this is realted to ofc used in electronics

1. Optical Sources
• Two classes of light sources that are widely used for
fiber optic communications are heterojunction
structured semiconductor laser diodes (also referred
to as injection laser diodes or ILDs) and light-
emitting diodes (LEDs).
• A heterojunction consists of two adjoining
semiconductor materials with different bandgap
energies.
• light-emitting region of both LEDs and laser diodes
consists of a pn junction.
• When this junction is forward biased, electrons and
holes are injected into the p and n regions,
respectively.

2. • These injected minority carriers can recombine
either radiatively, in which case a photon of energy
hv is emitted, or nonradiatively, whereupon the
recombination energy is dissipated in the form of
heat.
• This pn junction is thus known as the active or
recombination region.
• A major difference between LEDs and laser diodes is
that the optical output from an LED is incoherent
(different frequency and phase difference), whereas
that from a laser diode is coherent (same frequency
and zero or constant phase difference).

3. • In a coherent source, the optical energy is produced
in an optical resonant cavity. The optical energy
released from this cavity has spatial and temporal
coherence, which means it is highly monochromatic
and the output beam is very directional.
• In an incoherent LED source, no optical cavity exists
for wavelength selectivity
• spatial coherence = is correlation between waves at
different points in space (tells directionality of laser
beams)
• temporal coherence= is a measure of average
correlation between the value of a wave and itself
delayed by time t. (tells how monochromatic a light
is)

4. Characteristics of Light Source of Communication
1) It must be possible to operate the device continuously at a variety of temperatures for
many years.
2) It must be possible to modulate the light output over a wide range of modulating
frequencies.
3) For fiber links, the wavelength of the output should coincide with one of transmission
windows for the fiber type used.
4) To couple large amount of power into an optical fiber, the emitting area should be
small.
5) To reduce material dispersion in an optical fiber link, the output spectrum should be
narrow.
6) The power requirement for its operation must be low.
7) The light source must be compatible with the modern solid state devices.
8) The optical output power must be directly modulated by varying the input current to
the device.
9) Better linearity to prevent harmonics and intermodulation distortion.
10) High coupling efficiency.
11) High optical output power.
12) High reliability.
13) Low weight and low cost.

5. Light-Emitting Diodes (LEDs)
• Conventional p-n junction is called as homojunction as,
same semiconductor material is used on both sides of
the junction.
• The electron-hole recombination occurs in relatively wide
layer = 10μm. As the carriers are not confined to the
immediate vicinity of junction, hence high current
densities can not be realized.
• The carrier confinement problem can be resolved by
sandwiching a thin layer ( = 0.1 μm) between p-type and
n-type layers.
• The carrier confinement occurs due to bandgap
discontinuity of the junction. Such a junction is call
heterojunction and the device is called double
heterostructure.

6. • LEDs are best suitable optical source for an optical
communication system when the requirements is –
i) Bit rate f 100-200Mb/sec.
ii) Optical power in tens of micro watts.
LED Structures
• To be useful in fiber transmission applications, an LED must
have
i. high radiance output,
ii. fast emission response time,
iii. high quantum efficiency.

7. • Its radiance (or brightness) is a measure, in watts, of
the optical power radiated into a unit solid angle
per unit area of the emitting surface. High radiances
are necessary to couple sufficiently high optical
power levels into a fiber.
• The emission response time is the time delay
between the application of a current pulse and the
onset of optical emission.
• The quantum efficiency is related to the fraction of
injected electron–hole pairs that recombine
radiatively.

8. • LED structure must provide a means of
confining the charge carriers and the
stimulated optical emission to the active
region of the pn junction where radiative
recombination takes place.
• Carrier confinement is used to achieve a high
level of radiative recombination in the active
region of the device.
• Optical confinement is of importance for
preventing absorption of the emitted
radiation by the material surrounding the pn
junction.

9. • The most effective of these structures is the
configuration shown in Fig. This is referred to as a
double-heterostructure (or heterojunction) device
because of the two different alloy layers on each side
of the active region.
• By means of this sandwich structure of differently
composed alloy layers, both the carriers and the
optical field are confined in the central active layer.
• The bandgap differences of adjacent layers confine
the charge carriers (as in b) while the differences in
the indices of refraction of adjoining layers confine
the optical field to the central active layer. (as in c)

11. • There are two main types of LED used in optical fiber
links –
1. Surface emitting LED.
2. Edge emitting LED.

12. Surface emitting LED

13. • In the surface emitter, the plane of the active light-
emitting region is oriented perpendicularly to the
axis of the fiber, as shown in Fig.
• In this configuration, a well is etched through the
substrate of the device, into which a fiber is then
cemented in order to accept the emitted light.
• The circular active area in practical surface emitters is
nominally 50 mm in diameter and up to 2.5 mm
thick.
• The emission pattern is essentially isotropic with a
120° half-power beam width.

14. • This isotropic pattern from such a surface emitter is
called a lambertian pattern.
• In this pattern, the source is equally bright when
viewed from any direction, but the power diminishes
as cos θ, where θ is the angle between the viewing
direction and the normal to the surface (this is
because the projected area one sees decreases as
cos θ).
• Thus, the power is down to 50 percent of its peak
when θ = 60°, so that the total half-power beam
width is 120°.

15. Edge emitting LED

16. • The edge emitter depicted in Fig.consists of an active junction
region, which is the source of the incoherent light, and two guiding
layers.
• The guiding layers both have a refractive index lower than that of
the active region but higher than the index of the surrounding
material.
• This structure forms a waveguide channel that directs the optical
radiation toward the fiber core.
• To match the typical fiber-core diameters (50–100 mm), the contact
stripes for the edge emitter are 50–70 mm wide. Lengths of the
active regions usually range from 100 to 150 mm.
• The emission pattern of the edge emitter is more directional than
that of the surface emitter, as is illustrated in Fig.
• In the plane parallel to the junction, where there is no waveguide
effect, the emitted beam is lambertian (varying as cos θ) with a
half-power width of θ parallel = 120°. In the plane perpendicular to
the junction, the half-power beam θ perpendicular has been made
as small as 25–35° by a proper choice of the waveguide thickness.

17. Features of ELED
• Linear relationship between optical output and current.
• Spectral width is 25 to 400 nm for λ = 0.8 – 0.9 μm.
• Modulation bandwidth is much large.
• Are more reliable.
• ELEDs have better coupling efficiency than surface
emitter.
• ELEDs are temperature sensitive.
Usage :
1. LEDs are suited for short range narrow and medium
bandwidth links.
2. Suitable for digital systems up to 140 Mb/sec.
3. Long distance analog links.

18. Light Source Materials
• For operation in the 800-to-900-nm spectrum, the
principal material used is the ternary alloy
• The ratio x of aluminum arsenide to gallium arsenide
determines the bandgap of the alloy and,
correspondingly, the wavelength of the peak emitted
radiation.
• An example of the emission spectrum of a
LED with x = 0.08 is shown in fig.

19. • The peak output power
occurs at 810 nm.
• The width of the
spectral pattern at its
half-power point is
known as the full-width
half maximum (FWHM)
spectral width.
• As shown in Fig. 4.12,
this FWHM spectral
width is 36 nm.

20. • At longer wavelengths the which is a
quaternary alloy is one of the primary material
candidates.
• Using the fundamental quantum mechanical
relationship between energy E and frequency v,
• The peak emission wavelength λ in micrometers can
be expressed as a function of the bandgap energy Eg
in electron volts by the equation

21. • The bandgap energy (Eg) can be controlled by two
compositional parameters x and y, within direct
bandgap region.
• In the ternary alloy GaAlAs the bandgap energy Eg in
electron volts for values of x between zero and 0.37
can be found from the empirical equation
• In the quaternary alloy expression
relating Eg and x,y are

22. Problem: Compute the emitted wavelength from a
ternary optical source having x = 0.07.
Soln:

23. Problem: Consider the material alloy
Find (a) the bandgap of this material;
(b) the peak emission wavelength
• Soln: Comparing the alloy with the quaternary alloy
composition it is found that x = 0.26 and y = 0.57

24. Quantum Efficiency and LED Power
• The internal quantum efficiency in the active region
is the fraction of the electron–hole pairs that
recombine radiatively.
• If the radiative recombination rate is Rr and the
nonradiative recombination rate is Rnr, then the
internal quantum efficiency ηint is the ratio of the
radiative recombination rate to the total
recombination rate

25. • The radiative recombination lifetime is τr = n/Rr
• The nonradiative recombination lifetime is
τnr = n/Rnr.
• Thus the internal quantum efficiency can be
expressed as

26. • If the current injected into the LED is I and q is
electron charge then total number of recombinations
per second is
Rr + Rnr = I/q
Substituting this in quantum efficiency equation

28. Advantages of LED
1. Simple design.
2. Ease of manufacture.
3. Simple system integration.
4. Low cost.
5. High reliability.
Disadvantages of LED
1. Refraction of light at semiconductor/air interface.
2. The average life time of a radiative recombination is
only a few nanoseconds, therefore modulation BW
is limited to only few hundred megahertz.
3. Low coupling efficiency.
4. Large chromatic dispersion.

29. Injection Laser Diode (ILD)
• The laser is a device which amplifies the light, hence
the LASER is an acronym for Light Amplification by
Stimulated Emission of Radiation.
• The operation of the device may be described by the
formation of an electromagnetic standing wave
within a cavity which provides an output of
monochromatic highly coherent radiation.
• Material absorb light than emitting. Three different
fundamental process occurs between the two energy
states of an atom.
1) Absorption 2) Spontaneous emission 3) Stimulated
emission.

30. • Laser action is the result of three process absorption
of energy packets (photons), spontaneous emission,
and stimulated emission. (These processes are
represented by the simple two-energy-level
diagrams).
Where E1 is the lower state energy level.
E2 is the higher state energy level.

31. • Quantum theory states that any atom exists only
in certain discrete energy state, absorption or
emission of light causes them to make a
transition from one state to another.
• The frequency of the absorbed or emitted
radiation f is related to the difference in energy E
between the two states.
• If E1 is lower state energy level, E2 is higher state
energy level.
• E = (E2 – E1) = h.f
• Where, h = 6.626 x 10-34 J/s (Plank’s constant).

32. • An atom is initially in the lower energy state, when
the photon with energy (E2 – E1) is incident on the
atom, it will be excited into the higher energy state
E2 through the absorption of the photon.

33. • When the atom is initially in the higher energy state
E2, it can make a transition to the lower energy state
E1 providing the emission of a photon at a frequency
corresponding to E = h.f. The emission process can
occur in two ways.
A) By spontaneous emission in which the atom
returns to the lower energy state in random manner.
B) By stimulated emission when a photon having
equal energy to the difference between the two
states (E2 – E1) interacts with the atom causing it to
the lower state with the creation of the second
photon.

35. • Spontaneous emission gives incoherent radiation while
stimulated emission gives coherent radiation.
• Hence the light associated with emitted photon is of
same frequency of incident photon, and in same phase.
• It means that when an atom is stimulated to emit light
energy by an incident wave, the liberated energy can add
to the wave in constructive manner.
• The emitted light is bounced back and forth internally
between two reflecting surface.
• The bouncing back and forth of light wave cause their
intensity to reinforce and build-up. The result in a high
brilliance, single frequency light beam providing
amplification.