Laser technology (excimer and femto)

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CHAPTER 24 Laser technology (excimer and femto)

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.

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.

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.

A simple form of the optical cavity can be created by two opposing mirrors. The mirrors force the radiation to oscillate and pass through the gain medium. One of the mirrors has 100% reflectivity and the other can be transparent to a small percentage of the incident radiation. Through this partially transparent mirror laser radiation is exported from the laser device. The output laser radiation can be either continuous or pulsed and this is one of the essential characteristics of the laser device. Pulsed emission is regulated with techniques incorporated in the device (as Q-switching, mode locking) that regulate the especial properties of the laser pulse, i.e. pulse duration, peak energy, repetition rate (Hz).

The pumping device can supply energy in several forms to the laser, depending on the type of the gain medium. It can be in the form of an electrical discharge, usually in gas mediums, or in the form of photons or other laser radiation, usually in solid state gain mediums.

The emitted laser radiation has a wavelength that depends mainly on the gain medium. It could belong to the visible spectrum, the ultraviolet, the infrared, etc. Other properties of the laser output radiation – energy, continuous or pulsed emission, pulse frequency – also depend on the gain medium and the other elements of the device that form the final output laser beam.

Excimer lasers

Excimer lasers are pulsed laser devices with gas gain medium. The word excimer is short for ‘excited dimer’, which refers to the type of the gain medium. The dimer is a molecule that consists of an atom of a noble gas (argon, krypton, xenon) and an atom of a halogen (fluorine, chlorine, bromine, iodine). These molecules exist only in the excited state during electric discharge and their lifespan is a few nanoseconds.

The laser radiation of excimer lasers is in the ultraviolet region. Each pulse has a duration of a few nanoseconds. In refractive surgery we use the ArF (argon–fluoride) at 193 nm wavelength. This wavelength is highly absorbed by the cornea. It is appropriate for corneal surgery because it has high accuracy and repeatability of tissue excision, it is not mutagenic, and it has minimal thermal effect and damage to the surrounding structures.

Concurrent effects of excimer laser energy

One of the important side effects of the laser radiation is the collateral thermal damage. It is caused by the direct absorption of laser radiation with energy lower than the ablation threshold by tissue surrounding the ablation, and also by thermal energy deriving by the rapidly moving splinters of collagen. It has been shown that the amount of thermal damage is related with haze formation after photorefractive keratectomy (PRK). The local rise of temperature in the tissue surrounding the ablation is higher when ablation is repeated in the same area with a high frequency. Due to this fact, lower repetition rate lasers are more suitable for PRK than high repetition rate lasers4. In modern devices each pulse targets a different position than the previous, and the incidence of subsequent pulses in the same point is avoided.

Other side effects are the potential mutagenesis and cataractogenesis caused by ultraviolet radiation. In the wavelength of 193 nm, both side effects are negligible due to the minimal penetration of the radiation. The secondary radiation between 260 and 290 nm, generated after ArF 193 nm photoablation, has been suspected of potential cataractogenesis and mutagenesis, although there are experimental and clinical findings that show no such danger.

The mechanical damage caused by an acoustic shock is also referred as a side effect of excimer laser photoablation. Acoustic waves are produced after each pulse of ablation and their energy depends on the fluence and the spot size of the excimer laser beam. The acoustic shock is considered to be responsible for the postoperative scar formation and endothelial cell loss, but the incidence in modern lasers with small spot size is negligible.

Femtosecond lasers

Femtosecond lasers are pulsed laser devices emitting laser radiation with ultra-short pulse duration in the domain of femtoseconds (10–15 seconds). A femtosecond is to a second what a second is to 32 million years. The creation of ultra-short pulses can be achieved with the use of the technique called mode locking. This technique takes advantage of the interference of light traveling in the optical cavity of the laser device. The gain medium in femtosecond lasers used in corneal surgery is Nd : glass and the pumping system is a diode laser.

In the typical femtosecond laser device, laser pulses are produced through a process called ‘chirped pulse amplification’. There are three main components in a unit used in refractive surgery: the diode pumped Nd : glass laser device that gives birth to the laser pulse, the pulse modulation system that amplifies and regulates the pulse, and the application system that applies the laser radiation on the cornea.

The laser radiation features when it reaches the corneal surface are: a pulse duration of a few hundred femtoseconds, repetition rate of up to 120 kHz, pulse energy of a few microjoules, and wavelength in the near infrared region (1053 nm). These parameters may differ between the commercially available femtosecond laser units (Fig. 24.3).