Nd:YAG laser has the characteristics of high gain, low threshold, high quantum efficiency, small thermal effect, good mechanical properties, suitable for various working modes (continuous, pulsed), etc. The effect of applying matter interaction in various solid-state lasers today is Different, Nd:YAG lasers with different wavelengths work in continuous and pulsed ways, so that the laser interacts with biological tissues in different parts, and good curative effects can be obtained. Medical Nd:YAG lasers are widely used in surgery, ophthalmology, dentistry, stomatology, otolaryngology, dermatology, cosmetology, etc., especially in the treatment of skin pigmented diseases, with unique advantages such as small trauma, good healing, and no scars. This paper mainly introduces the characteristics of Nd:YAG laser and its application in the treatment of skin diseases, so that readers can understand the performance of various lasers and the therapeutic effect of different types of laser therapy instruments.

A system that can generate laser light is called a laser. A simple laser usually consists of three parts: working substance, pump source and resonator. Since the birth of the first laser in 1960, hundreds of lasers have been introduced. Various types of lasers are very different from each other, depending on the working substance that generates the laser, there are gas, liquid, solid and semiconductor lasers, etc. Solid-state lasers are lasers with a small amount of active elements doped in a solid-state matrix as the working material. The physical and chemical properties of the working material are mainly determined by the host material, and its spectral characteristics are mainly determined by the energy level structure of the luminescent particles. However, the luminescent particles are affected by the host material, and their spectral characteristics will change to some extent, and some even change greatly. The main ones used as substrates are corundum, garnet crystals and various glasses. The luminescent particles are called activated ions, and the most commonly used activated ions are rare earth element ions such as neodymium and chromium. For example, the working material used in the world’s first laser is ruby, which is corundum doped with a very small amount of chromium ions. A laser that uses a yttrium aluminum garnet (YAG) crystal doped with a certain amount of neodymium ions (Nd3 + ) as the working substance is called a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser. Neodymium-doped lasers are one of the most widely used solid-state devices, and are widely used in laser processing, medical, military and other fields.

The Nd:YAG laser is a four-level system with multiple fluorescence lines at room temperature. Under normal working conditions (room temperature), the 1064 nm wavelength laser oscillates the strongest. The simplified energy level is shown in Figure 1. If an etalon or dispersive prism is inserted into the resonator, or a specially designed resonator mirror is used as the output mirror, and a mirror coated with a highly selective dielectric film is used to suppress the laser oscillation of the unwanted wavelength, the desired wavelength can be obtained. Laser transitions of wavelengths, such as 1319 nm, 1338 nm, 946 nm, etc. The Nd:YAG laser with output wavelength of 1064 nm can generate laser with wavelength of 532 nm after frequency doubling (KTP) crystal. The output light has the form of continuous and quasi-continuous.

The concentration of Nd3 + doped in Nd:YAG should be reasonable. If the doping concentration is high, the absorption rate will be high, the number of inverted particles will be high, and the efficiency of the laser will be high. However, when the doping concentration is too high, the conversion efficiency will not increase, but will increase. decrease, and even concentration quenching phenomenon occurs. The concentration of Nd atoms is generally within 0.15% to 115%. Higher concentrations will shorten the fluorescence lifetime, broaden the broadening line, and cause strain in the crystal, which will eventually lead to poor optical quality and reduced efficiency. In the application, the appropriate doping concentration can be selected according to the needs, so as to improve the performance of the laser. For Q-switch operation, a high concentration of dopant (112%) is selected to generate high energy storage; for continuous operation, a low concentration is usually selected. Dopants (015% ~ 018%) to obtain excellent beam quality. The size of the gain medium is usually selected according to the gain of the laser. For high-gain lasers, large-sized YAG rods are selected, and the length can reach 150 mm. The laser rod not only can not improve the gain, but will reduce the efficiency due to factors such as absorption loss.

The Nd:YAG laser uses a laser diode (LD) as the pump source, which is small in size, light in weight, high in efficiency, long in life, and does not require a cooling system, which provides favorable conditions for the miniaturization of the laser system. LD-pumped Nd:YAG lasers have two types: end-pumped and side-pumped. The end-face pumping method is flexible and convenient, and the pump light coupled through the fiber has a small divergence angle, which can match the fundamental mode of the solid-state laser. The side pump can use multiple LD arrays, which has good heat dissipation effect and can provide strong pump light, which is suitable for high-power operation.

LD-pumped all-solid-state Q-switched lasers store energy in the form of activated ions at the high laser energy level of the gain medium, and release them in a very short period of time, resulting in high repetition frequency, high peak power, and pulse width. Adjustable properties, such as in optical detection, atomic

It has great application value in the fields of molecular physics, spectroscopy, nonlinear optics, laser medical treatment, lidar and optoelectronic countermeasures. The Q-switched laser has short pulse, high peak power, and small thermal effect. When the Q-switched laser is concentrated on the target tissue, a shock wave will be generated, and the target tissue will be mechanically microblasted. There have been many reports about the application of Q-switched Nd:YAG laser in various fields of medicine at home and abroad, such as surgery, ophthalmology, dentistry, stomatology, ear and nose, and dermatology. Q-switched lasers work in a variety of ways, including active Q-switching, passive Q-switching, and mirror-switching Q-switching. Active Q-switched lasers mainly include acousto-optic Q-switched, electro-optical Q-switched, and saturable absorption Q-switched lasers are passive Q-switched lasers. Acousto-optic Q lasers have the advantages of small insertion loss, low modulation voltage, easy to cooperate with continuous lasers to obtain high repetition rate giant pulses, and good stability, but they have poor switching ability for high-energy lasers and can only be used for low-gain continuous laser. Electro-optic Q-switched lasers have the advantages of short switching time (10-9 seconds), narrow output pulse width (10-20 nanometers), high efficiency, precise control of Q-switching time, and high peak power (above tens of megawatts). The disadvantage is that the half-wave voltage is high, requiring high-voltage pulses of several thousand volts. Passive saturable absorption Q-switched lasers are suitable for high repetition rate, high power lasers to generate narrow pulse width giant pulses

At the same time of rushing, the spectral line is narrowed to achieve the effect of selecting photons of the same energy and momentum, but it is a passive Q switch and cannot be controlled manually.

The generation time of the Q-switching pulse. The mirror-turned Q-switched laser is a slow-switching type with no insertion loss and no optical damage, and is used for pulses with high energy

The laser can obtain giant pulses with a peak power of more than tens of megawatts and a pulse width of nanoseconds.

The theoretical basis of laser treatment of skin pigmentation diseases and laser beauty is Anderson (Anderson RR.) and Parry in 1983 in the United States

The theory of “selective photothermolysis” proposed by Dr. Parrish JA. Selective photothermolysis is that laser energy is selectively absorbed by some specific tissue components, and the heat generated by the thermal effect destroys these specific tissue components. The body’s own immune and metabolic systems can absorb and excrete these destroyed tissue debris, so as to achieve the purpose of treating pigmented diseases. This theory points out the difference in the absorption of different wavelengths of laser light by skin tissue, and the effect of laser pulses with different pulse widths on various skin diseases. This theory is the basis of laser treatment of skin diseases, which guides the development of laser medicine and promotes the rapid development of medical lasers. In the following decades, a large number of lasers with different wavelengths have emerged in the medical field. These lasers have achieved good results in the treatment of pigmented diseases (such as Ota nevus, birthmarks, freckles, chloasma, senile plaques) and vascular diseases (such as port-wine stains, hemangioma, telangiectasia). Laser uses the principle of selective photothermolysis to treat skin diseases, which can cause local high damage to the target tissue, and little damage to the surrounding tissue. In order to achieve selective photothermolysis, the appropriate wavelength, pulse width and energy density must be selected. Different tissues absorb lasers of different wavelengths. We usually hope that most of the laser photon energy can be absorbed by the target tissue and absorbed by the surrounding tissue. less.

Among the three main fluorescent lines of Nd:YAG, the laser with the center wavelength of 1064 nm is the easiest to oscillate, and the line fluorescence is the strongest, accounting for about 60% of the entire spectral line energy. Cells are highly absorbent and can more effectively remove darker or inflamed tissues. According to the principle of selective photothermolysis, it can be used to treat pigmented diseases in the deep layers of the skin. The output wavelength of today’s medical Nd:YAG lasers is mostly 1064 nanometers. The traditional treatment methods for facial pigmented lesions include cryosurgery, dermabrasion, chemical peeling, and carbon dioxide laser therapy, which have been gradually replaced by solid-state laser therapy. When 1064 nm Nd:YAG Q-switched laser is used to treat facial pigmented lesions, the treatment parameters should be reasonably discriminated according to the nature of the lesion, the location of the lesion, the age and gender of the patient and other factors, and should be adjusted in a timely manner during the treatment process to find out the specific parameters. According to the optimal treatment parameter standard, the diseased tissue will be pulverized after selectively absorbing the energy of the laser, and gradually cleared by autophagy cells, and finally achieve the purpose of removing the lesions without leaving scars.

The 1064 nanometer wavelength laser has deep penetrating power and can be selectively absorbed by melanocytes in the deep layer of the skin, and the thermal damage to the surrounding tissue is small when treating nevus of Ota. The Q switch uses the pulse wave of its shock wave to crush the pigment cells, and then phagocytose the pigment particles by the phagocytes.

It has the advantages of high selectivity, high efficiency, safety and reliability, and easy operation. Foreign body reactions caused by tattooing eyebrows are not uncommon. In the past, topical or local injection of corticosteroids could only relieve symptoms but not clear lesions, and local inflammation would recur. When using 1064 nanometer laser treatment, the pigment particles in the eyebrow tattoo liquid selectively absorb laser energy and then be crushed, and the reticuloendothelial system will phagocytose and excrete them, thereby eliminating the cause. In addition, because the instrument adopts Q-switching technology, the light energy is released within 5-10 seconds, the action time is short, the thermal damage to the surrounding tissue is small, scarring, atrophy or skin texture changes can be avoided, and there is no hypopigmentation or deposition. . The 1064-nanometer frequency-doubling crystal produces a 532-nanometer laser output, which is effective for brown and red skin lesions and pigments. The pigment particles in the skin are broken after absorbing the laser, while the cell framework is completely preserved, and the skin can recover quickly. Lasers of this wavelength are used to treat freckles with few side effects, easy operation, no scarring, and low postoperative care requirements.

1064 nm Nd:YAG laser removal of the anterior membrane of the intraocular lens is safe and effective, the patient has no discomfort, does not require general anesthesia, and reduces the risk of surgery. It has replaced the traditional surgical removal of the iris and has become the preferred treatment for glaucoma.

Laser surgery has a high temperature, has a good bactericidal effect, and is rarely infected after surgery; laser treatment is safe, with basically no complications, short operation time, good hemostatic effect, and fast wound healing; laser beam energy is high, and can reach ordinary scalpels The inaccessible part and the precise location can perform microsurgery, reducing the damage to the tissue other than the lesion. 1064 nm Nd:YAG Q-switched laser can also be used to treat laryngeal and laryngopharyngeal hemangioma, posterior cataract, periodontal disease and other diseases. Nd: YAG lasers with unique properties will be more and more widely used in the medical field.