Download Laser means LIGHT AMPLIFICATION BY STIMULATED EMISSION

Laser means LIGHT AMPLIFICATION BY
STIMULATED EMISSION OF RADIATION
Basic Concepts:
Laser is a narrow beam of light of a single wavelength
(monochromatic) in which each wave is in phase (coherent)
with other near it(figure 1). Laser is a narrow beam of light of a
single wavelength (monochromatic) in which each wave is in
phase (coherent) with other near it as show on in figure2.
Figure (1)
Figure(2)
1
Basic theory for laser (Einstein 1917)
Atom composed of a nucleus and electron cloud. If an incident
photon is energetic enough, it may be absorbed by an atom, raising
the latter to an excited state(figure3). It was pointed out by Einstein in
1917 that an excited atom can be revert to a lowest state via two
distinctive mechanisms:
Spontaneous Emission and Stimulated Emission.
Figure(3)
Spontaneous emission
Each electron can drop back spontaneously to the ground state
emitting photons. Emitted photons bear no incoherent(figure4). It
varies in phase from point to point and from moment to moment. e.g.
emission from tungsten lamp.
2
Stimulated emission:
Each electron is triggered into emission by the presence of
electromagnetic radiation of the proper frequency. This is known as
stimulated emission and it is a key to the operation of laser e.g.
emission from Laser .
Absorption:
Let us consider an atom that is initially in level 1 and interacts with an
electromagnetic wave of frequency n. The atom may now undergo a
transition to level 2, absorbing the required energy from the incident
radiation. This is well-known phenomenon of absorption.
Figure(4)
3
Population inversion
Generally electrons tends to (ground state). What would happen if a
substantial percentage of atoms could somehow be excited into an
upper state leaving the lower state all empty? This is known as a
population inversion. An incident of photon of proper frequency
could then trigger an avalanche of stimulated photon- all in phase
(Laser).
Figure(5)
Consider a gas enclosed in a vessel containing free atoms having a
number of energy levels, at least one of which is meta stable. By
shining white light into this gas many atoms can be raised, through
resonance, from the ground state to excited states (figure 5).
E1 = Ground state,
E2 = Excited state (short life time ns),
E3 = Meta stable state (long life time from ms to s).
4
Types of lasers
According to the active material: solid-state, liquid, gas, or
semiconductor lasers.
According to the wavelength:
(UV) or X-ray Lasers.
Infra-red (IR), Visible, Ultra-violet
Solid-state lasers have lasing material distributed in a solid matrix
(such as ruby or Nd-YAG). Flash lamps are the most common power
source. The Nd-YAG laser emits infrared light at 1.064 nm.
Semiconductor lasers, sometimes called diode lasers, are p-n
junctions. Current is the pump source. Applications: laser printers or
CD players.
Dye lasers use complex organic dyes, such as Rhodamine 6G, in
liquid solution or suspension as lasing media. They are tunable over a
broad range of wavelengths.
Gas lasers are pumped by current. Helium- Neon (He-Ne) lasers in
the visible and IR. Argon lasers in the visible and UV. CO2 lasers
emit light in the far-infrared (10.6 mm), and are used for cutting hard
materials.
Solid-state Laser:
Example: Ruby Laser
Operation wavelength: 694.3 nm (IR),3 level system: absorbs green/blue
5
Gain Medium: crystal of aluminum oxide (Al2O3) with small part of
atoms of aluminum is replaced with Cr3+ ions. Pump source: flash lamp
and the ends of ruby rod serve as laser mirrors(figure6) .
Figure(6)
A Helium-Neon (He-Ne) gas Laser
A helium–neon laser or He-Ne laser, is a type of gas laser whose gain
medium consists of a mixture of helium and neon(10:1) inside of a
small bore capillary tube, usually excited by a DC electrical
discharge. The best-known and most widely used He-Ne laser
operates at a wavelength of 632.8 nm, in the red part of the visible
spectrum.
6
figure(7)
The energy or pump source of the laser is provided by a high voltage
electrical discharge passed through the gas between electrodes (anode
and cathode) within the tube(figure 7). A DC current of 3 to 20 mA is
typically required for CW operation. The optical cavity of the laser
usually consists of two concave mirrors or one plane and one concave
mirror, one having very high (typically 99.9%) reflectance and the
output coupler mirror allowing approximately 1% transmission.
High and Low Level Lasers
1.High Level Lasers such as surgical lasers, hard lasers, and thermal
laser with
energy (3000-10000) mW.
2.Low Level Lasers such as medical lasers and soft lasers with
energy
(1-500) mW. Therapeutic (Cold) lasers produce maximum output of
90 mW or less (600-1000) nm light.
Laser Treatment & Diagnostics
Treatment cover everything from the ablation of tissue using high
power lasers to photochemical reaction obtained with a weak laser.
Diagnostics cover the recording of fluorescence after excitation at a
suitable wavelength and measuring optical parameters.
Laser Tissue Interaction
Energy is reflected, transmitted, absorbed and scattered(figure9).
Lambert Beer law
7
I = Io 10-aX
α= absorption coefficient
X = thickness of material
Io = incident intensity
Extinction length = 1/α = L; where 90%I = transmitted intensity of
the intensity is absorbed.
figure(9)
Medical use of laser:
Laser light waves penetrate the skin with no heating effect, no
damage to skin & no side effects. Laser light directs bio stimulation
light energy to the body’s cells which convert into chemical energy to
promote natural healing & pain relief.
Laser uses in surgery
8
Laser surgery to correct for
1.nearsightedness, and
2.farsightedness (figure10).
figure(10)
In surgery, lasers can be used to operate on small areas without
damaging delicate surrounding tissue(figure 11).
9
figure(11)
10