Dr Agarwals’ Step by Step LASIK Surgery Amar Agarwal, Athiya Agarwal, Sunita Agarwal
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Excimer Laser: Fundamental ConceptsChapter 1

Amar Agarwal
J Agarwal
2
 
INTRODUCTION
To understand what an excimer laser is one should understand the fundamental concepts of the excimer.1,2 The word LASER stands for light amplification by stimulated emission of radiation. The word EXCIMER stands for excited dimer. If we understand these two words, then only we can understand what an excimer laser is?
 
ATOM
Let us first of all understand what an atom is. In an atom (Fig.1.1) we have a nucleus. This nucleus contains protons and neutrons. The protons are positively charged and the neutrons are neutral. Around the nucleus we have electrons which are negatively charged. These electrons move around in orbits. These orbits are named K, L, M, N, etc. The first orbit K has 2 electrons. This is calculated from the formula 2n2, n stands for the number in the orbit. For example, in the first orbit the number of electrons present will be-
    K = 2n2
    K = 2 × 12 = 2
This means that in the first orbit K there can be a maximum of 2 electrons. Let us now look at the second orbit L.
    L = 2n2
    L = 2 × 22= 8
Thus, in the second orbit L there can be a maximum of 8 electrons.
 
EMISSION
There are two words, which we have to understand. They are spontaneous emission and stimulated emission. Whenever an electron from a lower orbit say K moves to a higher orbit say M it cannot stay there for a long time.
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Fig. 1.1: Atom
It has to come back to its original place. When that happens it gives off a photon of light. In spontaneous emission (Fig. 1.2) the electron at a lower orbit is energized spontaneously to a higher orbit. But it cannot stay there, so it decays back to the lower energy orbit level. At this time, it gives off a photon of light (Fig. 1.3). The wavelength of this light is inversely proportional to the energy lost by the electron, which, in turn, is dependent upon the type of atom.
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Fig. 1.2: Spontaneous emission
If the electron is energized and sent to a higher orbit by another photon that has an identical wavelength to that which the atom will produce, then we call it as stimulated emission (Fig. 1.4). The photon stimulates the electrons, which goes from the lower orbit to a higher orbit. Once again, it cannot stay there for a long time and so falls back to its original orbit level and in the process gives off another photon of light.
When this process continues, it leads to an amplification process (Fig. 1.5). One photon hits an atom producing the electron from that atom going to a higher orbit.5
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Fig. 1.3: Spontaneous emission—note the release of photon
When the electron falls back to its original level it, in turn, releases, photons. These then hit other atoms, thus, leading to an amplification process.
 
LASER MEDIA AND PUMPS
Now that we have understood what an amplification process is we should understand what the laser media and laser pumps are. The laser media contains the atoms that do the lasing.
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Fig. 1.4: Stimulated emission
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Fig. 1.5: Amplification process
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The laser media can be a gas medium, which is what happens in the excimer or the argon laser. It can be a liquid medium as in the dye laser or it can be a solid medium as in the neodymium: yttrium-aluminum-garnet (Nd:YAG) or diode lasers.
From what we have read, it is obvious that we need some energy to hit the atoms. The laser pump is a source of energy that is needed to energize the electrons. This can be in the form of electrical discharge or current. Alternatively, it could be light energy produced by either flash lamps or another lamp.
If lasing has to occur, population inversion must be present. In other words, the pumped medium must be in a state of population inversion for lasing to occur. Population inversion is when more than half of the atoms in the medium are energized to an excited state. An atom that is capable of being excited and then stimulated to emit a photon of light will also resonantly absorb a photon of the same wavelength when not excited. So, more atoms should be in the excited state than the nonexcited state. If there are more atoms in the nonexcited state than the excited state, the atoms will have the absorption process going rather than the stimulated emission process, and the amplification process will not occur. As one wants the amplification process to occur more atoms should be in the excited state than the nonexcited state. This is called population inversion.
 
EXCITED DIMER
The word excimer is a contraction of the term excited dimer. Let us first understand what the word dimer means. Molecules made up of two identical atoms are called dimers (Fig. 1.6). In excimer laser, we use two atoms—an inert gas and a halide, specifically argon and fluoride.
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Fig. 1.6: Dimer
These two elements are in very small concentrations in a helium mixture known as premix. Thus, in excimer laser we do not use an actual dimer, as the two atoms are not exactly identical. This is why, it is said that the word dimer is a misnomer as it applies to two of the same elements.
If two systems (atoms or molecules) do not form a strong chemical bond when they are in their ground states, but form a strong chemical bond when one of them is in an excited state, then the bound excited state is called an excimer. The excimer dissociates in the ground state and this helps to maintain population inversion necessary for lasing to occur. When the excited dimer (argon and fluoride) dissociates, it releases ultraviolet energy.
 
EXCIMER LASER
Thus an excimer laser is an excited dimer which produces a laser beam of ultraviolet energy. The excimer lasers can produce ultraviolet light energy at various wavelengths depending upon the gas elements utilized. If the ultraviolet energy released is of 248 nm, it results in potentially mutagenic behavior as evidenced by unscheduled DNA synthesis. If the ultraviolet energy is of 308 nm, it can lead to cataract formation. 9Thus, it was found that the ideal ultraviolet energy should be 193 nm. This is really suitable for corneal ablation. The excimer laser at this wavelength has submicron precision with submicron collateral damage and minimal thermal energy effect. The excimer laser removes 0.25 microns per pulse and damages an additional 0.25 microns of adjacent corneal tissue.
 
LASER VS LIGHT
The laser beam is a coherent light compared to a light beam. The differences between laser and light are shown in Table 1.1.
Table 1.1   Difference between laser and light
Laser
Light
Monochromatic
Polychromatic
Coherent
Incoherent
Laser beam is less divergent
Light beam is more divergent
Laser is a narrow beam with small spot size
Light beam is a broad beam with large spot size
 
EXCIMER MACHINE
Excimer lasers usually consist of a large elongated aluminum box. This box is filled with the appropriate gas mixture. Running the full lengths of this box are two metal electrodes spaced about 2 to 3 cm apart. At one end of the box, aligned with the gap between the electrodes, a mirror is mounted and, at the other end, a window. The window is usually an uncoated optic. Outside the box, there is usually a large bank of capacitors, and these are charged using a high voltage power supply to several tons of kilovolts. A special switch called a thyraton is used to dump the energy stored in these capacitors across the electrodes inside the box.
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Fig. 1.7: Excimer laser machine
The electrical discharge through the gas between the electrodes ionizes the gas and allows the excimer molecules to form. Lasing action usually occurs within nanoseconds. The laser beam goes through a power monitoring system and is then directed by mirrors onto the patient's eye (Fig. 1.7).
 
FLUENCY
Once the concept of an excimer laser is understood, we have to then understand what the word fluence means. Fluence means the amount of energy applied to the ablative area. In other words:
Fluence=Energy/Area=millijoules/sq cm
Fluence is defined as energy applied to a given area and is expressed in units of millijoules per square centimeter 11and varies depending upon the laser. Fluence is the primary determinant of the amount of tissue ablated with each pulse. Different excimer machines have different fluency values. For example, the fluence of:
  • Summit excimer—160 mJ/cm2
  • Chiron—130 mJ/cm2
  • Nidek—130 mJ/cm2
This means that when we are testing the fluence plates before starting lasik or photorefractive keratectomy (PRK), we first keep the fluence plates and convert the white plates to red in the Chiron machine. We check in the Chiron machine that once the area has become fully red, the figure in the computer shows 65 shots. If that happens then only we proceed with the case. This shows that the fluence is now 130 mJ/cm2.
 
HOMOGENEITY
Homogeneity indicates the pattern of energy distribution within the exposed area. This can be shown as microhomogeneity or macrohomogeneity. Microhomogeneity shows localized variability in the energy beam density, i.e. hot and cold areas within the beam. The hot areas represent highest energy density areas and the cold areas represent lowest energy density areas. Rather than defining localized energy patterns, macrohomogeneity tends to refer to the overall energy beam profile of the laser. Macrohomogeneity can be gaussian, homogeneous or reverse gaussian. In gaussian, the greatest energy density is centrally. In homogenous, the energy distribution is equal throughout the beam and in reverse gaussian, the energy density is lowest centrally.12
 
DELIVERY SYSTEMS
We have to remember with all this there has to be a delivery system, which delivers the laser beam onto the eye. The delivery systems can be broad beam or a scanning beam system. The broad beam system is similar in concept to a projection system for a 35-mm slide system. The broad beam delivery system projects the entire beam on the stromal surface. Then utilizing an iris diaphragm or variable aperture system, the broad beam is controlled to create the ablation pattern required. The scanning systems utilize a slit beam or small spot to scan the surface of the cornea. The advantages are the versatility for creating various topographical patterns—hyperopia, asymmetrical astigmatism, etc. In Fig. 1.8, one can see the slit beam scanning system. The slit beam scans the corneal surface.
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Fig. 1.8: Slit-beam scanning laser system
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In the flying spot system, instead of the slit beam, one has a flying spot, which delivers the excimer laser energy. In the scanning systems, one incorporates an eye tracking system, as these systems are less forgiving for even small decentrations. The advantage of the scanning beam is that as the beam is smaller, it allows for a more easily created homogeneous beam, requiring less optics and a less powerful laser head.
 
SUMMARY
An excimer laser has changed the management of refractive errors. It is nothing but an excited dimer, which creates a 193 nm wavelength of ultraviolet light. The excimer laser machine is a very sophisticated machine, and one should remember fluence, homogeneity and delivery systems when understanding this machine.
REFERENCES
  1. Mcghee CNJ, Taylor HR, Garty DS et al: Excimer Lasers in Ophthalmology: Principles and Practice Martin Duntz:  London, 1997.
  1. Machat JJ: Excimer Laser Refractive Surgery Slack Inc:  Thorofare 1996.