part of the cornea, after which it is not possible to grow this part again.
2. The so-called dry eye syndrome, when the patient experiences redness and pain in the eye. This complication can occur in cases where, during the process of vision correction, the nerve endings that are responsible for the production of tears have been damaged.
3. Various vision disorders, for example, double vision or decreased vision in the dark, impaired color perception, or the appearance of a light halo.
4. Weakening or softening of the cornea, which can occur either a few months after surgery or several years later.

Excimer laser in dermatology

The effect of low-frequency laser on the skin is extremely positive. This happens due to the following effects:

• anti-inflammatory;
• antioxidant;
• pain reliever;
• immunomodulatory.

That is, there is a certain biostimulating mechanism of action of laser radiation with low power.

Successfully undergoes excimer laser treatment for vitiligo. Pigment spots on the skin are smoothed out very quickly.

(laser vision correction) and semiconductor manufacturing.

Laser emission from an excimer molecule occurs due to the fact that it has an “attractive” (associative) excited state and a “repulsive” (non-associative) ground state - that is, molecules do not exist in the ground state. This is because noble gases such as xenon or krypton are highly inert and do not usually form chemical compounds. When excited (caused by electrical discharge), they can form molecules with each other (dimers) or with halogens such as fluorine or chlorine. Therefore, the appearance of molecules in an excited bound state automatically creates a population inversion between the two energy levels. Such a molecule, in an excited state, can give up its energy in the form of spontaneous or stimulated emission, as a result of which the molecule goes into the ground state, and then very quickly (within picoseconds) disintegrates into its constituent atoms.

Even though the term dimer refers only to the joining of identical atoms, and most excimer lasers use mixtures of noble gases with halogens, the name has stuck and is used for all lasers of a similar design.

The wavelength of an excimer laser depends on the composition of the gas used, and usually lies in the ultraviolet region:

Excimer lasers usually operate in pulsed mode with a pulse repetition rate from 1 Hz to several hundred Hz; in some models the frequency can reach 2 kHz; generation of single pulses is also possible. Radiation pulses usually have a duration from 10 to 30 ns and an energy from units to hundreds of mJ. The powerful ultraviolet radiation of such lasers allows them to be widely used in surgery (especially eye surgery), in photolithography processes in semiconductor production, in microprocessing of materials, in the production of LCD panels, as well as in dermatology. Today, these devices are quite bulky, which is a disadvantage for widespread medical use (see LASIK), but their size is constantly decreasing due to modern developments.

see also

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Links

• EXCIMER LASER - Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-chief A. M. Prokhorov. 1988.
• Excimer lasers, ed. C. Rhodes, trans. from English, M., 1981

An excerpt describing the excimer laser

Balashev respectfully allowed himself to disagree with the opinion of the French emperor.
“Every country has its own customs,” he said.
“But nowhere in Europe is there anything like this,” said Napoleon.
“I apologize to your Majesty,” said Balashev, “besides Russia, there is also Spain, where there are also many churches and monasteries.”
This answer from Balashev, which hinted at the recent defeat of the French in Spain, was highly appreciated later, according to Balashev’s stories, at the court of Emperor Alexander and was appreciated very little now, at Napoleon’s dinner, and passed unnoticed.
It was clear from the indifferent and perplexed faces of the gentlemen marshals that they were perplexed as to what the joke was, which Balashev’s intonation hinted at. “If there was one, then we did not understand her or she is not at all witty,” said the expressions on the faces of the marshals. This answer was so little appreciated that Napoleon did not even notice it and naively asked Balashev about which cities there is a direct road to Moscow from here. Balashev, who was on the alert all the time during dinner, replied that comme tout chemin mene a Rome, tout chemin mene a Moscow, [just as every road, according to the proverb, leads to Rome, so all roads lead to Moscow,] that there are many roads, and that among these different paths there is the road to Poltava, which Charles XII chose, said Balashev, involuntarily flushing with pleasure at the success of this answer. Before Balashev had time to finish the last words: “Poltawa,” Caulaincourt began talking about the inconveniences of the road from St. Petersburg to Moscow and about his St. Petersburg memories.
After lunch we went to drink coffee in Napoleon’s office, which four days ago had been the office of Emperor Alexander. Napoleon sat down, touching the coffee in a Sevres cup, and pointed to Balashev’s chair.
There is a certain after-dinner mood in a person that, stronger than any reasonable reason, makes a person be pleased with himself and consider everyone his friends. Napoleon was in this position. It seemed to him that he was surrounded by people who adored him. He was convinced that Balashev, after his dinner, was his friend and admirer. Napoleon turned to him with a pleasant and slightly mocking smile.
– This is the same room, as I was told, in which Emperor Alexander lived. Strange, isn't it, General? - he said, obviously without doubting that this address could not but be pleasant to his interlocutor, since it proved the superiority of him, Napoleon, over Alexander.
Balashev could not answer this and silently bowed his head.
“Yes, in this room, four days ago, Wintzingerode and Stein conferred,” Napoleon continued with the same mocking, confident smile. “What I cannot understand,” he said, “is that Emperor Alexander brought all my personal enemies closer to himself.” I do not understand this. Didn't he think that I could do the same? - he asked Balashev with a question, and, obviously, this memory pushed him again into that trace of morning anger that was still fresh in him.
“And let him know that I will do it,” said Napoleon, standing up and pushing his cup away with his hand. - I will expel all his relatives from Germany, Wirtemberg, Baden, Weimar... yes, I will expel them. Let him prepare refuge for them in Russia!
Balashev bowed his head, showing with his appearance that he would like to take his leave and is listening only because he cannot help but listen to what is being said to him. Napoleon did not notice this expression; he addressed Balashev not as an ambassador of his enemy, but as a man who was now completely devoted to him and should rejoice at the humiliation of his former master.

Excimer laser system WaveLight EX500

WaveLight EX500 is the latest generation excimer laser unit, the use of unique advantages of which allows the patient to achieve the best visual acuity in the most comfortable and safe way.

The operating pulse frequency is 500 Hz, which makes the WaveLight EX500 one of the fastest excimer laser systems in the world. Due to the high speed of the laser, the cornea is not subjected to excessive thermal effects, which prevents its dehydration during the procedure - accordingly, the recovery period after laser correction is shortened and proceeds as comfortably as possible.

The new excimer laser installation features full integration with the diagnostic complex - a single server for diagnostic equipment and a surgical laser allows for fully automated data transfer, which minimizes the human factor. The built-in pachymeter provides additional control of the depth of laser exposure, allowing you to measure the thickness of the cornea on-line at all stages of surgery.

The infrared tracking system, which monitors the center of the pupil and is synchronized with the laser source itself, allows you to accurately determine the laser impact area. The reaction time of the eye tracking system is less than 3 milliseconds. The frequency of the eye tracking system is 1050Hz. Controlling the position of the eye at the center of the pupil, the edge of the cornea, and the iris allows you to track the slightest movements of the eye in such a way that it does not affect the accuracy of the correction.

Thanks to the use of optimized and controlled wavefront technologies, the risk of spherical aberrations is prevented, and patients have virtually no problems associated with impaired twilight and night vision.

Application limits of the WaveLight EX500 excimer laser system:

• myopia from -0.25 to -14.0 D;
• myopic astigmatism from -0.25 to -6.0 D;
• farsightedness from +0.25 to +6.0 D;
• hypermetropic astigmatism from +0.25 to +6.0 D.

VISX Star S4 IR laser

The VISXStarS4 IR laser is significantly different from other models - it allows excimer laser correction for patients with complicated forms of myopia, farsightedness and higher order aberrations (distortions).

The new integrated approach implemented in the VISX Star S4 IR installation allows us to guarantee the most smooth surface of the cornea formed during laser correction, to monitor possible minor movements of the patient’s eye during surgery, and to compensate as much as possible for the most complex distortions of all optical structures of the eye. Such characteristics of the excimer laser significantly reduce the likelihood of postoperative complications, significantly shorten the rehabilitation period, and guarantee the highest results.

Application limits:

• Myopia (myopia) up to -16 D;
• Farsightedness (hypermetropia) up to +6 D;
• Complex astigmatism up to 6 D.

Femtosecond lasers

Femtosecond laser FS200 WaveLight

The FS200 WaveLight femtosecond laser has the fastest corneal flap formation speed - in just 6 seconds, while other laser models create a standard flap in 20 seconds. In excimer laser correction, the FS200 WaveLight femtosecond laser creates a corneal flap by applying very fast pulses of laser light.

The femtosecond laser uses a beam of infrared light to precisely separate tissue at a specific depth through a process called photodisruption. A pulse of laser energy is focused to a precise location within the cornea, with thousands of laser pulses placed side by side to create an access plane. Due to the application of multiple laser pulses according to a certain algorithm and at a certain depth in the cornea, it is possible to cut out a corneal flap of any shape and at any depth. That is, the unique characteristics of the femtosecond laser enable the ophthalmic surgeon to form a corneal flap, fully controlling its diameter, thickness, alignment and morphology with minimal disruption of the architecture.

Most often, a femtosecond laser is used during excimer laser correction using the FemtoLasik technique, which differs from other techniques in that the corneal flap is formed using a laser beam rather than a mechanical microkeratome. The absence of mechanical impact increases the safety of laser correction and several times reduces the risk of acquired postoperative corneal astigmatism, and also allows laser correction to be performed on patients with thin corneas.

The FS200 WaveLight femtosecond laser is combined into a single system with, and therefore the time required for the excimer laser correction procedure using these two laser units is minimal. Due to its unique properties for creating an individual corneal flap, the femtosecond laser is also successfully used during keratoplasty when forming a corneal tunnel for subsequent implantation of an intrastromal ring.

Femtosecond laser IntraLase FS60

The IntraLase FS60 femtosecond laser has a high frequency and short pulse duration. The duration of one pulse is measured in femtoseconds (one trillionth of a second, 10-15 s), which makes it possible to separate the layers of the cornea at the molecular level without generating heat or mechanical impact on the surrounding eye tissue. The process of forming a flap using the FS60 femtosecond laser for laser vision correction occurs in a few seconds, absolutely contact-free (without a corneal incision).

The IntraLase FS60 femtosecond laser is part of the complete line of equipment for the iLasik system. It works in conjunction with the VISX Star S4 IR excimer laser and WaveScan aberrometer. This complex makes it possible to perform laser vision correction, taking into account the slightest features of the patient’s visual system.

Microkeratomes

The result of laser correction depends on many parameters. This includes the experience of the specialist, the treatment method used, and the laser used during the correction. But no less important in the treatment process is a device such as a microkeratome. The microkeratome is necessary for excimer laser correction using the LASIK technique. A special feature of the microkeratomes used in Excimer clinics is the highest safety. They can work autonomously, regardless of power supply. During LASIK treatment, it is not the outer layers of the cornea that are exposed, but the inner ones. In order to separate the upper layers of the cornea, a microkeratome is needed. The Excimer Clinic uses microkeratomes from the world-famous Moria company. It was one of the first to produce not manual, but automatic models, which made it possible to minimize risks when carrying out excimer laser correction and significantly improve its quality.

Moria Evolution 3

This type of microkeratome allows the preparatory stage before excimer laser vision correction (namely, the formation of a flap) to be carried out in the least painful way for the patient and to reduce discomfort to a minimum. The device is equipped with reusable heads, fixing vacuum rings, as well as a directly automatic rotational keratome. The design of the microkeratome rings and heads allows for flexible adjustment of the equipment to the individual characteristics of the patient’s eye, which leads to more accurate and guaranteed results.

In modern refractive surgery, 2 types of laser systems are used for laser vision correction: excimer and femtosecond units, which have a number of distinctive features and are used to solve various problems.

Excimer lasers

An excimer laser is a gas laser device. The working medium in this laser is a mixture consisting of inert and halogen gases. As a result of a special reaction, excimer molecules are formed.

The word excimer is an acronym that can be literally translated as excited dimer. This term refers to an unstable molecule that forms when stimulated by electrons. With the further transition of molecules to their previous state, photons are released. In this case, the wavelength depends on the gas used in the device. In medical practice, excimer lasers are usually used, which emit photons in the ultraviolet spectrum (157-351 nm).

For medical purposes, high-power pulsed light is used, which leads to ablation of tissue in the affected area. So, in some cases, an excimer laser can replace a scalpel, as it causes photochemical destruction of surface tissues. At the same time, the laser does not lead to an increase in temperature and subsequent thermal destruction of cells, which affects deeper tissues.

History of excimer lasers

In 1971, the excimer laser was first presented at the P.N. Lebedev Physical Institute. in Moscow by several scientists (Basov, Popov, Danilichev). This device used bi-xenon, which was excited by electrons. The laser had a wavelength of 172 nm. Later, mixtures of various gases (halogens and inert gases) began to be used in the device. It was in this form that the laser was patented by the Americans Hart and Searles from the Navy laboratory. This laser was first used to engrave computer chips.

Only in 1981, scientist Srivanson discovered the ability of a laser to produce ultra-precise tissue cuts without causing damage to surrounding cells due to high temperatures. When tissue is irradiated with a laser with a wavelength in the ultraviolet range, intermolecular bonds are broken, as a result of which the tissue turns from solid to gaseous, that is, it evaporates (photoablation).

In 1981, lasers began to be introduced into ophthalmic practice. In this case, the laser was used to influence the cornea.

In 1985, the first laser correction was performed using the PRK technique using an excimer laser.

All excimer lasers used in modern clinical practice operate in a pulsed mode (frequency 100 or 200 Hz, pulse length 10 or 30 ns) with the same wavelength range. These devices differ in the shape of the laser beam (flying spot or scanning slit) and the composition of the inert gas. In cross-section, the laser beam looks like a spot or slit; it moves along a certain trajectory, removing specified layers of the cornea. As a result, the cornea takes on a new shape, which has been programmed taking into account individual parameters. In the photoablation zone there is no significant (more than 6-5 degrees) increase in temperature, since the duration of laser irradiation is insignificant. With each pulse, the laser beam evaporates one layer of the cornea, the thickness of which is 0.25 microns (about five hundred times less than human hair). This precision allows you to get excellent results when using an excimer laser for vision correction.

Femtosecond lasers

Ophthalmology, like many other areas of medicine, has been actively developing in recent years. Thanks to this, methods of performing eye surgeries are being improved. About half of the success of the operation depends on modern equipment, which is used during diagnosis and directly during the intervention. During laser vision correction, a beam is used that contacts the cornea and changes its shape with high precision. This allows the operation to be bloodless and as safe as possible. It was in ophthalmology that lasers began to be used for surgical interventions earlier than in other areas of medical practice.

In the treatment of eye diseases, special types of laser devices are used, which differ in the source of study, wavelength (krypton lasers with a red-yellow luminescence range, argon lasers, helium-neon units, excimer lasers, etc.). Recently, femtosecond lasers have become widespread, which are distinguished by a short luminescence pulse of only a few (sometimes several hundred) femtoseconds.

Advantages of femtosecond lasers

Femtosecond lasers have a number of advantages that make them indispensable for use in ophthalmology. These devices are highly accurate, so you can get a very thin layer of cornea with predetermined flap parameters.

During the operation, the contact lens of the installation momentarily comes into contact with the cornea, as a result of which a flap is formed from the superficial layers. The unique capabilities of the femtosecond laser help create a flap of any shape and thickness depending on the surgeon’s needs.

The area of ​​application of the femtosecond laser in ophthalmology is the correction of ametropia (astigmatism, myopia, hypermetropia), corneal transplantation and the creation of intrastromal rings. It is the operations that use a femtosecond laser that allow you to obtain stable and high results. After surgery, the flap is placed in its original place, so the wound surface heals very quickly without suturing. Also, when using a femtosecond laser, discomfort during surgery and pain after it are reduced.

7 facts in favor of femtosecond laser

• The surgical operation does not require the use of a scalpel, and the manipulation itself is very quick. It only takes 20 seconds to create a flap using a laser. The laser scale is ideal for ophthalmic interventions. During and after the procedure, the patient does not experience pain, because the tissue is practically not damaged (the layers of the retina are stratified under the influence of air bubbles).
  Immediately after separating the corneal flap, you can begin direct vision correction by evaporating the stromal substance. Moreover, the entire operation takes no more than six minutes for one eye. If you use another laser, it may take time for all the air bubbles to disappear (about an hour).
• The operation is carried out under the control of Eye-tracking, which is a tracking system for the displacement of the eyeball. Thanks to this, all pulses of the laser beam hit exactly the point at which it was programmed. As a result, vision after surgery is restored to high values.
• Visual acuity in the dark when performing surgery with a femtosecond laser also reaches high values. Dark vision is restored especially well after correction using the FemtoLasik method, which takes into account the individual parameters of the patient’s cornea and pupil.
• Fast recovery. After laser vision correction, you can go home immediately, but experts recommend staying at the clinic for at least a day. This will reduce the risk of infection and corneal injury along the way. Visual function is restored as quickly as possible. The next morning, visual acuity reaches its maximum values.
• Incapacity for work only for a day. Complete healing of the cornea takes about a week, but in most cases the patient can return to work the very next day after femtosecond laser surgery. During the recovery period, special drops should be instilled, and physical activity and increased visual stress should be avoided.
• Technical excellence in performing FemtoLasik is made possible thanks to extensive experience in performing such operations. The femtosecond laser has been used since 1980, and during this time all errors and inaccuracies of the technique have been corrected.
• The predictability of results with this type of laser vision correction reaches 99%. It is extremely rare, due to the individual characteristics of the patient, that after surgery there is undercorrection, which requires re-intervention or spectacle correction.