Light is to be understood, as all known forms of emitted energy, which is visible as well as invisible for the human eye. In theory, there are two ways of describing the light: as a wave or as a beam of photons. According to the first mentioned above theory, a wave is characterised by its length (ë), frequency (f) and amplitude (A). The wave amplitude determines the light power or energy. The wave frequency is connected with so called wave period (T) and this relation can be defined as follows: f= 1/T.
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| Fig.1. Basic parameters characterising laser and electromagnetic wave. |
Fig.1. illustrates schematically a wave amplitude (A) and period (T). The oscillation time (T) is usually measured in seconds while frequency - in units called Hertz. In fact, 1 Hz is the inverse of wave time, i.e. 1/s. In other words, 1 Hz is one light pulse emitted within 1 second. The wavelength (ë) means the distance the wave goes within one full cycle (priode). It is convenient to measure the wavelength of laser radiation in meter derivative unit called nanometer. Nanometer is the bilionth part of meter (1nm=10-9m=0.000000001m). In practice also other wavelength and frequency units of measure are used. The frequency of the laser radiation f in Hertzs or the radiation wavelength in nanometers (micrometers) allow to situate the laser radiation on the spectrum scale of the known in the nature other radiation sources (cosmic radiation, X-ray, etc.) or the known lengths of radiowaves and microwaves, etc. The typical scheme of the spectrum of laser radiation on the background of electromagnetic waves is shown in fig. 2. It can be seen that the visible area covers only a small part of wide radiation spectrum. In this visible area wavelengths are in range from 385 (400) up to 700 (760)nm. The human eye perceives a change of wavelength in this area as a change of colour. Wavelengths exceeding 760nm (more often 700nm) belong to the invisible infrared area (IR), whereas wavelengths below 380nm (rarely 400nm) are covered by invisible ultraviolet area (UV).
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| Fig. 2. Illustration of wavelength and frequency range for known kind of radiation - from radio waves through visible light up to cosmic radiation. Definition of laser radiation wavelength ranges. |
To put it more clearly, there are some important definitions and units used in dental and med-ical laser practice. Laser energy (E) is one of them. Energy is a kind of radiation dose measured in Joules (abbr. 1 J). As light is a beam of photons we can consider each of them as a particle trans-mitting its own defined energy. Single photon energy can be described as follows:
where: h stands for the Planck's constant equal to 6.63 x 10-34Js. Total energy (E) in photon beam will be equal to the sum of energy of all photons (Ep).
Units derived from 1J are: 1mJ = 0.001J = 10-3J
1kJ = 1000J = 103J
Laser power (P) is a physical quantity characterised by speed of energy flow. It is defined by energy value transmitted within time (t):
Radiation power is measured in Watts (abbr. 1 W). Basic units are derivative from 1W are as follows: 1mW = 0.001W = 10-3W
1kW = 1000W = 103W.
Superficial energy density (D) is determined by energy dose applied on the defined surface (S):
Energy density is measured in Joules per square centimetre (J/cm2). Volume energy density is measured in J/cm3. In practice such units as 1W/cm2 or 1W/cm3 are useful, too. Measure units of light physical quantities are also used to define laser radiation characteristics.
As it is commonly known the first laser was invented in 1960. Schemes of typical laser design are shown in fig. 3 and 4. Fig. 3 presents a scheme of three-level atom system showing process of stimulation emission of laser radiation. Fig. 4 illustrates a scheme of design of any laser out of 1000 different types.
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Fig. 3. Scheme of three-level atom system, illustrating the principle |
It is known that each atom has electrons orbiting around its nucleus. Negatively charged electrons are attracted by positively charged protons in the atom nucleus. Only respective, precisely defined energy can keep some distance between these particles. Spacing between electrons and nucleus is defined by their so-called energy state. Atoms and molecules can be in different energy states or levels. The level with minimum energy value is called the ground state. Levels with high-er energy values are called excited states. There is multiplicity of energy levels in atoms and mole-cules and infinite number of transfers between them. A transfer from the higher level, for instance En to the lower one En - 1 means a loss of energy equal to En - En-1. A portion of this energy is released outside as the measurable radiation quantum. The quantum energy (photon energy) has already been defined above:
As appears from the above a radiation wavelength generated as a result of such energetic process of atomic (molecular) transfer will be equal to:
The following conclusions can be drawn from the above formula:
1. The radiation wavelength generated from atomic system depends only on the difference
between energy En and En-1 (since h x c is a constant quantity).
2. If there is a considerable spacing between energy level En and En-1 (considerable energy difference), the generated radiation wave will be shorter.
3. If energy levels are near each other, the generated radiation wave will be longer.
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Fig. 4. Simplified scheme of typical laser radiation generator. |
From a practical point of view, especially taking into consideration the dental applications, it's worth getting familiar with laser classification in respect of the active material used, power supply systems, wavelengths and power of generated light. The type of laser material used allows to divide lasers into the following groups:
. gas lasers
. liquid lasers
. solid state lasers
. semiconductor lasers.
The type of active material determines a wavelength (a colour) of generated radiation. Various chemical elements in different forms of active materials allow for the identification of more than 1000 types of lasers. They generate radiation of various wavelengths in range from 157nm (UV), through 385-760nm (visible) up to >300ìm (far infrared). Nowadays, the LASER conception, reserved after all for light, is more often used in relation to other spectrum areas. For instance, the definition: 'X-RAY LASER' is used in the case of waves shorter than those of ultraviolet radiation, and in the case of waves longer that those of infrared radiation the definition: 'Millimetre-wave Laser' is used. The basic types or groups of lasers with regard to wavelengths are shown in fig. 5. This classification is supplemented with specification shown in table 1.
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Fig. 5. Scheme diagram showing the placement of main laser groups and their typical wavelengths. |
Let us choose from this classification the most suitable and safest lasers in medical and dental applications. The best known gas lasers in the dental applications are among others CO2 laser and He-Ne laser. Excimer and Argon lasers are also used. Nd:YAG laser belonging to the group of lasers based on crystals is the most popular in dentistry applications. The marvellous results were repor- ted on procedures performed on hard tissues with Er:YAG laser. The Ho:YAG laser is also used quite often. Many therapeutic procedures involve semiconductor lasers especially high-power diode lasers. This classification requires some explanations concerning atypical designations and defini-tions used in literature on dental and medical laser aplications [1 - 11].
CO2 laser is a gas laser based on CO2 + He + N2. Energetic transfers in CO2 molecule determine strong emission of laser radiation in the infrared area (from 9.6 to 11m). Wavelength of 10.6m = 10600nm is the strongest one and its energetic efficiency reaches up to 20%, and even up to 40% in theory.
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Table 1. A presentation of the main types and groups of lasersmost commonly used |
Nd:YAG, Ho:YAG, Er:YAG, Er:YSGG lasers are based on YAG or YSGG crystals doped with atoms or ions i.e. Nd, Ho, Er:YAG crystal consists of Yttrium Aluminium Garnet.
Excimer laser belongs to the group of gas lasers and typically includes three components, for example Xe + He + F2 or Xe + He + HCl or Kr + Ne + F2. Excimer = EXCIted diMER is the name of excited molecule from XeF*, XeCl*, KrF*, etc. XeCl* molecule consisting of noble gas atom (Xe) and halogen (Cl). It can exist only in the excited energy state enabling effective inversion of populations.
Other types of lasers and their names are connected directly with the laser material used in a particular type of laser, for example ruby, argon, helium-neon, semiconductor lasers. There are many different semiconductor lasers dependent on material included into a given type of semicon-ductor. For medical and dental practice purposes there should be known that one type of semicon-ductor material produces one wavelength of laser light.
Lasers can also be classified with respect to the power supply. There are devices operating in the continuous and pulsed mode. In the continuous mode of operation, the laser output power meas-ured in Watts or milliWatts remains the same during the whole time of laser operation: see a scheme in fig. 6 which enables also to define radiation energy dose. To determine energy dose we should multiply a value of continuous power and time of laser operation. The below drawing defines dose of 1J for laser emitting power of 5mW during 200 seconds.
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Fig. 6. Illustration of continuous wave laser operation. |
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Fig. 7. Scheme of theorethical pulsed laser operation. |
For the pulsed laser illustrated in fig. 7, in order to determine how large dose of energy can be delivered within the time unit, one has to know three of its parameters:
1) power in the pulse (Pi),
2) pulse width (ti),
3) number of single pulses emitted during 1 sec. called pulse repetition frequency (frep)
By multiplying these three parameters we obtain so-called average laser power (Pa)
For instance, the peak power of 10W, the single pulse duration of 200ns and the frequency of 2500Hz, give the following average power:
The average power equals 5mW of continuous wave power. Energy dose is calculated as pre-viously (1J = 5mW x 200s). From the point of view of radiation power (continuous or average), there are following medical and dental lasers:
1) low power lasers in range from 1 up to 5mW,
2) mid power lasers in range from 6 up to 500mW,
3) high power lasers >500mW.
Low power lasers are often called 'soft lasers'. Mid power lasers are defined as 'mid lasers', and those of high power - 'hard lasers'. Soft and mid lasers belong to the group of 'biostimulating' lasers.
Below, there are three examples describing how to calculate the radiation dose equal to 1J in the case of three different values of laser power:
1J = 1mW x 1000sec.
1J = 10mW x 100sec.
1J = 50mW x 20sec.
In most situations it is worth to know the intensity of the laser beam. This can be described as the energy density (J/cm2) or the power density (W/cm2). In practice the energy or power density incident on the volumetric tissue surface (for instance on 1 cubic metre 1J/cm3) is the most impor-tant. However, the effect of the laser radiation dose will be determined by depth of light penetration at a given light wavelength and power.