Introducing the Excimer Laser

Introducing the Excimer Laser

Introduction

Over the last decades the excimer laser has obtained the key position among lasers in various sectors of health care (especially in eye medicine & surgery). In the more than 35 years after their discovery excimer lasers have been used increasingly in a variety of R&D, medical and industrial applications. Various revolutionary and evolutionary technological achievements have translated into signi?cant improvements of the parameters, the performance, the ease of handling and the reliability of excimer lasers.

What is an excimer laser?

Excimer lasers are pulsed gas lasers which produce ultraviolet radiation in the range of 351nm to 193 nm depending upon the composition of the gas mix use. Excimer lasers can also be defined as the gas lasers that emit pulses of light with duration of 10 ns to several 10 ns in the ultraviolet (UV) spectral range. They are the most powerful lasers in the UV.

History & background

The name excimer comes from excited dimer (The name excimer comes from the words excited and dimmers). The first experimental evidence of excimer lasing was obtained by N. G. Basov et al. in 1970. They did use a high current electron beam to excite liquid Xe. The Xe2 excimer laser emitted around 172 nm. With this experimental proof the excimer lasers were invented as a new class of lasers. At the end of 1975 excimer lasers with all important UV wavelengths had been demonstrated experimentally. In addition to fundamental investigation of the excimer laser and its properties itself, scientists started to built up excimer lasers which could be used in potential research applications. Lambda Physik developed and manufactured its ?rst commercial excimer laser–the EMG 500–which was introduced into the market in 1977.

What is so special about an excimer?

The key of the success of the excimer laser is its unbeatable UV performance with power levels not available from any other laser source. The largest application of excimer lasers for medical use is in refractive laser surgery. Excimer lasers are capable of emitting different wavelengths in the ultraviolet region, including 193, 223, 248, 308, and 351 nm. For corneal surgery, 193 nm is the wavelength of choice, because it produces the most precise cuts with the minimum amount of thermal distortion of adjacent tissue. In simple words, the excimer laser creates a “cool” beam of ultraviolet light. This makes the excimer laser ideal for corneal surgery because it can vaporize (ablate) corneal tissue without causing thermal damage to surrounding tissue.  By means of intense excimer pulses at 193 nm the surface of the human cornea is reshaped to change its refractive power and thus to correct for short or long sightedness.

Utility of excimer for the smallest and largest areas it can vaporize

A focused excimer beam will vaporize any known material. In laser ablation, a focused laser beam from a pulsed excimer laser strikes a target. The area vaporizes and is projected in a narrow forward plume to a substrate. For example, LASIK makes use of the excimer laser to vaporize small items of corneal tissue, so as to accurate the corneal form and surface area. Then the light rays emphasis sharply on the retina and we see plainly. If the vaporized area is small (narrow), 10 µm to 100 µm, tissues can be cut. As for the known smallest area, excimer has the ability to focus a beam as small as 0.25 microns.

On the other hand, if the vaporized area is large and has diameter of several millimeters, excimer laser can destroy tumors or infectious processes.

Types of excimer laser surgeries and which one is the best?

In the field of eye surgery (ophthalmology), excimer laser-based surgery can be of many types. However, it can be broadly divided into three main types: Photorefractive Keratectomy (PRK): This conventional type of excimer laser is best suitable for patients who suffer from certain corneal abnormalities that do not allow them to undergo regular surgery. In PRK, an eye surgeon (ophthalmologist) removes excess corneal tissue from the eye by an excimer laser. However, due to certain associated complications such as pain, longer recovery time and resulting vision problems of haze or blurriness in some patients, PRK is now was being largely replaced by LASIK and other procedures.

Laser-Assisted in Situ Keratomileusis (LASIK): It is the most frequently practiced or common type of excimer eye laser. Those people who have a normal cornea that can resist a cut through both the outer and middle layer are good candidates for Lasik. Hence, LASIK uses a blade to create a small flap in the cornea to provide laser access, It is commonly performed to correct farsightedness. LASIK surgery is completed quickly (within 2 minutes) and is relatively less risky as compared to other eye surgeries. The recovery time is usually one to two days. For the same reason, LASIK is generally considered as the “best excimer laser surgery” to date. Laser Assisted Sub-Epithelial Keratomileusis (LASEK): Those with thin corneas may be better suited LASEK, which combines PRK and LASIK technologies to treat the eye. LASEK also cuts a flap in the eye and reshapes the exposed eye with a laser. There is less risk of hazing and more corneal tissue is preserved. There is also less risk of dry eye syndrome that is sometimes found with LASIK patients. The full recovery time is about six to eight weeks.

Why do excimer lasers have to be FDA approved?

The Food and Drug Administration (FDA) first approved the excimer laser for patients with mild to moderate nearsightedness in October 1995. The FDA approved the excimer laser to correct astigmatism in April 1997. Excimer lasers, being “medical or health care devices” need FDA approval. However, it must be remembered that the FDA approves medical devices, not medical procedures. For example, the combination of Automated Lamellar Keratoplasty (ALK) and the excimer laser to create Lasik does not require specific FDA approval. This is considered an "off label" use of the excimer laser and is totally appropriate.

Will the excimer laser be replaced anytime soon with a more modern technology?

It must be noted that since the invention of the laser in 1960, the development of excimer laser lithography has been highlighted as one of the major milestones in the 50-year history of the laser. For applications in laser eye surgery in particular, excimer lasers have been highly industrialized, making them extremely reliable and easily serviceable. With ongoing research and development, their sizes are rapidly decreasing and various improvements have been consistently suggested and focused on. Therefore, it can be rightly concluded that the excimer laser will not be replaced anytime soon with a more modern technology except by its own advanced versions.

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