[Laser Resurfacing] #3.Lasers for Skin Resurfacing and Their Effects

Ablation-type and Contraction-type Lasers

Lasers are widely used for medical purposes; especially when a surgical knife cannot be used for a skin lesion, laser alone can yield excellent therapeutic effects. With the discovery of selective photothermolysis (SPTL) of laser on the skin, it has become available to choose an appropriate laser for effective treatment of various skin diseases, without many side effects.

Laser resurfacing is available in ablation mode, where the irradiated laser vaporizes the moisture in the skin exfoliating the skin surface, or in contraction mode, where thermal activity induces skin contraction, thereby improving wrinkles and scars.



Lasers for skin resurfacing targets the moisture in the skin (chromophore) and uses CO₂ laser (10,600nm) and Er:YAG laser (2,940nm), which are in infrared region that absorbs well in the moisture.

Laser resurfacing in ablation mode is useful for epidermal lesions, including various keratosis and nevi, as it exfoliates the skin surface. In this case, it is recommended to maximize the ablation on the skin surface so as to reduce thermal injury of the skin. As for the laser setting, the power should be increased (400-500mJ) and the pass number should be lowered for less thermal injury and maximal tissue ablation.

[Ad. ▶ MAGNUM(Q-switched Nd:YAG Laser) - Manufacturer: (www.i-dana.com)]

Laser resurfacing in contraction mode is skin remodeling for skin contraction and increased resilience, not for exfoliating the skin. It induces collagen contraction in the dermis, thereby increasing the resilience of the aged skin and straightening wrinkles. The contraction mode is also available for various scar remodeling. In this case, lower output (250-300mJ) with multiple irradiation (2-3 passes) setting can induce stronger resilience effect.

	Ablation mode	Contraction mode
Purpose	skin lesions	wrinkle, scar
Vaporization	maximum	minimum
Thermal damage	minimum	moderate
Power(mJ)	high(400~500)	lower(250~300)
Pass	1-2 pass	multiple

Table 1. Comparison between ablation mode and contraction mode

CO₂ Laser and Er:YAG Laser

Early CO₂ lasers in continuous mode created scars more frequently from thermal injury. Pulse frequency was developed later and then the later ultra-high frequency Ultrapulse mode is capable of yielding maximum skin ablation and papillary dermis contraction at the same time by high output (500mJ) for a short period of time (1/1,000 sec).

Figure 1. Removal of skin lesion using ablation mode of resurfacing laser.

Figure 2. Improvement of wrinkles using contraction mode of resurfacing laser.

High power CO₂ laser is useful for skin resurfacing but may interfere with the patient’s social life due to long-lasting erythema and frequent pigmentation (postinflammatory hyperpigmentation; PIH) after resurfacing.

Er:YAG laser is favorable for skin ablation due to 10 times stronger water absorption coefficiency than CO₂ laser. Early Er:YAG lasers have short irradiation time (pulse) on the skin, resulting in less thermal injury, but the lack of hemostasis and the hemorrhage absorbing laser energy prevents deeper resurfacing. In order to prevent such disadvantages, Er:YAG lasers with longer pulse has been used with CO₂ laser for skin resurfacing.



The ablation threshold is 0.5-1.5J/cm² for Er:YAG lasers, which is lower than 4.5-5J/cm² for CO₂ lasers.

Er:YAG lasers cause less thermal injury, induces faster wound healing, causes less erythema and pigmentation, and provides more safety and comfort for patients, making it more appropriate for shallow laser resurfacing.



I used Ultrapulse CO₂ laser in the 1990s, and from the 2000s, when the device was no longer any use, I have been using Er:YAG laser. In order to achieve deep skin resurfacing as the CO₂ laser, Er:YAG laser needs 2-3 times more irradiation for 2-3 times longer period of time. Therefore, I personally prefer Ultrapulse CO₂ laser for full facial resurfacing.



Patients often question about the difference between CO­₂ laser and Er:YAG laser; I would explain to them that how to use the devices, rather than the difference of laser models, is more important. I would also describe CO₂ laser as a sharp knife, which is effective when used well but may accompany side effects when used wrong, and Er:YAG laser as a relatively dull knife, which is safe but requires multiple use for deeper procedure.

Lasers Characteristics	Er:YAG	CO2
wave length	2,940nm	16,000nm
water absorption coefficient	12,800cm-1	800cm-1
pulse duration	200~300μsec	-950μsec
albation depth / pass	2~5μm	20~30μm
thermal damage	10μm	25~70μm
usual power setting	5~15J/㎠	250~500mJ/pulse
immediate collagen shrinkage	12%	25%
late collagen contraction	-14%	-43%
epithelization period	3~5days	5~10days
erythema lasting	1~2week	1~3month
hyperpigmentation	30~50%	50~80%
delayed hypopigmentation	rare(in Caucacian)	none
infection	less than CO2	2~7%

Table 2. Comparison between CO₂ laser and Er:YAG laser

For some clinicians who ask which laser is more beneficial for resurfacing, I remark the fact that the purpose of resurfacing is more important and that a specialized costly laser is not a necessity simply for removing nevus. Lasers recently developed in Korea also seem to have excellent cost efficiency.



Reference: Laser Plastic Surgery, Koonja Publishing, 2008, Seoul

- To be continued -

▶ Previous Artlcle : #2. Total Rejuvenation

[Laser Resurfacing] #2. Total Rejuvenation

Lasers are definitely effective for rejuvenation, and various lasers are being used for the purpose. ‘Juvenile’ means young and ‘rejuvenation’ means becoming young again. This word can be easily confused with ‘anti-aging’, which means preventing the process of aging, slightly different from rejuvenation. Anti-aging is mostly focused on medical treatments, such as hormone therapy, prevention and treatment of metabolic diseases and aging-associated diseases. On the other hand, rejuvenation is a more aggressive method to turn back the youth. Everybody is bound to experience the aging process when they grow old; people lose confidence in their appearance and don’t even want to look at themselves in the mirror or take a picture. People think of facial wrinkles first among the aging process, but aging is related not only to the facial skin but also to other physical changes, including subcutaneous fat, musculoskeletal system, hormone and metabolic change, as well as psychological and social changes.

Aging epidermis of the facial skin becomes withered, shows lentiginosis, age spot and blemishes, and melanin pigment is increased, making the skin color more yellowish than in the younger age. Dermis also becomeswithered, thin and less resilient, the appendages become atrophied, and the skin is dried. Resilient skin contains a lot of collagen fibers, but aging skin has a thick degenerated elastic tissue layer (elastosis). The subcutaneous fat also becomes atrophied and thin in parts, not as evenly distributed as in the younger age, and the facial skin adheres to the muscle, forming deep wrinkles. Nasolabial folds between the cheeks and lips or glabellar frown lines between the eyebrows are deepened. The face also becomes partially sagged, making nasolabial folds, jowl, marionette line and dark circles under eyes. There are retaining ligaments in the face that supports skin and soft tissues; aging face becomes sagged due to the gravity, with the exception of the areas with retaining ligaments. The forehead, eyelids, cheeks and jowl also becomes sagged.



The musculoskeletal system also changes; muscles are generally reduced, the bone is reduced in parts, the alveolar bone becomes atrophied and the mouth is puckered. The bone is not constantly fixed but partly absorbed and partly accumulated by new calcium, keeping the balance. When more bone is absorbed then generated, the bone becomes smaller. Aged people have reduced hormone and metabolic change. Growth hormones are necessary not only for growth but also for the maintenance of the body; growth hormone plateaus around the age of 16 and decreases to 20% of the maximum level at the age of 60.



Despite the lack of understanding on the accurate reason of aging, there are several theories explaining the reason of aging. The first theory argues that exposure to harmful environments, such as pollution, alcohol, cigarette, overwork and stress, triggers early aging. Programmed aging theory states that aging is imprinted on DNAs and cells are programmed to divide a certain number of times. Aged cells should go through apoptosis, and denaturized cells may turn into a carcinoma. Aging by DNA changes has been partially demonstrated by experiments.

[Ad. ▶ MAGNUM(Q-switched Nd:YAG Laser) - Manufacturer: (www.i-dana.com)]

Before and after laser skin resurfacing (Left, before; right, after)

Aging becomes accelerated in old people by hormone and metabolic changes. Anti-aging medicine focuses on medication to maintain hormone and metabolism in younger people.

Aging also leads to psychological and emotional changes; aging people has decreased memory ability, depressed mood, loss of confidence, psychological withdrawal, contributing the increased risk of depression.

As aging includes all these physical, psychological and social changes, total rejuvenation should bring all possible measures to recover youth.

In plastic surgery, face lifting is the most traditional but invasive method. For lessinvasive methods, other procedures such as forehead lift by minimum incision or endoscopy are available. Blepharoplasty is commonly performed for sagged eyelids in middle-aged patients.



Among laser therapies, ablative laser is an invasive method because it requires more than 1 month recovering the ablated skin. Non-ablative infrared low-output lasers are non-invasive, and fractional lasers are less invasive. As for the effectiveness, the ablative laser is the most effective; while the infrared low-output laser is weak and the fractional laser yields moderate effectiveness.

As the saying ‘no pain, no gain’, the ablative laser provides the most dramatic effect but erythema and pigmentation last for about 1-2 months making the facial color uneven. The infrared low-output laser does not cause a discomfort in social life but provides less effect. The fractional laser is recently popular because it has the advantages of both the ablative and non-ablative lasers, providing considerable effect with -less discomfort

Rejuvenation Methods

As aged face has atrophied subcutaneous fat, face lifting with fat grafting can recover facial volume and rejuvenate the face. Filler injection can be handy when injecting small amount of fat instead of fat graft. A simple method of relieving wrinkles is to inject botulinum toxin, which paralyzes facial expression muscles. However, the anatomy and physiology of the facial muscles should be fully understood to achieve a proper effect, and misuse may lead to side effects, such as ptosis or long-term inappropriate facial expression around the mouth.

Other methods of total rejuvenation include exercise and diet therapy; exercise increases the level of growth hormone, enhances muscular strength and is very helpful for mental health. Bad eating habits and social habits can be improved, and psychology consultation and psychiatric drug treatment can be also helpful, if necessary, to maintain young and healthy mental status. Aging occurs in every part of human body. Therefore, just one method is not enough for rejuvenation and multiple methods selected according to the patient’s needs are most recommended.



Among laser therapies for rejuvenation, ablative laser is most effective. After removing the aged epidermis, the skin is regenerated with more regular and thicker epidermal cells and reduced melanin pigments. The ablated dermis also regenerates to thicker collagen fiber layers, enhancing the skin resilience. Among ablative lasers, CO₂ laser is appropriate for deep ablation but is invasive, while Er:YAG laser is safe and induces less erythema and pigmentation. Among non-invasive lasers, low-output infrared laser and IPL are available, although the expected extent of rejuvenation is not enough to improve wrinkles. The fractional laser can achieve rejuvenation effect by delivering micro-thermal zones to the skin, but the effect is weak and requires repeated treatments. Ablative fractional laser is an intermediate form between the ablative laser and the fractional laser, and the micro-ablative column causes less discomfort in social life. Radiofrequency and ultrasound can also increase the elasticity of the dermis without peeling the skin.

.

Reference: Laser Plastic Surgery, Koonja Publishing, 2008, Seoul

- To be continued -

▶ Previous Artlcle : #1. History of Laser Resurfacing

▶ Next Artlcle : #3.Lasers for Skin Resurfacing and Their Effects

[Laser Resurfacing] #1. History of Laser Resurfacing

Laser resurfacing is safer and more effective than dermabrasion or chemical peeling and has wide applications. Laser resurfacing often brings drastic improvements in scar or facial wrinkle removal. Professor Park Seungha, the bestselling author or ‘Laser Plastic Surgery’ is an expert in laser resurfacing in Korea. Professor Park is working hard to promote positive image of laser treatments in patients through education on correct use of laser. With this article, Professor Park intends to share his extensive experience in laser resurfacing technology with our readers. This article will provide helpful tips to plastic surgeons who perform laser resurfacing procedures.

The first time I witnessed the procedure of facial laser resurfacing was when I visited Dr. Fitzpatrick’s clinic in La Jolla, California in 1995. The CO₂laser (Ultrapulse), which wasprohibitively expensive at the time was first introduced in the university hospital which opened the era of laser resurfacing in Korea. Before this I had only witnessed laser resurfacing on videos but watching the procedure in person witnessing the subsequent effect made me trust. Laser did not have a very positive public image in the past. The Argon laser that was introduced in Korea in the 80s was effective against nevus or mole removal but also caused depigmentation or scar and was not in high demand despite its novelty.



During my trainee days, my professor used dermabrasion with low conviction on patients with smallpox scars and often warned us that any dermatologist with a good track record could suffer a major career setback with one mistake with this procedure. Dermabrasion caused a lot of bleeding as it surgically removes the skin layer. The patient suffered a lot of pain as the blood clots and crusts remained after the procedure, not to mention the rather unappealing and sickly appearance of the face post-procedure.



Patients with flame burn in the face often visited the plastic surgery clinic. I could see that during the course of the treatment, their skin first developed blisters and redness, but in about a week, the recovered skin was cleaner, more elastic and free from blemishes than the pre-burn skin. This not only pleased the patient and his or her caretaker but many doctors would have thought if they could burn the skin just enough to cause this kind of recovery, they would bring back magically clean and rejuvenated skin.

[Ad. ▶ MAGNUM(Q-switched Nd:YAG Laser) - Manufacturer: DANA(www.i-dana.com)]

The introduction of CO₂ laser in dermatology

Laser resurfacing uses adjusted burning of the skin to give rejuvenating and clarifying effect. I have been performing laser resurfacing since the mid-90s and held several symposia on laser resurfacing at our hospital to educate Korean doctors the benefits of the procedure. I also performed live surgery to allow real time observation of the procedure in the operation room as they do in the US. However, a more than expected number of people signed up for the observation and we had to air part of the procedure on a screen in the auditorium. At the time, laser resurfacing was a new field and a lot of the professionals related with this field were greatly interested in the live surgery presentation which was a rare occasion in Korea.



A patient of mine who received the laser resurfacing on the entire face about 10 years ago came back recently for a repeat procedure. This patient received the same procedure from other clinics but the effect was not as good as 10 years ago and wanted to get the same procedure. She said she recommended the laser resurfacing procedure to those around her. Immediately after getting laser resurfacing and before the skin recovers, the face still has red splotches. This may give people a negative idea about laser. However, if you wait 2-3 months after the procedure you can see a dramatic improvement of the skin.



Chemical peeling was once popular as it was believed to rejuvenate and clarify the skin. Chemical peeling was actually performed centuries ago in Europe as a secret treatment and disappeared over time. Some had positive results, however, it was not a safe procedure at the time and resulted in horrible scars and serious sequelae in some cases.

[laser resurfacing before and after (left shows skin before laser resurfacing, right shows skin after laser resurfacing)]

Peeling involves removal of the outer layer of the skin to allow skin regeneration by skin appendages. Just like the onion, the skin resurfaces in the same way regardless of how many times the outer layer is removed. The most important principle in peeling is to maintain the appropriate depth of removal. If one fails to do so, skin regeneration is not obtained and irreversible scar may result.



I once watched a movie titled ‘Faceless.’ Most people would have found this movie rather unrealistic and farfetched but as a plastic surgeon, I could relate to the story and it left quite a lasting impression on me. The main character was a plastic surgeon who ran a famous rejuvenation clinic in Paris. He brought youthful energy back to elderly patients and applied secret substance on their faces to make them appear younger. He makes a mistake with one female patient and turns her face into a large lump of scar tissue. The aggravated female patient decides to take a revenge on him, and pours hydrochloric acid on his younger sister’s face making her horribly disfigured. In this story, the magical substance that the doctor applied on his patients’ faces, could be comparable to chemical peel. To treat his sister’s face, he kidnaps a young woman at night and transplants her healthy skin to his sister’s face. He performs total facial skin graft just as in the movie ‘Face Off’. However, the patient’s body keeps rejecting the graft and the doctor brings in a German doctor who’s an expert in in vivo experiments. He continues to try facial graft on his sister, however, his efforts keep failing. The plastic surgeon murders the young women whom he kidnaps and uses their blood to inject into elderly patients for rejuvenation effect. Finally, he gets arrested by the police who have been investigating a case of serial kidnapping and murder. This movie dealt with rejuvenation therapy, chemical peeling, sequalae of chemical peel, facial graft and graft rejection, which were all pertinent topics in plastic surgery. Whenever I hear of chemical peeling, I think of this movie.



The most important aspect of peeling is adjusting the depth of penetration as the facial skin has differing thickness depending on the area. The thinnest skin is the eyelids and the skin in the neck is also thinner than one might expect. The thickness of the skin is determined by the thickness of the dermis rather than epidermis. The thickness of epidermis varies from 0.11mm at the thinnest to 0.15mm in thicker regions. The thickness of dermis is about 0.2mm in the eyelid and neck and 1.0~1.5mm in the forehead and cheek. Including the hypodermis, thin skin is about 0.5mm and thick skin is around 2.0mm in depth.



Chemical peeling has serious side effects

The eyelid and neck where the dermis is thin and thus more susceptible to have problems after chemical peeling have less pilosebaseous units which are dermal appendanges that regenerate the skin. Deep peeling in these regions results in scars without achieving skin regeneration.



As the eyelid and neck have thin skin, dermabrasion is dangerous and can cause the skin to roll in. In these areas, chemical peeling is not suitable as it could create irreversible scar. On the contrary, laser resurfacing can adjust penetration depth with laser output and can perform peeling in weaker skins with optimal depth. In laser resurfacing, as the irradiation depth is maintained at 0.2mm~0.5mm, it is safe and can be performed with desired depth. As it is safe and unlikely to leave scars, it can be used to perform deep penetration peeling as well.  When irradiated on the skin, the laser coagulates the capillaries and lymph nodes minimizing bleeding and edema. It also blocks nerve endings thereby causing little pain after the procedure. There is also none of the bleeding or edema that often follow dermabrasion and the patient feels more comfortable during the procedure.



Chemical peel results in thick crusts on the skin and the penetration depth can be measured only when the thick crusts come off. The depth varies depending on the concentration of the peeling agent, number and force of rubbing and existence of additives. In general, chemical peeling is suitable as mild skin scaling or esthetic care of the skin, however, is riskier as a deeper form of peeling. Laser resurfacing allows a markedly safer and simpler procedure compared to chemical peeling and has excellent rejuvenating effect. Thus, it has a wide variety of applications in esthetic treatment of the skin. 



Laser resurfacing treats skin diseases through the principle of skin rejuvenation and brings marked improvement of wrinkle, elasticity, and scar removal, etc. With accurate understanding of the principle, indications and contraindications of laser resurfacing as well as pre- and post-procedural skin care methods, it can be used to bring satisfactory results for both patients and doctors.  



Reference: Laser Plastic Surgery, Koonja Publishing, 2008, Seoul

- To be continued-

[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.’

[Ad. ▶HELIOSⅡ/LOTUSⅡ/HYPERION - Manufacturer: LASEROPTEK(www.laseroptek.com)

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.

- To be continued -

▶ Next Artlcle : #2. Development Process and Types of Lasers