Laser technology (excimer and femto)

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CHAPTER 24 Laser technology (excimer and femto)

George D. Kymionis, Georgios A. Kontadakis

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 changes 1,2. 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.

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.

Interaction of laser with ophthalmic tissues

There are three main types of laser tissue interaction that are utilized in ophthalmology. The thermal effect of laser radiation is used to treat retinal tissue in photocoagulation. In photoablation the photochemical effect of laser radiation is used to remove corneal tissue with high accuracy. In photodisruption, the effect of photoionization is used to create plasma inside ophthalmic tissue and thus create cuts such as in YAG laser capsulotomy and in corneal surgery applications of femtosecond lasers 3. Each type of effect takes place with different laser applications depending on several laser radiation properties such as the absorbance of the wavelength by the tissue, the pulse energy and duration (Fig. 24.2).

Fig. 24.2 Penetration of electromagnetic laser radiation in ophthalmic tissue.

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.

Excimer laser tissue interaction

Tissue excision is accomplished through photoablation. This procedure takes place when the laser pulse is absorbed by the collagen molecules and induces the breaking of the molecular bonds and a local rise of pressure. Due to the high pressure, the splinters of the collagen molecules are removed in an explosive way by the corneal surface while a small pit of about 0.25 µm depth is created.

The amount of tissue removed with each pulse of laser radiation is called ablation rate. The depth of the pit which is created by the laser pulse depends on the energy density that is applied by each pulse on the corneal surface. The energy density is called fluence and it is calculated as energy per area. There is a minimum amount of energy per area that could cause photoablation and it is called the ablation threshold. Typically the ablation rate of excimer laser is about 0.3 µm/pulse and the fluence of the pulse about 150 mJ/cm².

The laser beam in excimer lasers usually has a Gaussian or top-hat distribution. A beam with Gaussian profile has higher energy in the center and lower in the periphery. In top-hat energy distribution the energy is equal throughout the beam. The diameter of the beam in the focal area is called the spot size. The small laser spot size is mandatory to produce fine complex treatment patterns. The ideal customized ablation pattern can be produced with smaller spot size Gaussian beam with optimized spot overlap.

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 lasers 4. 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.

Eye tracking devices

Modern excimer lasers incorporate an eye tracking system that keeps the laser beam on target. Eye tracking is responsible for following eye movements during the ablation. In passive eye tracking when a movement is detected the ablation is interrupted. In active eye tracking the laser detects a change in global location and moves the next laser pulse in an attempt to meet that change. Both systems increase the accuracy of the ablation. Eye tracking systems utilize cameras that register either the patient’s pupil or limbus.

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).

Fig. 24.3 Basic differences between excimer and femtosecond laser properties.

Femtosecond laser tissue interaction

Femtosecond laser may tear corneal tissue through a non-linear phenomenon called photodisruption or laser-induced optical breakdown (LIOB) which leads to plasma formation. Photodisruption, or plasma formation, begins with a process called multiphoton ionization, which creates free electrons. These electrons then absorb more photons and, finally, they produce more free electrons through impact ionization. This process is called avalanche or cascade ionization. Plasma formation is accomplished when an energy threshold is attained. This energy threshold depends on tissue properties and the wavelength, and it is lower for smaller pulse duration. In transparent materials, plasma formation takes place in any depth of the material, given that the energy threshold is achieved. The created plasma bears energy that is strongly related to the power of the mechanical effects of the phenomenon to the collateral tissue. This energy is significantly less when the pulse duration is decreased.

Mechanical effects of plasma formation

The main mechanical effect of photodisruption that is used in corneal surgery is the bubble formation. With the explosive phenomenon of plasma formation, there is a shock wave emission, and a cavitation bubble formation. In corneal surgery, this bubble tears corneal tissue and the juxtaposition of several bubbles creates incisions in the tissue. The mechanical effects of plasma formation in femtosecond pulses are tenuous due to the low plasma energy in ultra-short pulses. Consequently the cavitation bubble is restricted to the focal area of the laser beam and the shock wave weakens significantly.

Advantages of femtosecond lasers in corneal surgery

The main properties of the laser pulses used in corneal surgery are wavelengths in the near infrared region and pulse durations of a few hundred femtoseconds. In the near infrared region, the cornea is transparent and the laser beam can be focused in deep corneal layers and produce bubble formation. Due to the non-linear nature of the phenomenon there are no thermal side effects. At the same time a small pulse duration gives low plasma energy and slight mechanical effects. The accuracy of the bubble positioning is increased since it is restricted in the focal area due to the low plasma energy while the laser beam is focused through lenses with large numerical apertures.

Currently, in refractive surgery femtosecond lasers are used for flap creation in LASIK, pocket creation for inlay insertion, and other applications such as sutureless anterior lamellar keratoplasty, astigmatic keratotomy, diagnostic corneal biopsy, anterior lamellar corneal staining-tattooing, and intrastromal vision correction 5–10. In keratoplasty applications, these devices are able to create several customized patterns of incisions. Commercially available devices are capable of controlling the spot size, pulse energy, separation, and position of pulse placement. Therefore they can create precise cuts of almost any possible geometry within the cornea.

1 Trokel SL, Srinivasan R, Braren B. Excimer laser surgery of the cornea. Am J Ophthalmol. 1983;96(6):710-715.

2 Munnerlyn C, Koons S, Marshall J. J Cataract Refract Surg. 1988:14.

3 Krauss JM, Puliafito CA. Lasers in ophthalmology. Lasers Surg Med. 1995;17(2):102-159.

4 Kymionis GD, Diakonis VF, Kounis G, et al. Effect of excimer laser repetition rate on outcomes after photorefractive keratectomy. J Cataract Refract Surg. 2008;34(6):916-919.

5 Soong HK, Malta JB. Femtosecond lasers in ophthalmology. Am J Ophthalmol. 2009;147(2):189. 197.e2

6 Coskunseven E, Kymionis GD, Tsiklis NS, et al. One-year results of intrastromal corneal ring segment implantation (KeraRing) using femtosecond laser in patients with keratoconus. Am J Ophthalmol. 2008;145(5):775. 779.e1

7 Kymionis GD, Ide T, Galor A, et al. Femtosecond-assisted anterior lamellar corneal staining-tattooing in a blind eye with leukocoria. Cornea. 2009;28(2):211-213.

8 Kymionis GD, Yoo SH, Ide T, et al. Femtosecond-assisted astigmatic keratotomy for post-keratoplasty irregular astigmatism. J Cataract Refract Surg. 2009;35(1):11-13.

9 Yoo SH, Kymionis GD, Koreishi A, et al. Femtosecond laser-assisted sutureless anterior lamellar keratoplasty. Ophthalmology. 2008;115(8):1303. 1307.e1

10 Yoo SH, Kymionis GD, O’Brien TP, et al. Femtosecond-assisted diagnostic corneal biopsy (FAB) in keratitis. Graefe’s Arch Clin Exp Ophthalmol. 2008;246(5):759-762.

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