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LASER

The word laser is an acronym for "Light amplification by stimulated emission of radiation", which translates into French by: "Amplification of light by stimulated emission of radiation".

Lasers occupy an important place in ophthalmology, especially in the therapeutic arsenal of the ophthalmologist, for the realization of various eye operations. Their wavelength, delivery mode and power differ according to the desired application. If ophthalmology is a broad field of application for the use of therapeutic laser radiation, refractive surgery is the domain in which excimer and femtosecond lasers are mastered for the Chirurgiale correction of visual defects.

Its many practical applications tend to trivialize the use of laser light, which is the result of a double feat: theoretical and technological.  The idea of a directional and coherent monochromatic radiation emitted by an stimulated emission was envisaged by Einstein as early as 1917; It was only nearly half a century later, in 1960, that the first laser light was emitted in the laboratory by Maiman. It took only a few more years to develop various industrial and medical applications.

Thus, the laser was first a theoretical prediction: its concrete realization materializes the prowess accomplished by the instrumental optics at the 20e century, and the mastery acquired in relation to the interactions between light and matter. It is also remarkable that the wave and corpuscular aspects of light are each necessary and indispensable for understanding the functioning, and designing a laser: this instrument is the result of advances in quantum physics and modern engineering.

This page is devoted to the peculiarities that distinguish the laser light from the other lights, and to the fundamental and common aspects that govern the emission: pages are particularly relevant to the Characteristics of the laser light, the mechanisms peculiar to the design of an instrument capable of emitting such light and which concern The Laser Show and Laser Amplification. They are voluntarily written in language accessible to non-physicists, but the few necessary simplifications should not alter the essence of the subject.

The laser: an energetic light

Most of us have at least once held a laser in hand, in the form of a red laser "pointer", usually used during the illustrated lectures of a slide show.

If one considers the corpuscular aspect of light, this pointer can be likened to a "photon cannon".  Unlike the beam of an electric torch, these photons seem to have special properties: they go "straighter" and "farther", and are confined to a range of particular colored radiation (red, or green, etc).

The energy transported by the laser beam is the accumulation of the energy of each of the emitted photons.

However, the energy transported by a photon is quantified, and equal to: E = h ν or h is the Planck constant (6.626068 × 10-34 m2 kg/s), and ν is the wave frequency associated with the photon (in Herz) This frequency corresponds to the number of oscillations that the electric field performs in one second. This formula connects the corpuscular aspect (photon energy) to the wave aspect of the light.

The energy unit is the joule (a joule = 0.2389 calorie). Physicists often substitute the electron volt to describe the energy associated with the photon: 1 EV = 1.602 x 10-19 : This is the energy acquired by an electron accelerated by a potential difference of 1 V: It is also the energy associated with an infrared photon with a wavelength of 1.24 microns.  The power is the amount of energy delivered in a given time: A watt corresponds to the issuance of a joule over a period of one second. The power delivered by a laser pointer is typically 3 milli-joules (MJ) per second, or 3 milli-watts (3 MW). If the energy of each of the photons emitted by the pointer is equal to 3 x 10 -19 Joules, every second, 10 000 million million photons are emitted by the pointer. If this energy is focused on 1 mm2, the local illumination is greater than the hazard threshold for the eye (this threshold of danger is 2 mw/mm2). This is why the accidental exposure of the retina of our eyes to laser pointers is potentially dangerous.

 

Photon energy is proportional to their frequency (inversely proportional to their wavelength). The radiation emitted by an excimer laser is located in the ultraviolet (higher frequency than the visible light). The photons delivered have an energy of 6.4 ev for the wavelength classically delivered by the Excimer laser (193 nanometers). This energy is sufficient to break the interatomic bonds, and allow the phenomenon of "photoablation", used in Refractive surgery corneal (PKR- LASIK).

The particular energy of the laser light is that it can be concentrated in space and time

A power lamp equal to 100 W has a yield close to 5%: it produces about 5 W of luminous energy, which is displaced in all directions of the surrounding space. At a distance of one metre, the illumination provided is 0.04 mw/cm2– 0.0004 mw/cm2. A laser that emits a comparable power (5w) but focuses it on a surface of 1 mm2 Provides local illumination 10 000 times more intense, in the order of 5w/mm2. The laser allows light to be concentrated on a small surface; This property is useful for targeted action. At a minimum, the diameter of the laser focusing spot can reach that of its wavelength!

The energy of the delivered photons can also be concentrated in time. The ultra short pulses allow to deliver very "powerful" impacts, despite a low total energy by impact. For example, a pulse of 100 micro-joules delivered in 20 femto seconds (20 x 10-15 seconds) corresponds to a peak power (pulse energy divided by pulse duration) of 5 gigawatt! The pulses delivered by the femtoseconds lasers used in corneal surgery have a duration of a few hundred femto seconds, and a power of the order of a micro joule. The intensity of the electric field created at the point of focus allows to overcome the force that connects is electrons to the atomic nuclei, which creates an ionization without heat emission (the pulse duration is too low to communicate the energy delivered in thermal form). By juxtaposing a series of impacts within the cornea, one can locally create juxtaposed tissue ruptures allowing to cut without heating the corneal tissue.

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