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Femtosecond laser

Laser for corneal surgery femtosecond

The femtosecond laser is a laser that delivers ultra short pulses, of the order of a few hundred femtoseconds for medical applications.  In ophthalmic surgery, it is mainly used to achieve the precise cutouts of the cornea, without thermal effects. The LASIK benefits of the femtosecond technology that allows to create flaps whose thickness and dimensions are adjusted with precision. The cataract surgery is a scope more recent, where the interest of the femtosecond technology for fragment cristallinien kernel is being evaluated.

 

Definition of the femtosecond

A femtosecond is 0.000000000000001 10-15 seconds, i.e. a millionth of a billionth of a second !

Some data can be a idea of the brevity of a femtosecond :

-If in a second, the light travel a distance equivalent to 7 times around the Earth in a femtosecond she doesn't have time to go through the thickness of a hair,.. .to have a virus!

-There are more femtosecond in a second, as hours passed since the big bang, about 14 billion years ago

-Pulse issued by its brief, of the order of some nanoseconds excimer lasers (10-9 (seconds). Their duration is, however... a million times higher than a ultra-breve pulse of a few femtoseconds.

 

Interest of the femtosecond laser pulses

It was during the 1990s the development of compact ultrabrefs lasers to solid medium has allowed to consider their use in ophthalmic surgery. The ability to perform specific ultra cutouts of the cornea was particularly interesting for LASIK surgery, then booming. Currently, 5 femtosecond lasers are available in ophthalmology for cornneenne surgery:

-the IntraLase FS (Abbott Medical Optics, USA) platform, first airline to offer a late 1990s femtosecond laser

-the Femtec (Perfect Vision, Germany) platform, acquired by the company Technolas, then more recently Bausch and Lomb

-The Femto LDV (Ziemer, Switzerland) platform

-platform Visumax (Zeiss, Germany)

-the platform FS 200 (Wavelight/Alcon, Germany - United States)

The brevity of the delivered laser pulses allows to reach very high powers, despite a modest output energy. The power is defined as the amount of energy delivered per unit of time. (the energy is equal to the intensity corresponding to the ultra short amplitude variations of the electromagnetic field). For the same energy, as time is short, the more the power output. In eye surgery, the energy of pulses issued is of the order of the micro-Joule, for an irradiance of the order of a joule/cm2. The wavelengths used to generate radiation able to interact with healthy corneal tissue are in the near infra red (800 nm to 1 micrometer).

The power delivered by the ultra short pulses can cause spectacular effects in materials: in addition, the effects produced by these impulses are localized and deterministic. Pulses them laser cause a 'ionisation', i.e. breaking the links between electrons and atomic nucleus. When a femtosecond pulse is focused within the corneal tissue (stroma), it causes the ionization, which is followed by the appearance of a bubble cavitation linked to the expansion of this plasma consisting of ions. This plasma then broadcasts in the adjacent stroma, and a collapse occurs over a 'lack of continuity' or 'disruption', IE a localized rupture of the fabric. If we wisely juxtaposes these femtosecond pulses in a contiguous and regular manner, we can achieve a cutting path which the geometry is adjustable and accurate.

femtosecond laser cutting

Principle of focusing of the femtosecond laser in corneal stroma to the realization of a cutting of flap of LASIK with thickness 120 microns. The corneal surface is smoothed under low suction, in contact with a glass of unfortunately. Pulses them emitted by the femtosecond laser can be focused to get a flap of near-constant thickness contiguously to the depth desired.

 

This is accomplished without thermal effect, so without burning, due to the extremely short duration of femtosecond pulses.

laser effect on biological tissue based on the exposure time

Photochemical effects on biological tissue based on the time of exposure to laser radiation. As time is short, the more the power output. Photoablation (excimer laser) and "plasma-disruption" (femtosecond laser) are tinted.

 

Thus, to create a flap of LASIK with femtosecond laser requires to issue a series of impacts including spacing, depth, whose distribution in the three-dimensional space of the corneal tissue must be perfectly controlled.

femtosecond flap LASIK laser cutting

The dimensions of the flap, the location and the width of the hinge can be adjusted with precision with femtosecond technology. case of cutting 'raster' mode (line by line) the flap is pre-cut with a series of impacts focused at an identical depth. The shots are made line after line, sweeping the cornea attached to the licensing system of the laser of the hinge (in top) down. Each line is a series of impacts, spaced about 7 to 9 microns. The spacing between the lines is also customizable, and usually identical to one located between the impacts. The size of the impact depends on the used wavelength, and the programmed energy cavitation bubble (in practice, cavitational bubbles may reach a diameter of a few microns).

The following video shows a cut-out of flap of LASIK by a last generation, clocked femtosecond to 200 000 impacts per second (200 K Hz) laser. The camera is located across the original unfortunately that comes in contact with the eyeball and allows for deliver impacts at constant preset depth corneal surface, glass.

This video shows also sequences of femtosecond laser cutting, seen from the camera of an iPhone that is placed in the line of fire.

 

This page is dedicated to the more fundamental aspects of the femtosecond technology used for eye surgery. Specific pages are dedicated to the clinical and as the 100% laser LASIKla cataract surgery laser.

 

Technical characteristics of femtosecond lasers

To generate a continuous laser radiation or producing 'long' impulses, we need to get a radiation emitted by stimulated emission (which is monochromatic and "coherent". (''), then amplify. The monochromatic nature of the collected light is "relative" because the laser cavity in fact amplifies a "comb" of frequencies (see...), that can be purified by various techniques.

Obtaining of pulse ultra short is based instead on the use of a wide range of frequencies, but which must be perfectly synchronized to the output of the laser instrument. Transported power by the pulses ultra short is very important: in fact, amplification classic, tied to the gain obtained at each "back and forth" in the cavity can quickly lead to the deterioration of the. To resolve this issue, synchronization of a wide range of frequencies and its amplification are 'separated '; they assigned groups of distinct elements.

Relationship among time and frequency

The need to use a broad spectrum of frequencies of radiation is explained by the relation between the duration of a pulse (T) and the necessary to produce this frequency range pulse (dF):

dF x T ≈ 1

Thus, duration and range of frequencies required are proportionally inverse: more T is low, the more dF. If you want to produce the pulses of a close period of the femtosecond (10 -15  (seconds), to use a wide range of radiation of the order of... 10 15 Hz! The brevity of the femtosecond pulses inevitable corollary is a large spectral width.

To get this frequency range, the femtosecond laser use one of the properties of the laser cavities: for many reasons, amplify not a single frequency of light, but a range of radiation centered on one or a few specific frequencies. The cavity has a physical length finished, and several frequencies say longitudinal can there be amplified - for example, a simple condition is that a finite number of half the length of the wave is equal to the length of the cavity (see:) amplification laser). When you want to emit a radiation the more 'pure' laser in spectral terms, seeks to "turn off" the resonant frequencies around the desired frequency, using various devices like the insertion of prisms of elements diffractants in the cavity. The case of the femtosecond laser, try instead to amplify a very high number of frequencies, it will also synchronize.

To get a pulse lasting 100 femtoseconds, but use a range of wavelength which the difference between the shortest and longest must reach 15 nm. In ophthalmology, the Central wavelength of the femtosecond lasers currently available on the market is located in the infrared.

 

The resonant frequencies getting

The techniques used to generate the first impacts femtoseconds are designed in the 1980s; in particular, the discovery of a new material made up of doped the titanium (Ti ion (Sapphire) aluminum oxide 3+: Al2O3). The common name of this material is Ti: Sapphire: transitions are obtained through the Ti ion 3+which is located in the matrix Al2O3.  The thermal conductivity of the Ti: Sapphire is good and allows a heat dissipation. Its emission spectrum is wide and located in the infrared (680 to 1100 nm).

We have seen that more spectrum was wide, and more pulses could be brief: they can be as short as 5 femtoseconds to experimental conditions with the Ti:saphir. In practice, femtosecond lasers used in eye surgery issue of pulses of some hundreds of femtoseconds duration.

The Ti: Sapphire has an absorption spectrum located in Green: this environment must therefore be 'excited' by a diode whose frequency is doubled (the wavelength divided by 2). The development of femtosecond lasers used in ophthalmological applications is especially in the use of other crystals doped with Ytterbium, which allow to increase the compactness of the facilities (use of optical fibers), and reduce the cost. These crystals can be excited by diodes that emit in the infrared. The 'central' wavelength used by currently available femtosecond lasers is close to 1.05 micrometer.

 

Phase-locking

The implied relationship between the frequency of the radiation range and duration of impulses is the basis of the pulse system. But there is a key requirement: a pulsed radiation of ultra-short duration, to synchronize a high number of longitudinal resonance modes of the electromagnetic field. This is achieved by what is called a "phase lock" longitudinal modes.

This lock is needed to get the Ultrashort pulse, and thus must synchronize in time a large number of longitudinal modes; a subtle game of interference locally very constructive and destructive elsewhere allows to amplify the signal next to the box where the modes are centered, and mitigate it elsewhere.

locking modes

Schematic representation of a group of resonant frequencies synchronized (central line). There is a gap of half a period of oscillation of the electric field between the highest frequency mode and low frequency mode, at the level of the edge of the pulse product. The colors do not match "colors" of the visible spectrum: the frequencies used to generate impulses minidresses are located in the infra red

Most of femtosecond lasers used in the laboratory have an oscillator coupled with a titanium-Sapphire Crystal where various combined 'non-linear' so-called optical effects to the use of special mirrors contribute to this synchronization. He must use dispersive mirrors, lengthening trains of waves reflected based on frequencies in different ways. This allows to compensate for shifts that occur in the environment (Titanium Sapphire Crystal, which itself is "pumped" with a laser wavelength in green).

 

Stretching and pulse compression

Because of the power carried by pulses them femtosecond, it is not possible to amplify them "such what" in the cavity, damage it. The Middle oscillator, where are produced and pré-synchronisées the frequency of the radiation is separated from the Middle amplifier. Generally used a method of amplification to "frequency drift". We can reduce the peak pulse power "stretching them" in time. Before undergoing the amplification, the duration of pulses is elongated by a "anti-roll": which is equivalent to shift the radiation (i.e. to modulate the phase) of various frequencies. Once the amplification is obtained, a resynchronization is performed output. Thus, stretched and then amplified by simulated emission, the pulse is finally "recompressed" and is then emitted in the form of a pulse utra-brief.

Stretch compression of pulses femtoseconds for amplification

It is not possible to amplify without damage pulses them femtosecond oscillator of the fact of the peak obtained on these extremely short duration power-generated. A "anti-roll" allows to shift the phase of resonant frequencies, before their amplification. A re-synchronization allows obtaining a pulse ultra high energy brief.

 

The ball stretcher and compressor are made of mirrors that diffract light, thus separating the various frequencies which the route is changed: the lengthening or shortening of the route based on the wavelength (same as some optical effects non-linear as the Kerr effect) are involved in obtaining of pulse amplified.

Etierur and compressor femtosecond laser

Representation of an anti-roll (top) and a compressor (bottom) pulse that use pairs of gratings. The ball stretcher is an afocal system where the photons 'blue' (more short wavelength, high frequency) follow a path longer than "red photons. At the entrance, the frequencies are synchronous to the output, blue is "late". In a compressor, where mirrors diffractant its Parallels, it's the opposite. By adjusting the distance between the mirrors, can be compensated in a perfect way the stretching and're compress' the pulse whose constituent frequencies were previously amplified.

 

 

Characteristics and use of femtosecond pulses

The duration of pulses issued is between 200 and 800 femtosecond laser models used in ophthalmic surgery. The energy of pulses is close to the microJoule for corneal applications (LASIK, cutouts of tunnels for intra corneal rings, transplants, dissections of pockets, etc.), but 10 to 20 times higher than for the cataract surgery assisted by the femtosecond laser, where them pulses are delivered to realize circular cutting of the capsule (capsular rhexis) and fragmentation of the cristallinien kernel.

The creation of a LASIK flap, to deliver a number of impacts on the order of 1 million: this imposes high shooting speed, of the order of the kilo Herz (100 000 impacts per second!). The time interval that separates two pulses (on the order of 10 – 5 seconds) is extraordinarily long for the duration of a pulse (a few hundreds of femtosecond is 10-13 (seconds). If a pulse lasted the equivalent of one day (24 hours), the time between a pulse of the pulse following would be of... 30 million years! The spacing between pulses them and lines of pulses is chosen by the surgeon, and the delivered energy per pulse.

In cataract surgery, shooting rates are a little smaller, the order of 30 to 50 Hz.

Here are two among the most recent machines, used in corneal and refractive eye surgery

-Laser Wavelight FS200 Alcon

Femtosecond laser Alcon/Wavelight FS150 before and after removal of the skin shells. The laser cavity is located in the upper part of the main part.

Femtosecond laser Alcon/Wavelight FS150 before and after removal of the skin shells. The laser cavity is located in the upper part of the main part.

-Laser Intralase IFS 150

Laser intralase IFS 150

Laser femtosecond IFS 150 dressed and partially "bare." The upper part has oscillator, amplifier, and the system of stretching / compression of pulses.

Here is a detail of the optical bench used by the IFS 150 laser, where we find the main elements described above:

"Aerial" view of a part of the optical path between the oscillator and amplifier (laser Intralase IFS 150)

"Aerial" view of a part of the optical path between the oscillator and amplifier (laser Intralase IFS 150)

Side view of the wheel of polarization, which allows to modulate the energy of the beam before the exit to the arm of the licensing system.

The wheel of polarization allows to modulate the energy of the transmitted laser beam (the angle of the polarizer is modulated to preferentially attenuate laser radiation, from the angle of polarization of it)

The wheel of polarization allows to modulate the energy of the transmitted laser beam (the angle of the polarizer is modulated to preferentially attenuate laser radiation, from the angle of polarization of it)

 

Profile view of the ball stretcher / compressor:

 

anti-roll compressor femtosecond laser

View horizontal part of the compressor anti-roll using Diffractive element to shift frequencies before amplification, and resynchronize them prior to issuance of the pulse.

 

Ball stretcher / compressor that has a network of diffraction. In box, visualization with an infrared camera of the same compressor ball stretcher on the infrared optical path.

Ball stretcher / compressor that has a network of diffraction. In box, visualization with an infrared camera of the same compressor ball stretcher on the way optical (laser radiation has a frequency range that is located in the infrared).

Benefits of lasers in corneal refractive surgery (LASIK) femtosecond

The development of technology for the corneal refractive chiurgie femtosecond laser goes back in the early 2000s.  At that time, LASIK is the technique of reference for medium and high myopia correction. The cutting of the stromal flap is performed by a mechanical system called microkeratome.

During the 90s, experimental work conducted at the University of Michigan suggests that the use of pulses femtoseconds allows tinted contours of biological tissue: the cornea is a target tissue particularly interesting because of its accessibility and transparency to infrared radiation. The Intralase® company was incorporated in 1997 to Irvine, from a technology developed at the University of Michigan. The first cuts of flap on human eyes was completed in 2001, and the year 2003 has seen a growing number of publication of refractive surgery centers Americans adopting the laser cutting for the realization of the LASIK flaps. These studies show that the Visual results are equivalent to those obtained with the microkeratomes.

At the time, firing rates were of the order of 15-30 KHz (a cut of flap lasted more than a minute), and the higher issued energies. Over the years, the rates increased to 150 or even 200 KHz (cut in 10 seconds), and the energy of pulses below the micro Joule.

Numerous publications attest to the advantage provided by the femtosecond lasers, including the main:

-Low dependence of the cutting towards the cornea's biomechanical and geometric characteristics (keratometry): cutting is performed under unfortunately flat or slightly curved, allowing to achieve a path parallel to the corneal surface. With the microkeratomes, the unfortunately is carried out progressively, and cutting subject to shear constraints

-Better predictability of the depth of cut: the resulting thickness is close to the programmed thickness (adjustable in steps of 5 microns with the latest generation of laser like the 200 FS). The standard deviation is about 10 microns, against 20 to 30 with mechanical microkeratomes

-Improved regularity of cutting: studies in the LASIK flaps optical coherence tomography show sometimes significant variations in the thickness of the flaps at different points with mechanical microkeratomes: the thickness of the flaps cut laser is much more homogeneous

-Possibility of personalization of the trace of the cuts: the diameter of the flaps can be selected with a precision of 0.1 mm. The perimeter can be circular, or circular (correction of the)astigmatism). The angulation of the edges of the flap is also ajsutable.

-Opportunity to refocus the trace of the flap to the center of the pupil: before cutting, the control screen pre visualize the route of cutting; a refocusing is possible if the unfortunately reveals a shift of pupillary Center (effect of the kappa angle or the eccentricite of the pupil). This balance allows to optimize the dimensions of the stromal bed offered to the sculpture made by the excimer laser, and translates into a better optical quality in post operative.

-Lack of projection of debris in the interface: some studies have shown the presence of foreign material in the interface of the operated corneas of LASIK with microkeratome (blade debris, oil lubrication of the mechanical parts particles). Creating the interface with the laser is "firm globe.

-Possibility to immediately resume a partial cut in case of interruption (desertion of the mammals with uncoupling of the eye towards the system unfortunately).  Just resolidariser the system unfortunately eye and repeat the procedure.

 

The disadvantages of the femtosecond technology are related to:

-its cost, the price of a femtosecond laser is close to that of the laser excimer (500,000 euros). There is a cost for the use of the order of a few hundred euros by procedures. Femtosecond LASIK procedures are more expensive than mechanical LASIK procedures.

-the appearance of opaque white bubble (opaque bubble layer) for cutting: incident linked to the accumulation of products of degassing of the laser impacts in the corneal stroma sometimes have to wait a few minutes before proceeding to the sequence of correction excimer.

-the rainbow glare: this often transient side effect (a few months) is very rare and is characterized by the perception of halos colored around the white light sources, whose distribution is usually vertical and iridescent (from Red inside to blue outside). It is related to a phenomenon of diffraction by the regular network created by the successive impacts within the corneal stroma at the level of the posterior face of the flap.

 

Femtosecond technology is meant to evolve. Work are currently carried out to realize femtosecond lasers which the central peak would be located in the ultraviolet. Their development requires the use of crystals for double or triple the frequency of the initial radiation. Ultraviolet radiation used to reduce the energy of the impact and produce more accurate cuts, even if the dissemination of ultraviolet radiation in the depth of the corneal tissue is less than for the infrared. The sites of action of sight refractive corrections being superficial, femtosecond technology combined with the use of ultraviolet promises to be particularly interesting for refractive surgery.

 

 

One response to "Laser femtosecond"

  1. Sandrine says:

    Beautiful tribute to the laser pioneers rewarded for their work by the Nobel Prize in Physics 2018, with current and upcoming extraordinary applications.

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