Transcript Document

IN The Name of GOD
Basic of femtosecond laser
A . Abolhasani MD
A . Shojaee MD
BASIR EYE CENTER
TERHRAN – IRAN
There is not a financial interest in the products or companies mentioned herein
www.iranophthalex.com
Although laser light is perhaps the purest form of light, it is not of a
single, pure frequency or wavelength. All lasers produce light over
some natural bandwidth or range of frequencies
A typical helium-neon (HeNe) gas laser has a gain bandwidth of
approximately 1.5 GHz (a wavelength range of about 0.002 nm at a
central wavelength of 633 nm), whereas a titanium-doped sapphire
(Ti:Sapphire) solid-state laser has a bandwidth of about 128 THz (a 300 nm
wavelength range centred around 800 nm).
In a simple laser, each of these modes will oscillate independently,
with no fixed relationship between each other, in essence like a set of
independent lasers all emitting light at slightly different frequencies.
The individual phase of the light waves in each mode is not fixed, and
may vary randomly due to such things as thermal changes in materials
of the laser. In lasers with only a few oscillating modes, interference
between the modes can cause beating effects in the laser output,
leading to random fluctuations in intensity; in lasers with many
thousands of modes, these interference effects tend to average to a
near-constant output intensity, and the laser operation is known as a
c.w. or continuous wave.
If instead of oscillating independently, each mode operates with a fixed
phase between it and the other modes, the laser output behaves quite
differently. Instead of a random or constant output intensity, the modes of
the laser will periodically all constructively interfere with one another,
producing an intense burst or pulse of light.
Such a laser is said to be mode-locked or phase-locked
Δt pulse duration
N modes locked
Δν frequency separation
the HeNe laser with a 1.5 GHz spectral width, the shortest Gaussian
pulse consistent with this spectral width would be around 300
picoseconds; for the 128 THz bandwidth Ti:sapphire laser, this spectral
width would be only 3.4 femtoseconds
What is a Femtosecond?
millisecond
microsecond
nanosecond
picosecond
femptosecond
1/1000
1/1000000
1/1000000000
1/1000000000000
1/1000000000000000
The number of femtoseconds in a second is far greater than the number of
seconds in a human lifespan
In one second, a beam of light travels 186,000 miles, seven times around the Earth,
or three quarters of the way to the Moon.
In one femtosecond, light travels 0.3 microns (a fraction of a human hair).
The shortest directly produced optical pulses are generally produced
by Kerr-lens mode-locked Ti-sapphire lasers, and are around
5 femtoseconds long
produce optical features with durations as short as 100 attoseconds
(10−18 second) in the extreme ultraviolet spectral region (i.e. <30 nm )
Applications
Nuclear fusion
Optical Data Storage
Femtosecond laser micromachining – drilling the silicon jet surface of ink jet
printers
Two-photon microscopy
Corneal Surgery. Femtosecond lasers can create bubbles in the cornea, if multiple
bubbles are created in a planar fashion parallel to the corneal surface then the
tissue separates at this plane and a flap like the one in LASIK is formed (Intralase:
Intralasik or SBK (Sub Bowman Keratomileusis) if the flap thickness is equal or
less than 100 micrometres). If done in multiple layers a piece of corneal tissue
between these layers can be removed (Visumax: FLEX Femtosecond Lenticle
Extraction).
A laser technique has been developed that cutting steel , tooth enamel to very
soft materials like heart tissue
Photonic Sampling, using the high accuracy of lasers over electronic clocks to
decrease the sampling error in electronic ADCs
Femtochemistry
Medical imaging
laser-matter interaction
The course of a photodisruptive process is shown. Due to multiphoton
absorption in the focus of the laser beam, plasma develops (A).
Depending on the laser parameter, the diameter varies between 0.5 μm to
several micrometers. The expanding plasma drives as a shock wave, which
transforms after a few microns to an acoustic transient (B).In addition to the
shock wave’s generation, the expanding plasma has pushed the surrounding
medium away from its center, which results in a cavitation bubble (C).
The maximum diameter of the cavitation bubble can reach 10 to 100 μm.
Its lifetime is only a few microseconds. After the collapse of the cavitation
bubble, a gas bubble is left behind, containing carbon dioxide ,water, nitrogen,
and other gas molecules (D).
Mechanism of corneal femtodissection
Typical applications of photodisruption and their
effects as a function of pulse energy.Nd:YAG lasers (10-ns pulse
duration) are used to produce posterior capsulotomies at milliJoule energies (A); femtosecond lasers can cut LASIK flaps with
a microJoule of energy (approximately 930 femtoseconds) (B);
almost bubbleless LASIK flaps at 100 nanoJoules (200 femtoseconds)
(C); and cutting of mitochondria within a living cell
with femto-scissors at 1 nanoJoule (90 femtoseconds) with a
very high numerical aperture lens is shown(D).
dissection of a chromosome within a
living cell using 80 MHz femtosecond
THz imaging
THz radiation provides a means of identification of specific materials,
including biomedical materials such as DNA. This is because molecular
rotations, vibrations or librations occur in this frequency range
Group Velocity Dispersion (GVD)
Optical pulse in a transparent medium stretches because of GVD
• Because of GVD, red components (longer wavelengths) of the pulse propagate
faster than blue components (shorter wavelengths) leading to pulse
stretching (“chirp”).
• Uncompensated GVD makes fs laser operation impossible
• GVD can be compensated by material with abnormal dispersion
GVD Compensation
The first femtosecond laser approved for bladeless LASIK in the
United States was the IntraLase laser ( AMO ), which gained FDA
approval in 2001
Femtec. This femtosecond laser from 20/10 Perfect Vision received
FDA clearance in 2004
VisuMax. FDA-approved in 2007, from Carl Zeiss Meditec
Ziemer Femto LDV Ophthalmic Systems received FDA approval in
March 2008 for its portable femtosecond laser
The threshold for optical breakdown (photodisruption)
is inversely related to the laser’s intensity. The
shorter the pulse’s duration and the smaller the diameter
(and volume) of the spot, the lower the energy needed
for photodisruption.
Laser pulses of longer duration require greater energy to
generate optical breakdown
The side effect of greater pulse energy is disruption of the
surrounding tissue.
The slower the laser pulse, the more excess heat, shock, and
acoustic waves can singe,
melt, or otherwise alter the In matter around it.
Unique spherical Patient Interface of the FEMTEC workstation,
Which does not applanate the cornea The laser's cuts are also
curved, Following the stromal lamella
FEMTEC uses tracker-friendly circular patterns, Patient Interface
Suction is computerized and permanently controlled
Other femtosecond lasers do applanate the cornea with a flat
Contact glass, compressing the cornea and inducing higher
Intraocular pressure . Central bubble layer scan interfere with Excimer tracker
systems. Suction is typically applied manually without system monitoring and control