Excimer laser is a gas laser with a working substance of argon fluoride and a wavelength of 193 cm. It is characterized by low penetration of the cornea, low thermal effect, precise removal of corneal tissue and a very smooth cutting surface, which can be used to change the surface curvature of the cornea to alter the refractive index or to remove superficial corneal scarring. As an important component of modern medicine, excimer laser refractive surgery plays an important role in the treatment of myopia, hyperopia and astigmatism. I. Excimer laser refractive keratectomy (PRK) PRK corrects refractive errors by applying an excimer laser to cut the central anterior surface of the cornea, i.e., removing the anterior elastic layer and superficial stroma of the cornea to flatten the central cornea to correct myopia or steepen it to correct hyperopia. Contraindications: cone cornea, severe dry eye, active viral keratitis, glaucoma and iridocyclitis, etc. Excimer laser in situ keratomileusis (LASIK) LASIK is the mainstream procedure for the correction of refractive error worldwide, characterized by the use of microkeratome to create a diameter of about 8.5-9.0mm, thickness of about 130-160μm corneal flap, and then excimer laser cutting the central area of the cornea to make it flat, so as to achieve the purpose of correcting myopia, or cutting the central paracorneal steepening The flap will be reset after laser cutting. The indications for surgery are broader than PRK, and are suitable for myopia from -1.00D to -14.00D, astigmatism up to 6D and hyperopia up to 6D. The procedure is not recommended for people with corneal thickness thinner than 450 μm or high refractive error. LASIK is the most popular procedure among patients and doctors because it corrects a large number of refractive errors, the surface epithelium and Bowman’s layer are not damaged, the structural integrity of the cornea is well maintained, the patient has no pain after surgery, the wound heals quickly, and there is no Haze (haze clouding of the cornea). Excimer laser subepithelial keratomileusis (LASEK) The principle is to soften the corneal epithelium with alcohol to separate the basal cells of the corneal epithelium from the corneal Bowman’s membrane, flip the epithelial flap, and reset the flap after conventional PRK. This procedure only cuts on the corneal surface and preserves the corneal epithelium, therefore, it is more suitable for myopic eyes with thin corneas, small lid fissures, those who are not suitable for negative pressure suction rings, those with severe corneal neovascularization and those who wear corneal contact lenses for a long time. The disadvantages of LASEK are that the procedure is more complicated, there is still postoperative eye pain, and the recovery of vision is slower than LASIK, taking about a week. Haze may occur in some patients, especially in those with high myopia. Epi-LASIK Epi-LASIK is actually a special form of LASEK, in which a device similar to a corneal lamellar knife is used to separate the corneal epithelium from the Bowman layer to form an epithelial flap, then laser cutting is performed, and the flap is reset and a corneal contact lens is put on. This procedure has not been used in clinical practice for a long time, and further clinical observation and research are needed. Femtosecond laser LASIK surgery This is an all-laser LASIK procedure that uses a femtosecond laser to make a corneal flap. Femtosecond laser is a laser operating in the form of pulses, the duration is very short, only a few femtoseconds, a femtosecond is 10-15 seconds, that is, one trillionth of a second, it is thousands of times shorter than the shortest pulse obtained by using electronic methods, is the shortest pulse available to mankind under experimental conditions. This is the first characteristic of the femtosecond laser. The second feature of the femtosecond laser is that it has a very high instantaneous power, which can reach one million billion watts, a hundred times more than the total power generated worldwide. The third characteristic of the femtosecond laser is that it can focus to a region of space smaller than the diameter of a hair, making the electromagnetic field several times stronger than the force of an atomic nucleus on its surrounding electrons. In the medical field, the femtosecond laser can be used as an ultra-precise surgical scalpel and has been successfully applied in the field of ophthalmology, where it is used to create corneal lamellar flaps in excimer laser treatment of myopia, as well as other corneal surgeries Femtosecond laser surgery is different from conventional mechanical corneal lamellar knife excimer laser surgery, mainly in terms of corneal flap creation: Femtosecond laser surgery uses a laser to create a corneal flap without a blade, while Conventional excimer laser surgery uses a mechanical corneal lamellar knife to create the corneal flap, and the excimer laser cutting after the flap is completed is the same. The biggest advantage or specialty of the femtosecond laser for LASIK is that it produces very precise flap thicknesses that are three times more accurate (fluctuating from 10 to 15 um) than the laminar knife (fluctuating from 30 to 45 um), and the actual probability of flap complications is tens of thousands of times lower. Therefore, the use of femtosecond laser to create the corneal flap can greatly improve the safety of excimer laser surgery for myopia, especially for myopic patients with thin corneas and large corneal curvature variants. Pre-operative examination: 1. 2.Refractive examination: including computerized optometry, dilated optometry and re-testing three steps. The correctness of optometry directly affects the effect of surgery. 3.Ocular segment and fundus examination: focus on corneal transparency, whether the cornea has scars, whether the crystal is cloudy, and whether there is vitreous turbidity and fundus lesion in fundus examination. 4.IOP examination: to exclude the possibility of high intraocular pressure and glaucoma. 5.Corneal thickness measurement: for those whose central corneal thickness is less than 500μm, if the patient is highly myopic, attention should be paid to the central corneal cutting thickness and the situation should be explained to the patient before surgery. 6.Corneal topography examination: The main purpose is to have an understanding of the regularity and symmetry of the entire corneal surface and to exclude various abnormal factors. 7, A ultrasound to determine the axial length of the eye. 8.Measure visual contrast sensitivity and wavefront surface aberration if available. 9.Tear secretion test or tear film rupture time measurement to rule out the possibility of dry eye.