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Laser Physics

Laser

Light, Laser and its basic principles

The light can be described as an electromagnetic emission, and because of that, it has some characteristics that identify it completely. These emissions are generically known as, radiations or electromagnetic waves, and are contained in a great band or track, which is subdivided according to some peculiar physical characteristics. There are those ones that we cannot see, such as sound waves emitted by someone who speaks or sings and the AM and FM radio waves (Figure 3), and there are those ones that we can see, such as luminous ones, formed by photons, like the light emitted by the lamps of a house’s chandeliers.

The emissions are organized following what is called Electromagnetic Radiations Spectrum, based on a particular characteristic: The wavelength (Figure 4). This spectrum is formed by infrared radiations, visible radiations, ultraviolet radiations, non-ionizing radiations (x-ray and gamma ray), besides other types of radiation which are not related to this work. The lasers for medical, odontological and veterinarian treatment (what we call Life Sciences) emit radiations that are located in the range of visible radiations, infrared and ultraviolet and those ones that are ionizing.

In order to identify in which part of the spectrum is classified a certain radiation, we need to know its wave-length, which is nothing more than the measured distance between two consecutive peaks of a waving trajectory (wake like) (Figure 5). The used unit to express this greatness is a meter’s fraction, normally the nanometer, which is equivalent to 0.000000001 meters (or 10-9).

A very simple way of understanding the spectrum concept is to observe the rainbow (Figure 4). This natural phenomenon is formed by the decomposition of the white light in seven basic colors. These seven colors, which we can see, are part of the spectrum of the electromagnetic radiations, are defined by their wavelength and when they are mixed they generate the white color. Each emitted color has got its own wavelength, and that happens with other colors that we cannot see, but whose effects can be felt.

Figure 3 – Oscillations, radiations or electromagnetic waves, are expressions that can be synonym.

Figure 4 – Electromagnetic radiations spectrum.

Figure 5 – Mensuration of length of an electromagnetic wave.

In the scale of wavelength, below the range of emissions of what we call visible, we have the ultraviolet, which is a very broad range. The ultraviolet emission is responsible for the darkening of our skin when we are exposed to the sun.

Above this range of emissions we call visible, we have the infrared, which is also a very broad er range than the broad we can see. This type of emission is responsible for the heating we observe in the light generated by the photopolymerizers that use halogen light source, and which is commonly called heat.

The Laser is nothing more than light, and thus it has the light behavior, that is it can be reflected, absorbed or transmitted, suffering or not the spreading in the process (Figure 6). However, it is a light with very special characteristics, such as, unidirectionality, coherence and monochromaticity.

Figure 6 – The laser has the light behavior.

The light can be understood as small packages of energy (photons) traveling according to a wavy trajectory.

The laser is a type of light whose photons are identical and they propagate over parallel trajectories, differently from the common light, where the photons with different wavelength are emitted and they propagate in a chaotic way, in all directions (Figure 7). It is still a coherent light, where the peaks and valleys of all wave like trajectories of photons that form it, coincide in terms of direction and sense, amplitude, length and phase. These aspects are the ones that make it different from the common light, where there is no synchronicity in the emitted photons (Figure 8). Like all the photons emitted by a laser device, the patterns are identical, they propagate according to identical trajectories, direction, sense, amplitude and phase. They are device capable of emitting light with unique and defined wavelength. Then we can say that these photons are of pure color (Figure 9).

Figure 7- The laser is a passible light to suffer collimation, this is, walks in a parallel way different than the common light that gets lost in time and space.

Figure 8- The laser is a coherent light.

Figure 9 – The laser is a monochromatic light.

For a laser production, some special conditions are necessary. Firstly an Active Means is needed, formed by (gaseous, liquid, solid or yet by their associations) substances that generate light when they are excited by an external light source. This excitement process is called Pumping and its function is to transform the active means into a radiation amplifier means, since it causes in this phenomenon called inversion of population, that is, the electrons of the valence layer of the means absorb the Dumped energy and they jump to a more external level of energy. Since this second level is more distant from the core influence, its level of energy is greater. We call this situation Meta stable state. When the first electron goes down, returning to the level with less energy (Original energy), the liberation of the small package of highly concentrated energy takes place, and we call it Photon (Figure 10). This photon ends up exciting the going down of the remaining atoms that already were in excited state (meta stable). This generates a waterfall process and with a growth in geometrical progression, which results in the stimulated emission of radiation (Bagnato, 2001).

The active means must be contained in a reservatory called Resounding Cavity. In the internal extremities of this cavity there must be mirrors, being one of them of total reflection and the other of partial reflection. This will assure that the system formed by optical reaction and active means be the seat of a laser oscillation. Since the laser cavity is formed by mirrors in its extremities, this radiation is amplified, that is, the photons emitted by stimulation go into phase (all photons take up the same direction) and make it possible to occur an increment in each trip (multiple reflections) that is completed in the cavity.

There are a lot of types of laser, but, the basic principle to produce a laser bundle is the same for all of them, no matter if it is surgical, therapeutic or diagnosis laser.

Figure 10- Photon formation.

Figure 11- Diagram of the resonant cavity of a generic laser.

For the identification of the laser, we need to know its generating source (characterized by the active means that will generate the laser light) and its intensity (characterized by the density of the optical potency produced or energy generated by the laser). The same way the domestic lamps are identified by their potency, normally expressed in Watts, we also use this measuring unit (or a fraction of it), to identify the lasers potency (mW = miliWatt = 0,001 W).

The last relevant characteristic of the lasers is related to their functioning process, that is, there are those ones that when they are activated, they remain turned on continuously until they are turned off (Continuous Lasers, CW) and there are other types that work in a pulsating or switched form (Figure 12), that is, they are part of the time turned on and part of the time off. Most of the Therapeutic Lasers operate in a continuous mode

Figure 12- Different types of emissions of a laser diode.

Laser of semiconductor

The lasers of semiconductor are the emitters of the smallest dimensions that exist and they can be produced in large quantities. Thanks to their effectiveness and the small size they are especially adequate to be used in Dentistry Clinics.

The simplest active means is formed by a diode (P-N junction) with high concentration of donating impurities (doping) in the N zone and receiving ones in the P zone, and for which the base material is the same for both zones (for example GaAs or InP).

This type of arrangement is known by the name of homo junction. The basic configuration of this type of diode is indicated in the Figure 13 A.

When an electrical tension V is applied, directly polarizing this union, a very narrow area is created around it, where the inversion of population is produced. It happens when there is a greater probability of electrons to be in the conduction band, than in the valence band. The direct polarization produces an electrical current, which is translated into a passage of electrons to the Pe Zone, and blanks to the N zone. The luminous radiation is produced by a recombination of electrons and blanks in the junction zone.

The wavelength of the transition depends on the energetic leap between the Valence and the conduction bands. The potential energy needed for an electron to leap from the valence band to the conduction band is equal to the photon’s energy that is produced after its recombination.
Normally the commercial diode lasers are of the hetero junction type (Figure 1 3 B), that is, they are formed by the union of two distinct materials (for instance GaAs e AIGaAs). This type of structure presents some technical advantages in relation to the homo junction and that is why it is more routinely used. In order to obtain the laser action, two faces of the semiconductor element are cut and polished in parallel (to function as mirrors), being that in the two other faces it is necessary to have a rugous working, as to avoid that the laser phenomenon be produced between them. Frequently, the two polished surfaces are not covered with anti reflexive coatings, since a semiconductor’s level of refraction is high, and there is enough reflexivity (around 35%) on the surface between the semiconductor and the air to produce the acceptable optical feeding.

The active region where the laser energy circulates has got a rectangular section, with typical dimensions of 0,5 :m x 10 :m in the hetero junction lasers. The output laser beam has got an elliptical section, with different divergences in the plan parallel to the union and in the perpendicular plan (Figure 1 3 B). With proper optical systems, this section can be converted to circular, more convenient for a further focalization.
The Diode lasers applications are very diversified, but above all those in the medical – odontological area, communication by optical fiber field, dimensional recognition, Bar codes reading, compact disk reading, Office printers, and sharpener among others.

Figure 13A- Basic configuration of a diode laser.

Figure 13B- Basic configuration of a diode laser of double heterojunction.

Laser’s Historical Aspects

The application of light as photo-therapeutic treatment is a very procedure. In 1903, Finsen received the Nobel Prize for the advances in the Vulgar Lupus’ treatment using an ultraviolet light source. Specifically for lasers, everything started with Einstein, who has postulated the theoretical basis about the controlled manipulation of light waves, and has published his ideas in 1917. This theory was checked by Landberg in 1928, but only between 1933 and 1934 Townes and Weber for the first time talked about micro waves amplification. At the same time there was a great advance in the development of the optical fibers and material in a general way. The theory about the amplification of stimulated emission was patented in 1951 by Fabrikant (a Russian Physicist) and his team, however it remained unpublished until 1959.

The first device that has used stimulated emission has been named MASER (another English acronym formed from Microwave Amplification by Stimulated Emission of Radiation), built by Townes in 1952.

Weber proposed in the same year the MASER amplification, theory that was published in 1953.

Theodore Maiman, an American Scientist in the United States, constructed the first laser in history in 1960. This first laser was developed from a synthetic ruby bar, which produced a short-lived light, and with high density of energy, operating in 694,3 nm when an intense common light fell upon it. It was developed in the Hughes Aircraft Research Laboratory in Malibu, and on this date presented to the press. In 1961, Gould obtained the application’s patent, which caused a great confusion about who its inventor would be. He published the biomedical indications of the high-density energy laser light. The first application was performed in the Ophthalmology field, and it was where the first clinical complication was observed. In 1962, Dulberger published a work about the lesions produced by focusing of the light over the retina and thus causing vision loss.

In 1961, Leon Goldman, in the University of Cincinnati, founded the first laser laboratory for medical applications, where the first in-vivo experiments were performed.

In 1962, Patel developed the first laser, which, later on would be used with therapeutic objective, a device whose active means was a mixture of the Helium and Neon gases (He-Ne), generating a laser light bundle with a wavelength of 632,8 nm (Pontinen, 1992).

In the ancient Soviet Union, different scientists have simultaneously worked on the laser development. Basov and Prokhorov have made great progress in this area, and together with Townes they won the Nobel Prize in 1964.

In 1966, the first clinical applications with laser operating in low potency were reported by Endre Mester from Budapest, Hungary, when the first reports of clinical cases about Laser Bio stimulation of chronic ulcers of the lower members using ruby and argon lasers were presented (Mester, 1966). He produced a great volume of scientific, clinic and experimental works, having the He-Ne Laser as the central subject.

The most used therapeutic Lasers in the 70’s and 80’s decades were the He-Ne lasers, with emission in the red region (632,8 nm). In this region of the electromagnetic spectrum, the laser radiation presents little penetration in the biological tissues, which limited its utilization. For the application of this kind of laser in deeper lesions, it would be necessary an optical fiber to conduct the light to the inner part of the body oncontextmenu=”return false” onselectstart=”return false” onmousedown=”return false” , thus limiting and counter indicating many times this type of therapy, for being such an invasive technique. Another Limitation of the He-Ne lasers was its great physical dimension and the fact that its active means was in glass ampoules that could easily break. The helium gas itself, formed by very small atoms, quickly migrates through the ampoule’s walls drastically reducing the work life of these devices.

In 1973, following the same line of Mester, Heinrich Plogg from Fort Coulombe, Canada, presented a work about the The use of laser in needleless acupuncture, for pain relief (Baxter, 1994).

From the end this decade, diode lasers started to be developed, originating this way the first diode laser operating in the infrared region next to (<img src=”lambida.jpg” width=”20″ height=”20″> = 904 nm), constituted by a crystal of gallium arsenate (As-Ga). The main advantages of this laser over the He-Ne laser are smaller dimensions and greater penetration in the biological tissues. Another advantage is that this device can operate in either a continuous or a pulsating mode, while the He-Ne laser can only operate in the continuous mode. The bio stimulation effect using the pulsating laser has been the subject of different works, but Morrone et al., in 1998, demonstrated that for in-vivo applications the continuous radiation shows better results than the pulsating radiation, what Almeida-Lopes confirmed in 2003, although this fact is only true exclusively for the soft tissues’ cicatrisation, but not for the osseous cicatrisation or for the pain treatment.

In 1981, the first report of the clinical application of a As-Ca-AI diode laser appeared, published by Glen Calderhead, from Japan, who compared the pain relief that a diode laser promoted to the Nd:YAG laser, (Yttrium and Aluminum, doped with Neodymium), 1064 nm.

In the same year the Nobel Prize was granted to Schawlow, Bloemberger and Siegmahn, for their studies about spectroscopy applied to the laser technology.

From the 90’s different dopers (doping agents = impurity that alters a pure substance’s properties) were introduced aiming to obtain different diode lasers, which were capable of generating different wavelengths. With the availability of this technology, nowadays we can count on small apparatus, easily handled and transported, with high durability and low cost.

 

Theoretical Aspects: Therapeutic laser, Concept of irradiance, flow and deposited energy and Wavelength.

Therapeutic Lasers

The therapeutic lasers of low intensity maybe are the most widely studied in the world, and for sure they are already part of the daily routine of a large number of clinics in countries such as Spain, Russia, Japan. Germany and Brazil. One of the reasons of the popularity of this kind of laser is related to the effectiveness and low cost of the equipment, besides the objectivity and simplicity of the therapeutic and clinical procedures tomwhich it is directed.

The first studied therapeutic lasers, as we’ve said before, were the lasers in which the active means were a gaseous mixture of Helium and Neon (He-Ne), with potency varying between 5 and 30 mW, and wavelength of 632,8 nm, which is located within the visible range of the light spectrum, more precisely in the red color region.

It was formed by a glass reservatory (tube) filled with the mentioned gas, which was activated by a generating source of high tension electrical feeding. The light conduction up to the application point occurred through a flexible optical fiber cable of a bundle type (similar to the cables used in the photopolymerizers of the first generation), which would give a low optical yield to the system, that is, very little light reached the application point.

Along with the characteristics of the low optical yield, there is also the fact that this wavelength is highly absorbed by soft tissue, what compromises extremely the penetrability of the light.

These technical limitations have imposed the necessity of searching for low cost, with higher potency levels and with wavelengths that could go through soft tissues, without however, compromising the integrity of these tissues. That has been possible with the appearing of diode lasers, which according to what has been previously discussed , are relatively simple and low cost electronic devices.

The most used diode laser s in Dentistry have as an active means a compound of GaAIAs, with wavelength ranging between 760 and 850 nm (currently the most used is the one of 830 nm).

Which is located out of the visible range of the light spectrum, more precisely in the range of the near infrared, with potencies varying between 20 and 1000 mW.

Another type of active means used if the InGaAlP compound that produces light with wavelength varying between 635 and 690 nm, which is located within the visible range of the light spectrum, more specifically in the red color region, with potencies varying between 1 and 250 mW.

The generated light by this type of emitter has the same characteristics described for the He-Ne emitter and, thus, the same limitations in terms of penetrability.

 

Irradiance concept, fluency and energy deposited

Irradiance is the term that the photo biologists use as a synonym for potency density (PD), which is defined as being the laser’s working optical potency, expressed in Watts (W), divided by the irradiated area, expressed in squared centimeters (cm²). It is through an irradiance control that the surgeon can cut, vaporize, coagulate or weld the tissue, when surgical lasers are used. The proper potency density can also create the photo activation from a laser of low intensity of energy (therapeutic laser).

Fluency is the tern used to describe the energy rate that is being applied in the biological tissue.

When we multiply the irradiance (expressed in Watts) by squared centimeters (W/cm²), by the exposition time (expressed in seconds) we obtain the fluency or density of the energy or yet the energy dosage (ED) expressed in Joules by squared centimeter (J/cm²).

Energy is a physical greatness that, in the laser therapy case, represents the amount of laser light that is being deposited in the tissue, and it is defined multiplying the working optical potency of the laser device (expressed in Watts) by the exposition time (expressed in seconds).

The obtained result has as a representation the Joule unit (J).

The discussion about the mathematical aspects will be retaken in the further topics, because in this stage, the question that really matters for the professionals of the dentistry area is what these greatnesses mean, and how they relate to each other. We believe that through examples we can make these important concepts clear:

1. For a given potency, variations in the irradiance can produce effects on the biological tissue that clearly differentiated. For example, a laser with an output potency of 10 W, irradiating an area of 10 cm², will present irradiance equal to 1 W/cm². If the same laser is focused on an area of 1 cm², the irradiance will be increased ten times, probably generating thermal damages to the biological tissue, depending on the exposition time.

Conclusion: In fact, in order to define if the laser device can cause thermal damages, we should assess the generated irradiance, and not the working optical potency of the laser device in discussion.

2. For a given amount of energy to be deposited, variations of the fluency can produce effects over the biological tissue that is clearly differentiated.

For example, let’s suppose that we should apply a total dosage of 30J over a point. In a first hypothesis, let’s imagine that 30J are applied for one second, over an area of 1 cm². We will then have, irradiance equal to 30 W/cm², and that probably it cause thermal damage to the biological tissue. Let’s imagine now that the 30J are applied over the same area for 30 seconds. We will then have for this situation, irradiance equal to 1 W/cm², which will not cause thermal damage to the biological tissue.

Conclusion: the quantity of energy to be ministered is important; for the tissues will respond better to a proper dosage of energy, however, the way this energy is deposited is also very important.

Using as conventional systematic analogies adopted by Odontology or Medicine, when we prescribe an antibiotic, the medicine dosage is ministered as the following example: Amoxilina, 500 mg, 1 tablespoon every 8 hours, that is, the name of the active principle and its dosage (concentration of the active principle, milligrams, quantity and frequency of the mentioned drug use).

When we refer to laser therapy, it will be indicated the dosage expressed in Joules (energy, which is the quantity of laser light deposited in the tissue), the fluency is expressed in Joules per squared centimeter (cm²), which is the rate of deposition of this energy (the way how the energy will be deposited) and the estimate number of sessions, following the same adopted principle in the antibiotic prescription of the previously mentioned example.

The energy (quantity of laser light applied) and the fluency. Are essential concepts for the Biomedicine, but for Dentistry and Medicine, the term used to define the concept is dosage. Still using the analogy of the antibiotic prescription, in order to obtain a certain medicine effect, the ministered therapeutic dosage is vital, that is, a too low dosage prescription by kilogram/weight of the patient will bring the expected result. On the other hand, a too high dosage prescription, may lead the patient to intoxication, or even to an anaphylactic shock. The same holds true for the prescription of a therapy with low intensity laser, which means, too low dosages do not cause satisfactory effects on the tissues, whereas too high dosages in soft tissues, may lead to inhibition of the cicatrizing process (this is only true for soft tissue).

 

Wavelength

The wavelength is an extremely important characteristic, because it is what defines the penetration depth in the target tissue (Figure 14). Different wavelengths show different coefficient of absorption for the same tissue. Jacques, in 1995 (Figure 15), summarized the different coefficients of absorption for different chromophores depending on the wavelength (chromophores are molecular agglomerate capable of absorbing light).

As we can observe, the emitted radiations in the ultraviolet and in the middle infrared region.

Show high coefficient of absorption by the skin, causing the radiation to be absorbed in surface.

While in the region of the near infrared (820 nm and 840 nm) a low coefficient of absorption can be noticed, resulting in a maximum penetration in the tissue (Karu, 1985, 1987).

The tissues are heterogeneous from the optical point of view and thus, absorb and reflect energy in distinct ways. The importance of absorption occurring in diversified ways, according to the tissue in which the laser energy is deposited, is in the fact that, depending on the wavelength, this tissue either absorbs energy more superficially or allows the energy to go through it, acting on a target installed in the tissue’s intimacy (generally a cellular membrane). To this we give the name of laser’s selectivity.

Once the luminous energy is absorbed by the cell, it will be changed into another type of energy. When we use lasers operating in high intensity of energy, most of the times it will be changed into heat. When we use lasers operating in low intensity of energy, the low wavelengths are capable of electronically excite the molecules activating the cell respiratory chain, while for the higher wavelengths the excitation will take place through the cellular membrane.

Figure 14- Didactic drawing illustrated of the penetration of the laser in function of its Wavelength.

Figure 15- Absorption coefficients for different tissues in function of its Wavelength, proposed by Jacques in 1995.

As we can observe in the figure number 6, part of the light that falls upon a translucent surface is reflected back to the means where it came from, part is absorbed by the material which is falling upon and part goes through the material, and returns to the original means. The reflected light, as well as the transmitted one, does not have relevance from the clinical application point of view.

Only the absorption process will be considered, because when the light penetrates the tissues suffers a process called scattering or spreading, being then absorbed by the cells and converted in biological effects.

When a beam light falls upon a surface, a percentage of the light that will be reflected will depend on the incidence angle of this beam.

The smallest they formed angle is between the incidental beam and the irradiated surface, the greatest this beam reflection will be, and thus, we will have less energy absorption by the tissue (Figure 16). That is why it is so important to apply the laser with the light conductor always positioned in a perpendicular way to the tissue, avoiding this way the reflection and maximizing the laser absorption (Figure 17).

The reflection will still depend on the optical characteristics of the tissues, since they are very heterogeneous from this point of view, as each type of tissue absorbs and reflects light in distinct ways. Tissues with keratin, like the skin for example, reflect more the laser light than tissues without keratin, like the mucous!

What we search for in the treatment is the laser absorption by the tissue, for the laser light will only act if it is absorbed and thus converted in effects.

Figure 16 – The smallest they formed angle is between the incidental beam and the irradiated surface, the greatest the reflection of this beam will be, and thus we will have smaller energy absorption by the tissue.

Figure 17 – The hand held laser equipment must be always perpendicular to the target tissue so that to minimize the light reflection.