OPTICAL COMMUNICATION

1. Optical Sources
LASER
By: Prof. Snehal Laddha

2.  A laser is a device that generates light by a
process called STIMULATED EMISSION.
 The acronym LASER stands for Light
Amplification by Stimulated Emission of
Radiation
 Semiconducting lasers are multilayer
semiconductor devices that generates a
coherent beam of monochromatic light by laser
action. A coherent beam resulted which all of
the photons are in phase.
Prof. Snehal Laddha

3.  LED is chosen for many applications using
multimode fibers
 The lasers tends to find more use as a single
mode device in single mode fibers.
Prof. Snehal Laddha

4.  Lasers are all around us in many places you might
not realize. Besides being useful for pure science
like in a physics lab, lasers are found in many
real-world applications.
 The grocery store
 The Doctor’s office
 Manufacturing
 Telecommunication
 The Moon
 Weapons
Prof. Snehal Laddha

5. The scanner measures the brightness of the reflected light and converts this
information into numbers and letters.
Prof. Snehal Laddha

6. Lasers are also used by
dentists to fill cavities,
and by doctors as a
scalpel.
Lasers have the
advantage of being
more precise and much
less invasive than a
standard scalpel.
Lasers have become
widespread in the
treatment of cancer
(tumor removal)
Prof. Snehal Laddha

7. Lasers can cut and weld complex structures out of both hard and soft
materials, and at much smaller scales than traditional welding. This
type of welding/cutting can be computer controlled for ultra high
precision.
Prof. Snehal Laddha

8.  Fiber-optic communication is one of the most
important contributions from physics to our daily
lives.
 Telephone, internet, television are all transmitted
using fiber-optics.
 Laser light is used as a carrier for different types
of information, which can then be sent huge
distances with very little signal degradation and at
high speeds.
Prof. Snehal Laddha

9.  An example of application is for the light source for fiber
optics communication.
 Light travels down a fiber optics glass at a speed, = c/n,
where n = refractive index.
 Light carries with it information
 Different wavelength travels at different speed.
 This induce dispersion and at the receiving end the light
is observed to be spread. This is associated with data or
information lost.
 The greater the spread of information, the more loss
 However, if we start with a more coherent beam then
loss can be greatly reduced.
Prof. Snehal Laddha

10. Prof. Snehal Laddha

11. 1. Absorption
2. Spontaneous Emission
3. Stimulated Emission
1. Absorption
2. Spontaneous Emission
3. Stimulated Emission
For atomic systems in thermal equilibrium with their
surrounding, the emission of light is the result of:
Absorption
And subsequently, spontaneous emission of energy
For atomic systems in thermal equilibrium with their
surrounding, the emission of light is the result of:
Absorption
And subsequently, spontaneous emission of energy
There is another process whereby the atom in an upper energy
level can be triggered or stimulated in phase with the an
incoming photon. This process is:
Stimulated emission
It is an important process for laser action
There is another process whereby the atom in an upper energy
level can be triggered or stimulated in phase with the an
incoming photon. This process is:
Stimulated emission
It is an important process for laser action
Therefore 3 process
of light emission:
Prof. Snehal Laddha

12. E1
E2
When a photon of energy (E2-E1) is incident on the atom it
may be excited into the higher energy state E2 through
absorption of photon. This process is called as stimulated
absorption.
Prof. Snehal Laddha

13. When the atom in higher energy state E2 makes a transition in lower
energy state E1 in an entirely random manner, the process is called
spontaneous emission.
Prof. Snehal Laddha

14. When a photon of energy equal to the energy difference between
the two states (E2-E1) interacts with the atom in the upper energy
state causing it to return to the lower state with the creation of a
second photon, is called stimulated emission.
Prof. Snehal Laddha

15.  In 1917 Einstein predicted that:
 under certain circumstances a photon incident
upon a material can generate a second photon of
Exactly the same energy (frequency)
Phase
Polarisation
Direction of propagation
In other word, a coherent beam resulted.
Prof. Snehal Laddha

16.  Consider the ‘stimulated emission’ as shown
previously.
 Stimulated emission is the basis of the laser
action.
 The two photons that have been produced can
then generate more photons, and the 4
generated can generate 16 etc… etc… which
could result in a cascade of intense
monochromatic radiation.
Prof. Snehal Laddha

17. E1
E2
hυ
(a) Absorption
hυ
(b) Spontaneous emission
hυ
(c) Stimulated emission
In
hυ
Out
hυ
E2 E2
E1 E1
Absorption, spontaneous (random photon) emission and stimulated
emission.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Prof. Snehal Laddha

18. The frequency of the absorbed or emitted radiation f is
related to the difference in energy E between the
higher energy state E2 and lower energy state E1 by
expression:
E=E2-E1=hf
Where, h= 6.626x10^-34 Js is planck’s constant
Prof. Snehal Laddha

19.  The random nature of spontaneous emission
process where light is emitted by electronic
transitions from a large number of atoms gives
incoherent radiation.
 Such process provides basic mechanism for light
generation within the LED
 The light generated from stimulated photon is in
phase and has same polarization, i.e. coherent
radiation.
Prof. Snehal Laddha

20.  Therefore we must have a mechanism where N2> N1
 This is called POPULATION INVERSION
 Population inversion can be created by introducing a so call metastable
centre where electrons can piled up to achieve a situation where more N2
than N1
 The process of attaining a population inversion is called pumping and the
objective is to obtain a non-thermal equilibrium.
 It is not possible to achieve population inversion with a 2-state system.
 If the radiation flux is made very large the probability of stimulated emission
and absorption can be made far exceed the rate of spontaneous emission.
 But in 2-state system, the best we can get is N1 = N2.
 To create population inversion, a 3-state system is required.
 The system is pumped with radiation of energy E31 then atoms in state 3 relax
to state 2 non radiatively.
 The electrons from E2 will now jump to E1 to give out radiation.
Prof. Snehal Laddha

21.  A thermal equilibrium Boltzmann equation shows
us that: N1 > N2 > N3 Thus, the population
numbers of higher energy levels are smaller than
the population numbers of lower ones. This
situation is called "Normal Population".
 In a situation of normal population a photon
impinging on the material will be absorbed, and
raise an atom to a higher level.
Prof. Snehal Laddha

22.  In population inversion, at least one of the higher
energy levels has more atoms than a lower
energy level. An example is described in the
Figure below.
 In this situation there are more atoms (N3) in an
higher energy level (E3), than the number of
atoms (N2) in a lower energy level (E2).
Prof. Snehal Laddha

23. Prof. Snehal Laddha

24.  A schematic energy level diagram of a laser with
three energy levels is the figure below. The two
energy levels between which lasing occur are: the
lower laser energy level (E1), and the upper laser
energy level (E2).
Prof. Snehal Laddha

25.  To achieve lasing, energy must be pumped into
the system to create population inversion. So that
more atoms will be in energy level E2 than in the
ground level (E1). Atoms are pumped from the
ground state (E1) to energy level E3. They stay
there for an average time of 10-8 [sec], and decay
(usually with a non-radiative transition) to the
metastable energy level E2.
Prof. Snehal Laddha

26.  Since the lifetime of the metastable energy level
(E2) is relatively long (of the order of 10-3 [sec],
many atoms remain in this level.
 If the pumping is strong enough, then after
pumping more than 50% of the atoms will be in
energy level E2, a population inversion exists, and
lasing can occur.
Prof. Snehal Laddha

27.  In a four level laser compared to the equivalent
diagram of a three level laser, there is an extra
energy level above the ground state. This extra
energy level has a very short lifetime.
Prof. Snehal Laddha

28.  The pumping operation of a four level laser is
similar to the pumping of a three level laser. This
is done by a rapid population of the upper laser
level (E3), through the higher energy level (E4).
 To create population inversion, there is no need to
pump more than 50% of the atoms to the upper
laser level.
Prof. Snehal Laddha

29. Prof. Snehal Laddha

30.  The lasing threshold of a four level laser is lower.
 The efficiency is higher.
 Required pumping rate is lower.
 Continuous operation is possible.
Prof. Snehal Laddha

31. When a sizable population of electrons resides in upper levels,
this condition is called a "population inversion", and it sets the
stage for stimulated emission of multiple photons. This is the
precondition for the light amplification which occurs in a LASER
and since the emitted photons have a definite time and phase
relation to each other, the light has a high degree of coherence.
Prof. Snehal Laddha

32.  Light amplification in the laser occurs when a photon
colliding with an atom in the excited energy state causes
the stimulated emission of a second photon and then
both these photons release two more.
 Continuation of this process effectively creates
avalanche multiplication, and when the electromagnetic
waves associated with these photons are in phase,
amplified coherent emission is obtained.
 To achieve this laser action it is necessary to contain
photons within the laser medium and maintain the
conditions for coherence.
Prof. Snehal Laddha

33.  This is accomplished by placing or forming mirrors
(plane or curved) at either end of the amplifying
medium, as illustrated in Figure
Prof. Snehal Laddha

34.  The optical cavity formed is more analogous to an oscillator than an
amplifier as it provides positive feedback of the photons by
reflection at the mirrors at either end of the cavity.
 Hence the optical signal is fed back many times while receiving
amplification as it passes through the medium. The structure
therefore acts as a Fabry– Pérot resonator.
 Although the amplification of the signal from a single pass through
the medium is quite small, after multiple passes the net gain can be
large.
 Furthermore, if one mirror is made partially transmitting, useful
radiation may escape from the cavity.
Prof. Snehal Laddha

35.  A stable output is obtained at saturation when the
optical gain is exactly matched by the losses
incurred in the amplifying medium.
 The major losses result from factors such as
absorption and scattering in the amplifying
medium, absorption, scattering and diffraction at
the mirrors and nonuseful transmission through
the mirrors.
Prof. Snehal Laddha

36.  Oscillations occur in the laser cavity over a small
range of frequencies where the cavity gain is
sufficient to overcome the above losses.
 Hence the device is not a perfectly
monochromatic source but emits over a narrow
spectral band.
 The central frequency of this spectral band is
determined by the mean energy-level difference of
the stimulated emission transition
Prof. Snehal Laddha

37.  Other oscillation frequencies within the spectral
band result from frequency variations due to the
thermal motion of atoms within the amplifying
medium (known as Doppler broadening) and by
atomic collisions.
 Hence the amplification within the laser medium
results in a broadened laser transition or gain
curve over a finite spectral width, as illustrated in
Figure
Prof. Snehal Laddha

38.  The amplifying medium the radiation builds up and becomes
established as standing waves between the mirrors.
 These standing waves exist only at frequencies for which the
distance between the mirrors is an integral number of half
wavelengths.
 Thus when the optical spacing between the mirrors is L, the
resonance condition along the axis of the cavity is given by
1
Prof. Snehal Laddha

39.  where λ is the emission wavelength, n is the
refractive index of the amplifying medium and q is
an integer. Alternatively, discrete emission
frequencies f are defined by:
Prof. Snehal Laddha

40.  where c is the velocity of light. The different
frequencies of oscillation within the laser cavity
are determined by the various integer values of q
and each constitutes a resonance or mode. Since
Eqs (1) and (2) apply for the case when L is along
the longitudinal axis of the structure, the
frequencies given by Eq. (2) are known as the
longitudinal or axial modes. Furthermore, from Eq.
(2) it may be observed that these modes are
separated by a frequency interval δf where:
Prof. Snehal Laddha

41. Hence substituting for δf from Eq. (3) gives:
Prof. Snehal Laddha

42. Prof. Snehal Laddha

43.  The radiative properties of a junction diode may
be improved by the use of heterojunctions.
 A heterojunction is an interface between two
adjoining singlecrystal semiconductors with
different bandgap energies.
 Devices which are fabricated with heterojunctions
are said to have heterostructure.
Prof. Snehal Laddha

44.  Heterojunctions are classified into either an isotype (n–n
or p–p) or an anisotype (p–n).
 The isotype heterojunction provides a potential barrier
within the structure which is useful for the confinement of
minority carriers to a small active region (carrier
confinement).
 It effectively reduces the carrier diffusion length and
thus the volume within the structure where radiative
recombination may take place.
 This technique is widely used for the fabrication of
injection lasers and high-radiance LEDs.
 Isotype heterojunctions are also extensively used in
LEDs to provide a transparent layer close to the active
region which substantially reduces the absorption of light
emitted from the structure
Prof. Snehal Laddha

45.  Alternatively, anisotype heterojunctions with
sufficiently large bandgap differences improve the
injection efficiency of either electrons or holes.
 Both types of heterojunction provide a dielectric
step due to the different refractive indices at either
side of the junction.
 This may be used to provide radiation
confinement to the active region (i.e. the walls of
an optical waveguide).
 The efficiency of the containment depends upon
the magnitude of the step which is dictated by the
difference in bandgap energies and the
wavelength of the radiation.
Prof. Snehal Laddha

46. Prof. Snehal Laddha

47.  A heterojunction is shown either side of the active
layer for laser oscillation.
 The forward bias is supplied by connecting a
positive electrode of a supply to the p side of the
structure and a negative electrode to the n side.
 When a voltage which corresponds to the
bandgap energy of the active layer is applied, a
large number of electrons (or holes) are injected
into the active layer and laser oscillation
commences.
 These carriers are confined to the active layer by
the energy barriers provided by the
heterojunctions which are placed within the
diffusion length of the injected carriers.
Prof. Snehal Laddha

48.  Stimulated emission by the recombination of the
injected carriers is encouraged in the
semiconductor injection laser (also called the
injection laser diode (ILD) or simply the injection
laser) by the provision of an optical cavity in the
crystal structure in order to provide the feedback
of photons
Prof. Snehal Laddha

49.  High radiance due to the amplifying effect of
stimulated emission. Injection lasers will generally
supply milliwatts of optical output power.
 Narrow linewidth on the order of 1 nm (10 Å) or
less which is useful in minimizing the effects of
material dispersion.
 Modulation capabilities which at present extend
up into the gigahertz range and will undoubtedly
be improved upon
Prof. Snehal Laddha

50.  Relative temporal coherence which is considered
essential to allow heterodyne (coherent) detection
in high-capacity systems, but at present is
primarily of use in single-mode systems.
 Good spatial coherence which allows the output
to be focused by a lens into a spot which has a
greater intensity than the dispersed unfocused
emission.
Prof. Snehal Laddha

51.  The basic structure of this homojunction device is
shown in Figure, where the cleaved ends of the
crystal act as partial mirrors in order to encourage
stimulated emission in the cavity when electrons
are injected into the p-type region.
 However, as mentioned previously these devices
had a high threshold current density (greater than
104 A cm−2 ) due to their lack of carrier
containment and proved inefficient light sources.
Prof. Snehal Laddha

52.  The DH laser structure provides optical
confinement in the vertical direction through the
refractive index step at the heterojunction
interfaces, but lasing takes place across the whole
width of the device.
Prof. Snehal Laddha

53. Prof. Snehal Laddha

54.  To overcome these problems while also reducing
the required threshold current, laser structures in
which the active region does not extend to the
edges of the device were developed.
 A common technique involved the introduction of
stripe geometry to the structure to provide optical
containment in the horizontal plane.
Prof. Snehal Laddha

55. Prof. Snehal Laddha

56.  Generally, the stripe is formed by the creation of high-
resistance areas on either side by techniques such as
proton bombardment or oxide isolation
 The stripe therefore acts as a guiding mechanism which
overcomes the major problems of the broad-area device.
 However, although the active area width is reduced the
light output is still not particularly well collimated due to
isotropic emission from a small active region and
diffraction within the structure.
 The optical output and far-field emission pattern are also
illustrated in Figure 2.
 The output beam divergence is typically 45°
perpendicular to the plane of the junction and 9° parallel
to it. Nevertheless, this is a substantial improvement on
the broad-area laser
Prof. Snehal Laddha

57.  The stripe contact device also gives, with the
correct balance of guiding, single transverse (in a
direction parallel to the junction plane) mode
operation, whereas the broad-area device tends to
allow multimode operation in this horizontal plane.
 Numerous stripe geometry laser structures have
been investigated with stripe widths ranging from
2 to 65 μm, and the DH stripe geometry structure
has been widely utilized for optical fiber
communications.
 Such structures have active regions which are
planar and continuous.
Prof. Snehal Laddha

58. The Nd : YAG laser
 The crystalline waveguiding material which forms the active medium
for this laser is yttrium–aluminum–garnet (Y3Al5O12) doped with
the rare earth metal ion neodymium (Nd3+ ) to form the Nd : YAG
structure.
 The energy levels for both the lasing transitions and the pumping
are provided by the neodymium ions which are randomly distributed
as substitutional impurities on lattice sites normally occupied by
yttrium ions within the crystal structure.
 However, the maximum possible doping level is around 1.5%. This
laser, which is currently utilized in a variety of areas
Prof. Snehal Laddha

59.  Single-mode operation near 1.064 and 1.32 μm,
making it a suitable source for single-mode
systems.
 A narrow linewidth (<0.01nm) which is useful for
reducing dispersion on optical links.
 A potentially long lifetime, although comparatively
little data is available
 The possibility that the dimensions of the laser
may be reduced to match those of the single-
mode fiber.
Prof. Snehal Laddha

60.  The device must be optically pumped.
 A long fluorescence lifetime of the order of 10^−4
seconds which only allows direct modulation of
the device at very low bandwidths. Thus an
external optical modulator is necessary if the laser
is to be usefully utilized in optical fiber
communications.
 The above requirements (i.e. pumping and
modulation) tend to give a cost disadvantage in
comparison with semiconductor lasers.
Prof. Snehal Laddha

61. It comprises an Nd : YAG rod with its ends ground flat and then
silvered. One mirror is made fully reflecting while the other is about
10% transmitting to give the output
Prof. Snehal Laddha

62.  The basic structure of a glass fiber laser is shown
in Figure
Prof. Snehal Laddha

63.  An optical fiber, the core of which is doped with
rare earth ions, is positioned between two mirrors
adjacent to its end faces which form the laser
cavity.
 Light from a pumping laser source is launched
through one mirror into the fiber core which is a
waveguiding resonant structure forming a Fabry–
Pérot cavity.
 The optical output from the device is coupled
through the mirror on the other fiber end face, as
illustrated in Figure
Prof. Snehal Laddha

64.  Thus the fiber laser is effectively an optical
wavelength converter in which the photons at the
pumping wavelength are absorbed to produce the
required population inversion and stimulated
emission;
 This provides a lasing output at a wavelength
which is characterized by the dopant in the fiber.
Prof. Snehal Laddha