UNIT 3

OPTICAL SOURCES
Introduction:
• The optical source is often considered to be the active component in
an optical fiber communication system.
• Its fundamental function is to convert electrical energy in the form of
a current into optical energy (light) in an efficient manner which
allows the light output to be effectively launched or coupled into the
optical fiber.
Major requirements for an optical
fiber
emitter
1. A size and configuration compatible with launching light into an
optical fiber. Ideally, the light output should be highly directional.
2. Must accurately track the electrical input signal to minimize
distortion and noise. Ideally, the source should be linear.
3. Should emit light at wavelengths where the fiber has low losses and
low dispersion and where the detectors are efficient.
4. Preferably capable of simple signal modulation over a wide
bandwidth extending from audio frequencies to beyond the
gigahertz range.
5. Must couple sufficient optical power to overcome attenuation in the
fiber plus additional connector losses and leave adequate power to
drive the detector.
6. Should have a very narrow spectral bandwidth (linewidth) in order
to minimize dispersion in the fiber.
7. Must be capable of maintaining a stable optical output which is largely unaffected
by changes in ambient conditions (e.g. temperature).
8. It is essential that the source is comparatively cheap and highly reliable in order
to compete with conventional transmission techniques.
LED(Light Emitting Diode)
• The normally empty conduction band of the semiconductor is
populated by electrons injected into it by the forward current
through the junction, and light is generated when these
electrons recombine with holes in the valence band to emit a
photon.
• This is the mechanism by which light is emitted from an LED.
• The LED can therefore operate at lower current densities than
the injection laser
• But the emitted photons have random phases and the device is
an incoherent optical source
• gives a much wider spectral linewidth.
• LED supports many optical modes within its structure and is
therefore often used as a multimode source.
DISADVANTAGES OVER LASER:
• (a) generally lower optical power coupled into a fiber
(microwatts);
• (b) usually lower modulation bandwidth;
• (c) harmonic distortion
Advantages:
• Simpler fabrication
• Cost. The simpler construction of the LED leads to much reduced
cost.
• Reliability. The LED does not exhibit catastrophic degradation and
has proved far less sensitive to gradual degradation than the injection
laser.
• Generally less temperature dependence. The light output against
current characteristic is less affected by temperature than the
corresponding characteristic for the injection laser.
• Simpler drive circuitry. This is due to the generally lower drive
currents and reduced temperature dependence which makes
temperature compensation circuits unnecessary.
• Linearity. Ideally, the LED has a linear light output against current
characteristic
 Optical emission from
semiconductors
• A perfect semiconductor crystal containing no impurities or lattice defects
is said to be intrinsic. The energy band structure of an intrinsic
semiconductor shows the valence and conduction bands separated by a
forbidden energy gap or bandgap Eg, the width of which varies for different
semiconductor materials.
• In the semiconductor at a temperature above absolute zero where thermal
excitation raises some electrons from the valence band into the con-duction
band, leaving empty hole states in the valence band. These thermally
excited electrons in the conduction band and the holes left in the valence
band allow conduction through the material, and are called carriers.
• When donor impurities are added, thermally excited electrons from the
donor levels are raised into the conduction band to create an excess of
negative charge carriers and the semiconductor is said to be n-type, with the
majority carriers being electrons.
• The Fermi level corresponding to this carrier distribution is raised to a
position above the center of the bandgap.
• When acceptor impurities are added, thermally excited electrons are
raised from the valence band to the acceptor impurity levels leaving an
excess of positive charge carriers in the valence band and creating a p-
type semiconductor where the majority carriers are holes. In this case
Fermi level is lowered below the center of the bandgap.
• The p–n junction diode is formed by creating adjoining p- and n-type
semiconductor layers in a single crystal, as shown in Figure 6.10(a). A
thin depletion region or layer is formed at the junction through carrier
recombination which effectively leaves it free of mobile charge carriers
(both electrons and holes).
• This establishes a potential barrier between the p- and n-type regions
which restricts the interdiffusion of majority carriers from their respective
regions, as illustrated in Figure 6.10(b). In the absence of an externally
applied voltage no current flows as the potential barrier prevents the net
flow of carriers from one region to another.
• The width of the depletion region and thus the magnitude of the potential
barrier is dependent upon the carrier concentrations (doping) in the p-
and n-type regions and any external applied voltage. When an external
positive voltage is applied to the p-type region with respect to the n-
type, both the depletion region width and the resulting potential barrier
are reduced and the diode is said to be forward biased.
• Electrons from the n-type region and holes from the p-type region can
flow more readily across the junction into the opposite type region.
These carriers are effectively injected across the junction by the
application of the external voltage and form a current flow through the
device as they continuously diffuse away from the interface.
• This situation in suitable semiconductor materials allows carrier
recombination with the emission of light.
• Excess carrier population is therefore decreased by recombination which
may be radiative or nonradiative.
• In nonradiative recombination the energy released is dissipated in the
form of lattice vibrations and thus heat. However, in band-to-band
radiative recombination the energy is released with the creation of a
photon
Spontaneous Emission:
Material Used in LED Sources
 LED Power and Efficiency
• The absence of optical amplification through stimulated
emission in the LED tends to limit the internal quantum
efficiency (ratio of photons generated to injected electrons)
of the device.
• The power generated internally by an LED may be
determined by consideration of the excess electrons and
holes in p and n type material respectively.
• The excess density of electrons Δn and holes Δp is equal
since the injected carriers are created and recombined in
pairs such that charge neutrality is maintained within the
structure.
• In extrinsic materials one carrier type will have a much
higher concentration than the other and hence in the p-type
region, for example, the hole concentration will be much
greater than the electron concentration.
• The excess carrier density decays exponentially with time t

where Δn(0) is the initial injected excess electron density

and t represents the total carrier recombination lifetime. In
most cases, however, Δn is only a small fraction of the
majority carriers and comprises all of the minority carriers.
• When there is a constant current flow into the junction diode, an
equilibrium condition is established.
• In this case, the total rate at which carriers are generated will be
the sum of the externally supplied and the thermal generation
rates.
• A rate equation for carrier recombination in the LED can be
expressed in the form of the externally supplied and the thermal
generation rates.
•
• ………….(1)
J (current density A/m2) written as J/ed in electrons per cubic meter
per second ; d is thickness of recombination region
• At equilibrium rate become zero in the above equation i.e injected
carrier rate becomes equal to recombination rate.
• The condition for equilibrium is obtained by setting the
derivative in Eq. (1) to zero.

• In the steady state the total number of carrier

recombinations per second or the recombination rate r t
will be
•

• where rr is the radiative recombination rate per unit

volume and rnr is the non-radiative recombination rate per
unit volume.
• when the forward-biased current into the device is i then the
total number of recombinations per second Rt becomes:

• Excess carriers can recombine either radiatively or non-

radiatively. While in the former case a photon is generated, in
the latter case the energy is released in the form of heat (i.e.
lattice vibrations).
• The LED internal quantum efficiency* ηint, which can be
defined as the ratio of the radiative recombination rate to the
total recombination rate,
• The internal quantum efficiency for the LED is obtained only
from the spontaneous radiation and hence is written as ηint.

• where Rr is the total number of radiative recombinations per

second. Substituting the value of Rt
• Since Rr is also equivalent to the total number of photons
generated per second and each photon has an energy equal to hf
joules, then the optical power generated internally by the LED,
Pint is:
•

• ηint must be multiplied by a factor representing the external

quantum efficiency* ηext to provide an overall quantum efficiency
for the device.
• In terms of carrier lifetime internal quantum efficiency can also
be expressed . The radiative minority carrier lifetime is τ r
=Δn/rr and the non-radiative minority carrier lifetime is
τnr=Δn/rnr
• The internal quantum efficiency is:

• The total recombination lifetime τ can be written as τ = Δn/r t

• ηint must be multiplied by a factor representing the external quantum
efficiency* ηext to provide an overall quantum efficiency for the device.
• The external quantum efficiency may be defined as the ratio of the
photons emitted from the device to the photons internally generated.
• However, it is sometimes defined as the ratio of the number of photons
emitted to the total number of carrier recombinations (radiative and
nonradiative).
• The radiation geometry for an LED which emits through a planar
surface is essentially Lambertian in that the surface radiance (the
power radiated from a unit area into a unit solid angle, given is
constant in all directions.
• The Lambertian intensity distribution where the maximum intensity
is perpendicular to the planar surface but is reduced on the sides in
proportion to the cosine of the viewing angle θ as the apparent area
varies with this angle.
• This reduces the external power efficiency to a few percent as most
of the light generated within the device is trapped by total internal
reflection
• External power efficiency ηep is defined as the ratio of the optical
power emitted externally Pe to the electric power provided to the
device P

• Also, the optical power emitted Pe into a medium of low refractive

index n from the face of a planar LED fabricated from a material of
refractive index nx is given approximately by

• Where F is the transmission factor of the semiconductor–external

interface
• A further loss is encountered when coupling the light output into a
fiber. Considerations of this coupling efficiency are very complex.
however, it is possible to use an approximate simplified approach
• All the light incident on the exposed end of the core within the
acceptance angle θa is coupled, then for a fiber in air,

• Also, incident light at angles greater than θa will not be coupled.

• For a Lambertian source, the radiant intensity at an angle θ, I(θ), is
given by:
• where I0 is the radiant intensity along the line θ = 0.
• Considering a source which is smaller than, and in close
proximity to, the fiber core, and assuming cylindrical symmetry,
the coupling efficiency ηc is given by:
The double-heterojunction LED
• The principle of operation of the DH LED
• The device shown consists of a p-type GaAs layer sandwiched
between a p-type AlGaAs and an n-type AlGaAs layer.
• When a forward bias is applied electrons from the n-type layer are
injected through the p–n junction into the p-type GaAs layer
where they become minority carriers.
• These minority carriers diffuse away from the junction,
recombining with majority carriers (holes).
• Photons are therefore produced with energy corresponding to the
bandgap energy of the p-type GaAs layer.
• The injected electrons are inhibited from diffusing into the p-type
AlGaAs layer because of the potential barrier presented by the p–p
heterojunction
The double-heterojunction
LED: (a) the layer structure,
shown with an
applied forward bias; (b) the
corresponding energy band
diagram
LED Structures
• Planar LED
• Dome LED
• Surface Emitter LED
• Edge Emitter LED
• Super luminescent LED
• Resonant Cavity and Quantum Dot
Only two extensively use in OFC(SLED and ELED)
Planar LED

• Simplest of the structures that are available.

• Fabricated by liquid or vapour phase epitaxial
processes over GaAs surface.
• This involves a p-type diffusion into the n-type
substrate in order to create the junction. Forward
current flow through the junction gives Lambertian
spontaneous emission and the device emits light
from all surfaces.
• only a limited amount of light escapes the structure
due to total internal reflection
• the Radiance low
Dome LED
• A hemisphere of n-type GaAs around p-region.
• Higher external power efficiency than planar LED.
• Geometry of the structure is such that the dome must be far
larger than the active recombination area, which gives a
greater effective emission area and thus reduces the radiance
Surface Emitter LEDs
• A method for obtaining high radiance is to restrict the emission
to a small active region within the device.
• Used an etched well in a GaAs substrate in order to prevent
heavy absorption of emitted radiation.
• Low thermal impedance in active region allowing high current
densities and giving high radiance emission into optical fiber.
• employing DH structures giving increased efficiency.
• The structure of a high-radiance etched well DH surface emitter
The emission from the active layer is essentially isotropic,
although the external emission distribution may be considered
• Lambertian with a beam width of 120° due to refraction from a
high to a low refractive index at the GaAs–fiber interface.
Edge emitter LEDs
• high-radiance structure currently used in optical
communications is the stripe geometry DH edge
emitter LED (ELED).
• Most of the propagating light is emitted at one end face
only due to a reflector on the other end face and an
antireflection coating on the emitting end face.
• The effective radiance at the emitting end face can be
very high giving an increased coupling efficiency into
small-NA fiber compared with the surface emitter.
Superluminescent LEDs
• Another device geometry which is providing significant benefits
over both SLEDs and ELEDs for communication applications is
the superluminescent diode or SLD.
• This device type offers advantages of: (a) a high output power;
(b) a directional output beam; and (c) a narrow spectral
linewidth
Resonant cavity and quantum-dot
LEDs

Structures of resonant cavity light-emitting diodes: surface

emitting using DBR mirrors;
LED characteristics
Optical output power:
• The ideal light output power against current characteristic for
an LED.
• LED is a very linear device in comparison with the majority
of injection lasers and hence it tends to be more suitable for
analog transmission where severe constraints are put on the
linearity of the optical source.
• The light output power against temperature characteristics for
three important LED structures operating at a wavelength of
1.3 μm are shown in Figure 7.19
• It may be observed that the edge-emitting device exhibits a
greater temperature dependence than the surface emitter and that
the output of the SLD with its stimulated emission is strongly
dependent on the junction temperature.
• This latter factor is further emphasized in the light output against
current characteristics for a superluminescent LED
Output spectrum
• The spectral linewidth of an LED operating at room
temperature in the 0.8 to 0.9 μm wavelength band

• Figure 7.22 LED output spectra: (a) output spectrum for an AlGaAs surface
emitter with doped active region (b) output spectra for an InGaAsP surface
emitter showing both the lightly doped and heavily doped cases
• The differences in the output spectra between InGaAsP
SLEDs and ELEDs caused by self-absorption along the
active layer of the devices are displayed in Figure 7.23.
• It may be observed that the FWHP points are around 1.6
times smaller for the ELED than the SLED
Modulation bandwidth:
• The modulation bandwidth in optical communications may be
defined in either electrical or optical terms. In electrical
definition where the electrical signal power has dropped to half
its constant value corresponds to the electrical 3 dB point. In
optical terms modulation bandwidth being the frequency range
between zero and high-frequency 3 dB point.
Reliability:
• LEDs are not generally affected by the catastrophic
degradation mechanisms which can severely affect
injection lasers.
• In addition, LEDs do exhibit gradual degradation
which may take the form of a rapid degradation mode*
or a slow degradation mode.