[The Principle and Application of Laser(with focus on medical lasers)] #3. Lasers and Soft Tissue Interaction Ⅰ

5-1. Optical process of energy transfer

Since the development of laser in 1960, it had not been used a lot for medical use as now for about 25 years due to the lack of fundamental understanding on the process of interaction between laser and soft tissue until mid-1980s. It was from 2000s when the subject had rapidly gained attention with more and more studies being published, and now laser has become the basic tool for surgery. Understanding the process would be of much help for appropriate use of medical laser. In order to understand the interaction of laser with soft tissue, it is most important to know what optical process occurs when the skin is irradiated by the laser light. Skin tissues exposed to light undergoes the following 4 optical processes:

1) Absorption

2) Scattering

3) Transmission

4) Reflection

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There are various forms of energy in the nature with different sizes determined by the physical condition. Thermal energy, chemical energy, sound energy, light energy and kinetic energy are the examples.

Among these, laser is a form of light energy which shifts its form when interacting with another material. What is common between medical and industrial lasers is the change in the form of energy by absorption. For example, when welding or cutting a steel plate with laser, the laser is absorbed to the steel plate in the form of light energy and then changes to thermal energy, melting the steel plate for welding or cutting. Likewise, for the treatment of vascular lesions in the face or legs, laser absorbed in the form of light energy should be changed to thermal energy for coagulation or contraction of blood vessels. Thus, without absorption, no physical process can occur. Given the importance of absorption, unfortunately, human skin absorbs different amount of laser depending on the wavelength, causing the hassle of having to choose the laser which emits a wavelength that absorbs well in the lesion to be treated. Figure 1 shows optical absorption coefficients of light-absorbing chromophores. The representative soft tissue components are protein, melanin, hemoglobin, water and collagen.

Figure 1. Optical absorption coefficients of principal tissue chromophores in the 0.1-12 um spectral region.

Absorption means light energy transferring to another form of energy when the light interacts with molecules or atoms. It is difficult to describe accurately the process of which the light is absorbed in the tissue due to the non-uniform and complex structure of skin tissue. This will be described later conceptually with a simple formula.

Scattering refers to the physical process where light is deviated from a straight trajectory by collision with molecules, atoms or particles smaller than the wavelength of light. Scattering occurs in a gas, liquid and solid, although the scattered light more frequently appear as refraction or reflection of light in a solid or liquid. Scattered light appears inside an object (such as in the skin tissue) only when the skin tissue has irregular density or orientation of the arranged molecules. For example, when irradiating a laser light in the visible spectrum to very non-uniform skin tissues, the area near the irradiated site appears bright due to backscatter phenomenon of light. As skin tissues cause much scattering effect to light, the laser light has limited penetration depth without proceeding deeper into the skin. Since the penetration depth also varies depending on the wavelength of light, the wavelength should be selected properly according to the depth of the lesion to be treated. The fluence irradiated on the skin should be increased to obtain a satisfactory therapeutic effect because the fluence per unit area decreases as scattering increases. The penetration depth in the skin becomes greater from visible light to infrared light until reaching the plateau with Nd:YAG laser wavelength (1064nm), after which the depth decreases again.

Transmission is a very important physical property for selective treatment. For example, Q-switched Nd:YAG laser (1064nm wavelength), which is used for pigmented lesions located deep in the dermis layer, is not therapeutic if it is absorbed in the epidermis layer and does not reach the dermis layer. The Q-switched Nd:YAG laser (1064nm wavelength) can be therapeutic because the absorption rate is smaller than the transmission rate in the epidermis layer, while the transmission rate is smaller than the absorption rate in the dermis layer. As every material on earth (including skin tissue) has its own absorption spectrum, selective treatment is available without inflicting damage to the other tissues if only the exact absorption spectrum can be found. This is the unique property of laser light.

Reflection is a physical property that has no interaction with a laser light and has no effect on a therapy. For example, hard tissues, such as enamel, reflect all laser lights from the surface, making it difficult to treat. However, enamel can be treated as well when Er:YAG laser (2940nm) is used with water. This is because the water on the surface becomes rapidly hot by absorbing laser light and then explodes, ablating the enamel layer.

The above-described 4 optical phenomena, absorption in relation to the wavelength of light and energy, scattering in relation to the light penetration depth and fluence, and transmission in relation to selective treatment, should be well understood for better use of medical laser. The links among these properties should be also well understood because they occur simultaneously when irradiating a laser to skin tissues.

5-2. Parameters of lasers

Among the methods of laser oscillation are continuous wave oscillation and pulse oscillation. Most of the current medical lasers adopt the pulse oscillation to minimize thermal damage to surrounding normal tissues. The following parameters are for pulse lasers:

1) Energy

2) Power (P)

3) Beam profile/pattern

4) Pulse duration

These 4 parameters should be considered first when evaluating lasers of the same wavelength.

Energy means the capability of work, or energy transfer between two objects (that is, laser energy delivering energy to skin tissues), and is presented as Joule [J]. The fluence mentioned above when describing scattering refers to energy per unit area [J/cm²]. For example, if output energy is 1 Joule and the diameter of the output beam is 4mm, the area of the output beam is 3.14x0.2x0.2cm², or approximately 0.13 cm², and the resulting fluence is 1/0.13=7.7J/cm². High fluence or energy means greater ability to deliver energy to the skin tissue, or even therapeutic injury of the epithelia beyond certain level of fluence or energy. Therefore, the epithelia should be cooled down properly to minimize thermal injury during a procedure.

Power/P means the amount of work per unit time, or the change of energy per unit time (P=Joule/time), and is presented as W[watt]. Power is represented by average power and peak power. The average power is energy [J] x pulse repletion rates [Hz], while the peak power is energy [J] / pulse duration [s]. For example, when pulse duration is 5ns (5x10^-9s), maximum pulse repetition rate is 10Hz and output energy is 1 Joule, the average power is 10W and the peak power is 2x10⁸ W (200MW).

Beam profile is determined by the structure of the laser resonator and there are roughly two types. One is the output from multimode resonator and the other is the output from single-mode resonator. Typically, the laser from single-mode has a bell-shaped Gaussian profile, while the laser from multimode has non-uniform beam profile. It is the dream of laser researchers to design and develop a resonator for flat-top beam profile, which is theoretically impossible. In case of Q-switched high power solid-state laser, the great thermal lens effect of the gain medium – Nd:YAG crystal, for example – makes it impossible to realize flat-top beam profile. Nevertheless, unstable resonator or a resonator which can compensate for the thermal lens effect inside the resonator can be used to produce relatively uniform laser, and these suggestions are often being used in practice.

Pulse duration is the time it takes to generate laser beam. The full width at half maximum on the axis of time is expressed as pulse duration. Lasers can be categorized to long pulse (>1ms), free-running (~tens or hundreds ns), short-pulse (~ns) or ultra-short –pulse (<1ns) depending on the pulse duration. In general, long pulse lasers are used for maximized thermal effect, while short pulse (Q-switched) lasers are used for minimized thermal effect with high peak power.

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[The Principle and Application of Laser(with focus on medical lasers)] #2. Development Process and Types of Lasers

3. Development Process and Components of Lasers

3-1. Development process of lasers

All lights are generated by atoms or molecules. An atom is composed of a nucleus surrounded by electrons. Electrons’ orbit around the nucleus various according to the type of the atom; the same type of atom has the same number of electrons of the same orbit. If an outermost electron is given a light energy from the outside or energy by collision with another electron or atom, the electron moves further to an outer orbit. This state where en electron moves further to the outermost orbit when the atom was given an additional energy is called an ‘excited state’ or ‘excited energy level’. Atoms in ground state have a longer lifetime without a time limit, but can maintain the excited state only for a limited time. As atoms are unstable in excited state, they tend to emit the received energy in the form of light energy and return to the original orbit where they maintain a stationary status. Atoms in stationary status are referred to as in ‘ground state’.That is, light is emitted when atoms or molecules return to the ground state or when the excited state with a higher energy returns to a state with a lower energy. The number of atoms that constitute materials is countless in effect.Since each atom absorbs energy and emits a light individually, the emitted lights have different phases and wavelengths. The process where electrons in the excited states emits photons while spontaneously turning to the ground state is called ‘spontaneous emission’. Light emission from bulbs, fluorescent lights and neon signs are all examples of spontaneous emission. These lights emitted by spontaneous emission are characterized by broad spectrum and incoherence between each other.

On the other hand, stimulated emission refers to the emission of photons in the course of transition from the excited state to the ground state induced by external incident photons. The photons generated by stimulated emission have the same frequency, phase and direction as the photons of the incident wave, presented as coherence. This process appears as if one incident light doubled, making two rays of light of identical nature. A laser is a device that amplifies light through the process of stimulated emission.

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In natural condition of thermal equilibrium, there are generally far more atoms in ground state than those in excited state, thus the probability of stimulated emission is only about 10^-12 . In order to increase the probability of stimulated emission, the number of atoms in excited state has to be higher than those in ground state. Some atoms have a longer lifetime when it becomes excited by absorbing light energy than the time it takes to make transition to a lower energy level. This property can be found in a synthetic ruby, which is made by adding a little bit of chromium ions (Cr³⁺) to a sapphire, crystalline alumina. This gain medium becomes excited when exposed to strong light energy by a Xe or Kr lamp. At this point, the number of atoms in excited state is greater than those in ground state (or in lower energy level), and this state is called ‘population inversion’. If the condition of population inversion is satisfied, stimulated emission becomes amplified like an avalanche while the light passes through the gain medium. However, the amount of stimulated emission is still not enough to generate a laser and a resonator is required to meet the required amount. The resonator is consists of a total mirror, which has 100% reflectivity for laser wavelengths, and an output coupler, which lets some of them transmitted. By placing the gain medium between the two mirrors, the light generated by stimulated emission repeatedly travels between the mirrors reflecting back on itself, until some of the amplified light is emitted through the output coupler as laser light. To put it simply, the following three components are essential to generate laser:

1) A laser gain medium with an energy level that can induce population inversion.

2) A pumping source required for inducing population inversion.

3) An optical resonator for amplifying the light generated by stimulated emission.

4. Laser Types

Lasers can be categorized systematically into solid-state lasers, gas lasers, liquid lasers, semiconductor lasers, etc. The gain medium of a laser defines its wavelength. Characteristics of lasers by systematic categories are described below.

4-1. Solid-state lasers

Solid-state lasers use (amorphous) glass or crystalline ‘host’ material to which active atoms (molecules) are uniformly dispersed as the gain medium. The first laser light in human history produced by Maiman was also a solid-state laser using a synthetic ruby. Solid-state lasers typically use photon pumping method, for which a flashlamp, arc lamp, or laser diode is used.

Solid-state lasers can be subdivided into several types, chiefly such as ruby laser (emission at 694nm), Nd:Glass laser (1.05㎛), Nd:YAG laser (1.06㎛), and tunable laser. Giant pulse can be obtained by Q-switching. Nd:YAG laser, first produced in 1964, is a four-level laser, which is composed of ① ground state level, ② the lower laser level, ③ the upper laser level of metastable state, and ④ pump band level. Nd:YAG is a compound word from the initial letters of Yttrium, Aluminium and Garnet, with some addition of Neodymium (Nd). YAG is the basic material of neodymium, the atoms of which emits strong laser beams at 1.06㎛ (near infrared), and YAG crystals are also excited by strong light such a flash lamp. Among solid-state lasers, ruby laser, alexandrite laser, Nd:YAG laser, Nd:Glass laser, Er:Glass fiber laser, Ho:YAG laser and Er:YAG laser are currently used for medical purposes.

4-2. Gas lasers

Gas lasers use active atoms (molecules) of gas or mixed gas containing them as the gain medium. In gas lasers, excited state can be induced by discharge (plasma) or by electron beam. He-Ne laser (emission at 633nm), carbon dioxide laser (10.6㎛), Argon-ion laser (488nm, 514nm) are commonly used. There are also copper (Cu) vapor laser and excimer laser.

The He-Ne laser, first produced in 1962, is the first gas laser. Although the output power is weak, it can induce stable continuous wave (CW) for a long time, making it still commonly used in measuring.

The carbon dioxide laser, first produced in 1964, is a very powerful laser, with scores of kilowatts of CW output. The energy efficiency is as high as 15-20%, making it useful for laser processing.

The argon laser was first produced in 1964 and is capable of continuous output of about 1W. It commonly uses water cooling system but air cooling system can be used in some cases. It is characterized by the presence of several oscillations between green (514.5nm) and blue (488nm) lasers, contributing to the wide range of visible lasers.

The excimer laser is a high-output UV laser and mostly used for chemical reaction process or medical purposes. Excimer molecules include ArF (emission at 193nm), KrF (248nm) and XeCl (308nm), and the pulsed light has average 50-200W power output. However, the energy efficiency is generally poor at about 0.5-1%. In the excimer laser, atoms of fluorine and krypton, which do not combine in the ground state, becomes excited and combines as an excimer molecule, which is unstable and immediately returns to the ground state, during which the ultraviolet is emitted. This is the principle of the excimer laser. The term ‘excimer’ is short for ‘excited dimer’, which refers to the molecule that exists only in the excited state.

Among gas lasers, excimer laser, copper vapor laser, He-Ne laser and carbon dioxide laser are currently used for medical purposes.

4-3. Liquid lasers (dye lasers)

Liquid lasers use active molecules, such as a dye, diffused through a liquid, such as alcohol, as a lasing medium. Liquid lasers include organic and inorganic lasers and dye lasers; recently, liquid lasers mostly refer to dye lasers. Excitation is typically induced by light. Liquid lasers are easy for tuning wavelengths and the peak output power at 370-720nm reaches scores of kW. The efficiencies of ion laser and excited laser energy are 10-20%. Among liquid lasers, the dye laser at around 600nm is currently used for medical purposes.

4-4. Semiconductor lasers

The semiconductor laser was first demonstrated in 1962 but continuous wave operation succeeded first in 1970. Since then, it went through a series of improvement, and the development of double heterostructure and stripe structure has led to its wide application in fiber-optic communications, CD players, laser printers, laser scanners and laser pointers. It is now produced the most among lasing devices. The basic structure is P-N junction, although the laser diode is of double hetero-structure that has light-emitting layer (active layer) inserted between  the cladding layers on both sides. The laser diode uses the cleaved facet as the total mirror (resonator). GaAs, GaAlAs orInGaAsP is used as the material. Laser diodes are highly efficient, small and lightweight, cheap, and have high energy efficiency of 20-40% for multiple-quantum-well structure and ~25% for P-N type. The continuous output covers from ultraviolet to visible lights, and the development of devices with the optical pulse output of 50W (100ns pulse width) has made it a very convenient laser oscillator for a laser radar and excitation light source.

Among semiconductor lasers, wavelengths at 405nm, 650nm, 810nm, 930nm and 980nm are widely used for medical purposes.

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[The Principle and Application of Laser(with focus on medical lasers)] #1. The birth of laser and its characteristics

Laser procedures in the field of dermatology and plastic surgery will continue to grow in importance in the future. It requires medical as well as general knowledge in laser engineering to repeat the benefits shown in clinical trials on laser in actual patients. D&PS publishes series of articles on the extensive and detailed views of an engineer, developer and manufacturer of lasers to provide doctors who use medical lasers with insightful tips. Starting with this article, we will cover all topics on the principle and application of laser as well as types of laser used in dermatology and plastic surgery. The below article was contributed by Dr. Chu Hong, an authority of laser engineering in Korea. Dr. Chu served as a senior researcher from 1985 to 2002 at Korea Institute of Science and Technology and has been the CEO of LASEROPTEC co., ltd. since 2000.

1. History behind the birth of laser

1-1. What is laser?

The term ‘laser’ was coined from putting together the initials from ‘Light Amplification by Stimulated Emission of Radiation.’ The direction, frequency, and phase of the light created by stimulated emission, that is the photon, is completely identical with those of the incident photon stimulating emission of radiation. Therefore, laser has excellent collimation, focus, and coherence as well as very high brightness.



Laser is one of the most notable inventions in human history and since its creation in 1960s, diverse types of laser have been developed to be widely used in basic sciences, engineering and medicine, etc. To better understand the phenomenon of laser, it may help to imagine the acoustic system. The sound system consists of the microphone, amplifier and speaker. The microphone converts the sound input into electronic signals and the amplifier magnifies these signals so that the speaker can produce sound with adjustable volume. If you place the microphone right in front of the speaker, you will hear a sudden burst of a loud noise. This can be explained as the following. When a very low volume of sound is input through a microphone, this sound is amplified and output through a speaker. This amplified sound is re-input into the microphone to be repeated in a positive feedback, the end result of which is the ear-splitting noise. This process resembles that of a landslide and also the production of great laser output. The details of this process will be discussed in ‘Generation of Laser.’

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1-2. History of laser

a)Niels H. D. Bohr (1885-1962): Bohr was a Danish physicist who contributed greatly to the advancement of quantum mechanics. In 1913, Bohr was able to accurately demonstrate the spectrum of a hydrogen atom using the hypothesis on a new atomic orbital and emission of photons, on which the new theory of quantization of matter was founded. This achievement won him the Nobel Prize in Physics in 1922.



b)Einstein (1879-1955): Einstein was the most respected physicist and intellect of the 20th century. He proposed revolutionary theories such as special theory of relativity, photoelectric, Brownian movement, and general theory of relativity, etc. and won the Nobel Prize of Physics in 1923.



Einstein provided a more systematic analysis of the hypothesis involving Niels Bohr’s atomic model. The hypothesis posits that an electron can jump between quantized orbits by absorbing(induced absorption) or emitting(spontaneous emission) a photon. In Bohr’s frequency condition, atoms’ emission of light in high energy levels was not related to the existence of external light.



In 1917, Einstein suggested that a second possibility of light emission was needed considering the thermal equilibrium. This introduced the concept of stimulated emission, light emission from the influence of surrounding light. At the time, this was a groundbreaking idea. Along with this concept of stimulated emission, Einstein was able to establish a comprehensive theory on the light absorption and emission of the atom and this provided an essential step toward invention of laser 40 decades later. Einstein incorporated the new concept of stimulated emission to Bohr’s hypothesis of the interactions between light and the atom (stimulated absorption, spontaneous emission) and discovered an important basic principle of laser.



When Einstein theoretically demonstrated the existence of stimulated emission as part of the interaction between light and matter in 1917, he laid the conceptual foundation for introduction of laser. However, there were no practical methods to empirically prove this idea and only 40 years later, in 1953, C.Townes and A.Schalow of the Bell Lab in USA empirically demonstrated for the first time that stimulation emission from ammonia gas (NH3) to microwave (24GHz) was possible.



In 1958, Townes and Schalow went on to show that light amplification through stimulated emission was possible in the visible light range. In 1960, T. Maiman at Hughes Research Laboratories brought into existence red ruby laser in the visible light range of 694.3nm. He inserted a synthetic ruby crystal created from dopping a small amount of chromium in aluminum oxide crystal( sapphire) in the center of a spiral-shaped flashlamp. By supplying high electric voltage and creating a spark across the electrodes in the flashlamp tube, he could input strong light into the ruby and succeeded in laser oscillation. This achievement led Townes to be jointly awarded the Nobel Prize in Physics in 1964 with Russian scientists Basov and Prokhorov. In 1961,immediately after ruby laser oscillation, oscillation of the world’s first gas laser, helium-neon laser (He-Ne Laser) was achieved by Javan, etc. Following this, research on laser saw explosive growth and by 1960s, most of the important lasers used today were developed. In the 70s and 80s, applied research on laser flourished and laser now took on a more important status in various areas and applications.



2. Characteristics of laser

Laser beams have three unique characteristics that set it apart from the natural light or lamp light; monochromatic, coherence, and collimation. Due to these characteristics, laser is being used extensively in various fields of medicine and industry. Therefore, accurate understanding of each of these properties of laser is necessary to find appropriate use of the laser. As laser is a tool, without the knowledge of the tool, predicting the results of using this tool becomes difficult. The following passage explains the unique characteristics of laser that are not found in general sources of light.



2-1. Monochromatic

The laser beam is single-wavelength, unlike other types of light and progresses straight forward without dispersing. Light from a burning material or light from fluorescent lamps are emitted from heated atoms or naturally created from individual atoms. This type of light, although originating from the same type of atoms or molecules, contains lights of widely different wavelengths. The light emitted from individual atoms or molecules is a collection of different lights that are not correlated to each other. However, the laser beam is monochromatic with a single wavelength and is continuous light with an even phase. Observing the spectrum of this light through a dispersing prism shows a very thin line spectrum on the screen. In other words, the laser beam is monochromatic with identical phase and it is also called coherent light or light with high coherence. Using a lens, the laser beam can be focused into a light beam of very small area (small enough to be measured by the wavelength).



Using this monochromatic characteristic, the laser can be selectively absorbed, reflected and transmitted on a material. For example, a medical laser, especially Q-switched Nd:YAG laser used in dermatology is easily absorbed in black and dark brown in the wavelength of 1064nm but is not readily absorbed in and penetrates other colors. That is, this laser does not respond to other pigments. Thus, the 1064nm wavelength is used to remove tattoos or pigmentary lesions such as melasma. In addition, the wavelength of 532nm is readily absorbed in yellow, red, dark brown, and blue, etc. making it ideal for treatment of Ota’s nevus and Becker’s nevus, etc. In order to treat pigmentary lesions, the knowledge of the pigment and the type of laser that is absorbed into that pigment is crucial for optimal treatment effect.



2-2. Coherence

Coherence refers to the degree of the wave phases that are in constant relative phase spatially and temporally. This is the most important property when it comes to application of light’s interference. In general, interference patterns are not visible with the light seen in everyday life, that is the sunlight or light from bulbs, as it has very low coherence. Coherence can be largely categorized as spatial and temporal as below.



1) Temporal coherence: the correlation of a light wave’s phase at different points along the direction of propagation of light. This is a measure of how close the light source is to single-wavelength(monochromatic).



2) Spatial coherence: the correlation of a light wave’s phase at different points transverse to the direction of propagation of light. This is a measure of how uniform the phase of the wavefront is.



Laser is in essence coherent, though in differing degree depending on the structure of a resonator. If the output of the laser is single mode, spatial coherence can be secured and the degree of temporal coherence depends on the wavelength bandwidth (△λ) of the laser. The degree of temporal coherence is also known as coherence length (Lc= λ2/△λ). This is related to the fact that temporal coherence is often used to create interference and interference is not observed in structures with larger optical path difference than this coherence length. Using this coherent property, measuring devices such OCT (Optical Coherence Tomography) can be operated for precise measurement of skin tissues.



2-3. Collimation

As only the light beam going parallel to the optical axis determined by the opposing mirrors (full reflection and partial reflection mirrors of the laser resonator) is amplified, it is collimated, or has linear progression in one direction. This property differs from thermal-radiated light that travels in all directions. Due to collimation, the laser beam can be used for precise alignment and it is possible to irradiate the laser beam accurately on the desired area using the reflection mirror. However, laser beam has the characteristics of a wave and can be affected by diffraction which results in some degree of divergence angle. The divergence angle differs depending on the structure of a resonator. In case of a gas laser (CO2 laser, He-Ne laser, etc.) the divergence angle generally does not exceed 1 mrad (0.05o).



Using a lens, a laser beam with strong collimation can be focused into a very narrow light (narrow enough to be measured by the wavelength), which allows maximal energy per unit area (Joule/ cm2) or power per unit area (Watt/ cm2) on the focal plane. On the contrary, the normal light travels in many different directions from the light source, creates light source image near the focus and cannot be condensed into a narrow beam using a lens. To minimize the light source image appearing near the focus, the lens should be placed infinitely far from the light source or the light source should be reduced to as small as a dot. However, this greatly weakens the light intensity. Therefore, it is very difficult to collect large light intensity into a small point using the normal light. As this is possible with the laser beam, it can be used to focus energy within only a few um of diameter for precision cutting of the skin or bone.

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