11 laser

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Laser vs Light
Laser Light
Stimulated Emission Spontaneous emission
Monochromatic Polychromatic
Highly energized Poorly energized
Parallelism Highly divergence
Coherence Non Coherence
Can be sharply focussed Can’t be sharply focussed

History of Laser
• 1960: The first laser was built by Theodore Maiman using a
ruby crystal medium.
• 1963: The first clinical ophthalmic use of Laser in human
• 1968: L Esperance developed the Argon Laser
• 1971: Neodymium Yttrium aluminum garnet (Nd:YAG) and
Krypton Laser develop
• 1983: Torkel developed the Excimer Laser
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What is laser?
 Laser is the acronym of
• L: Light
• A: Amplification by
• S: Stimulated
• E: Emission of
• R: Radiation

Laser physics
• Laser as electromagnetic waves emitting radiant energy in tiny
package called quanta/photon. Each photon has a characteristic
frequency and its energy is proportional to its frequency
• Three basic ways for photons and atoms to interact.
i. Absorption
ii. Spontaneous Emission
iii. Stimulated Emission

Properties of laser
1) Laser is monochromatic
2) A particular laser has single wavelength
3) This depends on the medium used
4) It cannot be white
5) It is always coloured, i,e green, blue-green etc
6) It is coherent, i,e each wave (photon) is in the same phase as
the next.

Properties of laser
7) It is collimated, i, e rays (photon) are exactly parallel
8) Polarization: The photons vibrate in the same plane
9) It produces bright light
10) It produces intense heat & energy at short distance
11) Laser can burn, coagulate, evaporate & disrupt
12) It can be concentrated in a very small area
(Ref: Manual of Optics & Refraction PM Mukherjee Page: 2.3.4)

Properties of laser
• The light emitted from a laser is monochromatic, that is, it
is of one wavelength (color). In contrast, ordinary white light
is a combination of many different wavelengths (colors).

Properties of laser
• Lasers emit light that is highly directional. Laser light is
emitted as a relatively narrow beam in a specific
direction. Ordinary light, such as coming from the sun, a light
bulb, or a candle, is emitted in many directions away from the
source.

Properties of laser
The light from a laser is said to be coherent, which
means the wavelengths of the laser light are in phase
in space and time

Different issues to know to understand laser
 In order to understand the basic principle of a
laser, it is
instructive to first consider a
passive resonator ("cavity"),such as an
arrangement of mirror that creates a closed path
for a light beam.
The simplest configuration is made with only
two mirror, one being flat and one being curved.

However, its optical power will decay, as some energy is lost in
every resonator round trip. A so-called gain medium can now be
inserted that, when supplied with energy ("pumped").
If the gain g is lower than the resonator losses l, the power
decay is only slowed down. For g = l, the optical power stays
constant; and for g > l, the power rises with each round trip.

• The latter condition can not be maintained
forever; sooner or later, the high intra cavity
intensity will saturate the gain.
• In the steady state, as reached after some time,
the gain will be exactly sufficient to compensate
for the resonator losses. We then have
continuous-wave laser operation with constant
optical power and g = l.

• For extracting a laser beam as a useful
output of the device, the left mirror, for
example, acts as an output coupler,
transmitting some percentage (say 10%)
of the intra cavity power.

1.Spontaneous absorption- electron will move from
low energy level to high energy level by absorbing
photon
2.Spontaneous radiation- electron will move from
high energy level to low one by releasing photon

Fig. 15.1. Energy levels of a simple
laser. (ELKINGTON: 216)

Construction of Laser

Fig. 15.2 Laser tube (ELKINGTON 217)

Three basic components of Laser
A Laser medium
• e,g solid, liquid or gas
Exciting method
• Light or electricity
Optical cavity (Laser tube)
• Around the medium acts as a resonator

• Laser consists of a cylinder that may be solid or
hollow; latter is filled with gas, liquid or a
combination.
• These substances should have ability to absorb
energy in one form and emit a new type of more
useful energy. The energy can be thermal,
mechanical, light or electrical. The process of
conversion is called lasing.

A cavity of the cylinder has two concave
mirrors at each end. One of them is fully
reflective. The mirrors are coated with thin
film of dielectric that reflects light close to
the wavelength of the laser light. The other
mirror is located on the other of the tube.

• The focal length of each mirror almost
coincides with the centre of the tube The
second mirror is partially reflective and is
considered to be leaky.

There are two slanting windows that close
each end of the tube.
The cavity or the rod is surrounded by
source of energy that raises the energy
level of the atoms within the cavity to a
high level in a very unstable state.
This is called population inversion.

The next step is spontaneous decay of the
energized atom to a lower energy level.
This phenomena is the basis behind the
release of high energy in the form of light
that is converted to suitable wavelength.

• Thus, to summarized, there are 2 steps:
1) Population inversion in active medium
2) Amplification of appropriate wavelength.

The energy stored in the laser material, i,
e, gas, liquid or solid, is released in a
narrow beam of monochromatic light.
This light is a source of high thermal
energy, which is used in ophthalmology
for various purposes.

Previously, we discuss that one mirror is
partially transparent, some of the light is
allowed to leave the tube.

• This light will be coherent (the wave fronts in
phase),
• monochromatic (one wave length) and
• collimated (all the rays parallel).
 Light is produced continuously, and such a laser
is said to be operating in continuous-wave (CW)
mode.

Laser mode
 Laser light is generally regarded as being
coherent, as a practical level not all the light
waves are preciously parallel as they resonate
between the two mirrors of the Laser tube.
 Cross-section of laser beam at different points
along its path reveals that it is very slightly
divergent, and that it is more intense at certain
points (called transverse electromagnetic modes)

 Transverse mode are not so important when
energy is delivered diffusely (retinal
photocoagulation)
 But for photo disruption (YAG) it is important to
have precisely focused energy a greater
disruptive effect and, consequently, the effects of
transverse modes need to be considered.

Units of
wavelength
Unit Symbol Length
Centimeter cm 10-2 meter
Angostrom 10-8 meter
Nanometer nm 10-9 meter
Micrometer μm 10-6 meter

Effects of laser energy on tissue

 The effects of laser energy on ocular tissues
depend upon the:
 Wavelength.
 pulse duration of laser light and the
 absorption characteristic of the tissue in
questions (largely determined by the pigments
contained within it).

Effects of laser energy on tissue
The effects can be
1) Thermal
2) Photochemical
3) Ionizing effect

Thermal effect
 Light energy is converted into heat energy if the wavelength
coincides with the absorption spectrum of the tissue pigment
on which it falls and if the pulse duration is between a few
microsecond and 10 s
 Melanin in the retina absorb most of the visible spectrum &
xanthophyll strongly absorb blue light, and hemoglobin absorb
blue, green and yellow wavelength.

Thermal effect
 In the retina, heat is transferred to the adjacent layers of the
retina to cause a 10-20 degree rise in tissue temp. The result is
photocoagulation and a localized burn.
 When visible or infrared light raises the tissue temp to 100 deg
water vaporizes and causes tissue disruption.
 Example: Carbon di oxide. Argon laser.

Photochemical effect
 When a pulse duration of 10 s or more is required to cause
damage, the mechanism is the formation of free radical ions
which are highly reactive and toxic to cells.
 Shorter wavelengths ( blue & UV) causes damage at lower
levels of irradiance and are therefore more harmful.

Ionisation
 Photon energy delivered in a nanosecond or less may be
sufficient to strip electrons from molecules to form a collection
of ions and electrons called a plasma.
 A plasma has a very high temperature and rapidly expands to
cause a mechanical shock wave sufficient to displace tissue.
 Energy released as photons may produce a flush.
 Example: Nd-YAG & Argon-fluoride excimer laser

• When the laser energy exceeds the
threshold for causing tissue damage, the
mechanism of any damage depends
largely upon the duration of exposure.

Laser tissue interaction
Laser
Tissue
Thermal effect
i. Photocoagulation
ii. Photodisruption
iii. Photovaporization
Photochemical
i. Photo radiation
ii. Photoablation
Ionizing
Effect

Photocoagulation
•
Target Tissue
Generate heat
Denatures Protein (Coagulation)
LASER LIGHT

Photodisruption (Mechanical effect)
Laser Light
Miniature lighting bolt
Optical breakdown
Vapor
Tissue damage
Quickly collapses
Acoustic shockwave
Thunder clap

Photoablation
 Breaks the chemical bonds that hold tissue together essentially
vaporizing the tissue e,g, photorefractive keratectomy. Argon-
Fluoride (ArF) excimer Laser.
 Usually
 Visible wavelength: Photocoagulation
 Ultraviolet: Photo ablation
 Infra red: Photodisruption & Photocoagulation

Photo ablation
• Vaporization of tissue to CO2 and water occurs when it’s temp
rise 60 – 100 deg or greater.
• Commonly used CO2
• Absorbed by water of cells
• Visible vapor (vaporization)
• Heat Cell disintegration
• Cauterization Incision

Photochemical effect
• Photo radiation
• Also called photo dynamic Therapy (PDT)
• Photochemical reaction following visible/infrared light
particularly after administration of exogenous chromophore
Commonly used photosensitizer:
• Hematoporphyrin
• Benzaporphyrin derivatives

Uses of Laser in ophthalmology
Mode Lesion Tissue treated
Photocoagulat
ion
Thermal burn Retina & TM
Photoablation Breakdown of chemical bonds
without thermal change
Cornea
Photodisruptio
n
Breakdown of form plasma
resulting in disruption of tissue
PCO
Photovaporiza
tion
Vaporization of fluid from the
tissue to cut
Small tumor

Commonly used Laser in Ophthalmology
Laser Wave
length
Effect
Argon Laser Green
Argon Laser Blue
514 nm
488 nm
Photocoagulation
Photocoagulation
Nd YAG single frequency
Nd YAG double frequency
1064 nm
532 nm
Photodisruption
Photocoagulation
Diode Laser 810 nm Photocoagulation
Excimer Laser 193 nm Photoablation
Ruby Laser 550 nm Photocoagulation
Krypton Laser Red
Krypton Laser Yellow
647 nm
568 nm
Photocoagulation
Photocoagulation

Modes of Laser Operation
• Continuous Wave Laser: It deliver their energy in a continuous
stream of photons
• Pulse Laser: Produce energy pulses of a few ten of micro to
few mili second
• Q Switches Laser: Deliver energy pulses of extremely shorter
duration (nanosecond)
• A mode locked Laser: Emits a train of short duration pulses
(picosecond)

11 laser

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