INTRODUCTION TO LASERS, CHARACTERISTICS

1. Basics in medical physics and electronics
Introduction to lasers:
 Atoms are the basic units of matter. Each atom consists of a nucleus and a
set of electrons orbiting the nucleus.
 Nucleus is formed as a result of strong nuclear forces between the
proton(positivecharge) and neutrons(no charge). So theoverall charge of
the nucleus is positive.
 Electrons havenegative charge and revolvearound the nucleus in different
orbits because of the electrostatic force of attraction.
 Each orbit has a unique energy level.
 The electrons orbiting at a larger distance from the nucleus have higher
energy levelwhereas theelectrons orbiting at a smaller distance from the
nucleus have lower energy level.
 The electrons in the lower energy need some extra energy to jump from
lower energy level to the higher energy level. This extra energy can be
supplied fromvarious types of energy sources such as heat, electric field or
light
 Candles and lamps are called as the man made light sources and sun rays is
called natural light source
 The first reliable artificial light source(incandescentlight bulb) was invented
in 1879 by Thomson Edison.
 In incandescent light bulb, When sufficientelectric currentis passed
through the filament, it gets heated up and emits visible light.
LASER :
 The word laser is an acronym for “Light Amplificationby Stimulated
Emissionof Radiation”.
 Laser emits light through a process called stimulated emission of radiation
which amplifies or increases the intensity of light and produces highly
directional light.
 Some lasers generate visible light but others generate ultraviolet or infrared
rays which are invisible.

2.  Towens and schawlow firstdeveloped a microwave amplifier basedon
stimulatedemissionof radiaiton it was called as MASER
 Maser operates on principles similar to laser but generates microwaves
rather than light radiation
 T.H. Maiman built the first laser device.
Absorption of radiation of light:
 The process of absorbing energy from photons is called Absorptionof
radiation
 The electrons orbiting at a larger distance from the nucleus have higher
energy levelwhereas theelectrons orbiting at a smaller distance from the
nucleus have lower energy level.
 Let us consider that the energy level of ground state electrons is E1 and the
next higher energy level is E2.
 When ground state electrons absorb sufficientenergy from photons they
jump into the next higher energy level of higher energy state.
 The absorbed energy should be equal to the energy difference between the
two energy state(E2-E1)
 The electrons in the higher energy are called excited electrons.
 The light or photons energy applied to excite the electrons can be
mathematically written as
Hv= E2-E1
Where,
H= planck’s constant
V= frequency of photon
E2= higher energy level electrons
E1= lower energy level electrons

3. Spontaneous emission:
 The process by which excited electrons emit photons while falling to the
ground level or lower energy is called spontaneous emission.
 The electrons in the excited state do not stay for a long period becausethe
lifetime of electrons in the higher energy state of excited state is very small,
of the order of 10-8.
 Hence after a shortperiod, they fall back to the ground state by releasing
energy in the formof photon or light.
 The energy of released photon is equal to the difference in energies between
the two electron shells or orbits.
 The electrons changing fromone state to another state occur naturally so the
photon emission also occurs naturally or spontaneously.
 And the emitted photon does not flow exactly inthe same direction of
incident photons, they flow in the random direction.

4. Stimulated emission:
 The process by which electrons in the excited state are stimulated to emit
photons while falling to the ground state is called stimulatedemission.
 Unlike the spontaneous emission, in this process the light energy of photon
energy is supplied to the excited electrons instead of supplying energy to the
ground state electrons.
 Itis an artificial process.
 In stimulatedemission, theelectrons in the excited state need not wait for
natural spontaneous emission to occur.
 An alternative method is used to stimulate excited electron to emit photons
and fall back to ground state.
 The incident photon stimulates or forces the excited electron to emit a
photon and fall into a lower state.
 The energy of a stimulating of incident photon mustbe equal to the energy
difference between the two electron shells.
 In this process theexcited electron released an additional photon of same
energy, same frequency samephase and in the same direction while falling
into the lower energy state.
 In stimulatedemission process, each incident photon generates two photons
 This is the Only known method to producecoherent light (beam of photons
with the samefrequency).
Population inversion:

5.  Population inversion is the state in which the number of atoms in higher
energy state is more thanthose in lower energy state
 Generally more number of atoms are in lower energy states, In stimulated
emission we require more number of electrons in excited state.
 So we optically pump the atoms from the ground state to a higher energy
state the atoms quickly decay non radioactively(withoutemitting a photon)
to a metastable state ( a state with longer lifetime).
 Stimulated emission happens between this metastable state and the ground
state.
 However in populationinversionstate, theprobability of photon causing
stimulated emission is more as more atoms are in higher energy state causing
amplificationof light.
Characteristics of laser:
Laser radiation has the following important characteristics over ordinary light
source. They are
1. Coherence
2. Directionality
3. Monochromaticity
4. Intensity
5. Brightness
1.Coherence:
 Visible light emitted when excited electrons jumped into the lower energy
level is called electrontransition.

6.  In ordinary light sourcethe electron transition is random the photons
emitted have different energies, frequencies, wavelengths or colours.
 Whereas in laser, the electron transition occurs artificially in which
photons emitted have the sameenergy, frequency and wavelength or
colour.
 Therefore the wavelength of the laser light is in phase.
 Thus light generated by laser is highly coherent. Becauseof this
coherence, a large amount of power can be concentrated in a narrow
space.
2. Directionality:
 In convetional light sourcephotons will travel in random direction, whereas
in laser all photons will travel in same direction this is called directionality
of laser light.
 The width of a laser beam is extremely narrow. Hence, a laser beam can
travel to long distances without spreading.

7. 3. Monochromaticity:
 Monochromatic light means a light containing a single colour or
wavelength.
 The photons emitted fromordinary light sources havedifferent energies,
frequencies, wavelength or colours.
 Whereas in laser all the emitted photons havethe same energy, frequency,
or wavelength .
 Therefore laser light covers a very narrow rangeof frequencies or
wavelengths.
4. High intensity:
 Intensity of a wave is the energy per unit time following througha unit
normal area.
 In ordinary light light spreads out uniformly in all directions.
 In laser, the light spreads in small regionof space and in a small
wavelength range, hence laser light has greater intensity when compared
to the ordinary light.
5. Brightness:
 Due to high intensity andhighdirectionality thelight emitted by the laser
are more brighter than ordinary light