Introduction

The interaction of the excimer laser with corneal tissue, which was initially described in early 1980s, is the fundamental aspect of refractive surgery. Corneal photoablation with excimer laser is a safe and effective technique for the modification of corneal curvature through tissue removal in order to achieve refraction changes1,2. The latest evolution in laser refractive surgery is the use of femtosecond laser technology, which is capable of creating cuts with high precision in the corneal stroma and replaces the use of the blade in several applications of corneal surgery. Both laser technologies, excimer and femtosecond, are commercially available with platforms created to fit the current standards of refractive surgery practice.

Laser principles

The term laser is an acronym for ‘Light Amplification by Stimulated Emission of Radiation’. Laser is a device that emits electromagnetic radiation. The main attributes of laser radiation are that it is spatially and temporally coherent. Spatial coherence enables the laser to be used for surgery because spatial coherent sources are able to be focused in very small regions with exceptional directionality. Temporal coherence is the property of laser radiation to provide monochromaticity. This means that the laser emits in a single wavelength of the radiation spectrum (ultraviolet, visible, or infrared) and consequently it may be absorbed in a very controlled way by various tissues.

Basic features of laser devices

A laser device consists of three fundamental elements: the optical cavity, the gain medium, and the pumping system (Fig. 24.1). The optical cavity is an arrangement of mirrors which allows the light to oscillate within it. An optical cavity can be created by two oppositely placed plane or concave mirrors. The gain medium is the material in which the process of stimulated emission takes place. It can be gas, solid, liquid, or semiconductor, and the critical properties of the emitted laser radiation depend on the material used. The gain medium generates and amplifies the radiation that travels through it guided by the mirrors of the optical cavity. The pumping system provides the necessary energy for the amplification of the laser radiation. Pumping can be either optical or electrical.

Fig. 24.1 Diagrammatic presentation of the main components of a laser device.

Stimulated emission, which is the fundamental process in the laser devices, is based on the behavior of electrons with various energy states in the atoms. When in a relaxed state, the electrons lie in predetermined orbits. When the electron absorbs energy, it can move further from the nucleus to high energy excited levels. This condition is unstable, so the electron can spontaneously return to the previous state by emitting the energy difference between the two levels in the form of a photon. This is called spontaneous emission. The electron can move in specific energy levels that depend on the material. Consequently the characteristics of the emitted photon depend on the material since its energy and frequency depend on the energy difference between the levels.

The transfer of the electron from the excited state to the low energy state also happens when a photon of the same energy is incident. This gives birth to an identical photon and it is called stimulated emission. This process takes place in the gain medium of the laser device where the energy from the pumping system forces the electrons to move to the higher excited level and creates a condition called population inversion, meaning that more electrons are in an excited state than in a low energy state. Since the atoms in the gain medium are in this state, when a photon with the proper energy passes by, it is very likely to produce the emission of another photon by stimulated emission. The emitted photon has exactly the same wavelength, direction, and phase with the incident photon; they are coherent. This is the principle of light amplification.

In laser devices, this process takes place inside an optical cavity, which forces the amplified radiation (the photons that are multiplied by stimulated emission) to oscillate inside it, by reflecting it with properly positioned mirrors. Each time the radiation passes through the gain medium it gets amplified, given that the pumping system provides enough energy to maintain the population inversion, and the energy that is gained is larger than the energy loss in the optical cavity during the oscillations. These are the parameters that determine the energy output of the laser.