Laser ii 3 ppt

THE WORKING PRINCIPLES OF FEMTOSECOND
LASER.
College of Science, Department of Physics
By:Getnet Tegenie
Advisor: Dr. GETASEW ADMASU(PhD)
June, 2021
(Bahir Dar University) PRINCIPLES OF FEMTOSECOND LASER June, 2021 1 / 28

Overview
1 Introduction To Laser
Introduction to femtosecond Laser
Femtosecond Laser
2 Mode-locked lasers
Pulse duration
Time-Frequency Relationship
Group Velocity Dispersion
Mode Lock in Laser
Mode Locking in Multimode Laser
Methods of Mode-Locking
3 Applications of femtosecond laser
4 Summary

Introduction To Laser
The acronym word laser means light amplification by stimulated
emission of radiation.
An optical source that emits photons in a coherent beam.
A device which produces any particles or electromagnetic radiations in
a coherent state is called Laser, e.g., Atom Laser.
In most cases laser refers to a source of coherent photons i.e., light or
other electromagnetic radiations.
It is not limited to photons in the visible spectrum.
Lasers are generally applied in commercial, industrial, bio-medical,
scientific and military applications.

Properties of Laser
I Monochromatic: with one color/wavelength
I Directionality: estimulated light propagate in particular direction
I Coherency: estimulated Laser lights are in phase and same direction

Introduction to femtosecond Laser
Femtosecond (FS) laser is an infrared laser with a wavelength of
1053nm.
FS laser like Nd:YAG laser works by producing photodisruption or
photoionization of the optically transparent tissue such as the cornea.
FS laser or Nd:YAG laser results in the generation of a rapidly
expanding cloud of free electrons and ionized molecules.
The acoustic shock wave so generated results in disruption of the
treated tissue.
FS laser has pulse duration in the femtosecond range (10−15 second).
Reducing the pulse duration reduces the amount of collateral tissue
damage.

Femtosecond Laser
Mode locking allows a laser with a broad gain bandwidth to generate
femtosecond pulses.
A mode-locked laser oscillates at the same time on a large number of
modes.
The fields of all modes are phase-locked to each other.
The femtosecond laser is the basis of a great variety of new areas of
research and applications.
Making use of methods of nonlinear optics, the distribution can be
broadened a frequency comb can consist of fields at equally spaced
frequencies corresponding to radiation from the near infrared to the
near ultraviolet.
The position of the frequencies generated by a particular laser can be
determined with a very high accuracy.

Femtosecond Laser Pulses
The recent development of all-solid state femtosecond lasers, tunable
in the visible and near-infrared spectral regions
has already shown an impact on spectroscopic investigations in
difierent areas in physics, chemistry and biology.
femtosecond laser pulses frequently used techniques to measure such
ultrashort pulses.
The operating principle of Ti:sapphire oscillators and Ti:sapphire
ampliflers together.
The second part is devoted to the diagnostic technique used in the
measurement of these ultra fast events.

Generation of Femtosecond Laser Pulses
nonlinear optics and provide basic information about the most-widely
used tunable femtosecond laser sources
in particular tunable Ti:sapphire oscillators and Ti:sapphire ampliflers
or optical parametric ampliflers.
In 1982, the flrst Ti:sapphire laser was built by Moulton.
The laser tunes from 680 nm to 1130 nm, which is the widest tuning
range of any laser of its class[1].
Nowadays Ti:sapphire lasers usually deliver several Watts of average
output power and produce pulses as short as 6.5 fs.

Mode-locked lasers
1 Mode-locked lasers (fs → ps range)
The laser output is pulsed because many laser modes are oscillating
simultaneously in the cavity and are coherently superposed.
Alternative laser name: femtosecond laser
2 Long pulse lasers (typically* µs→ ms range)
3 The laser output is pulsed because the pump is pulsed.
4 Q-switched lasers (ns → µs range) The laser output is pulsed because
a high amount of inversion is suddenly released: nanosecond laser.

Conditions to achieve lasing
The minimum condition to achieve lasing are:
Population inversion
Needs more than two energy level
Metastable state
The angular quatum number, ∆l = ±1
The total angular momentun number, ∆j = 0, ±1

Pulse duration
Considering pulse durations in the range of nanoseconds, the
interaction of the radiation with the sample has enough time to occur
with a significant contribution of thermal effects.
Thermal effects will prevail for long-duration pulses of a few
nanoseconds because the phonon relaxation after absorption of the
optical energy is in the order of tenths of a picosecond.

Time-Frequency Relationship
The pulse shape is a Gaussian function. It is known that the Fourier
transform of a Gaussian function is also a Gaussian function.
The general time and frequency Fourier transforms of a pulse can be
written
E(t) =
1
2π
Z +∞
−∞
E(ω)e−iωt
dω (1)
and
E(ω) =
Z +∞
−∞
E(t)eiωt
dt (2)
I where E(ω) and E(t) are the frequency and electric fleld of the pulse,
respectively.
∆v∆t ≥ K
I where ∆v is the frequency bandwidth at FWHM with ω = 2πv and
∆t is time of the pulse and K is a number which depends only on the
pulse shape.

Time duration of pulse
The minimum time duration of a pulse giving a spectrum with
∆λ (nm) at FWHM, central wavelength λo(nm) and C is the
speed of light (m/s):
∆t ≥
K
∆v
(3)
I But the frequency is given by
v =
C
λ
I The frequency bandwith will be derived as
∆v =
C∆λ
λ2
(4)
I Time is the reciprocal of frequency. Therefore, this will be derived as
∆t ≥ K
λ2
C∆λ
(5)

Group Velocity Dispersion
The Group Velocity Dispersion (GVD) is the propagation of different
frequency components at difierent speeds through a dispersive
medium.
This is due to the wavelength-dependent index of refraction of the
dispersive material.
GVD causes variation in the temporal proflle of the laser pulse, while
the spectrum remains unaltered.
Figure: Absorption and emission spectra of the Ti:sapphire laser.

CONT...
The broad character of the absorption/emission spectra is due to the
strong coupling between the vibrational energy states of the host
sapphire crystal and the electronic energy states of the active Ti3+
ions.
A transform-limited pulse is also called short pulse or unchirped pulse.
It is said that the initial short pulse will become positively chirped (or
upchirped) after propagating through a medium with ”normal”
dispersion (e.g. silica glass).
This corresponds to the situation when higher frequencies travel
slower than lower frequencies (blue slower than red).
The opposite situation, where the pulse travels through a medium
with ”anomalous” dispersion, leads to a negative chirp (or
downchirp).
Here the bluer frequencies propagate faster than the redder
frequencies. Sources of GVD are glass, prism sequences, difiraction
gratings, etc.

Mode Lock in Laser
Mode-locking - technique that generate ultrashort optical pulse in the
range of femto-second.
The secret of the femto second laser is the mode locking: the laser
oscillates at the same time on a large number of longitudinal
modeswith equal frequency separation between next-near modesand
all oscillations have fixed phases relative to each other.
The limitation here is the length of the cavity, which determines the
pulse length.
Ultrashort pulses with pulse widths in the picosecond or femto second
regime are obtained from solid-state lasers by mode locking.
Employing this technique, which phase locks the longitudinal modes
of the laser, the pulse width is inversely related to the bandwidth of
the laser emission.

Mode Locking in Multimode Laser
Lasers generally tend to oscillate on many modes because the
frequency separation of the modes is usually smaller, and often much
smaller, than the width of the gain profile.
The frequency difference between two consecutive longitudinal modes
is ∆v = c/2L.
where the laser gain profile is plotted against frequency, for increasing
values of the pump rate.
For simplicity, one cavity mode is assumed to be coincident with the
peak of the gain curve.
We further assume that oscillation occurs on the TEMoo mode, so
that all mode frequencies are separated by c/2L.
The laser gain coefficient is given by where the cross section of a
homogeneous line Oscillation starts in the central mode when the
inversion N = N2-N1.

Methods of Mode-Locking
Mode-Locking is fundamentally Multimode phenomenon.
1 Active Mode Locking
A modulation of the electromagnetic field is induced by-fast
modulating crystals. These are
1 Acousto-optic modulator
2 Synchronous pump mode-locking
2 Passive Mode Locking
Mode-locking or saturable absorbers are passive mode locking
1 Saturable absorber (dye, solid state)
2 Optical Kerr effect

Optical Kerr effect
Intensity dependent refractive index: n = n0 + n2I(x,t)
Spatial (self-focusing) provides loss modulation with suitable
placement of gain medium (and a hard aperture
Temporal (self-phase modulation) provides pulse shortening
mechanism with group velocity dispersion

Kerr Lens mode Locking
Kerr-lens mode-locking (KLM) is a method of mode-locking lasers via
the nonlinear optical Kerr effect.
This method allows the generation of pulses of light with a duration
as short as a few femtoseconds.
The optical Kerr effect is a process which results from the nonlinear
response of an optical medium to the electric field of an
electromagnetic wave.
The refractive index of the medium is dependent on the field strength.
Because of the non-uniform power density distribution in a Gaussian
beam (as found in laser resonators) the refractive index changes
across the beam profile
In the laser cavity short bursts of light will then be focused differently
from continuous waves (cw).

CONT...
Figure: Hard aperture Kerr-lens mode-locking principle: If the pulse is more
intense in the center, it induces a lens. Losses are too high for a low-intensity cw
mode to lase, but not for high-intensity fs pulse. Kerr-lensing is the mode-locking
mechanism of the Ti:Sapphire laser.
A mediums refractive index depends on the intensity.
n(I) = n0 + n2I (6)

Kerr Lens Mode-Locking principle
The axial modes of a laser cavity are separated by the intermode
frequency spacing v = c/2L.
The modes are separated in frequency by v = c/2L, L being the
resonator length, which also gives the repetition rate of the
mode-locked lasers:
τrep= 1
T = C
2L
(7)
Moreover the ratio of the resonator length to the pulse duration is a
measure of the number of modes oscillating in phase.
There are two ways of mode-locking a femtosecond laser: passive
mode locking and active mode-locking.

Advantages of Femto-second laser
The major advantages of femtosecond laser are:-
1 Reduced incidence of flap complications like buttonholes, free caps,
irregular cuts etc
2 Greater surgeon choice and control over flap diameter and thickness,
side cut angle, hinge position and length
3 Increased precision with improved flap safety and better thickness
predictability
4 Capability of cutting thinner flaps to accommodate thin corneas and
high refractive errors
5 Absence of moving parts

Applications of femtosecond laser
Many scientific, military, medical and commercial laser applications
have been developed since the invention of the laser in 1958.
The coherency, high monochromaticity, and ability to reach extremely
high powers are all properties which allow for these specialized
applications.

Commercial and industrial Applications
I Cutting, welding, marking,
I LIDAR / pollution monitoring,
I CD/DVD player,
I Laser printing, plates,
I Laser pointers, holography, laser light displays
I Laser marking
I Laser cleaning
I Laser cladding, a surface engineering mechanical components
I Optical communications over optical fiber or in free space Laser
I Guidance systems (e.g., ring laser gyroscopes)
I Laser rangefinder / surveying,
I Lidar / pollution monitoring,
I Digital minilabs, Barcode readers
I Laser engraving of printing plate etc.

Scientific Appliation
In science, lasers are used in many ways, including:
1 A wide variety of interferometric techniques
2 spectroscopy
3 Laser induced breakdown spectroscopy
4 Atmospheric remote sensing
5 Investigating nonlinear optics phenomena
6 Holographic techniques employing lasers also contribute to a number
of measurement techniques.
7 Laser based lidar (LIght raDAR) technology has application in
geology, seismology, remote sensing and atmospheric physics item
Lasers have been used aboard spacecraft such as in the
Cassini-Huygens mission.
8 In astronomy, lasers have been used to create artificial laser guide
stars, used as reference objects for adaptive optics telescopes.

Summary
FS laser is a short pulse laser operate using mode lock.
FS laser is being widely used in various ophthalmic surgical
procedures. item Advances in technology like lower energy systems
with faster firing rates will increase the versatility and precision of the
laser systems.
This will further reduce ancillary tissue damage and make the surgery
safer.
As with any other technology, competition will likely bring down the
cost of the equipment making the price per case less expensive.
FS lasers hold great promise and its applications are continuing to
evolve and expand in ophthalmology.

Laser ii 3 ppt

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